Residual thermal stress in wide band gap semiconductor thin films

An increasing interest has been shown lately in using wide energy band gap semiconductors for electronic and opto-electronic devices constructed by semiconductor heterojunctions, particularly GaN and SiC. To improve the electrical and optical properties of those semiconductors for high performance, films of high-quality and high-purity crystals should be produced. As a result, it is imperative to perform a true characterization of the properties at semiconductor heterostructures. While epitaxial growth of such films is a popular method of growing GaN and SiC semiconductors, the lattice mismatch and thermal expansion mismatch that exist between the epilayers and the underlying substrates result in crystalline misorientation, surface roughness, and wafer bending and cracking. This is due to accumulation of residual thermal stress during cooling from growth temperature to room temperature owing to the large difference in the thermal expansion coefficients between the epilayer and the substrate. The lattice mismatch further contributes to the residual stress. In this dissertation, three different methods to calculate residual thermal stress in the epitaxial layers, the model of Olsen and Ettenberg, Timoshenko, and Reinhart are reported. Detailed calculations of misfits at growth temperature and room temperature between GaN and its prospective substrates were performed. Properties and residual thermal stress in 1-micrometer epitaxial GaN film grown on other III-V compounds and substrates were also worked and tabulated using the method of Olsen. By employing the same method, the residual stress was calculated in 3-epitaxial layer structure of GaN and 3C-SiC grown on Al₂O₃ with a buffer layer of AlN used to alleviate the induced thermal stress. Residual thermal stress in SiC thin film grown on 6H-SiC substrate was obtained by X-ray diffraction method and compared with the theoretical values worked by the method of Olsen. In the calculations of theoretical residual thermal stress in thin films the standard method of employing the average CTE was not used and the step method, by which the CTE values employed at intervals of temperatures, was used instead. The final residual stress is the sum of stresses at different intervals from growth temperature to room temperature. By adopting the step method more accurate results were obtained. Comparison between standard and adopted step method is included. To further investigate the validity of the step method a comparison was made between the experimental values of residual stress in the 3C-SiC thin films of the 3C-SiC/Si system that was published previously and the experimental values of the 3C-SiC/6H-SiC system obtained in this research with the theoretical values obtained by the step method and found it to agree with the step method values.