Achieving porous germanium from both p- and n-type epitaxial Ge-on-Si via bipolar potentiostatic etching

Due to the large surface specific area, tunable porosity and refractive index, porous germanium (Ge) has wide applications in optoelectronics, photovoltaics, anode materials in batteries. Electrochemical etching is the most used approach to fabricate porous Ge. However, previous work focused on bulk Ge wafer etching, which is not suitable for device integration on a Si substrate, and illumination had to be used for n-type porous Ge formation. In this paper, we show that by using bipolar potentiostatic etching, porous Ge with the thickness of more than 100 nm and the pore diameter in the range of 10 to 100 nm, can be obtained from both p- and n-type Ge-on-Si layers without illumination. The pore formation mechanism is interpreted by hydrogen evolution effect and Space Charge Region (SCR) model where pore nucleation, growth and dissolution are determined by the total etching time. Our work paves the way for porous Ge applications in Si based integrated photonics and microelectronics.