Alternating-current Thin-film Electroluminescent Research Articles

Green electroluminescence from Zn 1– xMg xS:Mn alternating current thin film electroluminescent devices

The color shifting from yellow to green of electroluminescent emission from Zn 1– x Mg x S:Mn alternating current thin film electroluminescent (ACTFEL) devices has been achieved by changing the Mg composition in the phosphor layers. Zn 1– x Mg x S:Mn solid solution thin films in the composition range x=0–0.25 have been prepared by multi source deposition technique. The commission internationale de l'Eclairge (CIE) color co-ordinates of the ACTFEL devices prepared by these phosphor layers show a shifting from yellow ( x=0.55, y=0.45) towards green ( x=0.49, y=0.5).

Photo-, cathodo-, and electroluminescence studies of sputter deposited AlN:Er thin films

Green cathodoluminescence (CL), photoluminescence (PL) and electroluminescence (EL) have been obtained from Er-doped amorphous AlN thin films, 200nm thick, prepared by rf magnetron sputtering. All films were activated by annealing at 750°C for 10min in a nitrogen atmosphere. Three sharp bands at about 479, 538 and 559nm corresponding to the 4F7/2→4I15/2,2H11/2→4I15/2 and 4S3/2→4I15/2 transitions are observed. Fine structure is seen on the major transitions that does not change with temperature indicating that this structure is related to different local environments of the Er3+ ion. The PL spectrum revealed sharp peaks from Er3+ ions and a broad spectral profile that might be from defect states in the amorphous AlN. The results from EL measurements show that Er-doped amorphous AlN films can be used as a phosphor layer in alternating-current thin-film electroluminescent (ACTFEL) devices.

Visible emission from electroluminescent devices using an amorphous AlN:Er3+ thin-film phosphor

Electroluminescence (EL) studies of AlN:Er alternating-current thin-film electroluminescent (ACTFEL) devices were performed at 300 K. Thin films of Er-doped AlN, ∼200 nm thick, were grown on indium–tin–oxide/aluminum–titanium–oxide/glass substrates using rf magnetron sputtering in a nitrogen atmosphere. The turn-on voltage was found to be around 70–80 and 100 V for ACTFEL devices without and with a top insulator layer. Sharp emission lines in the visible region were observed which correspond to known transitions of the Er3+ ion. Temperature-dependent cathodoluminescence studies corroborate the EL results, and show that optimum device performance is attained near 300 K.

Alternating-current thin-film electroluminescent device modeling via SPICE Fowler-Nordheim diode

A novel SPICE model is proposed for simulating the electrical characteristics of alternating-current thin-film electroluminescent (ACTFEL) devices. The model consists of two capacitors, representing the top and bottom insulators of the ACTFEL device, connected to a Fowler-Nordheim diode shunted by a resistor, which account for conduction and leakage, respectively, in the ACTFEL phosphor layer. A software interface is developed so that the user may conveniently input the two adjustable model parameters (trap barrier height and phosphor shunt resistance) as well as five parameters corresponding to the physical structure of the ACTFEL device (electron effective mass, device area, insulator capacitance, phosphor thickness, and phosphor dielectric constant). Excellent agreement is obtained between measured and simulated electrical characteristics for evaporated ZnS:Mn ACTFEL devices.

Synthesis and characterization of some ZnS-based thin film phosphors for electroluminescent device applications

ZnS:Mn, ZnGaS:Mn and ZnS:CuCl 2 phosphor thin films for alternating-current thin-film electroluminescent (ACTFEL) devices were prepared by thermal evaporation from two and three sources, respectively. Films are polycrystalline, strongly oriented and stoichiometric or nearly stoichiometric with high optical transmission in the visible part of the spectrum. The band gap of ZnS:Mn, ZnGaS:Mn and ZnS:CuCl 2 films was found to be 3.63, 3.86 and 3.56 eV, respectively. The sheet resistance of the ZnS:Mn and ZnGaS:Mn films was greater than 100 MΩ. The resistivity of ZnS:CuCl 2 films was between 1.5–80 Ω cm. Photoluminescent (PL) and electroluminescent (EL) characteristics were also studied. The results indicate that after the optimization the investigated phosphor thin films will be suitable for ACTFEL device applications.

Electroluminescence thermal quenching in SrS:Cu thin-film electroluminescent devices

Electroluminesence (EL) thermal quenching refers a reduction in luminance, concomitant with a reduction in transferred charge, when an alternating-current thin-film electroluminescent (ACTFEL) device is operated at an elevated temperature. EL thermal quenching is found to be significant in SrS:Cu ACTFEL devices operated above ∼60-80 °C. Maximum transferred charge-maximum applied voltage (Qmax-Vmax) and transferred charge capacitance (i.e., dQmax/dVmax vs Vmax) measurements as a function of temperature in conjunction with ACTFEL device simulation are employed in order to establish that EL thermal quenching arises from a thermally activated annihilation of positive space charge and a corresponding increase in the threshold voltage.

Thin-Film Electroluminescent Device Physics Modeling

Previous and current approaches to alternating current thin-film electroluminescent (ACTFEL) device physics modeling are reviewed. The primary goal of the modeling efforts of interest to this review is to accurately simulate the electrical properties of two-terminal ACTFEL test structures when subjected to realistic applied voltage waveforms. A comprehensive review of previous ACTFEL modeling efforts is presented. Additionally, two modeling methodologies that are believed to hold the most promise for accurate ACTFEL device modeling are presented. These methodologies involve state-space simulation of a discretized phosphor and SPICE simulation.

Green Emission from Er-Doped AlN Thin Films Prepared by RF Magnetron Sputtering

ABSTRACTThin films of Er doped AlN, ∼ 200 nm thick, were grown on indium tin oxide/aluminum titanium oxide/glass substrates using RF magnetron sputtering in a pure nitrogen atmosphere. To optically activate Er all films were subject to post-deposition annealing in flowing nitrogen atmosphere at atmospheric pressure at temperatures between 1023-1223 K for 10-60 minutes. The visible cathodoluminescence (CL) in the green was detected at both 11 K and 300K. The strongest CL peaks were observed at 558 nm and 537 nm (11 K), which correspond to the transitions from 4S3/2 and 2H11/2 to the 4I15/2 ground level. Electroluminescence (EL) studies of AlN:Er alternating-current thin-film electroluminescent (ACTFEL) devices were performed at 300 K. The turn-on voltage was found to be around 80-100 V for our ACTFEL devices. The intensity of the EL emission rapidly increases with the voltage increasing in the investigated range of 110-130 V.

Lanthanide doping in ZnS and SrS thin-film electroluminescent devices

The relative high field transport efficiency and short wavelength electroluminescence (EL) potential of the phosphors ZnS and SrS for alternating-current thin-film electroluminescent (ACTFEL) device flat-panel display applications are assessed via a comparison of the EL spectra of ZnS and SrS ACTFEL devices prepared in a very similar manner and doped with the same lanthanide luminescent impurities: Dy, Er, Ho, Tb, and Tm. For all of the lanthanide luminescent impurities studied, it is found that the the higher energy EL peaks are much more intense for SrS than for ZnS, even though the average phosphor field in SrS is smaller than in ZnS. These observations show SrS to be a superior high-field electron transport material compared to ZnS. All of the ZnS EL spectra show a dramatic cut off in their EL intensities at about 440–460 nm; this suggests that ZnS is not an appropriate phosphor for blue light emission since its electron distribution does not appear to be adequately heated to efficiently excite blue luminescent impurities.

Alkali metal coactivators in SrS: Cu,F thin-film electroluminescent devices

A unique approach for obtaining bright and efficient saturated green phosphors for alternating-current thin-film electroluminescent (ACTFEL) device applications is presented. The approach involves color-shifting blue SrS:Cu,F ACTFEL phosphors into the green region of the spectrum via the incorporation of alkali metal ions into the SrS lattice. Alkali metals are incorporated into SrS:Cu,F phosphors by using LiF, NaF, KF, RbF, or CsF coactivators. The best result to date is obtained by using a KF coactivator and results in a saturated green brightness and efficiency of 52.7 cd/m2 and 0.973 lm/W (at a frequency of 60 Hz and an overvoltage of 40 V). In addition to providing a color shift, the alkali-metal fluorides improve the overall performance of the ACTFEL device by increasing the magnitude of the electric field and its uniformity across the phosphor through suppression of positive space charge.

Effects of rapid thermal annealing on electron beam evaporated SrS : Ce thin film electroluminescent devices made in H 2S ambient

Possible device structures for using rapid thermal annealing (RTA) for alternating current thin film electroluminescent (ACTFEL) devices have been evaluated. SrS : Ce ACTFEL structures have been made on a highly doped silicon substrate, which is used as supporting electrode. RTA was performed on these EL devices and the effect of different annealing temperature, annealing time and of ZnS buffer layers was investigated. Besides an increase in total brightness, a change in crystallinity and in device characteristics was seen. Time resolved optical measurements could be explained by assuming the role of interface states. Composition of the layers and diffusion phenomena have been investigated, using X-ray photo-electron spectroscopy. At annealing temperatures above 700°C, the Ce ion becomes mobile. The possible physical processes behind these effects have been discussed.

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.

Threshold voltage trends in ZnS : Mn-based alternating-current thin-film electroluminescent devices: role of native defects

We report on the effect of manganese concentration c Mn, substrate temperature T sub, and post-deposition annealing on threshold voltage V th of ZnS : Mn-based alternating-current thin-film electroluminescent (ACTFEL) devices. The increase in the V th and a shift of the edge absorption to higher energies are observed with increasing c Mn, T sub, and after annealing of the device. For the first time, the observed trends are consistently explained via the formation of native lattice defects.

Electrical characterization of alternating-current thin-film electroluminescent devices

We suggest a formalism built on the analogy between the pressure-volume (P–Ṽ) diagram of a gas system and the charge-voltage (Q–V) characteristics of the alternating-current thin-film electroluminescent (ACTFEL) device. The formalism extends the understanding of the processes in ACTFEL devices and the complete characterization of their internal parameters based on the Q–V diagram.

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.

Optimization of alternating-current thin-film electroluminescent displays

Different factors affecting the performance of alternating-current thin-film electroluminescent (ACTFEL) devices are discussed. The Chen-Krupka model of electron transport was applied to examine the case of non-uniform distribution of the dopant concentration in the active layer. Based on the results, a ZnS doping profile is proposed which incorporates higher doping at the interfaces than in the bulk. Also, the asymmetry in the excitation efficiency of electrons near the two insulator-semiconductor interfaces (ISIs) of ACTFEL devices was studied and was ascribed to the difference in the interface quality arising from the choice of insulating material and to the selection of the annealing process. The role of the space charge stored near the ISI and the influence of drive conditions on the operational characteristics of the device are discussed.

Static space charge in evaporated ZnS:Mn alternating-current thin-film electroluminescent devices

The operation of alternating-current thin-film electroluminescent (ACTFEL) devices may be strongly affected by the presence of dynamic or static positive space charge within the phosphor layer during device operation. Dynamic space charge is a positive charge in the phosphor layer which is periodically created and annihilated during each period of the applied voltage waveform. In contrast, static space charge is a positive space charge in the phosphor layer whose charge state does not change appreciably during steady-state operation of the ACTFEL device. The static space charge density of evaporated ZnS:Mn ACTFEL devices is estimated to be ∼7×1016 cm−3 from measured trends in the phosphor clamping field as a function of phosphor thickness. This static space charge density estimate implies a cathode clamping field of ∼ 2.2 MV/cm and a clamping interface trap depth of ∼1.5 eV. Furthermore, from transferred charge trends as a function of the phosphor thickness it is concluded that the static space charge in evaporated ZnS:Mn ACTFEL devices arises from metastable hole trapping in the phosphor.

Alternating-current thin-film electro-luminescent devices: Effect of fabrication conditions on aging and failure defect formation

Various types of failure defects can be encountered or unintentionally generated in the alternating-current thin-film electroluminescent (ACTFEL) devices by various growth, environmental, and driving conditions. The review summarizes the existing information on the failure formation and aging phenomena as well as methods of defect analysis and classification. The material reported in this paper is intended to serve as a useful tool for the new failure analyst as well as a convenient reference source for the expert in field of thin-film electroluminescent devices. In this study, different kinds of failures, such as localized destructive breakdown defects (LDBD), propagating defects, blistering, layer de-lamination, stress-induced defects, defects of packaging and metallization, and defects due to moisture and ion migration are covered. The optical observation is found to be one of the most valuable tools for large-area defect investigation. The layer-by-layer etch technique is applied to analyze the position of each type of defect in the stack. Most of these defects were found to start at the edges of the pixel. Both the material quality and uniformity of the semiconductor and insulator layers as well as quality of the front transparent electrode shoulders are found to be prerequisites of the failure defect-free structure. Some peculiarities of the active layer growth are found to be responsible for the stoichiometry perturbation and inclusion formation.

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.

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.