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Electron-beam-induced-current (EBIC) investigations of GaN structures grown by metal-organic chemical vapour deposition on (0001) sapphire substrates have been carried out. It is shown that the widths of the EBIC profiles for individual extended defects can be as small as about 100 nm. This width is observed to decrease with decreasing diffusion length and/or with increasing electron beam energy. The high spatial resolution is explained by the small diffusion length in the samples under study. The diffusion length is small even in structures with dislocation densities of about 10 8 cm −3 and carrier mobilities of about 600 cm 2 V −1 s −1 at 300 K and 1800 cm 2 V −1 s −1 at 125 K.

Irradiation with 6 MeV electrons of near-UV (peak wavelength 385-390 nm) multi-quantum-well (MQW) GaN/InGaN light emitting diodes (LEDs) causes an increase in density of deep electron traps near E c À0.8 and E c À1 eV, and correlates to a 90% decrease of electroluminescence (EL) efficiency after a fluence of 1.1 Â 10 16 cm À2 . The likely origin of the EL efficiency decrease is this increase in concentration of the E c À0.8 eV and E c À1 eV traps.

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Radiation-induced point defects and their annealing in silicon-doped n-GaN have been investigated by means of Hall effect measurements and Raman spectroscopy. Correlated compensation effects due to simultaneous introduction of donor and acceptor centres are observed in irradiated n-GaN. The defect production rate is dependent on the dopant concentration. This means that the model of all native defects immobile at room temperature is not true. The behaviour of radiation-induced defects upon heating is complicated, exhibiting two prominent stages of reverse annealing. The presence of radiation defects is still observable after annealing to T = 750 • C.

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The EBIC investigations of extended defect system in GaN epitaxial layers with different electron mobility and dislocation density have been carried out. The recombination strength and defect cylinder radius for threading dislocations have been estimated from their EBIC profiles. Some cellular structure is revealed in the samples with the less-ordered mosaic structure that could be associated with the enhanced recombination activity of domain boundaries in such layers. IntroductionThe increasing application of GaN based structures in light emitting diodes, lasers and high power transistors generates interest in the study of electrical and optical properties of extended defects in these materials [1]. Probably, the most important extended defects in these structures attracting essential attention are threading dislocations, the density of which in the recent GaN epitaxial layers varies typically in the range from 10 8 to 10 10 cm -2 . Now it is well documented that the screw threading dislocations in GaN act as nonradiative recombination centers [2][3][4][5][6][7], although other recombination defects determine the minority carrier diffusion length [6] and limit the device performance [8]. Nevertheless, studying the characteristics of individual threading dislocations is important because it could help to predict more reliably their effect on the device performance and to develop approaches for controlling the dislocation properties. But the threading dislocations in GaN epi-layers are only a part of extended defect system, which includes also the typical for III-nitrides mosaic structure, ordering of which could depend essentially on growth conditions. For the well-ordered mosaic structures the defect relaxation occurs via the coherent concordance of mosaic structure domains with a formation of dilatation boundaries [9]. This process is accompanied with decreasing the density of threading dislocations, which could partly change their direction and form new growth steps. For such mosaic structures the small twist angles of domains, small values of roughness and low dispersion of surface domain sizes are typical [10]. For the less-ordered mosaic structures relaxation of defect system occurs via the formation of numerous domain dislocation boundaries, the edge dislocation density in domain boundaries exceeding essentially that of screw threading dislocations [9]. Such mosaic structures are characterized by large-sized domains with the larger twist angles and have the higher values of roughness and the higher dispersion of surface domain sizes [10]. Close correlation between mosaic structure ordering and surface roughness allows the quantitative characterization of ordering by the multifractal analysis [11] of atomic force microscopy data.Ordering of mosaic structure in hexagonal III-nitrides affects essentially their macroscopic electrical and optical properties that could be determined by the effect of edge dislocations located in the disor-