No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Thermally activated double-carrier transport in epitaxial graphene on vanadium-compensated 6H-SiC as revealed by Hall effect measurements
... 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]. ...
... We attribute this term to the unique property of restoring the QFS properties of the system exposed to high-energy neutrons with a total fluence (dose) of 2.0 × 10 17 cm -2 , followed by a sequential temperature treatment in the 423 K -623 K range. The post-NR investigations of the system are performed for the first time using a semi-insulating vanadium-compensated nominally on-axis 6H-SiC(0001) substrate, previously suggested for application in high-temperature Hall effect sensors with a record-breaking current-mode sensitivity as compared to its metallic counterparts [34]. ...
... The test elements were manufactured using graphene-on-SiC technology [32] under the geometry and type available with the GET ® platform [44]. 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]. ...
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
... Unfortunately, the electrical properties of QFS graphene on SI 6H-SiC(0001):V and SI HP 4H-SiC(0001) are stable up to only ∼573 K, at which point the single-carrier conductivity governed by spontaneouspolarization-induced holes in graphene evolves into a double-carrier conductivity that involves holes in graphene and thermally-activated electrons in the SiC substrate [11,12] (schematic in Fig. 1). Thermal progression of the SiC-related electron channel was quantified through dark Hall effect measurements. ...
... Transfer-free p-type hydrogen-intercalated [1][2][3][4][5] quasi-free-standing (QFS) epitaxial Chemical Vapor Deposition [6] (CVD) graphene on semi-insulating (SI) vanadium-compensated nominally on-axis 6H-SiC(0001) [7][8][9] and SI high-purity (HP) nominally on-axis 4H-SiC(0001) [10] has been verified as a high-sensitivity platform, suitable for magnetic field detection at significantly elevated temperatures, as high as 770 K [11,12], and under neutron radiation [13,14]. It is a likely alternative to thin-film active layers based on bismuth [15][16][17][18], antimony [19,17], chromium [17], ceramic-chromium [20], gold [21,17], molybdenum [17], tantalum [17], copper [17], NiFe [22,23], InSb [24][25][26][27][28], AlGaN/GaN [29][30][31], and La-Sr-Mn-O [32]. ...
... Conceptual diagram of the thermally-activated double-carrier conductivity at the interface between p-type QFS graphene and hydrogen-saturated semi-insulating on-axis SiC(0001). ascribed to a deep acceptor level of vanadium in the hexagonal (ℎ) site of the SI 6H-SiC:V substrate [11], or to a deep acceptor level related to silicon vacancy 2−∕− occupying the cubic ( ) site of the SI HP 4H-SiC lattice [12]. This observation led us to conclude that high-temperature electric charge transport at the interface between QFS graphene and SI SiC depends on the SiC defect structure; hence, its thermal stability could improve if one intentionally suppressed the build-up of the substrate-bound electron channel. ...
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
... This letter compares the HT performance of two magnetic field sensor platforms with the potential for use in energy industry: a semiconductor (InSb) thin-filmbased platform (ITP) and a 2-D, epitaxial-graphene-based platform (EGP). Both selected platforms are already defined in the literature as having both the potential to work in HT [13], [14], [15]; and the potential resistance to neutron radiation [10], [12], [16], which is the main motivation for the selection of platforms for comparison, significantly different in terms of construction and components. Due to the very high mobility of the carriers, InSb is the best material for the production of Hall sensors, and thin InSb layers heavily doped with donor tin are already characterized by weak temperature dependencies [13]. ...
... The EGP and ITP views are presented in Fig. 1 and is representative of all samples from a given platform. An EGP fabrication technology is described in detail in [14], [15], and [17], and ITP is described in [13] and [18], for the convenience of the reader, we only provide a brief description here. The EGP active layer was grown on semi-insulating 4H-SiC(0001) using the epitaxial chemical vapor deposition method, with hydrogen atom intercalation. ...
... Reference samples representing EGP and ITP were subjected to Procedures 1 and 2. One representative was allocated to each thermal procedure. The EGP sample G1 based on the 4H SiC(0001) substrate (prepared according to [15]) and the ITP sample I1 (prepared according to [13]) were subjected to Procedure 1. Two independent samples were used to carry out Procedure 2: EGP sample G2, based on the 6H SiC(0001) substrate (produced according to [14]) and ITP sample I2 (fabricated according to [13], but derived from a different technological process than I1). The input electrical parameters of the samples subjected to both procedures are listed in Table 1. ...
Modern two-dimensional carbon materials are being increasingly studied as potential magnetic field sensors for use in environments with harmful radiation, such as neutron radiation present in future fusion reactors. Potential radiation resistance is also demonstrated by classical semiconductor thin-film materials, after appropriate doping. A necessary condition for the potential neutron-resistant sensor is high-temperature stability. In this letter, we bring together two leading high-temperature sensing platforms: graphene-based and Sn-doped InSb-based. Our study focuses on their thermal stability under identical high-temperature and time conditions. We utilized long-term annealing procedures combined with the simultaneous measurement of the Hall effect to reflect both, the temperature conditions during radiation tests of these platforms performed recently in the MARIA research nuclear reactor, as well as post-radiation temperature treatment. We showed that long-term annealing at fixed temperature can affect the graphene-based platform to a greater extent, however, variable temperature tests showed better stability of this system. The InSb-based platform, on the other hand, exhibits much better temperature stability when operating up to
</inline-formula
... There are several substrates for the growth of graphene [27,[36][37][38][39][40][41]. 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). ...
... QFS properties of the graphene sheets can be achieved through hydrogen intercalation [44,45] of the carbon buffer layer. The latter approach was reported particularly effective in the production of magnetic field sensors for extreme temperatures supporting high sensitivity and exceptional thermal stability up to 500 • C [13,41]. This is well in excess of the operational temperature of ITER and DEMO, which makes the hydrogen-intercalated graphene sheets an optimal structure for our investigation. ...
... The substrate was processed into a batch of 25 1.6 mm × 1.6 mm Hall effect sensors [13,41] featuring a cross-shaped [39] 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 ALD reactor [49]. The type of dielectric layer was selected for its noninvasive character, lack of energetic or reactive species, sub-nanometer precision, and layer uniformity. ...
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.
... Recently, we have published on a Hall effect sensor fabricated on quasi-free-standing (QFS) monolayer graphene grown on vanadium-compensated SI on-axis 6H-SiC(0001) substrate in an epitaxial chemical vapor deposition (CVD) process [16]. We demonstrated that when passivated with a 100-nm-thick aluminum oxide layer, the sensor's current-mode sensitivity is ...
... where T 1 = 300 K and T 2 = 573 K, is negative and equals −0.02%/K, which is exactly the previously reported value for passivated graphene on vanadium-compensated SI on-axis 6H-SiC. 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]. ...
... In the 23 passivated elements, the polarization effect of the positively charged oxygen vacancies present in the 100-nmthick aluminum oxide layer [28] suppresses the influence of the spontaneous polarization of SiC and pins the arithmetical mean hole concentration at p Smean ≈ 7.5 × 10 12 cm −2 with σ ≈ 3.6×10 11 cm −2 at 300 K, p Smean ≈ 7.9×10 12 cm −2 with σ ≈ 2.9×10 11 cm −2 at 573 K, and p Smean ≈ 9.4×10 12 cm −2 with σ ≈ 6.5 × 10 11 cm −2 at 770 K [ Fig. 4(a)]. The decrease in the room-temperature hole concentration to approximately 60% of its value without passivation is quantitatively consistent with our previous report on a single sensor fabricated in the same technology on vanadium-compensated SI 6H-SiC (0001) [16]. ...
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.
... Recently, transfer-free p-type hydrogen-intercalated quasifree-standing (QFS) epitaxial chemical vapor deposition (CVD) graphene grown on semi-insulating (SI) nominally on-axis hexagonal SiC(0001) and passivated with amorphous, atomic-layer-deposited (ALD) aluminum oxide, has been suggested as a 2-D platform for magnetic field detection at elevated temperatures, up to 770 K [3], [4], likely beyond 770 K [5], and under neutron radiation [6]. The very low vulnerability of the graphene-on-SiC technology to critical temperatures opens the room to experimental electronic-transport-based evaluation of the effect of abnormally high-temperature stresses, i.e., above the 770 K limit. ...
... It was grown at 1873 K in an Aixtron VP508 reactor on a 20 × 20 mm sample diced from a 4-in, 500-µm-thick, SI, vanadium-compensated, nominally on-axis 6H-SiC(0001) wafer (II-VI Inc.), using thermally decomposed propane [26], and processed into a standard batch of 96 devices [27]. The individual device was a 1.4 × 1.4 mm four-terminal van der Pauw structure [4] featuring an equal-arm, cross-shaped [28] 100 × 300 µm QFS graphene mesa (50 000 µm 2 ), Ti/Au (10/110 nm) ohmic contacts, and a 100-nm-thick, ALD, amorphous Al 2 O 3 passivation [29], [30] synthesized from trimethylaluminum and deionized water at 770 K in a Picosun R200 Advanced ALD reactor. It was mounted and bonded to in-house-made 6.6 × 6.6 mm sapphire holders equipped with four Ti/Au (10/190 nm) corner contacts enabling direct-current electrical characterization at I = 1 mA in a 0.556-T Ecopia AHT55T5 automated Hall effect measurement system between 300 and 770 K (on par with the upper limit of the projected DEMO-class reactor requirements). ...
We studied the effect of abnormal thermal stress on a graphene-on-SiC Hall effect sensor dedicated to elevated temperatures. After subsequent thermal stresses at 823, 873, and 923 K provided by rapid thermal processing (RTP), we monitored the transport parameters of a sensor (sheet resistance, mobility, and carrier concentration) and its low-frequency noise (LFN). We showed that RTP increases the average carrier concentration and widens its distribution across the device, as confirmed by Raman spectroscopy. We observed that the LFN magnitude significantly increases after subsequent stresses, much more than the average resistance, which decreases. The evaluation of thermal stress in electronic devices should include the noise-based method because it seems to be a much more sensitive indicator of thermal degradation than typically monitored electronic transport parameters. We showed that the RTP promotes nonhomogeneity within the sensor, the presence of which is directly exposed by the LFN measurements. Moderate thermal degradation (noise and resistances) up to 873 K suggests that the graphene-on-SiC Hall platform is promising for magnetic field detection at elevated temperatures. Furthermore, the methodology of 1/ f noise analysis is universally applicable whenever the noise model of the active layer can be represented by a resistance network.
... 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]. ...
... 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]. ...
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.
... With the growing interest in epitaxial Chemical Vapor Deposition (CVD) of graphene on semiinsulating (SI) silicon carbide and noticeable progress in applied activities [1][2][3][4][5][6], it has become crucial to reliably quantify, compare and categorize graphene's properties. ...
... 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. The choice of the sensor's structure was grounded on three premises. ...
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.
Share Link: https://authors.elsevier.com/c/1dMSw4xMlkEtr3
... Magnetic field diagnostics have always been discussed in the context of cross-cutting technologies that enable the use of specific devices and components in space and on Earth. However, in recent years, the problem has received increased interest, following the rise of high-temperature electronics [1][2][3][4] and electronics for extreme environments [5]. Applications for extreme environments include the defense, oil, gas, automotive, aerospace, and geothermal industries. ...
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 x 10^17 cm2. 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.
... 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
... 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]. ...
... The technology of graphene epitaxy through Chemical Vapor Deposition (CVD) from decomposed propane [1] on semiinsulating (SI) on-axis SiC(0001) has already been verified in a number of potential applications, including broad-temperature magnetic field sensing [2,3] and monolithic microwave integrated circuits (MMICs) [4][5][6]. Historically, first to synthesize on SiC was n-type monolayer graphene in the form of a covalently bonded to the substrate carbon buffer layer [7][8][9][10] topped with a single graphene layer. ...
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
... 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.
... Furthermore, due to its intrinsic properties, such as excellent carrier mobility [2] and anomalous quantum Hall effect [3], graphene could revolutionize the future of electronics [4]. The material has already been used to create highly effective devices that can outperform their established semiconductor counterparts including: infrared photodetectors [5], Halleffect magnetic field sensors [6,7], pressure and gas sensors [8,9]. ...
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.
<<https://authors.elsevier.com/c/1cCvOcXa~wKAx>>
Anyone clicking on the above link before January 28, 2021, will be taken directly to the final version of the article on ScienceDirect, which they are welcome to read or download. No sign-up, registration, or fees are required.
... The concentration of carriers in the films is improved steadily with the increase in carrier gas flow rate, with a maximum value around 7.18 × 10 13 cm -3 at 0.15 Pa. The decreasing hall mobility from 17 to 11 and decreasing of bandgap from 2.85 to 2.65 eV is believed that, the alkali metal ions diffused from the substrate at the higher carrier gas flow are electrically active and have major effect on the optoelectronic properties [38,39]. The present analysis on the optoelectronic properties of SnS2 thin films as a function of carrier gas flow rate reveals that the device quality, uniform, highly transparent and low resistive SnS2 thin films can be deposited at the 0.15Pa. ...
Magnetic field sensors, based on the Hall effect, in mass production have currently a standard operating temperature limit of 150
. There are sensors designed for specific purposes that can function within a limited range of up to 225
, or even at the temperature of liquid nitrogen or liquid helium. The technology for the production of magnetic field sensors enabling operation in industrial conditions at temperatures significantly exceeding 225
or in the full range of temperatures from cryogenics to elevated temperatures (reaching 350
) has not yet been developed. In this letter, we present a reliable and high-quality magnetic field sensor that is capable of functioning under a wide range of temperatures. The sensor was developed using mainly the academic infrastructure of the Poznań University of Technology and can be suitable for industrial use.
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.
We present an overview of the main techniques for production and processing of graphene and related materials (GRMs), as well as the key characterization procedures. We adopt a ‘hands-on’ approach, providing practical details and procedures as derived from literature as well as from the authors’ experience, in order to enable the reader to reproduce the results.
Section I is devoted to ‘bottom up’ approaches, whereby individual constituents are pieced together into more complex structures. We consider graphene nanoribbons (GNRs) produced either by solution processing or by on-surface synthesis in ultra high vacuum (UHV), as well carbon nanomembranes (CNM). Production of a variety of GNRs with tailored band gaps and edge shapes is now possible. CNMs can be tuned in terms of porosity, crystallinity and electronic behaviour.
Section II covers ‘top down’ techniques. These rely on breaking down of a layered precursor, in the graphene case usually natural crystals like graphite or artificially synthesized materials, such as highly oriented pyrolythic graphite, monolayers or few layers (FL) flakes. The main focus of this section is on various exfoliation techniques in a liquid media, either intercalation or liquid phase exfoliation (LPE). The choice of precursor, exfoliation method, medium as well as the control of parameters such as time or temperature are crucial. A definite choice of parameters and conditions yields a particular material with specific properties that makes it more suitable for a targeted application. We cover protocols for the graphitic precursors to graphene oxide (GO). This is an important material for a range of applications in biomedicine, energy storage, nanocomposites, etc. Hummers’ and modified Hummers’ methods are used to make GO that subsequently can be reduced to obtain reduced graphene oxide (RGO) with a variety of strategies. GO flakes are also employed to prepare three-dimensional (3d) low density structures, such as sponges, foams, hydro- or aerogels. The assembly of flakes into 3d structures can provide improved mechanical properties. Aerogels with a highly open structure, with interconnected hierarchical pores, can enhance the accessibility to the whole surface area, as relevant for a number of applications, such as energy storage. The main recipes to yield graphite intercalation compounds (GICs) are also discussed. GICs are suitable precursors for covalent functionalization of graphene, but can also be used for the synthesis of uncharged graphene in solution. Degradation of the molecules intercalated in GICs can be triggered by high temperature treatment or microwave irradiation, creating a gas pressure surge in graphite and exfoliation. Electrochemical exfoliation by applying a voltage in an electrolyte to a graphite electrode can be tuned by varying precursors, electrolytes and potential. Graphite electrodes can be either negatively or positively intercalated to obtain GICs that are subsequently exfoliated. We also discuss the materials that can be amenable to exfoliation, by employing a theoretical data-mining approach.
The exfoliation of LMs usually results in a heterogeneous dispersion of flakes with different lateral size and thickness. This is a critical bottleneck for applications, and hinders the full exploitation of GRMs produced by solution processing. The establishment of procedures to control the morphological properties of exfoliated GRMs, which also need to be industrially scalable, is one of the key needs. Section III deals with the processing of flakes. (Ultra)centrifugation techniques have thus far been the most investigated to sort GRMs following ultrasonication, shear mixing, ball milling, microfluidization, and wet-jet milling. It allows sorting by size and thickness. Inks formulated from GRM dispersions can be printed using a number of processes, from inkjet to screen printing. Each technique has specific rheological requirements, as well as geometrical constraints. The solvent choice is critical, not only for the GRM stability, but also in terms of optimizing printing on different substrates, such as glass, Si, plastic, paper, etc, all with different surface energies. Chemical modifications of such substrates is also a key step.
Sections IV–VII are devoted to the growth of GRMs on various substrates and their processing after growth to place them on the surface of choice for specific applications. The substrate for graphene growth is a key determinant of the nature and quality of the resultant film. The lattice mismatch between graphene and substrate influences the resulting crystallinity. Growth on insulators, such as SiO2, typically results in films with small crystallites, whereas growth on the close-packed surfaces of metals yields highly crystalline films. Section IV outlines the growth of graphene on SiC substrates. This satisfies the requirements for electronic applications, with well-defined graphene-substrate interface, low trapped impurities and no need for transfer. It also allows graphene structures and devices to be measured directly on the growth substrate. The flatness of the substrate results in graphene with minimal strain and ripples on large areas, allowing spectroscopies and surface science to be performed. We also discuss the surface engineering by intercalation of the resulting graphene, its integration with Si-wafers and the production of nanostructures with the desired shape, with no need for patterning.
Section V deals with chemical vapour deposition (CVD) onto various transition metals and on insulators. Growth on Ni results in graphitized polycrystalline films. While the thickness of these films can be optimized by controlling the deposition parameters, such as the type of hydrocarbon precursor and temperature, it is difficult to attain single layer graphene (SLG) across large areas, owing to the simultaneous nucleation/growth and solution/precipitation mechanisms. The differing characteristics of polycrystalline Ni films facilitate the growth of graphitic layers at different rates, resulting in regions with differing numbers of graphitic layers. High-quality films can be grown on Cu. Cu is available in a variety of shapes and forms, such as foils, bulks, foams, thin films on other materials and powders, making it attractive for industrial production of large area graphene films. The push to use CVD graphene in applications has also triggered a research line for the direct growth on insulators. The quality of the resulting films is lower than possible to date on metals, but enough, in terms of transmittance and resistivity, for many applications as described in section V.
Transfer technologies are the focus of section VI. CVD synthesis of graphene on metals and bottom up molecular approaches require SLG to be transferred to the final target substrates. To have technological impact, the advances in production of high-quality large-area CVD graphene must be commensurate with those on transfer and placement on the final substrates. This is a prerequisite for most applications, such as touch panels, anticorrosion coatings, transparent electrodes and gas sensors etc. New strategies have improved the transferred graphene quality, making CVD graphene a feasible option for CMOS foundries. Methods based on complete etching of the metal substrate in suitable etchants, typically iron chloride, ammonium persulfate, or hydrogen chloride although reliable, are time- and resource-consuming, with damage to graphene and production of metal and etchant residues. Electrochemical delamination in a low-concentration aqueous solution is an alternative. In this case metallic substrates can be reused. Dry transfer is less detrimental for the SLG quality, enabling a deterministic transfer.
There is a large range of layered materials (LMs) beyond graphite. Only few of them have been already exfoliated and fully characterized. Section VII deals with the growth of some of these materials. Amongst them, h-BN, transition metal tri- and di-chalcogenides are of paramount importance. The growth of h-BN is at present considered essential for the development of graphene in (opto) electronic applications, as h-BN is ideal as capping layer or substrate. The interesting optical and electronic properties of TMDs also require the development of scalable methods for their production. Large scale growth using chemical/physical vapour deposition or thermal assisted conversion has been thus far limited to a small set, such as h-BN or some TMDs. Heterostructures could also be directly grown.
Section VIII discusses advances in GRM functionalization. A broad range of organic molecules can be anchored to the sp² basal plane by reductive functionalization. Negatively charged graphene can be prepared in liquid phase (e.g. via intercalation chemistry or electrochemically) and can react with electrophiles. This can be achieved both in dispersion or on substrate. The functional groups of GO can be further derivatized. Graphene can also be noncovalently functionalized, in particular with polycyclic aromatic hydrocarbons that assemble on the sp² carbon network by π–π stacking. In the liquid phase, this can enhance the colloidal stability of SLG/FLG. Approaches to achieve noncovalent on-substrate functionalization are also discussed, which can chemically dope graphene. Research efforts to derivatize CNMs are also summarized, as well as novel routes to selectively address defect sites. In dispersion, edges are the most dominant defects and can be covalently modified. This enhances colloidal stability without modifying the graphene basal plane. Basal plane point defects can also be modified, passivated and healed in ultra-high vacuum. The decoration of graphene with metal nanoparticles (NPs) has also received considerable attention, as it allows to exploit synergistic effects between NPs and graphene. Decoration can be either achieved chemically or in the gas phase. All LMs, can be functionalized and we summarize emerging approaches to covalently and noncovalently functionalize MoS2 both in the liquid and on substrate.
Section IX describes some of the most popular characterization techniques, ranging from optical detection to the measurement of the electronic structure. Microscopies play an important role, although macroscopic techniques are also used for the measurement of the properties of these materials and their devices. Raman spectroscopy is paramount for GRMs, while PL is more adequate for non-graphene LMs (see section IX.2). Liquid based methods result in flakes with different thicknesses and dimensions. The qualification of size and thickness can be achieved using imaging techniques, like scanning probe microscopy (SPM) or transmission electron microscopy (TEM) or spectroscopic techniques. Optical microscopy enables the detection of flakes on suitable surfaces as well as the measurement of optical properties. Characterization of exfoliated materials is essential to improve the GRM metrology for applications and quality control. For grown GRMs, SPM can be used to probe morphological properties, as well as to study growth mechanisms and quality of transfer. More generally, SPM combined with smart measurement protocols in various modes allows one to get obtain information on mechanical properties, surface potential, work functions, electrical properties, or effectiveness of functionalization. Some of the techniques described are suitable for ‘in situ’ characterization, and can be hosted within the growth chambers. If the diagnosis is made ‘ex situ’, consideration should be given to the preparation of the samples to avoid contamination. Occasionally cleaning methods have to be used prior to measurement.
The encapsulation of graphene in hexagonal boron nitride provides graphene on
substrate with excellent material quality. Here, we present the fabrication and
characterization of Hall sensor elements based on graphene boron nitride
heterostructures, where we gain from high mobility and low charge charier
density at room temperature. We show a detailed device characterization
including Hall effect measurements under vacuum and ambient conditions. We
achieve a current- and voltage-related sensitivity of up to 5700 V/AT and 3
V/VT, respectively, outpacing state-of- the-art silicon and III/V Hall sensor
devices. Finally, we extract a magnetic resolution limited by low frequency
electric noise of less than 50 nT/pHz making our graphene sensors highly
interesting for industrial applications.
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.
We have performed sensitivity mapping of graphene Hall devices with the width of 0.6–15 micron
operating in the diffusive regime under non-uniform, local magnetic and electric fields induced by a
scanning metallic magnetic probe. The transverse voltage was recorded, while tuning the magnitude
and orientation of the bias current, the probe-sample distance, and orientation of the probe
magnetization. A strong two-fold symmetry pattern has been observed, as a consequence of
capacitive coupling between the probe and the sample. The effect is particularly pronounced in small
devices (<1 micron), where the dominating electric field contribution significantly lowers the effective
area of the magnetic sensor. We show that implementation of the Kelvin probe feedback loop in the
standard scanning gate microscopy setup drastically reduces parasitic electric field effects and
improves magnetic sensitivity.
Hall elements fabricated on chemical vapor deposited graphene exhibited high current- and voltage-related sensitivities due to its low intrinsic carrier density and high mobility about 5000 cm2/V s. Electric noise of the Hall elements was measured at room temperature and found to be largely Flicker noise at low frequency which can be well described by Hooge's empirical relation with a low noise parameter of about 1.8 × 10-4. The combination of high sensitivity and low noise in graphene Hall elements leads to a high room temperature magnetic resolution of about 5 × 10-3 G/Hz0.5 at 3 kHz.
Hall sensors with the width range from 0.5 to 20.0 μm have been fabricated out of a monolayer graphene epitaxially grown on SiC. The sensors have been studied at room temperature using transport and noise spectrum measurements. The minimum detectable field of a typical 10-μm graphene sensor is ≈2.5 μT/√Hz, making them comparable with state of the art semiconductor devices of the same size and carrier concentration and superior to devices made of CVD graphene. Relatively high resistance significantly restricts performance of the smallest 500-nm devices. Carrier mobility is strongly size dependent, signifying importance of both intrinsic and extrinsic factors in the optimization of the device performance.
High-resolution photoinduced transient spectroscopy (HRPITS) has been applied to studying defect centers controlling the charge compensation in semi-insulating (SI), vanadium- free, bulk 6H- SiC. The photocurrent relaxation waveforms were digitally recorded in the temperature range of 50 - 750 K and a new approach to extract the parameters of defect centers from the temperature-induced changes in the time constants of the waveforms has been implemented. It is based on a two-dimensional analysis using the numerical inversion of the Laplace transform. As a result, the images of spectral fringes depicting the temperature dependences of the emission rate of charge carriers for defect centers are created. Using the new procedure for the analysis of the photocurrent relaxation waveforms and the new way of the visualization of the thermal emission rate dependences, a number of shallow and deep defect levels ranging from 80 to 1900 meV have been detected. The obtained results indicate that defect structure of undoped SI bulk 6H-SiC is very complex and the material properties are affected by various point defects occupying the hexagonal and quasi-cubic lattice sites.
Silicon diffusion from the substrate through Al1-xSixOy thin films was investigated by ToF-SIMS depth profiling. Two types of substrate stacks were analyzed: Si wafer with either native oxide or with additional silicon nitride layer. The amount of diffused silicon depends strongly on the type of the substrate. The activation energy for pure alumina was found to be 2.27 ± 0.02 eV and 3.73 ± 0.02 eV for SiO2/Si and Si3N4/SiO2/Si samples, respectively.
Furthermore it was proved that the activation energy increases with higher concentration of Si for SiO2/Si samples whereas it decreases for Si3N4/SiO2/Si. Detailed analysis of SIMS depth profiles provide satisfactory explanation of this phenomenon: SiO2 has much stronger tendency to react with Al1-xSixOy material forming an interface layer that restrain further diffusion of Si from the substrate (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
We report on the preparation of the first complete extreme temperature Hall sensor. This means that the extreme-temperature magnetic sensitive semiconductor structure is built-in an extreme-temperature package especially designed for that purpose. The working temperature range of the sensor extends from -270 °C to +300 °C. The extreme-temperature Hall-sensor active element is a heavily n-doped InSb layer epitaxially grown on GaAs. The magnetic sensitivity of the sensor is ca. 100 mV/T and its temperature coefficient is less than 0.04 %/K. This sensor may find applications in the car, aircraft, spacecraft, military and oil and gas industries.
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.
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.
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.
We have investigated the transport and noise properties of a micron-sized Hall probe, fabricated on chemical vapor deposited (CVD) graphene, from 300 K to 4.2 K. The field sensitivity of the Hall probe was tunable within ∼0.031-0.12 Ω/G, while the field resolution could reach ∼0.43-0.09 G/Hz1/2 at room temperature. The characteristics of graphene Hall probes (GHPs) were found to be comparable to present Hall sensors. Our results indicate that the fundamental limitation of the field sensitivity and the field resolution are respectively restricted by intrinsic and extrinsic defects. Our study paves the way for the use of CVD GHPs for scanning Hall probe with high field sensitivity and submicron spatial resolution at room temperature.
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.