Adrian H. Kitai

Point defects and luminescence centres in zinc oxide and zinc oxide doped with manganese

Abstract The green and yellow luminescence centres in ZnO and Mn-doped ZnO are investigated. Water vapour enhances green luminescence of ZnO in the surface, and causes a two band luminescence phenomenon (green and yellow35in the bulk. Mn behaves as quencher for both the green and the yellow luminescence of ZnO. Positron lifetime measurements show the existence of at least two distinct vacancy-type defects in all materials. At the present time it seems that a VZn · V0 divacancy exists, and that luminescence is due to interstitial zinc and oxygen.

Electroluminescence of the oxide thin film phosphors Zn2SiO4 and Y2SiO5

Mn‐activated Zn2SiO4 and Ce‐activated Y2SiO5 multilayer thin film electroluminescent (EL) devices were prepared by rf magnetron sputtering. The EL response of the devices under different voltages, frequencies, and pulse widths, as well as the transferred charge and decay characteristics were studied. The EL device using Zn2SiO4:Mn as the phosphor layer is shown to achieve a brightness of over 200 cd/m2 at 400 Hz and a field of 3×106 V/cm (twice the threshold field) and a maximum efficiency of 0.78 lm/W, with a decay time of 0.6 ms. The main characteristics of the oxide phosphors include (a) strong trailing edge excitation in nonsymmetrical EL devices, (b) a narrow transferred charge loop, and (c) a time response sensitive to device structures. Compared with the nonsymmetrical structure, the more symmetrical double‐insulated structure is shown to increase the efficiency of the EL device by generating strong EL excitation from both positive and negative voltage pulses.

Temperature‐dependence of the growth orientation of atomic layer growth MgO

High‐quality MgO thin films deposited on Si(111) substrates by atomic layer growth (ALG) are formed by a hydrolysis surface reaction of Mg(C2H5)2 and H2O. The growth orientation of MgO changes from (111) to (100), when the temperature of the silicon substrate changes from 600 to 900 °C. The growth orientation difference of MgO grown by ALG is rationalized in terms of the surface diffusion coefficients on MgO.

Rare-earth-doped transparent yttrium silicate thin film phosphors for colour displays

Abstract We report the preparation and cathodoluminescent properties of rare-earth-doped Y2O3 SiO2 thin films. The thin films were prepared by magnetron sputtering. By doping with Eu3+, Tb3+ and Ce3+, most as-deposited amorphous thin films show luminescence. High temperature annealing above 800 °C increases the luminescence intensity 5–10 times. This reveals that the crystal field is dominated by the nearest atomic neighbours. The primary colours needed for a full-colour display can be realized by doping with Eu3+, Tb3+ and Ce3+.

A novel atmospheric pressure technique for the deposition of ZnS by atomic layer epitaxy using dimethylzinc

Abstract We report on the results of a novel atomic layer epitaxy (ALE) deposition system which performs deposition at atmospheric pressure. Polycrystalline thin films of ZnS have been deposited using the gaseous sources dimethylzinc and hydrogen sulphide. Depositions were carried out at substrate temperatures in the range 25 to 500 ° C. The results of several bulk and surface analytical techniques are presented which suggest that the films are stoichiometric and of high purity and crystal quality. The methodology of system operation is discussed as is the dependence of film quality upon process parameters.

ZnGa2 O 4 : Mn phosphors for Thin-Film Electroluminescent Displays Exhibiting Improved Brightness

Thin films of ZnGa 2 O 4 :Mn films have been deposited by radio frequency magnetron sputtering in order to study the effects of annealing temperatures less than 1000°C on the thin-film electroluminescent properties. Energy-dispersive X-ray compositional analysis showed a loss of zinc during sputtering, with the film composition being Zn 0.9 Mn 0.03 Ga 2 O 4 . All films showed strong (111) and (222) X-ray reflections relative to the power standard. As the annealing temperature was raised, the texture rotated toward that of the powder material. The as-deposited films showed no photoluminescence; however, once annealed at T ≥ 750°C, a single emission band at 504 nm was observed. Emission wavelength was independent of annealing temperature. The electroluminescent brightness of the devices peaked at an annealing temperature of 900°C. Peak brightness and efficiency were 350 cd/m 2 and 0.55 lm/W at 60 Hz, and 1500 cd/m 2 and 0.30 lm/W at 600 Hz. These high brightness values have been attributed to the roughness of the substrates.

Luminescence concentration quenching due to energy migration in ZnS : Mn with fixed trap density

Abstract By fixing the trap concentration, the Blumen and Silbey dynamic model for energy migration is applied to ZnS:Mn. We treat trap concentration as independent of Mn concentration for a given ZnS host, and obtain the dependence of both the decay time and intensity on Mn concentration. These correlate with known experimental data, and explain the empirical decay dependence on Mn concentration. We are able to quantify the probability of energy loss to traps using Dexters equation for dipole-dipole transfer. It is suggested that energy loss to the traps can be influenced by the growth technique.

The electrodeposition of thin film zinc sulphide from thiosulphate solution

Abstract We have investigated the physical properties of thin film zinc sulphide electrodeposited from aqueous thiosulphate solution. The as-deposited films are nearly stoichiometric in composition and have a 〈200〉 texture when deposited on transparent indium tin oxide coated glass substrates. Scanning electron micrographs indicate some discontinuity in surface coverage of as-deposited films. After annealing to 973 K in nitrogen the absorption characteristics and bandgap agree well with single crystal and evaporated thin film data. Annealing thicker films results in coalescence of the zinc sulphide into small islands on the surface.

Effect of Mn concentration on the cathodo‐ and photoluminescence of ZnS:Mn

A novel method of calibrated doping of Mn in ZnS thin films has been used to study room‐temperature cathodo‐ and photoluminescence characteristics of ZnS:Mn films for a Mn concentration range of 0.07–26.4 wt. %. It was observed that the luminescent intensity increases with Mn concentration up to ∼2 wt. %, beyond which intensity decreases. Emission spectra revealed a lower energy peak in addition to the 580‐nm yellow peak for higher dopant concentration. The occurrence of this red peak and quenching of yellow emission is probably at approximately the same activator concentration (∼2 wt. %). We attribute these to the phenomenon of energy transfer to energy sinks via unexcited Mn.

Characterization of the electro-optical behavior of Zn2Si0.5Ge0.5O4:Mn thin-film electroluminescent devices

Zn2Si0.5Ge0.5O4:Mn was investigated as a green-emitting oxide phosphor for thin-film electroluminescent (EL) display applications. Detailed electro-optical characteristics of thin-film EL devices with SrTiO3 dielectrics on a glass substrate (TFEL) and EL devices with a single ceramic insulator (CIEL) have been analyzed. The steeper brightness–voltage dependency of the TFEL device is ascribed to the capability to provide high current densities and efficient light generation for both voltage polarities. The CIEL device on the other hand suffered from asymmetrical behavior, related with the lack of a top dielectric. It was found that the use of high-permittivity dielectrics in the TFEL device results in a small voltage drop over the insulating layers, enhancing the amount of charge transferred through the phosphor. Transient charge–voltage (αQp–V) measurements on the TFEL device evidence that a high positive space charge density of ∼0.68 μC/cm2 is present in the oxide phosphor during steady-state operation. ...