ION-ASSISTED PULSED-LASER DEPOSITION OF CUBIC BORON-NITRIDE FILMS

Ion-assisted pulsed laser deposition has been used to produce films co ntaining >85% sp3-bonded cubic boron nitride (c-BN). By ablating from a target of hexagonal boron nitride (h-BN), BN films have been deposit ed on heated (50-800-degrees-C) Si(100) surfaces. The growing films ar e irradiated with ions from a broad beam ion source operated with Ar a nd N2 source gasses. Successful c-BN synthesis has been confirmed by F ourier transform infrared (FTIR) spectroscopy, high-resolution transmi ssion electron microscopy (TEM), selected-area electron diffraction, e lectron energy-loss spectroscopy, and x-ray diffraction. The films are polycrystalline and show grain sizes up to 300 angstrom. In addition, Rutherford backscattering, elastic recoil detection, and Auger electr on spectroscopies have been used to further characterize the samples. The effects of varying ion current density, substrate growth temperatu re, growth time, and ion energy have been investigated. It is found th at stoichiometric films with a high c-BN percentage can be grown betwe en 150 and 500-degrees-C. Below approximately 150-degrees-C, the c -BN percentage drops dramatically, and the deposited film is completely r esputtered at the current densities and ablation deposition rates used . As the deposition temperature rises above approximately 500-degrees- C the c-BN percentage also drops, but less dramatically than at low te mperatures. In addition, the IR-active c-BN mode narrows considerably as the deposition temperature increases, suggesting that the c-BN mate rial has fewer defects or larger grain size. It is found that films wi th a high c-BN percentage are deposited only in a narrow window of ion /atom arrival values that are near unity at beam energies between 800 and 1200 eV Below this window the deposited films have a low c-BN perc entage, and above this window the deposited film is completely resputt ered. Using FTIR analysis, it is found that the c-BN percentage in the se samples is dependent upon growth time. The initial deposit is essen tially all sp2-bonded material and sp3-bonded material forms above thi s layer. Consistently, cross-section TEM samples reveal this layer to consist of an amorphous BN layer (approximately 30 angstrom thick) dir ectly on the Si substrate followed by highly oriented turbostratic BN (approximately 300 angstrom thick) and finally the c-BN layer. The h-B N/t-BN interfacial layer is oriented with the 002 basal planes perpend icular to the plane of the substrate. Importantly, the position of the c-BN IR phonon changes with growth time. Initially this mode appears near 1130 cm-1 and decreases with growth time to a constant value of 1 085 cm-1. Since in bulk c-BN the IR mode appears at 1065 cm-1, a large compressive stress induced by the ion bombardment is suggested. Possi ble mechanisms are commented on for the conversion process to c-BN bas ed upon the results.