Characterization of ion-assisted pulsed laser deposited cubic boron nitride films

Abstract

Cubic boron nitride films were deposited by pulsed laser deposition from a boron nitride and a boron target using a KrF-excimer laser, where the growing films were bombarded by a nitrogen or a nitrogen/argon ion beam. The variation of the film properties with laser and ion beam parameters and substrate temperature has been investigated by infrared spectroscopy, cross-section and plan-view high-resolution transmission electron microscopy, electron-energy-loss spectroscopy and in situ ellipsometry. It will be shown that c-BN films with high phase purity can be prepared at sufficiently strong ion bombardment as well as substrate temperatures above 160°C. The c-BN phase was found to grow exclusively on top of the well-known hexagonal interlayer with c-axis orientation parallel to the substrate surface. Two types of nucleation were observed, the first characterized by c-BN (111) and the second by c-BN (001) lattice planes growing parallel to the (0002) lattice planes of the initially formed h-BN layer. c-BN films were prepared at maximum growth rates of 16 nm/min. Additional UV-photon irradiation of the growing films results in distinct modifications of the microstructure of the BN films. Using laser pulse energy densities on the substrate surface above 200 mJ/cm2 the laser irradiation leads to the formation of turbostratic h-BN even though the unirradiated film regions of the same sample show the cubic structure. In contrast, films irradiated at 100 to 160 mJ/cm2 are cubic. Electron microscopic observations show that in this range the mean diameter of crystallite in the excimer laser irradiated regions increased by a factor of two in comparison with unirradiated regions of the same sample. The experimental results will be discussed in context with the results of temperature field calculations.