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Photodecomposition and photobactericidal activities of anatase/rutile-mixed phase TiO2 nanoparticles annealed with low-temperature O2 plasma were clarified by comparing them with those annealed in ambient air. The photocatalytic activities of plasma-assisted-annealed sample greatly enhanced as compared with the untreated sample, under not only ultraviolet irradiation but also visible-light irradiation. The photocatalytic activities of air-annealed samples did not enhance under ultraviolet irradiation but enhanced under visible-light irradiation. The enhanced photocatalytic activities due to the plasma-assisted annealing (PAA) originated from the increased photoexcited carrier concentration. This enhancement was discussed from PAA-induced characteristic factors. PAA facilitated the phase transformation to anatase, contributing directly to extending the photoexcited carrier lifetime. PAA introduced more oxygen vacancies, contributing to trapping more photogenerated electrons. PAA also introduced more bridging/terminal oxygen groups adsorbed on the surface, increasing the upward band-bending, the depletion layer width at the surface, and the charge transfer from rutile to anatase. These two introductions contributed to facilitating the separation of photoexcited carriers. Furthermore, PAA reduced the aggregate size of TiO2 nanoparticles formed on the surface, contributing to increasing optical absorptions. More reactive oxygen species produced from the bridging/terminal oxygen groups by the photoexcited carriers would also enhance the photocatalytic activities.

We succeeded in fabricating InGaN-based laser diodes (LDs) with a wavelength of 518 and 488 nm under continuous wave (cw) operation. The both LDs structures were grown on conventional c-plane GaN substrates by metal organic chemical vapor deposition (MOCVD). The threshold current and threshold voltage were 45 mA and 5.5 V at 518 nm, 30 mA and 4.5 V at 488 nm, respectively. The lifetime test of these LDs was carried out under high driving temperature up to 80 °C in cw operation. Lifetime was estimated to be over 5000 h with an optical output power of 5 mW at 80 °C in 515–518 nm LDs from 1000 h operation, and was estimated to be over 10,000 h with an output power of 60 mW at 60 °C in 488 nm LDs from 2000 h operation.

We fabricated three types of high luminous efficiency white light emitting diodes (LEDs). The first is the white LED, which had a high luminous efficiency (ηL) of 161 lm/W with the high luminous flux (ϕv) of 9.89 lm at a forwardbias current of 20 mA. The blue LED had a high power (ϕe) of 42.2 mW and high external quantum efficiency (ηex) of 75.5%. The second is the high luminous efficiency white LED with a low voltage (Vf) of 2.80 V, which was almost equal to the theoretical limit. ηL and wall-plug efficiency (WPE) is 169 lm/W and 50.8%, respectively, at 20 mA. They are approximately twice higher than those of a tri-phosphor fluorescent lamp (90 lm/W and 25%). The third is the high power white LED fabricated from the high power blue LED with high ϕe of 651 mW at 350 mA. ϕv, ηL and WPE of the high power white LED are 145 lm, 134 lm/W and 39.6% at 350 mA, respectively. Moreover, at 1 A, ϕv and ηL were 361 lm and 97 lm/W, respectively. Thus ϕv is equivalent to that of a 30 W-class incandescent lamp. And, ηL is slightly higher than that of a tri-phosphor fluorescent lamp. Moreover, we fabricated the high power near ultra-violet, bluish-green and green LEDs, whose ϕe at 350 mA were 675 mA, 325 mW, and 236 mW, respectively. ϕv of the green LED was 128 lm at 350 mA. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We investigate the gain formation properties of In x Ga 1-x N laser diodes emitting at 404 nm and 470 nm (sample A and B, respectively). Employing the Hakki-Paoli method, their optical gain spectra were measured at 298 K and 333 K. Since spatial fluctuations of the well width and In composition are considerable for long-wavelength laser diodes, broadening of the optical gain and luminescence spectra are clearly enhanced in sample B when compared with sample A. Moreover, population inversion occurs in at least two discrete energy levels for the gain spectra of sample B. It was found that only the modal gain of the higher gain peak can overcome the mirror loss, therefore the lower one does not contribute to lasing action. We also estimate internal loss of A and B to be 25 cm -1 and 30 cm -1 , respectively. The maximum gain value of A increases linearly with increasing injected current even at 333 K, while that of B grows sub-linearly.

We fabricated three types of white light emitting diodes (LEDs). The first is the white LED, which has a high general color rendering index (R 0 ) of 97 and CRI-No. 9 of 96. The CRI-No. 9 denotes the color reproduction in the red region. These values are higher than those of a tri-phosphor fluorescent lamp (R a = 85 and CRI-No. 9 = 8). The second is the high efficiency white LED fabricated from the small-size high efficiency blue LED chip. The output power (P 0 ), the external quantum efficiency (η ex ) and wall-plug efficiency (WPE) of the small-size blue LED were 35.0 mW, 63.3% and 56.3%, respectively, at a forward-bias current of 20 mA. The luminous flux (Φ), luminous efficiency (η L ) and WPE of the second white LED are 8.6 lm, 138 Im/W and 41.7%, respectively. The luminous efficiency is 1.5 times greater than that of a tri-phosphor fluorescent lamp (90 lm/W). The third is the high power white LED fabricated from the larger-size blue LED chip. Po, η ex and W.P.E. are 458 mW, 47.2% and 39.7%, respectively, at 350 mA. Φ, η L and WPE of the third white LED are 106 lm, 91.7 Im/W and 27.7% at 350 mA, respectively. Moreover, Φ of 247 lm and 402 lm at 1 A and 2 A are obtained, respectively. Φ at 2 A is equivalent to the total flux of a 30 W incandescent lamp.

Nitride laser diodes and light emitting diodes were irradiated with 200-, 80- and 10-MeV protons. The threshold current of violet laser diodes increased with increasing proton fluence to 1 x 10 14 p/cm 2 . The increase in threshold current is considerably small compared with that of AlGaAs laser diodes. Proton energy dependence of the change of threshold current was a little bit different from that of GaN NIEL. Oscillation wavelength did not change before and after irradiation. The defect levels made by proton irradiation do not seem to be radiative recombination centers.

We review the recent progress of nitride-based light-emitting diodes (LEDs) and discuss the relation between dislocation density and quantum efficiency. We also discuss how to improve the external quantum efficiency of nitride-based LEDs. Secondly, the group-III nitride laser diodes (LDs), which emit from near-ultraviolet to pure-blue, are reviewed. Reducing threading dislocations can increase the lifetime of nitride LDs. Using the epitaxial lateral overgrowth technique, a dislocations density of the order of 105 cm−2 has been obtained. The relation between the lifetime of nitride LDs and the density of dislocations is discussed. Finally, near-ultraviolet LDs and pure-blue LDs are described. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have remarkably improved the emission efficiency of 365 nm ultraviolet (UV) light emitting diode (LED). Rugged pattern is fabricated on the surface of LED chips in order to enhance the extraction efficiency. As a result, the extraction efficiency of this LED approximately increased by 50%. When this UV LED was operated at a forward-bias pulsed current of 500 mA at room temperature (RT), the peak wavelength, the output power (Po), the operating voltage (Vf) and the external quantum efficiency (ηex) were 365 nm, 240 mW, 4.5 V and 14.1%, respectively. On the other hand, at a forward-bias direct current of 500 mA at RT, Po, Vf and ηex were 210 mW, 4.3 V and 12.4%, respectively. The ηex at 365 nm increased to double that of the reported value before. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

It is well known that GaN-based semiconductors are the most excellent material for short wavelength light-emitting devices. In this paper, we review the development of GaN-based laser diodes (LDs). 400 nm InGaN-LDs are mainly described. Additionally, ultraviolet (UV) and purely-blue LDs are demonstrated, and the emission-wavelength dependence of the threshold current density is studied.

First, we review the recent progress on nitride-based light-emitting diodes (LEDs) and discuss the relation between dislocation density and luminous efficiency. We also discuss a method to improve the external quantum efficiency of nitride-based LEDs. Secondly, group-III nitride laser diodes (LDs) which emit from near-ultraviolet to pure-blue are reviewed. Reducing the threading dislocation density can increase the lifetime of nitride LDs. Using an epitaxial lateral overgrowth technique, a dislocation density of the order of 10 5 cm -2 has been obtained. The relation between the lifetime of nitride LDs and the dislocation density are discussed. Finally, near-ultraviolet LDs and pure-blue LDs are described.

Electroluminescence (EL) mapping has successfully been performed for In x Ga 1-x N single quantum well (SQW)-based light emitting diodes (LEDs) by employing scanning near-field optical microscopy (SNOM) at room temperature. The relative EL intensity fluctuates in the range from 1 to 4 with the spatial scale less than a few hundred nanometers. Clear correlation has been found between EL peak wavelengths and EL intensities where the regions of long wavelength correspond to those of strong EL intensity, suggesting that the injected carriers redistribute toward local potential minima within In x Ga 1-x N SQWs.

Violet laser diodes (LDs) with higher power and longer lifetime are required for some applications such as read/write laser light sources of digital versatile disks (DVDs). We achieved that the reliability of the LDs for DVDs was improved by using low dislocation density free standing GaN substrates. Moreover, we fabricated blue and ultraviolet (UV) LDs in order to spread the emission wavelength. The details of LD characteristics in the blue and UV region were investigated. In the blue LDs, there is a strong relationship between the threshold current density and the emission wavelength of LDs. For near UV LDs, we fabricated three types of LDs whose active layers are (I) ternary InGaN, (II) quaternary AlInGaN, (III) binary GaN single quantum well structures (SQW). We demonstrated the binary GaN SQW LDs under continuous-wave operation for the first time. From our experiments, we successfully expanded the range of GaN-based LDs emission wavelength, from UV (365 nm) to pure blue (465 nm).

Time-resolved optical characterization is an indispensable tool to study the recombination mechanisms of excitons and/or carriers based on radiative, non-radiative, localization and many-body processes. In this paper, we review the instrumentation of various spectroscopic techniques for the assessment of In x Ga 1 -x N-based semiconductors such as time-resolved photoluminescence (TRPL), time-resolved electroluminescence (TREL), transient grating (TG) method to probe photothermal processes, microscopic TRPL using optical microscope, submicroscopic TRPL using scanning near field optical microscopy (SNOM) and pump-and-probe spectroscopy for the measurement of transient absorption/gain spectra. The obtained results are cited in the references.

Optical spectra of GaN, In-doped GaN, and InGaN single quantum well (SQW) structures were compared to explore the role of In for the emission mechanisms in the SQW light emitting diodes (LEDs). The internal electric field, F, due to spontaneous and piezoelectric polarization in strained quantum wells (QWs) naturally induces the quantum confined Stark effect (QCSE) that reduces the electron–hole (e–h) wavefunction overlap due to rapid ionization of excitons causing a serious decrease of the emission efficiency especially for the GaN SQW, although a clear QW excitonic absorption peak was observed. The low energy tail of the emission spectrum of InGaN amber SQW LEDs extends below the bulk InN bandgap, indicating that F is not completely screened by injected carriers. However, doping and alloying of In in GaN improve the emission efficiency despite the fact that the effective bandgap inhomogeneity is increased with increasing InN mole fraction. In is considered to support effective localization of QW excitons, and the Coulomb interaction between the e–h pair is maintained provided that the QW thickness is smaller than the free exciton Bohr radius.