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The defects and strain of GaN(0001) films as virtual substrate on Si(111) with step-graded Sc2O3(111)/Y2O3(111) buffers were investigated by means of transmission electron microscopy. The misfit dislocation network identified in the interfaces nearly fully compensates the lattice mismatch. Inversion domains and pinholes occur within the closed GaN film. The atomic structure of the inversion domain boundaries is identified. Major parts of the films were found to be N-polar. Threading dislocations were formed as remains from the coalescence of initial GaN islands. Furthermore, the formation of small cubic inclusions is found to be restricted to the vicinity of the interface only. [ABSTRACT FROM AUTHOR]

The final film quality of GaN on foreign substrates is known to crucially depend on the initial GaN interface and nucleation characteristics. To shed light on these characteristics of recently pioneered virtual, hexagonal GaN(0001) substrates on Si(111) via step graded Sc2O3(111)/Y2O3(111) buffers, a complex GaN(0001)/Sc2O3(111) interface structure model and the initial nucleation scenario is derived from a combined experimental (reflection high energy electron diffraction and X-ray photoelectron spectroscopy) and theoretical ab initio study. It is shown that the GaN/Sc2O3 interface chemistry is determined by a N-Ga-O-Sc atomic arrangement leading to N-polar GaN films. However, the atomic GaN(0001)/Sc2O3(111) interface configuration is complex and local perturbations might be at the origin of Ga-polar inversion domains in the mainly N-polar GaN films. The initial growth of GaN on Sc2O3 is characterized by an ultrathin N-Ga-O-Sc wetting layer which carries tensile strain and relaxes with increasing thickness. Further GaN deposition results in the formation of 3D islands which fully relax before island coalescence occurs. The implications of the GaN/Sc2O3 interface configuration, the 3D nucleation growth mode, and the coalescence process of misaligned islands are discussed with respect to the defect characteristics (inversion domains, cubic inclusions, threading dislocations) of the final GaN layer. [ABSTRACT FROM AUTHOR]

We report on a characteristic photoluminescence feature of the substitutional Mn in high quality GaMnN layers. The lattice site was identified using atom localization by channeling enhanced microanalysis with a transmission electron microscope. It shows that 96.5%±5.0% of the Mn atoms are incorporated on the substitutional Ga site. In photoluminescence a feature appears at 1.41 eV with a phonon sideband related to the GaN matrix. The temperature evolution is characteristic of an intra-atomic transition and it is assigned to the internal transition 5E→5T2 of the Mn3+ ion. The assignment is supported by absorption experiments. The persistence of the clear PL signal up to about 1% Mn concentration is proposed to be a fingerprint of high quality diluted GaMnN. [ABSTRACT FROM AUTHOR]

While electron holography in the transmission electron microscope offers the possibility to measure maps of the electrostatic potential of semiconductors down to nanometer dimensions, these measurements are known to underestimate the absolute value of the potential, especially in GaN. We have varied the dose rates of electron irradiation over several orders of magnitude and observed strong variations of the holographically detected voltages. Overall, the results indicate that the electron beam generates electrical currents within the specimens primarily by the photovoltaic effect and due to secondary electron emission. These currents have to be considered for a quantitative interpretation of electron holographic measurements, as their negligence contributes to large parts in the observed discrepancy between the measured and expected potential values in GaN. [ABSTRACT FROM AUTHOR]

Abstract: A model-based approach to estimate lattice constants from an atomically resolved HRTEM image is presented. The approach only utilizes the inherent periodicity of these images and does not require a centrosymmetric structure of the specimen. This allows the evaluation of, for instance, wurtzite-based materials like InGaN/GaN heterostructures. The lattice constants are determined within precisions below 3pm from areas only a few unit cells large. This makes this method suitable for further strain/compositional analysis. Furthermore, the impact of the approximations of the true detector''s covariance matrices on the assessment of the model-based approach is investigated, and insights into the quality of these noise models of the detector are gained. [Copyright &y& Elsevier]