On-axis 6H-SiC Research Articles

Quasi-free-standing epitaxial graphene on 4H-SiC(0001) as a two-dimensional reference standard for Kelvin Probe Force Microscopy

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=0.64V, 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.

Surface morphology of 3C–SiC layers grown on 4H–SiC substrates using TCS as silicon precursor

The hetero-epitaxial growth of 3C–SiC layers on on-axis 4H–SiC substrates has been demonstrated in a hot-wall CVD reactor using trichlorosilane and ethylene as precursors. The additional chlorine is supplied by hydrogen chloride for variation of Cl/Si ratio. The influence of temperature, C/Si ratio and Cl/Si ratio process parameters on the morphology is studied. Double-position-boundaries free 3C–SiC epitaxial layers have been successfully grown at the optimized condition, which is at a temperature of 1340 °C, with C/Si = 0.6 and Cl/Si = 6 using a carbon-rich pretreatment. Low temperature near bandgap photoluminescence shows a good optical property of the obtained 3C–SiC epitaxial layers. Compared to the standard chemistry, a higher growth rate of 12 μm/h could be achieved by utilizing the chlorinated precursors. This study provides a feasible way to grow double-position-boundaries free 3C–SiC epitaxial layers using TCS as silicon precursor.

Defect-engineered graphene-on-silicon-carbide platform for magnetic field sensing at greatly elevated temperatures

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.

Spectroscopic properties of close-to-perfect-monolayer quasi-free-standing epitaxial graphene on 6H[sbnd]SiC(0001)

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 6HSiC(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 SiH 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.

CVD growth of 3C-SiC layers on 4H-SiC substrates with improved morphology

Chloride-based CVD growth of 3C-SiC epitaxial layers on on-axis 4H-SiC substrates has been studied using chloromethane and silane as precursors. Hydrogen chloride is utilized as additional chlorine source. Process parameters, such as growth temperature, C/Si ratio, Cl/Si ratio and temperature ramp-up condition, are investigated. Smooth 3C-SiC layers without DPB defects on the surface could be obtained with optimized process at a growth rate of 14 μm/h. Low temperature photoluminescence measurement shows sharp near bandgap emission, which verifies the good optical properties of the epitaxial layer. The growth window is wide in this process by using chlorinated precursors compared to the standard chemistry.

Development of an angle detection system for channeling implantation to the c-axis of SiC using birefringence phenomenon

We developed an angle detection system for channeling ion implantation in 4H-SiC using the birefringence phenomenon. Our optical method detects the c-axis direction in 4H-SiC due to its uniaxial optical properties. The system, consisting of a laser, polarizer, gonio stage, and analyzer, is simple and cost-effective. We conducted experiments on both on-axis and off-axis 4H-SiC (0001) samples, presenting angular dependence results around the [1–100] and [11–20] rotations. Despite the need for consideration of light incident angles, the performance was comparable to Rutherford backscattering spectrometry. These findings suggest the potential application of our system in channeling implantation to the c-axis of 4H-SiC.

High-Quality AlN Grown by a Combination of Substrate Pretreatment and Periodic Growth Mode Control

The growth of high-quality AlN films has always been the focus of research. Combined with NH3 in situ pretreatment before growth, with optimized growth conditions, we demonstrated a periodic growth method, which alternates between three-dimensional (3D) growth mode and two-dimensional (2D) growth mode, to grow a thick crack-free AlN film with good crystal quality on the on-axis 4H-SiC substrate by metal–organic chemical vapor deposition (MOCVD). X-ray rocking curves (XRCs) indicated that full width at half-maximum (FWHM) of the (0002) and (10–12) reflections were 241.3″ and 474.4″, respectively. The film thickness was over 1.4 μm. Lattice constant and residual stress results showed residual compressive stress in the sample. The periodic two-step growth and substrate pretreatment both play an important role in controlling the residual stress of films to avoid cracking. This work can serve as an important reference for growing high-quality crack-free AlN on SiC.

High quality epitaxial graphene on 4H-SiC by face-to-face growth in ultra-high vacuum

Epitaxial graphene on SiC is the most promising substrate for the next generation 2D electronics, due to the possibility to fabricate 2D heterostructures directly on it, opening the door to the use of all technological processes developed for silicon electronics. To obtain a suitable material for large scale applications, it is essential to achieve perfect control of size, quality, growth rate and thickness. Here we show that this control on epitaxial graphene can be achieved by exploiting the face-to-face annealing of SiC in ultra-high vacuum. With this method, Si atoms trapped in the narrow space between two SiC wafers at high temperatures contribute to the reduction of the Si sublimation rate, allowing to achieve smooth and virtually defect free single graphene layers. We analyse the products obtained on both on-axis and off-axis 4H-SiC substrates in a wide range of temperatures (1300 °C–1500 °C), determining the growth law with the help of x-ray photoelectron spectroscopy (XPS). Our epitaxial graphene on SiC has terrace widths up to 10 μm (on-axis) and 500 nm (off-axis) as demonstrated by atomic force microscopy and scanning tunnelling microscopy, while XPS and Raman spectroscopy confirm high purity and crystalline quality.

Investigation on dislocation and deflection morphology of PVT-grown on-axis 4H-SiC crystals

The morphologies of dislocation etch pits and dislocation deflections of on-axis 4H-SiC substrate etched by molten KOH were observed with the help of a microscope. Based on experimental observation and etch mechanism, a method for the identification of threading edge dislocations, threading screw dislocations (TSDs) and threading mixed dislocations was proposed. The details about the inner micro-structure of threading edge dislocations and TSDs were observed by laser scanning confocal microscope and scanning electron microscopy. The morphologies and the cross-sectional views of the basal plane dislocation formed by threading edge dislocation were observed and two models were formed to explain it.

Contamination-induced inhomogeneity of noise sources distribution in Al2O3-passivated quasi-free-standing graphene on 4H-SiC(0001)

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.

The Comparison of InSb-Based Thin Films and Graphene on SiC for Magnetic Diagnostics under Extreme Conditions.

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 ≈ 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.

Investigation on Step-Bunched Homoepitaxial Layers Grown on On-Axis 4H-SiC Substrates via Molten KOH Etching

Wafer-scale on-axis 4H-SiC epitaxial layers with very low roughness were obtained in this study. By performing carbon-rich hydrogen etching and epitaxial growth of the epitaxial layer at different temperatures, local mirror regions (LMRs) with root mean square (RMS) roughness less than 0.2 nm were obtained on the epitaxial layer surface. The LMRs’ length is tens of millimeters, and the width is sub-millimeters. The step-flow growth induced by threading screw dislocations (TSDs) was observed on the epitaxial layer surface by atomic force microscopy (AFM), together with the double bi-atomic step-flow growth induced by the step bunch, which was the cause of LMRs. Furthermore, the growth mechanism was investigated by wet etching. The etching pits were found to be associated with 3C-SiC and their effect on the growth rate of epitaxial layers was further explored.

Atomic layer deposition of AlN on different SiC surfaces

Thin AlN films were grown using a Picosun R-200 atomic layer deposition (ALD) reactor on SiC surfaces with different crystallographic orientation: on-axis 4H-SiC (0001) and 8° off-axis 4H-SiC. TMA (trimethylaluminium) and NH3 were used as precursors while hydrogen and nitrogen plasma were applied for in-situ substrate cleaning. The substrate temperatures were 400 °C and 450 °C, with 20 ALD cycles. The surface morphology was investigated by scanning electron microscopy (SEM), which revealed nanometer-sized islands in all films. The AlN films deposited on on-axis 4H-SiC at 450 °C substrate temperature exhibited a relatively small roughness of about 0.255 nm. The chemical composition and bonding states were investigated by X-ray photoelectron spectroscopy. For all layers, high-resolution XPS showed Al 2p and N 1s spectra that are characteristic of AlN. These results are a good prerequisite of establishing the growth conditions of AlN films for surface acoustic wave (SAW) devices.

Morphological evolution of thin AlN films grown by atomic layer deposition

Thin AlN films were grown using a Beneq TFS-200 ALD reactor. TMA (trimethylaluminium) and NH3 were used as precursors. The substrate temperature was 330 °C, the ALD cycles, 550. The TMA and NH3 doses (pulses) lasted 180 ms and 90 ms, followed by 2-s and 9-s nitrogen gas purge, respectively. In order to study the morphological evolution of the thin AlN films, substrates providing different surface kinetics were used: Si-face and C-face of 4°-off axis and on-axis 4H-SiC, and graphene grown on 4H-SiC by sublimation. As revealed by atomic force microscopy (AFM), the lowest RMS surface roughness of about 0.8 nm was exhibited by the AlN film deposited on Si-face on-axis 4H-SiC due to its higher surface energy which provides for better film nucleation. The chemical composition and bonding states were investigated by X-ray photoelectron spectroscopy (XPS). The existence of AlN is justified by the presence of the XPS peaks of Al 2p and N 1s at about 73.3 eV and 396.6 eV, respectively. These results are promising in view of further studies of thin AlN films properties for application in surface acoustic wave devices (SAW).

Determining the number of graphene layers based on Raman response of the SiC substrate

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.

Comparison of the Thermal Stress Behavior of AlN Single Crystal Growth on AlN and SiC Seeds via the Physical Vapor Transport Method through Three-Dimensional Numerical Modeling and Simulation

We extend a thermal-elastic stress model by the finite element method to evaluate anisotropic three-dimensional thermal stress in AlN bulk crystals grown on on-axis 2H-AlN and 6H-SiC seeds. The dis...

Synchrotron X-ray topographic image contrast variation of screw-type basal plane dislocations located at different depths below the crystal surface in 4H-SiC

The contrast features of synchrotron X-ray topographic images of screw-type basal plane dislocations (BPDs) in on-axis 4H-SiC wafers have been studied. Screw BPD images are categorized into two types: one exhibiting a white line bordered by black lines and the other a pure black line contrast. Similar images for off-axis specimens and the corresponding ray-tracing simulations demonstrate that these contrasts can be attributed to the depth of the screw BPDs below the crystal surface. The correlation of the contrast features between simulations and the screw BPD topography images can be used to estimate the depth.

Substrate-mediated growth of oriented, vertically aligned MoS2 nanosheets on vicinal and on-axis SiC substrates

The layer- and morphology-dependent properties of two-dimensional molybdenum disulfide (MoS2) have established its relevance across broad applications in electronics, optoelectronics, sensing, and catalysis. Understanding how to manipulate the material growth to achieve the desired properties is the key to tailoring the material towards a specific application. In this work, we investigate the growth of vertically standing MoS2 nanosheets by chemical vapor deposition on vicinal and on-axis 4H-SiC (0001) substrates. In both cases the MoS2 flakes exhibit three preferred orientations, aligning with the 〈112¯0〉 substrate directions due to strain minimization of a MoO2 intermediate phase. Whereas MoS2 grown on vicinal SiC substrates exhibits strict near-vertical alignment, scanning electron microscopy and near-edge X-ray absorption fine structure (NEXAFS) measurements indicate a near-random vertical orientation when MoS2 is grown on on-axis SiC. Photoemission spectroscopy and NEXAFS measurements indicate the presence of defects and disordered edges which establish the suitability of the material for applications in sensing and catalysis.

Synchrotron X-Ray Topographic Image Contrast Variation of BPDs Located at Different Depths Below the Crystal Surface in 4H-SiC

The contrast features of synchrotron X-ray topographic images of screw-type basal plane dislocations (BPDs) in on-axis 4H-SiC wafers have been studied. Screw BPD images are categorized into two types: one exhibiting a white line bordered by black lines and the other a pure black line contrast. Similar images for off-axis specimens and the corresponding ray-tracing simulations demonstrate that these contrasts can be attributed to the depth of the screw BPDs below the crystal surface. The correlation of the contrast features between simulations and the screw BPD topography images can be used to estimate the depth.

Epitaxial synthesis of graphene on 4H-SiC by microwave plasma chemical vapor deposition

Epitaxial graphene (EG) on semi-insulating SiC prepared by a thermal decomposition method is the most promising strategy for graphene application in large-scale integrated circuits due to compatibility with current semiconductor processes. In this study, high-quality few-layer graphene (FLG) was epitaxially grown on the semi-insulating on-axis 4H-SiC by microwave plasma chemical vapor deposition (MPCVD). Both sides of the SiC substrate were etched with H2 plasma at ∼1000 °C to promote the nucleation and the growth of graphite before epitaxial growth. The surface morphology and properties of EG on SiC (0001) were measured by energy-dispersive x-ray (EDX), x-ray photoelectron spectrogram (XPS), and atomic force microscope (AFM). The qualities of EG grown on the two surfaces of SiC at various temperatures were checked by Raman spectroscopy. Furthermore, the EG growth was controlled by the effect of Ar plasma in the MPCVD and the few-layer (1–3 layers) high-quality EG films were formed on SiC (000). The room temperature Hall mobility of EGs up to 2790 cm2 V−1 s−1 on SiC(000) was realized.