Sintering and reorganization of electrochemically etched mesoporous germanium layers in various atmospheres

We investigate sintering and reorganization of electrochemically etched mesoporous Ge layers. Sintering in nitrogen, forming gas, or argon atmospheres always yields a reorganized and oxidized porous layer. Water-insoluble Ge native oxides produced during etching do not hinder thermal reorganization, but evolve to water soluble oxides during annealing. Reorganized porous layers hence dissolve in subsequent water or HF dips. Sintering in hydrogen atmospheres removes native Ge oxides and prevents oxidation of porous layers. Porous layers with porosities less than 30% exhibit a compact closed surface after sintering, whereas porous layers with porosities exceeding 50% collapse. The porosity decreases linearly in a range of 57%–45% for electrolyte concentrations and in a range of 35–50 wt% for a specific resistivity of 0.013 Ω cm and an etching current density of 5 mA cm−2. We obtain porosities below 30% by using substrate resistivities in the 0.015–0.030 Ω cm range, etching current densities below 2.5 mA cm−2 and an electrolyte concentration of HF 50 wt%.