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.