Comparison study of temperature dependent direct/indirect bandgap emissions of Ge1-x-ySixSny and Ge1-ySny grown on Ge buffered Si

Abstract

Temperature-dependent photoluminescence (PL) of two sets of ternary samples with fixed tin concentrations of ~5.2% (Ge0.924Si0.024Sn0.052, and Ge0.911Si0.036Sn0.053) and ~7.3% (Ge0.900Si0.027Sn0.073, and Ge0.888Si0.04Sn0.072) were measured along with their binary counterparts (Ge0.948Sn0.052 and Ge0.925Sn0.075). The variations of direct bandgap emission (ED) and indirect bandgap emission (EID) with temperature were studied for both ternary and binary alloys by means of Gaussian curve fitting, and the results are compared. The bandgap widths of ternaries clearly increase after Si incorporation into the GeSn with similar Sn concentrations. It is found that for the ternaries both ED and EID peak energies are blue shifted, and the energy separation of ED and EID peaks becomes larger than that of binaries for similar Sn concentrations. Moreover, both ED and EID peaks appear at room temperature (RT) in the GeSiSn spectra, but the ED peak position is greater than EID, indicating these ternaries are indirect bandgap materials. Low temperature PL validates the existence of indirect PL emission in Ge0.90Si0.027Sn0.073 and direct gap behavior in Ge0.925Sn0.075, indicating GeSn becomes a direct bandgap material at lower Sn concentration than GeSiSn. The PL intensities of these ternaries are generally weaker and the spectra become more complicated than those of binaries, probably due to increased strain and defects in the ternaries. Finally, it is found that the effect of large differences in strain of ternary samples on PL peak positions can be greater than that of small Si composition differences in ternaries. A large compressive strain in ternaries can also make splitting of the ED into ED,HH (conduction band minimum-Γ valley to heavy hole maximum) and ED,LH (conduction band minimum-Γ valley to light hole maximum) transitions more observable in the PL spectra.