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.

Ultraviolet anti-Stokes photoluminescence (ASPL) is observed under continuous visible light excitation in a GaN single crystal at 4 and 77 K. The ASPL spectra are almost the same as the Stokes photoluminescence (SPL) spectra under band-to-band excitation. Namely, we observe ASPL due to bound and free excitons at 4 and 77 K, respectively. Based on the temporal decay, the excitation spectrum and the excitation intensity dependence of the ASPL intensity, we propose a two-step two-photon absorption mechanism through an electron trapped at a deep ionized donor with a lifetime of 1.45 ms.

Porous barium titanate (BaTiO 3 , BT) ceramics were prepared by a conventional sintering method using two kinds of BT particles. The relationship between pore structure (porosity and pore size) of BT ceramics and their sensor properties was investigated. Since a piezoelectric constant d 33 of BT depends largely on the pore structures, the microstructure control of porous BT ceramics is important to improve the figure-of-merit of piezoelectric stress sensor application (g 33 /µ). In this study, the maximal piezoelectric g 33 constant value of 14.8 © 10 ¹3 V•m•N ¹1 and the maximal g 33 /µ value of 3.14 © 10 3 V•m 4 •N ¹1 •g ¹1 were obtained at a porosity of approximately 23%.

Excitonic emission was observed at 3.357 eV by photoluminescence only from the (0001) Ga‐face of GaN grown by an ammonothermal method. The emission intensity varied over the crystal surface. The temperature dependence of the emission intensity is similar to that of excitons bound to neutral donors for hexagonal (2H) GaN. The origin of the emission is discussed. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Deep‐level luminescence at 1.93 eV (red luminescence) was observed from GaN grown by the acidic ammonothermal method. Typical deep emission at 2.2 eV (yellow luminescence) was not observed. The intensity decreased linearly with increasing temperature, which is different from the red and yellow luminescence reported thus far. The time decay of the luminescence appeared partially exponential, i.e. with the both possibilities of a simple two‐level transition and donor–acceptor pairs. The origin of the red luminescence is probably different from the previously reported one [J. Appl. Phys. 97, 061301 (2005)]. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)