Multi-Scale Electronic Structure Analysis of Direct-Gap Group-IV Alloys: Implications for Device Applications

We present a comparative theoretical analysis of the group-IV semiconductor alloys Ge1-x(C,Sn,Pb) x, and quantify the character and evolution of their electronic structure with composition. Our calculations reveal (i) dilute Ge1-xCx admits a quasi-direct band gap, retaining predominantely indirect character, (ii) the indirect- to direct-gap transition in Ge1-x Snx is characterised by strong band mixing, evolving continuously from indirect to direct with increasing x, and (iii) less pronounced band mixing effects characterise Ge1-xPbx, with the indirect- to direct-gap transition occurring for Pb compositions close to where the alloy becomes zero-gap. We describe the consequences of these effects for proposed device applications of group-IV alloys.