The microstructure of gallium nitride monocrystals grown at high pressure

Abstract

This work shows the results of X-ray diffractometric measurements performed on gallium nitride (wurtzite structure, (00.1) oriented plates) bulk crystals grown using the high-pressure (12–15 kbar)-high-temperature (about 1800 K) method. The examinations included: rocking-curve analysis, reciprocal lattice mapping, topography and measurements of lattice parameters. Monocrystals of size up to about 1 mm exhibit a very high crystallographic perfection (rocking curves of 20–30 arc sec). Bigger crystals possess a mosaic structure (0.1–1 mm crystallites separated by 1–3 arc min angle boundaries) visualised by X-ray topography. A model of the creation of those low-angle boundaries is proposed. It is based on the following observations: (i) the mosaic crystals are dome-shaped; (ii) the concave side is a “nitrogen-terminating” one, which grows faster; (iii) this side possesses smaller lattice parameters with respect to the “gallium-terminating” side. We have related the decrease of the lattice parameters to the gallium precipitation (observed in electron microscopy) beneath the “nitrogen-terminating” side. The difference between the lattice parameters on the two sides of the crystal causes its bending, which is then relaxed by emission of the low-angle boundaries.