III-V semiconductors on SiGe substrates for multi-junction photovoltaics

The epitaxial integration of III-V semiconductors with Si is of interest for photovoltaics since Si substrates offer a lighter, stronger, and cost effective platform for device production. By using compositionally step-graded SiGe layers to 100% Ge, the 4% lattice-mismatch between Si and GaAs and In _0.49 Ga _0.51 P is accommodated; this method has produced record low threading dislocation densities (TDD) of 1x10 ⁶ cm ^-2 in fully relaxed the Ge/SiGe/Si (SiGe) substrates. In this dissertation, this method of III-V/Si integration is used for the development of GaAs and In _0.49 Ga _0.51 P single junction (SJ) solar cells and In _0.49 Ga _0.51 P /GaAs dual junction (DJ) solar cells, integrated on a Si platform. As such, we report that the minority carrier electron lifetime in p-type GaAs grown on Si is lower than that of holes in n-type GaAs at a given TDD and is a consequence of the higher mobility of electrons. This lower lifetime produced higher reverse saturation currents and lower open-circuit voltages for n+/p compared to p+/n configuration GaAs cells grown on SiGe with the same TDD. The higher performance of the p+/n GaAs/Si cell, by virtue of its higher open-circuit voltage, has demonstrated a record terrestrial efficiency of 18.1% and has been produced in areas up to 4 cm ² with no degradation in cell performance. In _0.49 Ga _0.51 P SJ cells were integrated on Si substrates and an increase in depletion region recombination component of the reverse saturation current with TDD was also measured. A p+/n polarity preference for In _0.49 Ga _0.51 P on Si was demonstrated, although, the lower mobility of both carriers in In _0.49 Ga _0.51 P compared to GaAs, suggests a greater TDD tolerance. Based on these SJ results, the first realization of an In _0.49 Ga _0.51 P/GaAs DJ cell on Si with an output voltage greater than 2 V was demonstrated. A comparison with an identical DJ cell on GaAs found that the DJ cell on Si retained 91% of open-circuit voltage and 99% of short-circuit current; moreover, the p+/n In _0.49 Ga _0.51 P/GaAs DJ cell grown on Si demonstrated an open-circuit voltage that was consistent with the value predicted by the metamorphic dual junction device model developed in this thesis.