High Photo-to-Dark-Current Ratio in SiGe/Si Schottky-Barrier Photodetectors by Using an a-Si:H Cap Layer

SiGe/Si heterojunction infrared Schottky-barrier photodetectors (PDs) with different thicknesses (30-60 nm) of amorphous silicon (a-Si:H) cap layers were successfully fabricated. The SiGe and a-Si:H layers were deposited by using ultrahigh-vacuum chemical vapor deposition and plasma-enhanced chemical vapor deposition systems, respectively. In the fabricated PDs, it was observed that the a-Si:H cap layer can effectively suppress a dark current and that the thicker a-Si:H cap can obtain lower dark current. The lowest dark current was observed in the 60-nm a-Si:H capped PDs. When compared with a capless a-Si:H device, the dark current in the 60-nm a-Si:H capped PD was reduced by a magnitude of 317 at 4-V reverse-bias voltage. However, by increasing the a-Si:H thickness, the photocurrent was reduced. Therefore, a compromise in a-Si:H thickness should be chosen. We discovered that the photo-to-dark-current ratio was enhanced by a factor of 850 for 30-nm a-Si:H capped PDs as compared to that of capless a-Si:H devices. Thus, a PD with a higher noise-rejection ability was achieved by merely inserting a thin a-Si:H layer. Possible mechanisms are discussed in detail by the use of spectrum and band diagrams.