Masakazu Kuwabara

Demonstration of an ultraviolet 336 nm AlGaN multiple-quantum-well laser diode

We have demonstrated laser operation of an AlGaN multiple-quantum-well (MQW) laser diode (LD) with a peak wavelength of 336.0 nm under pulsed current mode at room temperature. The LD was fabricated on a low-dislocation-density Al0.3Ga0.7N grown on a sapphire substrate using a hetero-facet-controlled epitaxial lateral overgrowth method. The laser emission is strongly transverse electric polarized with a peak output power of 3 mW and a differential external quantum efficiency of 1.1%. This demonstration of the LD lasing in ultraviolet-AII spectral band (320–340 nm) suggests that the AlGaN MQW LDs can be potent devices opening a path to deeper ultraviolet LDs.

AlGaN-based laser diodes for the short-wavelength ultraviolet region

We have demonstrated the room-temperature operation of GaN/AlGaN and indium-free AlGaN multiple-quantum-well (MQW) laser diodes under the pulsed-current mode. We have successfully grown low-dislocation-density AlGaN films with AlN mole fractions of 20 and 30% on sapphire substrates using the hetero-facet-controlled epitaxial lateral overgrowth (hetero-FACELO) method. GaN/AlGaN and AlGaN MQW laser diodes have been fabricated on the low-dislocation-density Al0.2Ga0.8N and Al0.3Ga0.7N films, respectively. The GaN/AlGaN MQW laser diodes lased at a peak wavelength ranging between 359.6 and 354.4?nm. A threshold current density of 8?kA?cm?2, an output power as high as 80?mW and a differential external quantum efficiency (DEQE) of 17.4% have been achieved. The AlGaN MQW laser diodes lased at a peak wavelength down to 336.0?nm far beyond the GaN band gap. For the GaN/AlGaN MQW laser diodes, the modal gain coefficient and the optical internal loss are estimated to be 4.7?0.6?cm?kA?1 and 10.6?2.7?cm?1, respectively. We have observed that the characteristic temperature T0 ranges from 132 to 89?K and DEQE shows an almost stable tendency with increase of temperature. A temperature coefficient of 0.049?nm?K?1 is also found for the GaN/AlGaN MQW laser diode. The results for the AlGaN-based laser diodes grown on high-quality AlGaN films presented here will be essential for the future development of laser diodes emitting much shorter wavelengths.

Entirely Crack-Free Ultraviolet GaN/AlGaN Laser Diodes Grown on 2-in. Sapphire Substrate

We have succeeded in fabricating ultraviolet (UV) GaN/AlGaN laser diodes without any crack generation on a whole 2-in. sapphire substrate using a hetero-facet-controlled epitaxial lateral overgrowth (hetero-FACELO) method. The UV laser diodes lased in the peak wavelength range from 355.4 to 361.6 nm under a pulsed current operation at room temperature. We have also investigated both parameters, material gain and optical-internal loss in GaN/AlGaN multiple quantum wells (MQWs). The actual threshold currents of the UV laser diodes were practically in agreement with the estimated threshold current from these parameters. This layer structure is one of the solutions for the purpose of high-yield production of UV photonic devices.

Radiative and nonradiative recombination in an ultraviolet GaN/AlGaN multiple-quantum-well laser diode

We have experimentally investigated the radiative and nonradiative recombination in a GaN/AlGaN multiple-quantum-well laser diode. The each carrier lifetime has been evaluated based on a rate equation analysis of light output-current characteristics of the laser diode. The estimated nonradiative carrier lifetime is 830 ps, and the Auger recombination is negligibly small at room temperature. At a threshold current density of 8 kA cm−2, the carrier density and the internal quantum efficiency are estimated to be 2.6×1019 cm−3 and 34%, respectively. These results are responsible for experimental and theoretical analysis of optical and electrical properties in AlGaN-based laser diodes.

Time-Resolved 1.5 µm-Band Photoluminescence of Highly Oriented β-FeSi2 Films Prepared by Magnetron-Sputtering Deposition

Temporal decay characteristics of highly (110)-oriented β-FeSi2 films were explored at 77 K and at room temperature. The 1.5 µm-band photoluminescence (PL) decayed as fast as that of the laser pulse, giving a characteristic decay time of sub-nanosecond, which is four orders faster than that of β-FeSi2 precipitates previously reported. In addition, the PL intensity of our β-FeSi2 films suffered a much weaker thermal quenching from 77 K to room temperature. These results indicated possible promising applications in light-emitting and high-speed response devices.

A 350-nm-band GaN/AlGaN multiple-quantum-well laser diode on bulk GaN

We have demonstrated the pulsed operation of a 350-nm-band ultraviolet laser diode with a vertical current path. The laser structure was grown on a (0001)-face bulk GaN substrate. The lasing wavelength was 356.6 nm and the peak output power reached to 10 mW from the one side of uncoated facets under pulsed current operation with a pulse duration of 10 ns and a repetition frequency of 5 kHz at room temperature. The GaN substrate is expected to provide a cleaved facet configuration leading to an excellent far-field pattern as well as an advantageous thermal management solution of the devices relative to sapphire substrates. The far-field pattern of actual device on GaN substrate has been improved dramatically compared with distorted one on that of sapphire substrates.

A 340-nm-band ultraviolet laser diode composed of GaN well layers

We have demonstrated the laser operation of a short-wavelength ultraviolet laser diode with multiple-quantum-wells composed of GaN well layers. The laser action has been achieved in 340-nm-band far from the wavelength corresponding to GaN band gap under the pulsed current mode at room temperature. The device has been realized on the Al(0.2)Ga(0.8)N underlying layer. The AlN mole fraction of the underlying layer is 0.1 lower than that of the underlying layer which was used for the previously reported 342 nm laser diode. These results provide a chance to the next step for a shorter-wavelength ultraviolet laser diode.

AlN/GaN Near Infrared Quantum-Cascade Structures with Resonant-Tunneling Injectors Utilizing Polarization Fields.

AlN/GaN quantum-cascade structures for near-infrared-laser application, which utilize the polarization field for electron injection into higher subbands, are proposed. Multiple-quantum-well resonant-tunneling injection layers are necessary for near-infrared quantum-cascade lasers, and the design of the injection layer is highly simplified in the quantum-cascade structure with superlattice light-emitting layers by utilizing the polarization field.

Over 1 W record-peak-power operation of a 338 nm AlGaN multiple-quantum-well laser diode on a GaN substrate

We have demonstrated the high-peak-power operation of an AlGaN-based ultraviolet laser diode (UV-LD) with a lasing wavelength of 338.6 nm. The UV-LD structure was fabricated on a bulk GaN(0001) substrate. The broad-area and vertical conductive structure of the UV-LD, whose ridge width and cavity length were 50 and 600 µm, respectively, was employed. The threshold current density and differential external quantum efficiency were estimated to be 38.9 kA/cm2 and 8.5%, respectively. The characteristic temperature of threshold current was estimated to be 119 K, and the temperature dependence of lasing wavelength was obtained to be 0.033 nm K−1. A peak power of over 1 W has been achieved in 338.6 nm under pulsed operation at room temperature, which is the highest peak power ever obtained for AlGaN-based UV-LDs.

Laser Operation of Nitride Laser Diodes with GaN Well Layer in 340 nm Band

We have reported the laser operation of a short-wavelength ultraviolet laser diode with multiple quantum wells composed of GaN well layers. The GaN well-width is estimated to be around 1–1.5 nm. We have simulated whole laser-diode structure, and calculated wave-function overlap integrals. It is provided the integral becomes the maximum value in the well-width of 1.5 nm. The laser operation has been achieved in 340-nm-band under the pulsed current mode at room temperature. The wavelength is far from the wavelength corresponding to band gap of GaN, and the shortest lasing wavelength ever reported for a semiconductor laser composed of binary compound well layer. Moreover, the device has been realized on an Al0.2Ga0.8N underlying layer with 0.1 lower AlN mole fraction margin than that of a previous reported 342 nm laser-diode with an Al0.3Ga0.7N underlying layer. These results provide a chance to the next stage for a shorter-wavelength ultraviolet laser diode.