Role of a-Si in enhancing the current gain of Si-based MSM photodetector using conventional method

Metal-Semiconductor-Metal Photodetector (MSM-PD) is a well known structure for its higher response speed due to its ultralow capacitance per area compare to other type of photodetectors. It is also compatible with FET technology and is a potential candidate as a photodetector in receiver optoelectronics integrated circuits (OEICs) [1]. Conventional MSM-PD structures have a major disadvantage of high dark current which results in higher power consumption. This problem is particularly serious for low-bandgap material. Many works have been done to suppress dark current of MSM-PD. A common approach is to incorporate Schottky barrier enhancement layer. Recent paper has reported the use of silicon-carbon (Si:C) Schottky barrier enhancement layer to suppress dark current in Ge-on-SOI MSM photodetector by more than four orders of magnitude and leading to dark current of ∼11.5 nA for an applied bias of 1.0 V [2]. Another way of reducing dark current is by using passivation layer such as a-Si:H layer. Deposition of a single or multiple layers of amorphous Si or amorphous Si alloy on crystalline silicon substrate to suppress dark current were also reported [3], [4]. In this paper, the effect of a thin un-hydrogenated amorphous silicon layer (a-Si) on the performance of Si-based MSM photodetector (MSM-PD) was studied. Thermal evaporation technique was used to deposit the a-Si layer on crystalline silicon substrate. a-Si layer was shown to decrease dark current of Si-based MSM-PD up to five orders of magnitude. a-Si layer was used as barrier enhancement layer in one MSM structure and capping layer in another structure. At applied bias of 5V, the latter structure exhibits lower dark current leading to photocurrent-to-dark current ratio of ∼ 800 (white light 15W).