The effect of an electron blocking layer (EBL) on the efficiency droop in InGaN/GaN multiple quantum well light-emitting diodes (LEDs) is investigated. At low current density, the LEDs with a $p$ -AlGaN EBL show a higher external quantum efficiency (EQE) than LEDs without an EBL. However, the EQE of LEDs without an EBL is higher than LEDs with an EBL as injection current density is increased. The improved EQE of LEDs without an EBL at high current density is attributed to the increased hole injection efficiency.

Phosphorus-doped $p$ -type ZnO thin films were grown on sapphire by radio-frequency magnetron sputtering. The photoluminescence (PL) spectra revealed an acceptor bound exciton peak at 3.355 eV and a conduction band to the acceptor transition caused by a phosphorus related level at 3.310 eV. A study of the dependence of the excitation laser power density and temperature on the characteristics of the PL spectra suggests that the emission lines at 3.310 and 3.241 eV can be attributed to a conduction band to the phosphorus-related acceptor transition and a donor to the acceptor pair transition, respectively. The acceptor energy level of the phosphorus dopant was estimated to be located 127 meV above the valence band.

We report the growth of high-quality GaN on a Si(111) substrate using a five step-graded AlxGa1−xN (x=0.87–0.07) interlayer between GaN epilayer and AlN buffer layer by ultrahigh vacuum chemical vapor deposition. The crack density and the surface roughness of the GaN layer grown on the graded AlxGa1−xN interlayer were substantially reduced, compared to those of GaN grown on an AlN buffer layer. Significant improvement in the structural and optical properties of the GaN layer was also achieved by the use of a graded interlayer. These results are attributed to the decrease of the lattice mismatch between GaN and AlN layer, and the reduction of the thermal stress by the graded interlayer.