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A three-step growth process is developed for depositing high-quality aluminium-nitride (AlN) epilayers on (001) sapphire by low pressure metalorganic chemical vapour deposition (LP-MOCVD). We adopt a low temperature (LT) AlN nucleation layer (NL), and two high temperature (HT) AlN layers with different V/III ratios. Our results reveal that the optimal NL temperature is 840–880°C, and there exists a proper growth switching from low to high V/III ratio for further reducing threading dislocations (TDs). The screw-type TD density of the optimized AlN film is just 7.86 × 106cm−2, about three orders lower than its edge-type one of 2 × 109cm−2 estimated by high-resolution x-ray diffraction (HRXRD) and cross-sectional transmission electron microscopy (TEM).

Canti-bridged epitaxial lateral overgrowth (CBELO) of GaN is performed by metalorganic chemical vapour deposition (MOCVD) on maskless V-grooved sapphire substrates prepared by wet chemical etching with different mesa widths. The wing tilt usually observed in ELO is not found in the CBELO GaN with wide mesa widths, while it can be detected obviously in the GaN with narrow mesa widths. The wing tilt of CBELO GaN grown on a grooved sapphire substrate with narrow mesa can be controlled by adjusting the thickness of the nucleation layer. The dependence of the wing tilt on the nucleation layer thickness is studied. Cross-sectional scanning electron microscopy is used to characterize the geometry of the wing regions, and double crystal x-ray diffraction is used to analyse the structural characteristics and to measure the magnitude of the crystalline wing tilt. It is found that the crystalline wing tilt can be eliminated completely by first growth of a thin nucleation GaN layer then the CBELO GaN. Possible reason and the origin of the wing tilt in CBELO GaN films are also discussed.

InN and In0.46Ga0.54N films are grown on sapphire with a GaN buffer by metalorganic chemical vapour deposition (MOCVD). Both high resolution x-ray diffraction and high resolution transmission electron microscopy results reveal that these films have a hexagonal structure of single crystal. The thin InN film has a high mobility of 475 cm2V−1s−1 and that of In0.46Ga0.54N is 163 cm2V−1s−1. Room-temperature photoluminescence measurement of the InN film shows a peak at 0.72 eV, confirming that a high quality InN film is fabricated for applications to full spectrum solar cells.

InGaN/GaN multiple quantum wells (MQWs) are grown on planar and maskless periodically grooved sapphires by metal organic vapour phase epitaxy (MOCVD). High-resolution x-ray rocking curves and transmission electron microscopy (TEM) are adopted to characterize the film quality. Compared with the MQWs grown on planar sapphire, the sample grown on grooved sapphire shows better crystalline quality: a remarkable reduction of dislocation densities is achieved. Meanwhile, the MQWs grown on grooved sapphire show two times larger PL intensity at room temperature. Temperature-dependent PL measurements are adopted to investigate the luminescence properties. The luminescence thermal quenching based on a fit to the Arrhenius plot of the normalized integrated PL intensity over the measured temperature range suggests that the nonradiative recombination centres (NRCs) are greatly reduced for the sample grown on grooved sapphire. We assume that the reduction of dislocations which act as NRCs is the main reason for the sample grown on pattern sapphire having higher PL intensity.

ZnO nanotubes were epitaxially grown on a Si(I 1 1) substrate by plasma-assisted molecular beam epitaxy using a two-step process. First, a low-temperature ZnO layer was deposited on the Si substrate at a temperature of 300 degrees C, which was treated by an O-2(+) -plasma for 30 min. Second, a ZnO layer that showed tubular-like nanostructure was grown on this treated layer at 550 degrees C using the same flow rate of the 02 and zinc. No catalyst or templates were required for the formation of the ZnO nanotubes. Field-emission scanning electron microscopy (SEM) and X-ray power diffraction (XRD) experiments showed that the ZnO nanotubes were single crystals of pure hexagonal wurtzite structure growing along the [0 0 0 2] direction. The diameters of the nanotubes were in a range of 10-90 nm, and the average size of the nanotubes was 50 nm. Room-temperature photoluminescence (PL) measurements of the ZnO nanotubes showed ultraviolet peaks at 3.290 eV with a full-width at half-maximum (FWHM) of 104 meV, which was comparable to those found in high-quality ZnO films. The PL spectrum at 81 K showed a sharp free-exciton emission with an FWHM of 22.1 meV. The SEM, XRD and PL results indicated that the prepared ZnO nanotubes have potential applications in optoelectronic devices. Possible reasons for the growth of ZnO nanotubes were discussed. (c) 2005 Elsevier B.V. All rights reserved.