BUFFER CONCEPTS OF ULTRATHIN SIMGEN SUPERLATTICES

Short-period Si(m)Ge(n) strained-layer superlattices (SLS) were grown on (100) silicon substrate by molecular beam epitaxy. The SLSs had 145 repeat periods, each having a length of around 10 monolayers (ML) (total thickness 200 nm), and were grown on a Si1-ybGeyb alloy buffer layer serving as a strain symmetrizing substrate. Different buffer concepts for optimum growth quality and strain adjustment of the SLS were employed, and their influence on the photoluminescence spectrum of a Si6Ge4 SLS was investigated. The most recently grown buffer layer consisted of two parts: the first part had a rather thick alloy layer with a linear increase of the Ge content in the growth direction and was grown at a high temperature (T(g) almost-equal-to 600-degrees-C); the second part had a constant Ge concentration and was also grown at a high temperature (T(g) = 500-600-degrees-C). The shallow increase of the Ge content and the high growth temperature lead to full relaxation of each subsequent layer, with an incremental higher Ge concentration which results in a three to four orders of magnitude lower threading dislocation density in the buffer layer.