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Formation mechanism of graphene buffer layer on SiC(0 0 0 1)
Abstract
The initial stage of the growth of graphene on SiC with the underlying mechanism of carbon layer early stage formation on the single crystal silicon carbide surface was studied using silicon sublimation technique. The obtained buffer layer is organized in a form of carbon regions with 10% of sp3 defects separated 10–15 Å. Raman spectroscopy was used to assess the degree of the buffer layer’s disorder. The intensity of I(D) and I(GB) buffer peaks was found to be proportional to the number of defects. Although the layer is not fully saturated with carbon atoms, it remains impenetrable. However, sublimation from the steps side walls which are not covered by the buffer layer is possible. It was observed that in the vicinity of the macro-step edges the sublimation is more effective, which leads to the production of additional free C atoms, filling the buffer layer structure, subsequently decreasing sp3 hybridization, to about 1–2%. This healing process which also continues during the graphene layer growth is reflected in a decrease in D line intensity and finally in formation of the well-organized buffer layer.
... Introducing Ar at different temperatures during the graphitization process may provide an alternative pathway to influence the phase transition temperature between different surface reconstructions, and hence enable the growth of smooth MLG without the need of special pre-treatment. However, this approach has not been explored despite the intense investigation of buffer layer properties and optimization [4,[31][32][33][34]. ...
... Figure 2 shows µ-Raman spectra of buffer layers on n-type 4H-SiC, for which the Ar gas was introduced at T Ar = 800 • C, 900 • C, 1150 • C and 1300 • C, respectively (BL1-BL4, Table 1). The Raman spectra reveal features in the range of 1200-1700 cm −1 , typical for the buffer layer [31,33,42]. The band around 1330 cm −1 appears to be on par in terms of intensity with the band around 1580 cm −1 for all samples. ...
... It has been argued that the buffer layer Raman spectrum is not composed of discrete peaks but rather reflects the vibrational density of states [42]. The integrated intensity ratio of the D-band around 1330 cm −1 (D BL ) and the G-band 1580 cm −1 (G BL ) can be used to evaluate the content of sp 3 hybridization [31] or discuss correlations associated with buffer structure in general [33]. We will come back to this question when comparing buffer layers grown on n-type and SI 4H-SiC. ...
In this work we have critically reviewed the processes in high-temperature sublimation growth of graphene in Ar atmosphere using closed graphite crucible. Special focus is put on buffer layer formation and free charge carrier properties of monolayer graphene and quasi-freestanding monolayer graphene on 4H–SiC. We show that by introducing Ar at higher temperatures, TAr, one can shift the formation of the buffer layer to higher temperatures for both n-type and semi-insulating substrates. A scenario explaining the observed suppressed formation of buffer layer at higher TAr is proposed and discussed. Increased TAr is also shown to reduce the sp3 hybridization content and defect densities in the buffer layer on n-type conductive substrates. Growth on semi-insulating substrates results in ordered buffer layer with significantly improved structural properties, for which TAr plays only a minor role. The free charge density and mobility parameters of monolayer graphene and quasi-freestanding monolayer graphene with different TAr and different environmental treatment conditions are determined by contactless terahertz optical Hall effect. An efficient annealing of donors on and near the SiC surface is suggested to take place for intrinsic monolayer graphene grown at 2000 ∘C, and which is found to be independent of TAr. Higher TAr leads to higher free charge carrier mobility parameters in both intrinsically n-type and ambient p-type doped monolayer graphene. TAr is also found to have a profound effect on the free hole parameters of quasi-freestanding monolayer graphene. These findings are discussed in view of interface and buffer layer properties in order to construct a comprehensive picture of high-temperature sublimation growth and provide guidance for growth parameters optimization depending on the targeted graphene application.
... The XPS results presented above show a clear indication of a buffer layer between the graphene and the SiC substrate. Several groups have investigated the evolution of the Raman spectrum during the formation of graphene by sublimation of Si from SiC [36][37][38][39]. Both Schumann et al. [36] as well Fig. 3. C1s XPS spectra obtained using a) the bulk and b) the surface analysis modes. ...
... as Strupinski et al. [37] report Raman spectra for the buffer layer with supposedly no graphene overlayer. These spectra are essentially replicas of that shown in Fig. 4b with respect to both peak broadening and relative intensities for the D, G, and two phonon region. ...
... In contrast, Fromm et al. [38] observe the D and G peak broadening equivalent to that reported here, but they do not observe any peaks in the two phonon region. As pointed out by Strupinski et al. [37], this discrepancy may be partially due the discontinuous nature of the graphene films formed by Si sublimation from SiC and the fact that the laser spot may cover both buffer and graphene areas at the same time in some cases. Fromm et al. [38] make the crucial observation and demonstrate that the Raman spectra for the buffer layer represent a mapping of the vibrational density of states (vDOS) of the buffer layer. ...
A two-step process using halogen based plasma etching combined with atmospheric pressure rapid thermal annealing has been used to synthesize few layer graphene films on 6H-SiC (0001) surfaces. In this process, the 6H-SiC substrates were etched under different plasma conditions to produce a carbon rich surface layer. This was followed by rapid thermal annealing in a flow of atmospheric pressure Ar to produce the graphene films. The effects of different etching conditions, heating rates, and average and maximum annealing temperatures were investigated. Changes in surface and near-surface composition after each step were characterized by x-ray photoelectron spectroscopy and overall film quality was assessed using Raman spectroscopy. Film defects associated with this synthesis method included a buffer layer between the SiC substrate and graphene as well as oxygen-based defects on the graphene surface. Electrical characterization of these films was performed using both two and four point methods. In the two point measurements, these films exhibited back-to-back Schottky behavior from which the Schottky barrier height, carrier density and mobility were determined. The four point measurements were used to determine the contact and film resistivity as well as the transfer length.
... To this purpose, a sample has been produced at a growth temperature of 1600°C with a temperature ramp of 0.33°C/s and an annealing time of 300 s under a low Ar pressure of 10 mbar. We emphasize that back to the time when we produced this sample, we regarded this sample as a true BL based on the comparison of its Raman signature with the literature [163,164]. However, our recent results suggest that this sample is more alike with a carbon structure between the buffer layer phase and amorphous carbon phase [165]. ...
... The lack of 2D mode is a signature that no or only small portion of well-crystallized honeycomb structure is formed on this sample. Indeed, the lineshape of this spectrum is very comparable to the spectra reported for the BL in the literature [144,163,166,167], as we discussed in section 2.2 ( Fig. 2.8 (b)). Most of the BL peaks are situated between the region of 1100 and 1700 cm -1 (outlined in red dashed rectangular in Fig. 3.1 (c)) with some low broad bands around 2D-peak region. ...
... At that moment, we have noticed the variety of BL Raman signature in terms of peak integrated intensity and position in the studied samples. In the literature, despite Raman signature of BL being reported and the inhomogeneity of these spectra having been noticed [122,144,163,164], a systematic and statistical analysis of BL characteristic is still missing. Moreover, only few works have investigated the coupling between graphene and BL by Raman technique. ...
This manuscript presents a work aiming to optimize a reproducible and controlled growth process of a monolayer graphene on Si-face of SiC (SiC (0001)) by sublimation under low argon pressure, i.e. 10 mbar. This low pressure process is challenging regarding the results in the literature. Various complementary techniques as optical microscopy, Raman spectroscopy, atomic force microscope, scanning tunneling microscope, and Hall Effect measurements have been performed on the samples in order to validate the monolayer graphene growth and investigate their surface morphology, their structural and electronic properties. All the results obtained from these measurements confirm the control of homogeneous, continuous and large-size (6×6 mm²) monolayer graphene from our optimized growth process. More than 50 monolayers graphene were produced during this thesis, validating a reproducible process in a prototype furnace developed by Annealsys, local company in Montpellier. The step-flow growth mode which encourages the formation of step-terrace surface structures is obtained under this unclassical growth condition contrary as established in the literature. Moreover, we have investigated the effect of the temperature ramp on the SiC morphology to evaluate the impact of the width of the terraces on electronic properties of graphene. Samples with terraces larger than 10 µm have been obtained allowing original transport measurements localized on only one terrace.Thanks to the reproducibility of our optimized growth process, further characterization studies on epitaxial graphene were investigated. The first carbon layer grown on SiC (0001) is a buffer layer covalently linked to SiC. Then a second buffer layer grows under the first one that becomes graphene. This well-known buffer layer at graphene / SiC (0001) interface has been investigated in this thesis to complete the poor literature on this topic. Statistically buffer Raman signatures have been obtained and compared to the literature demonstrating an inhomogeneous buffer layer. Furthermore, we have developed two graphene transfer techniques aiming to exfoliate graphene layer and leave behind only the buffer layer on the sample surface. The Raman signatures of buffer layer in these two cases (with or without graphene coverage) have been compared. We believe the evidenced evolution could be related to the coupling between graphene and buffer layer. Two major results illustrate this coupling: (i) the Raman signature of buffer layer increases in integrated intensity after the graphene transfer and (ii) two fines peaks are observed only in epitaxial graphene spectra and not in uncovered buffer layer spectra.The last part of this work concerns the electrical properties of monolayer graphene on SiC (0001). Contrary to the typical n-type doping of epitaxial graphene, the low p-type residual Hall concentration observed in our samples has been related to the atmospheric effect. More precisely, the charged impurities deposited on the sample surface could lead to the formation of electron-hole puddles, resulting in an inhomogeneous doping. The potential fluctuation has been estimated by fitting the experimental data using a model of two types of charges. Moreover, we have shown that the doping type change from p-type to n-type under vacuum condition or under UV illumination. This could be explained by desorption of the charged absorbents during the pumping or UV illumination. These results demonstrate the possibility of tuning the electrical properties of our samples by external factor such as UV light.
... The spectrum (Fig. 1b) is comparable to the buffer layer, one reported in the literature. [22][23][24][25] The structure of the buffer layer which is different from the graphene layer consists of a reconstructed C-rich layer in which covalent bonds between some of the carbon atoms of the buffer layer and underlying silicon atoms exist. Several authors [22][23][24][25] have reported different characteristics for this buffer layer such as the number of peaks used for fitting and the peak positions ( Table 1). ...
... [22][23][24][25] The structure of the buffer layer which is different from the graphene layer consists of a reconstructed C-rich layer in which covalent bonds between some of the carbon atoms of the buffer layer and underlying silicon atoms exist. Several authors [22][23][24][25] have reported different characteristics for this buffer layer such as the number of peaks used for fitting and the peak positions ( Table 1). The Raman spectrum of sample A (inset Fig. 1b) was fitted with four Raman and AFM analysis of sample A. (a) 9 mm  6.5 mm Raman map of integrated intensity (scale in counts s À1 cm À1 ) of the buffer signal from 1000 to 1750 cm À1 ; (b) average Raman spectrum of the Raman map in (a) and buffer layer Raman signal fitted by four Gaussian functions (inset); the black and magenta lines represent the raw data and total fit, respectively; four Gaussian peaks are shown in red, green, light magenta, and brown; (c) topographic AFM image (scale in nm) of the same area as Raman map in (a); (d) the extracted height profiles from black line (1) and blue line (2) in (c); (e) phase AFM image (scale in degrees) corresponding to (c), the red rectangle indicates the Raman map area; (f) Raman map of 2D band integrated intensity (counts) from 2650 to 2800 cm À1 . ...
... Tiberj et al. 22 Strupinski et al. 24 Fromm et al. 25 Growth components as Fromm et al. 25 but at different peak positions. Our buffer spectrum line shape is similar to both, the one reported by Strupinski et al. 24 in which growth conditions were close to ours, and the one in ref. 22 where higher temperature and pressure were used. ...
Silicon carbide (SiC) sublimation is the most promising option to achieve transfer-free graphene at the wafer-scale. We investigated the initial growth stages from buffer layer to monolayer graphene on SiC (0001) as a function of annealing temperature at low argon pressure (10 mbar). A buffer layer, fully covering the SiC substrate, forms when the substrate is annealed at 1600°C. Graphene formation starts from the step edges of the SiC substrate at higher temperature (1700°C). The spatial homogeneity of the monolayer graphene was observed at 1750°C, as characterized by Raman spectroscopy and magneto-transport. Raman spectroscopy mapping indicated an AG-graphene/AG-HOPG ratio around 3.3%, which is very close to the experimental value reported for a graphene monolayer. Transport measurements from room temperature down to 1.7 K indicated slightly p-doped samples (p ≃ 10¹⁰ cm⁻²) and confirmed both continuity and thickness of the monolayer graphene film. Successive growth processes have confirmed the reproducibility and homogeneity of these monolayer films.
... Therefore the surface buffer layer actually plays a significant role as a precursor for monolayer graphene growth. However, there is just few report focusing on the quality of surface buffer layer [11,12], and thus more investigation of the surface buffer layer is necessary. ...
... The selective oxygen etching was carried out in a mixed atmosphere (O 2 : 10 −3 Pa, Ar: 1 atm) at 1300 • C for 15 min, and changes of the surface structures were observed by the in situ SEM and STM. All the SEM images were obtained with a primary electron beam of 2 keV, because the formation of buffer layer and graphene on SiC substrates are easily distinguished by low-voltage SEM contrasts [11,23,24]. Ex situ micro probe Raman spectroscopy was also carried out for graphene and buffer layer evaluation before and after the selective oxygen etching. ...
... Therefore, surface buffer layer more likely be etched at upper side of a terrace (B side). According to the report from Strupinski et al. [11], the surface buffer layer will form in higher quality with a more ordered structure and less sp 3 hybridization under a condition of faster Si sublimation (i.e., more supply of free C atoms). They also reported that buffer layer formed near an upper side of a step bunch has higher quality due to the abundant supply of C atoms form the step bunch. ...
Thermal decomposition of SiC has been used for the fabrication of high quality monolayer graphene and graphene nanoribbons on semi-insulating substrates. In this work, we propose a selective oxygen etching method to remove buffer layers on SiC surfaces that are connected to monolayer graphene formed from step edges. A thermal treatment in an extreme low partial pressure oxygen diluted by argon atmosphere was found to be effective to etch only the buffer layers and remain monolayer graphene areas intact, which might be significant for the application of graphene to electric/spintronic devices. The etching processes of surface buffer layer investigated by in situ scanning electron microscopy and scanning tunneling microscopy revealed an etching rate dependence on a distance from a step edges, suggesting a distribution of crystallinity of surface buffer layer on a terrace.
... The technology of graphene epitaxy through Chemical Vapor Deposition (CVD) from decomposed propane [1] on semiinsulating (SI) on-axis SiC(0001) has already been verified in a number of potential applications, including broad-temperature magnetic field sensing [2,3] and monolithic microwave integrated circuits (MMICs) [4][5][6]. Historically, first to synthesize on SiC was n-type monolayer graphene in the form of a covalently bonded to the substrate carbon buffer layer [7][8][9][10] topped with a single graphene layer. The technology's potential was further improved with the introduction of hydrogen atom intercalation [11][12][13][14][15][16] that converts the electrically inactive buffer [10] into quasi-free-standing (QFS) monolayer graphene [17] and monolayer graphene into QFS-bilayer graphene [11,18]. ...
... Historically, first to synthesize on SiC was n-type monolayer graphene in the form of a covalently bonded to the substrate carbon buffer layer [7][8][9][10] topped with a single graphene layer. The technology's potential was further improved with the introduction of hydrogen atom intercalation [11][12][13][14][15][16] that converts the electrically inactive buffer [10] into quasi-free-standing (QFS) monolayer graphene [17] and monolayer graphene into QFS-bilayer graphene [11,18]. ...
In this report we investigate structural and electrical properties of epitaxial Chemical Vapor Deposition quasi-free-standing graphene on an unintentionally-doped homoepitaxial layer grown on a conducting 4H-SiC substrate 4° off-axis from the basal [0001] direction towards [11-20]. Due to high density of SiC vicinal surfaces the deposited graphene is densely stepped and gains unique characteristics. Its morphology is studied with atomic force and scanning electron microscopy. Its few-layer character and p-type conductance are deduced from a Raman map and its layers structure determined from a high-resolution X-ray diffraction pattern. Transport properties of the graphene are estimated through Hall effect measurements between 100 and 350 K. The results reveal an unusually low sheet resistance below 100 Ω/sq and high hole concentration of the order of 10¹⁵ cm⁻². We find that the material’s electrical properties resemble those of an epitaxially-grown SiC PIN diode, making it an attractive platform for the semiconductor devices technology.
https://authors.elsevier.com/a/1cvqR5aLOStCsP
... 109 The surface morphology of the graphene and buffer layers was characterized by tapping mode were heated only to T gr = 1600 • C (zero growth time). The Raman spectra reveal features in the 140 1200-1700 cm −1 range, typical for the buffer layer [26,28,36]. The band around 1330 cm −1 appears to 141 be on par in terms of intensity with the band around 1580 cm −1 for all samples. ...
... [36] The integrated intensity ratio of the two bands around 1330 cm −1 (D B ) and 144 1585 cm −1 (G B ) can be used to evaluate the content of sp 3 hybridization [26] reflectivity mapping can also be employed to obtain information on the buffer layer uniformity on a 152 large-scale. ...
In this work we have critically reviewed the processes in high-temperature sublimation growth of graphene in Ar atmosphere using enclosed graphite crucible. Special focus is put on buffer layer formation and free charge carrier properties of monolayer graphene and quasi-freestanding monolayer garphene on 4H-SiC. We show that by introducing Ar at different temperatures, TAr one can shift to higher temperatures the formation of the buffer layer for both n-type and semi-insulating substrates. A scenario explaining the observed suppresed formation of buffer layer at higher TAr is proposed and discussed. Increased TAr is also shown to reduce the sp3 hybridization content and defect densities in the buffer layer on n-type conductive substrates. Growth on semi-insulating substrates results in ordered buffer layer with significantly improved structural properties, for which TAr plays only a minor role. The free charge density and mobility parameters of monolayer graphene and quasi-freestanding monolayer graphene with different TAr and different environmental treatment conditions are determined by contactless terahertz optical Hall effect. An efficient annealing of donors on and near the SiC surface takes place in intrinsic monolayer graphene grown at 2000∘C, and which is found to be independent of TAr. Higher TAr leads to higher free charge carrier mobility parameters in both intrinsically n-type and ambient p-type doped monolayer graphene. TAr is also found to have a profound effect on the free hole parameters of quasi-freestanding monolayer graphene. These findings are discussed in view of interface and buffer layer properties in order to construct a comprehensive picture of high-temperature sublimation growth and provide guidance for growth parameters optimization depending on the targeted graphene application.
... To the best of our knowledge, the only few Raman analysis with extensive statistics were obtained on disordered-like BL [28,29]. Usually, only one or few spectra are presented in the literature. ...
... 3b and 4b. Moreover, these spectra are comparable to BL Raman signature reported in litterature [6,29,31,47]. The successful graphene peeling exposed the now uncovered BL which will be compared to the BL 0 in the next section. ...
The so-called buffer layer (BL) is a carbon rich reconstructed layer formed during SiC (0001) sublimation. The covalent bonds between some carbon atoms in this layer and underlying silicon atoms makes it different from epitaxial graphene. We report a systematical and statistical investigation of the BL signature and its coupling with epitaxial graphene by Raman spectroscopy. Three different BLs are studied: bare buffer layer obtained by direct growth (BL0), interfacial buffer layer between graphene and SiC (c-BL1) and the interfacial buffer layer without graphene above (u-BL1). To obtain the latter, we develop a mechanical exfoliation of graphene by removing an epoxy-based resin or nickel layer. The BLs are ordered-like on the whole BL growth temperature range. BL0 Raman signature may vary from sample to sample but forms patches on the same terrace. u-BL1 share similar properties with BL0, albeit with more variability. These BLs have a strikingly larger overall intensity than BL with graphene on top. The signal high frequency side onset upshifts upon graphene coverage, unexplainable by a simple strain effect. Two fine peaks (1235, 1360 cm⁻¹), present for epitaxial monolayer and absent for BL and transferred graphene. These findings point to a coupling between graphene and BL.
... The other feature is the absence of 2D band, but two slight bands at around 2500-3000 cm -1 are observed. These two features are in accordance with previous reports, which demonstrates the formation of the BL [15,16]. The C1s spectrum of the BL sample by XPS is revealed in Fig. 1b. ...
... As shown in Fig. 2c, the positions of the G B peak are around 1595-1600 cm -1 . According to the previous report, the variations of G B peak position relate with the change of sp 3 hybridization contribution [16,23]. So, the little variations of G B peak position are likely correspond to the change of sp 3 hybridization contribution from 2 to\1% in the BL [23]. ...
Buffer layer (BL) was grown on the Si-face of semi-insulating SiC substrates by thermal decomposition. The Raman spectra were correlated with the surface potential obtained by Kelvin probe force microscopy to assuredly confirm the complete coverage of the BL on SiC substrates. Subsequently, quasi-free standing monolayer graphene (QFSMG) was achieved by hydrogen intercalation. And moreover, different hydrogen annealing temperature was chosen in order to study the process of hydrogen intercalation. Raman and X-ray photoelectron spectroscopy measurements distinctly revealed the changes of QFSMG with varied hydrogen annealing temperature. In particular, a large number of Raman data were collected to indicate the differences of uniformity. Additionally, the peak of Si–H bonding vibration mode was observed by surface enhanced Raman scattering, which was the direct evidence to show the success of hydrogen intercalation. All results indicated that the optimized annealing temperature was about 900 °C for obtaining uniform coverage of high-quality QFSMG with low density of defects.
... In those regions there is always some fit of 2D band—the area is then very small, but FWHM is outside the physical interpretation and may be very different (like black or white on the FWHM 2D map). So analyzing in such regions area of 2D band should clarify if there is a place without graphene or some errorsStrupinski et al., 2015). Nevertheless Raman spectra (Fig. 1d ) invalidate the existence of the buffer layer in the above mentioned regions (Strupinski et al., 2015). ...
... So analyzing in such regions area of 2D band should clarify if there is a place without graphene or some errorsStrupinski et al., 2015). Nevertheless Raman spectra (Fig. 1d ) invalidate the existence of the buffer layer in the above mentioned regions (Strupinski et al., 2015). Figures above are adequate for estimating the thickness of graphene structures, at the same time showing agreement between SEM and Raman maps and making it possible to do proper calibra-tion of the 2D area map. ...
Graphene grown by a sublimation technique was studied by Scanning Electron Microscopy (SEM) and micro-Raman spectroscopy. The measurement area of a sample was marked and investigated using both systems, as a result of which SEM images were directly compared with Raman maps. In this work we show that a correlative analysis of Energy Selective Backscattered electrons detector (EsB), In-Lens figures and Raman maps of shape and intensity of the 2D band is adequate to determine graphene layer thickness with the precision of SEM and reliability of Raman spectroscopy.
... The buffer layer is a first carbon layer which was generally found in the case of graphene growth on Si-face SiC due to the appearance of Si−C bonding between the first carbon layer and the topmost silicon layer. 23,24 Figure 3a,b displays the SEM images of the GFe at low and high magnification, respectively, revealing that the grain size of GFe was in the range of 20 to 100 nm. The elemental composition of the GFe was investigated by EDX. ...
The graphene-wrapped iron (GFe) was synthesized by annealing the sucrose-ferric chloride solution. The GFe was utilized as a magnetic efficient adsorbent to adsorb fuchsin from aqueous solution. Due to the presence of iron inside the GFe, the fuchsin-adsorbed GFe was separated easily from the aqueous solution using a permanent magnet. For the adsorbent characterization, the structure of graphene was investigated by X-ray diffraction and X-ray photoelectron spectroscopy. The grain size of graphene was acquired by field emission scanning electron microscope and Raman spectroscopy. The weight % of iron inside the graphene was estimated using energy-dispersive X-ray spectroscope. Adsorption kinetic, adsorption isothermals, influence of pH, and regeneration of the GFe were also investigated. The adsorption kinetic data was analyzed using pseudo-second-order, Elovich, and intraparticle diffusion models. In addition, the Langmuir, Freundlich, and D-R models were utilized to analyze the adsorption isothermal data. The results revealed that the adsorption process of fuchsin onto the surface of the GFe was due to the combination of chemisorption and physisorption. The maximum adsorption capacity of fuchsin by GFe was 12.71 mg/g. Besides, the adsorption capacity of fuchsin by the GFe was stable at the pH range of 3.56 to 9.47. The GFe could be regenerated by immersing in ethanol. The removal ratio decreased from 96% to 90% after 5 regeneration cycles.
... The graphene was transfer-free, in-situ fully [49] hydrogen-intercalated [50,51] at 1273 K (therefore quasi-free-standing and p-type), and epitaxial Chemical Vapor Deposition [30,31]. It was grown on an as-purchased, non-modified [7] semi-insulating, vanadiumcompensated, nominally on-axis 6H-SiC(0001) (II-VI, Inc.), using thermally decomposed propane as the carbon-sourcing gas [29,52]. An optical top-view of the structure, along with a schematic of the irradiation experiment and its effect on the QFS graphene structure along with the hydrogen layer (Si-H bond [32]), is presented in Fig. 1. ...
This article reveals a unique self-healing ability of the amorphous-aluminum-oxide-passivated p-type hydrogenintercalated quasi-free-standing epitaxial Chemical Vapor Deposition graphene on semi-insulating vanadiumcompensated nominally on-axis 6H-SiC(0001) system, exposed for 166 h to a destructive flux of 3.3 ×
E11 cm−2s−1 of mostly fast-neutrons (1–2 MeV), resulting in an accumulated fluence of 2.0 × E17 cm−2. Postirradiation room-temperature Hall effect characterization proves that the a-Al2O3/QFS-graphene/6H-SiC(0001) is n-type, which implies the loss of the quasi-free-standing character of graphene and likely damage to the SiC(0001)-saturating hydrogen layer. Micro-Raman spectroscopy suggests an average defect density in graphene of 𝑛𝐷 = 3.1 × 1010 cm−2 with an 𝐿𝐷 = 32-nm inter-defect distance. Yet, a thermal treatment up to 623 K eliminates defect-related Raman peaks and restores the original p-type conductance. At the same time, 623 K is not enough to recover the initial transport properties in a sample irradiated for 245 h with a total fluence of 2.0 × E18 cm−2. A Density Functional Theory model explains the self-healing phenomenon and restoration of the quasi-free-standing properties through thermally-activated lateral diffusion of the remaining population of hydrogen atoms and re-decoupling of the graphene sheet from the SiC(0001) surface. The thermal
regime of 623 K fits perfectly into the operational limits of the a-Al2O3/QFS-graphene/6H-SiC(0001) system, defined as 300 K to 770 K. The finding constitutes a milestone for two-dimensional, graphene-based diagnostic and control systems designed for operation in extreme environments
... [26][27][28][29] When EG buffer layers are used for metal intercalation, the metal can remain stabilized as metallic [28,29] or transform into a disordered oxide [27,30] based on the structural properties (level of disorder) of the starting EG buffer layer. [31][32][33][34][35] Hence, controlled synthesis of ordered and disordered EG buffers, and its impact on the intercalated 2D layers should be understood such that the interface can be engineered for next-generation applications. ...
Novel confinement techniques facilitate the formation of non‐layered 2D materials. Here it is demonstrated that the formation and properties of 2D oxides (GaOx, InOx, SnOx) at the epitaxial graphene (EG)/silicon carbide (SiC) interface is dependent on the EG buffer layer properties prior to element intercalation. Using 2D Ga, it is demonstrated that defects in the EG buffer layer lead to Ga transforming to GaOx with non‐periodic oxygen in a crystalline Ga matrix via air oxidation at room temperature. However, crystalline monolayer GaO2 and bilayer Ga2O3 with ferroelectric wurtzite structure(FE‐WZ') can then be formed via subsequent high‐temperature O2 annealing. Furthermore, the graphene/X/SiC (X = 2D Ga or Ga2O3) junction is tunable from Ohmic to a Schottky or tunnel barrier depending on the interface species. Finally, using vertical transport measurements and electron energy loss spectroscopy analysis, the bandgap of 2D gallium oxide is identified as 6.6 ± 0.6 eV, significantly larger than that of bulk β‐Ga2O3 (≈4.8 eV), suggesting strong quantum confinement effects at the 2D limit. The study presented here is foundational for development of atomic‐scale, vertical 2D/3D heterostructure for applications requiring short transit times, such as GHz and THz devices.
... It is similar to the growth of epitaxial graphene on Si-terminated SiC that the rst carbon layer is buffer layer which contains Si-C bonds between the rst carbon layer and the silicon layer underneath. 36,37 Fig. S3(a) † shows XRD patterns of the GFeNi0 prepared with and without vaporization process. The intensity ratio of graphene peak ($26 ) to Fe peak ($44 ) (I graphene /I Fe ) for the GFeNi0 prepared with and without vaporization process are 0.72 and 0.33, respectively, revealing that the GFeNi0 prepared with vaporization process contains a higher quantity of graphene since only the carbon atoms near the metal catalyst can be dissolved and formed graphene aer the calcination at 700 C for 6 hours. ...
Monolayer graphene has excellent electrical properties especially a linear dispersion in the band structure at the K-point in the Brillouin zone. However, its electronic transport properties can be degraded by surface roughness and attachment of charge impurities. Although multilayer graphene can reduce the surface roughness and attachment of charge impurities, the increase in the number of graphene layers can degrade the electronic transport properties due to interlayer interactions. Turbostratic graphene can significantly reduce the effect of interlayer interaction of multilayer graphene resulting in electrical properties similar to those of monolayer graphene. In this report, we have demonstrated the growth of turbostratic stacked graphene using waste ferric chloride solution as a feedstock by vaporization and calcination at 700 °C for 6 hours under an argon atmosphere. SEM images and EDX elemental distribution maps showed graphene can be grown on iron and nickel catalysts. XRD results and Raman spectra confirmed the presence of turbostratic stacked graphene with the interlayer spacing in the range of 3.41 Å to 3.44 Å. The Raman spectra in all samples also displayed a weak intensity peak of iTALO- and a well-fitted 2D band by a single Lorentzian peak indicating the presence of turbostratic stacked graphene. In addition, XPS spectra reveal the growth mechanism of the turbostratic stacked graphene. This synthesis process of turbostratic stacked graphene is not only simple, low-cost, and suitable for large-scale production but also decreases the environmental issues from releasing waste ferric chloride solution with improper disposal.
... Although it is very similar to graphene, approximately 30 % of its carbon atoms are sp 3 hybridized and are covalently bound to the substrate. Thus, the π-bands of the buffer layer are distorted and the buffer layer becomes electrically isolating [Str15]. A sketch of epitaxial monolayer graphene is shown in figure 3.3 (a). ...
This thesis is dedicated to the investigation of current transport in Schottky diodes from DC to PHz frequencies presenting first semiconductor devices for lightwave electronics. Epitaxial graphene on n-doped 4H-silicon carbide (SiC) is chosen as a material system because of its outstanding electric and mechanical properties. As a first step, the response of the diodes to radiation in the low terahertz (THz) range is studied. Here, the diodes are connected to antennas, which transform the electric field of the THz radiation into an AC voltage that is rectified by the Schottky contact. However, due to impedance mismatches and the RC-roll off this AC voltage is severely damped leading to a 1/f² decrease of the rectified current. Thus, the detectable frequency range is limited to 580 GHz with a maximum responsivity of 1.6 A/W at 80 GHz. The frequency dependency of the responsivity of the graphene/\ac{SiC} Schottky diodes can be fully modelled employing parameters determined from the DC IV-characteristics. Only one parameter remains unknown: the device capacitance, which serves as a fit parameter. The graphene/SiC diodes are compared to nickel/SiC and state-of-the-art Schottky detectors. Although the latter have higher responsivities and a larger detectable frequency range, the graphene/SiC diodes stand out due to their extraordinary robustness. In the low THz regime the detection mechanism is limited by the RC-roll off and the impedance mismatch of the diode and its external circuitry. In order to study the current response of the diodes at higher frequencies, intense mid-infrared (MIR) pulses are directly focussed onto the structure so that their electric field adds to the static field of the diode. Two regimes are identified from the current induced by the MIR pulses: In forward direction, a thermally induced current is detected which is caused by two effects. At first, the pulses are absorbed in the SiC crystal due to the excitation of optical phonons. Furthermore, electrons in the graphene sheet are heated by the in-plane component of the electric field. Thereby, the mean temperature of the diode rises approximately 20 mK. In addition, a polarisation dependent current is observed in reverse direction. Due to the electric field of the MIR pulses, the potential at the Schottky contact is modulated leading to a severe decrease of the barrier in the course of the negative half cycle. This results in a tunnelling current that is rectified by the diode. A simple model based on the DC parameters of the diode is presented taking the tunnelling current and the subsequent electron dynamics in the space charge region into account. It reproduces the measurement data correctly giving insight to the physical origin of the rectified current. This effect is observed in the frequency range from 18 THz to 78 THz. As no frequency limit was examined in the MIR regime, Fourier-transform limited near-infrared (NIR) pulses with a center frequency of 350 GHz are focussed onto the same Schottky diodes. Due to the larger photon energy ranging from 1.2 eV to 1.9 eV electrons are excited to energy states above and below the barrier by two photon absorption. The electrons excited to states above the barrier are subsequently rectified by the electric field of the NIR pulse that adds to the static field of the diode leading to a clear carrier envelope phase dependence of the induced current. This carrier envelope dependency was observed in the experiments. Furthermore, experiments employing two pulses with perpendicular polarisation support this interpretation. As a side topic, a new detector for homodyne detection of THz radiation based on the material system InGaAs is presented. Homodyne detection is advantageous compared to other detection methods as it is not only sensitive to the amplitude of the THz radiation but also to its phase. In this thesis, devices are fabricated and their photoresponse to infra-red radiation is characterised. As they show the desired response, first frequency dependent THz spectra are recorded revealing that the detectable frequency range is limited to 150 GHz due to RC damping. Possible ways to increase this frequency limit are suggested. Finally, various whispering gallery mode resonators (WGMR) are characterised and their response to local changes of their dielectric environment is studied. As a first step, disk-shaped resonators made of high density polyethylene are examined. When a small scatterer, i.e. a hole, is added to the resonator, the center frequency of the resonances are shifted by a few hundred megahertz. By filling the hole with the specimen, this frequency shift is altered. Hence, the resonators can act as detectors for biomolecules. Yet, the sensitivity of this system is limited due to the low quality factors of the resonators. In order to increase the sensitivity, sphere-shaped sapphire resonators and disk-shaped silicon resonators with higher refractive indices are employed and characterized. The experiments show that they are indeed sensitive to local changes of their dielectric environment. An optimum is reached for disk-shaped silicon resonators with a thickness below the wavelength. At last, the suitability of those resonators as detectors of gaseous and liquid biomolecules is critically discussed and a second application of the WGMRs is proposed: adjustable, narrow-banded frequency filters for THz radiation.
... The samples were successively investigated via micro-Raman spectroscopy. Given the difficulty in extracting the Raman spectrum of the facet-graphene alone due to the limited spatial resolution (∼1 µm) of the equipment used, we prepared and analyzed two samples: one with a carbon layer covering the whole surface, i.e., both the macroterraces (in the form of the buffer layer 24,25 ) and macrofacets, and the other with a carbon layer only on the macroterraces (obtained at a reduced growth time) (Fig. 4(b)). Fig. 4(a) shows the measured spectra of both samples and the one of the facet-graphene alone, obtained by subtracting the blue spectrum from the red one. ...
Thermal decomposition of vicinal 6H-SiC(0001) surfaces with off-angles toward the direction results in the appearance of pairs of (0001) macroterraces and macrofacets covered with graphene, as follows. A carpet-like carbon layer grows on the surface, covering both the macroterraces and macrofacets; it forms buffer layer on the former ones, whereas its partial periodic bonding with the SiC steps on the latter ones generates a pseudo-graphene nanoribbon (pseudo-GNR) array. The nanoribbons have a width of 2 nm and are aligned in the direction with a spatial periodicity of 3.3 nm. Here, the Raman spectroscopy analysis of the pseudo-GNR array showed the absence of the 2D peak and the polarization dependence of the G and D peaks, which is typical of the armchair edge nanoribbon.
... In this buffer layer, however, around 30 % of the carbon atoms are sp 3 hybridized and bound to the substrate surface. As a result, the π bands are strongly distorted and consequently the buffer layer is electrically insulating [Str15]. The first true graphene layer resides as a second graphitic layer on top of the buffer layer (see Fig. 2.6). ...
This work is dedicated to the investigation of electrically driven light emission at nano contacts, i. e. point and tunnel contacts. The phenomenon is not a peculiarity of the used material system but a fundamental process of light matter interaction. It was first observed at thin, island like metal films in the 1960s and afterwards especially in scanning tunneling microscopes (STMs). Despite the long standing observation, there is still an active scientific discussion about its origin and multiple competing models exist. The novelty of this work is the use of graphene point contacts (GPC) and tunnel contacts (GTC) made out of epitaxial graphene on silicon carbide. This type of nano contacts also exhibits a current driven light emission. In contrast to many other experiments, however, graphene nano contacts provide a flat geometry and a transparent material system. This enables direct optical access to the contact avoiding nano-optics or other obstructing phenomena and thus allows to characterize the transmission function of the entire optical setup. The spectra measured at GPCs perfectly follow Planck's law in the whole observed spectral range from 550nm to 1600nm. Surprisingly, the temperatures necessary to produce such thermal spectra turned out to be extremely high: at air, temperatures of up to 1500K are extracted from Planck fits and under cryogenic vacuum conditions even higher temperatures of up to 3000K were measured. These values exclude a simple heating of the junction in analogy to light bulbs as the observed temperatures by far exceed the damage threshold of the used material system. Nevertheless, the contacts proved to be stable. This apparent contradiction is explained by the presence of an overheated electronic subsystem with a temperature well above the temperature of the actual crystal lattice. Due to the spatially sharp drop in electric potential at the point or tunneling contact, hot electrons are injected from one electrode into the other. These hot electrons thermalize due to electron-electron interaction on a short timescale while cooling to the lattice occurs on a significantly longer timescale. Additionally, the lattice is well coupled to the surrounding bath temperature due to the high thermal conductivity of graphene and SiC. This leads to a stationary state in which the temperature of the electron system surpasses the temperature of the crystal lattice by far. The observed thermal radiation is a signature of this high electronic temperature. The advantages of the used GPCs compared to similar experiments are their excellent stability, large parameter space (enabling voltages up to 8V) and also the absence of resonant spectral signatures (e. g. by plasmons) in the relevant spectral range. The latter allows for an unobscured observation of the thermal effects. In total more than 100 nano contacts were characterized, giving a consistent and conclusive data set. Nevertheless, the question remains how these thermal spectra are linked to the resonant signatures observed in many other similar experiments. It has to be noted, that thermal radiation depends on the photonic density of states of the environment. If it equals to the density of states of the vacuum, Planck's law is obtained. However, if it is modified e. g. by a resonant environment, the thermal spectra contain resonant contributions. Such an environment can be created by bringing a rough metallic edge into close proximity of the junction: an additional signal arises in the spectra which is a fingerprint-like signature of the metal edge. After removing the metal, also the additional signal vanishes. Also at GTCs resonant contributions to the thermal spectra were observed. The origin of this signature could not be conclusively explained but nevertheless it can be shown that it is of thermal origin: the additional spectral weight of the feature is in both cases strictly proportional to the underlying Planck spectrum. From this experiments it cannot unambiguously be answered to what extent these results can be transferred to other geometries as for example present in the STM. It is nevertheless apparent that an overheated electron system should also be present in STM and other point and tunnel contact experiments and may also there be the driving force of the observed electroluminescence. The aim of this work is to provide a thorough and consistent data set measured at a clear model system to aid in the answer of this long standing physical question. Due to the high temperatures observed by the optical experiments, electrons should be thermionically emitted from the nano contact. During this work this has indeed been observed and characterized. The measured thermionic emission current fits to the Richardson law with temperatures in agreement to the optical measurements. This is a strong further evidence of the overheated electron gas. As a side topic, the improvement of the epitaxial graphene growth process on SiC by an additional carbon source was investigated. The homogeneity of the graphene layer can be enhanced by applying a thin layer of graphite onto the sample prior to the growth process. This affects the microscopic structure as shown by electron microscopy and also magnetotransport measurements. Lastly, the development of flexible measurement software for laboratory use is discussed with special focus on the upcoming challenges of research data management and the German Nationale Forschungsdateninfrastruktur (national research data infrastructure). Efficient research data management enforces the collection of a considerable amount of meta-data which needs to be supported by the measurement software. The development and usage of unified frameworks is a promising route to meet these requirements.
... QFS-graphene necessary for the study has been obtained through in-situ hydrogen atom intercalation [18] of sole buffer layer [19] epitaxially grown on a semiinsulating vanadium-compensated on-axis 20 mm × 20 mm 6H-SiC(0001) substrate cut from a 4-in wafer purchased at II-VI Inc., in a hot-wall Aixtron VP508 reactor using the CVD method [20] and thermally decomposed propane as carbon source. Shortly after the process, the sample was fed with I = 500-μA direct current and characterized with a 0.55-T Ecopia HMS-3000 Hall effect measurement system to prove hole sheet concentration p s = 8.8 × 10 12 cm −2 , hole mobility p = 3032 cm 2 /Vs, and sheet resistance R s = 234 Ω/sq. ...
In this report we demonstrate a method for direct determination of the number of layers of hydrogen-intercalated quasi-free-standing epitaxial Chemical Vapor Deposition graphene on semiinsulating vanadium-compensated on-axis 6H-SiC(0001). The method anticipates that the intensity of the substrate’s Raman-active longitudinal optical A1 mode at 964 cm^(−1) is attenuated by 2.3 % each time the light passes through a single graphene layer. Normalized to its value in a graphene-free region, the A1 mode relative intensity provides a greatly enhanced topographic image of graphene and points out to the number of its layers within the terraces and step edges, making the technique a reliable diagnostic tool for applied research.
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... The insets of Fig. 1(e) and (f) depict typical examples of background subtracted Raman spectrum which is characteristics for EG layer grown on Si-terminated and C-terminated faces of SiC, respectively. In order to confirm that the Raman signals do not belong to the buffer layer (the interface layer on SiC formed prior to the growth of first graphene layer), we compared our Raman measurements with the data presented in Ref. [20]. It has been shown that the characteristics of the Raman spectrum of the buffer layer is subsequently different from those of actual first graphene layer. ...
The friction coefficients of single-layer epitaxial graphene grown on the Si-terminated and C-terminated faces of Silicon Carbide (SiC) substrate were measured under ambient conditions using Friction Force Microscope (FFM). The lateral friction force measurements acquired in the applied normal force range between 4.0 – 16.0 nN showed that the friction coefficient of graphene on the C-terminated face of SiC is about two times smaller than the one grown on its Si-terminated face. The lateral friction was found to be decreased as the average of root mean square roughness increases suggesting the observed difference in the friction coefficients cannot be related to the roughness of the graphene layers. DFT calculations demonstrated that the altered periodicity of charge distribution on graphene due to the specific interactions with two distinct polar faces of SiC substrate might explain the observed difference in the friction coefficients.
... The distortions are caused by covalent bonds forming between a part of the carbon atoms (about 1/3) in the buffer layer and the underlying silicon atoms. [83,84] Quasiperiodic (6 × 6) domain pattern emerges out of a larger commensurate (6√3 × 6√3)R30° periodic interfacial reconstruction. [79] In a growth model by Norimatsu et al. [85], it was argued that because the area densities of the C atoms in ideal graphene and a single SiC bilayer have values of 32.9 nm -2 and 10.5 nm −2 , respectively, the formation of one graphene layer requires about three SiC bilayers. ...
(See the complete abstract within the thesis in both English and German versions) In this thesis, the process conditions of the epitaxial graphene growth through a socalled polymer-assisted sublimation growth method are minutely investigated. Atomic force microscopy (AFM) is used to show that the previously neglected flow-rate of the argon process gas has a significant influence on the morphology of the SiC substrate and atop carbon layers. The results can be well explained using a simple model for the thermodynamic conditions at the layer adjacent to the surface. The resulting control option of step-bunching on the sub-nanometer scales is used to produce the ultra-flat, monolayer graphene layers without the bilayer inclusions that exhibit the vanishing of the resistance anisotropy. The comparison of four-point and scanning tunneling potentiometry measurements shows that the remaining small anisotropy represents the ultimate limit, which is given solely by the remaining resistances at the SiC terrace steps. ... The precise control of step-bunching using the Ar flow also enables the preparation of periodic non-identical SiC surfaces under the graphene layer. Based on the work function measurements by Kelvin-Probe force microscopy and X-ray photoemission electron microscopy, it is shown for the first time that there is a doping variation in graphene, induced by a proximity effect of the different near-surface SiC stacks. The comparison of the AFM and low-energy electron microscopy measurements have enabled the exact assignment of the SiC stacks, and the examinations have led to an improved understanding of the surface restructuring in the framework of a step-flow mode. ...
... The distortions are caused by covalent bonds forming between a part of the carbon atoms (about 1/3) in the buffer layer and the underlying silicon atoms. [83,84] Quasiperiodic (6 × 6) domain pattern emerges out of a larger commensurate (6√3 × 6√3)R30° periodic interfacial reconstruction. [79] In a growth model by Norimatsu et al. [85], it was argued that because the area densities of the C atoms in ideal graphene and a single SiC bilayer have values of 32.9 nm -2 and 10.5 nm −2 , respectively, the formation of one graphene layer requires about three SiC bilayers. ...
Abstract (English):
The electrical quantum standards have played a decisive role in modern metrology, particularly since the introduction of the revised International System of Units (SI) in May 2019. By adapting the basic units to exactly defined natural constants, the quantized Hall resistance (QHR) standards are also given precisely. The Von Klitzing constant RK = h/e2 (h Planck's constant and e elementary charge) can be measured precisely using the quantum Hall effect (QHE) and is thus the primary representation of the ohm. Currently, the QHR standard based on GaAs/AlGaAs heterostructure has succeeded in yielding robust resistance measurements with high accuracy <10−9.
In recent years, graphene has been vastly investigated due to its potential in QHR metrology. This single-layer hexagonal carbon crystal forms a two-dimensional electron gas system and exhibits the QHE, due to its properties, even at higher temperatures. Thereby, in the future the QHR standards could be realized in more simplified experimental conditions that can be used at higher temperatures and currents as well as smaller magnetic fields than is feasible in conventional GaAs/AlGaAs QHR.
The quality of the graphene is of significant importance to the QHR standards application. The epitaxial graphene growth on silicon carbide (SiC) offers decisive advantages among the known fabrication methods. It enables the production of large-area graphene layers that are already electron-doped and do not have to be transferred to another substrate. However, there are fundamental challenges in epitaxial graphene growth. During the high-temperature growth process, the steps on the SiC surface bunch together and form terraces with high steps. This so-called step-bunching gives rise to the graphene thickness inhomogeneity (e.g., the bilayer formation) and extrinsic resistance anisotropy, which both deteriorate the performance of electronic devices made from it.
In this thesis, the process conditions of the epitaxial graphene growth through a so-called polymer-assisted sublimation growth method are minutely investigated. Atomic force microscopy (AFM) is used to show that the previously neglected flow-rate of the argon process gas has a significant influence on the morphology of the SiC substrate and atop carbon layers. The results can be well explained using a simple model for the thermodynamic conditions at the layer adjacent to the surface. The resulting control option of step-bunching on the sub-nanometer scales is used to produce the ultra-flat, monolayer graphene layers without the bilayer inclusions that exhibit the vanishing of the resistance anisotropy. The comparison of four-point and scanning tunneling potentiometry measurements shows that the remaining small anisotropy represents the ultimate limit, which is given solely by the remaining resistances at the SiC terrace steps.
Thanks to the advanced growth control, also large-area homogenous quasi-freestanding monolayer and bilayer graphene sheets are fabricated. The Raman spectroscopy and scanning tunneling microscopy reveal very low defect densities of the layers. In addition, the excellent quality of the produced freestanding layers is further evidenced by the four-point measurement showing low extrinsic resistance anisotropy in both micro- and millimeter-scales.
The precise control of step-bunching using the Ar flow also enables the preparation of periodic non-identical SiC surfaces under the graphene layer. Based on the work function measurements by Kelvin-Probe force microscopy and X-ray photoemission electron microscopy, it is shown for the first time that there is a doping variation in graphene, induced by a proximity effect of the different near-surface SiC stacks. The comparison of the AFM and low-energy electron microscopy measurements have enabled the exact assignment of the SiC stacks, and the examinations have led to an improved understanding of the surface restructuring in the framework of a step-flow model.
The knowledge gained can be further utilized to improve the performance of epitaxial graphene quantum resistance standard, and overall, the graphene-based electronic devices. Finally, the QHR measurements have been shown on the optimized graphene monolayers. In order to operate the graphene-based QHR at desirably low magnetic field ranges (B < 5 T), two known charge tuning techniques are applied, and the results are discussed with a view to their further implementation in the QHR metrology.
Abstract (German):
Elektrische Quantennormale spielen eine wichtige Rolle in der modernen Metrologie, besonders seit der Einführung des revidierten Einheitensystems (SI) im Mai 2019. Durch die Zurückführung der Basiseinheiten auf exakt definierte Naturkonstanten sind auch die quantisierten Werte von Widerstandsnormalen (QHR) exakt gegeben. Die Von-Klitzing-Konstante RK = h/e2 (h Planck-Konstante und e Elementarladung) lässt sich mittels des Quanten-Hall-Effekts (QHE) präzise messen und ist somit die primäre Darstellung des Ohm. Die Quanten-Widerstandsnormale bestehen aktuell aus robusten GaAs/AlGaAs-Heterostrukturen, die eine Genauigkeit <10−9 für die Widerstands-Messung erlauben.
In den letzten Jahren wird verstärkt Graphen auf sein Potenzial für die Widerstandmetrologie untersucht. Der einlagige hexagonale Kohlenstoffkristall bildet ebenfalls ein zweidimensionales Elektrongas aus, das den Quanten-Hall-Effekt zeigt – und dies auf Grund seiner Eigenschaften schon bei höheren Temperaturen. Damit könnten in Zukunft Widerstandsnormale für vereinfachte experimentelle Bedingungen realisiert werden, die bei höheren Temperaturen und Strömen oder kleineren Magnetfeldern eingesetzt werden können, als es mit konventionellen GaAs/AlGaAs- QHR möglich ist.
Für den Einsatz als Widerstandsnormal ist die Qualität des Graphens von entscheidender Bedeutung. Unter den bekannten Herstellungsmethoden bietet das epitaktische Wachstum von Graphen auf Siliciumcarbid (SiC) entscheidende Vorteile. Es lassen sich damit großflächige Graphenschichten herstellen, die nicht auf ein anderes Substrat übertragen werden müssen. Allerdings gibt es grundlegende Herausforderungen beim epitaktischen Wachstum. So tritt bei hohen Prozesstemperaturen eine Bündelung der Kristallstufen auf der SiC-Substratoberfläche auf (Step-bunching), was zu einer bekannten extrinsischen Widerstandsanisotropie führt und darüber hinaus die Bildung von Bilagen-Graphen begünstigt. Beides verschlechtert die Eigenschaften der daraus hergestellten Widerstandsnormale.
In dieser Dissertation werden zunächst die Prozessbedingungen des mittels der sogenannten Polymer-Assisted-Sublimations-Growth-Methode hergestellten epitaktischen Graphens auf SiC genauer untersucht. Mithilfe der Rasterkraft-Mikroskopie (Atomic-Force-Microscopy, AFM) wird gezeigt, dass es einen erheblichen Einfluss der bisher wenig beachteten Flussrate des Prozessgases Argon auf die Morphologie des SiC-Substrates und der oberen Kohlenstoffschichten gibt. Anhand eines einfachen Modells für die thermodynamischen Verhältnisse in einer oberflächennahen Schicht lassen sich die Ergebnisse hervorragend erklären. Die sich daraus ergebende Kontrollmöglichkeit des Step-bunching auf Sub-Nanometer-Skalen wird genutzt, um ultraflache, monolagige Graphenschichten ohne Bilageneinschlüsse herzustellen, die eine verschwindende Widerstandsanisotropie aufweisen. Der Vergleich von Vierpunkt-Messungen und Scanning-Tunneling-Potentiometery-Messungen zeigt, dass die verbleibende geringe Anisotropie das ultimative Limit darstellt, die allein durch die verbleibenden Widerstände an den SiC-Terrassenstufen gegeben ist.
Dank der fortschrittlichen Wachstumskontrolle werden auch großflächige, homogene quasi-freistehende Monolage- und Bilage-Graphenschichten hergestellt. Die Raman-Spektroskopie und die Rastertunnel-Mikroskopie zeigen sehr geringe Defektdichten der Schichten. Darüber hinaus wird die hervorragende Qualität der hergestellten quasi-freistehenden Schichten durch die Vierpunkt-Messung unter Beweis gestellt, die eine geringe extrinsische Widerstandsanisotropie zeigt.
Die präzise Kontrolle des Step-bunching mittels Ar-Fluss ermöglicht auch die gezielte Präparation von periodischen, nicht-identischen SiC-Oberflächen unter der Graphenlage. Anhand von Messungen der Austrittsarbeit mit Kelvin-Probe-Force-Microscopy und X-ray Photoemission-Electron-Microscopy konnte erstmals gezeigt werden, dass es eine Variation der Graphendotierung, induziert durch einen Proximity Effekt der unterschiedlichen oberflächennahen SiC-Stapel, gibt. Der Vergleich von AFM und Low-Energy-Electron-Microscopy-Messungen ermöglicht die genaue Zuordnung der SiC-Stapel und die Untersuchungen führen insgesamt zu einem verbesserten Verständnis der Oberflächen-Umstrukturierung im Rahmen eines adäquaten Step-Flow-Modells.
Die gesammelten Erkenntnisse können zur Verbesserung der Eigenschaften von Graphen-Quantennormalen und auch allgemein von graphenbasierten Bauteilen genutzt werden. Abschließend werden QH-Widerstandsmessungen an optimierten Graphen-Monolagen gezeigt. Um den Magnetfeldbereich (B < 5 T) einzuschränken, werden zwei bekannte extrinsische Dotiertechniken verwendet und die Ergebnisse werden im Hinblick auf den weiteren Einsatz in der QH-Metrologie diskutiert.
... Here, the three prominent Raman modes of graphene are visible, the A 1g -phonon related D and 2D peaks as well as the E 2g -phonon related G peak. An exemplary Raman spectrum of a pristine buffer layer is shown in Fig. 1a was taken from a sample after the preconditioning step and is consistent with other investigations [23,40]. One can see, that the BL contributes mainly to the underground of the D and G mode. ...
In this work a comprehensive study is presented for the analysis of epitaxial graphene layers using Raman spectroscopy. A wide range of graphene types is covered, from defective/polycrystalline single layer graphene to multilayer graphene with low defect density. On this basis the influence of strain type, Fermi level and number of layers on the Raman spectrum of graphene is investigated. A detailed view on the 2D/G dispersion and the respective slopes of uniaxially and biaxially strained graphene is given and its implications on the asymmetry of the G peak analyzed. A linear dependency of the phonon mode asymmetry on uniaxial strain is found in addition to the known Fermi level dependence. Additional impacts on the asymmetry are found to be arising from the defect density and transfer doping of adsorbates. The discovered transfer doping mechanism is contrary to pure phonon excitation through excitons and exhibits increasing asymmetry with increasing Fermi level. A new characteristic correlation between the 2Dmode line width and the inverse I(D)/I(G) ratio is presented that allows the determination of the strain type and layer number and explains the difference between Raman line widths of monolayer graphene on different substrates.
... QFS graphene samples were synthesized on HPSI on-axis 4H-SiC (0001) (Cree Inc.) substrate using the epitaxial Chemical Vapor Deposition (CVD) method in a standard hot-wall Aixtron VP508 reactor [8]. The growth process of the carbon buffer layer [9] was followed by in-situ hydrogen intercalation at 1000 � C in argon atmosphere. ...
... 24 Epitaxial growth of graphene on SiC(0001) is connected with formation of a buffer layer underneath the graphene. 25,26 The buffer layer comprises a carbon layer that is covalently bonded to the underlying SiC substrate and does not show graphitic electronic properties. The electronically inactive reconstructed buffer layer on SiC(0001) may be converted into quasi-free-standing monolayer graphene after hydrogen intercalation. ...
A method of growing highly oriented MoS2 is presented. First, Mo film is deposited on graphene/SiC(0001) substrate and the subsequent annealing it at 750oC leads to intercalation of Mo underneath the graphene layer which is confirmed by secondary ion mass spectrometry (SIMS) measurements. Formation of highly oriented MoS2 layers is then achieved by sulfurization of the graphene/Mo/SiC system using H2S gas. The X-ray diffraction reveals that MoS2 layers are highly oriented and parallel to the underlying SiC substrate surface. Further SIMS experiments reveal that the intercalation process occurs via the atomic step edges of SiC and Mo and S atoms gradually diffuse along SiC atomic terraces leading to the creation of the MoS2 layer. This observation can explain a mechanism of highly oriented growth of MoS2: nucleation of crystalline MoS2 phase occur underneath the graphene planes covering the flat parts of SiC steps and Mo and S atoms create a crystallization fronts moving along terraces.
... At 200 and 400 Pa, F 2D is 158 and 114 cm À1 , respectively, which are much larger than those at 600e800 Pa. As the previous studies reported, this discrepancy in F 2D may be caused by two factors: the increase of defect density [45] and/or the increase of N (N > 3) [46], [47]. The ratio of I G /I D is usually used to estimate the grain size (L) of graphene by Eq. (1) [48]. ...
Transparent and highly oriented 3C-SiC bulks were speedily fabricated at deposition temperature (Tdep) of 1623 K by halide laser chemical vapor deposition (HLCVD). The effect of total pressure (Ptot) on the optical transmittance, preferred orientation, microstructure, deposition rate (Rdep) and micro-hardness were investigated. The maximum Rdep of the transparent 3C-SiC reached 2450 μm/h at Ptot = 10 kPa. With an increase in Ptot, the transmittance of 3C-SiC bulks increased firstly, and then decreased. At Ptot = 10 kPa, 3C-SiC bulk, a highly <111>-oriented and low density of defects, showed the highest transmittance, greater than 55% in the wavelength range of 800–1100 nm. At Ptot = 4 kPa and 20 kPa, 3C-SiC bulks showed much lower transmittance, in which contained poorly oriented grains and numerous defects. The Vickers micro-hardness of 3C-SiC bulks increased with increasing Ptot and showed the highest value of 34.8 GPa at Ptot = 40 kPa.
... On Si-face SiC, the residual carbon recombines to a hexagonal monolayer of (6 √ 3 × 6 √ 3) R30°symmetry and mixed sp 2 and sp 3 hybridization with 10-30% covalent bonds. [43,44] This hybrid layer is often called buffer layer and known to exhibit a bandgap around 0.5 eV. [45] Similarly, a ( √ 43 × √ 43) R±7.6°symmetric buffer is appearing on the C-face. ...
Graphene nanoribbons show unique properties and have attracted a lot of attention in the recent past. Intensive theoretical and experimental studies on such nanostructures at both the fundamental and application-oriented levels have been performed. The present paper discusses the suitability of graphene nanoribbons devices for nanoelectronics and focuses on three specific device types – graphene nanoribbon MOSFETs, side-gate transistors, and three terminal junctions. It is shown that, on the one hand, experimental devices of each type of the three nanoribbon-based structures have been reported, that promising performance of these devices has been demonstrated and/or predicted, and that in part they possess functionalities not attainable with conventional semiconductor devices. On the other hand, it is emphasized that – in spite of the remarkable progress achieved during the past 10 years – graphene nanoribbon devices still face a lot of problems and that their prospects for future applications remain unclear.
... There exists evidence that up to 30% of the carbon atoms in the semiconductor-like transition carbon layer (called also the buffer layer, zero graphene layer or interfacial layer) are covalently bonded to the Si atoms (belonging to SiC) by sp 3 hybridized bonds [1][2][3]. It is believed that the first graphene layer fits into a (6 √ 3 × 6 √ 3) R30 • surface reconstruction on SiC, and the (6 √ 3 × 6 √ 3) R30 • unit cell ideally coincides with a graphene unit. ...
In spite of the great expectations for epitaxial graphene (EG) on silicon carbide (SiC) to be used as a next-generation high-performance component in high-power nano- and micro-electronics, there are still many technological challenges and fundamental problems that hinder the full potential of EG/SiC structures and that must be overcome. Among the existing problems, the quality of the graphene/SiC interface is one of the most critical factors that determines the electroactive behavior of this heterostructure. This paper reviews the relevant studies on the carrier transport through the graphene/SiC, discusses qualitatively the possibility of controllable tuning the potential barrier height at the heterointerface and analyses how the buffer layer formation affects the electronic properties of the combined EG/SiC system. The correlation between the sp2/sp3 hybridization ratio at the interface and the barrier height is discussed. We expect that the barrier height modulation will allow realizing a monolithic electronic platform comprising different graphene interfaces including ohmic contact, Schottky contact, gate dielectric, the electrically-active counterpart in p-n junctions and quantum wells.
... The singlelayer graphene (SLG) produced by this method on the buffer layer presents large crystalline atomic terraces and it is much less sensitive to SiC surface defects, resulting in higher electron mobility than those grown by Si sublimation process. Moreover, intercalation of hydrogen under the buffer layer of 4H-SiC(0001) is known to convert the buffer into graphene, the so called Quasi Free Standing Monolayer Graphene (QFSMG) 18,19 . The QFSMG samples used for this study present Hall mobility values above 8,000 cm 2 V À 1 s À 1 . ...
Graphene functionalization with organics is expected to be an important step for the development of graphene-based materials with tailored electronic properties. However, its high chemical inertness makes difficult a controlled and selective covalent functionalization, and most of the works performed up to the date report electrostatic molecular adsorption or unruly functionalization. We show hereafter a mechanism for promoting highly specific covalent bonding of any amino-terminated molecule and a description of the operating processes. We show, by different experimental techniques and theoretical methods, that the excess of charge at carbon dangling-bonds formed on single-atomic vacancies at the graphene surface induces enhanced reactivity towards a selective oxidation of the amino group and subsequent integration of the nitrogen within the graphene network. Remarkably, functionalized surfaces retain the electronic properties of pristine graphene. This study opens the door for development of graphene-based interfaces, as nano-bio-hybrid composites, fabrication of dielectrics, plasmonics or spintronics.
... This leads to the formation of a high density of 2D-islands that tend to coalesce and form continuous domains. In the literature, the growth of the buffer layer as well as of graphene is also often described to start with the formation of stripes at the upper side of terrace edges by step-flow growth [7,42,43]. This seems to be especially the case when the step height is significantly higher and broader compared to the ≤ 0.75 nm steps discussed in this section. ...
We investigate the epitaxial growth of the graphene buffer layer and the involved step bunching behavior of the silicon carbide substrate surface using atomic force microscopy. The results clearly show that the key to controlling step bunching is the spatial distribution of nucleating buffer layer domains during the high temperature graphene growth process. Undesirably high step edges are the result of local buffer layer formation whereas a smooth SiC surface is maintained in the case of uniform buffer layer nucleation. The presented polymerassisted sublimation growth method is perfectly suited to obtain homogenous buffer layer
nucleation and to conserve ultra-flat surfaces during graphene growth on a large variety of silicon carbide substrate surfaces. The analysis of the experimental results is in excellent agreement with the predictions of a general model of step dynamics. Different growth modes are described which extend the current understanding of epitaxial graphene growth by emphasizing the importance of buffer layer nucleation and critical mass transport processes.
... For these reasons many graphene researchers have focused on this polytype. Strupinski et al [88] fabricated graphene on semiinsulating on-axis 4H-SiC substrates cut out from a 4 inch wafer. The formation mechanism of graphene was investigated using micro-Raman and SEM studies. ...
Being a true two-dimensional crystal, graphene possesses a lot of exotic properties that would enable unique applications. Integration of graphene with inorganic semiconductors, e.g. silicon carbide (SiC) promotes the birth of a class of hybrid materials which are highly promising for development of novel operations, since they combine the best properties of two counterparts in the frame of one hybrid platform. As a specific heterostructure, graphene on SiC performs strongly, dependent on the synthesis method and the growth modes. In this article, a comprehensive review of the most relevant studies of graphene growth methods and mechanisms on SiC substrates has been carried out. The aim is to elucidate the basic physical processes that are responsible for the formation of graphene on SiC. First, an introduction is made covering some intriguing and not so often discussed properties of graphene. Then, we focus on integration of graphene with SiC, which is facilitated by the nature of SiC to assume graphitization. Concerning the synthesis methods, we discuss thermal decomposition of SiC, chemical vapor deposition and molecular beam epitaxy, stressing that the first technique is the most common one when SiC substrates are used. In addition, we briefly appraise graphene synthesis via metal mediated carbon segregation. We address in detail the main aspects of the substrate effect, such as substrate face polarity, off-cut, kind of polytype and nonpolar surfaces on the growth of graphene layers. A comparison of graphene grown on the polar faces is made. In particular, growth of graphene on Si-face SiC is critically analyzed concerning growth kinetics and growth mechanisms taking into account the specific characteristics of SiC (0001) surfaces, such as the step-terrace structure and the unavoidable surface reconstruction upon heating. In all subtopics obstacles and solutions are featured. We complete the review with a short summary and concluding remarks.
... The presence of the buffer layer can be detected by the Raman spectrum; the buffer layer has broad peaks around 1300-1600 cm À1 and no 2D band around 2700 cm À1 . [27][28][29][30] There were actually no broad peaks around 1300-1600 cm À1 , indicating the absence of the buffer layer. ...
We have investigated the relation between the step-bunching and graphene growth phenomena on an SiC substrate. We found that only a minimum amount of step-bunching occurred during the graphene growth process with a high heating rate. On the other hand, a large amount of step-bunching occurred using a slow heating process. These results indicated that we can control the degree of step-bunching during graphene growth by controlling the heating rate. We also found that graphene coverage suppressed step bunching, which is an effective methodology not only in the graphene technology but also in the SiC-based power electronics.
... The progress in wafer-scale growth of epitaxial graphene on dielectric substrates has seen two leading technologies e silicon atom sublimation from SiC surface [1e3] and Chemical Vapor Deposition (CVD) growth on SiC [4] from methane, propane or acetylene as the carbon source. The CVD epitaxy has been studied for both Si-terminated and C-terminated SiC; however, the growth kinetics of the Si-face enable synthesis of a pre-defined and uniform number of carbon layers, including a sole buffer layer [5]. The buffer layer [6e8] remains electrically inactive unless it is decoupled from the substrate through hydrogen atom intercalation [9e13]. ...
... Growing graphene on the Si-face of SiC always involves the formation of a buffer layer [7] which is the first layer of carbon atoms covalently bonded to the substrate8910. The buffer layer can be decoupled from the substrate through hydrogen atom intercalation1112131415. ...
... Such contribution could be a consequence of residual Si atoms aggregated in the step edge area 55 or growth disorder near the step edges leading to deterioration of graphene's quality and promoting short-range scattering. 72 ...
The transport properties of quasi-free-standing (QFS) bilayer graphene on SiC depend on a range of scattering mechanisms. Most of them are isotropic in nature. However, the SiC substrate morphology marked by a distinctive pattern of the terraces gives rise to an anisotropy in graphene's sheet resistance, which may be considered an additional scattering mechanism. At a technological level, the growth-preceding in situ etching of the SiC surface promotes step bunching which results in macro steps ∼10 nm in height. In this report, we study the qualitative and quantitative effects of SiC steps edges on the resistance of epitaxial graphene grown by chemical vapor deposition. We experimentally determine the value of step edge resistivity in hydrogen-intercalated QFS-bilayer graphene to be ∼190 Ωμm for step height hS = 10 nm and provide proof that it cannot originate from mechanical deformation of graphene but is likely to arise from lowered carrier concentration in the step area. Our results are confronted with the previously reported values of the step edge resistivity in monolayer graphene over SiC atomic steps. In our analysis, we focus on large-scale, statistical properties to foster the scalable technology of industrial graphene for electronics and sensor applications.
Thermal decomposition of vicinal 6H-SiC(0001) surfaces with miscut angles toward the [11¯00] direction results in the appearance of pairs of (0001) macroterraces and (11¯0n) macrofacets covered with graphene, as follows. A carpetlike carbon layer grows on the surface, covering both the macroterraces and macrofacets; it forms a (63×63) buffer layer on the former ones, whereas its partial periodic bonding with the SiC steps on the latter ones generates a pseudographene nanoribbon (pseudo-GNR) array. The nanoribbons have a width of 1.7–1.8 nm and are aligned in the [112¯0] direction with a spatial periodicity of 3.3 nm. Scanning tunneling spectroscopy at a nanoribbon indicated a 0.4–0.5 eV energy gap and the Raman spectroscopy analysis of the pseudo-GNR array showed the absence of the 2D peak and the polarization dependence of the G and D peaks, which is typical of the armchair-edge nanoribbon.
The paper presents an interaction of Ti/Ti-oxides with: Si(100), HOPG(0001) and graphene/4H-SiC(0001) substrates. A thin layer (~3 nm) of Ti was deposited by means of DC sputtering technique on all the considered substrates. XPS, AFM and Raman spectroscopy were applied to find out the differences in interaction of Ti/Ti-oxides with selected substrates under UHV annealing. In the case of Si(100), substrate, apart from the expected TiO2, the presence of TiSi2 and TiSiOx components and the easiest reduction towards Ti2O3 was observed under UHV annealing. The sample exhibited also substantial evolution of surface morphology without the change of surface roughness which was attributed to formation of TiSi2 and TiSiOx components. This was in contrast with HOPG substrate where an annealing-induced agglomeration of the deposited material increased the surface roughness substantially. However, in the case of Ti/Ti-oxides deposited on graphene/4H-SiC(0001), UHV annealing caused no noticeable change of surface morphology. Agglomerates were not formed which was attributed to degradation of graphene (confirmed by Raman spectra) and supressing van der Waals' interactions responsible for easy surface diffusion. The differences in morphology were also discussed in the context of the surface energy of selected substrates.
We present an overview of the main techniques for production and processing of graphene and related materials (GRMs), as well as the key characterization procedures. We adopt a ‘hands-on’ approach, providing practical details and procedures as derived from literature as well as from the authors’ experience, in order to enable the reader to reproduce the results.
Section I is devoted to ‘bottom up’ approaches, whereby individual constituents are pieced together into more complex structures. We consider graphene nanoribbons (GNRs) produced either by solution processing or by on-surface synthesis in ultra high vacuum (UHV), as well carbon nanomembranes (CNM). Production of a variety of GNRs with tailored band gaps and edge shapes is now possible. CNMs can be tuned in terms of porosity, crystallinity and electronic behaviour.
Section II covers ‘top down’ techniques. These rely on breaking down of a layered precursor, in the graphene case usually natural crystals like graphite or artificially synthesized materials, such as highly oriented pyrolythic graphite, monolayers or few layers (FL) flakes. The main focus of this section is on various exfoliation techniques in a liquid media, either intercalation or liquid phase exfoliation (LPE). The choice of precursor, exfoliation method, medium as well as the control of parameters such as time or temperature are crucial. A definite choice of parameters and conditions yields a particular material with specific properties that makes it more suitable for a targeted application. We cover protocols for the graphitic precursors to graphene oxide (GO). This is an important material for a range of applications in biomedicine, energy storage, nanocomposites, etc. Hummers’ and modified Hummers’ methods are used to make GO that subsequently can be reduced to obtain reduced graphene oxide (RGO) with a variety of strategies. GO flakes are also employed to prepare three-dimensional (3d) low density structures, such as sponges, foams, hydro- or aerogels. The assembly of flakes into 3d structures can provide improved mechanical properties. Aerogels with a highly open structure, with interconnected hierarchical pores, can enhance the accessibility to the whole surface area, as relevant for a number of applications, such as energy storage. The main recipes to yield graphite intercalation compounds (GICs) are also discussed. GICs are suitable precursors for covalent functionalization of graphene, but can also be used for the synthesis of uncharged graphene in solution. Degradation of the molecules intercalated in GICs can be triggered by high temperature treatment or microwave irradiation, creating a gas pressure surge in graphite and exfoliation. Electrochemical exfoliation by applying a voltage in an electrolyte to a graphite electrode can be tuned by varying precursors, electrolytes and potential. Graphite electrodes can be either negatively or positively intercalated to obtain GICs that are subsequently exfoliated. We also discuss the materials that can be amenable to exfoliation, by employing a theoretical data-mining approach.
The exfoliation of LMs usually results in a heterogeneous dispersion of flakes with different lateral size and thickness. This is a critical bottleneck for applications, and hinders the full exploitation of GRMs produced by solution processing. The establishment of procedures to control the morphological properties of exfoliated GRMs, which also need to be industrially scalable, is one of the key needs. Section III deals with the processing of flakes. (Ultra)centrifugation techniques have thus far been the most investigated to sort GRMs following ultrasonication, shear mixing, ball milling, microfluidization, and wet-jet milling. It allows sorting by size and thickness. Inks formulated from GRM dispersions can be printed using a number of processes, from inkjet to screen printing. Each technique has specific rheological requirements, as well as geometrical constraints. The solvent choice is critical, not only for the GRM stability, but also in terms of optimizing printing on different substrates, such as glass, Si, plastic, paper, etc, all with different surface energies. Chemical modifications of such substrates is also a key step.
Sections IV–VII are devoted to the growth of GRMs on various substrates and their processing after growth to place them on the surface of choice for specific applications. The substrate for graphene growth is a key determinant of the nature and quality of the resultant film. The lattice mismatch between graphene and substrate influences the resulting crystallinity. Growth on insulators, such as SiO2, typically results in films with small crystallites, whereas growth on the close-packed surfaces of metals yields highly crystalline films. Section IV outlines the growth of graphene on SiC substrates. This satisfies the requirements for electronic applications, with well-defined graphene-substrate interface, low trapped impurities and no need for transfer. It also allows graphene structures and devices to be measured directly on the growth substrate. The flatness of the substrate results in graphene with minimal strain and ripples on large areas, allowing spectroscopies and surface science to be performed. We also discuss the surface engineering by intercalation of the resulting graphene, its integration with Si-wafers and the production of nanostructures with the desired shape, with no need for patterning.
Section V deals with chemical vapour deposition (CVD) onto various transition metals and on insulators. Growth on Ni results in graphitized polycrystalline films. While the thickness of these films can be optimized by controlling the deposition parameters, such as the type of hydrocarbon precursor and temperature, it is difficult to attain single layer graphene (SLG) across large areas, owing to the simultaneous nucleation/growth and solution/precipitation mechanisms. The differing characteristics of polycrystalline Ni films facilitate the growth of graphitic layers at different rates, resulting in regions with differing numbers of graphitic layers. High-quality films can be grown on Cu. Cu is available in a variety of shapes and forms, such as foils, bulks, foams, thin films on other materials and powders, making it attractive for industrial production of large area graphene films. The push to use CVD graphene in applications has also triggered a research line for the direct growth on insulators. The quality of the resulting films is lower than possible to date on metals, but enough, in terms of transmittance and resistivity, for many applications as described in section V.
Transfer technologies are the focus of section VI. CVD synthesis of graphene on metals and bottom up molecular approaches require SLG to be transferred to the final target substrates. To have technological impact, the advances in production of high-quality large-area CVD graphene must be commensurate with those on transfer and placement on the final substrates. This is a prerequisite for most applications, such as touch panels, anticorrosion coatings, transparent electrodes and gas sensors etc. New strategies have improved the transferred graphene quality, making CVD graphene a feasible option for CMOS foundries. Methods based on complete etching of the metal substrate in suitable etchants, typically iron chloride, ammonium persulfate, or hydrogen chloride although reliable, are time- and resource-consuming, with damage to graphene and production of metal and etchant residues. Electrochemical delamination in a low-concentration aqueous solution is an alternative. In this case metallic substrates can be reused. Dry transfer is less detrimental for the SLG quality, enabling a deterministic transfer.
There is a large range of layered materials (LMs) beyond graphite. Only few of them have been already exfoliated and fully characterized. Section VII deals with the growth of some of these materials. Amongst them, h-BN, transition metal tri- and di-chalcogenides are of paramount importance. The growth of h-BN is at present considered essential for the development of graphene in (opto) electronic applications, as h-BN is ideal as capping layer or substrate. The interesting optical and electronic properties of TMDs also require the development of scalable methods for their production. Large scale growth using chemical/physical vapour deposition or thermal assisted conversion has been thus far limited to a small set, such as h-BN or some TMDs. Heterostructures could also be directly grown.
Section VIII discusses advances in GRM functionalization. A broad range of organic molecules can be anchored to the sp² basal plane by reductive functionalization. Negatively charged graphene can be prepared in liquid phase (e.g. via intercalation chemistry or electrochemically) and can react with electrophiles. This can be achieved both in dispersion or on substrate. The functional groups of GO can be further derivatized. Graphene can also be noncovalently functionalized, in particular with polycyclic aromatic hydrocarbons that assemble on the sp² carbon network by π–π stacking. In the liquid phase, this can enhance the colloidal stability of SLG/FLG. Approaches to achieve noncovalent on-substrate functionalization are also discussed, which can chemically dope graphene. Research efforts to derivatize CNMs are also summarized, as well as novel routes to selectively address defect sites. In dispersion, edges are the most dominant defects and can be covalently modified. This enhances colloidal stability without modifying the graphene basal plane. Basal plane point defects can also be modified, passivated and healed in ultra-high vacuum. The decoration of graphene with metal nanoparticles (NPs) has also received considerable attention, as it allows to exploit synergistic effects between NPs and graphene. Decoration can be either achieved chemically or in the gas phase. All LMs, can be functionalized and we summarize emerging approaches to covalently and noncovalently functionalize MoS2 both in the liquid and on substrate.
Section IX describes some of the most popular characterization techniques, ranging from optical detection to the measurement of the electronic structure. Microscopies play an important role, although macroscopic techniques are also used for the measurement of the properties of these materials and their devices. Raman spectroscopy is paramount for GRMs, while PL is more adequate for non-graphene LMs (see section IX.2). Liquid based methods result in flakes with different thicknesses and dimensions. The qualification of size and thickness can be achieved using imaging techniques, like scanning probe microscopy (SPM) or transmission electron microscopy (TEM) or spectroscopic techniques. Optical microscopy enables the detection of flakes on suitable surfaces as well as the measurement of optical properties. Characterization of exfoliated materials is essential to improve the GRM metrology for applications and quality control. For grown GRMs, SPM can be used to probe morphological properties, as well as to study growth mechanisms and quality of transfer. More generally, SPM combined with smart measurement protocols in various modes allows one to get obtain information on mechanical properties, surface potential, work functions, electrical properties, or effectiveness of functionalization. Some of the techniques described are suitable for ‘in situ’ characterization, and can be hosted within the growth chambers. If the diagnosis is made ‘ex situ’, consideration should be given to the preparation of the samples to avoid contamination. Occasionally cleaning methods have to be used prior to measurement.
Epitaxial Chemical Vapor Deposition graphene on silicon carbide is a promising material for application in electronics. Its most developed form, i.e. quasi-free-standing monolayer that comes through hydrogen atom intercalation of carbon buffer layer grown on the Si-face of SiC, constitutes an excellent platform for sensor technology. Raman spectroscopy allows for a non-destructive analysis of graphene properties. The G and 2D bands widths and positions bring information on the number of graphene layers as well as charge carrier type and doping level. Their intensity is strong. However, the analysis of Si-H bonds remains a challenge. In this work we present how analytical methods can improve the visibility of hydrogen-related Raman bands and help monitor hydrogen presence in quasi-free-standing graphene.
De par ses propriétés physiques remarquables, le GaN est un matériau très attrayant pour la fabrication de composants photoniques. Sa synthèse est en revanche très complexe et reste un obstacle à son utilisation. L’hétéroépitaxie est, pour l’heure, la technique de synthèse la plus employée mais l’absence de substrats cristallins aux propriétés proches de celles du GaN conduit à l’élaboration de couches minces épitaxiées très défectueuses. Bien que les dispositifs à base de GaN soient d’ores et déjà fonctionnels, une augmentation de la qualité cristalline du matériau permettra une amélioration de leurs performances.L’épitaxie Van der Waals (VdW) est une alternative qui se différencie de l’épitaxie classique par la nature de l’interaction à l’interface entre substrat et matériau déposé. Cette dernière n’est alors plus régie par des forces fortes (liaisons covalente, ionique, etc) mais par des forces faibles, de type VdW. L’hétéroépitaxie VdW qui prône une interface de croissance compliante, apparait ainsi comme une méthode de synthèse alternative judicieuse pour l’amélioration de la qualité cristalline des couches épitaxiées. Ces travaux de thèse proposent d’explorer, en détail, la faisabilité de l’épitaxie VdW dans le cas particulier de la croissance de GaN sur graphène par EPVOM.L’utilisation d’un nouveau type de surface de très basse énergie pour supporter l’épitaxie du GaN nécessite le développement d’une nouvelle stratégie de croissance. Dans ce travail, un procédé en trois étapes a été mis en place pour la germination du GaN sur le graphène. Les cristaux microniques qui en résultent présentent une qualité cristalline remarquable, sont entièrement relaxés et adoptent une orientation cristallographique commune. Une relation d’épitaxie peut ainsi être mise en place à travers une interface faible qui est alors une interface d’épitaxie compliante. La faisabilité et les atouts de l’épitaxie VdW de GaN sur graphène sont donc démontrés expérimentalement. Plus précisément, nous avons démontré le rôle du substrat sous-jacent au graphène dans larelation d’épitaxie. Son caractère polaire, en particulier, semble indispensable pour qu’une relation d’épitaxie à distance puisse exister à travers le graphène.Cette étude exploratoire a à la fois permis d’illustrer tout le potentiel de l’épitaxie VdW de matériaux 3D sur 2D, d’en identifier certaines limites mais aussi de démontrer les possibilités liées à la création de nouvelles interfaces d’épitaxie 3D / 2D.
For a better comprehension of hydrogen intercalation of graphene grown on a silicon carbide substrate, an advanced analytical technique is required. We report that with a carefully established measurement procedure it is possible to obtain a reliable and reproducible depth profile of bi-layer graphene (theoretical thickness of 0.69 nm) grown on the silicon carbide substrate by the Chemical Vapor Deposition method. Furthermore, we show that with depth resolution as good as 0.2 nm/decade, both hydrogen coming from the intercalation process and organic contamination can be precisely localized. As expected, hydrogen was found at the interface between graphene and the SiC substrate, while organic contamination was accumulated on the surface of graphene and did not penetrate into it. Such a precise measurement may prove to be invaluable for further characterization of 2D materials.
While the remarkable efficiency of microwave heating is widely exploited in many branches of chemistry, it has been barely considered in relation to the synthesis of epitaxial graphene. In this study, an advanced technique is presented for the rapid synthesis of quality few-layer epitaxial graphene on 4H-SiC(0001). A piece of SiC cut from a single crystal wafer is directly annealed by microwaves at high temperatures in a vacuum using a customized multimode domestic microwave oven. Various temperature/irradiation time combinations are investigated, with extensive surface coverage by the graphene obtained after microwave annealing at 1700 °C for just 1 min. The ramp-up time to the required temperature is extraordinarily fast, occurring within seconds. The annealing is not only selective and volumetric, but also, because the substrate itself acts as a heater, removes the need for heat transport to the sample. This, in turn, reduces the thermal burden placed on the reactor and minimizes contamination levels. Thus, this study presents a novel route for the preparation of quality graphene that has multiple advantages and guarantees the saving of energy through greater heating efficiency.
Conductivity Contrast in SEM Images of Hydrogenated Graphene Grown on SiC - Volume 21 Issue S3 - Iwona Jozwik, Jacek M. Baranowski, Kacper Grodecki, Pawel Dabrowski, Wlodzimierz Strupinski
Atomic-layer 2D crystals have unique properties that can be significantly modified through interaction with an underlying support. For epitaxial graphene on SiC(0001), the interface strongly influences the electronic properties of the overlaying graphene. We demonstrate a novel combination of x-ray scattering and spectroscopy for studying the complexities of such a buried interface structure. This approach employs x-ray standing wave-excited photoelectron spectroscopy in conjunction with x-ray reflectivity to produce a highly resolved chemically sensitive atomic profile for the terminal substrate bilayers, interface, and graphene layers along the SiC[0001] direction.
Thermally induced growth of graphene on the two polar surfaces of 6H-SiC
is investigated with emphasis on the initial stages of growth and
interface structure. The experimental methods employed are
angle-resolved valence band photoelectron spectroscopy, soft x-ray
induced core-level spectroscopy, and low-energy electron diffraction. On
the Si-terminated (0001) surface, the (63×63)R30°
reconstruction is the precursor of the growth of graphene and it
persists at the interface upon the growth of few layer graphene (FLG).
The (63×63)R30° structure is a carbon layer with
graphene-like atomic arrangement covalently bonded to the substrate
where it is responsible for the azimuthal ordering of FLG on SiC(0001).
In contrast, the interaction between graphene and the C-terminated
(0001¯) surface is much weaker, which accounts for the low degree
of order of FLG on this surface. A model for the growth of FLG on
SiC{0001} is developed, wherein each new graphene layer is formed at the
bottom of the existing stack rather than on its top. This model yields,
in conjunction with the differences in the interfacial bonding strength,
a natural explanation for the different degrees of azimuthal order
observed for FLG on the two surfaces.
We report a Raman study of the so-called buffer layer with
periodicity which forms the intrinsic
interface structure between epitaxial graphene and SiC(0001). We show that this
interface structure leads to a nonvanishing signal in the Raman spectrum at
frequencies in the range of the D- and G-band of graphene and discuss its shape
and intensity. Ab-initio phonon calculations reveal that these features can be
attributed to the vibrational density of states of the buffer-layer.
The interaction with a substrate can modify the graphene honeycomb lattice and, thus, alter its outstanding properties. This could be particularly true for epitaxial graphene where the carbon layers are grown from the SiC substrate. Extensive ab initio calculations supported by the scanning tunneling microscopy experiments demonstrate here that the substrate indeed induces a strong nanostructuration of the interface carbon layer. It generates an apparent 6×6 modulation different from the interface 6√3×6√3R30 symmetry used for the calculation. The top carbon layer roughly follows the interface layer morphology. This creates soft 6×6 ripples in the otherwise graphene-like honeycomb lattice. The wavelength and height of the ripples are much smaller than the one found in exfoliated graphene. Their formation mechanism also differs; they are due to the weak interaction with the interface layer and not to the roughening of the plane due to the instability of a strictly two-dimensional crystal.
The inability to grow large well-ordered ultra high vacuum (UHV) graphene with a specific number of layers on SiC(0001) is well known. The growth involves several competing processes (Si desorption, carbon diffusion, island nucleation, etc.) and because of the high temperatures, it has not been possible to identify the growth mechanism. Using scanning tunneling microscopy and a vicinal 6H-SiC(0001) sample, we determine that the Si desorption from steps is the main controlling process. Adjacent steps retract with different speeds and the released carbon produces large areas of bilayer graphene with characteristic “fingers” emanating from steps. If faster heating rates are used, the different Si desorption rates are avoided and single-layer graphene films extending over many microns are produced.
We report on the micro-Raman spectroscopy of monolayer, bilayer, trilayer, and many layers of graphene (graphite) bombarded by low-energy argon ions with different doses. The evolution of peak frequencies, intensities, linewidths, and areas of the main Raman bands of graphene is analyzed as function of the distance between defects and number of layers. We describe the disorder-induced frequency shifts and the increase in the linewidth of the Raman bands by means of a spatial-correlation model. Also, the evolution of the relative areas AD/AG, AD′/AG, and AG′/AG is described by a phenomenological model. The present results can be used to fully characterize disorder in graphene systems.
We have observed the formation of graphene on SiC by Si sublimation in an Ar atmosphere using low-energy electron microscopy, scanning tunneling microcopy, and atomic force microscopy. This work reveals unanticipated growth mechanisms, which depend strongly on the initial surface morphology. Carbon diffusion governs the spatial relationship between SiC decomposition and graphene growth. Isolated bilayer SiC steps generate narrow ribbons of graphene by a distinctive cooperative process, whereas triple bilayer steps allow large graphene sheets to grow by step flow. We demonstrate how graphene quality can be improved by controlling the initial surface morphology to avoid the instabilities inherent in diffusion-limited growth.
Using directly interpretable atomic-resolution cross-sectional scanning transmission electron microscopy, we have investigated the structure of few-layer epitaxial graphene (EG) on 6H-SiC(0001). We show that the buried interface layer possesses a lower average areal density of carbon atoms than graphene, indicating that it is not a graphenelike sheet with the 6×6R30° structure. The EG interlayer spacings are found to be considerably larger than that of the bulk graphite and the surface of the SiC(0001) substrate, often treated as relaxed, is found to be strained. Discontinuity of the graphene layers above the SiC surface steps is observed, in contradiction with the commonly believed continuous coverage.
Graphene, a monoatomic layer of graphite hosts a two-dimensional electron gas system with large electron mobilities which makes it a prospective candidate for future carbon nanodevices. Grown epitaxially on silicon carbide (SiC) wafers, large area graphene samples appear feasible and integration in existing device technology can be envisioned. This article reviews the controlled growth of epitaxial graphene layers on SiC(0001) and the manipulation of their electronic structure. We show that epitaxial graphene on SiC grows on top of a carbon interface layer that – although it has a graphite-like atomic structure – does not display the linear π-bands typical for graphene due to a strong covalent bonding to the substrate. Only the second carbon layer on top of this interface acts like monolayer graphene. With a further carbon layer, a graphene bilayer system develops. During the growth of epitaxial graphene on SiC(0001) the number of graphene layers can be precisely controlled by monitoring the π-band structure. Experimental fingerprints for in-situ growth control could be established. However, due to the influence of the interface layer, epitaxial graphene on SiC(0001) is intrinsically n-doped and the layers have a long-range corrugation in their density of states. As a result, the Dirac point energy where the π-bands cross is shifted away from the Fermi energy, so that the ambipolar properties of graphene cannot be exploited. We demonstrate methods to compensate and eliminate this structural and electronic influence of the interface. We show that the band structure of epitaxial graphene on SiC(0001) can be precisely tailored by functionalizing the graphene surface with tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) molecules. Charge neutrality can be achieved for mono-and bilayer graphene. On epitaxial bilayer graphene, where a band gap opens due to the asymmetric electric field across the layers imposed by the interface, the magnitude of this band gap can be increased up to more than double of its initial value. The hole doping allows the Fermi level to shift into the energy band gap. The impact of the interface layer can be completely eliminated by decoupling the graphene from the SiC substrate by a hydrogen intercalation technique. We demonstrate that hydrogen can migrate under the interface layer and passivate the underlying SiC substrate. The interface layer alone transforms into a quasi-free standing monolayer. Epitaxial monolayer graphene turns into a decoupled bilayer. In combination with atmospheric pressure graphitization, the intercalation process allows to produce quasi-free standing epitaxial graphene on large SiC wafers and represents a highly promising route towards epitaxial graphene based nanoelectronics.
With expanding interest in graphene-based electronics, it is crucial that high quality graphene films be grown. Sublimation of Si from the 4H- Si C (0001) (Si-terminated) surface in ultrahigh vacuum is a demonstrated method to produce epitaxial graphene sheets on a semiconductor. In this letter the authors show that graphene grown from the Si C (000 1 ) (C-terminated) surface are of higher quality than those previously grown on SiC(0001). Graphene grown on the C face can have structural domain sizes more than three times larger than those grown on the Si face while at the same time reducing SiC substrate disorder from sublimation by an order of magnitude.
ABSTRACT: Micro-Raman and micro-transmission imaging experiments have been done on epitaxial graphene grown on the C- and Si-faces of on-axis 6H-SiC substrates. On the C-face it is shown that the SiC sublimation process results in the growth of long and isolated graphene ribbons (up to 600 μm) that are strain-relaxed and lightly p-type doped. In this case, combining the results of micro-Raman spectroscopy with micro-transmission measurements, we were able to ascertain that uniform monolayer ribbons were grown and found also Bernal stacked and misoriented bilayer ribbons. On the Si-face, the situation is completely different. A full graphene coverage of the SiC surface is achieved but anisotropic growth still occurs, because of the step-bunched SiC surface reconstruction. While in the middle of reconstructed terraces thin graphene stacks (up to 5 layers) are grown, thicker graphene stripes appear at step edges. In both the cases, the strong interaction between the graphene layers and the underlying SiC substrate induces a high compressive thermal strain and n-type doping.
On the basis of first-principles calculations, we report that a novel interfacial atomic structure occurs between graphene and the surface of silicon carbide, destroying the Dirac point of graphene and opening a substantial energy gap there. In the calculated atomic structures, a quasiperiodic 6x6 domain pattern emerges out of a larger commensurate 6 sqrt [3] x 6 sqrt [3]R30 degrees periodic interfacial reconstruction, resolving a long standing experimental controversy on the periodicity of the interfacial superstructures. Our theoretical energy spectrum shows a gap and midgap states at the Dirac point of graphene, which are in excellent agreement with the recently observed anomalous angle-resolved photoemission spectra. Beyond solving unexplained issues in epitaxial graphene, our atomistic study may provide a way to engineer the energy gaps of graphene on substrates.
We calculate the double resonant (DR) Raman spectrum of graphene, and
determine the lines associated to both phonon-defect processes, and two-phonons
ones. Phonon and electronic dispersions reproduce calculations based on density
functional theory corrected with GW. Electron-light, -phonon, and -defect
scattering matrix elements and the electronic linewidth are explicitly
calculated. Defect-induced processes are simulated by considering different
kind of idealized defects. For an excitation energy of eV, the
agreement with measurements is very good and calculations reproduce: the
relative intensities among phonon-defect or among two-phonon lines; the
measured small widths of the D, , 2D and lines; the line shapes; the
presence of small intensity lines in the 1800, 2000 cm range. We
determine how the spectra depend on the excitation energy, on the light
polarization, on the electronic linewidth, on the kind of defects and on their
concentration. According to the present findings, the intensity ratio between
the and 2D lines can be used to determine experimentally the electronic
linewidth. The intensity ratio between the D and lines depends on the
kind of model defect, suggesting that this ratio could possibly be used to
identify the kind of defects present in actual samples. Charged impurities
outside the graphene plane provide an almost undetectable contribution to the
Raman signal.
We report on an investigation of quasi-free-standing graphene on 6H-SiC(0001)
which was prepared by intercalation of hydrogen under the buffer layer. Using
infrared absorption spectroscopy we prove that the SiC(0001) surface is
saturated with hydrogen. Raman spectra demonstrate the conversion of the buffer
layer into graphene which exhibits a slight tensile strain and short range
defects. The layers are hole doped (p = 5.0-6.5 x 10^12 cm^(-2)) with a carrier
mobility of 3,100 cm^2/Vs at room temperature. Compared to graphene on the
buffer layer a strongly reduced temperature dependence of the mobility is
observed for graphene on H-terminated SiC(0001)which justifies the term
"quasi-free-standing".
The temperature-induced shift of the Raman G line in epitaxial graphene on
SiC and Ni surfaces, as well as in graphene supported on SiO2, is investigated
with Raman spectroscopy. The thermal shift rate of epitaxial graphene on
6H-SiC(0001) is found to be about three times that of freestanding graphene.
This result is explained quantitatively as a consequence of pinning by the
substrate. In contrast, graphene grown on polycrystalline Ni films is shown to
be unpinned, i.e., to behave elastically as freestanding, despite the
relatively strong interaction with the metal substrate. Moreover, it is shown
that the transfer of exfoliated graphene layers onto a supporting substrate can
result in pinned or unpinned layers, depending on the transfer protocol.
Using scanning tunneling microscopy with Fe-coated W tips and first-principles calculations, we show that the interface of epitaxial graphene/SiC(0001) is a warped graphene layer with hexagon-pentagon-heptagon (H(5,6,7)) defects that break the honeycomb symmetry, thereby inducing a gap and states below E(F near the K point. Although the next graphene layer assumes the perfect honeycomb lattice, its interaction with the warped layer modifies )the dispersion about the Dirac point. These results explain recent angle-resolved photoemission and carbon core-level shift data and solve the long-standing problem of the interfacial structure of epitaxial graphene on SiC(0001).
The unique electronic properties of graphene offer the possibility that it could replace silicon when microelectronics evolves to nanoelectronics. Graphene grown epitaxially on silicon carbide is particularly attractive in this regard because SiC is itself a useful semiconductor and, by suitable manipulation of the growth conditions, epitaxial films can be produced that exhibit all the transport properties of ideal, two-dimensional graphene desired for device applications. Nevertheless, there is little or no understanding of the actual kinetics of growth, which is likely to be required for future process control. As a step in this direction, we propose a local heat release mechanism to explain finger-like structures observed when graphene is grown by step flow decomposition of SiC(0001). Using a continuum equation of motion for the shape evolution of a moving step, a linear stability analysis predicts whether a shape perturbation of a straight moving step grows or decays as a function of growth temperature, the background pressure of Si maintained during growth, and the effectiveness of an inert buffer gas to retard the escape of Si atoms from the crystal surface. The theory gives semi-quantitative agreement with experiment for the characteristic separation between fingers observed when graphene is grown in a low-pressure induction furnace or under ultrahigh vacuum conditions. Comment: 4 pages, 5 figures
Graphene is created through thermal decomposition of the Si face of 4H-SiC in high-vacuum. Growth temperature and time are varied independently to gain a better understanding of how surface features and morphology affect graphene formation. Growth mechanisms of graphene are studied by ex situ atomic force microscopy (AFM) and scanning tunneling microscopy (STM). On the route toward a continuous graphene film, various growth features, such as macroscale step bunching, terrace pits, and fingers, are found and analyzed. Topographic and phase AFM analysis demonstrates how surface morphology changes with experimental conditions. Step-bunched terraces and terrace pits show a strong preference for eroding along the {11 (2) over bar0} planes. Data from AFM are corroborated with STM to determine the surface structure of the growth features. It is shown that elevated finger structures are SiC while the depressed interdigitated areas between the fingers are comprised of at least a monolayer of graphene. Graphene formation at the bottom of terrace pits shows a dependence on pit depth. These features lend support for a stoichiometric view of graphene formation based on the number of decomposing SiC bilayers.
Graphene, a single monolayer of graphite, has recently attracted considerable interest owing to its novel magneto-transport properties, high carrier mobility and ballistic transport up to room temperature. It has the potential for technological applications as a successor of silicon in the post Moore's law era, as a single-molecule gas sensor, in spintronics, in quantum computing or as a terahertz oscillator. For such applications, uniform ordered growth of graphene on an insulating substrate is necessary. The growth of graphene on insulating silicon carbide (SiC) surfaces by high-temperature annealing in vacuum was previously proposed to open a route for large-scale production of graphene-based devices. However, vacuum decomposition of SiC yields graphene layers with small grains (30-200 nm; refs 14-16). Here, we show that the ex situ graphitization of Si-terminated SiC(0001) in an argon atmosphere of about 1 bar produces monolayer graphene films with much larger domain sizes than previously attainable. Raman spectroscopy and Hall measurements confirm the improved quality of the films thus obtained. High electronic mobilities were found, which reach mu=2,000 cm (2) V(-1) s(-1) at T=27 K. The new growth process introduced here establishes a method for the synthesis of graphene films on a technologically viable basis.
The early stages of epitaxial graphene layer growth on the Si-terminated 6H-SiC (0001) are investigated by Auger electron spectroscopy (AES) and depolarized Raman spectroscopy. The selection of the depolarized component of the scattered light results in a significant increase in the C-C bond signal over the second order SiC Raman signal, which allows us to resolve submonolayer growth, including individual, localized C=C dimers in a diamondlike carbon matrix for AES C/Si ratio of approximately 3, and a strained graphene layer with delocalized electrons and Dirac single-band dispersion for AES C/Si ratio >6. The linear strain, measured at room temperature, is found to be compressive, which can be attributed to the large difference between the coefficients of thermal expansion of graphene and SiC. The magnitude of the compressive strain can be varied by adjusting the growth time at fixed annealing temperature.
Graphene is the two-dimensional building block for carbon allotropes of every other dimensionality. We show that its electronic structure is captured in its Raman spectrum that clearly evolves with the number of layers. The D peak second order changes in shape, width, and position for an increasing number of layers, reflecting the change in the electron bands via a double resonant Raman process. The G peak slightly down-shifts. This allows unambiguous, high-throughput, nondestructive identification of graphene layers, which is critically lacking in this emerging research area.
Graphene grown epitaxially on SiC has been proposed as a material for carbon-based electronics. Understanding the interface between graphene and the SiC substrate will be important for future applications. We report the ability to image the interface structure beneath single-layer graphene using scanning tunneling microscopy. Such imaging is possible because the graphene appears transparent at energies of 1 eV above or below the Fermi energy. Our analysis of calculations based on density functional theory shows how this transparency arises from the electronic structure of a graphene layer on a SiC substrate. Comment: 18 pages, 5 figures
Graphitization of the 6H-SiC(0001) surface as a function of annealing temperature has been studied by ARPES, high resolution XPS, and LEED. For the initial stage of graphitization – the 6√3 reconstructed surface – we observe σ-bands characteristic of graphitic sp2-bonded carbon. The π-bands are modified by the interaction with the substrate. C1s core level spectra indicate that this layer consists of two inequivalent types of carbon atoms. The next layer of graphite (graphene) formed on top of the 6√3 surface at TA=1250°C-1300°C has an unperturbed electronic structure. Annealing at higher temperatures results in the formation of a multilayer graphite film. It is shown that the atomic arrangement of the interface between graphite and the SiC(0001) surface is practically identical to that of the 6√3 reconstructed layer.
The results of micro-Raman scattering measurements performed on three different ``graphitic'' materials: micro-structured disks of highly oriented pyrolytic graphite, graphene multi-layers thermally decomposed from carbon terminated surface of 4H-SiC and an exfoliated graphene monolayer are presented. Despite its multi-layer character, most parts of the surface of the graphitized SiC substrates shows a single-component, Lorentzian shape, double resonance Raman feature in striking similarity to the case of a single graphene monolayer. Our observation suggests a very weak electronic coupling between graphitic layers on the SiC surface, which therefore can be considered to be graphene multi-layers with a simple (Dirac-like) band structure. Comment: 4 pages, 3 Figures Structure of the paper strongly modified, small changes in Fig 2 and 3. Same interpretation and same results
We present a structural analysis of the graphene-4HSiC(0001) interface using surface x-ray reflectivity. We find that the interface is composed of an extended reconstruction of two SiC bilayers. The interface directly below the first graphene sheet is an extended layer that is more than twice the thickness of a bulk SiC bilayer (~1.7A compared to 0.63A). The distance from this interface layer to the first graphene sheet is much smaller than the graphite interlayer spacing but larger than the same distance measured for graphene grown on the (000-1) surface, as predicted previously by ab intio calculations. Comment: 1 tex file plus 6 figures
Transport in ultrathin graphite films grown on single-crystal silicon
carbide is dominated by the electron-doped epitaxial graphene layer at
the interface and shows graphene characteristics. Epitaxial graphene
provides a platform for studying the novel electronic properties of this
2D electron gas in a controlled environment. Shubnikov-de Haas
oscillations in the magnetoresistance data indicate an anomalous Berry's
phase and reveal the Dirac nature of the charge carriers. The system is
highly coherent with phase coherence lengths beyond 1 micrometer at
cryogenic temperatures, and mobilities exceeding 2.5 square meters per
volt-second. In wide structures, evidence is found for weak
anti-localization in agreement with recent graphene weak-localization
theory. Patterned narrow ribbons show quantum confinement of electrons.
Several Hall bar samples reveal anomalous magnetoresistance patterns
consisting of large structured non-periodic oscillations that may be due
to a periodic superlattice potential.
We present a detailed Raman study of defective graphene samples containing specific types of defects. In particular, we compared sp3 sites, vacancies, and substitutional Boron atoms. We find that the ratio between the D and G peak intensities, I(D)/I(G), does not depend on the geometry of the defect (within the Raman spectrometer resolution). In contrast, in the limit of low defect concentration, the ratio between the D′ and G peak intensities is higher for vacancies than sp3 sites. By using the local activation model, we attribute this difference to the term CS,x, representing the Raman cross section of I(x)/I(G) associated with the distortion of the crystal lattice after defect introduction per unit of damaged area, where x = D or D′. We observed that CS,D=0 for all the defects analyzed, while CS,D′ of vacancies is 2.5 times larger than CS,D′ of sp3 sites. This makes I(D)/I(D′) strongly sensitive to the nature of the defect. We also show that the exact dependence of I(D)/I(D′) on the excitation energy may be affected by the nature of the defect. These results can be used to obtain further insights into the Raman scattering process (in particular for the D′ peak) in order to improve our understanding and modeling of defects in graphene.
The atomic structure of the carbon nanomesh template (the so-called 63×63R30° reconstruction) on the 6H–SiC(0001) surface was investigated in detail by scanning tunneling microscopy (STM), low energy electron diffraction (LEED), synchrotron photoemission spectroscopy (PES) and density-functional theory (DFT) calculations. We propose that the origin of the carbon nanomesh template arises from the self-organization of excess carbon atoms forming a novel honeycomb arrangement atop the 6H–SiC(0001) surface after annealing at 1100°C. Two carbon nanomesh-related C 1s components are observed at binding energies of 285.1eV (S1) and 283.8eV (S2) by angle-resolved synchrotron PES experiments. We assign the S2 component to carbon atoms that bond to one underlying Si atom, and the S1 component to carbon atoms bonded to other carbon atoms without Si–C bond formation. Further annealing results in the formation of crystalline graphitic layers on top of the surface.
On the SiC(0001) surface (the silicon face of SiC), epitaxial graphene is obtained by sublimation of Si from the substrate. The graphene film is separated from the bulk by a carbon-rich interface layer (hereafter called the buffer layer) which in part covalently binds to the substrate. Its structural and electronic properties are currently under debate. In the present work we report scanning tunneling microscopy (STM) studies of the buffer layer and of quasi-free-standing monolayer graphene (QFMLG) that is obtained by decoupling the buffer layer from the SiC(0001) substrate by means of hydrogen intercalation. Atomic resolution STM images of the buffer layer reveal that, within the periodic structural corrugation of this interfacial layer, the arrangement of atoms is topologically identical to that of graphene. After hydrogen intercalation, we show that the resulting QFMLG is relieved from the periodic corrugation and presents no detectable defect sites.
Raman spectroscopy is an integral part of graphene research. It is used to determine the number and orientation of layers, the quality and types of edge, and the effects of perturbations, such as electric and magnetic fields, strain, doping, disorder and functional groups. This, in turn, provides insight into all sp(2)-bonded carbon allotropes, because graphene is their fundamental building block. Here we review the state of the art, future directions and open questions in Raman spectroscopy of graphene. We describe essential physical processes whose importance has only recently been recognized, such as the various types of resonance at play, and the role of quantum interference. We update all basic concepts and notations, and propose a terminology that is able to describe any result in literature. We finally highlight the potential of Raman spectroscopy for layered materials other than graphene.
Atomic-resolution structural and spectroscopic characterization techniques (scanning transmission electron microscopy and electron energy loss spectroscopy) are combined with nanoscale electrical measurements (conductive atomic force microscopy) to study at the atomic scale the properties of graphene grown epitaxially through the controlled graphitization of a hexagonal SiC(0001) substrate by high temperature annealing. This growth technique is known to result in a pronounced electron-doping (about 10^13 cm-2) of graphene, which is thought to originate from an interface carbon buffer layer strongly bound to the substrate. The scanning transmission electron microscopy analysis, carried out at an energy below the knock-on threshold for carbon to ensure no damage is imparted to the film by the electron beam, demonstrates that the buffer layer present on the planar SiC(0001) face delaminates from it on the (11-2n) facets of SiC surface steps. In addition, electron energy loss spectroscopy reveals that the delaminated layer has a similar electronic configuration to purely sp2-hybridized graphene. These observations are used to explain the local increase of the graphene sheet resistance measured around the surface steps by conductive atomic force microscopy, which we suggest is due to significantly lower substrate-induced doping and a resonant scattering mechanism at the step regions. A first-principles-calibrated theoretical model is proposed to explain the structural instability of the buffer layer on the SiC facets and the resulting delamination.
The model and theoretical understanding of the Raman spectra in disordered and amorphous carbon are given. The nature of the G and D vibration modes in graphite is analyzed in terms of the resonant excitation of π states and the long-range polarizability of π bonding. Visible Raman data on disordered, amorphous, and diamondlike carbon are classified in a three-stage model to show the factors that control the position, intensity, and widths of the G and D peaks. It is shown that the visible Raman spectra depend formally on the configuration of the sp2 sites in sp2-bonded clusters. In cases where the sp2 clustering is controlled by the sp3 fraction, such as in as-deposited tetrahedral amorphous carbon (ta-C) or hydrogenated amorphous carbon (a-C:H) films, the visible Raman parameters can be used to derive the sp3 fraction.
Homogeneous large-area graphene monolayers were successfully prepared ex situ on 6H-SiC(0001). The samples have been studied systematically and the results are compared with those from a sample cut from the same wafer and prepared by in situ heating. The formation of smaller graphene flakes was found on the in situ prepared sample, which is in line with earlier observations. Distinctly different results are observed from the ex situ graphene layers of different thicknesses, which are proposed as a guideline for determining graphene growth. Recorded C 1s spectra consisted of three components: bulk SiC, graphene (G), and interface (I), the latter being a 6√3 layer. Extracted intensity ratios of G/I were found to give a good estimate of the thickness of graphene. Differences are also revealed in micro low energy electron diffraction images and electron reflectivity curves. The diffraction patterns were distinctly different from a monolayer thickness up to three layers. At a larger thickness only the graphitelike spot was visible. The electron reflectivity curve showed a nice oscillation behavior with kinetic energy and as a function of the number of graphene layers. The graphene sheets prepared were found to be very inert and the interface between the substrate and the layer(s) was found to be quite abrupt. No free Si could be detected in or on the graphene layers or at the interface.
Using Co-decoration technique coupled with in situ scanning tunneling microscope (STM), the evolution of epitaxial graphene was found to preferentially begin at step edges of the silicon carbide surface and occurs with loss of Si and breakdown of C-rich (6×6)R30° template, which provides the C source for graphene growth. Interestingly, a new C-rich phase is also formed at the interface and it acts as a buffer layer for graphene from underlying bulk. STM reveals that graphene lies 2.3±0.3 Å above the buffer layer, larger than sp3 C–C bond length (1.54 Å) but shorter than graphite interlayer separation (3.37 Å), suggesting a pseudo-van der Waals interfacial interaction.
Graphene grown on SiC(0001) by Si depletion has a stepped surface with terraces and step heights up to 10 times larger than those observed in the original SiC surface. This is due to an additional step bunching that usually occurs during graphene formation. In this work, we show that such process can be suppressed by controlling the initial step structure of the SiC surface. In this case, the graphene monolayer is formed on the SiC without modification of the original surface morphology. We observe that the absence of step bunching during growth has no influence on the graphene structural quality.
We present Raman spectra of epitaxial graphene layers grown on 6×6 reconstructed silicon carbide surfaces during annealing at elevated temperature. In contrast to exfoliated graphene a significant phonon hardening is observed. We ascribe that phonon hardening to a minor part to the known electron transfer from the substrate to the epitaxial layer, and mainly to mechanical strain that builds up when the sample is cooled down after annealing. Due to the larger thermal expansion coefficient of silicon carbide compared to the in-plane expansion coefficient of graphite this strain is compressive at room temperature.
Manipulation of graphene-based systems is a formidable challenge, since it requires the control of atomic interactions over long timescales. Although the effectiveness of a certain number of processes has been experimentally demonstrated, the underlying atomic mechanisms are often not understood. An import class of techniques relies on the interaction between hydrogen and graphene, which is the focus of this research. In particular, the growth of epitaxial graphene on SiC(0001) is subject to a single-atom-thick interface carbon layer strongly bound to the substrate, which can be detached through hydrogen intercalation. Here we report that a nucleation phenomenon induces the transformation of this buffer layer into graphene. We study the graphenization dynamics by an ab initio based method that permits the simulation of large systems with an atomic resolution, spanning the time scales from nanoseconds to hours. The early evolution stage (∼ms time scale) is characterised by the formation of a metastable H layer deposited on the C surface. H penetration in the interface between the C monolayer and the SiC(0001) surface is a rare event due to the large penetration barrier, which is ∼2 eV. However, at high H densities, energetically favoured Si-H bonding appears on the substrate's surface. The local increase of the H density at the interface due to statistical transitions leads to the graphenization of the overlying C atoms. Thermally activated density fluctuations promote the formation of these graphene-like islands on the buffer layer: this nucleation phenomenon is evidenced by our simulations at a later evolution stage (>10(2) s at 700 °C for ∼3.6 × 10(15) at. cm(-2) s(-1) H flux). Such nuclei grow and quasi-freestanding graphene forms if the exposition to the H flux continues for a sufficiently long time (∼30 min for the same conditions). We have systematically explored this phenomenon by varying the substrate temperature and the H flux, demonstrating that the surface morphology during graphenization and post-graphenization anneals significantly depends on these variables. The computational findings are consistent with the experimental analyses reported so far and could serve as guidelines for future experimental works on graphene manipulation.
Transmission electron microscopy (TEM), scanning tunneling microscopy (STM), and micro-Raman investigations of epitaxial graphene on 4H -SiC on-axis and 4° off-axis are presented. The STM images show that there is superimposed on 1×1 graphene pattern the carbon nanomesh of honeycomb 6×6 structure with the lattice vector of 17.5 Å. The TEM results give evidence that the first carbon layer is separated by 2 Å from the Si-terminated SiC surface and that subsequent carbon layers are spaced by 3.3 Å. It is also found in TEM that the graphene layers cover atomic steps, present on 4° off-axis SiC(0001) surface, indicating a carpetlike growth mode. However, a bending of graphene planes on atomic steps of SiC apparently leads to generation of stress which leads to creation of edge dislocations in the graphene layers.
Raman scattering is used to study disorder in graphene subjected to low energy (90 eV) Ar+ ion bombardment. The evolution of the intensity ratio between the G band (1585 cm−1) and the disorder-induced D band (1345 cm−1) with ion dose is determined, providing a spectroscopy-based method to quantify the density of defects in graphene. This evolution can be fitted by a phenomenological model, which is in conceptual agreement with a well-established amorphization trajectory for graphitic materials. Our results show that the broadly used Tuinstra-Koenig relation should be limited to the measure of crystallite sizes, and allows extraction of the Raman relaxation length for the disorder-induced Raman scattering process.
We review recent work on Raman spectroscopy of graphite and graphene. We focus on the origin of the D and G peaks and the second order of the D peak. The G and 2D Raman peaks change in shape, position and relative intensity with number of graphene layers. This reflects the evolution of the electronic structure and electron–phonon interactions. We then consider the effects of doping on the Raman spectra of graphene. The Fermi energy is tuned by applying a gate-voltage. We show that this induces a stiffening of the Raman G peak for both holes and electrons doping. Thus Raman spectroscopy can be efficiently used to monitor number of layers, quality of layers, doping level and confinement.
LEED and AES experiments of the SiC{0001} crystal surfaces show that on heat-treatment these surfaces are easily “covered” with a layer of graphite by evaporation of silicon. The graphite layer, which has a distinct crystallographic relation to the SiC crystal, is monocrystalline on the Si-face and mostly polycrystalline on the C-face. A speculation about the mechanism of the initial graphitization of the basal faces of SiC is given.
We show experimentally that multilayer graphene grown on the carbon terminated SiC(0001[over ]) surface contains rotational stacking faults related to the epitaxial condition at the graphene-SiC interface. Via first-principles calculation, we demonstrate that such faults produce an electronic structure indistinguishable from an isolated single graphene sheet in the vicinity of the Dirac point. This explains prior experimental results that showed single-layer electronic properties, even for epitaxial graphene films tens of layers thick.
We present a Raman study of Ar(+)-bombarded graphene samples with increasing ion doses. This allows us to have a controlled, increasing, amount of defects. We find that the ratio between the D and G peak intensities, for a given defect density, strongly depends on the laser excitation energy. We quantify this effect and present a simple equation for the determination of the point defect density in graphene via Raman spectroscopy for any visible excitation energy. We note that, for all excitations, the D to G intensity ratio reaches a maximum for an interdefect distance ∼3 nm. Thus, a given ratio could correspond to two different defect densities, above or below the maximum. The analysis of the G peak width and its dispersion with excitation energy solves this ambiguity.
Local electrical characterization of epitaxial graphene grown on 4H-SiC(0001) using electrostatic force microscopy (EFM) in ambient conditions and at elevated temperatures is presented. EFM provides a straightforward identification of graphene with different numbers of layers on the substrate where topographical determination is hindered by adsorbates. Novel EFM spectroscopy has been developed measuring the EFM phase as a function of the electrical DC bias, establishing a rigorous way to distinguish graphene domains and facilitating optimization of EFM imaging.
We demonstrate the growth of high quality graphene layers by chemical vapor deposition (CVD) on insulating and conductive SiC substrates. This method provides key advantages over the well-developed epitaxial graphene growth by Si sublimation that has been known for decades. (1) CVD growth is much less sensitive to SiC surface defects resulting in high electron mobilities of ∼1800 cm(2)/(V s) and enables the controlled synthesis of a determined number of graphene layers with a defined doping level. The high quality of graphene is evidenced by a unique combination of angle-resolved photoemission spectroscopy, Raman spectroscopy, transport measurements, scanning tunneling microscopy and ellipsometry. Our measurements indicate that CVD grown graphene is under less compressive strain than its epitaxial counterpart and confirms the existence of an electronic energy band gap. These features are essential for future applications of graphene electronics based on wafer scale graphene growth.
The formation of surface phases on the Si-terminated SiC(0001) surface, from the Si-rich (3x3) structure, through the intermediate (1x1) and (sqrt[3]xsqrt[3])-R30 degrees structures, to the C-rich (6sqrt[3]x6sqrt[3]) phase, and finally epitaxial graphene, has been well documented. But the thermodynamics and kinetics of these phase formations are poorly understood. Using in situ low energy electron microscopy, we show how the phase transformation temperatures can be shifted over several hundred degrees Celsius, and the phase transformation time scales reduced by several orders of magnitude, by balancing the rate of Si evaporation with an external flux of Si. Detailed insight in the thermodynamics allows us to dramatically improve the morphology of the final C-rich surface phases, including epitaxial graphene.
Large and homogeneous layers of graphene are obtained by annealing silicon carbide in a dense noble gas atmosphere that controls the way in which silicon sublimates. Emtsev and co-workers have demonstrated an important step towards this goal for a specific fabrication strategy, graphene epitaxy on silicon carbide (SiC). They began by comparing the electron mobility, a measure of the charge carrier drift velocity in an applied electric field for samples prepared in vacuum and in ambient-pressure argon, patterned into simple test structures. Samples produced by the new process show a nearly twofold improvement over the previous record mobility in Si-face epitaxial graphene. This study prepares for their identification for epitaxial graphene on SiC by reducing roughness and increasing the graphene domain size. The improved properties demonstrated by Emtsev and coworkers provides an additional performance measurement for this technology.
We report results of Raman spectroscopy studies of large-area epitaxial graphene grown on SiC. Our work reveals unexpectedly large variation In Raman shift resulting from graphene strain inhomogeneity, which Is shown to be correlated with physical topography by coupling Raman spectroscopy with atomic force microscopy. We show that graphene strain can vary over a distance shorter than 300 nm and may be uniform only over roughly 1 μm. We show that nearly strain-free graphene is possible even in epitaxial graphene.
Momentum, heat, and mass transfer (McGraw-Hill chemical engineering series)
- C.O. Bennett
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Interaction, growth, and ordering of epitaxial graphene on SiC{0 0 0 1} [48] Ferrari AC, Basko DM. Raman spectroscopy as a versatile tool for studying the properties of graphene
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Emtsev KV, Speck F, Seyller T, Ley L. Riley JD, Interaction, growth, and ordering of epitaxial graphene on SiC{0 0 0 1} [48] Ferrari AC, Basko DM. Raman spectroscopy as a versatile tool for studying the properties of graphene. Nat Nanotechnol 2013;8:235–46.