EPITAXIAL STRUCTURES OF GAAS/GAALAS ON GE SUBSTRATES BY MOVPE FOR PHOTOVOLTAIC APPLICATIONS

To overcome some of the drawbacks of the GaAs solar cells, such as cost and fragility, in particular when these cells are utilized in space missions, germanium has been proposed as an alternative, robust and cheap substrate for metalorganic vapor phase epitaxy (MOVPE). In fact, Ge provides a good lattice mismatch with GaAs and large-area slices are available. The MOVPE growth conditions, optimized to grow GaAs on GaAs substrates, were modified to allow the deposition of good GaAs epitaxial layers on Ge substrates. In particular growth rate, growth temperature and substrate orientation were studied in order to minimize the diffusion of Ga, As and Ge across the interface. Several solar cells were fabricated and the influence of the Ge substrate on cell performance was evaluated in comparison with standard GaAs-on-GaAs MOVPE cells. The paper reports the cell characterization, including electron and proton irradiations carried out to simulate cell degradation in the space environment.