Heteroepitaxial GaN layers have been grown on many foreign substrates such as sapphire, silicon carbide or silicon. Therein, a common feature is that the GaN epitaxy proceeds via several stages, i.e., nitridation, growth of the low temperature buffer layer, annealing, and, finally, growth of the layer with the reduced density of dislocations.

The mechanism of the GaN growth has also been investigated. Despite many efforts, the microscopic mechanism of island creation, dominant growth of selected grains, their coalescence, and the strain reduction via creation of dislocations have been only partially elucidated.

Recently, alternative GaN nano-ceramic substrates were developed [1, 2]. They offer a unique possibility to investigate the growth of GaN layers originated on various crystallographic orientations in nano-scale which is the subject of the reported transmission electron microscopic (TEM) study. For comparison, GaN layers on silicon were also investigated by this technique.

It is shown that the MOCVD growth of GaN layers on the GaN nano-ceramic substrates is highly anisotropic. The disorientation of the grains in the layer is much higher causing larger strains as compared to the GaN layer on the silicon substrate. In consequence, the flat growth front can be attained with higher difficulty. However, an appropriately thick GaN layer can, eventually, develop flat surfaces suitable for construction of optoelectronic structures. This can be achieved at the cost of creation of the relatively large density of dislocations and stacking faults.

We want to acknowledge a generous support of the Polish Committee for Scientific Research KBN, Grant 3 T08D 043 26

[1] E. Grzanka, S. Stelmakh, S. Gierlotka, A. Swiderska-Sroda, G. Kalisz, B. Palosz, M. Drygas, J. F. Janik, R. T. Paine: "In situ X-ray diffraction studies of distribution of strain during simultaneous sintering of nanocrystalline GaN powders under high-pressure high-temperature conditions"; EPDIC-10, September 1-4, 2006; Geneve, Switzerland.

[2] S. Stelmakh, A. Swiderska-Sroda, G. Kalisz, S. Gierlotka, E. Grzanka, B. Palosz, M. Drygas, J. F. Janik, R. T. Paine: "Microstructure and mechanical properties of GaN nanoceramics sintered under high-pressure high-temperature conditions"; International Conference on Nanoscience and Technology 2006 (ICN+T 2006); July 30-August 4, 2006; Basel, Switzerland.