New experimental evidence for nature of the band gap of GeSn alloys

To harness the advanced fabrication capabilities and high yields of the electronics industry for photonics, monolithic growth and CMOS compatibility are required. One promising candidate which fulfils these conditions is GeSn. Introducing Sn lowers the energy of the direct G valley relative to the indirect L valley. The movement of the conduction band valleys with Sn concentration is critical for the design of efficient devices; however, a large discrepancy exists in the literature for the Sn concentration at which GeSn becomes a direct band gap. We investigate the bandgap character of GeSn using hydrostatic pressure which reversibility modifies the bandstructure. In this work we determine the movement of the band-edge under pressure using photocurrent measurements. For a pure Ge sample, the movement of the band-edge is dominated by the indirect L valley with a measured pressure coefficient of 4.26 +/- 0.05 meV/kbar. With increasing Sn concentration there is evidence of band mixing effects, with values of 9.4 +/- 0.3 meV/kbar and 11.1 +/- 0.2 meV/kbar measured for 6% and 8% Sn samples. For a 10% Sn sample the pressure coefficient of 13 +/- 0.5 meV/kbar is close to the movement of the direct bandgap of Ge, indicating predominately direct G-like character for this GeSn alloy. This further suggests a gradual transition from indirect to direct like behaviour in the alloy as also evidenced from theoretical calculations. The implications of this in terms of optimising device performance will be discussed in further detail at the conference.