Controlling Etch Anisotropy of Crystalline Germanium Nanostructures.

Orientation-dependent wet chemical etching of crystalline germanium (c-Ge) is essential for the fabrication of next-generation complementary metal oxide semiconductor (CMOS) devices. Here, using a combination of conventional and in situ liquid-phase transmission electron microscopy (TEM) imaging, we reveal the details of the wet etching process of c-Ge nanostructures and identify critical parameters that control the etching rates along different crystalline directions. We demonstrate that etching behavior can be changed from isotropic to anisotropic etching (i.e., from crystal-orientation-independent to orientation-dependent etching) by introducing hydrochloric acid (HCl) into a commonly used hydrogen peroxide (HO) etchant. The observations reveal that the relative etching rates along different crystal directions can be tuned by adjusting the HCl concentration, allowing for full control over the etch anisotropy. The study provides important insights into the nanoscale details of the wet etching of c-Ge and presents a new level of control required for the fabrication of advanced nanoelectronic devices.