Papers by Author: W.C. Mitchel

Abstract: Graphene growth on SiC in atmospheric pressure argon exhibits large terrace sizes and coverage over the entire substrate surface. Graphene growth and the resulting morphology are correlated with the characteristics of the growth chamber and the surface quality of the starting SiC substrate. Without in-situ surface preparation prior to growth, we observe “wrinkles” in the graphene surface. Graphitic-like disordered structures are formed at 1500°C while atomically flat graphene terraces are formed above 1600°C.

Abstract: D-band electron paramagnetic resonance (EPR) measurements as well as X and Q-band field-swept Electron Spin Echo (ESE) and pulsed Electron Nuclear Double Resonance (ENDOR) studies were performed on a series of n-type 4H-SiC wafers grown by different techniques including sublimation sandwich method (SSM), physical vapor transport (PVT) and modified Lely method. Depending on the C/Si ratio and the growth temperature the n-type 4H-SiC wafers revealed, besides a triplet due to nitrogen residing on the cubic site (Nc), two nitrogen (N) related EPR spectra with g||=2.0055, g⊥=2.0010 and g||=2.0063, g⊥=2.0005 with different intensities. In the samples with low C/Si ratio the EPR spectrum with g|| =2.0055, g⊥=2.0010 consists of a triplet with low intensity which is tentatively explained as a N-related complex, while in the samples with high C/Si ratio the triplet is transformed into one structureless line of high intensity, which is explained as being due to an exchange interaction between N donors. In the samples grown at low temperature with enhanced carbon concentration the EPR line with g||=2.0063, g⊥=2.0005 and a small hyperfine (hf) interaction dominates the EPR spectrum. It is attributed to N on the hexagonal lattice site. The interpretation of the EPR data is supported by activation energies and donor concentrations obtained from Hall effect measurements for three donor levels in this series of 4H-SiC samples.

Abstract: Ohmic contacts on SiC have been investigated extensively in the past decade. However, the mechanism for ohmic contact formation has been a troublesome issue. The interfacial structures at the atomic scale responsible for forming ohmic contacts have not been revealed. Our previous results have shown that carbon can form ohmic contacts on SiC after thermal annealing, and that an interfacial carbon layer between Ni and the SiC improves the contacts significantly. In this study, we have investigated the interactions between Ni and carbon, and ohmic contact formation on SiC using x-ray diffraction (XRD) and Raman spectroscopy. After annealing, ohmic behavior was observed and Ni graphite intercalated compounds (GICs) were found on Ni/C/SiC structures. Unlike conventional graphite intercalated compounds, the Ni atoms substitute for carbon atoms in the graphitic networks in these Ni-GICs. XRD peaks at 21.6° due to the Ni graphitic intercalation compound (Ni-GIC) and at 26.3° due to graphite have been observed. The distance between graphitic sheets is 0.403nm in the Ni graphite intercalated compounds, whereas it is ~20% larger in the graphite. The thickness of the interfacial carbon layer does not affect the formation of Ni-GIC.

Abstract: Al based alloys, such as Ti/Al, are commonly used for ohmic contacts on p-type SiC. The interfacial structures of a metal alloy film on SiC are very complicated after annealing. Al is considered as the key element responsible for forming ohmic contacts on p-type SiC, and reacts with C from SiC and forms Al4C3 and Si during annealing. In this study, we have investigated ohmic contact formation of a single component Al4C3 film on p-type SiC. Based on the stoichiometric formation of Al4C3 between Al and C at high temperatures, several samples with various Al/C mole ratios have been examined for ohmic contact formation after different annealing temperatures. Carbon rich and stoichiometric Al4C3 films form ohmic contacts on p-type 4H-SiC (~2.8 x1018 cm-3 ) after annealing at 800 and 900°C. X-ray diffraction (XRD) data have shown that a single component Al4C3 is formed when an ohmic contact on p-type SiC is activated. Al/SiC, as the control sample, does not form ohmic contacts under the same conditions. This study reveals that Al4C3 can be responsible for forming ohmic contacts on p-type SiC. However, its chemical instability requires that the secondary metal is necessary to form stable ohmic contacts when Albased films are used.

Abstract: Aligned carbon nanotubes (CNT’s) are formed on the surface of silicon carbide (SiC) wafers during high temperature anneals. The exposed 4H SiC surface transforms into CNT’s for temperatures in the range of 1400-1700°C and under moderate vacuum conditions (10-2 – 10-5 torr). The rate of formation on the C-face (0001,‾) is about three times the rate on the Si-face (0001), but both rates increase with anneal temperature. SEM, TEM and Raman scattering measurements have confirmed the presence of both single-wall and multi-wall CNT’s. The carbon source is believed to be residual carbon from the SiC left on the surface after preferential evaporation of Si. CNT formation is believed to be catalyzed by low concentrations of residual oxygen in the chamber. Subsequent I-V measurements provide insight into the electrical characteristics of the CNT’s and the SiC/CNT interface.

Abstract: Carbon nanotubes (CNTs) grown on SiC are metal-free, well-aligned, and with low structural defects. In this study, CNT formation on SiC is examined in high vacuum (10-5torr) and ultra-high vacuum (10-8torr). Multi-wall carbon nanotubes and graphitic structures are the main products on the SiC surface at 1400-1800°C in 10-5torr. Under ultra-high vacuum, the decomposition rate of SiC is much lower than in high vacuum, indicating that SiC is decomposed by oxidation reaction. Using X-ray photoelectron spectroscopy (XPS), the intensity of the O1s peak at 530.3 eV decreases with increasing take-off angle, indicating that this oxygen species exists on the walls of CNTs. The results show that oxygen with a low pressure not only oxidizes SiC, but also forms a highly thermally stable carbon-oxygen compound, and interacts with the CNTs at high temperatures.

Abstract: A multiple data point version of the industry standard, two data point raster-changing procedure is employed to measure low levels (< 1 x 1017 atoms/cm3) of nitrogen (N) in silicon carbide (SiC) by SIMS (Secondary Ion Mass Spectrometry). A current-changing procedure is also employed. Together, these are used evaluate the assumptions of the standard method, to separate and measure the components of background signal, and to improve upon the precision and accuracy of the standard method. The risk of poor precision in the two-point method is demonstrated, as is the improvement provided by the multiple-point method. Results show that, in addition to the wellknown N memory background, adsorption background can contribute significantly to the N signal. In general, establishing the presence of adsorption gas in this way can be used to warn of the presence of ionization background, which is not measurable per se.

Abstract: The decay kinetics of a persistent photoconductivity (PPC) in undoped semi-insulating 4H SiC and intercenter charge transfer were studied with EPR, photo-EPR and optical admittance spectroscopy (OAS). A thermally activated charge transfer process that occurs in the dark has been observed. The PPC effect was observed directly in changes in the quality factor of the EPR cavity before and after illumination and by the decay of the OAS signal for deep levels, and indirectly by the excitation and decay of the nitrogen and boron EPR lines that were not observed in the dark before illumination. The decay kinetics of the PPC and photo-induced carrier capture by nitrogen and boron levels were found to follow a stretched exponential form. The PPC in the temperature range from 77 to 300K was found to be produced by a thermally induced charge transfer process involving deep trap levels.

Abstract: The high resistivity of SiC required for many device applications is achieved by compensating residual donors or acceptors with vanadium or intrinsic defects. This work addresses the defect levels of substitutional vanadium and the positively charged carbon vacancy (VC +) in semiinsulating (SI) SiC. After reviewing the earlier studies related to both defects, the paper focuses on temperature-dependent Hall measurements and photo-induced electron paramagnetic resonance (EPR) experiments of 4H and 6H SI SiC. In vanadium-doped samples, a V3+/4+ level near Ec-1.1 eV (4H) and Ec-0.85 eV (6H) is estimated by a comparison of dark EPR spectra and the activation energy determined from the Hall data, assuming that vanadium controls the Fermi level. In high purity semiinsulating substrates, analysis of time-dependent and steady-state photo-EPR data suggests that the plus-to-neutral transition of the carbon vacancy involves a structural relaxation of about 0.6 eV.