In the last decade, the III-N compounds have attracted much interest because of their applications in blue, violet and ultraviolet optoelectronics. Most of the devices and research use sapphire as a substrate for epitaxy of nitrides. However, these epi-structures contain a very high dislocation density induced by the 16% lattice mismatch between GaN and sapphire. In our laboratory, we grow single crystals of GaN at a high hydrostatic pressure of 10 kbar of nitrogen. These crystals have an ultra-low dislocation density and are successfully used for construction of violet laser diodes. This work presents experimental data of high resolution x-ray diffractometry and photoluminescence. This work consist of three parts: (i) comparison between GaN substrates and GaN and sapphire templates; (ii) influence of AlGaN layers on bowing of samples; (iii) microstructure of InGaN/GaN multiple quantum wells.