Blue−Laser MOVPE Structures Grown on Bulk GaN Crystals

The single crystals of GaN grown at high hydrostatic pressures (about 15 kbar) and high temperatures (about 1800K) possess a very low dislocation density (below 103 cm−2) and sizes of about 100 mm2. These crystals are used as substrates for MOVPE growth of laser structures. The most important question we had to answer was if the ternary compounds InGaN and AlGaN layers on GaN would be strained or relaxed by misfit dislocations. To answer this question we grew a series of these ternary layers of various compositions and thickness and examined them using high−resolution X−ray diffraction sensitive to dislocation density larger then 104 cm−2. These experimental results indicated that the laser structure grown on bulk GaN crystal should be fully strained (without mismatch dislocations). The further research confirmed this prediction. Atomic force microscopy (AFM) scans do not show any threading dislocations. The perfect atomic steps are not disturbed by the presence of dislocations and indicate a step−flow growth mode on a slightly misoriented substrate. However, significant deviation from perfect epitaxial structure could be observed by diffuse scattering of X−ray diffraction around InGaN peaks (both, thick layers and quantum wells) what meant a certain indium segregation or variation of well/barrier thickness. Even more important problem for fully strained epitaxial layers on relatively thin GaN substrates is the sample bowing. In extreme cases, for very thick AlGaN claddings, the bowing radius i