Improving the Performance of SiGe Metal–Semiconductor–Metal Photodetectors by Using an Amorphous Silicon Passivation Layer

SiGe metal-semiconductor-metal photodetectors (MSM-PDs) with a thin amorphous silicon (a-Si:H) passivation layer have been fabricated by an ultrahigh-vacuum chemical vapor deposition (UHVCVD) system. It was found that the thin (30 nm) a-Si:H passivation layer could effectively suppress the dark current of SiGe MSM-PDs. As compared to the unpassivated devices, the dark current for devices with a-Si:H passivation layers was drastically reduced by 1.7 times 10⁵, and the photo-to-dark current ratio was enhanced by 1.33 times 10⁶. We attribute this result to the passivation effect of a-Si:H films on SiGe surfaces by hydrogen diffusion, which can compensate the dangling bonds on the SiGe surface.

I. Introduction

The requirements of silicon-based photonic devices in the next generation of chip technologies have caused extensive studies on SiGe devices because strained SiGe exhibits several advantages, such as higher electron and hole mobility than Si, longer detection wavelength (up to 1.6 ) for near-infrared optical detection, and the possibility for integration into an existing silicon technology [1], [2]. In the past years, various types of SiGe-based optoelectronic devices [3]–[6] have been proposed. Among these devices, the metal–semiconductor–metal photodetectors (MSM-PDs) exhibit the highest response speed, owing to their ultralow intrinsic capacitance [12], [13]. The fabrication process of MSM-PDs is also compatible with that of a field-effect transistor (FET). Thus, one can easily integrate SiGe MSM-PDs with SiGe FET-based electronics to realize optoelectronic integrated circuits. However, up to now, the reported planar-type MSM-PDs are few, to the best of our knowledge. In planar-type MSM-PDs, both metals (anode and cathode) are on the surface of SiGe, which may contain many surface defect states caused by Ge atoms that were segregated during the growth of SiGe [7]. These surface defect states will play a detrimental role in the generation of a dark current because the current flows through the top SiGe layer in planar-type MSM-PDs.