GaN has received much attention over the past few years because of several new applications, including light emitting diodes, blue laser diodes and high-power microwave transistors. One of the biggest problems is a high density of structural defects, mostly dislocations, due to a lack of a suitable lattice-matched substrate since bulk GaN is difficult to grow in large sizes. So far, the substrate of choice has been sapphire (Al2O3), which has a 14% lattice-size mismatch and a 34% mismatch in thermal expansion coefficient. As a result of growth along (0001) GaN on Al2O3, high concentrations of misfit and threading dislocations are formed. The main concerns are threading dislocations because they will propagate to the active parts of devices grown on top of the underlying GaN layers, due to the fact that dislocations cannot terminate inside the material unless they form half-loops. One of the growth techniques that give smaller dislocation density is hydride vapor-phase epitaxy (HVPE). The lower density is due to the fact that large thickness of GaN can be grown, allowing more interactions between dislocations and lowering their density.
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