Today there is a strong drive towards higher efficiency light emitters and devices for power electronics based on GaN and its ternary compounds. Device performance can be improved in several ways on the material level. Development of bulk GaN to substitute sapphire and SiC as substrate materials can allow lower defect density epitaxial GaN layers to be grown. Using nonpolar homoepitaxial layers alleviates the problem of polarization fields present in polar GaN epilayers. This thesis advances the field by attacking outstanding problems related to doping and its influence on structural and optical properties of GaN. Optical and structural investigations were performed on bulk GaN grown by halide vapor phase epitaxy (HVPE) and on polar and nonpolar epitaxial GaN grown by metal organic chemical vapor deposition (MOCVD), doped with different impurities: Mg, Si, O or C. Optical characterization was done using photoluminescence (PL), time-resolved photoluminescence (TRPL), and cathodoluminescence (CL) in-situ scanning electron microscope, whereas structural properties were studied by means of transmission electron microscopy (TEM) and atom probe tomography (APT).

A correlation between Mg doping levels and stacking fault (SF) concentration in highly Mg-doped c-plane homoepitaxial GaN layers is found. Increasing Mg concentrations, from 2×10¹⁸ cm^-3 to 5×10¹⁹ cm^-3, coincides with increasing density of small, 3-10 nm-sized, SFs. Emission lines ascribed to SFs are observed in CL in all the studied samples. The observed SF-related luminescence can be explained by a model where Mg atoms interacting with the nearby SF changes the confinement for holes and leads to a pronounced defectrelated luminescence. Non-polar m-plane homoepitaxial GaN layers with Mg concentration of 2×10¹⁸ cm^-3 and 3×10¹⁹ cm^-3 exhibits high density of basal SFs as well as a number of prismatic SFs. Instead of normally observed in nonpolar GaN SF-related broad lines several sharp lines are detected in the 3.36-3.42 eV region. Their relation to donor-acceptor pair recombination (DAP) was dismissed by calculating the DAP energies and fitting with the measured spectra. The sharp lines are tentatively explained by some impurities bound to point defects or SFs. The origin of two Mg related acceptor bound exciton (ABE) peaks in the emission spectra is also proposed: narrower ABE1 peak at 3.466 eV is identified as coming from a substitutional Mg atom. Broader emission at 3.454 eV is deemed to be coming from a Mg acceptor atom perturbed by a nearby SF. Additionally, Mg cluster formation in the highest doped sample ([Mg] = 1×10²⁰ cm^-3) was revealed by APT.

Simultaneous doping by Si and O was studied for HVPE grown bulk GaN. Doping with O concentration from 10¹⁷ cm^-3 leads to a decrease in the Si concentration to less than 1016 cm-3. Si incorporation is believed to be suppressed by the competing Ga-vacancy-O incorporation process. Bandgap narrowing by 6 meV due to high doping was observed. Donor bound exciton (DBE) lifetime was obtained from TPRL experimental data and it is found to decrease with increasing doping. In non-polar m-plane homoepitaxial GaN Si doping influences the SF-related luminescence. At moderate Si concentrations excitons are bound to the impurity atoms or impurity-SF complex. Proximity of impurity atoms changes the potential for SF creating localization for charge carriers resulting in SF-related emission. At dopant concentrations higher than the Mott limit screening destroys the carrier interaction and, thus, the exciton localization at impurity-SF complex.

Finally, C-doped HVPE grown bulk GaN layers were studied by TEM, CL, and TRPL. Enhanced yellow line (YL) luminescence was observed with increasing C doping. Stability of YL in a wide temperature range (5-300 K) confirms that YL is due to a deep defect, likely C_N-O_N complex. Low-temperature CL mapping reveals a pit-like structure with different luminescence properties in different areas. DBE emission dominates in CL spectra within the pits while in pit-free areas, in contrast, two ABE lines typical for Mg-doped GaN are observed.