The most important advantage of the growth of GaN from high temperature gallium solutions under high nitrogen pressure is that the crystals are almost free of dislocations (100 cm-2). The low density of dislocations is of crucial importance for the high power optoelectronic devices based on GaN and their ternaries. Therefore, the application of the pressure grown crystals as substrates for epitaxial deposition of blue-violet InGaN laser diode structure should give high quality devices. This will be shown by comparing of some physical properties of the quantum structures grown on both near defect free GaN and typical GaN/Al2O3 substrates containing high density of dislocations. Also the recent progress in the construction of high power blue and violet lasers on the pressure grown substrates achieved at the HPRC will be rewieved.
The most important disadvantage of the high pressure growth of GaN is that the method gives individual 100 mm thick platelets of 10-15 mm in their lateral size. The strong anisotropy of the crystallization process will be discussed in terms of two dimensional nucleation on particular crystallographic planes to show that this is rather an intrinsic property of the system. The relations between supersaturation and the nucleation rates explain both the anisotropy and difficulties with control of the growth in directions parallel to the c-axis of the crystal. The application of directional crystallization methods is a way to increase the rate of stable growth of near dislocation free GaN in c-direction. The results of both the high pressure solution growth and the low pressure HVPE (Hydride Vapor Phase Epitaxy) with the use of GaN platelets as substrates are very promising and will be also presented.

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1. Growth anisotropy of GaN single crystals by high pressure and HVPE methods
2. Recognition of chemical (electrically active) non-homogeneities in thick HVPE-grown GaN layers
3. Microstructure of homoepitaxially grown InGaN/GaN, violet light emitting laser diodes.
4. Anomalous behaviour of the photoluminescence from GaN/AlGaN quantum wells
5. Built-in electric fields in group III-nitride light emitting quantum structures
6. Bowing of epitaxial structures grown on bulk GaN substrates
7. Interaction of Mn and Ti atoms with GaN surface - a resonant photoemission study
8. The optimalisation of the GaN and GaN/AlGaN heterojunctions on bulk crystals using plasma-assisted molecular beam epitaxy
9. Optical Detection of 2DEG in GaN/AlGaN Structures - High Magnetic Field Studies
10. Gallium nitride surface formation and modification by Mn deposition - photoemission studies with use of synchrotron radiation