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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.

next up previous
Next: Discussion Up: Title page Previous: Experimental Details (Alison Chaiken)
Wed Oct 11 09:49:01 PDT 1995