We report on the optical properties of the InGaN-based red LED grown on a c-plane sapphire substrate. Blue emission due to phase separation was successfully reduced in the red LED with an active layer consisting of 4-period InGaN multiple quantum wells embedding an AlGaN interlayer with the Al content of 90% on each quantum well. The light output power and external quantum efficiency at a dc current of 20 mA were 1.1 mW and 2.9% with the wavelength of 629 nm, respectively. This is the first demonstration of a nitride-based red LED with the light output power exceeding 1 mW at 20 mA.

High-efficiency InGaN light-emitting diodes (LEDs) in the “green gap” range were fabricated on c-face sapphire (0001) substrates. Optical properties were enhanced by band engineering of active layers and optimization of growth conditions. Output power and external quantum efficiency of 11.0 mW and 24.7% for a 559 nm green-yellow LED and 4.7 mW and 13.3% for a 576 nm yellow LED with the injection current of 20 mA were achieved, respectively.

We have studied the electronic structure of the diluted magnetic semiconductor Ga 1−x Mn x N ͑x = 0.0, 0.02, and 0.042͒ grown on Sn-doped n-type GaN using photoemission and soft x-ray absorption spectroscopy. Mn L-edge x-ray absorption have indicated that the Mn ions are in the tetrahedral crystal field and that their valence is divalent. Upon Mn doping into GaN, new states were found to form within the band gap of GaN, and the Fermi level was shifted downward. Satellite structures in the Mn 2p core level and the Mn 3d partial density of states were analyzed using configuration-interaction calculation on a MnN 4 cluster model. The deduced electronic structure parameters reveal that the p-d exchange coupling in Ga 1−x Mn x N is stronger than that in Ga 1−x Mn x As.

High‐efficiency green‐yellow InGaN‐based light‐emitting diodes (LEDs) were grown by metal organic chemical vapor deposition on conventional c‐plane sapphire (0001) substrates. The crystal quality of the InGaN quantum well was improved by optimizing the growth temperature and rate. Fabricated LEDs exhibited a high output power of 11.2 mW and external quantum efficiency (EQE) of 25.5% for 560 nm green‐yellow LEDs with an injection direct current of 20 mA. The highest EQE of over 30% was achieved with an injection direct current of 3 mA at an emission wavelength of 568 nm. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The magnetic properties of Zn 1−x Co x O ͑x = 0.07 and 0.10͒ thin films, which were homoepitaxially grown on a ZnO͑0001͒ substrates with varying relatively high oxygen pressure, have been investigated using x-ray magnetic circular dichroism ͑XMCD͒ at Co 2p core-level absorption edge. The line shapes of the absorption spectra are the same in all the films and indicate that the Co 2+ ions substitute for the Zn sites. The magneticfield and temperature dependences of the XMCD intensity are consistent with the magnetization measurements, indicating that except for Co there are no additional sources for the magnetic moment, and demonstrate the coexistence of paramagnetic and ferromagnetic components in the homoepitaxial Zn 1−x Co x O thin films, in contrast to the ferromagnetism in the heteroepitaxial Zn 1−x Co x O films studied previously. The analysis of the XMCD intensities using the Curie-Weiss law reveals the presence of antiferromagnetic interaction between the paramagnetic Co ions. Missing XMCD intensities and magnetization signals indicate that most of Co ions are nonmagnetic probably because they are strongly coupled antiferromagnetically with each other. Annealing in a high vacuum reduces both the paramagnetic and ferromagnetic signals. We attribute the reductions to thermal diffusion and aggregation of Co ions with antiferromagnetic nanoclusters in Zn 1−x Co x O.