III-V semiconductors on silicon-germanium substrates for multi-junction photovoltaics.

摘要

The epitaxial integration of high quality III-V semiconductors with Si is of fundamental interest for photovoltaic devices since Si substrates offer a lighter, stronger, and cost effective platform for device production. However, the lattice-mismatch between conventional III-V photovoltaic materials and Si generates threading dislocations in the epitaxial device layers, which can limit solar cell performance, depending of the density of such defects, the particular III-V material, and the device design. By using compositionally step-graded SiGe interlayers up to 100% Ge, which is lattice-matched to GaAs, the ∼4% lattice-mismatch between Si and GaAs and In0.49 Ga0.51P is accommodated in the Group IV alloy system; this has produced defect densities less than 1 x 106 cm -2 in fully relaxed the Ge/SiGe/Si (SiGe) virtual substrates. This unique approach to III-V/Si integration is employed in this dissertation for the development of GaAs and In0.49Ga0.51P single junction (SJ) solar cells and ultimately In0.49Ga0.51P/GaAs dual junction (DJ) solar cells, integrated on a Si platform.; The residual threading dislocation density (TDD) present in the SiGe substrates transfers to the epitaxially grown III-V layers and thus can influence III-V solar cell performance. In this dissertation we report, for the first time, on the impact of TDD on the minority carrier electron lifetime in GaAs grown on SiGe. The electron lifetime in metamorphic p-type GaAs was found to be lower than that of holes in n-type GaAs at a given TDD. This resulted from the higher mobility of electrons compared to holes and thus enhanced interactions with the TD array. Incorporating a TDD dependent lifetime into metamorphic GaAs solar cell device models, higher reverse saturation current densities and lower open-circuit voltages for n+/p compared to p+/n were predicted. This result was experimentally confirmed in this dissertation by diode and solar cell device measurements of both n +/p and p+/n GaAs cells grown on GaAs and SiGe substrates. The higher performance of the p+/n GaAs-on-SiGe solar cell, by virtue of its higher opencircuit voltage, offers great potential for both space and terrestrial photovoltaic applications. The extension of this technology to space applications has lead to the development of large area GaAs-on-SiGe solar cells (up to 4 cm2) with no degradation in cell performance. These large area cells will be flown on the International Space Station to test their actual space performance, which indicates their technological importance. Meanwhile, record terrestrial performance was measured and suggests efficiencies higher than 20% are realizable with current SiGe substrate technologies. (Abstract shortened by UMI.)