Electron energy loss spectroscopy—An additional tool to characterize thin films of cubic boron nitride

Abstract

Applying Reflection Electron Energy Loss Spectroscopy (REELS) to boron nitride (BN) films in principle should allow us to distinguish between different crystal structures, i.e. between the cubic (c-) and the hexagonal (h-) phase, respectively. This possibility results from the fact that the two phases exhibit well separated plasmon loss lines. In practice, however, largely varying line positions are reported in the literature for these plasmon losses leading to contradictory conclusions on the reliability of the REELS method. It is the aim of the present contribution to resolve the above inconsistencies and to demonstrate that REELS can reliably be used to characterize c-BN films if the sampling depth of the spectroscopy is properly adjusted via the energy of the primary electron beam. For this purpose, BN films with various contents of the cubic phase (0–95%) as determined by infra-red (IR)-spectroscopy have been prepared by ion beam assisted sputter deposition onto Si (100) substrates. The as-prepared films were analyzed in situ by REELS followed by ion irradiation (3 keV Ar + ) to investigate the influence of the sputter cleaning process frequently applied to ex situ prepared samples prior to spectroscopic measurements. As the main result, an h-BN-like surface layer on top of the c-BN phase can clearly be identified for the as-prepared samples, yielding a thickness of this top layer of approx. 0.5 nm. In addition, it will be shown that the plasmon loss line of c-BN is continuously shifted towards the corresponding line of h-BN by introducing lattice defects as produced by, e.g. Ar + ion bombardment, offering a simple explanation for the variety of spectroscopic results found for sputter-cleaned ex situ prepared specimens.