Results

Boron and nitrogen core-level photoabsorption was analyzed using the methods described by Stühr[8] where resonances in the NEXAFS are interpreted as transitions from bound, localized core-levels with discrete angular-momentum to quasi-bound or continuum states. Dipole selection rules, which govern these optical transitions, predict allowed transitions for 1s initial states into p-like empty final states. These p-like final states - or p* states - are associated with anti-bonding molecular orbitals or empty bands in systems with p-bonding.[18] These empty states are predicted for the hexagonal BN 19 , much like structurally similar graphite.

In Figure 2, the most prominent feature in the B 1s NEXAFS spectrum for hBN is the narrow and intense transition at 192.0 eV. This resonance has been observed and assigned as a 1s-p* transition in previous photoabsorption measurements.[4,20] These p-bond related resonances are associated with sp² hybridized, planar bonding and should be absent in sp³ tetrahedrally bonded materials. In fact, the cubic phase BN with the diamond structure pictured in Figure 1 is predicted to have sp³ bonding and should not have any strong pi^* NEXAFS resonances.[19] Figure 2 clearly supports this by illustrating that cBN does not exhibit this sharp 1s-p^* resonance at 192.0 eV, but instead has an absorption maximum into sigma^* continuum states at 194 eV as its dominant feature. The same conclusion can be made for the n-edge results in Figure 3; the feature at 399 eV for the hBN sample can be attributed to a pi^* resonance and is absent in the cBN NEXAFS. This striking difference in the absorption cross section between two phases of the same material make NEXAFS measurements a valuable tool for better understanding the relationship amongst the bonding, atomic, and electronic structure in solids.

The rhombohedral phase of BN is formed by heat treating a mixture of NaBH4, NH4CL, and KCN.[10] Its structure (pictured in Figure 1) can be viewed as a distortion of the hBN structure. Hexagonal BN has A-B-A stacking of hexagonally bonded planes, but rBN has A-B-C-A stacking. Because the intra-planar bonding/electronic interaction is greater than the inter-planar interaction - and the intra-planar bonding is identical - for these materials, the B 1s NEXAFS of rBN should be similar to that of hBN. In fact, the rBN B 1s photoabsorption in Figure 2 exhibits a strong 1s - pi^* resonance indicative of sp2 bonding and is nearly identical to the hBN NEXAFS. Using the core-level photoabsorption alone, it would be difficult to distinguish between the hexagonal and rhombohedral phases of BN. This is not surprising because of the nearly identical local bonding and chemical environment surrounding each photoabsorber. For the nitrogen edges illustrated in Figure 3, a similar conclusion can be drawn: there is little to distinguish the hexagonal and rhombohedral phases in the NEXAFS structure. Again, this can be explained by the dominance of the 1s - pi^* transition in the near-edge structure. As mentioned, both phases would have a similar sp², conjugated p-bond leading to these resonances.

The wurtzite BN was formed by a shock compression process[9] and has a structure that can be viewed as a distorted cubic structure (Figure 1). Qualitatively, this would imply that the bonding in this phase of the material would be more sp³ -like than sp² and should not have quasi-bound p-states. Band calculations predict[19] ample dispersion along all three dimensions and supports this qualitative view that the bonding in wBN is three dimensional - like cBN - and dissimilar to the planar hBN - which has little or no electronic-state dispersion along the c-axis of the material. This assessment is supported by the B 1s NEXAFS in Figure 2. In these data, there is no evidence of the 1s -p^* resonances such as those found in hBN, and the edge onset is close to that of sp³ - bonded cBN (194 eV). The resonances present in wBN are similar to those found in cBN and can be correlated with 1s - sigma^* transitions. Comparing the B 1s photoabsorption structure of cBN and wBN, we observe many similarities, which is expected for similarly bonded material with the same chemical composition, but near the excitation edge there is a more prominent resonance at 196 eV in the wBN NEXAFS than in the cBN. This comparison is not as striking as the difference between hBN and cBN, but at the edge - and between 210 and 216 eV - the dissimilarity is clear. Calculations that model the NEXAFS excitation process, and take into account the final-state core-hole interaction, should be able to model the effects of structural changes on the core-level photoabsorption features.[11]

Using the NEXAFS measured from these four phases of BN, we can then proceed to qualitatively assess the bonding of the BN/Si thin film. Immediately it is evident from the B 1s photoabsorption in Figure 2 that the BN/Si sample has the strong 1s - pi^* resonance at 192 eV indicative of sp² bonding as is observed in the hexagonal phase of BN. The same is true for the nitrogen 1s photoabsorption edge shown in Figure 3. One difference is noticeable between the BN/Si and hBN boron 1s spectra: the resonance at 200 eV is more pronounced in the BN/Si film than in the hBN sample. In the nitrogen spectrum in Figure 3, the resonance at 407 eV is higher than the edge peak for the BN/Si sample, but the opposite is true for the hBN sample. One explanation for this could be contribution of cubic- phase, or sp³ related, resonances to the NEXAFS. This is supported by infrared absorption measurements on this BN/Si film which indicate the presence of approximately 15% cBN.[11] While there is no unique evidence of cBN in this qualitative assessment of the near-edge structure of BN/Si, the NEXAFS measurements are useful for determining the sp² bonding present in the thin film.

In our earlier report on using NEXAFS to determine the bonding in a BN thin film, we were able to determine that the film was mainly sp² bonded material, but was not entirely amorphous. It was determined through a polarization dependent NEXAFS measurement that the BN film was preferentially ordered with the BN hexagonal planes lying nearly perpendicular to the surface normal (within 20 degrees). This method of determining the angular orientation of the p^* states in a material has been described previously. 17 Our earlier observation of the hBN - like film being preferentially oriented with the BN c-axis lying near the surface plane was also seen in this film (data not shown). This counterintuitive film structure was supported by recently published TEM results[21] that show laser ablated BN films can have such an oriented hBN-like component. In fact, electron diffraction measurements of our film confirm this perpendicular stacking of hexagonal BN sheets.[22] McKenzie[23] has also suggested that during the growth of carbon films in-plane compressive stresses due to ion irradiation can cause the c-axis of graphite to orient parallel to the substrate surface.