A rather unknown application of Ge is the use of Ge wafers in Ge/GaAs solar cells which provide the electrical supply of telecommunication satellites. The Ge wafers are used not only as a substrate for the epitaxially grown GaAs-based layers but also as a photovoltaic absorber layer contributing to the photocurrent and thus to the total efficiency of the solar cell. For the latter function the minority carrier lifetime of the Ge wafers is of major importance. Since most minority carrier lifetime measurements on semiconductor wafers have been made on Si (in particular commercial instrumentation and surface passivation) it was necessary to explore this domain for Ge wafers. Two types of lifetime measurements were used in this study: Photoconductive decay measurements (PCD) with contacts on rectangular wafer pieces and contactless microwave detected PCD (μ-PCD) on complete wafers. In order to measure the bulk recombination lifetime, passivation of the Ge surface is necessary. In this paper the chemical passivation of the Ge wafer surface is studied.
Chemical surface passivation
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We present the development, optimization and fabrication of high carrier mobility materials based on GeOI wafers co-doped with Sn and P. The Ge thin films were fabricated using plasma-enhanced chemical vapour deposition followed by ion implantation and explosive solid phase epitaxy, which is induced by millisecond flash lamp annealing. The influence of the recrystallization mechanism and co-doping of Sn on the carrier distribution and carrier mobility both in n-type and p-type GeOI wafers is discussed in detail. This finding significantly contributes to the state-of-the-art of high carrier mobility-GeOI wafers since the results are comparable with GeOI commercial wafers fabricated by epitaxial layer transfer or SmartCut technology.