Morphological and chemical evolution of monocrystalline porous germanium over time in various storage environments

Abstract

Mesoporous germanium (MP-Ge) emerges as a very appealing material for many applications such as anode material for Lithium-Ion batteries due to it high specific area and large void spaces or, in optoelectronics as sacrificial layer for III-V materials growth and detachment, allowing notably several uses of a single Ge substrate. These porous nanostructures are distinguished by a large specific surface area and are prone to degradation with time due to exposure to the environment. To understand and be able to reduce this effect, we studied the chemical and morphological evolution of porous germanium layers under various ambient storage conditions for 3 months to identify the main parameters responsible for material degradation. This study demonstrates that the ambient air environment leads to the growth of native oxide, leading to major morphology changes. Scanning electrons microscope (SEM) showed the formation of clusters and the enlargement of the pores after 90 days. These structural modifications are caused by the oxidation of Ge, and more specifically by the creation of GeO2 matrices due to the synergy of dioxygen (O2) and humidity (H2O(g)). The energy brought by light can exacerbate these phenomena and thus accelerate the degradation rate of the pore morphology. Based on these experimental results, we propose efficient solutions to limit the GeO2 proportions and the clusters' appearance, by storing them under a dry neutral atmosphere (Ar) or by adding a hydrogen halide pre-treatment (10s 1% HBr solution).