Toward High Performance N/P GaAs Solar Cells Grown on Low Dislocation Density p-Type SiGe Substrates

GaAs solar cells grown on SiGe/Si virtual substrates in the n/p configuration are of interest to develop III-V/Si cell technologies with high radiation-tolerance and to demonstrate the general applicability of SiGe/Si substrates for transfer of standard multi-junction configurations. This paper reports the first study of minority carrier electron lifetimes in p-type GaAs base materials grown on low dislocation density (1×10 ⁶ cm ^-2) SiGe/Si substrates and the first study of n/p III-V cells grown on SiGe as a function of threading dislocation density. Minority carrier diffusion lengths of ∼4 μn, well in excess of a typical n/p cell base thickness, are demonstrated and correlations between diffusion length and dislocation density are made. Preliminary cell results match theoretical predictions, and n/p GaAs cell efficiencies on Si in excess of 15% have been achieved. In parallel developments for p/n cells, GaAs cell areas on SiGe have been increased from 0.36 cm ² to 4 cm ² with no decrease in cell performance. This indicates that thermal stress induced microcracks are not limiting cell performance on SiGe/Si substrates at this stage of development. The cumulative impact of these results indicate the growing promise of SiGe virtual substrates for achieving high performance III-V solar cells grown on Si substrates utilizing SiGe buffer layers.