Strain and Thermal Stability Analysis of Silicon - and Silicon-Germanium Alloys on SILICON(001)

The alloy layers of both silicon-germanium (SiGe) and silicon-germanium-carbon (SiGeC) grown by chemical vapor deposition (CVD) on Si(001) were studied using ion beam analysis, high resolution x-ray diffraction and Fourier transform infrared absorption spectroscopy (FTIR). The results of FTIR qualitatively showed that some of the C in the SiGeC alloys grown by CVD was located on substitutional sites. The amount of substitutional C was quantified using ion channeling at a C resonance energy. Approximately 50-80% of the C was determined to be located on substitutional sites using this method which was in good agreement with the results of FTIR. The strain of binary SiGe alloys was compared to the strain of ternary SiGeC alloys to determine the effect of C on the structure of the alloy. The presence of substitutional C was shown to decrease the misfit strain in SiGeC samples as compared to SiGe samples. For samples below a certain thickness, substitutional C reduced the lattice constant of the SiGeC as compared to the lattice constant of SiGe. For samples above a certain thickness, the lower misfit as a result of substitutional C allowed the thickness of the SiGeC alloy to extend well beyond that of the SiGe alloy without any dislocations being formed at the interface. The critical thickness of CVD grown SiGe alloys was shown to be in reasonable agreement with previously reported critical thicknesses of molecular beam epitaxy (MBE) grown SiGe alloys. It was also shown that the addition of 1 at.% C (approximately 60% of which is substitutional) to Si_{0.80}Ge_ {0.20} increases the critical thickness at least three times that of the critical thickness of a SiGe alloy with the same Si and Ge composition. The experimentally measured values of strain for binary SiGe alloys follow the predictions of macroscopic theory of elasticity quite well when used in conjunction with Vegard's law. The ternary alloy is more difficult due to the nature of C in the alloy. Initially it appears that macroscopic theory of elasticity (MTE) is followed quite well when using Vegard's law for the relaxed lattice constant of the ternary alloy. However, more elaborate theories suggest a much larger compensating effect of the C when it is located at substitutional sites. The results indicate there is a major discrepancy between the theory and the experiment which needs to be resolved. It is concluded that care must be taken in suggesting the validity of Vegard's law only because the strain values for the ternary alloy fall on the calculated values. The stability of the SiGeC alloys was studied for three temperatures. The greater stability of SiGeC compared to SiGe at 850^circC further confirmed the idea that the presence of C reduces the strain energy in the ternary alloy. It was shown that the defect density in the SiGeC samples increased with time for each of the three temperatures studied indicating that the alloys are no longer metastable at these temperatures.