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The luminous flux and temperature characteristics of InGaN-based white light-emitting diodes (LEDs) intended for automotive headlamps were greatly enhanced by bonding a yellow phosphor to the LED chip using surface activated bonding. The performance of these LEDs was found to be suitable for use as headlamps.

AlGaN-based light-emitting diodes (LEDs) are attractive candidates for next-generation light sources in the deep ultraviolet (DUV) region because they can cover the spectral range of 210 – 365 nm through control of the Al content in the epitaxial layers. Moreover, they are environmentally friendly, having zero mercury content and a long lifetime. These excellent properties of AlGaN-based DUV LEDs are suitable for sterilization and polymer curing applications, however, their output power is not yet sufficient for practical applications. One method to improve output power of DUV LEDs is the enhancement of the light extraction efficiency from the LEDs chips to the air by connecting graded refractive index medium outside the chips. The refractive index medium should have the long-term reliability against DUV. We designed and fabricated the hemispherical lensmade of inorganic materials as a refractive index medium, and bonded the hemispherical lenses to the LEDs using room temperature bonding methods. We demonstrated high output power 255 nm and 280 nm LEDs with lenses [M. Ichikawa et al., APEX, to be contributed]. For this study, the excellent properties of fabricated LEDs with lenses are reviewed in connection with the bonded interface structure in light of new experimental data. The following figures (A) and (B) show schematic illustrations of LED chips and hemispherical lenses. We fabricated AlGaN-based 255 nm and 280 nm LEDs on sapphire substrates with a size of 1 mm square. A flip-chip type LED was produced to extract DUV light through the sapphire substrate. A simulation study predicted that the refractive index of the hemispherical lens was optimum when the lens was made of sapphire, which was the same material as that of LED substrates. Moreover, the light extraction efficiency was most enhanced when the lens diameter exceeded 3 mm. Therefore, we fabricated hemispherical lenses made of sapphire, with diameters of 3 or 5 mm. The LED chips were bonded to sapphire lenses at room temperature using two different bonding methods as shown in figure (C). One was surface activated bonding (SAB). In this method, the bonding surfaces of sapphire are sputter-etched and activated using an Ar fast ion beam. Transmission Electron Microscopy (TEM) images revealed that the surfaces of sapphire are consisted of several nanometer-thick amorphous layers, which were the damaged layers of the sapphire by the Ar ion beam irradiation. A large loading force was necessary to bond sapphire to sapphire. However, no light absorption was observed at the SAB interface. Another bonding method was atomic diffusion bonding (ADB). In this method, metal films are fabricated on two flat sapphire surfaces using sputter deposition with subsequent bonding of the two films on the sapphire in vacuum. Very thin Al films were used as the metal films for this study because the extinction coefficient of aluminum is small. A light absorption was slightly observed at the bonded interface in the DUV region. However, sapphire could be bonded without applying too high loading pressures. Moreover, ADB is applicable to bonding any mirror-polished substrates, in addition to sapphire, which is expected to extend the applications. The LEDs with lenses had higher light extraction efficiency than conventionally structured LEDs without lenses. In SAB devices, the maximum external quantum efficiency of the 255 nm LED was 4.56% at 100 mA, and that of the 280 nm LED was 10.1% at 1 A. As a result, at a forward current of 350 mA, the output power of a 255 nm LED with a lens increased by a factor 2.8, reaching 73.6 mW. Furthermore, that of the 280 nm LED increased by a factor of 2.3, reaching 153 mW. The output powers of ADB devices were also significantly higher than those of conventionally structured LEDs without lenses but were slightly lower than those of SAB devices because of a light absorption at the bonding interface. A further enhancement of output powers of ADB devices can be expected by tuning bonding process. The findings in this study would contribute not only to the research in DUV LEDs but also to the research of visible light LEDs. Figure 1

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 high-power pure blue laser diodes (LDs) by using GaN-based material for full-color laser display. The operating output power, voltage and wall-plug efficiency of the LDs at forward current of 1.0A were 1.17W, 4.81V and 24.3%, respectively. The estimated lifetime of the LDs was over 30, 000 hours under continuous-wave 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 succeeded fabricating high-output-power blue (445nm) laser diodes (LDs) with an output power of 500mW. The operating current, voltage, wall-plug efficiency of these LDs were 480mA, 4.8V, and 21.7%. Estimated lifetime of these LDs was over 30,000 hours under continuous wave operation at 25°C.

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.

High output power deep-ultraviolet light-emitting diodes (LEDs) are reported. First, two series devices were operated at a forward current of 350 mA. We measured powers of 45.2, 93.3, and 65.8 mW for 255, 280, and 310-nm LEDs, respectively. Next, single-chip devices with a modified epitaxial structure and electrode pattern were evaluated. Peak wavelengths, output powers, forward voltages, and external quantum efficiencies were respectively 258.0 nm, 34.8 mW, 8.25 V, 2.07% for the 255-nm LED, 282.8 nm, 40.5 mW, 5.69 V, 2.64% for the 280-nm LED, and 312.3 nm, 40.1 mW, 6.36 V, 2.89% for the 310-nm LED. The 70% lifetime of the 280-nm LED was estimated to be over 3000 h at a junction temperature of 45°C.

We demonstrate the fabrication of blue, green, and amber InGaN/GaN light-emitting diodes (LEDs) on semipolar {11-22} bulk GaN substrates. The {11-22}GaN substrates used in this study are produced by cutting out from a c-oriented GaN bulk crystal grown by hydride vapor epitaxy. The LEDs have a dimension of 320 ×320 µm2 and are packed in an epoxide resin. The output power and external quantum efficiency (EQE) at a driving current of 20 mA are 1.76 mW and 3.0%, respectively, for the blue LED, 1.91 mW and 4.1% for the green LED, and 0.54 mW and 1.3% for the amber LED. The maximum output powers obtained with a maximum current of 200 mA are 19.0 mW (blue), 13.4 mW (green), and 1.9 mW (amber), while the maximum EQEs are 4.0% at 140 mA (blue), 4.9% at 0.2 mA (green), and 1.6% at 1 mA (amber). It is confirmed that the emission light is polarized along the [1-100] direction, reflecting the low crystal symmetry of the {11-22} plane.

We use surface plasmons to increase the light emission efficiency from InGaN/GaN quantum wells by covering these with thin metallic films. Large luminescence enhancements were measured when silver or aluminum layers are deposited 10 nm above an InGaN light emitting layer, whereas no such enhancements are obtained from gold coated samples. The internal quantum efficiencies of quantum wells before and after metallization were determined from the temperature dependence of the photoluminescence intensity. Our results indicate that the use of surface plasmons will lead to a new class of very bright light emitting diodes, and highly efficient solid-state light sources. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Starting with next generation high density optical disc, commercial use of GaN based violet laser diodes (LD) has been adopted in many new fields, such as bio-medical, reprographic, exposure and etc. Recently, the lasing wavelength has broadened to cover from ultraviolet (UV) to blue-green region, which enabled other new applications. In this paper, current status of GaN based LDs in UV to Blue-green region are reported and next target is discussed.

We review the recent progress of GaN-based light emitting diodes (LEDs) and laser diodes (LDs) and discuss the availability for ultraviolet (UV) emission optoelectronic devices by using GaN related compound semiconductors. We fabricated UV LEDs and LDs with an emission wavelength of 365 nm which are useful for various industrial uses because of same wavelength as i-line of high-pressure mercury vapor lamps. Regarding UV LEDs, practical 365 nm UV LED with high efficiency was realized by optimizing device structure. For fabricating the ultraviolet LDs, we used the AlInGaN active layer instead of InGaN one. It was investigated that the relationship between the threshold current density and the lasing wavelength in the UV region. We demonstrated the shortest LDs with a lasing wavelength 365 nm under the continuous-wave operation.

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)

We fabricated a high-color-rendering, red-enhanced white-light-emitting diode (LED) using a new red phosphor and a short-wavelength YAG phosphor. When the new white-LED was operated at a forward-bias current of 20 mA at room temperature (RT), color temperature (Tcp), the general color rendering index (Ra) and luminous efficiency were 4670 K, 87.7 and 25.5 lm/W, respectively. Most of the color-rendering indexes (CRIs) of the new white-LED were larger than those of current white-LEDs, in which only YAG is used. In particular, the CRI-No.9 value, which shows the color reproduction in the red region, is improved from -2.5 to 62.6.

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.

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).

Ultraviolet (UV) laser diodes (LDs) were grown on epitaxially laterally overgrown GaN substrates by a metalorganic chemical vapor deposition method. We fabricated three types of UV LDs whose active layers were (I) ternary InGaN, (II) quaternary AlInGaN and (III) binary GaN single quantum well (SQW) structures. We investigated the LD characteristics in detail in the near UV region. LDs whose active layers were quaternary Al0.03In0.02Ga0.95N were demonstrated under the pulsed-current-biased conditions at room temperature. The emission wavelength of these LDs was the shortest wavelength (366.4 nm) in our experiments. We also demonstrated binary GaN SQW LDs for the first time. The threshold current density, voltage and emission wavelength of these LDs under 25°C continuous-wave (cw) operation were 3.5 kA/cm2, 4.6 V and 369.0 nm, respectively. The estimated lifetime was approximately 2000 h under 25°C cw operation at an output power of 2 mW.

We review the progress in the field of InGaN-based light-emitting diodes (LEDs) and discuss the issue of threading dislocations and the luminous efficiency. The first candela-class blue LEDs have been developed. An InGaN layer was used to produce these LEDs instead of a GaN active layer. The quantum-well structure InGaN active layer dramatically improved the external quantum efficiency. There are a number of threading dislocations in nitride-based LEDs. InGaN LEDs, however, have quite high external quantum efficiency. With regard to this, it is thought that the fluctuation of the indium mole fraction is strongly related to the high external quantum efficiency. Considering the density of threading dislocations in the nitride-based LEDs, we discuss what can improve the external quantum efficiency of nitride-based LEDs.

Optical gain of InGaN quantum well laser diodes and its internal field dependences are investigated. Gain spectra are experimentally measured by the Hakki-Paoli method. Inhomogeneous broadening of the gain spectra as well as of the electroluminescence spectra increases with In content of the active layers due to potential fluctuation. From measured carrier decay times and current density, we estimated the carrier density for which modal gain overcomes internal loss. We present gain calculation including quantum confined Stark effect, Coulomb screening and valence band structure simultaneously. Calculated and measured carrier densities are in good agreement. In InGaN quantum well laser diodes working at 470 nm, the quantum confined Stark effect reduces the optical transition probability down to 30% of its value without internal field. Wavelength (nm) 520 500 480 460 440 420 400 380 2.4 2.6 2.8 3.0 3.2 3.4 Photon energy (eV) Typical electroluminescence spectra of InGaN quantum well laser diodes with different peak wavelengths. Injected current is 10 mA.

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 (v) of 9.89 lm at a forward-bias current of 20 mA. The blue LED had a high output power (e) of 42.2 mW and a high external quantum efficiency (ηex) of 75.5%. The second is the high luminous efficiency white LED with a low forward-bias voltage (Vf) of 2.80 V, which was almost equal to the theoretical limit. ηL and wall-plug efficiency (WPE) were 169 lm/W and 50.8%, respectively, at 20 mA. They were 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 e of 651 mW at 350 mA. v, ηL, and WPE of the high power white LED were 145 lm, 134 lm/W, and 39.6% at 350 mA, respectively.

Ultraviolet (UV) InGaN/AlGaN double-heterostructure (DH) light-emitting diodes (LEDs) with an external quantum efficiency of 7.5%, an output power of 5 mW and an emission wavelength of 371 nm were developed. High-power UV LEDs are obtained using an InGaN active layer with a thickness of 400 A instead of a GaN active layer. The localized energy states caused by In composition fluctuation in the InGaN active layer are related to the high efficiency of the InGaN-based LEDs.

We fabricated the high luminous efficiency white light emitting diode (LED) and the high power white LED by using the patterned sapphire substrates and an indium–tin oxide (ITO) contact as a p-type electrode. The high luminous efficiency white LED was the yellow YAG-phosphors-coated small-size (240 ×420 µm2) high efficiency blue LED with the quantum efficiency of 63.3% at a forward-bias current of 20 mA. The luminous flux (Φ), the forward-bias voltage (Vf), the correlated color temperature (Tcp), the luminous efficiency (ηL), and the wall-plug efficiency (WPE) of the high luminous efficiency white LED are 8.6 lm, 3.11 V, 5450 K, 138 lm/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 high power white LED was fabricated from the larger-size (1 ×1 mm2) blue LED with the output power of 458 mW at 350 mA. Φ, Vf, Tcp, ηL, and WPE of the high power white LED are 106 lm, 3.29 V, 5200 K, 91.7 lm/W, and 27.7%, respectively, at 350 mA. The WPE is greater than that of a fluorescent lamp (25%) in the visible region. Moreover, the luminous flux of the high power white LED reaches to 402 lm at 2 A, which is equivalent to the total flux of a 30 W incandescent lamp.

The first-oreder AlInGaN 405 nm distributed feed-back (DFB) laser diodes were grown on the c-face sapphire substrates by a metalorganic chemical vapor deposition method. The diffractive grating, which was shaped by 108 nm deep grating of an 85.7 nm period with a sidewall angle 89°, was formed into a p-type waveguiding layer. As a result, we succeeded in demonstrating the first-order AlInGaN based 405 nm DFB laser diodes by pulsed current injection. The threshold current and the peak lasing wavelength of the DFB laser diode were 190 mA and 403.7 nm under pulsed current operation at 25 °C, respectively. The lasing spectrum was kept the single longitudinal mode emission up to an output power of 30 mW.

We fabricated high-output-power 255 and 280 nm light-emitting diodes (LEDs) using direct bonding. The LED chips were bonded to sapphire lenses at room temperature using either atomic diffusion bonding or surface-activated bonding. The LEDs with lenses had a higher light extraction efficiency than conventionally structured LEDs. As a result, at a forward current of 350 mA, the output power of the 255 nm LED increased by a factor of 2.8, reaching 73.6 mW, while that of the 280 nm LED increased by a factor of 2.3, reaching 153 mW.

Ultraviolet (UV) laser diodes (LDs), whose active layer were quaternary AlxInyGa1-x-yN single-quantum well structure, were grown on epitaxial lateral overgrown GaN substrates by a metalorganic chemical vapor deposition method. We investigated the lasing wavelength dependence of UV LDs on the threshold current density. As a result, we succeeded in fabricating 365 nm UV LDs which are useful for various industrial uses because their wavelength is similar to that of the i-line of high-pressure mercury vapor lamps. The threshold current and voltage of the 365 nm UV LD under continuous wave (cw) operation at 25°C were 50 mA and 4.8 V, respectively. The estimated lifetime of the 365 nm UV LDs was approximately 2000 h at an output power of 3 mW under cw operation at 30°C. The shortest lasing wavelength was achieved at 354.7 nm under pulse current injection at 25°C.

We have fabricated high-power ultraviolet (UV) light emitting diodes (LEDs). Epi-layers of UV LEDs were grown on high-quality GaN templates with sapphire substrates, and then the GaN templates and the sapphire substrates were removed by using laser-induced liftoff and polishing techniques in order to reduce the absorption of UV light by the GaN layer. As a result, we obtained the GaN-free UV LEDs. 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, 118 mW, 4.9 V and 6.9%, respectively. On the other hand, at a forward-bias direct current of 500 mA at RT, Po, Vf and ηex were 100 mW, 4.6 V and 5.9%, respectively.

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.

We fabricated two types of high-efficiency white light-emitting diodes (LEDs) composed of an InGaN multi-quantum well (MQW), which emit light of two or three different colors without phosphors. The Type-1 white LED emits light of two colors (blue and yellow) from the MQW active layer, while the Type-2 LED emits light of three colors (blue, green and red). When the Type-1 white LED was operated at a forward current of 20 mA at room temperature, the color temperature (Tcp), average color rendering (Ra) and luminous efficiency were 7600 K, 42.7 and 11.04 lm/W, respectively. When the Type-2 white LED was operated at a forward current of 20 mA at room temperature, Tcp, Ra and luminous efficiency were 5060 K, 80.2 and 7.94 lm/W, respectively.

InGaN multi-quantum-well-structure laser diodes (LDs), whose emission wavelengths are in the pure blue region, were grown on epitaxially laterally overgrown GaN on a free-standing GaN substrate by the metaorganic chemical-vapor deposition method. The wavelength dependence of the InGaN LD characteristics was investigated. These results indicated that there is a strong relationship between the threshold current density and the emission wavelength of LDs. LDs with an emission wavelength of 460 nm were demonstrated. The threshold current density and voltage of these LDs were 3.3 kA/cm2 and 4.6 V, respectively. The estimated lifetime was approximately 3000 h under 50 °C continuous-wave operation at an output power of 5 mW.

Current-modulated electroluminescence spectroscopy was developed to investigate the mechanism of the current-induced spectral blueshift in InGaN single-quantum-well blue and green light-emitting diodes. The evidence was obtained that tail states exist in InGaN active layers, and also the blueshift was found to be dominated by the tail-state filling effect.

Blue InGaN single-quantum-well-structure laser diodes (LDs) with an emission wavelength of 450 nm were grown on an epitaxially laterally overgrown GaN substrate by a metalorganic chemical vapor deposition method. The threshold current density and voltage were 4.6 kA cm−2 and 6.1 V, respectively. The estimated lifetime was approximately 200 h under room-temperature continuous-wave operation at an output power of 5 mW. When the number of InGaN well layers of the LDs with emission wavelengths longer than 435 nm varied from one to three, the lowest threshold current density was obtained when the number of well layers was one.

Monolithic multi-color light-emitting diodes (LEDs) based on micro-structured InGaN/GaN quantum wells are demonstrated. The microstructure is created through regrowth on SiO2 mask stripes along the [1¯100] direction and consists of (0001) and {11¯22} facets. The LEDs exhibit polychromatic emission, including white, due to the additive color mixture of facet-dependent emission colors. Altering the growth conditions and mask geometry easily controls the apparent emission color. Simulations predict high light extraction efficiencies due to their three-dimensional structures. Furthermore, we demonstrate that the apparent emission colors can externally be controlled over a wide spectral range that encompasses green to blue or white at a color temperature of 4000 K to blue along the Planckian locus. The controllability relies on the facet-dependent polychromatic emissions; the pulsed current operation with the appropriate duties varies their relative intensities and the consequent apparent colors without seriously affecting the total number of emitted photons, particularly for the blue to green variation. The proposed LEDs can be fabricated through a simple process and, therefore, may be a key device for advanced solid-state lighting.

The first-order AlInGaN 405 nm distributed feed-back (DFB) laser diodes were grown on the low dislocation freestanding GaN substrates by a metal organic chemical vapor deposition method. The first-order diffractive grating whose period was 80 nm was formed into an n-type cladding layer. The fine tooth shape grating was obtained by the EB lithography and the dry etching. No additional threading dislocation could be found at the regrowth interface. As a result, we succeeded in demonstrating the first-order AlInGaN based 405 nm DFB laser diodes under cw operation. The threshold current and the slope efficiency were 22 mA and 1.44 W/A under continuous wave operation at 25 °C, respectively. The single longitudinal mode emission was maintained up to an output power of 60 mW. The fundamental transverse mode operation with a single longitudinal mode was observed in the temperature range from 15 °C to 85 °C at an output power of 30 mW. The lifetime was estimated to be 4000 h by the lifetime test which was carried out under the condition of a constant output power of 30mW at 25 °C for 1000 h. The single longitudinal mode emission was maintained for the life tested DFB laser diodes.

We fabricated two types of high luminous efficiency white light emitting diodes (LEDs). The first one 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 forward- current of 20 mA. Used blue LED had a high output power (φe) of 42.2 mW and high external quantum efficiency (ηex) of 75.5%. The second one is the luminous-efficiency-maximized white-LED with a low forward-bias voltage (Vf) of 2.80 V, which is almost equal to the theoretical limit. ηL and wall-plug efficiency (WPE) were 169 Lm/W and 50.8%, respectively, at 20 mA. They were approximately twice higher than those of a tri-phosphor fluorescent lamp (90 Lm/W and 25%). Moreover, we succeeded in fabricating longer wavelength laser diodes (LDs) with an emission wavelength of 488 nm under CW current condition by optimizing the growth conditions and structure of LDs. To our knowledge, this wavelength is the longest for under CW current condition in GaN-based LDs.

We demonstrate that the apparent emission colors of InGaN-based light-emitting diodes using microstructured multifacet quantum wells as active layers can externally be controlled over a wide spectral range that encompasses green to blue or white at a color temperature of 4000K to blue along the Planckian locus. The controllability relies on facet-dependent polychromatic emissions. The pulsed current operation with the appropriate duties varied their relative intensities and the consequent apparent colors without seriously affecting the total number of emitted photons, particularly for the blue to green variation.

We measure gain spectra for commercial (Al,In)GaN laser diodes with peak gain wavelengths of 470 nm, 440 nm,405 nm, and 375 nm, covering the spectral range accessible with electrical pumping. For this systematic study weemploy the Hakki Paoli method, i.e. the laser diodes are electrically driven and gain is measured below thresholdcurrent densities. The measured gain spectra are reasonable for a 2D carrier system and understandable whenwe take into account homogeneous and inhomogeneous broadening. While inhomogeneous broadening is almostnegligible for the near UV laser diode, it becomes the dominant broadening mechanism for the longer wavelengthlaser diodes. We compare the gain spectra with two models describing the inhomogeneous broadening. The “rstmodel assumes a constant carrier density, while the second model assumes a constant quasi Fermi level. Bothare in agreement with the experimental gain spectra, but predict very dierent carrier densities. We see ourmeasurements as providing a set of standard gain spectra for similar laser diodes covering a wide spectral rangewhich can be used to develop and calibrate theoretical manybody gain simulations.Keywords: InGaN laser diodes, optical gain, inhomogeneous broadening, localized states, Hakki Paoli method

High-power pure blue laser diodes (LDs) are expected to be adopted to the light sources for full color laser display systems. We have succeeded in fabricating high-power blue (445nm) LDs with an output power of 500mW. The typical operating current, voltage and wall-plug efficiency of these LDs were 480mW, 4.8V and 21.7%, respectively. The lifetime of these LDs was estimated to be over 10,000 hours under continuous-wave operation. Moreover, we succeeded in fabricating the high-luminance white light source by combining the high-power blue LD, optical fiber, and phosphor. In this paper, we report recent progress and future prospects of the high-power GaN-based blue LDs and the new concept of high-luminance white light source.

Since the first demonstration of a pulsed InGaN laser diodes (LDs) grown on sapphire substrate in 1995, we have been developing longer lifetime and higher optical output power LDs in the 400 - 410 nm wavelength range. Moreover, we have already succeeded in the expansion of the lasing wavelength range from ultraviolet (UV) to blue-green. In this paper, we reported the recent progress of high-power and wide wavelength range GaN-based LDs with an optical output power of 20 mW single mode (375nm), 160 mW single mode (405nm), 200 mW multi mode (405nm), 50 mW single mode (445nm), 300 mW multi mode (445nm), and 20 mW single mode (473nm).

Nine years has passed since the initial development of GaN-based violet laser diodes (LDs) in the 405 nm wavelength range in 1995. Starting with next-generation high-density optical discs, the commercial use of violet LDs has been adopted in many new fields, such as biomedical, reprographic, and exposure fields. Recently, lasing wavelength has broadened to cover from the ultraviolet (UV) to blue-green regions, which enabled other new applications. In this paper, the current status of GaN-based LDs from the UV to blue-green regions is reported.

We fabricated high power ultraviolet (UV) light emitting diodes (LEDs), whose emission wavelength is around 365 nm. We found that, in order to improve the external quantum efficiency (ηex) of UV LEDs, it is very important to reduce the optical self-absorption and the threading dislocation density (TDD) of epi-layers. Therefore, at first, UV LEDs epi-layers were grown on high-quality GaN templates (TDD = 2x 108/cm2) with sapphire substrates, and then the GaN templates and the sapphire substrates were removed by using laser-induced lift-off and polishing techniques. As a result, we obtained the low self-absorption and low TDD UV LEDs. 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 forward voltage (Vf) and the ηex were 365 nm, 410 mW, 5.3 V, 24%, respectively. Moreover, at a forward-bias direct current of 500 mA at RT, Po, Vf and ηex were 360 mW, 5.0 V, 21%, respectively.

High-efficient light emitting diodes (LEDs) emitting red, amber, green, blue and ultraviolet light have been obtained through the use of an InGaN active layers. The localized energy states caused by In composition fluctuation in the InGaN active layer seem to be related to the high efficiency of the InGaN-based emitting devices in spite of having a large number of threading dislocations (TDs). InGaN single-quantum-well-structure blue LEDs were grown on epitaxially laterally overgrown GaN (ELOG) and sapphire substrates. The characteristics of both LEDs was almost same. These results indicate that the dislocation doesn't affect the efficiency practically. Recently, the development of high-power light source using GaN-based LEDs has become active. In such high-power LEDs, the density of forward current is much higher than that of past LEDs. Therefore, an advantage of carrier localization in InGaN active layer becomes small, because of band filling under high injection level. This means that reducing the density of TDs becomes important, just like GaN-based laser diodes. Also, we show recent results of GaN-based LEDs.

GaN-based laser diodes (LDs), which emit from UV to blue-green, are reviewed. For fabricating the UV LDs, we used the AlInGaN active layer instead of InGaN one. We demonstrated the UV LDs with a lasing wavelength 368nm under continuous-wave (cw) operation. Moreover, we fabricated the blue-green LDs whose lasing wavelength was 480 nm. It was investigated that the relationship between the threshold current density and the lasing wavelength. From our experiments, we successfully expanded the range of GaN-based LDs lasing wavelength, from UV (368 nm) to blue-green (480 nm). Regarding high power 405 nm-LDs, we could demonstrate the cw operated LD array devices with an output power of 1W by decreasing the thermal resistance of the LD chips.

Wide-bandgap group-ill nitride lasers, which emit from near-ultraviolet to pure-blue, are reviewed. Characteristics of 405 nm wavelength laser diodes (LDs) are discussed. Reducing threading dislocation can increase the lifetime of nitride LDs. Using epitaxial lateral overgrowth technique, the dislocation density of the order of 105 cm-2 has been obtained. The relation between the lifetime of nitride LDs and the density of dislocation are discussed. Some optical and electrical properties are very important for optical disk system such as digital versatile disk (DVD) system. In use of the DVD system, important properties of nitride LDs are discussed. Furthermore, near-ultraviolet LDs and pure-blue LDs are described. The near-ultraviolet LDs uses GaN or AlInGaN active layer instead of In GaN.

InGaN/GaN double‐heterostructure light‐emitting diodes were fabricated. The output power was 90 μW and the external quantum efficiency was as high as 0.15% at a forward current of 20 mA at room temperature. The peak wavelengths of the electroluminescence(EL) varied between 411 and 420 nm with changes in the growth temperatures of an InGaN active layer between 820 and 800 °C. The full widths at half maximum of EL were between 22 and 25 nm.

255/280/310 nm deep ultraviolet light-emitting diodes (DUV LEDs) suitable for high-current operation are reported. Newly developed 1 mm sized chips are installed in a commercial package with a two-series configuration. At a forward current of 350 mA, we measured powers of 45.2, 93.3, and 65.8 mW for the 255, 280, and 310 nm LEDs, respectively. The corresponding external quantum efficiencies per serial circuit were 1.3, 3.0, and 2.4%, and successful chip scalability was demonstrated. The 50% lifetime of the 280 nm LED die was estimated to be 3000 h at a junction temperature of 30 °C.