GaN is one of the most widely used III/V semiconductor in optoelectronic applications. Single crystals of GaN are usually grown as the thick layers on foreign substrates. However, these substrates have typically different lattice parameters and this lattice misfit leads to the creation of microstructure defects, mostly threading dislocations (TDs). For (001) sapphire substrate (Al2O3), the in-plane misfit between GaN layer and substrate is almost 15 %.
High-temperature vapor phase epitaxy (HTVPE) is a very suitable deposition technique for production of specific microstructure features, which interact with TDs and can lead to their bunching and annihilation [1]. The most promising way to reduce the TD density was to produce a multilayer structure starting with GaN which was then covered by another structured GaN layer and overgrown by a GaN top layer [2]. The microstructure and morphology of individual layers is controlled mainly by the temperature of the gallium melt, by the temperature of substrate and by growth rate. By this procedure, the density of microstructure defects in the very top GaN layer should be low.
To probe the depth dependence of dislocation density, residual stresses and lattice parameters, the synchrotron X-ray diffraction reciprocal space mapping in reflection mode was performed on various samples prepared by the abovementioned procedure. The idea of the experiment was to use three different wavelengths of the primary beam to modify the absorption coefficient and thus to probe different depths of the samples. The change of the lattice parameters effects the position diffraction peaks in reciprocal space and the dislocation density influences the line broadening. Moreover, the influence of threading and screw dislocation is distinguishable, since their effect on the line broadening is different with respect to the corresponding Burgers vector and position of the peak in reciprocal space. The synchrotron experiment was performed at PETRA III: P24 beamline in Hamburg, Germany.