RAMAN-SPECTROSCOPY INVESTIGATION OF SIZE EFFECTS IN CUBIC BORON-NITRIDE

Raman spectra were taken of cubic boron nitride (c-BN) crystals with d iameters between 100 nm and 1 mm. The Raman line shape of the optical phonons was found to become increasingly asymmetric towards lower freq uency shifts, broader and weaker with decreasing crystal diameters. Th e results can be explained in terms of a spatial correlation model. Th e corresponding correlation lengths Lie in the nanometer range, i.e. s everal orders of magnitude below the actual crystallite sizes as deter mined by electron microscopy, thus revealing a high defect density. An additional examined typical c-BN film on a Si(100) substrate exhibits even weaker and broader structures consistent with an even higher def ect density. (C) 1997 American Institute of Physics.