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Abstract: Compared with titanium oxide (TiO2) and zirconium chloride (ZrCl4), cerium oxide (CeO2) significantly affects the dehydrogenation properties of lithium alanate (LiAlH4). CeO2-doped LiAlH4 samples obtained in different decomposition states were characterized by in situ X-ray diffraction, synchrotron radiation powder X-ray diffraction (PXD) data, and X-ray absorption fine structure (XAFS). PXD and XAFS analyses reveal that CeO2 is present as a cubic CeO2−x phase and behaves as a true catalyst. Ce4+ reduction affects the ability of Li to donate its charge to AlH4, thereby weakening the Al–H bond and causing hydrogen to desorb at lower temperatures. Beyond 180 °C, approximately 50% of the O atoms of the CeO2 catalyst are removed, which results in the formation of long-range disorder rhombohedral Ce7O12 phase and an opposite effect on the dynamics. [Copyright &y& Elsevier]

In order to understand the growth feature of GaN on GaAs (0 0 1) substrates grown by metalorganic chemical vapor deposition (MOCVD), the crystallinity of GaN buffer layers with different thicknesses was investigated by using double crystal X-ray diffraction (DCXRD) measurements. The XRD results showed that the buffer layers consist of predominantly hexagonal GaN (h-GaN) and its content increases with buffer layer thickness. The nominal GaN (1 1 1) reflections with χ at 54.74° can be detected easily, while (0 0 2) reflections are rather weak. The integrated intensity of reflections from (1 1 1) planes is 4–6 times that of (0 0 2) reflections. Possible explanations are presented. [Copyright &y& Elsevier]

Investigates periodicity fluctuations in strained (GaNAs)[sub 1](GaAs)[sub m] superlattices by the kinematical simulation of x-ray diffraction. Development of a method dependent on the width of satellite peaks to assess periodicity of fluctuations of a superlattice.

The structural characteristic of cubic GaN (C-GaN) nucleation layers on GaAs(0 0 1) substrates by metalorganic chemical vapor deposition was in detail investigated first by X-ray diffraction (XRD) measurements, using a Huber five-circle diffractometer and an intense synchrotron X-ray source. The XRD results indicate that the C-GaN nucleation layers are highly crystallized. Φ scans and pole figures of the (1 1 1) reflections give a convincing proof that the GaN nucleation layers show exactly cubic symmetrical structure. The GaN(1 1 1) reflections at 54.74° in χ are a measurable component, however (0 0 2) components parallel to the substrate surface are not detected. Possible explanations are suggested. The pole figures of {1 0 1¯ 0} reflections from H-GaN inclusions show that the parasitic H-GaN originates from the C-GaN nucleation layers. The coherence lengths along the close-packed 〈1 1 1〉 directions estimated from the (1 1 1) peaks are nanometer order of magnitude. [Copyright &y& Elsevier]

Abstract: The influence of well thickness on the electroluminescence (EL) of InGaN/GaN multiple quantum wells (MQWs) grown by metalorganic chemical vapor deposition is investigated. It is found that the peak wavelength of EL increases with the increase of well thickness when the latter is located in the range of 3.0–5.1nm. The redshift is mainly attributed to the quantum confined Stark effect (QCSE). As a contrast, it is found that the EL intensity of InGaN/GaN MQWs increases with the increase of well thickness in spite of QCSE. The result of X-ray diffraction demonstrates that the interface become smoother with the increase of well thickness and suggests that the reduced interface roughness can be an important factor leading to the increase of EL intensity of InGaN/GaN MQWs. [Copyright &y& Elsevier]

Abstract: The defect evolution and its correlation with electrical properties of GaN films grown by metalorganic chemical vapor deposition are investigated. It is found that the dislocation density decreases gradually during the growth process, and the dislocation reduction rate in the island coalescence process is especially rapid. The changes in electron mobility of GaN with the increase of growth time are mainly dependent on the dislocations acting as scattering centers. Furthermore, the variation of carrier concentration in GaN may be related with the point defects and their clusters. The quality of GaN could be improved by suitably increasing the film thickness. [Copyright &y& Elsevier]

Abstract: Both cracked and crack-free GaN/Al0.55Ga0.45N multiple quantum wells (MQWs) grown on GaN template by metalorganic chemical vapor deposition have been studied by triple-axis X-ray diffraction, grazing-incidence X-ray reflectivity, atomic force microscope, photoluminescence spectroscopy and low-energy positron annihilation spectroscopy. The experimental results show that cracks generation not only deteriorates the surface morphology, but also leads to a period dispersion and roughens the interfaces of MQWs. The mean density of dislocations in MQWs, determined from the average full-width at half-maximum of ω-scan of each satellite peak, has been significantly enhanced by the cracks generation. Furthermore, the measurement of annihilation-line Doppler broadening reveals a higher concentration of negatively charged vacancies in the cracked MQWs. The combination of these vacancies and the high density of edge dislocations are assumed to contribute to the highly enhanced yellow luminescence in the cracked sample. [Copyright &y& Elsevier]

The influence of low-temperature AlN buffer layer thickness on GaN epilayer was investigated by triple-axis X-ray diffraction (XRD) and photoluminescence measurements. A method was proposed to measure the screw and edge dislocation densities by XRD. It was found that the buffer layer thickness was a key parameter to affect the quality of GaN epilayer and an appropriate thickness resulted in the best structural and optical properties except the lateral grain size. After the thickness exceeding the appropriate value, the compressive stress in the epilayer decreased as the thickness increased, which led to the redshift of the near-band edge luminescence. The experimental results showed the buffer layer thickness had more influence on edge dislocation than screw type and the former was perhaps the main source of the yellow band. [Copyright &y& Elsevier]

The effects of pre-TMIn flow prior to QW growth and TMIn flow rates during QW growth on the interface and optical properties of InGaN/GaN MQWs were investigated. Pre-depositing indium prior to QW growth and an appropriate TMIn flow rate can improve the interface abruptness and increase the EL intensity. InGaN/GaN MQWs with improved interface abruptness have increasing emission intensity and wavelength. We attribute the interface improvement and the increase of EL intensity to the improvement of the indium compositional profiles. [Copyright &y& Elsevier]

AlInGaN quaternary epilayers have been grown with various TMGa flows by metalorganic chemical vapor deposition to investigate the influence of growth rate on the structural and optical properties. Triple-axis X-ray diffraction measurements show AlInGaN epilayers have good crystalline quality. Photoluminescence (PL) measurements show that the emission intensity of AlInGaN epilayers is twenty times stronger than that of AlGaN epilayer with comparable Al content. V-shaped pits are observed at the surface of AlInGaN epilayers by atomic force microscopy (AFM) and transmission electron microscopy (TEM). High growth rate leads to increased density and size of V-shaped pits, but crystalline quality is not degraded. [Copyright &y& Elsevier]

The influences of AlN buffer thickness on the optical and the crystalline properties of metalorganic chemical vapor deposition wurtzite GaN layers on Si(1 1 1) substrate have been investigated. High-resolution X-ray diffraction and photoluminescence measurement reveal that the thickness of AlN buffer exerts a strong influence on the distribution of dislocation and stress in GaN epilayer. The evidence is further reinforced by atomic force microscopic observation of AlN nucleation process. The optimum thickness of AlN buffer to effectively suppress Si diffusion has been determined by secondary-ion mass spectroscopy to be in the range of 13–20 nm. In addition, it is found that appropriate Si diffusion in AlN buffer helps to compensate the tensile strain in GaN, which subsequently improves the optical quality of GaN on Si(1 1 1), and reduces the cracks over the GaN surface. [Copyright &y& Elsevier]

A new method of measuring the thickness of GaN epilayers on sapphire (0 0 0 1) substrates by using double crystal X-ray diffraction was proposed. The ratio of the integrated intensity between the GaN epilayer and the sapphire substrate showed a linear relationship with the GaN epilayer thickness up to 2.12 μm. It is practical and convenient to measure the GaN epilayer thickness using this ratio, and can mostly eliminate the effect of the reabsorption, the extinction and other scattering factors of the GaN epilayers. [Copyright &y& Elsevier]

The crystallographic tilt of the lateral epitaxial overgrown (LEO) GaN on sapphire substrate with SiNx mask is investigated by double crystal X-ray diffraction. Two wing peaks beside the GaN 0002 peak can be observed for the as-grown LEO GaN. During the selective etching of SiNx mask, each wing peak splits into two peaks, one of which disappears as the mask is removed, while the other remains unchanged. This indicates that the crystallographic tilt of the overgrown region is caused not only by the plastic deformation resulted from the bending of threading dislocations, but by the non-uniformity elastic deformation related with the GaN/SiNx interfacial forces. The widths of these two peaks are also studied in this paper. [Copyright &y& Elsevier]