Direct observation of hexagonal boron nitride at the grain boundary of cubic boron nitride by high resolution electron microscopy

Cubic boron nitride (c-BN) film deposited on a Si substrate was observed by high resolution electron microscopy. Thin layers, 1-2 nm, of hexagonal boron nitride (h-BN) phase were often found at the boundaries of c-BN grains. The observed interplanar spacing was about 0.33 nm, which coincided with that of the (0002) plane of h-BN. The nucleation mechanism of c-BN film is briefly discussed based on the formation of the h-BN phase at the grain boundaries. The existence of the h-BN phase at the boundaries of c-BN grains may be the reason for the occurrence of compressive stress and cracks in c-BN thin films.