Sung Wook Huh

Lifetime-limiting defects in n− 4H-SiC epilayers

Low-injection minority carrier lifetimes (MCLs) and deep trap spectra have been investigated in n− 4H-SiC epilayers of varying layer thicknesses, in order to enable the separation of bulk lifetimes from surface recombination effects. From the linear dependence of the inverse bulk MCL on the concentration of Z1∕Z2 defects and from the behavior of the deep trap spectra in 4H-SiC p-i-n diodes under forward bias, we conclude that it is Z1∕Z2 alone that controls the MCL in this material.

Bulk growth of high-purity 6H-SiC single crystals by halide chemical-vapor deposition

High-purity 6H-SiC single crystals were grown by the halide chemical-vapor deposition process. Growth was performed in a vertical hot-wall reactor with a separate injection of a silicon precursor (silicon tetrachloride) and a carbon precursor (propane). Typical growth rates were between 100 and 300μm∕h. The crystals contain very low concentrations of residual impurities. The main contaminants, namely, nitrogen and boron, are in the 1014atomscm−3 range. Crystals grown under Si-rich conditions were n type with low room temperature electron concentrations in the 1014–1015atomscm3 range and with room-temperature electron mobilities approaching 400cm2∕Vs. The resistivity of the material increased up to 1010Ωcm with increasing C∕Si ratio. Deep levels spectra show that the electron traps density decreases with increasing C∕Si ratio.

Growth of Bulk SiC by Halide Chemical Vapor Deposition

A novel halide chemical vapor deposition (HCVD) process has been developed for bulk growth of high purity, single crystal 6H SiC. The effects of major process parameters including furnace temperature over the range of 1900-2150°C, reactor pressure over the range of 20-400 torr, reactant concentrations, and flow rates on the growth rate, crystallinity, and electrical properties of the single-crystal 6H boules grown by HCVD are described. Typical growth rates for the 6H polytype are on the order of 100-125 μm/h with a maximum observed rate of 180 μm/h. Thicknesses up to 1 mm have been demonstrated. GDMS analyses of the purity of HCVD grown material is discussed and compared to 6H SiC produced by commercial PVT and HTCVD processes. Boron and aluminum concentrations less than 1.8 E 15 atoms/cm 3 were demonstrated. Introduction The HCVD process was developed for growth of bulk, high purity, 6H SiC. This process has significant advantages over conventional physical vapor transport (PVT) processes [1] for manufacturing semi-insulating SiC. Foremost is the ability to maintain a constant gas phase chemistry at the growth surface. During PVT growth the solid source material sublimes incongruently leading to variations in the Si/C ratio during the growth process. In addition, impurities in the source material and furnace components evaporate at different rates resulting in a transient flux of impurities to the growth surface. Variations in the gas phase chemistry lead to variations in electrical properties along the length of the boule. CVD-based processes provide a means for carefully controlling the chemical composition of the gas phase and the growing crystal over time. This is accomplished by using high purity source gases and through independent control of the Si and C precursor flow rates. For example, in the HTCVD process [2] SiH4 and C3H8 are mixed and reacted at temperatures above 2000°C to grow crystals with very low impurity concentrations and high electrical resistivity. The highly reactive precursors used in this process can result in deposition of SiC in the gas inlet and outlet ports if gas flows and thermal gradients are not optimized. This can lead to reduced process times and shorter boules. The geometry of the HCVD reactor and the use of thermally stable precursors results in significant reductions of parasitic deposits in the gas inlet and outlet ports. Use of semiconductor grade precursors and dilution of contaminants from the furnace by the carrier gases results in the growth of high purity material.

Minority carrier diffusion length measurements in 6H–SiC

Minority carrier diffusion lengths were measured as a function of temperature and position along the growth axis of lightly nitrogen doped boules of 6H–SiC grown by the physical vapor transport technique. It is shown that the diffusion lengths increase from 1to2microns in the seed portion of the boule to about 4microns in the tail portion of the boules. Deep levels transient spectroscopy measurements revealed the presence of deep electron traps with the activation energies of 0.35eV, 0.5eV, 0.65eV, and 1eV. The densities of all these traps decrease when moving from seed to tail of the boules. A good correlation between the change of the lifetime values and the density of the 0.65eV and 1eV electron traps was observed. The measured lifetimes show an increase with temperature following a power law that suggests that the hole capture could be determined by cascade capture process.

Residual impurities and native defects in 6H‐SiC bulk crystals grown by halide chemical-vapor deposition

A variety of defect-sensitive techniques have been employed to detect, identify, and quantify the residual impurities and native defects in high-purity (undoped) 6H‐SiC crystals grown by halide chemical-vapor deposition technique. The incorporation efficiencies of N and B are determined by the site-competition effect. Most notably, material with low residual N levels (∼1014cm−3) can be produced. In addition, the nitrogen concentrations obtained from Hall-effect measurements and low-temperature photoluminescence are systematically lower than those determined from secondary-ion-mass spectrometry. The difference is ascribed to nitrogen forming complexes with native defects. The energy level of this complex is approximately 0.27eV below the conduction band. Four major electron traps with activation energies of 0.4, 0.5, 0.65, and 1eV and five hole traps with activation energies of 0.3, 0.4, 0.55, 0.65, and 0.85eV were observed by deep-level transient spectroscopy. The concentration of all traps decreased stro...

Halide-CVD Growth of Bulk SiC Crystals

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.

Doping-induced strain and relaxation of Al-doped 4H-SiC homoepitaxial layers

Aluminum-doped 4H-SiC epilayers with Al concentrations in the 7.4×1018–3.8×1020cm−3 range were deposited on off-orientation (0001) wafers by chemical vapor deposition method and analyzed using high-resolution x-ray diffraction, transmission electron microscopy, and KOH etching. Reciprocal space maps of (0008) reflection revealed two distinct peaks originating from the substrate and doped epilayer. For Al concentration below 3.3×1020cm−3, 10μm thick layers were fully strained with the a-lattice parameter of the layer matching that of the substrate. The equilibrium c-lattice parameter change versus doping was determined to be 1.3±0.3×10−24cm3. The basal planes of the epilayers were tilted in respect to the substrate in the direction of the offcut with the tilt magnitude proportional to the doping concentration. The 10μm thick layers with Al concentration above 3.3×1020cm−3 underwent partial relaxation. The a-lattice parameter of the epilayer was higher than that of the substrate, the width of ω and 2θ scans...

Deep Traps and Charge Carrier Lifetimes in 4H-SiC Epilayers

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.

Relationship between the EPR Si-5 signal and the 0.65 eV electron trap in 4H-and 6H-SiC polytypes

We used electron paramagnetic resonance (EPR) and deep-level transient spectroscopy (DLTS) to quantitatively compare the concentrations of the EPR signal originally known as SI-5 and the commonly observed DLTS signal at E c -0.65 eV in bulk and epitaxial 4H- and 6H-SiC.

Growth Kinetics and Polytype Stability in Halide Chemical Vapor Deposition of SiC

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.