(Invited) Fabrication of Pure-GaB Ge-on-Si Photodiodes for Well-Controlled 100-pA-Level Dark Currents

The selective epitaxial growth of Ge-on-Si followed by in-situ deposition of a nm-thin Ga/B layer stack (PureGaB) has previously been shown to be a robust CMOS-compatible process for fabrication of Ge-on-Si photodiodes. In this paper, strategies to improve the control and reproducibility of PureGaB Ge-on-Si photodiode fabrication by reducing the local loading effects during the depositions are presented. As compared to the earlier PureGaB devices, the elimination of parasitic Ge and concomitant in-situ As-doping from oxide regions surrounding the deposition windows leads to a well-controlled process flow that improves photodiode electrical and optical characteristics. For micrometer-sized diodes, ideality factors of less than 1.1 and dark current densities in the range of 15 µA/cm2 at room temperature are now achieved. Moreover, improvements in the flatness of the Ge-island surface facilitated a process flow for contacting the diode perimeter while leaving a large oxide-covered PureGaB-only light-entrance window on the central photosensitive region. The optical characteristics of the photodiodes at the low temperature of 180 K display dark current densities of less than 150 pA/cm2 and increased sensitivity towards infrared wavelengths.