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The radiative recombination efficiency (RRE) of AlGaN quantum wells on epitaxially laterally overgrown AlN/sapphire templates was investigated by picosecond-laser–excited photoluminescence (PL) spectroscopy under selective and non-selective excitation conditions. The PL efficiency, which was deduced by excitation-power-dependent PL measurements at low temperature (LT) and room temperature (RT), was unity at LT under both excitation conditions; However, at RT, the PL efficiency under non-selective excitation conditions was lower than that under selective excitation conditions. Time-resolved PL measurements revealed that, under non-selective excitation conditions, additional carriers are provided from the surrounding layers to the quantum-well layers, especially at LT. Therefore, at RT, the PL efficiency does not correspond to the RRE under non-selective excitation conditions. We propose a model to explain carrier dynamics under the two excitation conditions showing that the PL efficiency equals the RRE under selective excitation conditions.

In this study, the exciton binding energies of AlN are calculated by the anisotropic effective mass theory, which considers the strong electron–hole exchange and Frohlich-type exciton–phonon interactions. Our calculated results are in good agreement with recent experimental results, indicating that the electron–hole exchange and exciton–phonon interactions play essential roles in describing the excitonic structure of AlN. We estimate that the exciton binding energies of AlN are 53.7 and 67.3 meV, for the spin-singlet and the spin-triplet states, respectively. In addition, a universal correlation has been found in compound semiconductors including AlN, where the effect of the electron–hole exchange and Frohlich-type exciton–phonon interactions almost compensates each other on the binding energy of a spin-singlet exciton.

The optical properties of wurtzite AlN under large compressive strain are investigated by photoluminescence and optical reflectivity measurements with two different geometries. The AlN layer was coherently grown on 6H-SiC(0001), resulting in strains of exx=eyy=-9.6×10-3 and ezz=5.12×10-3, as confirmed by high-resolution X-ray diffraction. Free exciton transitions were clearly observed. The transition energy of A free exciton (with Γ1 symmetry) was estimated to be 6.246 eV at 10 K. The large energy shift of the free exciton transition with respect to the transition in unstrained AlN was well explained by the reported deformation potentials of AlN.

AlN layers are homoepitaxially grown on (0001) AlN substrates. The surfaces are atomically smooth, and the X-ray diffraction rocking curves for the symmetric and asymmetric planes indicate narrow line widths in the range of 10–30 arcsec. The oxygen, silicon, and carbon concentrations are below the detection limits of secondary ion mass spectroscopy. Due to these superior structural properties and low impurity concentrations, sharp free and donor-bound excitons dominate the photoluminescence spectra at low temperatures, while free excitons dominate at elevated temperatures.

Recently, we performed reflectance spectroscopy for nonpolar and semipolar bulk GaN substrates under uniaxial stress and experimentally determined all the exciton deformation potentials in GaN. Our findings indicated a breakdown of the quasicubic approximation. Herein its impact on the electronic band structures in GaN/AlGaN strained quantum wells is theoretically investigated. The calculated results significantly differ from those using the widely accepted exciton deformation potentials. A simplified Hamiltonian is given to describe the in-plane optical polarization properties of GaN/AlGaN quantum wells.

A simple apparatus to control water flow through a hydrogel at a fixed temperature was designed, in which the hydrogel was mechanically constrained in a glass microcapillary at gelation. An evaluation of the friction between the polymer network of polyacrylamide gels and the solvent water measured by this simple technique is presented here. The effects of the experimental conditions, the pressure applied to the solvent, the temperature and the gel size (the length and the cross-sectional area), on the friction coefficients were examined. The results agreed well with the model of the flux of water flow in a capillary based on the Hagen–Poiseuille equation.