Mesoporous Germanium Formation by Electrochemical Etching

Abstract

Uniform thick mesoporous germanium layers are reproducibly formed on 4 in. p-type Ge wafers by electrochemical etching in highly concentrated HF electrolytes. Pore formation by anodic etching in germanium leads to a constant dissolution of the porous layer. The growth rate of the porous Ge layer is therefore given by the difference between the etch rate at the porous layer/substrate wafer interface and the dissolution rate at the electrolyte/porous layer interface. The growth rate lies in the range of 0.071–2.7 nm/min for etching current densities of 0.1–80mA/cm2 , while both the etch rate and the dissolution rate lie in the range of several micrometers per minute. We define the substrate usage as the ratio of the growth rate and the etch rate. This substrate usage determines the growth efficiency of the porous layer and lies in the range of 0.2–2%. Thus, the substrate wafer is thinned substantially during anodic porous layer formation. Constantly alternating from an anodic to a cathodic bias prevents the thinning of the substrate. The dissolution rate decreases, and the usage increases up to 98%.