Theory of Optical Transitions in Si sub N/Ge sub N(001) Strained Layer Superlattices

Systematic trends for the minimum gap and low-lying direct optical transitions for superlattices alternating 2, 4 or 6 atomic layers of Si and Ge grown on Si(001) and Ge(001) substrates are calculated using the local density functional approach. Results of a many-body self energy calculation for the optical transition energies of the Si sub 4 Ge sub 4/Si (001) case are compared to photocurrent and electroreflectance data. Emphasis is placed on the narrow type II heterostructure formed by the Si sub 4 Ge sub 4 region between the Si(001) substrate and cap layer.