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Experimental Details

The three BN films used in this study were produced by ion-assisted pulsed laser deposition onto heated silicon substrates, as detailed previously.[6] Deposition conditions are summarized in Table 1. Infrared transmission measurements (not shown) on these films revealed absorption at wavelengths characteristic of both the cubic and hexagonal phases, allowing an estimate of their volume fraction. The purity of the three types of BN powder was determined using x-ray diffraction (not shown) to be greater than 90%.

Table: Parameters obtained from fits to the B K-edge spectra of two types of BN powder and three BN/Si films prepared by ion-assisted pulsed laser deposition under slightly different conditions. The sp3 fraction in the films was estimated using infrared transmission measurements. NA = not applicable.

B and N K-edge x-ray absorption spectra were obtained at both the 8-1 and 8-2 beamlines at SSRL, as well as at the IBM/U8 beamline at NSLS. Details of the data acquisition have been reported previously.[5,7] A simple polynomial was adequate to fit the background of the powder spectra but it was necessary to use a gaussian-broadened step function[1] to fit the background intensity for all the films' B K-edge spectra. The position of the step was fixed at 2.125 eV above the B 1s pi* peak, the energy where sigma * absorption begins in cBN. The absolute energies of spectral features are reproducible to within 0.3 eV from run to run, but the relative energies of the features are reproducible to within 0.05 eV. (Alison Chaiken)
Wed Oct 11 09:49:01 PDT 1995