RESULTS

Figure 1a shows the B K-edge spectra of the three BN/Si films, while Figure 1b shows the spectra of hBN, rBN and cBN powders, which have been described in detail previously.[5] The most notable feature of the hBN and rBN powder data is the presence of the pi^* feature at 192.0 eV, which is characteristic of sp² bonding.[8,9] This pi^* feature is absent in cBN powder due to the sp³ nature of the bonding. In the sp²-bonded layered materials, the pi^* feature has been

described as a core exciton whose position below the conduction-band edge is a result of a 1.3 eV excitonic binding energy.[10]

Comparison of the data Figures 1a and 1b shows that the bonding in the BN/Si films is predominantly sp², similar to the hBN powder. However, there are three sharp peaks that appear in the B spectra of all three BN films just above the pi^* peak but which don't appear in the hBN data. Parameters obtained from fits to these smaller peaks are collected in Table 1 along with fit parameters from the hBN and rBN powders. The BN/Si films' spectra also have a peak at 199 eV in the middle of the empty sigma band which is not present in any of the powder spectra.

N K-edge spectra for the BN/Si films and the three BN powders are shown in Figure 2. The first peak at 402 eV in the sp²-bonded materials is again a pi^* feature, although it is not as intense or as far below the conduction-band edge as in the B K-edge spectra. In the Wannier model of excitonic behavior, the intensity and binding energy of an exciton are reduced when the core hole is created on the anionic site.[10] Overall the film data is similar to that for hBN and rBN, although the intensity of the pi^* peak at 402 eV is reduced with respect to that of the the sigma^* peak at 408.5 eV in the films compared to the hBN or rBN standards.