Sang Ho Sohn

Electroluminescence in Li‐codoped ZnS: TmF3 thin‐film devices

The effects of Li codoping in ZnS:TmF3 thin‐film electroluminescent devices are reported. ZnS:Tm, Li thin films are prepared at several Li concentrations and substrate temperatures. The electroluminescent characteristics of ZnS:Tm,Li devices depend on Li concentrations, substrate temperatures, the annealing, and drive voltages.

Electroluminescence in oxygen co-doped ZnS:TmF3 and ZnS:Tm, Li thin-film devices

The effects of oxygen co‐doping in ZnS:TmF3 and ZnS:Tm, Li thin‐film electroluminescent devices are reported. Active layers are deposited in oxygen atmosphere at substrate temperatures of 200 and 300 °C. It is found that by oxygen codoping the luminance of ZnS:TmF3 and ZnS:Tm, Li devices increases, and that this phenomenon becomes marked in the films prepared at a higher substrate temperature.

A Model for Emissions from ZnS:Ce3+ and SrS:Ce3+ Thin-Film Electroluminescent Devices

ZnS:CeF3 thin-film electroluminescent devices are prepared in the vacuum and the oxygen atmosphere by means of an electron-beam evaporation method. The Auger depth profiles for the ZnS films prepared in the oxygen atmosphere show that oxygen substitutes for sulfur. The luminescent decay time for 475 nm and 530 nm emissions of Ce3+ ions reveals an increase, particularly in the 530 nm emission when the films are prepared in the oxygen atmosphere. The experimental results suggest that a nonradiative energy transfer via a certain defect occurs in ZnS:Ce3+ films. A simple model for emissions from ZnS:Ce3+ films is proposed, based on the Auger type-nonradiative energy transfer via sulfur vacancies. The electroluminescent characteristics seen in SrS:Ce3+ films are also discussed on the basis of the same mechanism.

Excitation and deexcitation of ac‐driven thin‐film ZnS electroluminescent devices

Theoretical formulas accounting for the excitation and deexcitation processes of the alternating current‐driven thin‐film electroluminescent devices have been obtained, which include both the impact excitation and the energy‐transfer mechanisms. The empirical equations for the conduction current duration time and the luminescent decay time related to the tunneling emission of electrons at the interface, the capture of holes in traps, and the light emission of luminescent centers lead to the analytical formulas for the transferred charge ΔQ, the luminance L, and other quantities of physical interest as a function of the electric field. The estimates for ΔQ and L in ZnS:Mn and ZnS:TbF3 devices have been made on the basis of Wolff’s distribution function and found to be in good agreement with the experimental data. From the estimated results, it is found that the energy‐transfer mechanism depends on various material parameters and drive conditions, and that it plays a role in improvement of the luminance in the low‐electric‐field region. In the high‐electric‐field region of interest, the energy transfer from Cu‐related sensitizers to luminescent centers in ZnS:Mn and ZnS:TbF3 devices yields an increase of luminance by a factor of about 1.5 and 3, respectively.

Effects of oxygen on electroluminescent characteristics of ZnS:TbOF and ZnS:TmOF devices

ZnS:TbOF and ZnS:TmOF active layers for thin‐film electroluminescent devices were deposited in oxygen atmosphere by an electron‐beam evaporation method and the effects of oxygen in active layers on electroluminescent (EL) characteristics were studied. With increasing oxygen deposition pressures, the luminance and the luminescent decay time increased and the threshold voltage shifted to lower values in ZnS:TbOF devices, while they showed no notable variations in ZnS:TmOF devices. X‐ray‐diffraction studies for ZnS thin‐films showed that by the deposition in oxygen atmosphere the film orientation of (111) plane, the film uniformity, and the grain size are improved. By an analysis based on the Auger electron spectroscopy spectra for ZnS thin films and EL emission spectra, it was confirmed that a small amount of oxygen is incorporated into films and suppresses the recombination related to zinc and sulfur vacancies. It was found that contrary to ZnS:TmOF devices, ZnS:TbOF devices show a change in the peak inten...

Effective charges in II–VI semiconductors

Abstract The effective charges accounting for the partially ionic binding character of II–VI semiconductors are calculated in a simple theory based on the cohesive energy per unit cell, which is expressed as a sum of the Madelung energy and the Morse potential. From the relation of cohesive energy and the bulk modulus, the effective charges for six zincblende, four wurtzite and ten rock-salt crystals of II a –VI b and II b –VI b semiconductors are estimated. The estimated results are in reasonable agreement with some experimental data.

An effect of oxygen content on ZnS:TbOF green color thin film electroluminescent devices prepared by electron-beam evaporation method

Abstract A systematic experimental investigation on an effect of oxygen content on the electroluminescence (EL) performance has been made on ZnS:TbOF EL devices prepared by electron-beam (EB) evaporation method. The results show that doping of oxygen with a high oxygen concentration of 3.5 mol% at 3 mol% Tb improves luminance. X-ray diffraction studies have identified an improvement in the phosphor layer crystallinity with increasing oxygen content. Through these experimental trials, it has been clarified that doping of oxygen is an effective method for an improvement in luminance on the EL devices prepared by EB evaporation method as well as those prepared by sputtering method.

Anion Vacancy States in the Insulator-ZnS Interface

The ground state energy of a trapped electron in an anion vacancy near the insulator-ZnS interface is estimated in a point-ion lattice approximation including an interface image potential. Assuming the (111) interface to be the halfly intersected plane of a Zn-S bond growned to the (111) crystal orientation and the insulator as a classical dielectric medium, the interface Madelung constant which can be used in a point-ion lattice approximation is calculated as a function of the position of an anion vacancy from the interface and the permittivity of insulator by the Evjen method. Taking the appropriate value 1.033 for the effective charge, the estimation shows the ground state energy levels of anion vacancies whose positions are in the 1st and 3rd atomic layers from the interface arise in the range of 1.18±0.3~1.55±0.3 eV below the lowest conduction band whereas, in the 2nd and 4th atomic layers, of 1.51±0.3~2.46±0.3 eV.

Electroluminescence in ZnS1-xTex:CeF3 Thin-Film Devices Prepared in Oxygen Atmosphere

The effects of oxygen codoping on ZnS1-xTex:CeF3 thin-film electro-luminescent devices are reported. ZnS1-xTex:CeF3 (x=0 and 1.25×10-3) films are prepared at substrate temperatures of 200°C and 300°C in vacuum and in oxygen remarkably. X-ray diffraction studies showed that the film orientation of the (111) plane and film uniformity are markedly improved when the films are prepared at 300°C in oxygen atmosphere. The emissive peak position and intensity of ZnS1-xTex:CeF3 devices change with substrate temperature and annealing, depending on the background atmosphere during film deposition. Oxygen codoping at a higher substrate temperature leads to an enhancement of luminance.

Electronic Polarizabilities of Isoelectronic Impurities in II-VI Compounds

A theoretical estimate for the electronic polarizabilities of isoelectronic impurities in II-VI compounds has been made in the frame of Ruffas theory. By a new analysis of the crystal ionization energy and the mean excitation energy of isoelectronic impurities, Ruffas theory for the electronic polarizabilities of alkali halides yields useful results for those of isoelectronic impurities in II-VI compounds. The estimated results show a large deviation from the additivity rule and a fluctuation in the electronic polarizabilities of any given impurities in the different crystalline environments.