Kenichiro Takeda

Internal Quantum Efficiency of Whole-Composition-Range AlGaN Multiquantum Wells

We analyze the internal quantum efficiency (IQE) of whole-composition-range AlGaN multiquantum wells (MQWs) on AlGaN with various dislocation densities (DDs) by excitation-density-dependent photoluminescence measurement. IQEs of deep ultraviolet/ultraviolet (DUV/UV) MQWs are strongly dependent on the DD. IQE with an excess carrier density of 1×1018 cm-3 changes from 4 to 64% when the DD changes from 6×109 to 2×108 cm-2. This trend is almost the same for DUV/UV MQWs with emission wavelength ranging from 230 to 350 nm. Thus, the reduction of the DD is very important for the realization of a high-IQE DUV/UV active layer.

Indium?Tin Oxide/Al Reflective Electrodes for Ultraviolet Light-Emitting Diodes

We investigated indium–tin oxide (ITO)/Al reflective electrodes for improving the light extraction efficiency of UV light-emitting diodes (LEDs). The ITO layer showed high transparency in the UV region upon optimization of the thickness and annealing temperature. As a result, the ITO/Al electrode exhibited both high reflectivity in the UV region and good contact characteristics simultaneously. Using this electrode, we succeeded in improving the light output power of a 350 nm UV-A LED.

AlGaN/GaN Heterostructure Field-Effect Transistors on Fe-Doped GaN Substrates with High Breakdown Voltage

AlGaN/GaN heterostructure field-effect transistors (HFETs) were fabricated by metalorganic vapor phase epitaxy (MOPVE) on c-plane Fe-doped GaN (GaN:Fe) substrates with different growth conditions of GaN buffer layers. The GaN buffer layers were grown at a V/III ratio of 272 at 300 Torr to realize highly resistive GaN buffer layers with automatic carbon doping. HFETs with carbon-doped GaN buffer layers showed the best characteristics among all the samples in this study. The carbon concentration was 5?1016 cm-3. In the sample with a gate-drain length of 50 ?m, the off-state breakdown voltage was 2 kV or higher and the on-resistance was 18.5 m??cm2. We found that the carbon-doped GaN buffer layers are necessary for high breakdown voltages even using semi-insulating GaN:Fe substrates.

Combination of Indium–Tin Oxide and SiO2/AlN Dielectric Multilayer Reflective Electrodes for Ultraviolet-Light-Emitting Diodes

We have investigated novel reflective electrodes by combining an indium–tin oxide (ITO) layer and a SiO2/AlN dielectric multilayer (DM) for UV-light-emitting diodes (LEDs). The reflectance of 10 pairs of SiO2/AlN DM reached 98.5% at 350 nm. As a result, the ITO/DM electrodes simultaneously satisfied the requirements of high reflectivity in the UV region, good contact characteristics, and current spreading. The light output power of the UV LED with an ITO/DM electrode is 1.2 and 1.6 times higher than those of LEDs with ITO/Al and Ni/Au electrodes, respectively.

Reduction in operating voltage of UV laser diode

We fabricated UV laser diodes (LDs). A high-temperature annealing at 850 °C for a short duration was found to be very effective for reducing the series resistance of an n-type contact. By applying high-temperature annealing, the operating voltage of an UV LD was reduced from 10.7 V to 9.5 V at a forward current of 300 mA.

Activation of Mg-Doped p-Type Al0.17Ga0.83N in Oxygen Ambient

Annealing in oxygen ambient was found to be effective for realizing a high hole concentration in p-type Al0.17Ga0.83N:Mg. The maximum hole concentration obtained was 1.3×1016 cm-3 at room temperature. Hydrogen dissociation from Mg-doped Al0.17Ga0.83N is found to be enhanced by annealing in a flow of O2 compared with annealing in a flow of N2.