On the Initial Cubic Phase Nucleation in Boron Nitride Films

Abstract

Boron nitride thin films were prepared on Si (100) substrates by r.f. magnetron sputtering from an hBN target with pure argon as working gas. During the deposition, the growing film was ion bombarded via an r.f. bias (27.12MHz ) at the substrate. The deposited films were characterized ex situ by Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (ATM), electron energy loss spectroscopy (EELS) and ellipsometry. For appropriate bias voltages at the substrate (-180...-240V, at a working pressure of 1x10 -3 mbar), the films were found to consist up to vol 80% of the cubic phase BN. To investigate the growth dynamics of the films, a series of samples was produced with different deposition time, while the other deposition parameters were kept constant such that cBN growth is facilitated (1x10 -3 mbar of Ar, -220V substrate bias and 500°C substrate temperature). At the initial stage, sp 2 bonded BN grew rapidly with a deposition rate of 0.35nm/s. The cubic phase began to appear when a film thickness of 70nm was reached, and it developed steadily thereafter with increasing deposition time. However, it was found that the amount of sp 2 bonded BN, as measured from the IR absorbance at 1390 cm -1 , decreased at the moment when cBN started to grow. This evidently indicates that the initial cBN nucleation happens via a transformation of the sp 2 phase in the surface region of the growing film. An inspection of the evolution of film thickness with deposition time shows that the film shrank concurrently with the reduction of the sp 2 phase, which further confirms the conversion of the sp 2 structure into the denser cubic phase. It is concluded that the initial nucleation of the cubic phase is a subsurface phenomenon, which is induced by appropriate ion bombardment and, hence, requires a minimum thickness of a sp 2 -bonded BN-layer.