High responsivity and fast speed n-MoSe₂/p-GeSn/n-GOI phototransistor with a composition-graded GeSn base for short-wave infrared photodetection

In this work, a short-wave infrared (SWIR) n-MoSe₂/p-GeSn/n-germanium-on-insulator (GOI) heterojunction phototransistor (HPT) with Sn composition-graded GeSn base is proposed for improvement of overall performance at low cost. The Sn composition-graded GeSn base layers are grown using magnetron sputtering epitaxy technique for improvement of crystal quality with a high Sn content of 15.2% in the top layer, rendering the extension of the cutoff wavelength beyond 2400 nm and significant suppression of dark current. The enormous electron/hole injection ratio, resulting from the large bandgap offset between the MoSe₂ emitter and the GeSn base, enables the harvesting of a high photocurrent gain of HPT. By optimizing the device parameters, a considerable responsivity of 23.79 A/W and an excellent specific detectivity of 8.24 × 10¹⁰ Jones at the peak wavelength of 2030 nm were achieved for the HPT with the dark current density of 261 mA/cm² under the emitter-collector bias voltage of 1.0 V at room temperature. The fast response speed is obtained for the HPT in terms of rising/falling times of 2.8 μs/9.3 μs at 1550 nm, surpassing those of most van der Waals (vdW) junction-based devices. Those results demonstrate that GeSn HPTs are suitable for SWIR optoelectronic imaging and microwave photonics applications.