Comparative analysis of electronic structure evolution in Ge1-xSnx and Ge1−xPbx alloys

We present a comparative analysis of electronic structure evolution and the indirect- to direct-gap transition in Ge1−x(Sn,Pb)x group-IV semiconductor alloys. We present first principles disordered alloy electronic structure calculations for Ge1−x(Sn,Pb)x special quasi-random structures, and find a significantly larger band gap reduction in Ge1−xPbx than in Ge1−xSnx at fixed x. We calculate that Ge1−xPbx becomes a direct gap semiconductor close to the composition (x ≈ 7%) at which it also becomes a zero-gap semiconductor. The indirect-to direct-gap transition in Ge1−xSnx occurs over an extended composition range, also centered around 7%, and is driven by Sn-induced mixing of Ge Γ and L conduction states. The pronounced band mixing effects present in Ge1−xSnx alloys will have significant implications for optical and transport properties.