Structural stability of short-period Si/Ge superlattices studied with Raman spectroscopy

Annealing effects on strain-symmetrized Si/Ge superlattices with period lengths of 6 to 24 monolayers are studied by Raman spectroscopy in the temperature range of 430°C to 780°C. Using a simple microscopic diffusion model, the evolution of the concentration profiles in the growth direction is calculated in order to understand the energy shift of the optical phonons and the intensity decay of the folded acoustic phonons. The analysis of the latter provides values of the interdiffusion coefficient D*. The strong dependence of the tracer-diffusion constants on the Si concentration causes different intermixing behavior of the Si and Ge layers, which results in a dependence of the interdiffusion coefficient on the degree of homogenization. A variation of D* with the average Si content and the superlattice period is obtained and is compared to the standard interdiffusion analysis.