Ultrahigh Sensitive Phototransistor Based on MoSe2/Ge Mixed-Dimensional Heterojunction for Visible to Short-Wave Infrared Broadband Photodetection

Germanium (Ge) photodetectors (PDs) have attracted much attention for their possible application in integrated photonic circuits, which have been incorporated in Si device process line. However, the performance of Ge PD is compromised due to its indirect band nature and high surface state density. In this work, a vertical-stacked n-MoSe2/p-Ge/n-Ge mixed-dimensional van der Waals (vdW) heterojunction phototransistor (HPT) is proposed and demonstrated for broadband photodetection. The p-Ge/n-Ge base–collector junction separates the photogenerated electron–hole pairs through its built-in potential to form the primary photocurrent, whereas the n-MoSe2/p-Ge emitter–base heterojunction provides large electron/hole injection ratio due to the large bandgap offset of about 0.44 eV, resulting in a considerable photocurrent gain. A broadband photoresponse spectrum from 450 to 1600 nm is realized with the maximum responsivity of 18.53 A/W at 1550 nm for the two-terminal phototransistor under 2 V bias voltage. The minimum specific detectivity of larger than $2.62\times 10^{{9}}$ Jones with fast response time of less than $590~\mu \text{s}$ is obtained for the device operating in the range of 650–1550 nm. Our work here provides a feasible strategy for developing high-performance Ge-based PDs.