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.

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Photoluminescence and reflectance spectra of Si-, Ge-and Sn-doped GaN epilayers grown on sapphire substrates at 10 K have been investigated, by which ionization energies on the Ga site were determined as 29, 30 and 33 meV, respectively. To investigate the coherent properties of excitons, the dephasing times of excitons in undoped and Si-doped GaN on sapphire substrates and freestanding GaN were measured using the degenerate four-wave mixing technique. The resulting homogeneous broadenings in undoped and Si-doped GaN on sapphire substrates were about twice as large as that in freestanding GaN, which indicates that defect-induced scattering is stronger in the former than in the latter.Introduction Recently, high-efficiency GaN-based light emitting diodes (LEDs) and laser diodes (LDs) have been developed and commercialized as products [1]. It is a next target of the research and development of nitride semiconductors to apply them to illumination equipment. For this purpose, GaN-based LEDs are required to have higher quantum efficiency than current products. One of the origins of the reduction of quantum efficiency is strain-induced internal fields via the quantum confined Stark effect that originates from the mismatch of the lattice constants and the thermal expansion coefficients between GaN and sapphire. This problem, however, can be overcome by doping so that the supplied carriers can screen the internal field. Therefore the techniques of doping are important to control internal fields and the optical properties changed by doping impurities, such as donor levels or coherent properties of excitons, should be revealed. Quantum efficiency also depends on the crystal quality. To assess the crystal quality, one can estimate the degree of exciton-defect scattering using the four-wave mixing (FWM) technique.Si has been commonly used as a donor to make n-type GaN and InGaN, since it substitutes for Ga. The optical properties in Si-doped GaN were investigated to determine the Si donor level [2 -4]. However, the ionization energy reported so far is varied [2 -4]. As for the other group IV dopants, little is known about the optical properties in Ge-and Sn-doped GaN.To assess crystal quality and investigate the coherent exciton properties of GaN, some authors measured the dephasing time in undoped GaN using the FWM technique [5][6][7]. This is because the dephasing time reflects the degree of scattering by excitons, biexcitons, and impurities. However, the coherent properties in n-type GaN have not been investigated.

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 E c À 0:37 eV involved in the DAP emission.