Development of Ge Isotropic Wet Etching Solution and its Application to High Quality Ge-on-Insulator Fabrication through the Etchback Method

Ge has many unique characteristics, such as high carrier mobility and a narrow bandgap corresponding to near-infrared wavelengths. To take advantage of the attractive characteristics of Ge, Ge-on-Insulator (GOI) structures are necessary. In this study, we focus on a direct wafer bonding and etchback method to fabricate GOI structures and explore appropriate etching solutions for the etchback. An HF + H2O2 + CH3COOH solution can isotropically etch Ge and improve surface uniformity. The resulting surfaces were sufficiently flat to achieve Schottky and MOS diodes showing good electrical characteristics of the same level as devices based on commercial mirror-polished Ge surfaces. We discuss the role of the chemicals in the etching solution in achieving the flat surface. We fabricated GOI structures and a back-gate GOI capacitor through direct wafer bonding of SiO2/Si and Al2O3/Ge with the etchback method using the solution. The resulting electrical characteristics are also explained using theoretical calculations. This approach might offer an alternative route to high-quality GOI fabrication.