Low-temperature (100–200 °C) growth of phosphorus nitride (P3N5) on InP surfaces has been successfully developed using a mixture of PCl3 and NH3 gases by a direct photochemical vapor deposition. The films have a resistivity of 1×1014 Ω cm and a breakdown voltage of 1×107 V/cm. The minimum density of interface trap states for the aluminum (Al)-P3N5-InP metal-insulator-semiconductor structure after the in situ processes is about 3.6×1010 cm−2 eV−1 near the midgap of InP. Auger electron spectroscopy and x-ray photoelectron spectroscopy measurements were used to evaluate the film and the film/InP interface.