Enabling tailored nanoporous germanium by quantifying the evolution of structural properties during self-irradiation

Abstract

Porous materials are essential for a wide range of applications in energy storage, catalysis, and environmental solutions. Material fabrication methods which can tailor the porous structures in a deterministic manner are highly valuable for the development of the field. In this paper, we use ion irradiation to synthesize large-scale germanium (Ge)-based porous structures. We furthermore demonstrate the structure to consist of pores interconnected by ultra-small channels that effectively allow transport of gaseous molecules throughout the whole network. Aside qualitative assessment of the evolution of the structure, the combination of transmission electron microscopy, microbeam Rutherford backscattering spectrometry and a Ge-on-Si sample design enables the quantification of properties such as mass density and sputtering yields during the evolution of porosity. We find progression of pore formation from the surface with a constant rate with increasing dose at otherwise constant density of the porous phase. Proximity to the substrate terminates pore formation well before reaching the interface with higher doses, eventually inducing a depth-independent densification of the porous structure. Experimental evidence challenges vacancy clustering as a commonly referred-to mechanism for pore-formation. Instead, relaxation in structurally weakened, i.e. near-surface volumes of the material by micro-explosions can conclusively explain all observations. The porous germanium synthesized in our approach is considered more resilient at high temperature and strong pH conditions than polymeric and hybrid materials, indicating great potential for applications in such environments.