Minority carrier lifetime of Ge film epitaxial grown on a large-grain seed layer on glass

Abstract

To develop high-efficiency multijunction solar cells onto inexpensive substrates, an innovative technique for growing a large-grained Ge layer on glass is strongly desired. We investigated the epitaxial growth of a light absorbing Ge film (500-nm thickness) on a large-grained (>100 μm) Ge seed layer (50-nm thickness) formed on glass by Al-induced layer exchange. After examining molecular beam epitaxy (MBE) and solid-phase epitaxy (SPE) for both methods, Ge layers with a low Al concentration were epitaxial grown at 350 °C. Microwave photoconductivity decay revealed that the MBE-Ge layer exhibited a short minority carrier lifetime owing to the rough surface. Conversely, the SPE-Ge layer was relatively flat and exhibited a long bulk minority carrier lifetime (5.6 μs), which is close to that of a single-crystal Ge. Therefore, the seed layer concept that combines SPE with Al-induced layer exchange is a promising way for fabricating ideal bottom cells for high-efficiency multijunction solar cells based on inexpensive substrates.