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In this study, AlGaN-based ultraviolet-B band laser diodes with 150-mW peak output power in pulsed operation were demonstrated at room temperature. The oscillation wavelength, differential quantum efficiency, and slope efficiency of a laser diode were 300 nm, 3.6%, and 0.15 W/A, respectively. These results were obtained by increasing the injection efficiency and decreasing the positive fixed polarization charge formed at the interface between a p-side waveguide layer and an electron blocking layer when polarization doping is formed in a p-AlGaN cladding layer.

Thin films of AlN, AlGaN, and AlGaN-based device structures of approximately 1 cm2 formed on a sapphire substrate were successfully exfoliated from the substrate by immersion in heated (115 °C) and pressurized (170 kPa) water. These thin films were crystalline, grown on periodically formed AlN nanopillars. The water was permeated through intentional voids formed in the AlGaN or AlN crystalline layers by using periodic AlN nanopillars. The exfoliated AlGaN exhibited clear X-ray diffraction peaks from its (0002) plane diffraction. Transmission electron microscopy (TEM) confirmed that exfoliation introduced few additional dislocations and that the device structure was maintained.

Ultraviolet (UV) semiconductor lasers are widely used in medical and industrial applications, and their commercialization is strongly desired. Recently, laser oscillation by current injection in the UV-C and UV-B regions has been reported . From now on, it is necessary to demonstrate CW operation, which is indispensable for practical applications. In order to realize this, it is important to understand the internal loss and optical gain of the current devices. In this presentation, we report the details of our UV-B lasers. Specifically, we would like to discuss the results of the variable stripe length analysis of the internal loss of the obtained device and the performance of the UV-B lasers. The results show that the internal losses of the optimized UV-B laser are relatively low and good values are obtained. We also discuss the details of polarization doping, which is very effective in the realization of these UV lasers.

Ultraviolet-B laser diode at 298 nm was realized by developing the technologies; growth method of lattice-relaxed Al 0.6 Ga 0.4 N layer on AlN/sapphire template and III-group composition-graded p-AlGaN cladding layer. The threshold current, current density and voltage were 5.0 A, 25 kA/cm2, and 34.4 V, respectively.

The centimeter-scale laser lift-off (LLO) of a UVB laser diode structure on nano-patterned AlN was demonstrated by using a 257 nm pulsed laser. The mechanism of this LLO, which can be used for vertical light-emitting device fabrications, was analyzed in detail from the structural and optical properties. The large-area high-yield LLO without cracks was found to be enabled by taking advantage of the intentional in-plane periodic and nanometer-scale inhomogeneous distribution of the AlN molar fraction in the AlGaN layer introduced by growing AlGaN on nano-patterned AlN.

We have successfully fabricated vertical LEDs by separating a 1 × 1 cm2 wafer composed of deep-ultraviolet LED on a sapphire substrate from the substrate using a laser liftoff (LLO) method. Reproducible substrate separation was achieved by the LLO method using an Al0.68Ga0.32N underlayer film on an AlN template with periodic pillars. The fabricated vertical LED successfully demonstrated notable luminescence (peak wavelength: 298 nm) characteristics up to a current density of ∼43 kA cm−2 at room temperature and in pulsed drive, which is expected to be used in high-power LEDs and laser diodes.

This review paper describes the historical development, current issues, and future expectations of UV-B laser diodes, which are expected to be adopted in various applications such as in microfabrication and biotechnology in the near future. To achieve RT operation of UV-B laser diodes, several challenges need to be addressed, including the development of a crystal growth technique for high-crystalline-quality AlGaN films that enables laser oscillation with a low excitation carrier density, and the development of a semiconductor layer structure for simultaneous formation of a desirable optical cavity and injection of high-density carriers (operation of high current density), allowing laser oscillation in the active layer. These challenges and the corresponding technologies that have overcome them are reviewed. The current status of device characteristics and future challenges are also discussed.

We report the results of crystal growth of thick AlGaN films on periodical 1 μm spacing AlN pillar concave-convex patterns and their impact on the performance of UV-B laser diodes. The formation of voids in the AlGaN film by increasing the AlN pillar height and the use of high-quality AlN templates were effective in improving the quality of AlGaN, and the dislocation density in the AlGaN film was reduced to approximately 3.4 × 108 cm−2. A UV-B laser diode was fabricated on the optimized AlGaN, demonstrating a threshold current density of ∼12 kA cm−2 and a high yield rate of over 90%.

Room-temperature pulsed oscillation with a laser wavelength of 290 nm and a threshold current density of 35 kA cm−2 was achieved by fabricating a UV-B laser diode on a thick AlGaN film formed on a 1 μm periodic concavo–convex pattern AlN (PCCP-AlN) on a sapphire substrate. The advantage of this method using PCCP-AlN is that it promotes the nucleation of AlGaN crystals. Planarization of this growth nucleus with AlGaN reduces the threading dislocation density at the top of the AlGaN growth layer while suppressing the formation of giant micrometer-sized hillocks and V-shaped pits that appear irregularly.

To reduce the threshold current density (Jth) of ultraviolet (UV)-B AlGaN-based laser diodes, we investigated the critical parameters aiming to increase the injection efficiency ηi and the optical confinement factor Γ. Optimization of the thickness of the waveguide layer, the average Al content of the p-type AlGaN cladding layer, and the film thickness of the cladding layer demonstrated that the device characteristics can be improved. This optimization achieved a reduction in Jth to 13.3 kA cm−2 at a lasing wavelength of 300 nm, thus offering the lowest Jth value yet achieved for a UV-B laser diode.

We demonstrated GaN-based vertical-cavity surface emitting lasers (VCSELs) with 5–30 μm wide nano-height cylindrical waveguide formed by BCl3 etching. A 5 nm-depth etching with BCl3 showed the most efficient current blocking at the interface of the etched p++-GaN and an ITO electrode among the cases with BCl3, Ar, or O2, which could be due to not only etching damages but also diffused B atoms into the etched surface. While room-temperature continuous-wave operations of the VCSELs with the large apertures were demonstrated, maximum light output power values of the large aperture VCSELs seemed limited by nonuniform current injection and device thermal resistances.

We obtained a high-quality 40-pair AlInN/GaN distributed Bragg reflector with a high growth rate of the AlInN layers (500 nm h−1), showing almost no threading dislocations and a peak reflectivity of 99.9% at 413 nm, by using a 0.3 nm GaN cap layer grown on the AlInN layer at low growth temperature. We also found that the threading dislocations generated at the interfaces between the bottom AlInN and the top GaN in the cases of 5–10 nm GaN cap layers which were typically used. Excess In atoms on the AlInN surfaces seem to cause the generation of the threading dislocations.

The UV semiconductor-based laser sources are important for a variety of fields, including medical, mechanical processing, chemical processing, biology, and photonics. However, the development of UV-B and UV-C laser diodes is strongly hampered because of the difficulties with current injection technology such as the realization of both a high hole concentration and low resistivity p-type AlGaN with a high AlN molar fraction. Because laser oscillation from AlGaN, with a high AlN molar fraction, can be obtained under optical pumping, UV lasers with controllable wavelengths should be realized if this problem can be solved. One promising technique for avoiding these problems is the use of electron beam excitation. Till date, nitride semiconductor-based lasers have been designed to achieve population inversion of the carrier and to oscillate due to current injection. However, as previously discussed, it is difficult to achieve wavelengths in UV-B and UV-C using this method. The conductivity control of nitride semiconductors is unnecessary using electron beam excitation. Therefore, it would be possible to expand the wavelength region for the laser action of nitride semiconductor-based lasers from deep UV to infrared if a nitride semiconductor-based laser could be oscillated via electron beam excitation. In this study, nitride-based lasers excited by electron beam were investigated, and laser emission was observed for the first time from a GaInN/GaN and GaN/AlGaN-based MQWs excited by an electron beam.

This study investigated electron beam laser excitation in the UV region using a GaN/AlGaN multiquantum well (MQW) active layer. Laser emission was observed when the GaN/AlGaN MQW was excited by an electron beam, with a wavelength of approximately 353 nm and a threshold power density of 230 kW/cm2. A comparison of optical pumping and electron beam pumping demonstrated that the rate of generation of electron-hole pairs when using electron beam excitation was approximately one quarter that of light excitation.

We investigated the dependence of the lasing threshold power density, optical gain, and internal loss on the dislocation density of an optically pumped AlGaN-based ultraviolet-B band laser. Reducing the dislocation density was found to not only increase the optical gain by reducing the non-radiative recombination centers but also reduce the internal loss. Furthermore, this reduction in internal loss was appropriately explained using an increased scattering model based on an increase in the refractive index fluctuations formed by the dislocations.

An AlGaN ultraviolet-B laser diode at 298 nm was realized at room temperature using pulse operation. The laser diode has a lattice-relaxed Al0.6Ga0.4N layer from the underlying AlN/sapphire template and a composition-graded p-AlGaN cladding layer. The multimodal laser spectrum with proper polarization properties at 298 nm was obtained over the threshold current at 0.90 A corresponding to 67 kA cm–2. By broadening the width of the p-electrode to 11.5 μm, the threshold current density decreased to 41 kA cm–2.

We aimed to further improve performances of GaN-based vertical cavity surface emitting lasers (VCSELs) by applying a combination of conducting distributed Bragg reflectors (DBRs) and lateral optical confinement structures simultaneously, generally used in GaAs-based VCSELs. Si-doped conducting AlInN/GaN DBRs and buried SiO2 apertures were adopted in the GaN-based VCSELs. By comparing the VCSELs and micro LEDs to those with undoped non-conducting DBRs, we found that lower device resistances and more uniform lateral current distributions were obtained with the conducting DBRs. At the same time, the maximum light output power of 2.6 mW was observed from the VCSEL with the conducting DBR while 4.4 mW was obtained from the VCSEL with undoped DBR. Inferior characteristics of a GaInN quantum well active region was found on the Si-doped conducting DBR.

We developed a model for in-situ wafer curvature values of AlInN/GaN distributed Bragg reflectors (DBRs) and determined InN mole fractions in the DBRs with the model. In order to develop the model, we experimentally investigated contributions of substrate temperature ramping and a GaN growth to changes in the in-situ curvature values during the AlInN/GaN DBR growth. We found that an increase of curvature changes at the substrate temperature ramping steps was explained by an increase of the total epitaxial layer thicknesses. Another finding was that strain in the GaN layers at the GaN layer growth steps was almost zero. Finally, we determined the InN mole fractions in the AlInN layers by using the model, showing excellent agreements with the values estimated from ex-situ X-ray diffraction measurements. The model reveals not only the entire in-situ curvature change profile but also the InN mole fraction under the precisely lattice-matched condition of AlInN/GaN DBRs.

In this paper, we investigated the dependence of threshold power density on the Al0.55Ga0.45N underlying layer film thickness in ultraviolet-B band (UV-B) lasers on various AlN wafers (four types). We also prepared and compared AlN templates for AlN freestanding substrates, AlN films fabricated by metalorganic vapor phase epitaxy, and annealed sputtered AlN templates at high temperature. The initial growth of AlGaN became three-dimensional by inserting a homoepitaxial Ga-doped AlN layer between the AlN template and Al0.55Ga0.45N, before it shifted to two-dimensional growth. It is possible to reduce the dislocation in Al0.55Ga0.45N using this mode. The dependence of AlGaN film thickness and that of the AlN template on samples with an inserted homoepitaxial Ga-doped AlN layer were studied. Compared with Al0.55Ga0.45N having a thickness of 5 μm, there was almost no noticeable difference between the dark spot density characterized by cathodoluminescence and the threshold power density in UV-B lasers for the AlN template. Besides, the characteristics were noticeably different for the film thickness of Al0.55Ga0.45N. The threshold power density in UV-B laser and dark spot density were reduced by increasing the film thickness. Through the optimization of the crystal growth condition, the threshold power density in UV-B laser and dark spot density were reduced to 36 kW cm−2 and 7.5 × 108 cm−2, respectively.

A 4.0λ-cavity GaN-based vertical-cavity surface-emitting laser (VCSEL) using an n-type conductive AlInN/GaN bottom distributed Bragg reflector (DBR) showed a light output power of 1.8 mW with a low differential resistance of 90 Ω. In order to obtain low resistive AlInN/GaN DBRs, 5 nm AlGaInN graded interfaces were introduced. A 40-pair AlInN/GaN DBR with the graded interfaces showed a peak reflectivity over 99.8% and a series resistance of 17 Ω. At the same time, the maximum light output power of a 1.5λ-cavity GaN-based VCSEL with the conductive DBR was only 0.03 mW due to a high thermal resistance of 2700 K W−1 caused by a high thermal resistivity of AlInN. We also conclude that a short cavity typically used in GaAs-based VCSELs is not appropriate in GaN-based VCSELs.

We have systematically investigated acceptor (Mg) and donor (Si) profiles at GaN tunnel junction interfaces with the aim of a low resistivity under reverse bias. We found that an overlap between Mg and Si at the tunnel junction interface was effective in helping to obtain a lower resistivity of the GaN tunnel junctions, which contradicts the typical picture of conventional semiconductor-based tunnel junctions. We demonstrated a LED with the GaN tunnel junction prepared in a single growth run by metalorganic vapor phase epitaxy, showing a differential resistivity of 2.4 × 10−4 Ω cm2 at 5 kA cm−2.

In this study, an electron beam excitation laser using a GaInN-based multiquantum well (MQW) active layer was investigated, and laser emission was observed for the first time from a GaInN-based MQW excited by an electron beam. This technology has the potential to provide access to an expanded wavelength region for the laser action of nitride-semiconductor-based lasers from deep UV to infrared.

In this study, an electron beam excitation laser using a GaInN-based multiquantum well (MQW) active layer was investigated, and laser emission was observed for the first time from a GaInN-based MQW excited by an electron beam. This technology has the potential to provide access to an expanded wavelength region for the laser action of nitride-semiconductor-based lasers from deep UV to infrared.

The GaN-based multiple quantum wells (MQW) laser diodes have been improved excellently by introducing GaN/GaInN optical- guiding layers and by reducing dislocation density. The lifetime of continuous wave operation has been improved to longer than 300 hours with 3 mW at the wavelength of 409 nm.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The GaN-based MQW laser diodes have been improved excellently by introducing GaN/GaInN optical-guiding. The continuous wave laser operation at room temperature has been achieved at the wavelength of 410 nm. The lifetime of room temperature continuous wave operation is longer than 60 minutes at around 1 mW output. The external efficiencies of GaInN/GaN MQW blue and green light emitting diodes (LEDs) have been increased by newly developed flip-chip (FC) type LED lamp structure. The luminous intensities of the FC-type blue and green LEDs were typically 6 cd and 14 cd at 20 mA, respectively. The FC-type blue and green LEDs are the brightest levels in the world currently. The peak wavelengths and full widths at half maximums were typically 464 nm and 27 nm for the blue LEDs, and 515 nm and 32 nm for the green LEDs.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

GaN-based short wavelength laser diodes are the most promising key device for a digital versatile disk. We have been improving the important points of the laser diodes in terms of optical guiding layers, mirror facets. The continuous wave laser irradiation at room temperature could be achieved successfully by reducing the threshold current to 60 mA (4 kA/cm2). We have tried to apply the multi low temperature buffer layers to the laser diodes for the first time to reduce the crystal defects.

We report the successful fabrication of a compact deep ultraviolet emission device via a marriage of AlGaN quantum wells and graphene nanoneedle field electron emitters. The device demonstrated a 20-mW deep ultraviolet output power and an approximately 4% power efficiency. The performance of this device may lead toward the realization of an environmentally friendly, convenient and practical deep ultraviolet light source.

In this paper, we proposed the possibility of implementing a single chip device for realizing optoelectronic integrated circuits (OEICs). Micro-light-emitting-diode (LED) arrays and a complementary metal–oxide–semiconductor (CMOS) pulse width modulation (PWM) silicon driver were proposed, designed, and fabricated on a single chip. The micro-LED arrays were separated by a dry etching method into 64 pixels of 8×8, each with a size of 30×30 µm2 and operated in 3 V at 100 µA. The PWM Si driver was well operated and modulated using various control signals. Furthermore, to investigate the driver for handling massive parallel information, a simple multifunctional driver was designed, fabricated, and flip-chip-bonded using a gold compliant bump and anisotropic conductive adhesive with micro-LED arrays.