Current Thin Film Electroluminescent Devices Research Articles

Roughness of ZnS:Pr,Ce/Ta/sub 2/O/sub 5/ interface and its effects on electrical performance of alternating current thin-film electroluminescent devices

Roughness effects of neighboring dielectrics on electrical characteristics of thin-film electroluminescent devices were investigated in order to improve the understanding of physics for the devices. Atomic force microscopy analysis reveal that thicker bottom layer of Ta/sub 2/O/sub 5/ shows rougher surface resulting in the rougher surface of ZnS:Pr,Ce layer. It can be easily seen that the dc leakage current increases rapidly with increase of surface roughness. Furthermore, it is notable that the initiation field of Poole-Frenkel current conduction is lowered by increasing surface roughness of Ta/sub 2/O/sub 5/ thin film. Internal charge-phosphor field (Q/sub int/-F/sub p/) analysis and capacitance-ac voltage (C-V) analysis for ITO-Ta/sub 2/O/sub 5/-ZnS:Pr,Ce-Al and ITO-Ta/sub 2/O/sub 5/-ZnS:Pr,Ce-Ta/sub 2/O/sub 5/-Al show that the steady state phosphor field is smaller and C-V curve in transition region is less steep with increase of root-mean-square roughness between lower dielectric and phosphor layer in the alternating current thin-film electroluminescent (ACTFEL) devices. Therefore, we conclude that interface roughness is one of the physical factors to change the electrical performance of ACTFEL device.

Electroluminescence from CaS:TmF3 film prepared by radio frequency magnetron sputtering

Blue electroluminescence is reported from CaS:TmF3 thin films prepared by radio frequency magnetron sputtering method. The dependences of brightness on substrate temperature, concentration of TmF3, and the applied voltage were investigated on alternating current thin-film electroluminescent devices. The obtained maximum brightness was above the level of ZnS:TmF3 devices. The equivalent average energy of the excited electrons contributed to the electroluminescence in CaS:TmF3 alternating current thin-film electroluminescent devices and was estimated at around 4.23 eV by comparing the ratio of infrared to blue peak in electroluminescent spectra with that in photoluminescent spectra excited by different photon energy. The excitation processes of electroluminescence in CaS:TmF3 thin films were assumed to be energy transfers from the conduction band edge to Tm3+ centers.

Study on optical efficiency of alternating-current thin-film electroluminescent devices

An expression of the optical efficiency for alternating current thin film electroluminescent (ACTFEL) devices is given by a model of an optical microscopic cavity, and the optimization of the structure of the ACTFEL device is discussed. The model is reasonable since the radiation patterns developed by it coincide with the experimental results. The model indicates that in the ACTFEL device, the wide-angle interference is obvious, but the multiple-beam interference is slight.

Ce-doped TiO 2 Insulators in Thin Film Electroluminescent Devices

Cerium-doped TiO2 thin films have been prepared by reactive RF sputtering. At low Ce concentration, X-ray diffraction indicates that the films have the anatase structure. Ce concentrations higher than 1.2 at.% result in an amorphization of the film which remains stable up to 873 K. The TiO2 electrical properties have been stabilized and improved by cerium doping, resulting in a lower conductivity (10-9 Ω-1m-1), a higher electrical breakdown strength (2 ×107 V/m), and a high value of the permittivity (45±5). The implementation of amorphous TiO2:Ce thin films as insulator layers in ZnS:Mn alternating current thin film electroluminescent devices (ACTFELD) results in a significant drop in the threshold operating voltage and a notable increase in the device brightness compared with ACTFELD containing Y2O3 or BaTa2O6 insulator layers. Rapid thermal annealing further improves the performance of the electroluminescent device.

Volatile Metal β‐Diketonates: ALE and CVD precursors for electroluminescent device thin films

AbstractThe use of volatile β‐diketonate chelates as precursors for the deposition of thin films for electroluminescent devices is reviewed. Alternating current thin film electroluminescent (ACTFEL) devices consist of an emitting layer sandwiched between two dielectric layers, together with conducting and buffer layers. Besides various physical deposition techniques, the commonly applied methods for preparing thin films are chemical vapor deposition (CVD) and its particular variant atomic layer epitaxy (ALE). Alkaline earth β‐diketonates are used as precursors for deposition of the semiconducting alkaline earth sulfide and thiogallate films that provide the matrix in emissive layers, while β‐diketonates of lanthanides and a few other metals (Mn, Na, K) are precursors for dopants and codopants producing colors. β‐Diketonate precursors can also be used for the preparation of dielectric alkaline earth titanate and oxide layers. Current research on thin film electroluminescent materials is focused on improving the blue color produced by cerium doping of alkaline earth sulfide and thiogallate matrices. The synthesis and properties of β‐diketonate chelates and the growth and characterization of the thin films obtained with them are presented and discussed. This review of precursors for TFEL materials is also relevant for other materials, including oxide superconductors.

ELECTRICAL CHARACTERIZATION OF THIN-FILM ELECTROLUMINESCENT DEVICES

▪ Abstract Electrical characterization methods for the analysis of alternating current thin-film electroluminescent (ACTFEL) devices are reviewed. Particular emphasis is devoted to electrical characterization techniques because ACTFEL devices are electro-optic display devices whose performance is to a large extent determined by their electrical properties. A systematic procedure for ACTFEL electrical assessment is described. The utility of transient charge, voltage, current, and phosphor field analysis is explained. Steady-state electrical characterization methods discussed in this review include charge-voltage (Q-V), capacitance-voltage (C-V), internal charge-phosphor field (Q-Fp), and maximum charge-maximum applied voltage (Qmax-Vmax) analysis. These electrical characterization methods are illustrated by reviewing relevant results obtained from the analysis of evaporated ZnS:Mn and atomic layer epitaxy (ALE) SrS:Ce ACTFEL devices.

Analytical circuit model for thin film electroluminescent devices

A simple equivalent circuit is developed for alternating current thin film electroluminescent devices. With sinusoidal excitation voltage an exact analytical solution is presented for calculating the device current, transferred charge and the internal voltage across the phosphor layer. The circuit is used for modeling EL devices with widely varying phosphor layer materials that include ZnS, CaS, SrS, CaF/sub 2/, SrF/sub 2/, ZnF/sub 2/ and multilayer structures prepared with Atomic Layer Deposition. The equivalent circuit gives a straightforward way to characterize the electrical properties of these materials. The fluorides have inferior breakdown properties compared to sulfides although they are much more symmetric. The strong asymmetry of SrS is attributed to its nonuniform defect distribution. >

Evidence of donor center involvement in ac thin film electroluminescent devices exhibiting memory

Alternating current thin film electroluminescent (EL) devices exhibiting memory with low Mn concentrations have a temperature independent threshold, but a temperature dependent saturation brightness and memory margin. Based upon these observations we argue that EL emission is initiated via the usual tunneling process from interface states, but subsequent characteristics are determined via a second carrier generation process. Shallow donor sites are calculated to exist at a depth of 0.12 eV below the conduction band. These are believed to be involved in the second carrier generation process since brightness-voltage characteristics are associated with Poole–Frenkel emission.

Short-wavelength electroluminescence in diamond-like carbon thin films

Alternating current thin film electroluminescent devices were fabricated using diamond-like carbon (DLC) as an active electroluminescent layer. An enhanced short wavelength response was observed in such devices with electroluminescent signal intensity evident to energies in excess of 3.3 eV, well into the UV region of the spectrum. This enhanced short-wavelength response is attributed to the improved purity of the DLC layer associated with extended pumping of the deposition chamber prior to the DLC deposition.