Pulsed laser deposition and modification of cubic boron nitride films

Abstract

Pulsed laser ablation using an excimer laser of 248-nm wavelength was applied to prepare boron nitride films, where the ablation from elemental boron targets as well as boron nitride was studied. The growing films were bombarded by a nitrogen or a nitrogen/argon ion beam to obtain stoichiometric films and to investigate ion induced modifications of structure and properties. Moreover, in order to study the influence of 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 investigated by in-situ ellipsometry, infrared spectroscopy, transmission electron microscopy including diffraction measurements and electron-energy-loss spectroscopy. The results show that the boron nitride films have either predominantly hexagonal or predominantly cubic structure dependent on the laser and ion beam parameters and the substrate temperature. At the optimum laser and ion beam parameters, nearly pure cubic films could be prepared even at a relatively low substrate temperature of 200°C. However, those films prepared on silicon substrates exhibit 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 nanocrystallites up to 30 nm in diameter. Additional excimer laser irradiation 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 even though the unirradiated film regions of the same sample show the cubic structure. The most interesting range of laser fluence lies between 100 and 160 mJ/cm 2. Electron microscopic observations show that in this range, the mean diameter of crystallite in the excimer laser irradiated regions increased by a factor of 2 in comparison with unirradiated regions of the same sample.