Epitaxial PbGeSe thin films and their photoluminescence in the mid-wave infrared

PbSe is a narrow bandgap IV-VI compound semiconductor with application in mid-wave infrared optoelectronics, thermoelectrics, and quantum devices. Alkaline earth or rare earth elements such as Sr and Eu can substitute Pb to widen the bandgap of PbSe in heterostructure devices, but they come with challenges such as deteriorating optical and electronic properties, even in dilute concentrations due to their dissimilar atomic nature. We substitute Pb instead with column-IV Ge and assess the potential of rocksalt phase PbGeSe as a wider bandgap semiconductor in thin films grown by molecular beam epitaxy on GaAs substrates. Low sticking of GeSe adatoms requires synthesis temperatures below 260 {\deg}C to incorporate Ge, but this yields poor structural and compositional uniformity as determined by X-ray diffraction. Consequently, as-grown films in the range Pb0.94Ge0.06Se to Pb0.83Ge0.17Se (6-17% Ge) show much less bandgap widening in photoluminescence than prior work on bulk crystals using absorption. We observe that post-growth rapid thermal annealing at temperatures of 375-450 {\deg}C improves the crystal quality and recovers bandgap widening. Rapid interdiffusion of Ge during annealing, however, remains a challenge in harnessing such PbGeSe materials for compositionally sharp heterostructures. Annealed 17%-Ge films emit light at 3-3.1 um with minimal shift in wavelength versus temperature. These samples are wider in bandgap than PbSe films by 55 meV at room temperature and the widening increases to 160 meV at 80 K, thanks to sharply different dependence of bandgap on temperature in PbSe and PbGeSe.