Thermodynamic equilibrium limitations on the growth of SiC by halide chemical vapor deposition

Abstract

Single crystal SiC for semiconductor applications is commonly produced by physical vapor transport (PVT). Incongruent sublimation of SiC causes the gas phase composition in the PVT growth cell to drift from Si-rich to C-rich as growth proceeds. The change in C/Si ratio in the gas phase causes significant variations in deep center and dopant concentrations along the growth axis of the crystal. Growth of SiC by halide chemical vapor deposition (HCVD) provides direct control over the C/Si ratio by independently metering C and Si precursor gases to the growth environment. Thermally stable Si sources, such as SiCl4, are used instead of SiH4 to eliminate premature decomposition of the Si source. Use of chlorinated precursors, combined with the high precursor concentrations required for growth rates of 50–250 μm/h, impose thermodynamic limits on the maximum C/Si ratio that can be used for deposition of single crystal SiC. A thermodynamic model is provided for predicting the boundary between deposition of SiC and deposition of a mixture of C and SiC. The predicted location of the boundary between SiC and SiC+C, its abrupt nature, and the expected trends in growth rate with precursor flow rates, agree well with the experimental data. Increasing the H2 concentration was predicted to increase the C/Si ratio at which growth of single crystal SiC could be maintained by increasing the equilibrium concentration of C in the gas phase. This was verified experimentally by observing the transition from polycrystalline mixed phase deposits at low H2 concentrations to growth of single crystal SiC at higher H2 concentrations while maintaining constant Si and C precursor input flow rates and concentrations. The transition was characterized using a combination of x-ray diffraction and observations of the growth morphology by optical microscopy.