The AlGaN, GaN and InGaN based devices are composed of layers having both the distinctive lattice parameters and the different donor or acceptor doping.  The characteristics of corresponding localized states are affected by the layer tensions, formation of the compensating point defects, the threading dislocations as well as by the eventual interface region between layers. Actual problem is experimental analysis of the nature, characteristics and electron recombination mechanism of defects as well as understanding of their formation related to the layer composition and growth conditions.  Due to the semi-insulating character of these materials most electrical characterisation methods, e.g. DLTS, are not applicable. In order to get an experimental access to the defect states of strongly compensated wide band gap materials we applied thermostimulated luminescence (TL) and thermally stimulated current (TSC) techniques on the MOCVD grown  GaN and Al_0.18Ga_0.82N samples deposited  on sapphire. The TL (emission band at E_em=1.95 eV) of GaN excited by E=3.5 eV at 10K showed five, typical for different samples peaks at 32 K, (E=9 meV), 82 K (E=29 meV),  broad unresolved peak at 125-160 K and two overlapping peaks at 238 K (E=0.53 eV) and 273 K. The TL peaks at 32–82 K and 238–273 K was accompanied by a broad unresolved TSC peaks whereas no TSC was detected for the 125- 160 K TL peak. Observed essential change of the TL/TSC peak intensities ratio indicates that the released charge carriers have different mobility. Thus the peak at 125-160 K is due to the release of holes from Ga vacancy related centres. TL of the AlGaN (emission band at E_em=2.1 eV) excited by E=3.88 eV (absorption edge E=3,78 eV) showed intense broad peak from 15 K to 120 K (I_max=75 K), and two broad peaks at 150 K and 250 K indicating the compositional disorder. The TL of GaN underlying AlGaN due to presence of layer tensions showed broadening of all peaks compared to the freestanding GaN.

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