Hun Jae Chung

Structural instability of 4H–SiC polytype induced by n-type doping

Spontaneous formation of stacking faults in heavily nitrogen-doped 4H-polytype silicon carbide crystals have been observed by transmission electron microscopy (TEM). Faults were present in as-grown boules and additional faults were generated by annealing in argon at 1150 °C. All faults had identical structure consisting of six layers stacked in a cubic sequence as determined by high-resolution TEM, and were interpreted as a result of two Shockley partial dislocations gliding on two neighboring basal planes of SiC. It is argued that the energy of faulted 4H silicon carbide is lower than the energy of perfect heavily doped (n>1×1019 cm−3) crystal at typical processing temperatures, thus providing a driving force for transformation.

Spontaneous formation of stacking faults in highly doped 4H–SiC during annealing

4H–SiC samples doped with nitrogen at ∼3×1019 cm−3 were annealed in Ar for 90 min at 1150 °C. Transmission electron microscopy revealed stacking faults at a density of approximately 80 μm−1 where faults were not found to exist prior to annealing. All faults examined were double layer Shockley faults formed by shear on two neighboring basal planes. The structural transformation was interpreted as due to quantum well action, a mechanism where electrons in highly n-type 4H–SiC enter stacking fault-induced quantum well states to lower the system energy. The net energy gain was calculated as a function of temperature and nitrogen doping concentration through solution of the charge neutrality equation. Calculations showed that doping levels in excess of ∼3×1019 cm−3 should result in double layer stacking faults forming spontaneously at device processing temperatures, in agreement with our observations. Single layer faults are not expected to be stable in 4H–SiC at concentrations below 1×1020 cm−3, but are expec...

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.

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...

Stacking fault formation in highly doped 4H-SiC epilayers during annealing

Spontaneous stacking fault formation during annealing in n(+) 4H-SiC epilayers deposited on the n(-) 4H-SiC substrates has been analyzed by conventional and high-resolution transmission electron mi ...

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.