High Quality GaAs Growth by MBE on Si Using GeSi Buffers and Prospects for Space Photovoltaics

摘要

III- V solar cells on Si substrates are of interest for space photovoltaics since this would combine high performance space cells with a strong, lightweight and inexpensive substrate. However, the primary obstacles blocking III -V/Si cells from achieving high performance to date have been fundamental material incompatibilities, namely the 4 percent 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 compositionally graded GeSi intermediate buffers grown by ultra-high vacuum chemical vapor deposition (UHVCVD) to mitigate the large lattice mismatch between GaAs and Si. GaAs cell structures were found to incorporate a threading dislocation density ofo.9- 1.5 x 10~6cm~(-2), identical to the underlying relaxed Ge cap of the graded buffer, via a combination of transmission electron microscopy, electron beam induced current, and etch pit density measurements. AlGaA s/GaAs double heterostructures were grown on the GeSi/Si substrates for time-resolved photoluminescence measurements, which revealed a bulk 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 surface prior to growth of active device layers. We found that both the high lifetimes and low interface recombination velocities are maintained even after reducing the GaAs buffer to a thickness of only 0.1 #mu# m. Secondary ion mass spectroscopy studies revealed that there is negligible cross diffusion of Ga, As and Ge at the III- V/Ge interface, identical to our earlier findings for GaAs grown on Ge 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. Preliminary diodes and single junction AlGaAs heteroface cells were grown and fabricated on the Ge/GeSi/Si substrates for the first time. Diodes fabricated on GaAs, Ge and Ge/GeSi/Si substrates show nearly identical I-V characteristics in both forward and reverse bias regions. External quantum efficiencies of AlGaAs-/GaAs cell structures grown on Ge/GeSi/Si and Ge substrates demonstrated nearly identical photoresponse, which indicates that high lifetimes, diffusion lengths and efficient minority carrier collection is maintained after complete cell processing.