The influence of lattice parameter variation on microstructure of GaN single crystals

Free-electrons expand the lattice of GaN [M. Leszczynski, I. Grzegory, M. Bockowski, J. Crystal Growth 126 (1993) 601]. For the free-electron concentration of 5 × 1019 cm−3, the lattice parameters are increased by about 0.02%. Two examples will be given that even such a small lattice expansion can lead to serious modification of GaN bulk crystals what has important technological implications. In the first example, we will show that the GaN crystals (dislocation density of 100 cm−2) grown at high pressure of 10-15 kbar and temperature 1500-1600 °C possess internal strains due to an inhomogenous oxygen (donor) distribution. In the second example, we will show that the layers of GaN grown using hydride vapour phase epitaxy (HVPE) on these high-pressure GaN crystals cause significant bending of the samples. The radius of bending is as small as 10-20 cm for 30-40 μm layers. For thicker layers, the deformation becomes plastic and the bending remains constant even after etching the substrate away.