Zinc phosphide (Zn3P2) is a promising alternative to traditional materials (e.g. CIGS, CdTe, a-Si) for thin film photovoltaics. Open circuit voltage in Zn3P2 cells has been limited by Fermi-level pinning due to surfaces states and heterojunction interdiffusion, motivating the need to prepare interfaces that are electrically passive and chemically inert. We investigated the surface chemistry of Zn3P 2 via etching with bromine in methanol and passivation with ammonium sulfide in t-butanol. The treatment decreases surface oxidation as determined by x-ray photoelectron spectroscopy and provides a stable, low-defect interface as monitored by steady-state photoluminescence. Magnesium Schottky diodes fabricated with sulfur-passivated interfaces show evidence of enhanced barrier heights in comparison to control devices.