Optical properties of Ge-rich G e1-x S IX alloys: Compositional dependence of the lowest direct and indirect gaps

Ge-rich Ge1-xSix alloys have been investigated using spectroscopic ellipsometry and photoluminescence at room temperature. Special emphasis was placed on the compositional dependence of the lowest-energy interband transitions. For x≤0.05, a compositional range of particular interest for modern applications, we find E0=0.799(1)+3.214(45)x+0.080(44)x2 (in eV) for the lowest direct gap. The compositional dependence of the indirect gap is obtained from photoluminescence as Eind=0.659(4)+1.18(17)x (in eV). We find no significant discrepancies between these results and the extrapolations from measurements at higher Si concentrations. Such discrepancies had been suggested by recent work on Ge1-xSix films on Si. Accurate knowledge of the interband transition energies is an important requirement for the design of devices incorporating Ge-rich Ge1-xSix alloys and for the understanding of more complex systems, such as ternary Ge1-x-ySixSny alloys, in terms of its binary constituents.