Photoconductivity of germanium tin alloys grown by molecular beam epitaxy

Photocurrent spectroscopy was used to measure the infrared absorption of germanium-tin alloys grown by molecular beam epitaxy. To study dependence on Sn composition, the photocurrent was measured at 100 K on alloys of Ge1−xSnx with atomic percentages of Sn up to 9.8%. The optical absorption coefficient was calculated from the photocurrent, and it was found that the absorption edge and extracted bandgap energy decreased with increasing Sn content. For all Ge1−xSnx samples, a fundamental bandgap below that of bulk Ge was observed, and a bandgap energy as low as 0.624 eV was found for a Sn percentage of 9.8% at 100 K.