Mg is the most commonly used p-type dopant for GaN, however the impact of Mg incorporation on structural, morphological and optical properties of GaN is still not fully understood. Another research challenge is to understand and improve the properties of nonpolar GaN as it allows the fabrication of more efficient optoelectronic devices due to the absence of polarization fields.

Thus, the aim of this thesis was to explore the effect of Mg doping on polar c-plane GaN in Paper 1 and nonpolar m-plane GaN in Paper 2. The samples were grown by metal-organic vapor phase epitaxy with varying Mg content on free-standing GaN substrates. The studies were done by transmission electron microscopy (TEM) and low temperature cathodoluminescence (CL) with the aim to correlate the optical and structural data obtained by these techniques.

Polar c-plane GaN:Mg layers exhibit such structural defects as stackingfaults (SF) of a small size (5-10 nm). The basal plane SF (BSF)density was estimated to be ~ 10⁵ − 10⁶ cm⁻¹ increasing with Mgconcentrations. Comparison between as-grown and annealed sampleshas not shown significant difference in structural or optical properties.Characteristic broad emission lines observed in CL in the rangeof 3.29 − 3.41 eV have been attributed to SF-related emissions byanalogy with nonpolar undoped GaN films grown heteroepitaxially.Acceptor bound exciton (ABE) emission and SF-related peaks havedemonstrated metastability. CL mapping performed on the TEMsamples at the energies corresponding to SF-related peaks has confirmedthat the origin of these lines is associated with Mg-doped GaNlayers. In nonpolar m-plane GaN:Mg layers similar BSFs have been observed. In addition more extended BSFs and prismatic SFs wereidentified at the interface with the GaN substrate. For the m-planesamples with Mg concentration of ~ 10¹⁹ cm⁻³ a number of fine CLlines have been detected in the region of 3.3-3.4 eV. Their shape andappearance were unlike the SF-related emissions in the case of c-planeGaN discussed in Paper 1. These peaks are not likely to be associatedwith donor-acceptor pair (DAP) recombination as has been proved byestimation of the separated DAP energies and by CL mapping experiment.A tentative explanation is given to these peaks as being relatedto excitons bound to some low symmetry acceptor defect centers.