Optical absorption edge characteristics of cubic boron nitride thin films

Boron nitride films with different cubic phase contents were deposited in a radio frequency bias sputtering system by a two-stage deposition process. The Fourier transform infrared spectra and UV-visible transmittance and ref lection spectra were measured. The optical absorption edge of BN films cons ists of a low energy region in which alpha increases exponentially as incid ent photon energy h nu, and a high energy region, in which alpha varies as (h nu)(m), which is the characteristics of amorphous materials. These two r egions are fitted by the Urbach tail model and the band-to-band transition model with an effective medium approach, and the Urbach energy and the opti cal band gap are determined from these fits. With an increase in the cubic boron nitride (c-BN) content, the absorption edge shifts to the higher ener gy and the optical band gap increases. For a BN film with 88% cubic phase, the optical band gap exceeds 6.0 eV, which is comparable to that of c-BN si ngle crystal. (C) 1999 American Institute of Physics. [S0003-6951(99)02327- X].