A virtual substrate consisting of a Ge layer grown directly on Si without an intervening SiGe graded layer is characterized. The nominally 100% Ge overlayer is fully relaxed and contains a small amount (3%) of unintentional Si. A dislocation density of 10 8 cm −2 is estimated for the virtual substrate prior to GaAs epitaxial growth, which is reduced by a factor of 100 after the growth of GaAs. On this novel virtual substrate 1 cm 2 single-junction GaAs photovoltaic cells were realized with an efficiency of 11.7% under AM0 compared with 20.2% for cells grown on a crystalline Ge substrate. Due to the high dislocation density a 50-fold higher dark current is measured in the virtual substrate cells compared to the crystalline Ge cells, leading to a lower short circuit current and open-circuit voltage of the cells fabricated on the virtual substrates. The post-GaAs growth dislocation density is estimated as 1 × 10 7 cm −2 in the base region and 4 × 10 5 cm −2 in the emitter region based on modelling and measurements.

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The effect of growth temperature on the annihilation of antiphase domain boundaries (APBs) in GaAs grown on 300 mm Si(001) wafers via an industrial III-V metal organic chemical vapor deposition (MOCVD) tool is examined. Samples were grown with identical low temperature nucleation layers to set the APB density before growth of bulk layers at varied temperature. The APB annihilation rate with respect to GaAs film thickness is determined by profiling the APB density in the samples as a function of depth using a combination of chemical etchants. It is found that higher bulk growth temperatures enhance the annihilation of APBs. The increase in APB annihilation rate must be facilitated by a greater portion of APBs kinking from {110} type planes to higher-index planes such as {111} or {112}. The temperature dependence of APB annihilation indicates that there is an associated activation energy for the kinking of APBs.