K. Iida

350.9 nm UV Laser Diode Grown on Low-Dislocation-Density AlGaN

We demonstrated a UV-laser diode grown on low-dislocation-density AlGaN. The combination of a low-temperature-deposited AlN interlayer technology and heteroepitaxial lateral overgrowth yielded crack-free and partially low-dislocation-density AlGaN on a grooved GaN substrate. A ridge waveguide was fabricated in the low-dislocation-density region. The lasing wavelength under pulsed current injection at room temperature was 350.9 nm, which is the shortest wavelength ever reported.

High-Efficiency Nitride-Based Light-Emitting Diodes with Moth-Eye Structure

Nitride-based blue light-emitting diodes (LEDs) with a moth-eye structure on the back of a 6H–SiC substrate have been developed. The moth-eye LED has a roughness less than the optical wavelength at the back surface of the SiC substrate fabricated by reactive ion etching (RIE) with CF4 gas. The light extraction efficiency and corresponding output power have been increased to 3.8 times those of a LED with a conventional structure. The experimental findings agree with the results of a theoretical analysis of the effect of the moth-eye structure.

UV laser diode with 350.9-nm-lasing wavelength grown by hetero-epitaxial-lateral overgrowth technology

We have demonstrated a UV-laser diode with a lasing wavelength of 350.9 nm, which has a GaN-AlGaN multiquantum-well (MQW) active layer and was grown on low-dislocation-density Al/sub 0.18/Ga/sub 0.82/N template. The Al/sub 0.18/Ga/sub 0.82/N template was produced by the hetero-epitaxial lateral overgrowth technology on the low-cost sapphire substrate, and has partially low-dislocation density of approximately 2/spl times/10/sup 7/ cm/sup -2/. The lasing operation under pulsed current injection was achieved with the threshold current density of 7.3 kA/cm/sup 2/ and the operating voltage of 10.4 V.

Fabrication of UV devices on various plane substrates

We have fabricated UV-emitters such as UV-light emitting diode (UV-LED) and UV-laser diode (UV-LD) on sapphire substrates. The combination of low-temperature-deposited AlN interlayer and lateral seeding epitaxy (Hetero-ELO) yielded crack-free and low-dislocation-density AlGaN. The light output power of GaN/AlGaN multi-quantum wells active layer based UV-LED monotonically decreased with the increase of threading dislocations. Moreover, we have demonstrated a UV-LD grown on this low-dislocation-density AlGaN. The lasing wavelength under pulsed current injection at room temperature was 350.9 nm. We also present violet and UV-LEDs grown on ZrB2 substrate. The violet LED exhibits excellent linearity of L-I characteristic and sharp single spectrum, and vertical conduction through nitride and ZrB2 interface has been confirmed in the UV-LED. We also present the growth of AlN single crystals by sublimation method.

Comparison of the simulation and experiments of the nitride-based UV light emitting diodes

In attempt to prepare a high performance AlxGa1-xN based UV-B LED, a computer simulation has been performed on a typical UV-LED structure to find out the effect of threading dislocations on non-radiative recombination process. UVB LED structures were formed on using GaN and AlN based layers for comparison. Cracks were generated in the device structure formed on GaN underlayer. No cracks were observed on the device structure formed on AlN under layer. Much better structure was formed when the base AlN was grown by high temperature MOVPE.

Nitride-Based UV Lasers

The ultraviolet (UV) laser diode (LD) is attracting much attention for various novel applications such as in medical engineering, sterilization and high density optical storage. Group III nitrides are one of the most promising candidates to realize UV LD. The external quantum efficiency of group-Ill nitride-based light-emitting diodes (LEDs) with emission wavelength shorter than 360 nm is still far inferior to that of nitride-based visible-short-wavelength LEDs. Simulation results show that there is no theoretical barrier which hiders the realization of short wavelength UV LDs (Chow, 2005). At the moment, however, the shortest emission wavelength of nitride-based LD is limited to 343nm on SiC (Edmond, 2004) and 350.9 nm on a sapphire substrate (Iida, 2004). In order to overcome the barrier for emission wavelength and to realize much shorter wavelength UV LDs, in addition to the control of conductivity for both n-type and p-type layers, the growth of low-dislocation-density high-Al-content AlGaN with low internal loss is essential.

Growth and fabrication of nitride-based LED on metallic substrate for application to high-power device and flexible display

A new candidate for the lattice-matched metallic substrate, i.e. ZrB2, for the growth of group-III nitrides is proposed. A low-temperature-deposited-buffer layer is found to be essential for the growth of GaN on ZrB2. Highly luminescent violet-light-emitting diodes fabricated on ZrB2 perform as well as or even superior to those fabricated on sapphire. ZrB2 is easily etched by the solution of HF and HNO3. Fabrication of a nitride-based flexible display is expected using a thin free-standing GaN film.

High Temperature MOVPE Growth of AlxGa1−xN (0.2-1) Layers on Sapphire and SiC Substrates for the Fabrication Deep UV Optical Devices

Single crystalline AlN epitaxial layers have been grown on and (0001) sapphire and 6H-SiC substrates by MOVPE technique at high temperatures in the range of 1340-1500°C. The structural qualities of the high temperature grown AlN layers were found to be good as evidenced by X-ray diffraction analyses results. By transmission electron microscopic analysis, dislocation densities of the layers were found to be 6.2 × 107 cm−2 or lower and the formation of dislocation loops was confirmed. High temperature bridge layers of AlN and AlxGa1−xN layers were grown on linear-groove patterned sapphire based AlN templates and 6H-SiC substrates. AlxGa1−xN bridge layers exhibited different growth behaviours depending on the direction of groove patterns on the sub-strates.

Physics and simulation of group-III-nitride-based moth-eye LEDs

Group III nitride-based blue-light-emitting diodes having a moth-eye structure at the bottom of the semi-insulating transport 6H-SiC substrate were fabricated. The light extraction efficiency and the corresponding output power were increased 3.8 times compared with those of the LED having the conventional structure. We also study a theoretical simulation using based on the RCWA method with three-dimensional Maxwells equations. The results of theoretical analysis agree with these findings.

UV laser diode with 350.9 nm-lasing wavelength grown on AlGaN template

We have demonstrated a UV-laser diode with a lasing wavelength of 350.9 nm, which was grown on low-dislocation-density AlGaN template. The lasing operation under pulsed injection was achieved with the threshold current of about 200 mA.