Papers by Author: Sung Wook Huh

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: 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: Carrier lifetimes and the dominant electron and hole traps were investigated in a set of thick (9-104mm) undoped 4H-SiC epitaxial layers grown by CVD homoepitaxy. Deep trap spectra were measured by deep level transient spectroscopy (DLTS) with electrical or optical injection, while lifetimes were measured by room temperature time-resolved photoluminescence (PL). The main electron traps detected in all samples were due to Ti, Z1/Z2 centers, and EH6/EH7 centers. Two boron-related hole traps were observed with activation energies of 0.3 eV (boron acceptors) and 0.6 eV (boron-related D centers). The concentration of electron traps decreased with increasing layer thickness and increased toward the edge of the wafers. PL lifetimes were in the 400 ns-1800 ns range with varying injection and generally correlated with changes in the density of Z1/Z2 and to a lesser extent the EH6/EH7 electron traps. However, the results of DLTS measurements on p-i-n diode structures suggest that the capture of injected holes is much more efficient for the Z1/Z2 traps compared to the EH6/EH7 centers making the Z1/Z2 more probable candidates for the role of lifetime killers. A good fit of the thickness dependence of the measured lifetimes to the usual analytical form was obtained assuming that Z1/Z2 is the dominant hole recombination center and that the surface recombination velocity was 2500 cm/sec.

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.