The prospect of terawatt-scale PV requires a low-cost, earth abundant photovoltaic material with excellent optoelectronic properties. Zinc phosphide (Zn<SUB>3</SUB>P<SUB>2</SUB>) is a strong candidate for an efficient thin-film absorber, with a direct band gap of 1.5 eV and long minority-carrier diffusion lengths (>5μm). Due to difficulties in achieving n-type conductivity, previous research focused on the performance of Mg-Schotty diodes fabricated on Zn<SUB>3</SUB>P<SUB>2</SUB> wafers. However, device efficiencies were limited by reflection losses from Mg-metal and poor barrier formation resulting from interface states. In order to avoid these issues, we propose the fabrication of Zn<SUB>3</SUB>P<SUB>2</SUB> solar cells implementing an n-type heterojunction partner. Zinc sulfide (ZnS) is an attractive heterojunction partner with a wide band gap (3.6 eV), high achievable donor concentrations (>10<SUP>18</SUP>), and a low electron affinity (3.9 eV). In this work, we report epitaxial growth of Zn<SUB>3</SUB>P<SUB>2</SUB> and ZnS on GaAs(001) substrates by molecular beam epitaxy (MBE). GaAs was chosen as a growth substrate since the phosphorus sub-lattice of α-Zn<SUB>3</SUB>P<SUB>2</SUB> is similar to that of arsenic and the room-temperature lattice mismatch is less than 1.3%. Films of approximately 0.1 to 1 μm in thickness were grown with substrate temperatures ranging from 50 to 350°C. Prior to deposition, an arsenic-free substrate pretreatment was used to create a smooth epi-ready surface. This procedure was similar to those employed for epitaxial growth of II-VI compounds on GaAs. Surface evolution during growth was monitored using reflective high energy electron diffraction (RHEED). The optimum epitaxial growth window for each material was determined via ex situ structural and optoelectronic film characterization. The Zn<SUB>3</SUB>P<SUB>2</SUB> growth rate varied with both substrate temperature and Zn/P beam flux ratio, with typical rates falling between 0.05 and 0.2 nm-s<SUP>-1</SUP>. RHEED studies demonstrated both polycrystalline and amorphous growth at temperatures below 300°C. Highly crystalline three-dimensional growth was obtained via appropriate control of substrate temperature and Zn/P flux ratio. Epitaxial growth of zincblende ZnS was possible at substrate temperatures as low as 50°C. For both materials, the Zn sticking coefficient dominated the growth rate at higher temperatures. Film orientation was confirmed by high resolution X-ray diffraction (HRXRD) measurements. For Zn<SUB>3</SUB>P<SUB>2</SUB>/GaAs(001), (201) and (004) orientations were observed with the (004) peak dominating at higher growth temperatures. The Zn(002) peak was occasionally observed for films grown at sub-optimal Zn/P flux ratios. Alternatively, for ZnS/GaAs(001), only the (002) and (004) peaks were observed over the entire range of growth conditions. In addition to HRXRD, film orientation and composition were further investigated using transmission electron microscopy (TEM) and X-ray dispersive spectroscopy (EDS). Finally, Hall Effect measurements were performed to assess the electronic properties of the films. This approach allows us to achieve low defect, electrically active semiconductor films, and ultimately efficient ZnS/Zn<SUB>3</SUB>P<SUB>2</SUB> heterojunction solar cells.