Ultrathin Ge epilayers on Si produced by low-temperature PECVD acting as virtual substrates for III-V / c-Si tandem solar cells

Abstract

Ultrathin (20 nm) epitaxial films of germanium are deposited on crystalline silicon wafers, to act as virtual substrates for the growth of III-V materials, opening a low-cost approach to tandem solar cells. Such ultrathin layers allow for material cost reduction, along with the possibility of using the silicon wafer as the bottom cell in tandem devices. A simple plasma-enhanced chemical vapor deposition (PECVD) process at 175 °C has been optimized to deposit these heteroepitaxial germanium films, which grow directly on the silicon wafers without any intermediate silicon-germanium alloy. Thanks to an in-situ plasma cleaning step prior to Ge epitaxy, the films can sustain high-temperature annealing in vacuum (up to 800 °C) without any delamination. The suitability of the germanium heteroepitaxial films as virtual substrates is analyzed by depositing III-V layers on them by conventional growth methods like chemical beam epitaxy (CBE) and metalorganic chemical vapor deposition (MOCVD). The properties of the GaAs films deposited on the virtual substrates are comparable in terms of roughness, microstructure, and crystallinity to these of the III-V layers co-deposited on c-Ge wafers, pointing at the effectiveness of the ultrathin c-Ge epitaxial layers to act as virtual substrates for III-V epitaxial growth. Moreover, growing the c-Ge layers on c-Si substrates with 5° miscut avoids the formation of antiphase domains. These substrates are finally used to demonstrate proof of concept tandem solar cells, proving the suitability of our low temperature and ultrathin virtual substrate approach.