| Summary: | Due to the unavailability of bulk defect-free GaN substrates, GaN is grown hetero-epitaxially on substrates which have a substantial lattice mismatch. As a consequence, a large population of threading dislocations (TDs), typically 10<sup>9</sup> – 10<sup>10</sup> cm<sup>-2</sup>, and other defects are generated in the GaN thin film. TDs act as non-radiative recombination centres. The main aim of this thesis is to investigate the effect of <i>in situ </i>silane dosing (simultaneous flow of SiH<sub>4</sub> and NH<sub>3</sub>) and varying the growth parameters on reducing the density of threading dislocations. Optimising the growth parameters led to a TD density ~ 10<sup>8</sup> cm<sup>-2</sup>. Binding of threading dislocations through 90° has been found to be important to reduce their density and a new mechanism is described to explain the bending of all types of TDs through 90°. Moreover, a new modified growth technique for the reduction of TDs is proposed which might lead to a very low TD density in GaN during metal organic vapour phase epitaxy (MOVPE) growth. In recent developments, the combination of MOVPE and halide vapour phase epitaxy (HVPE) growth techniques show good promise for reducing the TD density below 10<sup>6</sup> cm<sup>-2</sup>: some of these results are also presented. Additionally, two new techniques based on transmission electron microscopy (TEM) and atomic force microscopy (AFM) are described for counting the density of all types of threading dislocations reliably. The origin of additional threading dislocations in AlGaN growth on GaN using a relaxed AIN interlayer is presented. The quality of the AIN templates grown on sapphire as a function of the initial growth conditions {i.e nitridation of substrate, tri-methyl ammonia (TMA) pre-dosing, etc} is described. The growth of GaN and AlGaN on this AIN template is briefly described. Finally, strain relaxation and the origin of misfit dislocations in ‘high’ indium content (20%) green single quantum well (SQW) and multi quantum well (MQW) hetero-structures are described and analysed.
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