Boron nitride (BN) thin films were deposited on monocrystalline Si(100) wafers using electron beam evaporation of boron with simultaneous bombardment by nitrogen and argon ions. The effect of film thickness on the resultant BN phase was investigated using Fourier transform infrared (FTIR) spectroscopy and high resolution transmission electron microscopy (HRTEM). These techniques revealed the consecutive deposition of an initial 20 Å thick layer of amorphous BN, 20–50 Å of hexagonal BN having a layered structure, and a final layer of the polycrystalline cubic phase. The growth sequence of the layers is believed to result primarily from increasing biaxial compressive stresses. Favorable surface and interface energy and crystallographic relationships may also assist in the nucleation of the cubic and the hexagonal phases, respectively. The presence of the amorphous and hexagonal regions explains why there have been no reports of the growth of 100% cubic boron nitride on Si.