Harumasa Yoshida

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.

Formation of GaN Self-Organized Nanotips by Reactive Ion Etching

We describe a new method of forming self-organized GaN nanotips by reactive ion etching using chlorine plasma and its mechanism. Nanotips with a density of approximately 8×109 cm-2 have been formed after etching. The nanotips have diameters between 10 and 30 nm, a length of about 0.7 µm and a high aspect ratio. It is revealed that nanotip formation is attributed to a nanometer-scale mask with a high etch selectivity to GaN. The structure simulated using our formation mechanism is almost similar to the experimental nanotip structure. These results prove the validity of the formation mechanism by nanomasking effect of ionized SiO2 sputtered by Cl+ ions.

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.

Fabrication of Deep-Ultraviolet-Light-Source Tube Using Si-Doped AlGaN

An ultraviolet (UV)-light-source tube using a Si-doped AlGaN film as a target of electron beam excitation was fabricated. The Si-doped AlGaN was grown on an AlN/sapphire substrate by low-pressure metalorganic vapor phase epitaxy (LP-MOVPE), and its optical properties were evaluated by excitation with a 10 kV electron beam (EB). Emission intensity was significantly improved by Si doping and optimization of the growth conditions. 247 nm deep-UV light was observed from the tube, and the lifetime of the light tube until 50% emission output of the initial strength was approximately 2000 h at an EB acceleration voltage of 10 kV with a current of 100 µA.

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.

Growth and Characterization of AlGaN Multiple Quantum Wells for Electron-Beam Target for Deep-Ultraviolet Light Sources

The structure of Si-doped AlGaN multiple quantum well (MQW) targets has been optimized for application to electron-beam (EB)-pumped deep-ultraviolet (UV) light sources. The deep-UV light emission from Si-doped AlGaN MQW targets pumped by a 10 kV EB has been evaluated. The targets exhibited a deep-UV light output power of over 15 mW at a peak wavelength of 256 nm for an EB input power of 2.0 W, and the conversion efficiency was estimated to be over 0.75%. These results demonstrate the advantageousness of using p-type-AlGaN-free AlGaN MQW targets as a material for application to EB-pumped deep-UV light sources.

Growth of High-Quality Si-Doped AlGaN by Low-Pressure Metalorganic Vapor Phase Epitaxy

In this paper we report the growth of Si-doped AlGaN on an AlN/sapphire substrate by low-pressure metalorganic vapor phase epitaxy (LP-MOVPE) with an in situ monitoring system to fabricate a high-quality film with controlled thickness. The AlN mole fraction in AlGaN can be controlled by adjusting the growth temperature. We also discuss the quality and growth mechanism of AlGaN on AlN. GaN and AlGaN with an AlN mole fraction of 0.28 were free from stress because of the occurrence of three-dimensional (3D) growth. The lattice constant a of AlGaN with an AlN mole fraction of 0.40 was larger than that at an AlN mole fraction of 0.28 despite the two-dimensional (2D) growth mode. This indicates that the lattice mismatch between AlGaN and the underlying AlN adversely affects the crystal quality. AlGaN with an AlN mole fraction of over 0.60 was coherently grown on AlN owing to the fact that the AlN/sapphire template used in this work was subjected to compression. For this reason, in the samples with an AlN mole fraction of over 0.6, the crystal quality was high because of the coherent growth on the underlying AlN. Moreover, Si doping was performed for AlGaN. The carrier concentration increased linearly up to a Si concentration of 2 ×1018 cm-3, indicating that the activation rate was approximately 1.

Field Emission from GaN Self-Organized Nanotips.

GaN self-organized nanotips were applied to the cold cathode field emitter, which was fabricated by reactive ion etching (RIE). Field emission from the GaN nanotips was observed for the first time. The microscopic current I through the anode hole (=20 µm) at the anode electrode was 0.1 nA at 900 V. The field enhancement factor βd, which is related to the top structure of the emitter and is the product of the field conversion factor β and the sample-anode gap d, was estimated to be 300 from the Fowler-Nordheim (F-N) plot. The microscopic current I was stable at 12.5 nA for 3600 s from the onset of voltage application (standard deviation 0.64 nA).

Antireflection effect of self-organized GaN nanotip structure from ultraviolet to visible region

A self-organized GaN nanotip structure was fabricated by means of reactive ion etching using Cl2 plasma. We have revealed, in Fourier transform analysis, that the self-organized nanotip structure has a two-dimensional isotropic distribution and broadband structure frequencies of the nanotips with the most frequent spatial frequency of 10.4 µm-1. It has been found that the nanotip surface, which is a fine structure with periods smaller than the wavelength of light, provides antireflection and enhanced transmission effects from the ultraviolet to the visible region (330 nm–800 nm). These excellent antireflection performances of the GaN nanotip structure are of interest for optoelectronic applications in GaN-based devices.