Poster abstract

Tetrahedral amorphous carbon films have attracted considerable interest because they can be used as an inexpensive and easily produced wide band-gap semiconductor (2.0-2.7 eV) in the fabrication of heterojunction devices, thin film transistors and large area flat-emitter display materials" [1]. Relaxation processes are vital for useful diamond or diamond-like films, which usually have large grown-in stresses. Stress relief reduces the chances of delamination. Post-growth annealing also changes the band gap. There is a supposition for diamond-like films that the reduction in band gap with post-growth annealing can be attributed to the encroachment of the gap by aromatic pi-pi* states. It is argued that the pi-pi* gap in DLC decreases as the size of the aromatic system increases, in the same way as it does for polyaromatic molecules. All sp3 systems eventually graphitise with temperature (sp3 to sp2 conversion), but there is evidence that sp2 carbon atoms can effectively aggregate during annealing, without further conversion of sp3 atoms [2].

We have used dislocations in crystalline diamond as models for some of the environments and processes occurring within diamond-like carbon and tetrahedral amorphous carbon. Our justification is that a dislocation core shares some of the features of the amorphous state (i.e. non-crystalline ring statistics) and that structural changes are easily identified with dislocation motion. Clearly, the model has most applicability for films with a density approaching that of diamond. We have investigated the possibilities that the cores of dislocations in diamond are graphitised, and that dislocations can glide together to form climb dipoles which nucleate graphitic regions. Furthermore, we draw on previous work which gave the activation energy for motion of dislocations in the presence and in the absence of hydrogen to deduce the activation energies of the process of sp2 aggregation within an sp3 carbon network [3].

[1] K. Zellama, Current opinion in Solid State and Materials Science, 4, 34 (1999). [2] R. Kalish, Y. Lifshitz, K. Nugent and S. Prawer, Appl. Phys. Lett., 74, 2936 (1999). [3] M. I. Heggie, S. Jenkins, C. P. Ewels, P. Jemmer, R. Jones and P. R. Briddon, J. Phys. Condens. Matter, 12, 10263 (2000).