The authors fabricated GaN-based light-emitting diodes (LEDs) on two different GaN templates with the same LED structure. One on thin GaN template (∼2μm) with high dislocation density [low (109cm−2)] grown by metal-organic vapor-phase epitaxy (sample A) and the other on thick GaN template (∼20μm) with comparatively low dislocation density [high (108cm−2)] by hydride vapor-phase epitaxy (sample B). In order to understand the mechanism of leakage current in LEDs, the correlation between current-voltage characteristics and etch pit density of LEDs was studied.

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We have investigated electron-trap centers in ZnO layers grown under different Zn∕O flux ratios by molecular-beam epitaxy. Frequency-dependent capacitance measurements show that ZnO layers grown under Zn-rich and stoichiometric flux conditions suffer from larger dispersion than a ZnO layer grown under an O-rich flux condition. Temperature-dependent capacitance measurements reveal that all the ZnO layers have shallow electron-trap centers ET1 and deep electron-trap centers ET2, while the Zn-rich ZnO layer has another shallow electron-trap center ET3 besides ET1 and ET2: the thermal activation energies of ET1, ET2, and ET3 are estimated to be 0.033–0.046, 0.12–0.15, and 0.065 eV, respectively. Moreover, it is exhibited that the trap density of ET2 is larger than those of ET1 or ET3 in all the cases and increases as the Zn∕O flux ratio increases. Consequently, it is suggested that the large dispersion effect observed in the Zn-rich and stoichiometric ZnO layers is ascribed to the large density of deep electron-trap center ET2.

We report on the growth of uniquely shaped ZnO nanowires with high surface area and patterned over large areas by using a poly(dimethylsiloxane) (PDMS) microfluidic channel technique. The synthesis uses first a patterned seed template fabricated by zinc acetate solution flowing though a microfluidic channel and then growth of ZnO nanowire at the seed using thermal chemical vapor deposition on a silicon substrate. Variations the ZnO nanowire by seed pattern formed within the microfluidic channel were also observed for different substrates and concentrations of the zinc acetate solution. The photocurrent properties of the patterned ZnO nanowires with high surface area, due to their unique shape, were also investigated. These specialized shapes and patterning technique increase the possibility of realizing one-dimensional nanostructure devices such as sensors and optoelectric devices.

We report on the lattice relaxation mechanism of ZnO films grown on c-Al2O3 substrates by plasma-assisted molecular-beam epitaxy. The lattice relaxation of ZnO films with various thicknesses up to 2000nm is investigated by using both in situ time-resolved reflection high energy electron diffraction observation during the initial growth and absolute lattice constant measurements (Bond method) for grown films. The residual strain in the films is explained in terms of lattice misfit relaxation (compression) at the growth temperature and thermal stress (tension) due to the difference of growth and measurement temperatures. In thick films (>1μm), the residual tensile strain begins to relax by bending and microcrack formation.

Structure and magnetic properties of the Ga1−xCrxN (x=0.013, 0.063, and 0.101) have been investigated. The lattice constant of the c axis is systematically decreased with increasing Cr content. The local structure around a Cr atom maintains tetrahedral symmetry up to x=0.101 the same as the local symmetry of GaN by x-ray absorption fine structure analysis. The analysis on x=0.013 indicated that the second nearest neighbor around the absorbing Cr atoms consists of only 12Ga atoms. However, for x=0.063, the second nearest neighbor around the absorber consists of Ga and Cr atoms with an unexpectedly high ratio of about Ga:Cr=2:1. In CrK-edge x-ray absorption near edge structure, we observed the oxidation state of Cr ion increases with increasing Cr content. Ferromagnetic behavior was observed in all Ga1−xCrxN films. However, the paramagnetic component also coexists with the ferromagnetic component. Total effective magnetic moment per Cr atom decreased from 3.17μB∕atom for x=0.013 to 1.05 and 0.79μB∕atom for x=0.063 and 0.101, respectively. The decreased magnetic moments of the x=0.063 and 0.101 is possibly caused by antiferromagnetic interaction of Cr–N–Cr networks in the high Cr content samples.

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