Strukturelle Untersuchungen dotierter GaN-Filme mit stehenden Röntgenwellenfeldern

Abstract

In the blue spectral region, GaN-based hetero-structures are most commonly applied in light-emitting diodes or laser-diodes. However, the incorporation of Si and Mg, which are most commonly used as n- and p-type dopant material respectively, induces a variety of defects, hence reducing the efficiency and lifetime of the devices. Thus, it is essential to investigate the interplay between dopant incorporation and defect formation. In this thesis, the incorporation of Mg and Si in GaN-films grown on (0001)-sapphire substrates is investigated in detail by applying several synchrotron radiation techniques.The segregational behavior of Si and Mg is investigated by x-ray-photoelectron-spectroscopy (XPS) and XPS-microscopy. For the first time, it is revealed that also Si segregates to the surface and stabilizes surface facets along high symmetry directions. Additionally, it is found that Mg also segregates to the crystal surface at concentrations far below the threshold condition for the formation of pyramidal defects.For the first time, the technique of x-ray standing waves (XSW), which allows to directly conclude on the crystal and defect structure on the atomic scale, is successfully applied to thick crystals with high defect density. In order to compensate for the high crystal mosaicity of GaN, which significantly reduces the accuracy of the XSW measurements, the experiments were performed in near backscattering geometry, as in this setup the intrinsic reflection width becomes large as compared to the tilt mosaicity. Influences of doped crystal material are suppressed by using a thick undoped high quality GaN-film for the formation of a standing wave field, which is used for the investigation of a thin doped film deposited on top. By performing the XSW measurements with probe signals of different surface sensitivity (fluorescence and photoelectrons) the incorporation behavior within the bulk crystal is compared to regions close to the surface.Recording photoelectron signals, non-dipole effects to the photoemission process need to be regarded. However, the non-dipole contributions of the Ga-2p-photoelectrons on the secondary yield can neither be calculated theoretically nor determined experimentally from the XSW photoelectron data, as the structural parameters, especially the coherent fraction, are not known exactly due to the high crystal mosaicity. Within this thesis a technique is suggested that allows to determine the non dipole contributions to the yield by comparing Auger electron and photoelectron secondary signals.The XSW-measurements on Si-doped samples reveal that Si is solely incorporated on substitutional Ga-sites both within the bulk crystal and close to the surface. In the bulk, Si is homogeneously distributed, distorting the crystal lattice in their vicinity. At the surface Si is preferentially incorporated at surface facets. The facet formation can be explained in terms of a strain relaxation mechanism induced by the Si lattice distortion.XSW measurements using Mg-KLL Auger electrons reveal that, with increasing dopant incorporation, Mg atoms progressively occupy non-substitutional sites. This is explained by Mg incorporation of Mg at the basal planes of rod-like inversion domains, which have been observed by scanning electron microscopy. Based on the XSW results, a structural model for the basal planar inversion domains is presented.