Papers by Author: Mary Ellen Zvanut

Abstract: Low temperature infrared photoluminescence (PL) performed on a large set of bulk SiC substrates has revealed distinct series of lines between 0.8 and 1.5 eV for samples with nitrogen levels between ~ 1016 and 1017 cm-3. Semi-insulating and intentionally N-doped wafers grown by PVT and HTCVD were investigated. Two groups of PL lines clustered near 1.0 and 1.35 eV, respectively, were observed in n-type 4H-SiC. Not surprisingly, a multiplicity of features at slightly different energy positions was found for this emission from the 6H- and 15R-SiC polytypes. Both sets of lines were not observed for substrates with N doping concentrations greater than 3x1017cm-3. Thus, it appears this IR emission can serve as optical “fingerprints” of bulk n-type substrate with moderate levels of N impurities. Models for the possible origins of these lines will also be discussed.

Abstract: The understanding of the structure and associated defect level of point defects in SiC is important because the material is to be used both as a semiconductor and semi-insulator. Production of the latter is achieved by compensation of unavoidable impurities using defects that require more energy for ionization than the unintentional donors or acceptors. The purpose of the present work is to measure the defect energy level of one center in high resistivity 4H SiC using photo-induced electron paramagnetic resonance (photo-EPR). The center is identified as SI-5, an EPR signal that others have attributed to the negative charge state of the carbon vacancy-carbon antisite pair, − C Si V C . Samples containing this defect exhibit two different photo thresholds, which depend on the resistivity activation energy, Ea. For samples with Ea less than 0.8 eV, a photothreshold at 0.75+/- 0.05 eV is observed, but for those with Ea greater than 0.8 eV, the threshold is between 2 and 2.5 eV. Previous work focused on the former case. Here, the SiC substrates with the larger Ea are emphasized, showing that the photo-threshold likely measures the neutral to negative defect level, − / 0 C Si V C .

Abstract: Halide chemical vapor deposition (HCVD) allows for rapid growth while maintaining the purity afforded by a CVD process. While several shallow and deep defect levels have been identified in 6H HCVD substrates using electrical techniques, here we examine several different point defects found in 4H n-type HCVD SiC using electron paramagnetic resonance (EPR) spectroscopy. One spectrum, which exhibits axial symmetry and broadens upon heating, may represent a collection of shallow defects. The other prominent defect has the g tensor of the negatively charged carbon vacancy, but additional hyperfine lines suggest a more complex center. The role of these defects is not yet determined, but we note that the concentrations are similar to those found for the electrically detected defect levels, making them a reasonable source of electrically active centers.

Abstract: The purpose of this study is to determine the vanadium defect levels in semi-insulating 4H-SiC and 6H-SiC using optical admittance spectroscopy (OAS). OAS data show several distinct peaks for the vanadium-doped SI 4H-SiC and 6H-SiC. Comparison of the data for the two polytypes suggests that peaks at 0.67 ± 0.02 eV and 0.70 ± 0.02 eV in 6H substrates and 0.75 ± 0.02 eV in 4H substrates are related to V3+/4+ levels at the cubic sites. A peak at 0.87 ± 0.02 eV in the 6H sample is assigned to the same defect level at the hexagonal site and the associated transition in 4H was observed at 0.94 ± 0.02 eV in our spectra. The donor levels are thought to be related to peaks at 1.94 ± 0.05 eV and 1.87 ± 0.05 eV in 4H and 6H samples, respectively. The differences between the values obtained from the optical admittance measurements and those reported in the literature are attributed to thermal relaxation and/or contributions from defect complexes.

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.