High quality GaAs growth by MBE on Si using GeSi buffers and prospects forspace photovoltaics

III-V solar cells on Si substrates are of interest for space photovoltaics since this would combine high performance space cells with a strong, lightw eight and inexpensive substrate. However, the primary obstacles blocking II I-V/Si cells from achieving high performance to date have been fundamental material incompatibilities, namely the 4% lattice mismatch between GaAs and Si, and the large mismatch in thermal expansion coefficient, In this paper , we report on the molecular beam epitaxial (MBE) growth and properties of GaAs layers and single junction GaAs cells on Si wafers which utilize compo sitionally graded GeSi intermediate buffers gl own by ultra-high vacuum che mical vapor deposition (UHVCVD) to mitigate the large lattice mismatch betw een GaAs and Si, GaAs cell structures were found to incorporate a threading dislocation density of 0.9-1.5 x 10(6) cm(-2), identical to the underlying relaxed Ce cap of the graded buffer, via a combination of transmission ele ctron microscopy, electron beam induced current, and etch pit density measu rements. AlGaAs/GaAs doable heterostructures were grown on the GeSi/Si subs trates for time-resolved photoluminescence measurements, which revealed a b ulk GaAs minority carrier lifetime in excess of 10 ns, the highest lifetime ever reported for GaAs on Si. A series of growths were performed to assess the impact of a GaAs buffer layer that is typically grown on the Ge surfac e prior to growth of active device layers. We found that both the high life times and low interface recombination velocities are maintained even after. reducing the GaAs buffer to a thickness of only 0.1 mu m. Secondary ion ma ss spectroscopy studies revealed that there is negligible cross diffusion o f Ga, As and Ge at the III-V/Ge interface, identical to our earlier finding s for GaAs gr(o)wn on Ce wafers using MBE, This indicates that there is no need for a buffer to 'bury' regions of high autodoping, and that either pn or np configuration cells are easily accommodated by these substrates. Prel iminary diodes and single junction AlGaAs heteroface cells were grown and f abricated on the Ge/GeSi/Si substrates for the first time. Diodes fabricate d on GaAs, Ge and Ge/GeSi/Si substrates show nearly identical I-V character istics in both forward and reverse bias regions. External quantum efficienc ies of AlGaAs-\GaAs cell structures grown on Ge/GeSi/Si and Ge substrates d emonstrated nearly identical photoresponse, which indicates that high lifet imes, diffusion lengths and efficient minority carrier collection is mainta ined after complete cell processing. Copyright (C) 2000 John Wiley & Sons, Ltd.