Papers by Author: Hun Jae Chung

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: Undoped 6H- and 4H-SiC crystals were grown by Halide Chemical Vapor Deposition (HCVD). Concentrations of impurities were measured by various methods including secondary-ion-mass spectrometry (SIMS). With increasing C/Si ratio, nitrogen concentration decreased and boron concentration increased as expected for the site-competition effect. Hall-effect measurements on 6H-SiC crystals showed that with the increase of C/Si ratio from 0.06 to 0.7, the Fermi level was shifted from Ec-0.14 eV (nitrogen donors) to Ev+0.6 eV (B-related deep centers). Crystals grown with C/Si > 0.36 showed high resistivities between 1053 and 1010 4cm at room temperature. The high resistivities are attributed to close values of the nitrogen and boron concentrations and compensation by deep defects present in low densities.

Abstract: We have employed low-temperature photoluminescence to estimate the total residual N concentration in semi-insulating (SI) SiC substrates where all N shallow donors are compensated in the dark. The ratio of the nitrogen-bound exciton line (Qo) to the free excitonic emission (I77) as a function of excitation power density (Pexc) was tracked for several SI 4H-SiC samples with varying residual N concentration (~ 7x1014 – 5.2x1016 cm-3). Most notably, a linear relationship was found between Qo/I77 and [N] for [N] < 1x1016 cm-3 while a sub-linear behavior was observed for samples with higher N levels. This technique should be particularly valuable to map [N] where the levels are close to or below the present SIMS detection limit of ~ 5-7 x 1014 cm-3. Results obtained for a limited number of low n-type and SI 6H-SiC substrates are also presented.

Abstract: Deep electron and hole traps were studied in a series of high purity 6H-SiC single crystals grown by Halide Chemical Vapor Deposition (HCVD) method at various C/Si flow ratios and at temperatures between 2000 oC and 2100 oC. Characterization included Low Temperature Photoluminescence (LTPL), Deep Level Transient Spectroscopy (DLTS), Minority Carrier Transient Spectroscopy (MCTS), and Thermal Admittance Spectroscopy (TAS) measurements. Concentrations of all deep traps were shown to strongly decrease with increased C/Si flow ratio and with increased growth temperature. The results indicate that the majority of deep centers in 6H-SiC crystals grown by HCVD are due to native defects or complexes of native defects promoted by Si-rich growth conditions. The observed growth temperature dependence of residual donor concentration and traps density is explained by increasing the effective C/Si ratio at higher temperatures for the same nominal ratio of C and Si flows.

Abstract: Growth rates and relative stability of 6H- and 4H-SiC have been studied as a function of growth conditions during Halide Chemical Vapor Deposition (HCVD) process using silicon tetrachloride, propane and hydrogen as reactants. The growth temperature ranged from 2000 to 2150 oC. Silicon carbide crystals were deposited at growth rates in the 100-300 μm/hr range in both silicon- and carbon-supply limited regimes by adjusting flows of all three reactants. High resolution x-ray diffraction measurements show that the growth on Si-face of 6H- and C-face of 4H-SiC substrates resulted in single crystal 6H- and 4H-SiC polytype, respectively. The growth rate results have been interpreted using thermodynamic equilibrium calculations.

Abstract: A novel approach to the high growth rate Chemical Vapor Deposition of SiC is described. The Halide Chemical Vapor Deposition (HCVD) method uses SiCl4, C3H8 (or CH4), and hydrogen as reactants. The use of halogenated Si source and of separate injection of Si and C precursors allows for preheating of source gases without causing premature chemical reactions. The stoichiometry of HCVD crystals can be controlled by changing the C/Si flow ratio and can be kept constant throughout growth, in contrast to the Physical Vapor Transport technique. HCVD was demonstrated to deposit high crystalline quality, very high purity 4H- and 6H-SiC crystals with growth rates comparable to other bulk SiC growth techniques. The densities of deep electron and hole traps are determined by growth temperature and C/Si ratio and can be as low as that found in standard silane-based CVD epitaxy. At high C/Si flow ratio, the resistivity of HCVD crystals exceeds 105
_cm. These characteristics make HCVD an attractive method to grow SiC for applications in high-frequency and/or high voltage devices.