Growth of Polycrystalline GaAs for Solar Cell Applications
by A. E. Blakeslee, S. M. Vernon
Films of polycrystalline GaAs have been grown on foreign substrates by the metal-organic process. The main objective was to produce films with as large a grain size as possible, so that high-efficiency photovoltaic devices may eventually be fabricated from such thin film/substrate structures. At 973 K the average grain size was less than 1 μm, and was unaffected by the choice of substrate. Increasing the deposition temperature to 1123 K, while maintaining all other conditions the same, resulted in grains as large as 10 to 20 μm in diameter. Grain sizes as large as 10 μm could be obtained by precoating the substrates with thin films of evaporated gold or tin. However, both of these methods gave films that were discontinuous. A two-step procedure in which the films were nucleated at 873 K prior to growth at 1123 K yielded continuous films with an average grain size of 5 μm. Schottky barrier solar cells fabricated from these films exhibited short-circuit current densities as high as 15.7 mA/cm2, even though the highest conversion efficiency (AM0, uncoated) was only 1.3 percent because of the low fill factor (0.28).