Ge/GeSn <i>p-n-i-p</i> heterojunction phototransistor for short-wave infrared applications

In this work, the proposed two-terminal <i>p-n-i-p</i> GeSn heterojunction bipolar phototransistors (HPT) are fabricated using the molecular beam epitaxy technique on a Si substrate with a Ge virtual substrate. The proposed device aims to address the current growth challenges and the low experimental responsivity value of the GeSn HPT in the short-wave infrared (SWIR) bands. To investigate the impact of Sn content on the optical and electrical characteristics of GeSn HPTs, we grow four different samples with Sn concentration varying between 0% and 6.5% for performance comparison. The electrical characterization measurements indicate a reduced dark current density and demonstrate breakdown at voltages exceeding 6 V during forward operation. This observation is favorable, as the bias voltage is significantly lower than required for avalanche photodiodes to enter the breakdown region. The electro-optical characterization indicates a high collector current when exposed to illumination at a wavelength λ = 1310 nm, demonstrating a notable increase in response with the rise in incident optical power, enhancing the optical responsivity. Moreover, as the Sn concentration increases in the i-GeSn active layer, the measured responsivity spectra of the fabricated HPTs exhibit an improvement because of the increased absorption coefficient. Furthermore, increased Sn concentration also leads to a redshift in the cut-off (detection) wavelength. At λ = 1310 nm, the fabricated HPTs (6.5% Sn) achieve an optical responsivity of 4.77 A/W with an optical power of P_opt = 1 mW and V_EC = 4 V, surpassing the performance of recently reported GeSn photodetectors and HPTs, making it the highest responsivity recorded among all reported PDs and HPTs. Therefore, the high-quality grown layer and electro-optical performance, along with the benchmark results, demonstrate the efficacy of our fabricated GeSn HPTs and indicate their potential applications in SWIR bands, including communication and sensing, among others.