ZnSiP 2 is a wide band gap material that is lattice matched with Si, offering the potential for Si-based optoelectronic materials and devices, including multijunction photovoltaics. We present a carbon-free chemical vapor deposition process for the growth of both epitaxial and amorphous thin films of ZnSiP 2 -Si alloys with tunable Si content on Si substrates. Si alloy content is widely tunable across the full composition space in amorphous films. Optical absorption of these films reveals relatively little variation with Si content, despite the fact that ZnSiP 2 has a much wider band gap of 2.1 eV. Post-growth crystallization of Si-rich films resulted in epitaxial alignment, as measured by X-ray diffraction and transmission electron microscopy. These films have an optical absorption onset near 1.1 eV, suggesting the possibility of band gap tuning with Si content in crystalline films. The optical absorption is comparably strong to pure ZnSiP 2 , suggesting a more direct transition than in pure Si. ZnSiP 2 is a wide band gap material lattice matched with Si, with potential for Si-based optoelectronics. Here, amorphous ZnSiP 2 -Si alloys are grown with tunable composition. Films with Si-rich compositions can be crystallized into epitaxial films.