Tomotsugu Mitani

InGaN-Based Near-Ultraviolet and Blue-Light-Emitting Diodes with High External Quantum Efficiency Using a Patterned Sapphire Substrate and a Mesh Electrode

We markedly improved the extraction efficiency of emission light from the InGaN-based light-emitting diode (LED) chips grown on sapphire substrates. Two new techniques were adopted in the fabrication of these LEDs. One is to grow nitride films on the patterned sapphire substrate (PSS) in order to scatter emission light. Another is to use the Rh mesh electrode for p-GaN contact instead of Ni/Au translucent electrode in order to reduce the optical absorption by the p-contact electrode. We fabricated near-ultraviolet (n-UV) and blue LEDs using the above-mentioned techniques. When the n-UV (400 nm) LED was operated at a forward current of 20 mA at room temperature, the output power and the external quantum efficiency were estimated to be 22.0 mW and 35.5%, respectively. When the blue (460 nm) LED was operated at a forward current of 20 mA at room temperature, the output power and the external quantum efficiency were estimated to be 18.8 mW and 34.9%, respectively.

Enhancement of blue emission from Mg-doped GaN using remote plasma containing atomic hydrogen

We have found for the first time that blue emission from Mg-doped GaN was greatly enhanced by remote plasma treatment (RPT) with plasma containing atomic hydrogen, in particular, water vapor plasma, at low temperatures of 300–400°C. The highest enhancing factor was twenty, achieved by water vapor RPT at 400°C for 30 min. The enhanced blue emission was stable up to 500°C, similarly to blue emission from as-grown samples, suggesting the same origin and mechanism. We have confirmed that the emission mechanism is donor–acceptor pair (DAP) recombination, and have concluded that RPT produces a hydrogen-related donor level at Ec-0.37 eV involved in the DAP emission.

Conductance Frequency Spectroscopy Study of a Low Resistive p-Type GaN Layer Highly Doped with Mg

A low resistive p-type GaN layer highly doped with Mg (1020 cm-3) was studied by conductance frequency spectroscopy. Three peaks were found in the G/f–f curve of the reversed-biased Mg doped p–n junction at temperatures ranging from 300 K to 77 K. The activation energies associated with two peaks are 63 meV and 90 meV, respectively around room temperatures, and decrease with decreasing temperatures. The activation energy of one more peak appearing at low temperatures is 16 meV, the value of which is very shallow compared to the reported value. Analysis using a multi-deep level model is given, and the experimental results are well explained by this analysis. These results suggest that the Mg acceptor in GaN has not a simple hydrogen-like structure, but a complex multi-level structure.