PULSED-LASER DEPOSITION AND MODIFICATION OF CUBIC BORON-NITRIDE FILMS

Pulsed laser ablation using an excimer laser of 248-nm wavelength was applied to prepare boron nitride films, where the ablation from elemen tal boron targets as well as boron nitride was studied. The growing fi lms were bombarded by a nitrogen or a nitrogen/argon ion beam to obtai n stoichiometric films and to investigate inn induced modifications of structure and properties. Moreover, in order to study the influence o f ultraviolet photon bombardment on the structure of the growing boron nitride films, a certain region of each film was irradiated using an excimer laser beam of 248-nm wavelength. The films have been investiga ted by in-situ ellipsometry, infrared spectroscopy, transmission elect ron microscopy including diffraction measurements and electron-energy- loss spectroscopy. The results shaw that the boron nitride films have either predominantly hexagonal or predominantly cubic structure depend ent on the laser and ion beam parameters and the substrate temperature . At the optimum laser and ion beam parameters, nearly pure cubic film s could be: prepared even at a relatively low substrate temperature of 200 degrees C. However, those films prepared on silicon substrates ex hibit the typical layered structure with a 5 to 10-nm thick amorphous layer formed initially on the substrate, a 10 to 30-nm thick layer of highly oriented h-BN and the c-BN layer. The c-BN layer consists of na nocrystallites up to 30 nm in diameter. Additional excimer laser irrad iation of the growing films resulted in distinct modifications of the microstructure of the BN films. Using laser fluences above 200 mJ/cm(2 ), the laser irradiation leads to the formation of turbostratic h-BN e ven though the unirradiated film regions of the same sample show the c ubic structure. The most interesting range of laser fluence lies betwe en 100 and 160 mJ/cm(2). Electron microscopic observations show that i n this range. the mean diameter of crystallite in the excimer laser ir radiated regions increased by a factor of 2 in comparison with unirrad iated regions of the same sample. (C) 1998 Elsevier Science B.V.