[en] Self-induced GaN nanowires are grown by plasma-assisted molecular beam epitaxy, with In_xGa_1-xN quantum wells inserted to form an axial superlattice. From the ω-2θ scans of a laboratory x-ray diffraction experiment, we obtain the superlattice period, the thickness of the quantum wells, and the In content in this layer. The axial growth rate of the In_xGa_1-xN quantum wells is significantly enhanced, which we attribute to increased Ga diffusion along the nanowire sidewalls in the presence of In.

[en] X-ray diffraction peak profiles of relaxed heteroepitaxial layers with randomly distributed misfit dislocations are simulated and compared with the experimental data for two systems, LPE-grown SiGe / Si and MBE-grown AlSb / GaAs heterostructures. Different types of correlations in the spatial distribution of misfit dislocations are considered. For relatively small dislocation densities in the SiGe / Si system, the observed peaks are broader than the calculated ones for uncorrelated distributions of 60 dislocations. This broadening is explained as a result of a non-uniform tilt of the layer due to variations in the densities of dislocations with different Burgers vectors. In the AlSb / GaAs system possessing a high density of misfit dislocations, the peak of the relaxed AlSb buffer layer is found to be essentially narrower than expected from the model of uncorrelated dislocations. This effect can be explained by the short-range spatial correlations in the positions of the dislocations

[en] LaLuO₃ layers are epitaxially grown on Si(111) by molecular beam epitaxy using high temperature effusion sources. Samples are prepared by simultaneous as well as alternating growth of La₂O₃ and Lu₂O₃. Grazing incidence x-ray diffraction indicates that the resulting crystal structure of the alloys is cubic. Simultaneous and alternating growth with a monolayer period lead to the same distribution of La and Lu with no preferential ordering. In all cases the lattice mismatch to Si is less than 0.6%. The experimental results are analyzed by studying the energetics of hexagonal, bixbyite, and perovskite (La_1-xLu_x)₂O₃ crystal structures employing density functional theory.

[en] We investigate the structural and optical properties of spontaneously formed GaN nanowires with different degrees of coalescence. This quantity is determined by an analysis of the cross-sectional area and perimeter of the nanowires obtained by plan-view scanning electron microscopy. X-ray diffraction experiments are used to measure the inhomogeneous strain in the nanowire ensembles as well as the orientational distribution of the nanowires. The comparison of the results obtained for GaN nanowire ensembles prepared on bare Si(111) and AlN buffered 6H-SiC(000 1-bar ) reveals that the main source of the inhomogeneous strain is the random distortions caused by the coalescence of adjacent nanowires. The magnitude of the strain inhomogeneity induced by nanowire coalescence is found not to be determined solely by the coalescence degree, but also by the mutual misorientation of the coalesced nanowires. The linewidth of the donor-bound exciton transition in photoluminescence spectra does not exhibit a monotonic increase with the coalescence degree. In contrast, the comparison of the root mean square strain with the linewidth of the donor-bound exciton transition reveals a clear correlation: the higher the strain inhomogeneity, the larger the linewidth. (paper)

[en] GaN nanowire ensembles with axial In_xGa_1−xN multi-quantum-wells (MQWs) were grown by molecular beam epitaxy. In a series of samples we varied the In content in the MQWs from almost zero to around 20%. Within the nanowire ensemble, the MQWs fluctuate strongly in composition and size. Statistical information about the composition was obtained from x-ray diffraction and Raman spectroscopy. Photoluminescence at room temperature was obtained in the range of 2.2 to 2.5 eV, depending on In content. Contrary to planar MQWs, the intensity increases with increasing In content. We compare the observed emission energies with transition energies obtained from a one-dimensional model, and conclude that several mechanisms for carrier localization affect the luminescence of these three-dimensional structures. (paper)

[en] A method is proposed to determine the concentration and relaxation depth profiles in graded epitaxial films from x-ray reciprocal space maps (RSMs). Various approximations in the kinematical x-ray diffraction from epitaxial films with the misfit dislocation density depth profile are developed. We show that a symmetric and an asymmetric RSM, or two asymmetric RSMs, contain enough information to obtain the concentration, relaxation, and lattice tilt depth profiles without any additional assumptions. The proposed approach is applied to In_xGa_1-xAs/GaAs and GaAs_1-xP_x/GaAs epitaxial graded films. The reconstructed concentration and dislocation density depth profiles are found to be in an agreement with the ones expected from the growth conditions.

[en] We find that the widths of double-crystal x-ray diffraction peaks in asymmetric reflections of relaxed GaAs/Si(001) heteroepitaxial layers in reciprocal diffraction geometries (glancing incidence and glancing exit) are notably different. This observation is in agreement with previous measurements on other heteroepitaxial systems but apparently contradicts the reciprocity principle of electrodynamics. We show that the apparent contradiction originates from the summation of the scattered waves that are collected by the detector in a double-crystal setup and resolve it by giving an appropriate description of the peak widths

[en] We study x-ray diffraction peak profiles from highly mismatched relaxed epitaxial films at momentum transfers exceeding the peak widths. Calculated profiles for misfit dislocations are compared with triple-crystal diffraction profiles from GaAs/Si(001) epitaxial films. We find that the longitudinal and transverse scans have a common q^-4 asymptote but approach it differently, so that their profiles are qualitatively different in the experimentally available momentum range. The possible contribution from threading dislocations is estimated