Meiso Yokoyama

Preparation and characteristics of ZnS thin films by intense pulsed ion beam

A new deposition system has been developed for the preparation of ZnS thin films by an intense pulsed ion beam (1 MeV, 80 ns). The maximum beam‐power density of ∼5 GW/cm2 is concentrated on ZnS, so that a high‐temperature plasma is easily produced. The plasma composed of Zn and S expands to be deposited onto a substrate kept at room temperature. Clear evidence has been obtained on the production of polycrystalline ZnS thin films with hexagonal structure. The deposition rate is estimated to be ∼108 A/s, which is several orders of magnitude higher than with any other method.

Molecular beam epitaxial growth of ZnS on a (100)-oriented Si substrate

Abstract ZnS films grown on (100) Si substrates were successfully obtained by molecular beam epitaxy. The dependence of the crystal quality on growth conditions was studied. The substrate temperature ( T s ) and the molecular beam flux ration of Zn to S ( J Zn / J S affect the crystal quality and the growth rate of the ZnS films profoundly. Crystallographic quality and surface morphology were significantly improved by growing at temperatures higher than 300°C. Below 300°C, growth orientation and growing planes of ZnS are different from others, namely ZnS(111) ‖ Si(511). The films grown at higher T s ( > 300°C) have the relation of ZnS(100) ‖ Si(100). By increasing J Zn / J S , the surface morphology became worse. It is found that the optimum growth conditions are T s = 340° C and J Zn / J S =1 or 2. The growth rate of ZnS decreases with incresing T s , and increases with increasing J Zn / J S and t hen saturates above J Zn / J S = 3. From these results, it is presumable that ZnS growth is controlled by the sticking coefficient of the minority molecular species.

High dielectric constant of RF-sputtered HfO2 thin films

Hafnium dioxide (HfO2) thin films are deposited on indium-tin-oxide (ITO)-coated glass substrates by the radio-frequency (RF) sputtering method using a HfO2 sintered target. The deposition conditions, dielectric loss and dielectric constant of HfO2 films before and after heat treatment are studied. The best deposition conditions for HfO2 films are RF power 200 W, substrate temperature 100°C and sputtering gas pure Ar. The maximum dielectric constants are about 150 and 45 with 1 kHz signal excitation before and after annealing, respectively. They are higher than any ever reported.

Enhanced emission in organic light-emitting diodes using Ta2O5 buffer layers

Abstract An ultra-thin hole-injection buffer layer using Ta 2 O 5 by RF magnetron sputtering in organic light-emitting diodes has been fabricated. With the property of high dielectric constant, Ta 2 O 5 can be expected to be a good buffer layer material and thus enhance the emission performance of the OLED. The Ta 2 O 5 film blocks part of the injected holes from the ITO anode to the organic layer (TPD) and improves the balance of hole and electron injections. The power efficiency of the device with the Ta 2 O 5 film can be increased by approximately 20%.

Quick Deposition of ZuS:Mn Electroluminescent Thin Films by Intense, Pulsed, Ion Beam Evaporation

Electroluminescent ZnS:Mn thin films have been guickly deposited by a high-density, high-temperature plasma produced by the irradiation of an intense (>GW/cm2), pulsed (~tens of ns) ion beam onto a ZnS:Mn target. The films prepared on a glass substrate have a polycrystalline hexagonal structure. The temperature of the target plasma was estimated to be ~2.7 eV, where the plasma was highly ionized. The instantaneous deposition rate was estimated to be ~4 cm/s, which is at least five orders of magnitude higher than that of any other conventional evaporation techique. We have prepared a ZnS:Mn electroluminescent device with a luminescent layer having a thickness of ~200 nm. The device has a threshold voltage of ~80 V0p (zero to peak value). The maximum luminance of 195 cd/m2 has been obtained under 10 kHz sinusoidal excitation at 144 V0p.

The Effect of Li, Cu and Zn Doping on the Luminance and Conductivity of Blue ZnGa2O4 Phosphor.

This study investigates the doping effect of Li, Cu or Zn on the luminescence and conductivity characteristics of ZnGa2O4 phosphor. The X-ray diffraction (XRD) patterns reveal a negligible difference in peak positions and values of full-width at half maximum (FWHM) under various synthetic parameters, indicating that an excellent solid solution exists between the additives and the ZnGa2O4 phosphor. Doping of Li or Zn increases the oxygen vacancy concentration in the phosphor. Therefore, Li or Zn doping enhances the luminance and conductivity of ZnGa2O4 phosphors. However, doping of Cu ions increases the scattering and trapping probabilities of electrons, thereby decreasing the conductivity, and results in a decreased luminescence of ZnGa2O4 phosphor. As-prepared low-voltage phosphors show blue-emission, appropriate to the application in the field emission display (FED).

High quality zinc sulfide epitaxial layers grown on (100) silicon by molecular beam epitaxy

ZnS films were successfully grown by molecular beam epitaxy on (100) Si substrates. These high quality films, grown at a substrate temperature of 340 °C with a molecular beam flux ratio of Zn to S of unity, exhibited high crystallographic quality and revealed a flat surface. Secondary ion mass spectroscopic analysis showed that the film contained little impurity. In photoluminescence, a broad peak was observed at 2.6 eV at room temperature while an additional sharp peak at 3.6 eV appeared at 77 K. The latter is considered to be band‐to‐band emission and the former may be due to the Cu‐blue emission.

Field emission characteristics of carbon nanotube emitters synthesized by arc discharge

The fabrication of carbon nanotube emitters with excellent emission properties is described. The nanotubes synthesized by arc discharge are used as electron emitters. The fibrous bundles containing nanotubes were crushed, mixed with conductive pastes, and slurries and then screen printed. The scanning electron microscopy images showed that the nanotubes were disordered and the average diameter was about several tens of nanometers. In a diode structure, the electron field emission can be turned on at a field as low as 2 V/μm and attains current density as large as 7.2 mA/cm2. No significant degradation of these performance is observed for thus made electron emitters, operated under 3 V/μm (J=2.8 mA/cm2) for tens of hours. The influence of the growth condition and post-treatment process on the emission characteristics of carbon nanotubes emitters will also be discussed in this article.

Optical and Electrical Characteristics of CO2-Laser-Treated Mg-Doped GaN Film

This work investigates the optical and electrical characteristics of CO 2-laser annealed Mg-doped GaN films to activate Mg-doped p-type GaN films. Results obtained from the CO 2 laser annealing investigation were similar to those of thermal annealing or low energy electron beam irradiation (LEEBI) treatment to activate the Mg-doped p-GaN films. The room-temperature photoluminescence (PL) intensity of the blue emission of the Mg-doped GaN film after 10 W laser annealing was approximately ten times stronger than that of the as-grown film. The resistivity of the Mg-doped GaN film decreased from 10 5 ˜¢cm to 2‐3˜¢cm as the laser annealing power rose above 6 W. The hole concentration of Mg-doped GaN film was approximately 1£ 10 17 cm i3 when the laser annealing power was 7.5 W.

The study of aging mechanism in ZnS:Mn thin-film electroluminescent devices grown by MOCVD

Evidence from DLTS and FTIR measurements strongly supports the assertion that the degradation mechanism of ZnS:Mn ACTFEL devices is mainly due to the deep electron trap, E 1 , which comes from the Mn activators reacting with surface water molecules. The photoluminescence measurements reveal that the Mn-related E t trap behaves like a nonradiative center. As a result, poor brightness characteristics including lower brightness and a higher threshold voltage were obtained when samples become aged.