Structure investigation of BN films grown by ion-beam-assisted deposition by means of polarised IR and Raman spectroscopy

We present an optical investigation, by means of polarised infrared (IR) sp ectroscopy and Raman scattering, of the microstructure and crystallinity of mixed films of hexagonal and cubic boron nitride (h-BN and c-BN, respectiv ely). The films were deposited on an unheated silicon substrate by the ion- beam-assisted deposition method (IBAD) at low energy (400-500 eV). The depo sition temperature, due to the ion bombardment, was in the range 200-250 de grees C at the end of the deposition process. Different film types were gro wn on a silicon substrate of dimensions 75 mm x 15 mm by changing the ion ( nitrogen + argon) to atom (thermal boron) arrival ratio, phi(ion)/phi(B), i n the range 0.69-3. Polarised IR reflectivity (PIRR) spectra were acquired at different positions on the BN film (different arrival ratios phi(ion)/ph i(B)) and show an important upwards shift of Transverse Optical (TO) and Lo ngitudinal Optical (LO) phonons of the twofold degenerated mode E-1u of the sp(2) phase at the transition zone from sp(2) to sp(3) phases. Several pro cesses can shift the IR phonon peaks, including the degree of crystallinity , film thickness, film stoichiometry and intrinsic stress. The micro-Raman results and the full-width at half-maximum values of TO phonons of the E-1u mode show that the BN film has a similar crystallinity in all regions. The effect of the film thickness was shown by using a microstructure-dependent model for the IR anisotropic effective dielectric function of thin films. In order to show the influence of the film stoichiometry in the E-1u(TO) pe ak positions, a series of samples was deposited at 100% of nitrogen by chan ging the arrival flux phi(N)/phi(B) in the interval 0.75-2.5. It have been observed that, in this range of flux ratio, the E-1u(TO) phonon shift is ne gligible in comparison with the shift observed in the PIRR measurements. Th is results suggest that this E-1u(TO) phonon shift is due to the intrinsic stress. If we consider the findings of Friedmann et al. [J. Appl. Phys. 76 (1994) 3088], who suggested that nucleation of the cubic phase occurs as a result of extremely high stress, and those of Medlin et al. [J. Appl. Phys. 70 (1996) 3567], who showed a direct route to c-BN formation via a transfo rmation of h-BN into rhombohedral BN (r-BN), we can conclude that, in our c ase, the shift observed is due to an intrinsic high-stress phase. Then. a s tructural modification of h-BN into r-BN phase might be involved as a precu rsor for nucleation of the cubic phase. (C) 1999 Elsevier Science S.A. All rights reserved.