Enhanced Performance of Ge Photodiodes via Monolithic Antireflection Texturing and α-Ge Self-Passivation by Inverse Metal-Assisted Chemical Etching.

Surface anti-reflection micro and nanostructures, normally formed by conventional reactive ion etching, offer advantages in photovoltaic and optoelectronic applications, including wider spectral wavelength ranges and acceptance angles. One challenge in incorporating these structures into devices is that optimal optical properties do not always translate into electrical performance due to surface damage, which significantly increases surface recombination. Here, we present a simple approach for fabricating anti-reflection structures, with self-passivated amorphous Ge (α-Ge) surfaces, on single crystalline Ge (c-Ge) surface using the inverse metal-assisted chemical etching technology (i-MacEtch). Vertical Schottky Ge photodiodes fabricated with surface structures involving arrays of pyramids or periodic nano-indentations show clear improvements not only in responsivity, due to enhanced optical absorption, but also in dark current. The dark current reduction is attributed to the Schottky barrier height increase and self-passivation effect of the i-MacEtch induced α-Ge layer formed on top of the c-Ge surface. The results demonstrated in this work show that MacEtch can be a viable technology for advanced light trapping and surface engineering in Ge and other semiconductor based optoelectronic devices.