Orientation of the growing crystal surface is determined by orientation of the seed and by growth conditions influencing rates and mechanisms of the crystallization process. Physical properties of crystals are dependent on growth direction (growth surface) due to differences in microscopic growth mechanisms determining point and structural defects in the crystal. For quantum structures the crystallographic orientation can decide about their fundamental properties, which follows from symmetry of a particular crystal lattice.

In this presentation, crystallization of GaN by High Pressure Solution (HPS) and by HVPE on HPS grown seeds of different shapes and on large, flat seeds (substrates) will be considered. Also the differences in physical properties of GaN-AlGaN quantum structures grown on GaN bulk substrates of different orientations by PA MBE will be discussed.

The ~10 h HVPE growth on near dislocation free seeds in the form of (0001) oriented relatively small (~1cm) and thin (~0.1mm) platelets, leads to the formation of a few mm thick bulk crystals bonded by {0001} polar and {1-101} semi-polar side faces. The crystals are result of simultaneous growth in directions both lateral and normal to the (0001) plane of the initial seed. These different growth directions are reflected in large differences in physical properties of corresponding sectors of bulk crystals. As follows from PL, photo etching and micro-Raman measurements, the free electron concentration can vary by more than two orders of magnitude between the material grown on (0001) surface of the seed (electron concentration < 10¹⁷ cm^-3) and the one grown laterally on semi-polar side faces of the crystal (electron concentration >10¹⁹ cm^-3). Crystal sectors grown on the semi-polar faces are always near dislocation free whereas sectors grown on (0001) face can be both almost defect free (like the seeds) or containing dislocations, depending on the applied growth conditions (growth rate and carrier gas were varied).

As follows from micro-Raman characterization, much more uniform crystals in terms of their physical properties can be grown on the seeds in the form of hexagonal needles where the HVPE growth occurs mainly in non-polar {10-10} directions. The crystals of free electron concentration of about 5 x 10¹⁸ cm^-3 were grown in this way.

The change of orientation of the crystallization front can be induced also on the large flat substrates by their patterning and the use of growth conditions allowing formation of the required crystal facets. Such crystals are non-uniform in distribution of both point defects which results in non-uniform electrical and optical properties and structural defects (dislocations). Such crystal often contain large almost dislocation free areas being result of lateral growth. These features of the crystals grown by HVPE on the patterned (0001) substrates will be shown with the use of photo-etching and defect selective etching methods.

The substrates of both (0001) polar and (11-20) non-polar orientations (5 x 10mm) were cut from the bulk crystals grown by HVPE and used for growth of GaN-AlGaN quantum structures by PA MBE. Optical properties of these structures will be compared.

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