Transparent and insulating thin films have been grown by the plasma decomposition of B2H6, NH3, and H2, at a substrate temperature of 250°C. From chemical composition, transmission electron microscopy, infrared absorption, and optical absorption measurements, the thin films are determined to be a mixed phase of crystalline cubic boron nitride and amorphous hydrogenated boron nitride. Also, the films have significantly more boron than nitrogen, a large concentration of hydrogen, a very large bandgap, strong infrared aborption due to both hexagonal boron nitride and boron icosahedra, and good adhesion to various substrates.