Zinc phosphide (Zn3P2) is an earth-abundant semiconductor with promising properties for applications as an absorber in photovoltaics. To beat the 40 years old record power conversion efficiency of solar cells made with this material, a deep understanding of the compound's electrical, optical, and crystal properties and their interplay with its growth conditions is required. In this thesis, we explore the crystalline and optical properties of zinc phosphide and the closely related zinc arsenide (Zn3As2) by means of Raman and photoluminescence spectroscopy. In the first part of this work, we establish that defect-free zinc arsenide nanosails grown by gold-catalyzed metal-organic vapor phase epitaxy display a metastable crystalline structure. The atomic lattice of these flat structures becomes isostructural to that of zinc phosphide likely due to the nanoscale nature of the system. We also determine that the nanosails are degenerate p-doped semiconductors with pure impurity band conduction at low temperatures. The second part of this thesis is dedicated to establishing a thorough understanding of the dispersion and symmetries of the Raman-active lattice vibrational eigenmodes of zinc phosphide. We unambiguously identify most Raman-active phonons in the lattice and show the zinc- and phosphorus-dominated modes to be separated by a real phonon gap, establishing a reference Raman spectrum for zinc phosphide and its family of isostructural compounds. In the final part of the thesis, we characterize the optical properties of zinc phosphide by photoluminescence of monocrystalline zinc phosphide grown by molecular beam epitaxy. We show that the photogenerated electron-hole pairs experience two main radiative recombination mechanisms. Emission attributable to electronic transitions involving an impurity band is observed, and we show the first measurement by photoluminescence of the crystal field splitting of zinc phosphide.