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The technological importance of boron nitride is evident from the increasing use of this material in tribological and electronic device applications.[1-4] In particular, BN's use in thin-films has drawn attention to it as an abrasive coating, or as an insulator / buffer layer in electronic device heterostructures. Recent work by Doll et al.[5] has shown that the wide band gap cubic phase can be grown on a silicon substrate using ion-assisted, pulsed laser deposition. Other routes of fabricating unique BN thin-films have been reported, including a hexagonal phase film synthesized from boron and nitrogen containing precursor gases using a photo-assisted process.[6] Unfortunately, these meta-stable thin- films of BN require non-equilibrium growth conditions that frequently produce amorphous, or incoherent, thin films. Therefore, quantitative structural methods would be instrumental in determining the morphology of these films and would reveal structural information that could lead to new thin film fabrication methods.

Recently, we have shown that core-level photoabsorption can be used to characterize the bonding and structure of fine-grained thin-films of boron nitride[7]. In this earlier report we showed that core-level photoabsorption can be used to identify the bonding in an incommensurate thin-film - a film that exhibits no long range order. The boron and nitrogen 1s photoabsorption , or Near Edge X-ray Absorption Fine Structure (NEXAFS), probes the element-specific empty electronic states.[8] This synchrotron-based spectroscopy has been shown to be a useful structural tool that has local electronic and bonding structure sensitivity. NEXAFS spectroscopy is not subject to the constraint of long range order, which makes it complementary to other structure determination methods.

In this work, we have characterized the bonding of the hexagonal, cubic, rhombohedral, and wurtzite phases of BN using core-level photoabsorption. In our measurements we can identify unique spectral features in the photoabsorption cross section and associate these resonances with each phase. Also, because core-level photoabsorption obeys dipole optical selection rules, we can concurrently identify the type of bonding present in the structurally characterized material, in particular, identify the presence of conjugated p bands indicative of sp2 -like bonding in an incoherent BN/Si(100) film.

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