Daisuke Morita

Demonstration of Blue and Green GaN-Based Vertical-Cavity Surface-Emitting Lasers by Current Injection at Room Temperature

We realized room-temperature lasing of blue and green GaN-based vertical-cavity surface-emitting lasers (VCSELs), for the first time, by current injection. The blue GaN-based VCSEL had a threshold current of 1.5 mA and a threshold voltage of 3.3 V under continuous-wave operation. Its maximum output power was 0.70 mW and its laser emission wavelength was 451 nm. The green GaN-based VCSEL had a threshold current of 22 mA and a threshold voltage of 6.3 V under pulsed current operation. Its maximum output power was estimated to be over 0.80 mW and the laser emission wavelength was 503 nm.

High Output Power 365 nm Ultraviolet Light Emitting Diode of GaN-Free Structure

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.

Watt-Class High-Output-Power 365 nm Ultraviolet Light-Emitting Diodes

Ultraviolet (UV) light-emitting diode (LED) epilayers were grown on low dislocation density (1×108/cm2) GaN templates with sapphire substrates. Then, the GaN templates and sapphire substrates were removed using laser-induced lift-off and polishing techniques. Additionally, to enhance the extraction efficiency, this LED chip used a higher reflectance Ag p-type electrode, patterned surface, and silicone resin molding package. When this UV LED was operated at a forward-bias pulsed current of 1 A at room temperature, the peak wavelength, output power, forward voltage, and external quantum efficiency were 365 nm, 1.5 W, 4.4 V, and 44%, respectively.

Radical Species Formed by the Catalytic Decomposition of NH3 on Heated W Surfaces

The catalytic decomposition processes of NH3 on heated W surfaces were examined by employing laser spectroscopic techniques. H atoms and NH2 radicals were identified as primary decomposition products on the catalyzer surfaces. The effective enthalpies for the production of these species were both determined to be 150 kJ/mol. NH radicals were also identified, but the production of this species is ascribed to secondary processes. N atoms are minor products in both the primary and secondary processes. The absolute density measurements show that the decomposition efficiency of NH3 is comparable to that of H2. The steady-state densities of NH3 and the stable products, H2 and N2, were also measured by mass spectrometry. When the catalyzer temperature is over 2000 K, the H2 density is comparable to that of residual NH3. H atoms are produced not only by the direct decomposition of NH3 but also by the decomposition of H2.

XPS study of ZnxMg1−xS:Mn ternary compound thin films

Abstract The chemical bonding of Zn, Mg, S and Mn atoms in Zn x Mg 1− x S:Mn thin films has been studied with X-ray photoelectron spectroscopy (XPS) in view to correlate the variation of electronic state with blue shift of Mn 2+ emission in Zn x Mg 1− x S. We have elucidated the clear dependence of binding energy, E B of core electron on the composition x for each element as follows: E B (Zn 2p 3/2 )=1022+0.6 x , E B (Mg 2p)=50+0.6 x , E B (S 2p 3/2 )=161+0.7 x , E B (Mn 2p 3/2 )=641+1.2 x (eV). The origin of the chemical shift of binding energies is discussed together with the effect of chemical shift on the photoluminescence.

Dependence of electroluminescent properties on cathode metalsin PVCz double-layered organic EL devices

Abstract We have investigated cathode metal dependence of electroluminescence (EL) characteristics of PVCz based EL devices by using Sr, Mg and Ag. The following has been clarified. First, we have obtained luminance as high as 1000 cd/m2 for Poly(Nvinylcarbazole) (PVCz) EL devices without electron transport molecules but only emissive dye by using Mg and Sr metals cathode; while the device with Ag cathode showed a brightness about half as high. Secondly, threshold voltage for electroluminescence and barrier height for charge carrier injection both showed clear dependence on metal work function. Thirdly, chemical shift of core electron binding energy by X-ray photoelectron spectroscopy showed oxidation of Sr metal electrode near PVCz surface even if deposited under background pressure of 10−10 Torr, while that of Mg showed negligible shift.

Charge transport in PVCz EL using multi-layered PVCz film doped with Cz

Abstract Carrier mobility and barrier height at negative electrode have been studied by considering temperature dependence of current–voltage characteristics for the electroluminescent (EL) devices fabricated from poly( N -vinylcarbazole) (PVCz) double-layered films doped with Cz. Luminance was more than 3000 cd/m 2 at 110 K and 1500 cd/m 2 at 300 K without electron transport molecules. Temperature dependence of the hole mobility in Cz doped PVCz has been analyzed and the activation energy for carrier hopping has been derived; 0.02 and 0.05 eV for undoped PVCz and 0.01 eV for Cz doped PVCz. The tunneling barrier height at negative electrode has been analyzed based on the position and applied-voltage-dependent electric field derived from the combination of space charge limited current and tunneling current schemes. The barrier height for undoped PVCz:perylene was 0.46 eV and that for Cz doped PVCz:perylene was 0.38 eV. The intrinsic tunneling has been confirmed from the temperature dependence of barrier height of heavily Cz doped PVCz:perylene device for the first time. In the case of undoped or lightly PVCz:perylene devices, tunneling was observed at lower temperature and charge injection became temperature-dependent with increasing temperature.