Koutoku Ohmi

Opposite bias polarity dependence of resistive switching in n-type Ga-doped-ZnO and p-type NiO thin films

A hypothesis based on the model that explains the resistance change effect of resistive random access memory by redox reaction is proposed. This hypothesis leads to the conclusion that the relationship between the polarity of the applied bias voltage and the resultant resistance change in p-type semiconductors is opposite to that for n-type semiconductors. The bias polarity dependence of the resultant resistance change in ZnO and Ga-doped ZnO (GZO), which are n-type semiconductors, and that in NiO, which is a p-type semiconductor, were investigated using conducting atomic force microscopy. Opposite bias polarity was confirmed to induce GZO and NiO into the same resistance state, which is consistent with the hypothesis.

Improvement of crystallographic and electroluminescent characteristics of SrS:Ce thin film devices by post‐deposition annealing in Ar‐S atmosphere

Post‐deposition annealing in an Ar‐S atmosphere at atmospheric pressure has been demonstrated to improve crystallographic properties and electroluminescent (EL) characteristics of SrS:Ce thin film EL devices. Crystallinity and degree of the orientation of the SrS:Ce thin films with polycrystalline columnar grains are improved by the annealing. It seems that recrystallization takes place between SrS grains. For the annealed devices, charge generation in the SrS:Ce layer is suppressed and relaxation of the phosphor field is decreased. As a result, an instantaneous EL efficiency at the leading edge emission is enhanced. The EL luminance and the average EL efficiency are increased. The annealed SrS:Ce thin‐film EL device showed a luminance of 800 cd/m2 and an efficiency of 0.42 lm/W at 1 kHz drive.

Transparent Y2O2S:Eu3+ phosphor thin films grown by reactive evaporation and their luminescent properties

Transparent Y 2 O 2 S:Eu 3+ phosphor thin films have been grown by a reactive evaporation method. Y metal vaporized by electron-beam irradiation was allowed to react with SO 2 gas molecules. Eu was simultaneously supplied from a resistively heated crucible containing EuCl 3 powder. By employing a heated W filament and an ac plasma cell to excite (crack) the SO 2 molecules, Y 2 O 2 S:Eu 3+ thin films and thin films containing both Y 2 O 3 and Y 2 O 2 S phases which showed intense red photoluminescence were obtained. These results suggest that the films obtained are crystallized and Eu 3+ ions substitute for Y ions in the Y 2 O 3 and Y 2 O 2 S lattices and hence form luminescent centers.

Control of ZnSe Film Stoichiometry at ZnSe/GaAs Interface Grown by MOCVD

ZnSe film stoichiometry at a ZnSe/GaAs interface was found to be critically dependent on the surface conditions of the GaAs substrate. It is shown for the first time that the main factor governing the interface stoichiometry is the competition between the Zn–As bondings and the Ga–Se bondings at the interface.

ZnS:Mn Electroluminescent Thin Films Prepared by Multisource Deposition under Controlled Sulfur Vapor Pressure

ZnS:Mn thin-film electroluminescent (TFEL) devices have been prepared by using a multisource deposition (MSD) method. A new technique of sulfur vapor pressure control has been developed for the thin-film growth at a high sulfur vapor pressure. Growth kinetics of ZnS:Mn thin films on the sulfur vapor pressure have been studied. It has been found that at sulfur vapor pressure higher than 0.1 Pa, the growth rate is limited by the effective amount of the Zn flux. The ZnS:Mn film grown at the sulfur vapor pressure of 0.85 Pa shows a smooth surface and has a uniform grain size of about 200 nm. The ZnS:Mn TFEL device shows the luminance of 300 cd/m2 and the efficiency of 3.5 lm/W at 30 V above the threshold voltage of 180 V with 60 Hz drive.

BLUE SRS:CU THIN-FILM ELECTROLUMINESCENT DEVICES GROWN BY HOT-WALL DEPOSITION USING SUCCESSIVE SOURCE SUPPLY

SrS:Cu, Cl thin-film electroluminescent (TFEL) devices have been prepared by hot-wall deposition using a successive supply of source materials. The TFEL devices show blue EL emission with the chromaticity coordinate of (0.19, 0.28). The luminance and the efficiency at 1 kHz drive are 100 cd/m2 and 0.1 lm/W, respectively. EL emission is observed at both the leading and the trailing edge of the applied pulse voltage. The presence of trailing edge emission implies that Cu+ centers are ionized during EL excitation, and then electrons are subsequently captured by ionized Cu2+ centers giving rise to the EL emission.

Relaxation of phosphor field in SrS:Ce thin film electroluminescent devices

Capacitance‐applied voltage and internal charge‐phosphor field characteristics of SrS:Ce thin film electroluminescent devices have been studied. The average phosphor field of the SrS:Ce films reaches the maximum of 1.3 MV/cm and relaxes to 0.7 MV/cm, although the applied voltage increases. The turn‐on voltage of the SrS:Ce thin film electroluminescent device decreases nonlinearly with increase of conduction charge and depends on the slew rate dV/dt. A large spike current is observed and is thought to correspond to the charge generation in the SrS:Ce layer, which results in the space charge, and causes a relaxation of phosphor field in the SrS:Ce films.

Lattice-Mismatch Enhanced Diffusion at a ZnSe/GaAs Interface - Increase of Thermal Stability in a Lattice-Matching System

ZnSe/GaAs interface diffusion, especially at an elevated temperature, was investigated. For ZnSe films of which stoichiometry was not completely maintained at the interface, the enhancement of Zn diffusion into GaAs was observed with an increase of film thickness. With preheating of GaAs substrates prior to growth, ZnSe films are stably deposited on GaAs. However, for thick ZnSe films of 7200 A, the original n-ZnSe/n-GaAs interface turned out to be converted to p-type at the interface after thermal annealing at 650°C for three hours. These interface diffusions which depend on the film thickness are estimated to be related with the misfit-induced lattice relaxation caused above a critical thickness. This hypothesis was confirmed by the extreme thermal stability found in the lattice-matched ZnS0.06Se0.94/GaAs interface.

Blue-Emitting Eu2+-Doped CaAl2O4 Phosphor Thin Films Prepared Using Pulsed Laser Deposition Technique with Post Annealing

Blue-emitting Eu2+-doped calcium aluminate phosphor thin films were obtained using the pulsed laser deposition technique with post annealing. As-deposited films were amorphous and showed weak red Eu3+ photoluminescence (PL). By annealing in reducing atmosphere (N2/H2:2% mixed gas) at 950°C for 3 h, the film was crystallized and showed a PL emission band peaking at about 447 nm, which originated from the 4f65d to 4f7 transition of Eu2+ ion. It is considered that the deposited film consists mainly of CaAl2O4 and partly of other binary compounds of the CaO–Al2O3 system. It was determined that the PL intensity of Eu2+ in CaAl2O4 can be controlled by the laser fluence, target-substrate distance and injection gas.

Luminescence and Energy Transfer in Thin Films of SrGa2 S 4 : Ce

Excitation and photoluminescence (PL) spectra of both nominally undoped and Ce 3+ -activated SrGa 2 S 4 thin films have been measured at various temperatures between 10 and 300 K. The undoped film shows a complex emission band which appears to involve emission from different defect states. Each defect center effects the recombination kinetics and luminescence spectra in a different temperature region. At 77 K, recombination is dominated by the highly associated defect centers producing an intense red emission band. At higher temperatures, thermalization effects reduce the net capture rates at the competitive defect centers, and recombination through the defect centers at room temperature produces a broad emission band with the peak in the blue spectral region. Evidence of the interaction and energy exchange between defect centers and Ce 3+ activator states is inferred from the rise and decay time measurements of PL emission in SrGa 2 S 4 :Ce thin films. As a result of thermalization, the efficiency of energy transfer to Ce 3+ ions increases at room temperature but at the expense of introducing a long afterglow into the activator decay. Measurements of temperature dependence of PL intensity suggest that the kinetic model for energy transfer in SrGa 2 S 4 :Ce combines aspects of long-range resonance transfer and indirect thermalization from defect states with subsequent retrapping at the activator sites. In addition, possible mechanisms are discussed which may explain the observed concentration dependence of the Ce 3+ fluorescence decay time. Preliminary results are reported for a thin-film electroluminescent device with a SrGa 2 S 4 :Ce, Li phosphor layer prepared by the low-temperature deposition from binary vapors process.