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Charge carrier concentration and offset voltage in quasi-free-standing monolayer chemical vapor deposition graphene on SiC
... This form of carbon material offers a range of properties that enable one to assign it a precise, theory-based work function. These include the two-dimensional character, atomic-flat surface within the (0001) terraces [17,18], elemental purity, lack of substitutional dopants or ...
... Critically, though, and contrary to gold and HOPG, the structural and transport properties of QFS-graphene/4H-SiC(0001) are directly and frequently measurable through spectroscopic ellipsometry [22] and the classical Hall effect [17,18]. Also, a lithographically patterned, wire-bonded QFS-graphene/4H-SiC(0001) structure enables efficient connection to the apparatus signal ground and, thus, charge sinking. ...
... This variation is considered low for graphene technology and possible only because direct epitaxial growth is superior to a polymer-assisted transfer [19]. The vestigial dispersion is a remnant of the fingerprint-like topography of epitaxial graphene, marked with a combination of micrometer-scale terraces and nanometer-high steps [17,18,25]. These, in turn, originate from the fact that although nominally on-axis, the as-purchased SiC wafer is never ideally oriented along the [0001] vector. ...
Kelvin Probe Force Microscopy is a method to assess the contact potential difference between a sample and the probe tip. It remains a relative tool unless a reference standard with a known work function is applied, typically bulk gold or cleaved highly oriented pyrolytic graphite. In this report, we suggest a verifiable, two-dimensional standard in the form of a photolithographically patterned, wire-bonded structure manufactured in the technology of transfer-free p-type hydrogen-intercalated quasi-free-standing epitaxial Chemical Vapor Deposition graphene on semi-insulating high-purity nominally on-axis 4H-SiC(0001). The particular structure has its hole density 𝑝𝑆 = 1.61 × 1013 cm−2 measured through a classical Hall effect, its number of the graphene
layers 𝑁 = 1.74 extracted from the distribution of the ellipsometric angle 𝛹, measured at the angle of incidence AOI = 50◦ and the wavelength 𝜆 = 490 nm, and its work function 𝜙𝐺𝑅 = 4.79 eV postulated by a Density Functional Theory model for the specific 𝑝𝑆 and 𝑁. Following the algorithm, the contact potential difference
between the structure and a silicon tip, verified at 𝛥𝑉𝐺𝑅−Si = 0.64 V, ought to be associated with 𝜙𝐺𝑅 = 4.79 eV and applied as a precise reference value to calculate the work function of an arbitrary material.
... Another alternative is to grow graphene by CVD directly on SiC substrate by graphitization or by epitaxial growth without SiC sublimation or by hydrogen intercalation [19][20][21][22][23][24][25]. Besides the large-scale fabrication, the main advantage of CVD on SiC is to avoid the transfer from copper and improve the electronic properties of the material by using hydrogen intercalation, as reported by Ciuk et al. [26,27]. One of the advantages of the bilayer graphene on SiC substrate is overcoming the lack of a band gap; other possibilities reported in the literature include introducing defects, doping, strain and chemical bounding to the substrate [28][29][30]. ...
... The carbon films were deposited using propane as the carbon precursor. Our method employs high-temperature and low-argon-pressure CVD, creating laminar argon flow dynamics to protect the SiC substrate from silicon sublimation and facilitate propane mass transport, thereby enabling graphene epitaxy, as detailed in references [22,25,26]. The growth process was followed by in situ hydrogen intercalation at 1000 • C in a 900 mbar Ar atmosphere. ...
... Previous work about Raman studies in graphene on SiO 2 /Si show that the shape of the 2D peak is an indicator of the number of layers [37]. For Raman analysis, the nature of the growth (exfoliation, CVD graphitization, CVD without graphitization) and the type of the substrates (SiO 2 or SiC) can affect the peaks' position and the value of the full width at half maximum (FWHM), which enhance the need to have more results about bilayer graphene on SiC [26,38,39]. Here, the full width at the half maximum of the 2D peak is around 59 cm −1 , comparable to the 41-62 cm −1 reported in bilayer graphene on SiC and other substrates [25,37]. ...
Epitaxial bilayer graphene, grown by chemical vapor deposition on SiC substrates without silicon sublimation, is crucial material for graphene field effect transistors (GFETs). Rigorous characterization methods, such as atomic force microscopy and Raman spectroscopy, confirm the exceptional quality of this graphene. Post-nanofabrication, extensive evaluation of DC and high-frequency properties enable the extraction of critical parameters such as the current gain (fmax) and cut-off frequency (ft) of hundred transistors. The Raman spectra analysis provides insights into material property, which correlate with Hall mobilities, carrier densities, contact resistance and sheet resistance and highlights graphene’s intrinsic properties. The GFETs’ performance displays dispersion, as confirmed through the characterization of multiple transistors. Since the Raman analysis shows relatively homogeneous surface, the variation in Hall mobility, carrier densities and contact resistance cross the wafer suggest that the dispersion of GFET transistor’s performance could be related to the process of fabrication. Such insights are especially critical in integrated circuits, where consistent transistor performance is vital due to the presence of circuit elements like inductance, capacitance and coplanar waveguides often distributed across the same wafer.
... Transfer-free p-type hydrogen-intercalated [1][2][3][4][5] quasi-free-standing (QFS) graphene grown on semi-insulating (SI), nominally on-axis, hexagonal SiC(0001) in the process of epitaxial Chemical Vapor Deposition (CVD) in argon flow [6], has been appreciated for its reproducible hole density [7,8], thermal stability of transport properties [9,10], scalable growth technology [11], and verified as the optimum graphene platform for monolithic microwave integrated circuits (MMICs) [12][13][14] and high-temperature Hall effect sensors [9,10,15]. ...
... Although nominally on-axis, the SiC wafer is marked with micrometer-scale terraces separated by fewnanometer-high steps [16,17]. It offers uneven growth conditions favoring additional graphene inclusions at SiC vicinal surfaces [7,8]. A detailed topographic analysis reveals further subtle structural inhomogeneities within the actual (0001) terraces [11,18]. ...
... Fed with direct current I of 1 mA, the sample already proved above-average electrical properties. Its sheet concentration of holes p s = 7.69 × 10 12 cm -2 , in agreement with the theoretically postulated [40][41][42] and experimentally verified [7,8] 6 ≈ 7.5 × 10 12 cm -2 , its room-temperature direct-current Hall-effect-derived hole mobility p = 5019 cm 2 /Vs. Both indicators herald high structural quality of the graphene layer, especially that p > 5000 cm 2 /Vs averaged over the area of 225 mm 2 falls within the 90th percentile of our production stock. ...
In this report, we present transfer-free p-type hydrogen-intercalated quasi-free-standing epitaxial Chemical
Vapor Deposition graphene on 15-mm × 15-mm semi-insulating vanadium-compensated on-axis 6H–SiC(0001),
characterized in that its room-temperature direct-current Hall-effect-derived hole mobility 𝜇p = 5019 cm2/Vs,
and its statistical number of layers (N), as indicated by the relative intensity of the SiC-related Raman-active
longitudinal optical A1 mode at 964 cm−1, equals N = 1.05. The distribution of the ellipsometric angle 𝛹
measured at an angle of incidence of 50◦ and 𝜆 = 490 nm points out to N = 0.97. The close-to-unity value of
N implies that the material under study is a close-to-perfect quasi-free-standing monolayer, which is further
confirmed by High-Resolution Transmission Electron Microscopy. Therefore, its spectroscopic properties, which
include the Si–H peak at 2131 cm−1, the histograms of 𝛹 and 𝛥, and the Raman G and 2D band positions,
widths, and the 2D-to-G band intensity ratios, constitute a valuable reference for this class of materials.
... In its hydrogen-intercalated [7] quasi-free-standing (QFS) form on on-axis SiC(0001) graphene is usually bilayer [8] or monolayer [9] within the SiC(0001) terraces but at the step edges it is decorated with additional sub-micron-wide inclusions located beneath the continuous graphene layers. Often Raman spectroscopy is applied to verify their number judging by width of the 2D peak around 2660-2700 cm −1 , an overtone of the transverse optical phonon near the K point of the Brillouin zone [10,11]. ...
... Often Raman spectroscopy is applied to verify their number judging by width of the 2D peak around 2660-2700 cm −1 , an overtone of the transverse optical phonon near the K point of the Brillouin zone [10,11]. However, in general the 2D peak's width is sensitive to the SiC face choice and the process of hydrogen intercalation (intercalated graphene on SiC(0001) [8,9], non-intercalated graphene on SiC(0001) [12,13], turbostratic graphene on SiC(000-1) [14]), leaving room for discussion. In this report we suggest a reliable protocol for the assessment of the number of graphene layers based on Raman analysis of the underlying SiC substrate. ...
... 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. with the theoretically postulated p s 6H ≈ 7.5×10 12 cm −2 [9], make us conclude on the correctness of the intercalation step. Then, in a series of classical optical lithography-based technological steps, the sample was processed into a batch of Hall effect sensors [4,6] passivated with a 100-nm-thick aluminum oxide layer synthesized from trimethylaluminum (TMA) and deionized water at 770 K in a Picosun R200 Advanced ALD reactor. ...
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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... In this report we investigate structural and electrical properties of epitaxial Chemical Vapor Deposition quasifree-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][12][13][14][15][16][17][18][19][20]. Due to high density of SiC vicinal surfaces the deposited graphene is densely stepped and gains unique characteristics. ...
... 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]. ...
... QFS graphene on semiinsulating (SI) on-axis 4H-SiC(0001) has been appreciated for its strictly-defined spontaneous-polarization-induced ptype conductance and high hole mobility [17,18]. Its only disadvantage has been the detrimental step-edge-induced resistance anisotropy [19][20][21]. ...
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
... In particular, the linear dispersion of π and π * is virtually unaffected by the broken inversion symmetry of the surface (compare Figs. 5g, and j). However, the bands experience a shift correlated with ptype doping, which has been reported in the experiment [49,50] and has been associated with spontaneous polarization [51]. Still, the observed shift is relatively small, and the dispersion of bands π and π * is unaffected, which results in a nearly unchanged Fermi velocity of 1.04 × 10 6 ms − 1 . ...
... Reduced quality of H passivation results in an increase of n-doping of graphene, which is a product of C-Si interaction and the resulting charge transfer (given in Fig. 4g). In the case of GR@H 0. 5 graphene epitaxially grown on SiC(0001) substrate has been reported to facilitate an effective p-doping of the monolayer [49][50][51]. This corresponds to the shift in the position of the Dirac point with respect to the Fermi level shown in Fig. 5j. ...
... Hence, as the full intercalation produces the highest p-doping, its reduction should coincide with decreased hole concentration. Indeed, a similar argument has been raised in experimental studies, where statistical analysis has been employed for a large sample-size of QFS hydrogen-intercalated graphene sheets on SiC(0001) [50,52]. The cases where samples showed reduced hole concentration have been attributed by the authors to incomplete hydrogen intercalation of the H buffer layer [50]. ...
Graphene has attracted huge attention due to its unique electronic properties, however, when supported those are significantly dependent on the interface interactions. One of the methods of decoupling graphene sheets from a substrate is hydrogen intercalation, which has been shown to produce quasi-free-standing (QFS) layers on a SiC (0001) surface. Still, the effects of incomplete H termination of SiC remain mostly unknown. This work in vestigates, employing density functional theory calculations, the impact of partial termination on the structural, and electronic properties of graphene. It is predicted that interfaces with partially damaged H layer or produced under a lower technological standard could still benefit from the intrinsic, however, quantitatively reduced, properties of QFS graphene.
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... When graphene needs to be transferred (in the form of exfoliated flakes or as a sheet grown by chemical vapor deposition on copper) onto a substrate prior to the fabrication of the top gate, the mobility might be reduced even more. The highest reported room-temperature mobility values in top-gated G-FET, utilizing different dielectric materials, including Al 2 O 3 , Y 2 O 3 , HfO 2 , BN, SiC, SiO 2 and polymers, are still below 2.4 Â 10 4 cm 2 /V s. [3][4][5][6][7] In literature, the carrier mobility of graphene has been extracted by different methods, some of which require additional structures in the form of Hall bars and van der Pauw structures. 5,[7][8][9] The disadvantage of these methods is, that the conditions under which the mobility is obtained are not the same as for a transistor structure. ...
... The highest reported room-temperature mobility values in top-gated G-FET, utilizing different dielectric materials, including Al 2 O 3 , Y 2 O 3 , HfO 2 , BN, SiC, SiO 2 and polymers, are still below 2.4 Â 10 4 cm 2 /V s. [3][4][5][6][7] In literature, the carrier mobility of graphene has been extracted by different methods, some of which require additional structures in the form of Hall bars and van der Pauw structures. 5,[7][8][9] The disadvantage of these methods is, that the conditions under which the mobility is obtained are not the same as for a transistor structure. Alternatively, a commonly accepted experimental method for finding mobility values by direct measurements on G-FETs is to fit a simplified expression for the drain resistance to drain resistance-gate voltage characteristics [10][11][12][13][14] R ¼ R c þ ðL=WÞðqlÞ À1 ððn 0 2 0 þ ðCðV g À V Dirac Þ=qÞ 2 Þ À1=2 : ...
... A possibility is the method of transfer length measurement (TML) to extract contact resistance and Hall and van der Pauw measurements to obtain mobility and carrier concentration in a graphene sheet. 5,[7][8][9] It was shown that the contact resistance extracted by TML and the fitting procedure of resistance characteristics does not give the same result for the extracted contact resistance, since the processing steps for the test structures differ from the processing steps of the transistors. 38 Especially, the fabrication of the top gate of the transistor is likely to introduce defects and impurities. ...
A semiempirical model describing the influence of interface states on characteristics of gate capacitance and drain resistance versus gate voltage of top gated graphene field effect transistors is presented. By fitting our model to measurements of capacitance-voltage characteristics and relating the applied gate voltage to the Fermi level position, the interface state density is found. Knowing the interface state density allows us to fit our model to measured drain resistance-gate voltage characteristics. The extracted values of mobility and residual charge carrier concentration are compared with corresponding results from a commonly accepted model which neglects the effect of interface states. The authors show that mobility and residual charge carrier concentration differ significantly, if interface states are neglected. Furthermore, our approach allows us to investigate in detail how uncertainties in material parameters like the Fermi velocity and contact resistance influence the extracted values of interface state density, mobility, and residual charge carrier concentration.
... Over the last decade, technology for the epitaxy of graphene has intensively developed, clarifying two competing trends. These are epitaxy by sublimation of the top-most silicon atoms from the SiC(0001) surface [26][27][28] and epitaxy through Chemical Vapor Deposition (CVD) from a gaseous carbon source under conditions inhibiting sublimation from SiC(0001) [29][30][31][32]. In the context of practical verification in the detection of magnetic fields [33], novel amorphous-Al 2 O 3 -passivated (a-Al 2 O 3 ) structures made of fully-hydrogen-intercalated quasi-freestanding (QFS) CVD graphene on a semi-insulating SiC(0001) substrate (QFS-GR@SiC) have demonstrated remarkable thermal stability assessed from room temperature (RT) up to 770 K [6,7,14,34,35]. ...
... Each of the structures was a 1.4 mm × 1.4 mm fourterminal van der Pauw device [34] featuring an oxygen-plasma-etched, equal-arm, cross-shaped 100-μm × 300-μm QFS graphene mesa [45], electron-beam-deposited Ti/Au (10 nm/110 nm) current feed and voltage readout contacts, and a 100-nm-thick, atomic-layer-deposited, amorphous, non-stoichiometric, oxygen-deficient [46] Al 2 O 3 encapsulation [47,48]. 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]. ...
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
... Directly after the deposition process, graphene RT transport properties were monitored at = 1 mA with a 0.55-T Ecopia HMS-3000 direct-current Hall effect measurement system to verify whether the charge carrier type and sheet concentration was consistent with the theoretically postulated [41][42][43] 6 ≈ 7.5×10 12 cm -2 and 4 ≈ 1.2×10 13 cm -2 and whether the charge carrier mobility was comparable with the one expected for conventional processes on unmodified substrates [44][45][46]. ...
... The post-growth RT Hall effect characterization of the QFS graphene on pre-epitaxially ion-implanted substrates was performed at = 1-mA direct current. The measurements revealed that the material is p-type, with the sheet hole concentration ∼10% higher than predicted by theory for unmodified SiC, yet consistent with the reference samples and the statistical data for conventional processes on unmodified SI 6H-SiC:V and SI 4HP 4H-SiC [44,45]. Further supported by hole mobility of 2000 -3000 cm 2 /Vs, the graphene transport properties implied that the CVD epitaxy followed by in-situ hydrogen intercalation was successful despite the substrate pre-epitaxial modification ( Table 2). ...
High-temperature electrical properties of p-type hydrogen-intercalated quasi-free-standing epitaxial Chemical Vapor Deposition graphene on semi-insulating vanadium-compensated on-axis 6H-SiC(0001) and high-purity on-axis 4H-SiC(0001) originate from the double-carrier system of spontaneous-polarization-induced holes in graphene and thermally activated electrons in the substrate. In this study, we pre-epitaxially modify SiC by implanting hydrogen (H+) and helium (He+) ions with energies ranging from 20 keV to 50 keV to reconstruct its post-epitaxial defect structure and suppress the thermally developed electron channel. Through a combination of dark current measurements and High-Resolution Photo-Induced Transient Spectroscopy between 300 K and 700 K, we monitor the impact of ion bombardment on the transport properties of SiC and reveal activation energies of the individual deep-level defects. We find that the ion implantation has a negligible effect on 6H-SiC. Yet in 4H-SiC, it shifts the Fermi level from ∼600 meV to ∼800 meV below the minimum of the conduction band and reduces the electron concentration by two orders of magnitude. Specifically, it eliminates deep electron traps related to silicon vacancies in the charge state (2-/-) occupying the h and k sites of the 4H-SiC lattice. Finally, we directly implement the protocol of deep-level defect engineering in the technology of amorphous-aluminum-oxide-passivated Hall effect sensors and introduce a mature sensory platform with record-linear current-mode sensitivity of approximately 80 V/AT with -0.03-%/K stability in a broad temperature range between 300 K and 770 K, and likely far beyond 770 K.
https://www.sciencedirect.com/science/article/pii/S2667056923000585
... However, non-hydrogen-intercalated graphene on SiC(0001) is rarely the choice for device-oriented technology. Numerous authors report on the application of hydrogen-intercalated p-type epitaxial graphene on SiC(0001) [14][15][16][17][18], widely appreciated for the high mobility of charge carriers and their well-defined sheet density [19,20]. ...
... At the moment the publication of Ref. [34] was issued all the original 23 sensors had their room-temperature sheet charge carrier concentrations p s at a uniform level of ∼ 7.5× 10 12 cm −2 ( Table 1). The decrease in the hole concentration to approximately 60% of its value predicted by theory of SiC spontaneous polarization acting on unpassivated QFS graphene on 4H-SiC(0001) [19,20,51,52] (p s 4H = 1.2 × 10 13 cm −2 ) was quantitatively consistent with our previous report on sensors fabricated in the same technology but on semi-insulating vanadium-compensated on-axis 6H-SiC(0001) passivated with a 100nm-thick Al 2 O 3 [53]. In that report we had tentatively ascribed the effect of hole concentration suppression to the presence of positively charged oxygen vacancies in the passivation layer. ...
In this report, we introduce a novel method based on low-frequency noise analysis for the assessment of quality and pattern of inhomogeneity in intentionally-aged Hall effect sensors featuring hydrogen-intercalated quasi-free-standing epitaxial Chemical Vapor Deposition graphene mesa on semi-insulating high-purity on-axis 4H-SiC(0001), all passivated with a 100-nm-thick atomic-layer-deposited Al2O3 layer. Inferring from the comparison of the measured noise and one calculated for a homogeneous sensor, we hypothesize about possible unintentional contamination of the sensors’ active regions. Following in-depth structural characterization based on Nomarski interference contrast optical imaging, confocal micro-Raman spectroscopy, high-resolution Transmission Electron Microscopy and Secondary Ion Mass Spectrometry, we find out that the graphene’s quasi-free-standing character and p-type conductance make the Al2O3/graphene interface exceptionally vulnerable to uncontrolled contamination and its unrestrained lateral migration throughout the entire graphene mesa, eventually leading to the blistering of Al2O3. Thus, we prove the method’s suitability for the detection of these contaminants’ presence and location, and infer on its applicability to the investigation of any contamination-induced inhomogeneity in two-dimensional systems. https://authors.elsevier.com/a/1eyP64xMlkIhhc
... However, considering the requirements and the ease of sensor fabrication, we believe that the best candidate is the Si-face of semiinsulating on-axis silicon carbide (SiC). Epitaxial Chemical Vapor Deposition (CVD) of graphene on SiC is already an established technology that allows for effective fabrication and a consistent sensor standard [42,43]. QFS properties of the graphene sheets can be achieved through hydrogen intercalation [44,45] of the carbon buffer layer. ...
... Directly after the growth process, the sample was fed with = 1 mA direct current and characterized with a 0.55-T Ecopia HMS-3000 Hall effect measurement system to prove hole sheet concentration = 1.33 × 10 13 cm −2 , hole mobility = 3294 cm 2 ∕Vs, and sheet resistance = 142 Ω∕sq. The correctness of the hydrogen intercalation step is evidenced by the charge carriers' p-type character and the consistency of their concentration with the theoretically calculated 4 ≈1.2 × 10 13 cm −2 [43]. ...
In this paper, we report on the first experimental study on the impact of neutron radiation on quasi-free-standing (QFS) graphene. For this purpose, we have fabricated hydrogen-intercalated QFS graphene on semiinsulating high-purity 4H-SiC(0001), passivated it with an Al2O3 layer,and exposed it to a fast-neutron fluence of ≈6.6×1017 cm⁻². The results have shown that the graphene sheet is only moderately affected by the neutron radiation with the estimated defect density of ≈4×1010 cm⁻². The low structural damage allowed the Al2O3/graphene/SiC system to maintain its electrical properties and an excellent sensitivity to magnetic fields characteristic of QFS graphene. Consequently, our findings suggest that the system may be a promising platform for magnetic diagnostics in magnetic-confinement fusion reactors. However, the scope of its use should be a subject of further study. In this context, we have explored possible modes of damage and have concluded that the main factor that affects the electrical parameters of the structure is the impact of neutrons on the layer of hydrogen atoms saturating the SiC(0001) surface. We have shown, employing density functional theory (DFT) computations, that damage to the intercalating layer could lower hole concentration in graphene via reduced charge polarization and local coupling on the interface.
... Graphene was epitaxially grown through the chemical vapor deposition (CVD) from hydrocarbons on the silicon face of a semi-insulating on-axis 6H-SiC substrate [38,40] (see also [41][42][43]). First an electrically inactive buffer layer was synthesized and only later in situ turned into a QFS monolayer through hydrogen atom intercalation [41] (see also [44][45][46]). ...
... As was mentioned above, after the MRS measurements the samples were passivated with a silicone encapsulate to prevent uncontrolled environmental influence [43,52]. The magnetotransport measurements were performed in a cryomagnet system equipped with a superconducting coil and generating up to 14 T magnetic field (perpendicular to the graphene layer) [53]. ...
The structure of magnetoresistance curves as a function of magnetic field from 0 to 14 T at temperatures from 0.4 to 6.0 K for macroscopic samples of the quasi-free-standing (QFS) graphene monolayer on SiC substrate, are observed and analyzed, and also the spatial and depth frequency distribution of phonons have been measured using the micro-Raman spectroscopy (MRS). That one enables us to interpret the obtained resonance magnetoresistance curves based on the electron-phonon (e-p) interaction taking into account the actually observed phonon spectrum in researched samples: in the case of a linear e-p interaction the observation of the corresponding peaks on the Rxx(B) curves is difficult because an uninterrupted background is created. While nonlinear MPR with simultaneous G-phonon emission and D-phonon absorption occur in magnetic fields below 5 T against the background of MPR due to linear e-p interaction as well as Shubnikov–de Haas oscillations.
... The material's parameters are altered not only by the surface's chemical composition, but also by its polytype, polar face, crystallographic orientation, and miscut [12,13]. The charge carrier density in graphene fabricated on 4H-SiC, for example, differs distinctly from graphene on 6H-SiC [14]. In GR/Cu systems Cu(111) plane promotes graphene with a lower number of layers compared to other orientations [15]. ...
... The doping, simultaneously, can be an effect of the surface reconstruction, as at the RT two-thirds of the atoms which were in the direct contact with the graphene, moved deeper modifying the electronic states of graphene. This mechanism can be described as an electrostatic doping, probably workfunction-induced, contrastingly to spontaneous polarization typical for various SiC polytypes [14,48,49]. This hypothesis, to some extent, could also explain the difference between our work and Kiraly's since the surface of germanium substrates may differ. ...
In this paper we study graphene synthesized on undoped (001), (110), and (111) Ge substrates, and show how the surface orientation and reconstruction affect its morphological and structural properties. The article presents the first attempt to explain the impact of germanium surface reconstruction on the shape and the density of graphene nuclei as well as on the material's stress and doping levels. Our findings suggest that graphene obtained on Ge(001) is the most uniform, with low doping (1.5 × 10¹² cm⁻²) and low strain level (−0.1%). Graphene on Ge(110) appears to be highly compressed (−0.5%) while graphene on Ge(111) exhibits doping reaching 5 × 10¹³ cm⁻². These results help to better understand the dynamics of graphene growth on germanium and indicate Ge(001), in terms of its structural properties, as the most promising orientation for the future CMOS applications.
... [5] This was attributed to a reduced interaction between carriers in QFMLG and substrate phonons. However, the mobility of QFMLG (~6600 cm 2 V -1 s -1 ) [6] is lower than in free-standing graphene, thus indicating the existence of remaining carrier-scattering centers. ...
Si dangling bonds without H termination at the interface of quasi-free standing monolayer graphene (QFMLG) are known scattering centers that can severely affect carrier mobility. In this report, we study the atomic and electronic structure of Si dangling bonds in QFMLG using low-temperature scanning tunneling microscopy/spectroscopy (STM/STS), atomic force microscopy (AFM), and density functional theory (DFT) calculations. Two types of defects with different contrast were observed on a flat terrace by STM and AFM. Their STM contrast varies with bias voltage. In STS, they showed characteristic peaks at different energies, 1.1 and 1.4 eV. Comparison with DFT calculations indicates that they correspond to clusters of 3 and 4 Si dangling bonds, respectively. The relevance of these results for the optimization of graphene synthesis is discussed.
... The Hall effect sensor was produced in the graphene-on-SiC technology (GET ® [14]). The graphene was transfer-free, p-type, in-situ hydrogen-intercalated [15], QFS, epitaxial CVD [16], [17], and statistically ∼1.5-layer, within the understanding of the relative intensity of the SiC-related Raman-active A 1 mode at 964 cm -1 [18], [19]. It was grown at 1873 K in an Aixtron VP508 reactor on a 20-mm × 20-mm sample diced from a 4-in, 500-µm-thick, semi-insulating, high-purity, nominally on-axis 4H-SiC(0001) wafer (Wolfspeed Inc.), using thermally decomposed propane as the carbon-rich gas [20]. ...
In this letter, we demonstrate a Hall effect sensor in the technology of amorphous-Al2O3-passivated transfer-free
p-type hydrogen-intercalated quasi-free-standing epitaxial Chemical Vapor Deposition graphene on semi-insulating high-purity on-axis 4H-SiC(0001), pre-epitaxially modified with 5-
keV hydrogen (H+) ions. The sensor operates between 305 K and 770 K, with a current-mode sensitivity of ∼75 V/AT and thermal stability below 0.15 %/K (⩽ 0.03 %/K in a narrower range between 305 K and 700 K). It is a promising two-dimensional platform for high-temperature magnetic diagnostics and plasma control systems for modern tokamak fusion reactors.
... QFS graphene [26][27][28][29] necessary for the graphene-based HTHS was grown on semiinsulating high-purity on-axis 4H-SiC(0001) (Cree Inc.) in a hot-wall Aixtron VP508 reactor using the epitaxial Chemical Vapor Deposition method [30], with thermally decomposed propane as a carbon source and in situ hydrogen atom intercalation [31]. The substrate was processed into a 1.6 mm × 1.6 mm Hall effect structure featuring a cross-shaped [32] 100 µm × 300 µm graphene mesa and four Ti/Au (10 nm / 60 nm) ohmic contacts, all passivated with a 100 nm-thick aluminum oxide (Al 2 O 3 ) layer synthesized from trimethylaluminum (TMA) and deionized water at 670 K in a Picosun R200 Advanced Atomic Layer Deposition (ALD) reactor [33,34]. ...
The ability to precisely measure magnetic fields under extreme operating conditions is becoming increasingly important as a result of the advent of modern diagnostics for future magnetic-confinement fusion devices. These conditions are recognized as strong neutron radiation and high temperatures (up to 350 °C). We report on the first experimental comparison of the impact of neutron radiation on graphene and indium antimonide thin films. For this purpose, a 2D-material-based structure was fabricated in the form of hydrogen-intercalated quasi-free-standing graphene on semi-insulating high-purity on-axis 4H-SiC(0001), passivated with an Al2O3 layer. InSb-based thin films, donor doped to varying degrees, were deposited on a monocrystalline gallium arsenide or a polycrystalline ceramic substrate. The thin films were covered with a SiO2 insulating layer. All samples were exposed to a fast-neutron fluence of ≈7×1017 cm⁻². The results have shown that the graphene sheet is only moderately affected by neutron radiation compared to the InSb-based structures. The low structural damage allowed the graphene/SiC system to retain its electrical properties and excellent sensitivity to magnetic fields. However, InSb-based structures proved to have significantly more post-irradiation self-healing capabilities when subject to proper temperature treatment. This property has been tested depending on the doping level and type of the substrate.
... Without the need for further transfer, this process can prepare large-scale and high-quality wafer-level graphene. Its electrical properties are similar to those of single-layer graphene produced by mechanical exfoliation, with mobility of up to 6600 cm /V s [138], which can be directly applied to the development of electronic devices. ...
Terahertz waves are expected to be used in next-generation communications, detection, and other fields due to their unique characteristics. As a basic part of the terahertz application system, the terahertz detector plays a key role in terahertz technology. Due to the two-dimensional structure, graphene has unique characteristics features, such as exceptionally high electron mobility, zero band-gap, and frequency-independent spectral absorption, particularly in the terahertz region, making it a suitable material for terahertz detectors. In this review, the recent progress of graphene terahertz detectors related to photovoltaic effect (PV), photothermoelectric effect (PTE), bolometric effect, and plasma wave resonance are introduced and discussed.
... After the MRS measurements the samples were passivated with a silicone encapsulate to prevent uncontrolled environmental influence [38,47]. The magneto-transport measurements were performed in a cryo-magnet system equipped with a superconducting coil and generating up to 14 T magnetic field (perpendicular to the graphene layer) [48]. ...
... While the temperature dependence of mobility in the epitaxial graphene (S1) is due to longitudinal acoustic phonons in graphene and SiC, the dominant scattering in QFMLG is attributed to Coulomb scattering induced by charged impurities. [229,318] Although higher values were already reported for micrometer-sized QFMLG confined Hall bars on the single terrace [108,119,229,231,319], this is still a remarkable result concerning the low concentration of hydrogen and the produced large-size sample. In contrast to QFMLG, the measurements on QFBLG (after intercalating a sample like S1) revealed noticeable higher mobility values of ~2700 cm 2 /Vs (RT) and ~3350 cm 2 /Vs (2.2 K). ...
(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. ...
... While the temperature dependence of mobility in the epitaxial graphene (S1) is due to longitudinal acoustic phonons in graphene and SiC, the dominant scattering in QFMLG is attributed to Coulomb scattering induced by charged impurities. [229,318] Although higher values were already reported for micrometer-sized QFMLG confined Hall bars on the single terrace [108,119,229,231,319], this is still a remarkable result concerning the low concentration of hydrogen and the produced large-size sample. In contrast to QFMLG, the measurements on QFBLG (after intercalating a sample like S1) revealed noticeable higher mobility values of ~2700 cm 2 /Vs (RT) and ~3350 cm 2 /Vs (2.2 K). ...
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.
... The epitaxial carbon films were deposited using propane gas as carbon precursor. The method used is based on high temperature and low argon pressure CVD by creating dynamic conditions of the laminar flow of argon which protect the SiC substrate against Si sublimation and enable mass transport of propane to the SiC surface, thus realizing graphene epitaxy as reported in [17][18][19]. ...
This paper presents the first graphene radiofrequency (RF) monolithic integrated balun circuit. It is composed of four integrated graphene field effect transistors (GFETs). This innovative active balun concept takes advantage of the GFET ambipolar behavior. It is realized using an advanced silicon carbide (SiC) based bilayer graphene FET technology having RF performances of about 20 GHz. Balun circuit measurement demonstrates its high frequency capability. An upper limit of 6 GHz has been achieved when considering a phase difference lower than 10° and a magnitude of amplitude imbalance less than 0.5 dB. Hence, this circuit topology shows excellent performance with large broadband performance and a functionality of up to one-third of the transit frequency of the transistor.
... The analysis of the three basic bands, namely D (1350 cm 1 ), G (1580 cm 1 ) and 2D (2680 cm 1 ) [2], their energy, intensity and full width at half maximum (FWHM) reveals the number of graphene layers, strain and free carriers concentration [3,4]. In-situ hydrogen intercalation turns the electrically inactive buffer layer into quasi-free-standing (QFS) monolayer with strictly defined p-type doping and hole mobility as high as 6600 cm 2 /V [5]. However, Raman signal from hydrogen is very weak and hardly visible in the spectrum. ...
... However, between 573 and 770 K, it drops to −0.06%/K only, as opposed to the −0.26%/K for 6H-SiC (0001) [16]. We had previously shown that the characteristic and inherent morphology of SiC surface marked with terraces and step edges introduces significant anisotropy into graphene's sheet resistance [22] and promotes an anisotropic offset voltage [23], [24]. This voltage occurs between the output electrodes and adds to the Hall voltage, thus constituting an operating point of the sensor in zero magnetic field. ...
In this report, we demonstrate a novel high-temperature Hall effect sensor that is based on quasi-free-standing monolayer graphene epitaxially grown on high-purity semiinsulating (SI) on-axis 4H-SiC(0001) substrate in a chemical vapor deposition process. To ensure statistical perspective, characteristics of 23 elements are determined as a function of temperature ranging from 300 to 770 K. Passivated with a 100-nm-thick atomic-layer-deposited aluminum oxide, the sensor offers current-mode sensitivity of 80 V/AT with thermal stability of -0.02%/K within the range between 300 and 573 K, and -0.06%/K between 573 and 770 K. The sensor's room-temperature output voltage is monitored in the magnetic field from -300 to +300 mT and its offset voltage at 0 T is assessed. Its high-temperature electrical properties are explained through a double-carrier transport involving spontaneous-polarization-induced holes in the graphene layer and thermally activated electrons emitted from a deep acceptor level related to silicon vacancy VSi^1-/2- occupying the k site of the 4H-SiC lattice. The sensor is compared with a previously reported one on vanadium-compensated SI on-axis 6H-SiC(0001). The new sensor's applicability to magnetic field detection at high temperatures is verified.
... While the temperature dependence of mobility in the epitaxial graphene (G 0 ) is due to longitudinal acoustic phonons in graphene and SiC, the dominant scattering in QFMLG is attributed to Coulomb scattering induced by charged impurities. 8,46 Although higher values were already reported for micrometer-sized QFMLG confined Hall bars on the single terrace, 2, 5,8,16,47 this is still a remarkable result concerning the low concentration of hydrogen and the produced large sample. In contrast to QFMLG, the measurements on QFBLG (after intercalating G 0 ) revealed noticeable higher mobility values of 2696 cm 2 /(V s) (room temperature, RT) and 3352 cm 2 /(V s) (2.2 K). ...
In this study, we first show that the argon flow during epitaxial graphene growth is an important parameter to control the quality of the buffer and the graphene layer. Atomic force microscopy (AFM) and low-energy electron diffraction (LEED) measurements reveal that the decomposition of the SiC substrate strongly depends on the Ar mass flow rate while pressure and temperature are kept constant. Our data are interpreted by a model based on the competition of the SiC decomposition rate, controlled by the Ar flow, with a uniform graphene buffer layer formation under the equilibrium process at the SiC surface. The proper choice of a set of growth parameters allows the growth of defect-free, ultra-smooth and coherent graphene-free buffer layer and bilayer-free monolayer graphene sheets which can be transformed into large-area high-quality quasi-freestanding monolayer and bilayer graphene by hydrogen intercalation. AFM, scanning tunneling microscopy, Raman spectroscopy and electronic transport measurements underline the excellent homogeneity of the resulting quasi-freestanding layers. Electronic transport measurements in four-point probe configuration reveal a homogeneous low resistance anisotropy on both μm- and mm scales.
... The laminar flow of argon inhibited silicon sublimation and enabled mass transport of hydrocarbon molecules to the SiC surface, thus realizing graphene epitaxy. The growth process was followed by in situ hydrogen intercalation at around 1000 ˚C in 900 mbar argon atmosphere [10]- [12]. Figure 1 represents atomic force microscopy (AFM) image of the surface at room temperature. ...
... So far the fabrication of free-standing graphene has been considered only on bulk SiC [31][32][33][34][38][39][40][41][42][43]; due to the technological importance of SiC/Si we explored the production of free-standing graphene by hydrogen intercalation on 3C-SiC thin films on Si(111). We combine, for the first time, synchrotron radiation near-edge X-ray absorption fine structure (NEXAFS) with core-level photoelectron spectroscopy (PES) and low energy electron diffraction (LEED), with the aim of obtaining a clear picture of the intercalation process on graphene grown on 3C-SiC/Si(111). ...
Growing graphene on SiC thin films on Si is a cheaper alternative to the growth on bulk SiC, and for this reason it has been recently intensively investigated. Here we study the effect of hydrogen intercalation on epitaxial graphene obtained by high temperature annealing on 3C-SiC/Si(111) in ultra-high vacuum (UHV). By using a combination of core-level photoelectron spectroscopy (PES), low energy electron diffraction (LEED), and near-edge X-ray absorption fine structure (NEXAFS) we find that hydrogen saturates the Si atoms at the topmost layer of the substrate, leading to free-standing graphene on 3C-SiC/Si(111). The intercalated hydrogen fully desorbs after heating the sample at 850 °C and the buffer layer appears again, similar to what has been reported for bulk SiC. However, the NEXAFS analysis sheds new light on the effect of hydrogen intercalation, showing an improvement of graphene's flatness after annealing in atomic H at 600˚C. These results provide new insight into free-standing graphene fabrication on SiC/Si thin films.
... [5] This was attributed to a reduced interaction between carriers in QFMLG and substrate phonons. However, the mobility of QFMLG (~6600 cm 2 V -1 s -1 ) [6] is lower than in free-standing graphene, thus indicating the existence of remaining carrier-scattering centers. ...
Si dangling bonds without H termination at the interface of quasi-free standing monolayer graphene (QFMLG) are known scattering centers that can severely affect carrier mobility. In this report, we study the atomic and electronic structure of Si dangling bonds in QFMLG using low-temperature scanning tunneling microscopy/spectroscopy (STM/STS), atomic force microscopy (AFM), and density functional theory (DFT) calculations. Two types of defects with different contrast were observed on a flat terrace by STM and AFM. Their STM contrast varies with bias voltage. In STS, they showed characteristic peaks at different energies, 1.1 and 1.4 eV. Comparison with DFT calculations indicates that they correspond to clusters of 3 and 4 Si dangling bonds, respectively. The relevance of these results for the optimization of graphene synthesis is discussed.
... 1)-layer QFSG (n = 0, 1, 2, 3). Ciuk et al. [13] presented a statistical perspective on the transport properties of QFSMG synthesized on Si-terminated 4H-SiC (0001) and 6H-SiC (0001) through epitaxial Chemical Vapor Deposition. Tanabe et al. [14] clarified the optimal temperature for obtaining high-mobility QFSMG using top-gated Hall bars with QFSMG as their channels and also elucidated the effects of annealing temperature on the transport properties of QFSMG. ...
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.
... The resultant characteristic and inherent morphology of the SiC surface defined by terraces and step edges introduces significant anisotropy into graphene's resistance [10][11][12] and fosters the presence of an anisotropic offset voltage. 8,13 Recently, the authors mastered the control of step bunching in the range between 2 nm and 10 nm; yet, the conclusions remain the subject of discussion. ...
In this report, we demonstrate a complete Hall effect element that is based on quasi-free-standing monolayer
graphene synthesized on a semi-insulating on-axis Si-terminated 6H-SiC substrate in an epitaxial
Chemical Vapor Deposition process. The device offers the current-mode sensitivity of 87 V/AT and low excess noise (Hooge's parameter αH < 2 × 10−3) enabling room-temperature magnetic resolution of 650 nT/Hz0.5 at 10 Hz, 95 nT/Hz0.5 at 1 kHz, and 14 nT/Hz0.5 at 100 kHz at the total active area of 0.1275 mm2. The element is passivated with a silicone encapsulant to ensure its electrical stability and environmental resistance. Its processing cycle is suitable for large-scale commercial production and it is available in large quantities through a single growth run on an up to 4-in SiC wafer.
In this report, we demonstrate a method for the enhancement of Raman active modes of hydrogen-intercalated quasi-free-standing epitaxial chemical vapor deposition graphene and the underlying semi-insulating 6H–SiC(0001) substrate through constructive signal interference within atomic-layer-deposited amorphous Al2O3 passivation. We find that an optimum Al2O3 thickness of 85 nm for the graphene 2D mode and one of 82 nm for the SiC longitudinal optical A1 mode at 964 cm–1 enable a 60% increase in their spectra intensities. We demonstrate the method’s efficiency in Raman-based determination of the dielectric thickness and high-resolution topographic imaging of a graphene surface.
2D material of graphene has inspired huge interest in fabricating of solid state gas sensors. In this work, epitaxial graphene, quasi-free-standing graphene, and CVD epitaxial graphene samples on SiC substrates are used to fabricate gas sensors. Defects are introduced into graphene using SF 6 plasma treatment to improve the performance of the gas sensors. The epitaxial graphene shows high sensitivity to NO 2 with response of 105.1% to 4 ppm NO 2 and detection limit of 1 ppb. The higher sensitivity of epitaxial graphene compared to quasi-free-standing graphene, and CVD epitaxial graphene was found to be related to the different doping types of the samples.
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.
The development of two-dimensional graphene layers has recently attracted considerable attention because of its tremendous application in various research fields. Semi-metal materials have received significant attention because of their excellent biocom-patibility as well as distinct physical, chemical, and mechanical properties. Taking into account the technical importance of graphene in various fields, such as complementary metal-oxide-semiconductor technology, energy-harvesting and -storage devices, biotechnology, electronics, light-emitting diodes, and wearable and flexible applications, it is considered to be a multifunctional component. In this regard, material scientists and researchers have primarily focused on two typical problems: i) direct growth and ii) low-temperature growth of graphene. In this review, we have considered only cold growth of graphene. The review is divided into five sections. Sections 1 and 2 explain the typical characteristics of graphene with a short history and the growth methods adopted, respectively. Graphene's direct growth at low temperatures on a required substrate with a well-established application is then precisely discussed in Sections 3 and 4. Finally, a summary of the review along with future challenges is described in Section 5.
Graphene has been widely used in many fields due to its unique excellent mechanical, optical, thermal and electrical properties. A simple approach for reducing graphene oxide (GO) with Tea polyphenols (TP) (TRG) was developed by ultrasonic stripping-chemical reduction method. The reduction products of TRG were obtained, and Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy and X-ray-diffraction were introduced to prove the elimination of oxygen-containing groups from GO. It was found that when the weight of TP was 0.225 g, the reduction degree of GO was the highest. Besides, the thermal gravimetric analysis results showed that there was a close relationship between the reduction degree of GO and thermal stability of TRG.
The intercalation of various atomic species, such as hydrogen, to the interface between epitaxial graphene (EG) and its SiC substrate is known to significantly influence the electronic properties of the graphene overlayers. Here, we use high-resolution X-ray reflectivity to investigate the structural consequences of the hydrogen intercalation process used in the formation of quasi-free-standing (QFS) EG/SiC(0001). We confirm that the interfacial layer is converted to a layer structurally indistinguishable from that of the overlying graphene layers. This newly formed graphene layer becomes decoupled from the SiC substrate and, along with the other graphene layers within the film, is vertically displaced by similar to 2.1 angstrom. The number of total carbon layers is conserved during the process, and we observe no other structural changes such as interlayer intercalation or expansion of the graphene d-spacing. These results clarify the under-determined structure of hydrogen intercalated QFS-EG/SiC(0001) and provide a precise model to inform further fundamental and practical understanding of the system. (C) 2014 AIP Publishing LLC.
We report on top-gated field effect devices built from quasi-freestanding monolayer graphene (QFMLG) on 6H–SiC(0001) in combination with a silicon nitride (SiN) gate dielectric. SiN was grown by plasma enhanced chemical vapour deposition. The composition of the dielectric was investigated by x-ray photoelectron spectroscopy (XPS). Spectroscopic and electrical characterization of the graphene layers were done by XPS, Raman spectroscopy and Hall effect measurements before and after SiN deposition. In contrast to previous reports on SiN/graphene, we observe that our dielectric layer induces strong n-type doping. With such a gate insulator, the neutrality level of the QFMLG could be accessed by an appropriate gate voltage.
The doping of quasi-freestanding graphene (QFG) on H-terminated, Si-face 6H-, 4H-, and 3C-SiC is studied by angle-resolved photoelectron spectroscopy close to the Dirac point. Using semi-insulating as well as n-type doped substrates we shed light on the contributions to the charge carrier density in QFG caused by (i) the spontaneous polarization of the substrate, and (ii) the band alignment between the substrate and the graphene layer. In this way we provide quantitative support for the previously suggested model of polarization doping of graphene on SiC (Ristein et al 2012 Phys. Rev. Lett. 108 [http://dx.doi.org/10.1103/PhysRevLett.108.246104] 246104 ).
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.
Raman spectroscopy and scanning tunneling microscopy/spectroscopy measurements are performed to determine the atomic structure and electronic properties of H-intercalated graphene/SiC(0001) obtained by annealing the as-grown epitaxial graphene in hydrogen atmosphere. While the as-grown graphene is found to be n-type with the Dirac point (E-D) at 450 and 350 meV below Fermi level for the 1st and 2nd layer, the H-intercalated graphene is p-type with E-D at 320 and 200 meV above. In addition, ripples are observed in the now quasi-free standing graphene decoupled from the SiC substrate. This causes fluctuations in the Dirac point that directly follow the undulations of the ripples, resulting in electron and hole puddles in the H-intercalated graphene/SiC(0001). (C) 2014 AIP Publishing LLC.
We investigate the magnetotransport properties of epitaxial graphene on
SiC(0001), which exhibits a stepped surface with regular terraces and
step edges. We observe an anisotropic behavior in the magnetoresistance
measured at high magnetic fields for narrow Hall bars patterned on
stepped surfaces. If the Hall bar is aligned parallel to the terraces,
the conventional quantum Hall effect behavior is observed. In contrast,
a predominantly positive magnetoresistance arises if the current crosses
many surface steps. The results can be explained by a model which takes
into account inter-Landau-level scattering, which is enhanced in the
step edge surface region and may enable electron backscattering from one
channel to another on the opposite side, resulting in a positive
magnetoresistance.
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 studied the in-plane magnetoresistance R(B,T) anisotropy in epitaxial multilayer graphene films grown on the Si face of a 6H-SiC substrate that originates from steplike morphology of the SiC substrate. To enhance the anisotropy, a combination of argon atmosphere with graphite capping was used during the film growth. The obtained micro-Raman spectra demonstrated a complex multilayer graphene structure with the smaller film thickness on terraces as compared to the step edges. Several Hall bars with different current/steps mutual orientations have been measured. A clear anisotropy in the magnetoresistance has been observed, and attributed to variations in electron mobility governed by the steplike structure. Our data also revealed that (i) the graphene-layer stacking is mostly Bernal type, (ii) the carriers are massive, and (iii) the carriers are confined to the first 2–4 graphene layers following the buffer layer.
It has been known for almost 25 years that high temperature treatment of polar faces of SiC crystals lead to the graphitisation of the surface as a consequence of Si preferential sublimation. The group of Walt de Heer in Atlanta has proposed to use this procedure to obtain macroscopic multilayer graphene samples. This has lead to a renewal of interest for the study of the graphitic layers grown on the SiC surfaces. Due to the polar nature of the material, the 6H(4H)-SiC substrate present two non-equivalent surfaces: the (0001) is known as the Si face, and the (000-1) one is the C face. It turns out that the structure and the properties of the multilayer graphene samples obtained on those two surfaces are rather different. In particular the multilayer graphene samples grown on the C-terminated face presents electronic properties that are rather similar to those of monolayer graphene with very high mobility. This is shown by transport and optical experiments under high magnetic field. This surprising result is explained by the rotational stacking sequence, observed experimentally. which leads to an electronic decoupling as demonstrated by ab initio band structure calculations.
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.
We explain the robust p-type doping observed for quasi-free standing graphene
on hexagonal silicon carbide by the spontaneous polarization of the substrate.
This mechanism is based on a bulk property of SiC, unavoidable for any
hexagonal polytype of the material and independent of any details of the
interface formation. We show that sign and magnitude of the polarization are in
perfect agreement with the doping level observed in the graphene layer. With
this mechanism, models based on hypothetical acceptor-type defects as they are
discussed so far are obsolete. The n-type doping of epitaxial graphene is
explained conventionally by donor-like states associated with the buffer layer
and its interface to the substrate which overcompensate the polarization
doping.
After the pioneering investigations into graphene-based electronics at Georgia Tech, great strides have been made developing epitaxial graphene on silicon carbide (EG) as a new electronic material. EG has not only demonstrated its potential for large scale applications, it also has become an important material for fundamental two-dimensional electron gas physics. It was long known that graphene mono and multilayers grow on SiC crystals at high temperatures in ultrahigh vacuum. At these temperatures, silicon sublimes from the surface and the carbon rich surface layer transforms to graphene. However the quality of the graphene produced in ultrahigh vacuum is poor due to the high sublimation rates at relatively low temperatures. The Georgia Tech team developed growth methods involving encapsulating the SiC crystals in graphite enclosures, thereby sequestering the evaporated silicon and bringing growth process closer to equilibrium. In this confinement controlled sublimation (CCS) process, very high-quality graphene is grown on both polar faces of the SiC crystals. Since 2003, over 50 publications used CCS grown graphene, where it is known as the "furnace grown" graphene. Graphene multilayers grown on the carbon-terminated face of SiC, using the CCS method, were shown to consist of decoupled high mobility graphene layers. The CCS method is now applied on structured silicon carbide surfaces to produce high mobility nano-patterned graphene structures thereby demonstrating that EG is a viable contender for next-generation electronics. Here we present for the first time the CCS method that outperforms other epitaxial graphene production methods.
The recent discovery of graphene has led to many advances in two-dimensional physics and devices. The graphene devices fabricated so far have relied on SiO(2) back gating. Electrochemical top gating is widely used for polymer transistors, and has also been successfully applied to carbon nanotubes. Here we demonstrate a top-gated graphene transistor that is able to reach doping levels of up to 5x1013 cm-2, which is much higher than those previously reported. Such high doping levels are possible because the nanometre-thick Debye layer in the solid polymer electrolyte gate provides a much higher gate capacitance than the commonly used SiO(2) back gate, which is usually about 300 nm thick. In situ Raman measurements monitor the doping. The G peak stiffens and sharpens for both electron and hole doping, but the 2D peak shows a different response to holes and electrons. The ratio of the intensities of the G and 2D peaks shows a strong dependence on doping, making it a sensitive parameter to monitor the doping.
We uncover the constitutive relation of graphene and probe the physics of its optical phonons by studying its Raman spectrum as a function of uniaxial strain. We find that the doubly degenerate E[subscript 2g] optical mode splits in two components: one polarized along the strain and the other perpendicular. This splits the G peak into two bands, which we call G+ and G−, by analogy with the effect of curvature on the nanotube G peak. Both peaks redshift with increasing strain and their splitting increases, in excellent agreement with first-principles calculations. Their relative intensities are found to depend on light polarization, which provides a useful tool to probe the graphene crystallographic orientation with respect to the strain. The 2D and 2D′ bands also redshift but do not split for small strains. We study the Grüneisen parameters for the phonons responsible for the G, D, and D′ peaks. These can be used to measure the amount of uniaxial or biaxial strain, providing a fundamental tool for nanoelectronics, where strain monitoring is of paramount importance University of Palermo Sultan Qaboos University MITRE Interconnect Focus Center European Research Council Royal Society
We calculate the spontaneous polarization of wurtzite-structure SiC, using the recently proposed supercell technique of Posternak, Baldereschi, Catellani, and Resta [Phys. Rev. Lett. 64, 1777 (1990)] and a first-principles pseudopotential approach. The macroscopic polarization is found to be 4.32×10-2 C/m2, and is mainly due to the electronic-charge-density redistribution rather than a relaxation of the positions of the ions. The valence-band offset at the interface between the wurtzite and cubic forms is determined using the same supercell calculations, giving 0.13 eV, with the wurtzite-structure SiC valence-band edge being higher in energy. Our calculations also predict the crystal-field splitting of the top of the valence band of wurtzite-structure SiC to be 0.12 eV, and give some insight into the nature of the dipole created by a single stacking fault. Finally, the effect of the macroscopic electric fields on the relative stability of SiC polytypes is discussed and found to be negligible.
We demonstrate electrochemical top gating of graphene by using a solid polymer electrolyte. This allows to reach much higher electron and hole doping than standard back gating. In-situ Raman measurements monitor the doping. The G peak stiffens and sharpens for both electron and hole doping, while the 2D peak shows a different response to holes and electrons. Its position increases for hole doping, while it softens for high electron doping. The variation of G peak position is a signature of the non-adiabatic Kohn anomaly at . On the other hand, for visible excitation, the variation of the 2D peak position is ruled by charge transfer. The intensity ratio of G and 2D peaks shows a strong dependence on doping, making it a sensitive parameter to monitor charges.
We uncover the constitutive relation of graphene and probe the physics of its optical phonons by studying its Raman spectrum as a function of uniaxial strain. We find that the doubly degenerate E(2g) optical mode splits in two components: one polarized along the strain and the other perpendicular. This splits the G peak into two bands, which we call G(+) and G(-), by analogy with the effect of curvature on the nanotube G peak. Both peaks redshift with increasing strain and their splitting increases, in excellent agreement with first-principles calculations. Their relative intensities are found to depend on light polarization, which provides a useful tool to probe the graphene crystallographic orientation with respect to the strain. The 2D and 2D(') bands also redshift but do not split for small strains. We study the Gruneisen parameters for the phonons responsible for the G, D, and D(') peaks. These can be used to measure the amount of uniaxial or biaxial strain, providing a fundamental tool for nanoelectronics, where strain monitoring is of paramount importance.
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.
Quasi-free standing graphene (QFG) obtained from the intercalation of a
hydrogen layer between a SiC surface and the graphene is recognized as an
excellent candidate for the development of graphene based technology. In
addition, the recent proposal of a direct equivalence between the p-type
doping typically found for these systems and the spontaneous polarization (SP)
associated to the particular SiC polytype, opens the possibility of tuning the
number of carriers in the Dirac cones without the need of external gate
voltages. However, first principles calculations which could confirm at the
atomic scale the effect of the SP are lacking mainly due to the difficulty of
combining a bulk property such as the SP with the surface confined graphene
doping. Here we develop an approach based on standard density functional theory
(DFT) slab calculations in order to quantify the effect of the SP on the QFG
doping level. First, we present a novel and accurate scheme to estimate the SPs
by exploiting the dependence of the slab's dipole moment with its thickness.
Next, and in order to circumvent the DFT shortcomings associated to polar slab
geometries, a double gold layer is attached at the C-terminated bottom of the
slab which introduces a metal induced gap state that pins the chemical
potential inside the gap thus allowing a meaningful comparison of the QFG
dopings among different polytypes. Furthermore, the slab dipole can be removed
after adjusting the Au-Au interlayer distances. Our results confirm that the SP
does indeed induce a substantial p-doping of the Dirac cones which can be as
large as a few hundreds of meV depending on the hexagonality of the polytype.
The evolution of the doping with the slab thickness or, equivalently, with the
number of stacking defects, reveals that at least ten SiC bilayers are required
to fully develop the SP and recover the macroscopic regime.
Ab initio density functional theory simulations were used to investigate the influence of hydrogen intercalation on the electronic properties of single and multiple graphene layers deposited on the SiC(0001) surface (Si-face). It is shown that single carbon layer, known as a buffer layer, covalently bound to the SiC substrate, is liberated after hydrogen intercalation, showing characteristic Dirac cones in the band structure. This is in agreement with the results of angle resolved photoelectron spectroscopy measurements of hydrogen intercalation of SiC-graphene samples. In contrast to that hydrogen intercalation has limited impact on the multiple sheet graphene, deposited on Si-terminated SiC surface. The covalently bound buffer layer is liberated attaining its graphene like structure and dispersion relation typical for multilayer graphene. Nevertheless, before and after intercalation, the four layer graphene preserved the following dispersion relations in the vicinity of K point: linear for (AAAA) stacking, direct parabolic for Bernal (ABAB) stacking and “wizard hat” parabolic for rhombohedral (ABCA) stacking.
Intercalation of H between the SiC surface and graphene is known to largely reduce the graphene–substrate interaction thus leaving a so called quasi-free-standing graphene monolayer (QFG) which preserves most of the properties of free-standing graphene (FG). Here, we investigate via large-scale density functional theory (DFT) based calculations point defects in FG and QFG in the form of single vacancies passivated by additional H atoms. For QFG our results reveal that the intercalated H layer interacts strongly with the defects attracting unsaturated C atoms but repelling the H-passivated ones thus leading to large reconstructions which, in turn, may induce drastic changes on the electronic and magnetic properties when compared against FG. We conclude that QFG with defect concentrations larger than 0.3% cannot be regarded in general as quasi-free-standing any more.
We describe a mechanism in which low-energy phonons of the SiC/graphene
interface modify the separation between the buffer layer and the
graphene overlayer, resulting in a deformation potential and hence
charge carrier scattering. We determine this deformation potential ab
initio, and then calculate transport properties within the Boltzmann
formalism. Our results show (i) a remarkable decrease in the resistivity
(ρ) near the Dirac point with increasing temperature (T), (ii) a
linear increase of ρ with T away from the Dirac point, and (iii)
good agreement with experiment for low temperature to room temperature
change in resistivity for epitaxial graphene on the SiC Si face.
We report on first-principles density functional calculations of nonpolar low-index surfaces of hexagonal silicon carbide. We provide an accurate analysis of the macroscopic bulk spontaneous polarization as a function of the hexagonality of the compound, and we describe in detail the electronic and structural properties of the relaxed surfaces. We revise the methodology to achieve a detailed description of the surface polarization effects. Our results on low-index surfaces reveal a strong in-plane polar contribution, opposing the spontaneous polarization field present in hexagonal polytypes. This in-plane surface polarization component has not been considered before, although it is of significant impact in adsorption experiments, affecting functionalization and growth processes, as well as the electronic properties of confined, low-dimensional systems.
We report on the interface between graphene and 4H-SiC0001 as investigated by scanning tunneling microscopy STM and low energy electron diffraction LEED. It is characterized by the so-called 6 3 6 3R30° reconstruction, whose structural properties are still controversially discussed but at the same time are crucial for the controlled growth of homogeneous high-quality large-terrace graphene surfaces. We discuss the role of three observed phases with periodicities 6 3 6 3R30°, 6 6, and 5 5. Their LEED inten-sity levels and spectra strongly depend on the surface preparation procedure applied. The graphitization process imprints distinct features in the STM images as well as in the LEED spectra. The latter have the potential for an easy and practicable determination of the number of graphene layers by means of LEED.
To study the structural and electronic properties of SiC, we have performed self-consistent pseudopotential calculations for three SiC structures, namely 6H, 8H and a supercell containing thick slabs of the wurtzite and cubic forms. The supercell calculations were performed with and without including the structural relaxation of the wurtzite structure. Our main findings are: (i) the dipole set up at the stacking boundary is not centered on the central bond but on the adjacent C atom, and the disturbance to the charge density ranges anout 3.5 Å on either side; (ii) the spontaneous polarization in SiC polytypes can be interpreted as a superposition of localized dipoles due to each of the stacking boundaries, and it is mainly due to the charge density redistribution; (iii) the valence-band offset between the cubic and wurtzite forms, determined using the same supercell, is 0.13 eV, with the valence band edge of the wurtzite structure being higher in energy; (iv) from (i), one can easily explain the MAS-NMR data concerning the inequivalent Si and C sites and the structural relaxation of SiC polytypes; (v) the effect of the macroscopic electric fields on the relative stability of SiC polytypes is discussed and found to be negligible.
The macroscopic electric polarization of a crystal is often defined as the dipole of a unit cell. In fact, such a dipole moment is ill defined, and the above definition is incorrect. Looking more closely, the quantity generally measured is differential polarization, defined with respect to a "reference state" of the same material. Such differential polarizations include either derivatives of the polarization (dielectric permittivity, Born effective charges, piezoelectricity, pyroelectricity) or finite differences (ferroelectricity). On the theoretical side, the differential concept is basic as well. Owing to continuity, a polarization difference is equivalent to a macroscopic current, which is directly accessible to the theory as a bulk property. Polarization is a quantum phenomenon and cannot be treated with a classical model, particularly whenever delocalized valence electrons are present in the dielectric. In a quantum picture, the current is basically a property of the phase of the wave functions, as opposed to the charge, which is a property of their modulus. An elegant and complete theory has recently been developed by King-Smith and Vanderbilt, in which the polarization difference between any two crystal states-in a null electric field-takes the form of a geometric quantum phase. The author gives a comprehensive account of this theory, which is relevant for dealing with transverse-optic phonons, piezoelectricity, and ferroelectricity. Its relation to the established concepts of linear-response theory is also discussed. Within the geometric phase approach, the relevant polarization difference occurs as the circuit integral of a Berry connection (or "vector potential"), while the corresponding curvature (or "magnetic field") provides the macroscopic linear response.
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.
Quasi-free-standing epitaxial graphene is obtained on SiC(0001) by hydrogen intercalation. The hydrogen moves between the (6 square root(3) x 6 square root(3))R30 degrees reconstructed initial carbon layer and the SiC substrate. The topmost Si atoms which for epitaxial graphene are covalently bound to this buffer layer, are now saturated by hydrogen bonds. The buffer layer is turned into a quasi-free-standing graphene monolayer with its typical linear pi bands. Similarly, epitaxial monolayer graphene turns into a decoupled bilayer. The intercalation is stable in air and can be reversed by annealing to around 900 degrees C.
We present self-consistent pseudopotential calculations of the charge-density redistribution around an effectively isolated stacking boundary (stacking fault) and a stacking boundary in 6H SiC. The major result is that the dipole setup is not centered on the central bond but nearly on the adjacent C atom, and the disturbance to the charge density ranges about 3.5 Å on either side. In light of these calculations, we interpret the magic-angle-spinning NMR data concerning the inequivalent Si and C sites, the spontaneous polarization, and the structural relaxation in SiC polytypes.
Gate-modulated low-temperature Raman spectra reveal that the electric field effect (EFE), pervasive in contemporary electronics, has marked impacts on long-wavelength optical phonons of graphene. The EFE in this two-dimensional honeycomb lattice of carbon atoms creates large density modulations of carriers with linear dispersion (known as Dirac fermions). Our EFE Raman spectra display the interactions of lattice vibrations with these unusual carriers. The changes of phonon frequency and linewidth demonstrate optically the particle-hole symmetry about the charge-neutral Dirac point. The linear dependence of the phonon frequency on the EFE-modulated Fermi energy is explained as the electron-phonon coupling of massless Dirac fermions.