This low mobility indicates a fairly high level of compensation or disorder. Figure 7 shows that the surface meshes for these two orientations are very similar. It remains to be seen whether the lower misfit can be translated into higher quality films, but it is fair to say that the quality of the films we have grown on LiAlO 2 is not yet as good as films we have grown on sapphire. The observed tilt of the films is somewhat surprising. The usual picture of misfit accommodation in epitaxial growth is that the film grows in a strained state until the film is so thick that the built-up strain energy surpasses the threshold for dislocation formation.
Dislocations often enter as pairs, or as loops.
The microscopic tilt associated with each dislocation averages out, resulting in no net tilt, although the small tilts contribute to the mosaic spread as observed in x-ray diffraction rocking curves. If the misfit is negative, coherent tilt would require a miscut surface. In addition, the coherent tilt process cannot explain the tilts we observe for AlN grown on LiAlO 2 ., since the relevant misfit is negative. Another way that a film might be tilted crystallographically with respect to a substrate is by grapho-epitaxy on a miscut substrate. Here the substrate acts as a physical template for epitaxy, but has very weak chemical bonding to the film.