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We describe an instrument that exploits the ongoing revolution in synchrotron sources, optics, and detectors to enable in situ studies of metal-organic vapor phase epitaxy (MOVPE) growth of III-nitride materials using coherent x-ray methods. The system includes high-resolution positioning of the sample and detector including full rotations, an x-ray transparent chamber wall for incident and diffracted beam access over a wide angular range, and minimal thermal sample motion, giving the sub-micron positional stability and reproducibility needed for coherent x-ray studies. The instrument enables surface x-ray photon correlation spectroscopy, microbeam diffraction, and coherent diffraction imaging of atomic-scale surface and film structure and dynamics during growth, to provide fundamental understanding of MOVPE processes.

The effects of GaN quantum barriers with changing growth temperatures on the interfacial characteristics of GaN/InGaN single quantum well (SQW) grown on GaN templates by metalorganic vapour phase epitaxy were in situ investigated by X-ray crystal truncation rod (CTR) scattering and X-ray reflectivity measurements at growth temperature using a laboratory level X-ray diffractometer. Comparing the curve-fitting results of X-ray CTR scattering spectra obtained at growth temperature with that at room temperature, the InxGa1-xN with indium composition less than 0.11 was stabile of the indium distribution at the interface during the whole growth processes. By using several monolayers thickness GaN capping layer to protect the InGaN well layer within temperature-ramping process, the interfacial structure of the GaN/InGaN SQW was drastically improved on the basis of the curve-fitting results of X-ray CTR scattering spectra, and the narrow full width at half-maximum and strong luminous intensity were observed in room temperature photoluminescence spectra.

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The thermal decomposition of c-plane GaN/sapphire templates was studied in a metalorganic vapor phase epitaxy (MOVPE) system installed in a laboratory-level X-ray diffractometer by using in situ X-ray reflectivity (XRR). GaN remained thermally stable in pure N2 up to 900 °C, while a significant decomposition occurred at 950 °C. Then, thin In x Ga1-x N epilayers were grown on the annealed templates at 830 °C. In situ XRR measurements were conducted before and after InGaN growth. By theoretical and experimental analyses of the XRR spectra, the sample structure change upon thermal annealing was clarified. Photoluminecescence (PL) and atomic force microscopy (AFM) results demonstrated that thermal annealing affected the optical properties and microstructures of InGaN films. The PL peaks from InGaN slightly blue-shifted with thermal annealing.

Ga1−xInxN epilayers (x = 0.09 or 0.14) grown on c-plane GaN layers with different densities of threading dislocations have been investigated by real-time x-ray reflectivity during metal-organic vapor phase epitaxial growth. We found that the density of pre-existing threading dislocations in GaN plays an important role in the strain relaxation of Ga1−xInxN. Critical thicknesses were obtained and compared with theoretical predictions using the mechanical equilibrium model and the energy balance model. The critical thickness of GaInN varies inversely with dislocation density in the GaN sublayer. When the threading dislocation density in the sublayer was reduced by three orders of magnitude, the photoluminescence intensity of the Ga0.86In0.14N epilayer was improved by a factor of ten.

Methods are described to measure divergence by imaging the apparent source size using either slits to form a pinhole camera or a compound refractive lens. The values of brightness and coherent flux obtained are compared with those calculated for the undulator source, to evaluate the effects of beamline optics.

The stacking sequence of hexagonal close-packed and related crystals typically results in steps on vicinal {0001} surfaces that have alternating A and B structures with different growth kinetics. However, because it is difficult to experimentally identify which step has the A or B structure, it has not been possible to determine which has faster adatom attachment kinetics. Here we show that in situ microbeam surface X-ray scattering can determine whether A or B steps have faster kinetics under specific growth conditions. We demonstrate this for organo-metallic vapor phase epitaxy of (0001) GaN. X-ray measurements performed during growth find that the average width of terraces above A steps increases with growth rate, indicating that attachment rate constants are higher for A steps, in contrast to most predictions. Our results have direct implications for understanding the atomic-scale mechanisms of GaN growth and can be applied to a wide variety of related crystals.