Integration of III-V semiconductors and Si is a very attractive means to achieve low-cost high-effi ciency solar cells. A promising configuration is to utilize a dual-junction solar cell, in which Si is employed as the bottom junction and a wide-bandgap III-V semiconductor as the top junction. The use of a III-V semiconductor as a top junction offers the potential to achieve higher efficiencies than today's best Si solar cell. Dilute nitride GaNP is a promising candidate for the top cell in dual-junction solar cells because it possesses several extremely important attributes: a direct-bandgap that is also tunable as well as easily-attained lattice-match with Si. As a first step towards integration of GaNP solar cells onto Si, the goal of this dissertation is to optimize and demonstrate GaNP solar cells grown by gas-source molecular beam epitaxy (GSMBE) on GaP (001) substrate.The dissertation is divided into three major parts. In the first part, we demonstrate ~ 2.05 eV ([N]~ 1.8%) dilute nitride GaNP thin film solar cells, in which the GaNP is closely lattice-matched to Si, on GaP substrates. From transmission electron microscopy (TEM), the device exhibits defects only at the GaNP/GaP interface, and no threading dislocations in an active layer are observed. Our best GaNP solar cell achieved an efficiency of 7.9% with anti-reflection (AR) coating and no window layer. This GaNP solar cell's efficiency is higher than the most efficient GaP solar cell to date and higher than other solar cells with similar direct bandgap (InGaP, GaAsP). Through a systematic study of the structural, electrical, and optical properties of the device, effi cient broadband optical absorption and enhanced solar cell performance using GaNP are demonstrated.In the second part, we demonstrate the successful fabrication of GaP/GaNP core/shell microwires utilizing a novel technique: top-down reactive-ion etching (RIE) to create the cores and MBE to create the shells. Systematic studies have been performed over a series of microwire lengths, array periods, and microwire sidewall morphology. For a fixed length, short circuit current (Jsc) increases as physical fill factor (PFF) of microwires increases, while, for a fixed array period, Jsc increases as microwire length increases. Our studies show that the open circuit voltage (Voc) is degraded primarily due to defects at the GaP/GaNP interface and in the shells, not surface recombination. The best efficiency we achieved using our microwire solar cell is 3.2% using an AR coating. Compared to thin film solar cells in the same growth run, the microwire solar cells exhibit greater Jsc but poorer Voc. This results from greater light absorption and a greater number of defects in the microwire structure, respectively.In the final part, vertical self-catalyzed GaP, GaNP, and GaNP/GaNP core/shell nanowires are demonstrated. The growth window of GaP nanowires is comparable to the growth window of GaNP nanowires. The diameter of nanowires (cores) can be controlled by adjusting substrate temperature (Tsub). The shells can be grown by decreasing Tsub and increasing the V/III incorporation ratio to reduce adatom mobility. The crystal structures of GaP and GaNP nanowires are mixtures of cubic zincblend phase and hexagonal wurtzite phase along the [111] growth direction. According to photoluminescence measurements, the broad spectrum of nanowire arrays do not result from the variation of N composition among nanowires, but from the mechanism of light emission of GaNP
