Design impact on the performance of Ge PIN photodetectors

Abstract

This article presents the impact of epitaxial quality, contact resistance and profile of Ge PIN photodetectors (PDs) on dark current and responsivity. The PD structures were processed with either selectively grown Ge with integrated waveguides on SOI wafer or globally grown Ge on the entire wafer. The contact resistance was lowered by introducing NiGe layer prior to the metallization. The n-type doped Ge PIN structure was formed by ion implantation and the contact resistivity was estimated to 2.6 × 10⁻⁴ Ω cm². This value is rather high and it is believed to be due to fomation of defects during implantation. The results show a minor difference in dark currents for selectively and globally grown PDs but in both types, it depends on detector area and the epitaxial quality of Ge. For example, the threading dislocation density (TDD) in non-selectively grown PDs with thickness of 1 µm was estimated to be 10⁶ cm⁻² yielding relatively low dark currents while it dramatically changes for PDs with thinner Ge layers where TDD increases to 10⁸ cm⁻² and the dark current levels increase almost by 1.5 magnitude. Surprisingly, for selectively grown PDs with Ge thickness of 500 nm, TDD was still low resulting in low dark currents. The dark current densities at − 1 V bias of non-selectively and selectively grown PDs with optimized profile were measured to be 5 mA/cm² and 47 mA/cm², respectively, while the responsivity of these detectors were 0.17 A/W and 0.46 A/W at λ ~ 1.55 µm, respectively. Excellent performance for selectively grown PD shows an appropriate choice for detection of 1.55 µm wavelength.