Floating-base germanium-tin heterojunction phototransistor for high-efficiency photodetection in short-wave infrared range.

The floating-base germanium-tin (Ge1-xSnx) heterojunction phototransistor (HPT) is designed and investigated as an efficient optical receiver in the short-wave infrared range. Simulations indicate that as the Sn content increases, the responsivity significantly increases due to a higher absorption coefficient and a larger valence band offset between Ge and Ge1-xSnx. Ge0.935Sn0.065 HPTs that incorporated high-quality Ge0.935Sn0.065 film grown by molecular beam epitaxy were fabricated, demonstrating optical response beyond wavelength of 2003 nm. At a low bias voltage of 1.0 V, optical response enhancement of ~10 times was achieved over the conventional Ge0.935Sn0.065 p-i-n photodiode. High responsivities of ~1.8 A/W at 1550 nm and ~0.043 A/W at 2003 nm were demonstrated with low dark current density of 0.147 A/cm2.