To move beyond 50% efficiency multijunction solar cells, additional higher energy junctions are needed, and none of the conventional semiconductors can fill this role. Because of a large absorption coefficient (alpha ~10 5 cm-1 for GaN) and a wide diode depletion (~300 nm), the potential for significant optical absorption in the depletion of a p-i-n diode is promising for a drift-based solar cell based on the III-nitride system. The work described in this thesis covers the development of InGaN material growth by ammonia molecular beam epitaxy (MBE) and its incorporation into photovoltaic devices.

A new regime of high V/III ratio and modulated growth was explored in ammonia MBE, leading to device quality material layers. It was observed that increasing ammonia overpressure during growth dramatically increased the indium incorporation efficiency for a given group-III flux and substrate growth temperature. Upon increasing the ammonia pressure, however, the growth rate was observed to decrease. The growth rate reduction was explained by a free-space particle scattering model wherein group-III source beams are attenuated through collisions with the high density of ammonia molecules above the surface of the substrate. By modulating the ammonia flow and substrate temperature during alternating growth of InGaN and GaN layers, the growth of high quality InGaN/GaN superlattice and multiquantum-well (MQW) structures was achieved. These observations and techniques allowed the extension of device quality layers to smaller bandgap quantum wells through the increased incorporation of indium.

The InGaN layers grown by this method were then incorporated into photovoltaic (PV) device structures. The physics of quasi-bulk and MQW PV devices was explored through empirical and theoretical means. The short minority carrier diffusion length in highly n and especially p-type GaN was predicted to result in poor collection efficiency for photogenerated carriers. Experiments with varying the p-window layer thickness resulted in dramatically increased short-wavelength external quantum efficiency improvement. EQE simulations of these devices confirm these results and give estimates of minority carrier diffusion lengths in p-GaN. Very high internal quantum efficiency (IQE) was confirmed, however, for material within the depletion region of a p-i-n diode, as measured by comparing device EQE data to measured optical absorption.

To extend the PV spectral response to longer wavelengths, a change to the MQW-based structures was found to prevent morphological breakdown and result in much higher open-circuit voltages. A significant barrier-thickness dependent behavior was observed in these devices, with increasing IQE for decreasing barrier thickness. Temperature-dependent L-I-V curves and temperature-dependent biased EQE curves were measured to probe this behavior. A reduction in carrier escape times was suggested to explain the increase in IQE for reduced barrier devices. For thin barriers, a temperature-independent mechanism was observed. Calculations of tunneling and thermionic emission carrier escape corroborated the measured results and suggested tunneling dominated for all devices at room temperature and lower, while thermionic emission dominated for thicker-barrier samples at elevated temperatures. The dramatically increased performance of the thinner barrier samples suggested a future direction for practical performance devices for multijunction integration.