Liubchenko O. High-resolution X-ray diffractometry of single crystals near-surface layers and multilayered structures under ion irradiation

The thesis presents the results of complex investigation of the influence of ion implantation on the strain state of III-nitride superlattices (SL), compositionally graded AlxGa1 xN layers, and ion-implanted InSb and GaN single crystals. New methods based on high-resolution X-ray diffraction analysis for the characterization of these effects have been developed. In particular, an approach for simulating the X-ray diffraction spectra of Be+ implanted InSb crystals was considered. The method is based on the semi-kinematic theory of X-ray diffraction in Bragg geometry. The depth profiles of strain and structural disordering in the ion-modified layers were determined by a fitting procedure that was developed on the base of the direct search minimization algorithms, simplex method and differential evolution algorithm. The thickness of the implanted layer and strain maximum value for the ion-modified layers of InSb were determined. For the two-stage implantation with energies of 66 keV and 80 keV, and doses of 1.563∙1014 at./cm2 and 3.125∙1014 at./cm2, the thickness of the buried layer is about 500 nm with a strain maximum about 0.1%. The developed model was also used to investigate the structural changes in the single crystals of GaN implanted with Ar+ and Mg+ ions with various doses and energies. A detailed analysis of non-ideal AlN/GaN superlattices grown on GaN substrates was carried out. The influence of thickness variation with depth and interfaces roughness on the symmetrical 2-ω scans were investigated. It was shown that the thickness changes in GaN quantum wells and AlN barriers with depth lead to an asymmetrical broadening of the satellite peaks, while roughness is the cause of their symmetrical broadening. This makes possible to distinguish the influence of these effects on the diffraction pattern. The effectiveness of the developed method is shown by numerical simulation of X-ray diffraction spectra. Based on the dynamical theory of X-ray diffraction and using the model of mosaic crystal, a new method to investigate the influence of dislocations on the high-resolution X-ray diffraction spectra has been developed. Simulation of the XRD spectra from the AlN/GaN SLs simultaneously takes into account the variation of the SL layers thickness and the presence of dislocations. This model explains well the broadening and the asymmetry of the satellite peaks, especially for higher-order reflections. It is established that the classical Williamson-Hall method for dislocation densities estimation confirms the correctness of the developed model at dislocation densities larger than 1∙108 cm−2. The developed methods allow to determine quickly and reliably the layers thicknesses, the density of dislocations and strain profiles in multi-layered structures. Ion implantation is a powerful method for transforming the structural and strain state (strain or polarization engineering) in compositionally-graded layers and SL. It has been shown that implantation with Ar+ ions in graded AlxGa1-xN layers and AlN/GaN SL leads to change of strain state and relatively low structural damage. After the implantation, the density of microdefects mainly increases, while the configuration of dislocations practically does not change. The density of microdefects is significantly reduced as a result of post-implantation annealing. The structural quality of the AlxGa1 xN layers is strongly depends on the Al concentration and deteriorates with the increase of Al. The structural changes induced by ion implantation in highly defected samples with larger dislocations densities are less pronounced. Ion-implantation leads to crystal quality deterioration of both AlxGa1–xN and GaN layers, which can be explained by migration of the point defects and redistribution of strain fields within the heterostructure