Porous germanium multilayers

Abstract

AbstractWe present the reproducible fabrication of porous germanium (PGe) single‐ and multilayers. Mesoporous layers form on heavily doped 4” p‐type Ge wafers by electrochemical etching in highly concentrated HF‐based electrolytes with concentrations in a range of 30‐50 wt.%. Direct PGe formation is accompanied by a constant dissolution of the already‐formed porous layer at the electrolyte/PGe interface, hence yielding a thinner substrate after etching. This effect inhibits multilayer formation as the starting layer is etched while forming the second layer. We avoid dissolution of the porous layer by alternating the etching bias from anodic to cathodic. PGe formation occurs during anodic etching whereas the cathodic step passivates pore walls with H‐atoms and avoids electropolishing. The passivation lasts a limited time depending on the etching current density and electrolyte concentration, necessitating a repetition of the cathodic step at suitable intervals. With optimized alternating bias mesoporous multilayer production is possible. We control the porosity of each single layer by varying the etching current density and the electrolyte (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)