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The impact of partial H intercalation on the quasi-free-standing properties of graphene on SiC(0001)
Abstract
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>>
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... 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 reasoning agrees well with previous experimental investigations on H intercalation of graphene in the 6H-SiC(0001) system [68]. C-Si bonds clusters can be activated at temperatures below 400 K [49], resulting in an elevated electron doping of graphene, which coincides with the initial effects of thermal treatment. ...
... This process, in turn, exposes previously unavailable parts of the surface to much faster hydrogen diffusion, taking approximately 100 μs. Thus, the thermal treatment should result in a more even distribution of hydrogen, which is energetically favorable if total coverage is at least 25% [49]. These suggest that after irradiation, the carrier concentration returns to its original value, indicating a limited loss of intercalation [13]. ...
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
... QFS hydrogen-intercalated [47] graphene was grown on a semiinsulating high-purity on-axis 15 mm×15 mm 4H-SiC(0001) substrate cut from a 4-in. wafer purchased at Cree Inc., in a hot-wall Aixtron VP508 reactor using the epitaxial Chemical Vapor Deposition method [48] and thermally decomposed propane as carbon source. ...
... It represents direct damage produced by neutrons and amounts to 15% of the total defects indicated by the Raman characterization. Thus, remaining defects must result from secondary effects like interdiffusion at the interface (the effect has been reported for a similar system of Al 2 O 3 /Si under irradiation [63]) and possibly chemisorption between graphene and SiC [47]. Table 1 presents three principal transport properties of the test sample pre-NR. ...
... Consequently, some parts of the interface could have too few hydrogen atoms for an effective decoupling of the graphene sheet [47]. ...
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.
... 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]. ...
... Each of the two reference samples (one SI 6H-SiC:V and one SI HP 4H-SiC), as well as the eight pre-epitaxially ion-implanted ones, had in-situ hydrogen-intercalated [5] quasi-free-standing (QFS) graphene (branded GET ® [36]) grown on them in a hot-wall Aixtron VP508 reactor using the epitaxial Chemical Vapor Deposition (CVD) method in argon flow at 1600 • C [6] and thermally decomposed propane as a source of carbon. The flow of argon was adjusted to create optimum conditions to form a boundary layer that simultaneously inhibits the sublimation [37][38][39][40] of the top-most silicon atoms from the SiC(0001) surface and enables mass transport of propane. ...
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
... 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]. ...
... The hydrogen-intercalated [5] QFS graphene (branded GET) was grown epitaxially on a semi-insulating vanadium-compensated nominally on-axis 500-μm-thick 15-mm × 15-mm 6H-SiC(0001) sample cut from a 4-in wafer purchased at II-VI Inc. The growth was conducted in a hot-wall Aixtron VP508 reactor at 1600 • C, using Chemical Vapor Deposition in argon flow [6] and thermally decomposed propane as the source of carbon atoms. ...
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.
... Second sample was treated as a substrate for graphene growth. In the same reactor, epitaxy of hydrogen-intercalated [11,31] quasi-free-standing (QFS) graphene was performed using a Chemical Vapor Deposition process from methane as the source gas [1]. Prior to growth, in-situ etching of the UID SiC surface was carried out in hydrogen atmosphere at 1600 • C and chamber pressure of 100 mbar. ...
... However, due to the nature of computational methods employed by Sławińska et al., the authors did not consider partial hydrogen intercalation or an off-axis substrate. The former has been associated with reduced hole concentration [31] and thus its contribution is unlikely. Consequently, the effect must have its origin in the high facet-to-terrace ratio at the surface of the substrate. ...
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
... Being prime representatives of the van der Waals (vdW) crystals family, multilayered graphene and TMDs consist of stacked, weakly interacting sheets, naturally making them excellent for intercalating various species. This interaction mod has been relatively broadly studied for graphene [21][22][23][24][25], but far less frequently addressed in the context of TMDs other than in a few selected cases [26]. ...
Molecular intercalation holds significant implication for the effective utilization of two-dimensional (2D) materials in a wide array of key application including gas detection and catalysis. However, its full potential remains...
... sp 2 to sp 3 [34][35][36][37], resulting in significant alterations to graphene's properties. Consequently, methods of decoupling have been extensively explored [38][39][40][41][42]. However, even when chemically decoupled, and thus retaining the linear dispersion of their bands, the properties of graphene remain affected. ...
Both intentional and unintentional doping of graphene is a common occurrence, as its carrier concentration can be modulated through various mechanisms. While extensively explored in electronics for achieving desirable conductivity, other aspects of doping remain largely untapped, presenting opportunities for further innovation. This study demonstrates that carrier concentration serves as a powerful and selective tool for modulating the interaction between molecular adsorbates and graphene. The effects are tunable and evident for both n-type and p-type doping, with low-to-medium modulation at doping levels of ±1012e/cm2, and substantial enhancements, with interaction strength increases exceeding 150% and hundreds of meV, at doping levels of ±1013e/cm2. These effects are also molecule specific, with significant enhancements for species such as water (H2O), ammonia (NH3), and aluminum chloride (AlCl3), while having minimal impact on species like hydrogen (H2). This finding not only elucidates the fundamental chemical behavior of graphene but also provides a versatile method to tailor its surface chemistry for applications in sensors, catalysis, and electronic devices. The insights from this research pave the way for advanced material design strategies, leveraging the tunable nature of graphene’s properties to optimize its interaction with various molecular species.
... The growth was conducted in an Aixtron VP508 reactor at 1600 • C, using Chemical Vapor Deposition (CVD) in argon flow [26] and thermally decomposed propane as the carbon-sourcing gas. The growth was preceded by in-situ etching of the SiC(0001) surface in a pure hydrogen atmosphere at 1600 • C and chamber pressure of 100 mbar, and followed by in-situ hydrogen intercalation [27] at 1000 • C under 900-mbar argon atmosphere. ...
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.
... 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.
... For example, it is well-documented that the chemical bonding between graphene and the substrate can induce a transition in the hybridization of carbon atoms from the original sp 2 to sp 3 [31][32][33][34], resulting in significant alterations to graphene's properties. Consequently, methods of decoupling have been extensively explored [35][36][37][38][39]. However, even when chemically decoupled, and thus retaining the linear dispersion of their π bands, the properties of graphene remain affected. ...
Both intentional and unintentional doping of graphene is a common occurrence, as its carrier concentration can be modulated through various mechanisms. While extensively explored in electronics for achieving desirable conductivity, other aspects of doping remain largely untapped, presenting opportunities for further innovation. This study demonstrates that carrier concentration serves as a powerful and selective tool for modulating the interaction between molecular adsorbates and graphene. The effects are tunable and evident for both n-type and p-type doping, with low-to-medium modulation at doping levels of ±1012 e/cm2 , and substantial enhancements, with interaction strength increases exceeding 150% and hundreds of meV, at doping levels of ±1013 e/cm2 . These effects are also molecule-specific, with significant enhancements for species such as water (H2O), ammonia (NH3), and aluminum chloride (AlCl3 ), while having minimal impact on species like hydrogen (H2 ). This finding not only elucidates the fundamental chemical behavior of graphene but also provides a versatile method to tailor its surface chemistry for applications in sensors, catalysis, and electronic devices. The insights from this research pave the way for advanced material design strategies, leveraging the tunable nature of graphene’s properties to optimize its interaction with various molecular species.
... One of the methods for separating graphene sheets from a substrate is hydrogen intercalation, which creates quasi-free layers on the SiC (0001) surface [16]. The quantum Hall resistance is consistent with the nominal value (half of the Klitzing background constant) within a standard deviation of 4.5 × 10 −9 , which makes this method suitable for the manufacture of electrical quantum standards. ...
Since the discovery of graphene in 2004 [...]
... 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.
... Quasi-free-standing (QFS) hydrogen-intercalated [32] graphene was grown on a semi-insulating high-purity nominally on-axis 15 mm × 15 mm 4H-SiC(0001) substrate cut from a 4-in wafer purchased at Cree Inc., in a hot-wall Aixtron VP508 reactor using the epitaxial Chemical Vapor Deposition (CVD) method [33] and thermally decomposed propane as carbon source. The sample was processed into a number of 1.6 mm × 1.6 mm Hall effect sensors featuring a cross-shaped [30] 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 layer synthesized from trimethylaluminum (TMA) and deionized water at 400 • C in a Picosun R200 Advanced ALD reactor. ...
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
... QFS-graphene necessary for the study has been obtained through in-situ hydrogen atom intercalation [18] of sole buffer layer [19] epitaxially grown on a semiinsulating vanadium-compensated on-axis 20 mm × 20 mm 6H-SiC(0001) substrate cut from a 4-in wafer purchased at II-VI Inc., in a hot-wall Aixtron VP508 reactor using the CVD method [20] and thermally decomposed propane as carbon source. Shortly after the process, the sample was fed with I = 500-μA direct current and characterized with a 0.55-T Ecopia HMS-3000 Hall effect measurement system to prove hole sheet concentration p s = 8.8 × 10 12 cm −2 , hole mobility p = 3032 cm 2 /Vs, and sheet resistance R s = 234 Ω/sq. ...
In this report we demonstrate a method for direct determination of the number of layers of hydrogen-intercalated quasi-free-standing epitaxial Chemical Vapor Deposition graphene on semiinsulating vanadium-compensated on-axis 6H-SiC(0001). The method anticipates that the intensity of the substrate’s Raman-active longitudinal optical A1 mode at 964 cm^(−1) is attenuated by 2.3 % each time the light passes through a single graphene layer. Normalized to its value in a graphene-free region, the A1 mode relative intensity provides a greatly enhanced topographic image of graphene and points out to the number of its layers within the terraces and step edges, making the technique a reliable diagnostic tool for applied research.
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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.
Revealing the detailed mechanism of metal intercalation on epitaxial graphene on SiC (EG/SiC) is difficult due to the complicacy and randomness of thermal decomposition of SiC substrate. In this context, the fabrication and mechanism of Pb-intercalated graphene (PbG) produced on SiC are investigated by scanning tunneling microscopy (STM) and X-ray Photoelectron Spectroscopy (XPS). It is reported that Pb intercalation prefers to happen at the interface between buffer layer and SiC substrate. Pb atoms penetrate into buffer layer through point defects on buffer layer or graphene edges during annealing, making the buffer layer transferred into an additional graphene layer. PbG regions are mainly exhibited as regular ordered moiré pattern because of the mismatch between Pb atoms and graphene layer. Some irregular PbG regions with scattered moiré pattern are also formed due to the insufficient intercalation of Pb atoms underneath. In addition, the intercalated Pb atoms arrange as two dimensional (2D) ultrathin structure underneath the newly formed PbG, and the thickness of PbG layer will increase by one after Pb intercalation. This study benefits to the modification of electronic properties of graphene on SiC, promoting the development of new 2D materials protected by graphene layer.
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.
In this paper, micro-Raman mapping and conductive atomic force microscopy (C-AFM) were jointly applied to investigate the structural and electrical homogeneity of quasi-free-standing monolayer
raphene (QFMLG), obtained by high temperature decomposition of 4H-SiC(0001) followed by hydrogen intercalation at 900 °C. Strain and doping maps, obtained by Raman data, showed the presence of sub-micron patches with reduced hole density correlated to regions with higher compressive strain, probably associated with a locally reduced hydrogen intercalation. Nanoscale resolution electrical maps by C-AFM also revealed the presence of patches with enhanced current injection through the QFMLG/SiC interface, indicating a locally reduced Schottky barrier height (ФB). The ФB values evaluated from local I-V curves by the thermionic emission model were in good agreement with the values calculated for the QFMLG/SiC interface using the Schottky-Mott rule and the graphene holes density from Raman maps. The demonstrated approach revealed a useful and non-invasive method to probe the structural and electrical homogeneity of QFMLG for future nano-electronics applications.
Quantum ESPRESSO is an integrated suite of open-source computer codes for quantum simulations of materials using state-of-the art electronic-structure techniques, based on density-functional theory, density-functional perturbation theory, and many-body perturbation theory, within the plane-wave pseudo-potential and projector-augmented-wave approaches. Quantum ESPRESSO owes its popularity to the wide variety of properties and processes it allows to simulate, to its performance on an increasingly broad array of hardware architectures, and to a community of researchers that rely on its capabilities as a core open-source development platform to implement theirs ideas. In this paper we describe recent extensions and improvements, covering new methodologies and property calculators, improved parallelization, code modularization, and extended interoperability both within the distribution and with external software.
Intercalation of hydrogen is important for understanding the decoupling of graphene from SiC(0001) substrate. Employing first-principles calculations, we have systematically studied the decoupling of graphene from SiC surface by H atoms intercalation from graphene boundary. It is found the passivation of H atoms on both graphene edge and SiC substrate is the key factor of the decoupling process. Passivation of graphene edge can weaken the interaction between graphene boundary and the substrate, which reduced the energy barrier significantly for H diffusion into the graphene-SiC interface. As more and more H atoms diffuse into the interface and saturate the Si dangling bonds around the boundary, graphene will detach from substrate. Furthermore, the energy barriers in these processes are relatively low, indicating that these processes can occur under the experimental temperature.
The ability to perform first-principles calculations of electronic and vibrational properties of two-dimensional heterostructures in a field-effect setup is crucial for the understanding and design of next-generation devices. We present here an implementation of density functional perturbation theories tailored for the case of two-dimensional heterostructures in field-effect configuration. Key ingredients are the inclusion of a truncated Coulomb interaction in the direction perpendicular to the slab and the possibility of simulating charging of the slab via field-effects. With this implementation we can access total energies, force and stress tensors, the vibrational properties and the electron-phonon interaction. We demonstrate the relevance of the method by studying flexural acoustic phonons and their coupling to electrons in graphene doped by field-effect. In particular, we show that while the electron-phonon coupling to those phonons can be significant in neutral graphene, it is strongly screened and negligible in doped graphene, in disagreement with other recent first-principles reports. Consequently, the gate-induced coupling with flexural acoustic modes would not be detectable in transport measurements on doped graphene.
QUANTUM ESPRESSO is an integrated suite of computer codes for electronic-structure calculations and materials modeling, based on density-functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave). The acronym ESPRESSO stands for opEn Source Package for Research in Electronic Structure, Simulation, and Optimization. It is freely available to researchers around the world under the terms of the GNU General Public License. QUANTUM ESPRESSO builds upon newly-restructured electronic-structure codes that have been developed and tested by some of the original authors of novel electronic-structure algorithms and applied in the last twenty years by some of the leading materials modeling groups worldwide. Innovation and efficiency are still its main focus, with special attention paid to massively parallel architectures, and a great effort being devoted to user friendliness. QUANTUM ESPRESSO is evolving towards a distribution of independent and interoperable codes in the spirit of an open-source project, where researchers active in the field of electronic-structure calculations are encouraged to participate in the project by contributing their own codes or by implementing their own ideas into existing codes.
Metal–graphene nanocomposite is a kind of potential radiation tolerant material. Graphene damage of
the composite is inevitable within radiation environments. In this paper, two kinds of copper–graphene
nanocomposite (CGNC) systems containing perfect graphene and prefabricated damage graphene,
respectively, were adopted to expound the influences of graphene damage on the properties (radiationresistance and configuration) of CGNC under irradiation by atomistic simulations. In the CGNC
containing perfect graphene, the increasing graphene damage induced by the increase of the number
of cascades, did not obviously impair the role of copper–graphene interface in keeping the properties
of CGNC. In the CGNC containing prefabricated damage graphene, the properties of CGNC would
significantly deteriorate once the radius of prefabricated damage exceeds 10Å, and even stacking fault
tetrahedral would occur in the CGNC. The results highlighted that prefabricated graphene damage have
greater effects on the change of the properties of CGNC. Therefore, it is very necessary to maintain the
structural integrity of graphene for improving the radiation-resistance and configuration of CGNC.
Abstract
In the past decade graphene has been one of the most studied material for several unique and excellent properties. Due to its two dimensional nature, physical and chemical properties and ease of manipulation, graphene offers the possibility of integration with the exiting semiconductor technology for next-generation electronic and sensing devices. In this context, the understanding of the graphene/semiconductor interface is of great importance since it can constitute a versatile standalone device as well as the building-block of more advanced electronic systems. Since graphene was brought to the attention of the scientific community in 2004, the device research has been focused on the more complex graphene transistors, while the graphene/semiconductor junction, despite its importance, has started to be the subject of systematic investigation only recently. As a result, a thorough understanding of the physics and the potentialities of this device is still missing. The studies of the past few years have demonstrated that graphene can form junctions with 3D or 2D semiconducting materials which have rectifying characteristics and behave as excellent Schottky diodes. The main novelty of these devices is the tunable Schottky barrier height, a feature which makes the graphene/semiconductor junction a great platform for the study of interface transport mechanisms as well as for applications in photo-detection, high-speed communications, solar cells, chemical and biological sensing, etc. In this paper, we review the state-of-the art of the research on graphene/semiconductor junctions, the attempts towards a modeling and the most promising applications.
Pressure sensors are a key component in electronic skin (e-skin) sensing systems. Most reported resistive pressure sensors have a high sensitivity at low pressures (<5 kPa) to enable ultra-sensitive detection. However, the sensitivity drops significantly at high pressures (>5 kPa), which is inadequate for practical applications. For example, actions like a gentle touch and object manipulation have pressures below 10 kPa, and 10-100 kPa, respectively. Maintaining a high sensitivity in a wide pressure range is in great demand. Here, a flexible, wide range and ultra-sensitive resistive pressure sensor with a foam-like structure based on laser-scribed graphene (LSG) is demonstrated. Benefitting from the large spacing between graphene layers and the unique v-shaped microstructure of the LSG, the sensitivity of the pressure sensor is as high as 0.96 kPa(-1) in a wide pressure range (0 ~ 50 kPa). Considering both sensitivity and pressure sensing range, the pressure sensor developed in this work is the best among all reported pressure sensors to date. A model of the LSG pressure sensor is also established, which agrees well with the experimental results. This work indicates that laser scribed flexible graphene pressure sensors could be widely used for artificial e-skin, medical-sensing, bio-sensing and many other areas.
We present an effective approach to eliminate SiO 2 substrate doping to graphene by hydrogen intercalation during thermal annealing treatments. Owing to the high temperature applied, the conventional vacuum annealing process creates oxygen deficiency defects and dangling bonds in the substrate, which change the p-doped graphene to the heavily n-doped type. In contrast, an annealing process in hydrogen ambient leads to hydrogen intercalation at the graphene–SiO 2 interface, which could effectively terminate the dangling bonds on the substrate formed during the high-temperature process. Hence, the supported graphene can stay “uncontaminated” by the substrate.
We measure the adsorption height of hydrogen-intercalated quasi-free-standing
monolayer graphene on the (0001) face of 6H silicon carbide by the normal
incidence x-ray standing wave technique. A density functional calculation for
the full ()-R30 unit cell, based on a van
der Waals corrected exchange correlation functional, finds a purely
physisorptive adsorption height in excellent agreement with experiments, a very
low buckling of the graphene layer, a very homogeneous electron density at the
interface and the lowest known adsorption energy per atom for graphene on any
substrate. A structural comparison to other graphenes suggests that hydrogen
intercalated graphene on 6H-SiC(0001) approaches ideal graphene.
In this letter, we report radiation stability of graphene under extreme condition of high energy density generated by 150 MeV Au ion irradiation. The experiment reveals that graphene is radiation resistant for irradiation at 1014 ions/cm2 of 150 MeV Au ions. It is significant to note that annealing effects are observed at lower fluences whereas defect production occurs at higher fluences but significant crystallinity is retained. Our results demonstrate applicability of graphene based devices in radiation environment and space applications.
Materials can, in principle, be imaged at the level of individual atoms with
aberration corrected transmission electron microscopy. However, such resolution
can be attained only with very high electron doses. Consequently, radiation
damage is often the limiting factor when characterizing sensitive materials.
Here, we demonstrate a simple and effective method to increase the electron
radiation tolerance of materials by using graphene as protective coating. This
leads to an improvement of three orders of magnitude in the radiation tolerance
of monolayer MoS2. Further on, we construct samples in different
heterostructure configurations to separate the contributions of different
radiation damage mechanisms.
QUANTUM ESPRESSO is an integrated suite of computer codes for electronic-structure calculations and materials modeling, based on density-functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave). The acronym ESPRESSO stands for opEn Source Package for Research in Electronic Structure, Simulation, and Optimization. It is freely available to researchers around the world under the terms of the GNU General Public License. QUANTUM ESPRESSO builds upon newly-restructured electronic-structure codes that have been developed and tested by some of the original authors of novel electronic-structure algorithms and applied in the last twenty years by some of the leading materials modeling groups worldwide. Innovation and efficiency are still its main focus, with special attention paid to massively parallel architectures, and a great effort being devoted to user friendliness. QUANTUM ESPRESSO is evolving towards a distribution of independent and interoperable codes in the spirit of an open-source project, where researchers active in the field of electronic-structure calculations are encouraged to participate in the project by contributing their own codes or by implementing their own ideas into existing codes.
Graphene is an outstanding electronic material, predicted to have a role in post-silicon electronics. However, owing to the absence of an electronic bandgap, graphene switching devices with high on/off ratio are still lacking. Here in the search for a comprehensive concept for wafer-scale graphene electronics, we present a monolithic transistor that uses the entire material system epitaxial graphene on silicon carbide (0001). This system consists of the graphene layer with its vanishing energy gap, the underlying semiconductor and their common interface. The graphene/semiconductor interfaces are tailor-made for ohmic as well as for Schottky contacts side-by-side on the same chip. We demonstrate normally on and normally off operation of a single transistor with on/off ratios exceeding 10(4) and no damping at megahertz frequencies. In its simplest realization, the fabrication process requires only one lithography step to build transistors, diodes, resistors and eventually integrated circuits without the need of metallic interconnects.
Graphene, a monoatomic layer of graphite hosts a two-dimensional electron gas system with large electron mobilities which makes it a prospective candidate for future carbon nanodevices. Grown epitaxially on silicon carbide (SiC) wafers, large area graphene samples appear feasible and integration in existing device technology can be envisioned. This article reviews the controlled growth of epitaxial graphene layers on SiC(0001) and the manipulation of their electronic structure. We show that epitaxial graphene on SiC grows on top of a carbon interface layer that – although it has a graphite-like atomic structure – does not display the linear π-bands typical for graphene due to a strong covalent bonding to the substrate. Only the second carbon layer on top of this interface acts like monolayer graphene. With a further carbon layer, a graphene bilayer system develops. During the growth of epitaxial graphene on SiC(0001) the number of graphene layers can be precisely controlled by monitoring the π-band structure. Experimental fingerprints for in-situ growth control could be established. However, due to the influence of the interface layer, epitaxial graphene on SiC(0001) is intrinsically n-doped and the layers have a long-range corrugation in their density of states. As a result, the Dirac point energy where the π-bands cross is shifted away from the Fermi energy, so that the ambipolar properties of graphene cannot be exploited. We demonstrate methods to compensate and eliminate this structural and electronic influence of the interface. We show that the band structure of epitaxial graphene on SiC(0001) can be precisely tailored by functionalizing the graphene surface with tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) molecules. Charge neutrality can be achieved for mono-and bilayer graphene. On epitaxial bilayer graphene, where a band gap opens due to the asymmetric electric field across the layers imposed by the interface, the magnitude of this band gap can be increased up to more than double of its initial value. The hole doping allows the Fermi level to shift into the energy band gap. The impact of the interface layer can be completely eliminated by decoupling the graphene from the SiC substrate by a hydrogen intercalation technique. We demonstrate that hydrogen can migrate under the interface layer and passivate the underlying SiC substrate. The interface layer alone transforms into a quasi-free standing monolayer. Epitaxial monolayer graphene turns into a decoupled bilayer. In combination with atmospheric pressure graphitization, the intercalation process allows to produce quasi-free standing epitaxial graphene on large SiC wafers and represents a highly promising route towards epitaxial graphene based nanoelectronics.
Popular modern generalized gradient approximations are biased toward the description of free-atom energies. Restoration of the first-principles gradient expansion for exchange over a wide range of density gradients eliminates this bias. We introduce a revised Perdew-Burke-Ernzerhof generalized gradient approximation that improves equilibrium properties of densely packed solids and their surfaces.
We report measurements of the cyclotron mass in graphene for carrier
concentrations n varying over three orders of magnitude. In contrast to the
single-particle picture, the real spectrum of graphene is profoundly nonlinear
so that the Fermi velocity describing the spectral slope reaches ~3x10^6 m/s at
n <10^10 cm^-2, three times the value commonly used for graphene. The observed
changes are attributed to electron-electron interaction that renormalizes the
Dirac spectrum because of weak screening. Our experiments also put an upper
limit of ~0.1 meV on the possible gap in graphene.
We report on an investigation of quasi-free-standing graphene on 6H-SiC(0001)
which was prepared by intercalation of hydrogen under the buffer layer. Using
infrared absorption spectroscopy we prove that the SiC(0001) surface is
saturated with hydrogen. Raman spectra demonstrate the conversion of the buffer
layer into graphene which exhibits a slight tensile strain and short range
defects. The layers are hole doped (p = 5.0-6.5 x 10^12 cm^(-2)) with a carrier
mobility of 3,100 cm^2/Vs at room temperature. Compared to graphene on the
buffer layer a strongly reduced temperature dependence of the mobility is
observed for graphene on H-terminated SiC(0001)which justifies the term
"quasi-free-standing".
It has been shown that the first C layer on the SiC(0001)(2{\times}2)C
surface already exhibits graphene-like electronic structure, with linear pi
bands near the Dirac point. Indeed, the (2{\times}2)C reconstruction, with a Si
adatom and C restatom structure, efficiently passivates the SiC(0001) surface
thanks to an adatom/restatom charge transfer mechanism. Here, we study the
effects of interface modifications on the graphene layer using density
functional theory calculations. The modifications we consider are inspired from
native interface defects observed by scanning tunneling microscopy. One H atom
per 4 {\times} 4 SiC cell (5 {\times} 5 graphene cell) is introduced in order
to saturate a restatom dangling bond and hinder the adatom/restatom charge
transfer. As a consequence, the graphene layer is doped with electrons from the
substrate and the interaction with the adatom states slightly increases. Native
interface defects are therefore likely to play an important role in the doping
mechanism on the C terminated SiC substrates. We also conclude that an
efficient passivation of the C face of SiC by H requires a complete removal of
the reconstruction. Otherwise, at variance with the Si terminated SiC
substrates, the presence of H at the interface would increase the
graphene/substrate interaction.
Graphene research so far has focused on electronic rather than photonic applications, in spite of its impressive optical properties. These include its ability to absorb approximately 2% of incident light over a broad wavelength range despite being just one atom thick. Here, we demonstrate ultrafast transistor-based photodetectors made from single- and few-layer graphene. The photoresponse does not degrade for optical intensity modulations up to 40 GHz, and further analysis suggests that the intrinsic bandwidth may exceed 500 GHz. The generation and transport of photocarriers in graphene differ fundamentally from those in photodetectors made from conventional semiconductors as a result of the unique photonic and electronic properties of the graphene. This leads to a remarkably high bandwidth, zero source-drain bias and dark current operation, and good internal quantum efficiency.
A plane-wave basis has great advantages for many calculations in the physics solids. To apply this basis to a wider class of materials, the atomic characteristic of a pseudopotential is identified which leads to rapid convergence in the solid, and a new method for generating pseudopotentials optimized according to this criterion is shown. As a test case, an ab initio plane-wave basis determination of the structural properties of fcc copper is performed. The results indicate that these optimized pseudopotentials will facilitate study of transition metals and first-row nonmetals.
In the next decade, advances in complementary metal-oxide semiconductor fabrication will lead to devices with gate lengths (the region in the device that switches the current flow on and off) below 10 nanometers (nm), as compared with current gate lengths in chips that are now about 50 nm. However, conventional scaling will no longer be sufficient to continue device performance by creating smaller transistors. Alternatives that are being pursued include new device geometries such as ultrathin channel structures to control capacitive losses and multiple gates to better control leakage pathways. Improvement in device speed by enhancing the mobility of charge carriers may be obtained with strain engineering and the use of different crystal orientations. Here, we discuss challenges and possible solutions for continued silicon device performance trends down to the sub-10-nm gate regimes.
This paper investigated the temperature effects on the RF performances of the epitaxial bilayer graphene field-effect transistors (EBG-FETs) on a SiC substrate over a temperature range of 25–200 °C under the atmospheric environment. The temperature dependence of the cutoff frequency fT and the maximum oscillation frequency fmax of the EBG-FETs with an identical gate-length and different gate-widths were measured by small-signal measurement up to 40 GHz. The results show that EBG-FETs are capable of operating at high temperature up to 200 °C with workable amounts of thermally induced performance degradation. To gain further insight into the underlying physics of the device parameters affected by temperature, the small signal device parameter variations with ambient temperature are discussed. This work reveals the great potential for graphene in high temperature device applications.
We demonstrate photovoltaic and photoconductive responses to near-infrared light in devices formed by depositing a film of gel permeation chromatography purified PbS quantum dots (QDs) on top of n-SiC epitaxial layers with natively grown, low-leakage 10-15 monolayer thick epitaxial graphene (EG) Schottky contacts. The QD-film layer was removable by selec-tive chemical etching, resetting the EG/SiC Schottky diode: sub-bandgap response could be restored in subsequent PbS-QD depositions. The EG in these devices simultaneously forms Schottky contacts to SiC and ohmic contacts to PbS-QD, ena-bling electrical screening and isolation of these interfaces from each other. After PbS-QD deposition, the diodes exhibit pho-tovoltaic and photoconductive response at photon energies far below the SiC bandgap, extending to the NIR gap of the QD film. Scanning photocurrent microscopy illustrates that this is due to charge transfer from the QD-film to the n-type 4H-SiC through a trap-limited, rectifying PbS-QD/SiC heterojunction with ideality n=2 in parallel with the EG/SiC Schottky diode. The photoconductive gain at this QD/SiC interface could be useful for IR detection in wide bandgap platforms. Response times as fast as 40 ms are suitable for imaging applications, although careful contact design is required to optimize work-function matching and spreading resistance.
We report the results of in-depth Raman study of quasi-free-standing monolayer graphene on the (0001) Si-face of 4H-SiC, which contains ~0.1-2·10 11 cm −2 sp 3 defects that have been introduced by hydrogen intercalation. The nature of the intercalation-induced defects is elucidated and ascribed to the formation of the C-H bonds. At the higher intercalation temperature in the formed monolayer graphene the defect-related Raman scattering displays a great enhancement and new spectral features attributed to D′ and D+D′ modes appear. Comprehensive statistical analysis of the Raman data enabled us to estimate the homogeneity of the Raman scattering processes and to separate strain and doping effects. Analysis of the compressive strain and carrier density maps revealed that the intercalation temperature of 900 °C and intercalation time of 1 h are more favorable conditions for conversion of the buffer layer to uniformly relaxed and p-doped monolayer graphene in comparison to annealing at 1100 °C for 30 min.
The idea of a raster pattern magnetoresistor made of thin films of III–V compounds and a metallic
layer has been known for over fifty years. Based on this knowledge, we present the construction of
a magnetoresistor made of combined graphene and metallic strip patterns. The presented device is
implemented using a monolayer of graphene epitaxially grown on a semi-insulating substrate. A
graphene strip pattern magnetoresistor gives a promising wide range of practical applications due
to its very high sensitivity combined with the mono-atomic thickness of the sensitive layer, the sim-
plicity of realization, and a very convenient principle of sensor operation assuming only the usage
of two electrical terminals.
Herein, we report the fabrication of a highly stretchable, transparent gas sensor based on silver nanowire-graphene hybrid nanostructures. Due to its superb mechanical and optical characteristics, the fabricated sensor demonstrates outstanding and stable performances even under extreme mechanical deformation (stable until 20% of strain). The integration of a Bluetooth system or an inductive antenna enables the wireless operation of the sensor. In addition, the mechanical robustness of the materials allows the device to be transferred onto various nonplanar substrates, including a watch, a bicycle light, and the leaves of live plants, thereby achieving next-generation sensing electronics for the 'Internet of Things' area.
A new approach to the construction of first-principles pseudopotentials is described. The method allows transferability to be improved systematically while holding the cutoff radius fixed, even for large cutoff radii. Novel features are that the pseudopotential itself becomes charge-state dependent, the usual norm-conservation constraint does not apply, and a generalized eigenproblem is introduced. The potentials have a separable form well suited for plane-wave solid-state calculations, and show promise for application to first-row and transition-metal systems.
A method is given for generating sets of special points in the Brillouin zone which provides an efficient means of integrating periodic functions of the wave vector. The integration can be over the entire Brillouin zone or over specified portions thereof. This method also has applications in spectral and density-of-state calculations. The relationships to the Chadi-Cohen and Gilat-Raubenheimer methods are indicated.
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.
The development of graphene electronic devices produced by industry relies on efficient control of heat transfer from the graphene sheet to its environment. In nanoscale devices, heat is one of the major obstacles to the operation of such devices at high frequencies. Here we have studied the transport of hot carriers in epitaxial graphene sheets on 6H-SiC (0001) substrates with and without hydrogen intercalation by driving the device into the non-equilibrium regime. Interestingly, we have demonstrated that the energy relaxation time of the device without hydrogen intercalation is two orders of magnitude shorter than that with hydrogen intercalation, suggesting application of epitaxial graphene in high-frequency devices which require outstanding heat exchange with an outside cooling source.
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 investigate the intercalation of hydrogen at the graphene/SiC(0001) interface through atomistic models characterized by very low strains both in the epitaxial graphene and in the SiC substrate. Adsorption of H at the interface is always stable but shows energy variations larger than 1 eV between different locations of the interface. An interface model presenting a strong interaction of graphene with the substrate, corresponding to the experimental situation, shows that adsorption at the interface is on average 0.75 eV less stable than at the surface of the buffer layer. At variance, a model having a much weaker graphene/SiC interaction results in hydrogenation energies that are comparable in the two cases. The structural modifications occurring upon H intercalation show a partial conversion of the buffer layer into quasi-free standing graphene, accompanied by a marked downward relaxation of the hydrogenated Si atom and a local steric repulsion between the latter and the overlying graphene.
Transport in ultrathin graphite films grown on single-crystal silicon
carbide is dominated by the electron-doped epitaxial graphene layer at
the interface and shows graphene characteristics. Epitaxial graphene
provides a platform for studying the novel electronic properties of this
2D electron gas in a controlled environment. Shubnikov-de Haas
oscillations in the magnetoresistance data indicate an anomalous Berry's
phase and reveal the Dirac nature of the charge carriers. The system is
highly coherent with phase coherence lengths beyond 1 micrometer at
cryogenic temperatures, and mobilities exceeding 2.5 square meters per
volt-second. In wide structures, evidence is found for weak
anti-localization in agreement with recent graphene weak-localization
theory. Patterned narrow ribbons show quantum confinement of electrons.
Several Hall bar samples reveal anomalous magnetoresistance patterns
consisting of large structured non-periodic oscillations that may be due
to a periodic superlattice potential.
We study the intercalation steps for epitaxial graphene exposed to a hydrogen-rich gaseous ambient with the aid of event-driven kinetic Monte Carlo (KMC) simulations. We appropriately formulate an ab initio calibrated KMC algorithm that generates the evolvement of the system dynamics. We discuss the kinetic stages leading to the formation of the intercalated layer and the stability of the resulting structure against the process parameters. We moreover investigate the dependence of the process timeline with respect to the concentration of defects at the surface prior to intercalation. Results complement the experiment and could serve as guidelines for future works on the intercalation of epitaxial graphene on SiC..
Schematic representation showing the early-stage intercalation of H atoms at the interface between buffer layer and SiC(0001).
The properties of epitaxial graphene on SiC substrates can be modified by intercalation of different atomic species. In this work, mechanisms of hydrogen intercalation into the graphene-SiC(0001) interface, and properties of hydrogen and fluorine intercalated structures have been studied with the use of density functional theory. Our calculations show that the intercalation of hydrogen and fluorine into the interface is energetically favorable. Energy barriers for diffusion of atomic and molecular hydrogen through the interface graphene layer with no defects and graphene layers containing Stone-Wales defect or two- and four-vacancy clusters have been calculated. It is argued that diffusion of hydrogen towards the SiC surface occurs through the hollow defects in the interface graphene layer. It is further shown that hydrogen easily migrates between the graphene layer and the SiC substrate and passivates the surface Si bonds, thus causing the graphene layer decoupling. According to the band structure calculations the graphene layer decoupled from the SiC(0001) surface by hydrogen intercalation is undoped, while that obtained by the fluorine intercalation is p-type doped.
The current transport across the graphene/ 4H-SiC interface has been investigated with nanometric lateral resolution by scanning current spectroscopy on both epitaxial graphene (EG) grown on (0001) 4H-SiC and graphene exfoliated from highly oriented pyrolytic graphite deposited on the same substrate [deposited graphene (DG)]. This study reveals that the Schottky barrier height (SBH) of EG/4H-SiC (0.36±0.1eV) is ˜0.49eV lower than the SBH of DG/4H-SiC (0.85±0.06eV) . This result is discussed in terms of the Fermi-level pinning ˜0.49eV above the Dirac point in EG due to the presence of positively charged states at the interface between the Si face of 4H-SiC and the carbon-rich buffer layer, which is the precursor for EG formation.
We directly demonstrate the importance of buffer elimination at the graphene/SiC(0001) interface for high frequency applications. Upon successful buffer elimination, carrier mobility increases from an average of 800 cm(2)/(V s) to >2000 cm(2)/(V s). Additionally, graphene transistor current saturation increases from 750 to >1300 mA/mm, and transconductance improves from 175 mS/mm to >400 mS. Finally, we report a 10× improvement in the extrinsic current gain response of graphene transistors with optimal extrinsic current-gain cutoff frequencies of 24 GHz.
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.
A semiempirical addition of dispersive forces to conventional density functionals (DFT-D) has been implemented into a pseudopotential plane-wave code. Test calculations on the benzene dimer reproduced the results obtained by using localized basis set, provided that the latter are corrected for the basis set superposition error. By applying the DFT-D/plane-wave approach a substantial agreement with experiments is found for the structure and energetics of polyethylene and graphite, two typical solids that are badly described by standard local and semilocal density functionals.
Generalized gradient approximations (GGA{close_quote}s) for the exchange-correlation energy improve upon the local spin density (LSD) description of atoms, molecules, and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental constants. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential. {copyright} {ital 1996 The American Physical Society.}