Processes of carrier localization, their scattering and trapping at impurities, but also at localization states, are very important for understanding transport mechanisms in III-nitrides based device oriented structures. In this communication we discuss results of PL and ODMR studies of GaN/AlGaN HEMT structures. We studied undoped and Si-doped GaN and AlGaN layers grown by MOCVD in (0001) orientated sapphire substrates. We start with discussion of PL spectra and then correlate their properties with growth conditions. For example, a parasitic yellow emission is observed in the samples grown with 30 nm GaN nucleation layer, containing high dislocation density.

We also found two new magnetic resonance signals in the ODMR spectra with g factors equal about 1.7 and 0.9. Their possible origin is proposed.

Results of the ODMR and PL investigations are compared with electrical measurements.