Black-silicon-structured back-illuminated Ge-on-Si photodiode arrays

Backside illumination enables an increase in photoactive area and numerical aperture of Ge-on-Si photodetectors for SWIR applications. The transparency of silicon in the infrared range (lambda > 1.1 mu m) allows a nearly lossless propagation of incoming light through the Si substrate and an application of various optical microstructures on the rear side of the Si substrate. Moreover, an aluminum front contact covering the whole top area serves as a mirror which extends the optical propagation of the detectable SWIR light through the absorbing layers and hence increases the quantum efficiency. We developed back-illuminated Ge-on-Si photodiodes to apply such microstructures. Especially the usage of light trapping structures to increase the quantum efficiency of the photodiodes shows great potential. Among the different microstructures we chose black silicon (b-Si) as a promising light trapping candidate. After the fabrication, photodiodes with different configurations were evaluated. The obtained results show a strong increase of the quantum efficiency due to both, the existence of an Al mirror and the application of b-Si.