He implantation at high fluence in elementary semiconductors leads to the formation of a layer of nanocavities. During subsequent annealing, they undergo a growing process and for sufficient temperature, He desorbs from the cavities, leading to the formation of voids able to trap metallic impurities, a useful mechanism for proximity gettering. To get a quantitative understanding of the cavity evolution under annealing, the estimation of the cavity parameters (pressure, diameter and He density) at each step is essential. Spatially resolved Electron Energy Loss Spectroscopy (EELS) has been shown to be a powerful tool to determine such parameters in He-induced cavities in metals and alloys. However, the transfer of this approach to other materials is not trivial to perform.

In this study we will show that it is also possible to fully characterize He cavities in Si and Ge; an estimation of the physical parameters of the cavities will be given. For that purpose, He is implanted at high fluence in Si and Ge and samples are annealed at high temperature to form cavities of different diameters. The cavities are then analyzed by Scanning Transmission Electron Microscopy-EELS (STEM-EELS) and Energy Filtered Transmission Electron Microscopy (EF-TEM) using a new generation TEM/STEM fitted with an omega filter (Schottky-FEG 0.7 eV, 0.2 nm probe size). The experimental results are further compared with ab initio dielectric function calculations based on the Density Functional Theory to calculate the He K-edges in cavities for various He density. From these results, the formation and evolution mechanisms of the cavities are discussed.