Materials Science Forum Vols. 600-603

Abstract: An orientation relationship between the cubic 3C and nH hexagonal structures was developed to identify low surface energy, close packed, conducive to growth hexagonal semi-polar planes. This was done to identify planes on which micropipe free SiC crystals, and/or hetero-epitaxial AlGaN structures with a reduced piezoelectric field and a smaller lattice mismatch could be grown. The nH (33 0 2n)H and (33 0 n )H hexagonal planes, which correspond to the (100)C and (111)C cubic planes are identified, and equations are developed to determine their planar structure and packing density to assess whether there is a reasonable chance that high quality material can be grown on these surfaces and whether it is likely that stacking faults will be formed. It is shown that stacking faults will likely form in SiC crystals; it is also shown that good epitaxy of AlGaN films should be possible on these planes although it is likely stacking faults will be formed.

Abstract: We identified regions with low Schottky barrier height on 4H-SiC surfaces by the electrochemical deposition of ZnO. When we adopt an appropriate deposition voltage, ZnO grew preferentially at the regions with the low Schottky barrier height. Thus, we were able to identify the ZnO film only at these regions if we stopped the deposition at a proper time. We compared positions of the deposited film and etch pit after molten NaOH etching. As a result, in a bulk 4H-SiC, the films were deposited around some of micropipe positions. On the other hand, in an epitaxial 4H-SiC layer, although approximately a half of deposited films seemed to grow at the etch-pit defect positions, other deposited films were grown at positions without etch-pit defects. Therefore the Schottky barrier heights were reduced by not only defects emerging as etch pits but also other kind of origins in epitaxial 4H-SiC.

Abstract: We report an experimental investigation of the pits formed at oxidation. The pits were formed by a long time oxidation in a dry or wet atmosphere at high temperature (~ 1200 oC). Although they were observed in (0001) face, they were not in (000-1) face. Comparing the points of oxidation pits to those of molten KOH etching pits on the same area of the substrate, we show that the oxidation pits are formed by oxidizing defects. In addition, the influence of the oxidation pits to the reliability of thermal oxide was researched.

Abstract: High-purity, semi-insulating 6H-SiC substrates grown by high-temperature chemical vapor deposition were studied by electron paramagnetic resonance (EPR). The carbon vacancy (VC), the carbon vacancy-antisite pair (VCCSi) and the divacancy (VCVSi) were found to be prominent defects. The (+|0) level of VC in 6H-SiC is estimated by photoexcitation EPR (photo-EPR) to be at ~ 1.47 eV above the valence band. The thermal activation energies as determined from the temperature dependence of the resistivity, Ea~0.6-0.7 eV and ~1.0-1.2 eV, were observed for two sets of samples and were suggested to be related to acceptor levels of VC, VCCSi and VCVSi. The annealing behavior of the intrinsic defects and the stability of the SI properties were studied up to 1600°C.

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: We have recently explored the nature and stability of native defects in high-purity semi-insulating 4H-SiC bulk substrates grown by PVT and HTCVD methods after post-growth anneal treatments up to 2400oC using electron paramagnetic resonance (EPR) and low-temperature photoluminescence (PL) experiments. In the present study we have extended these investigations to SI 4H-SiC subjected to the same post-growth high-temperature anneal treatments, where significantly enhanced carrier lifetimes have been reported for such conditions, but cooled at different rates ranging from ~2-25oC/min. Previously, the intensities of the native defects decreased monotonically with anneals from 1200–1800oC; however, it was recently observed that several of these defects reappear after annealing at 2100oC and above. Our results illustrate the effects of the post-growth anneal treatments and cool-down rates on the concentrations of native defects.

Abstract: In order to characterize traps in semi-insulating 4H-SiC that is regarded as an attractive semiconductor for X-ray detectors, we apply discharge current transient spectroscopy (DCTS) that is a graphical peak analysis method based on the transient reverse current of a diode. We have found at least three types of traps whose emission rates at 373 K are 4.9×10-3, 8.3×10-3 and 8.0×10-2 s-1. Since it is difficult to characterize traps in semi-insulating semiconductors by transient capacitance methods, it is demonstrated that DCTS is a powerful method for determining the densities and emission rates of traps in semi-insulating semiconductors.

Abstract: The photoluminescence (PL) of the UD-4 defect is observed in semi-insulating bulk 4H, 6H and 15R SiC. In 4H and 6H SiC the UD-4 defect consists of two families of no-phonon (NP) lines, Ua and Ub, and in 15R SiC it consists of three families, Ua, Ub and U15R. The Ua family in 4H, 6H and 15R all show similar temperature behavior with higher energy NP lines becomming observable at higher temperatures. In the case of the Ub and U15R families, a luminescence line with lower energy than the prominent luminescence line appears at higher temperatures. The polarization and Zeeman measurements suggest that the defect has C3v symmetry.

Abstract: Semi-insulating (SI) 4H-SiC substrates doped with vanadium (V) in the range 5.5×1015 –1.1×1017 cm–3 were studied by electron paramagnetic resonance. We show that only in heavily V-doped 4H-SiC vanadium is responsible for the SI behavior, whereas in moderate V-doped substrates with the V concentration comparable or slightly higher than that of the shallow N donor or B acceptor, the SI properties are thermally unstable and determined by intrinsic defects. The results show that the commonly observed thermal activation energy Ea~1.1 eV in V-doped 4H-SiC, which was previously assigned to the single acceptor V4+/3+ level, may be related to deep levels of the carbon vacancy. Carrier compensation processes involving deep levels of V and intrinsic defects are discussed and possible thermal activation energies are suggested.

Abstract: The novel technique microwave detected photo induced current transient spectroscopy (MD-PICTS) was applied to semi-insulating 6H-SiC in order to investigate the properties of inherent defect levels. Defect spectra can be obtained in the similar way to conventional PICTS and DLTS. However, there is no need for contacting the samples, which allows for non-destructive and spatially resolved electrical characterization. This work is focused on the investigation of semi-insulating 6H-SiC grown under different C/Si-ratios. In the corresponding MD-PICTS spectra several shallow defect levels appear in the low temperature range. However the peak assignment needs further investigation. Additionally different trap reemission dynamics are obtained for higher temperatures, which are supposed to be due to different compensation effects.