Polysilicon near-infrared photodetector with performance comparable to crystalline silicon devices

Abstract

We demonstrate high responsivity in a polysilicon (Poly-Si) near-infrared photodetector by reducing the adverse effects of grain boundaries. The presence of grain boundaries is a serious problem in polysilicon since they act as traps and degrade the electrical characteristics. However, the relation between grain boundaries and photodetector performance has not yet been reported. Here, the effect of grain boundary on the resistivity and responsivity of Poly-Si devices is investigated. The resistivity of Poly-Si is higher than that of crystalline Si (c-Si) at the same doping concentration due to the potential barrier of the grain boundary, and its degradation of electrical characteristics is mitigated here by increasing the doping concentration. The photodetector with a highly-doped polysilicon layer exhibits a responsivity of 0.48 A/W at 900 nm, which is almost the same as that of c-Si photodetectors. In contrast, polysilicon devices with the same doping concentration as c-Si devices show degraded responsivity due to grain boundary traps. In addition, to improve the responsivity at target wavelengths of 900 nm and 1,064 nm, we deposited an indium-tin-oxide layer, which reduces the surface reflectance on the Poly-Si photodetector. The responsivity is improved by 22.9% and 50.0% at 900 nm and 1,064 nm respectively compared to devices without the ITO. The results here confirm the ability to fabricate low cost Poly-Si photodetectors with high responsivity which show similar or superior optical response compared to commercial c-Si devices.