Current Thin Film Electroluminescent Devices Research Articles

Fabrication and characterization of green ZnS:Tb AC- electroluminescent devices by ultrasonic spray pyrolysis

MISIM (metal-insulator-semiconductor-insulator-metal) structures of alternating current thin film electroluminescent (ACTFEL) devices were deposited on ATO/glass substrates by the ultrasonic spray pyrolysis technique. Zirconium oxide and terbium doped zinc sulfide were used as dielectric and semiconductor phosphor layers, respectively. A focused ion beam equipment was used to prepare a lamella of the MISIM structure, for cross-sectional scanning electron microscopy analysis in the STEM mode. Chemical analysis by energy-dispersive X-ray spectroscopy (EDS) elemental maps was also made for the identification of the distribution of the elements through the layers composing the MISIM structure. Characteristic Tb3+ emission bands due the 5D4 ->7FJ transitions were obtained by AC voltage application at 10 kHz with the main peak centered at 542 nm. An increase in the electroluminescent intensity and a shift of the CIE coordinates toward a purer green color with increasing the applied voltage was observed. The threshold voltage of these green electroluminescent devices was 46 Vrms, and a luminance up to 41 cd/m2 was obtained for the maximum applied voltage of 76.9 Vrms.

The effect of surfactants and precursors on the structure and properties of ZnS:Cu nanocrystalline particles

Doped ZnS materials exhibit excellent optical and electrical properties, making them a superior material for alternating current (AC) thin film electroluminescent devices, solar panels, light-emitting diodes, laser diodes and other luminescent device designs and fabrications. This study involves the hydrothermal synthesis of nanocrystalline Cu-doped ZnS materials without a surfactant and using two surfactants – hexadecyltrimethylammonium bromide (HTAB) and sodium dodecyl sulfate (SDS). The structure, morphology and elemental content of the synthesized nanopowders were characterized by X-ray powder diffraction, X-ray absorption spectroscopy (XAS), scanning electron microscopy and energy-dispersive X-ray spectroscopy (EDX) techniques, while the optical properties were studied by photoluminescence (PL) spectroscopy. The sizes of the nanocrystallites in all the synthesized samples were below 20 nm. XAS results suggest that Cu ions substitute Zn ions in cubic zinc blende ZnS structure and are located in a locally distorted environment. The addition of a surfactant allows control of the morphology of the particles and their photoluminescent properties.

Fabrication, characterization and electroluminescence studies of SrS: Ce3+ ACTFEL device

The SrS: Ce3+ thin film active phosphor layer of different thickness based alternating current thin-film electroluminescent (ACTFEL) devices are fabricated by electron beam evaporation deposition (EBED) method. The morphology and chemical composition of the deposited films are investigated by field emission scanning electron microscopy (FESEM). The optical transmittance in visible range for the optimum film and optical band gap of SrS: Ce3+ film is described on the basis of Ce3+ doping concentration.

Synthesis and characterization of thin film electroluminescent devices all-prepared by ultrasonic spray pyrolysis

Alternating current thin film electroluminescent devices have been fabricated using aluminum-doped zinc oxide (ZnO:Al) as transparent conducting layer, aluminum oxide (Al2O3) as insulating layers, and manganese-doped zinc sulfide (ZnS:Mn) as electroluminescent layer. All these films were deposited by the ultrasonic spray pyrolysis technique at the same temperature (450°) on glass substrates, forming a standard MISIM (metal–insulator–semiconductor–insulator–metal) configuration. The electroluminescence of MISIM devices with a total thickness of ~1330nm was investigated by applying a sinusoidal voltage with a frequency of 10kHz. The devices showed orange-emission spectra centered at approximately 570nm, characteristic of 4T1→6A1 radiative transitions of Mn2+ ions in the ZnS host, with a sharp intensity increase upon increasing the root mean square voltage above a threshold of 25V and a rapid saturation for voltages higher than 38V. The electroluminescent emission of these MISIM structures can be observed with the naked eye under ambient illumination.

Characteristics of A.C Electroluminescence in ZnGa2O4:Mn2+ Thin film Devices

Green emitting alternating current thin film electroluminescent devices were fabricated on glass substrates in the conventional MISIM structure using ZnGa2O4:Mn as the active layer. The devices were fabricated varying the deposition time of the active/phosphor layer at two different substrate temperatures, 500 and 600°C. The active layer and top dielectric layer were deposited using rf magnetron sputtering technique. All devices gave an emission in the green region of the visible spectrum even without any post deposition annealing. The EL emission could be de-convoluted into two/three peaks attributed to the 4T1 - 6A1 transition of Mn2+ centers at various sites in the host lattice. The intensity of the 502 nm EL emission was found to increase with active layer thickness. The luminance versus voltage plots exhibit an increase in threshold voltage with increase in phosphor layer thickness. The purity of the EL emissions is gauged using CIE coordinates that are found to lie in the green region of the chromaticity diagram.

Enhanced outcoupling of electroluminescence from ZnS:ErF3 thin films by a photonic crystal

Enhanced 1550 nm near-infrared emission due to a photonic crystal (PC) in zinc sulfide (ZnS) doped with erbium trifluoride (ErF3) has been studied in sputter deposited alternating current thin film electroluminescent (ACTFEL) devices. The honeycomb structured PC was fabricated into the ZnS:ErF3 layer by electron beam lithography and argon sputtering. A thin film of yttria-stabilized zirconia (YSZ) was deposited into the honeycomb structure of holes, creating the photonic crystal structure with a dielectric constant mismatch between the ZnS:ErF3 layer and the YSZ of approximately 20. The photonic band gaps were modeled for different values of dielectric mismatch and the honeycomb structure hole diameter and lattice spacing. For the 1550 nm wavelength, the lattice constant and hole diameter typically were ∼748 and ∼342 nm, respectively. The YSZ structured PC ACTFEL device showed a twofold increase in 1550 nm light intensity versus a device without a PC structure. These results could not be explained by clas...

Alternating Current Driven Electroluminescence from ZnSe/ZnS:Mn/ZnS Nanocrystals

We present a novel technique for room temperature, solution-based fabrication of alternating current thin-film electroluminescent (AC-TFEL) devices using phosphor-doped nanocrystals. Synthesis for stable ZnSe/ZnS:Mn/ZnS nanocrystals that exhibit a quantum yield of 65 +/- 5% is outlined, and their electroluminescence is demonstrated in structures consisting of only wide band gap ceramic layers. Both the nanocrystal and the ceramic films have minimal absorption across the visible light spectrum, enabling us to demonstrate transparent AC-TFEL devices.

Effects of processing parameters on electroluminescence of rf magnetron sputter deposited ZnS:ErF3

Zn S : Er F 3 alternating current thin film electroluminescent (ACTFEL) devices are fabricated by rf plasma magnetron sputtering. In a full factorial design-of-experiment study, increasing deposition temperature, duty cycle on the doped target, and sputter gas pressure all result in increases in the 1.55μm near infrared (NIR) electroluminescence (EL) irradiance at 20V above threshold (B20). An increase in the EL threshold voltage (Vth) upon increasing the duty cycle of the undoped target is also reported. Postdeposition annealing of ACTFEL devices at 425°C for 1h improves the NIR EL irradiance by decreasing the F concentrations in the ZnS:Er films. The origins of these effects are discussed in terms of negative ion resputtering, surface mobility of sputtered species, crystallinity, and the effects of atomic concentration on the EL and radiative relaxation processes. A maximum irradiance, B20, of 147μW∕cm2 is measured for the 1.55μm NIR EL peak from ZnS:Er ACTFEL devices produced using a deposition temperature of 150°C, a duty cycle of 75%, an argon sputtering gas pressure of 24mTorr, and postdeposition annealing at 425°C for 1h in nitrogen.

The influence of insulator properties on the electro-optical performance of flexible ZnS:ErF3 alternating current thin film electroluminescent devices

We have developed flexible ZnS:ErF3 alternating current thin film electroluminescent devices on molybdenum foils, and quantified the effect of insulator properties for the case of alumina (Al2O3) and barium tantalate (BaTa2O6) on the device electrical characteristics, and infrared emission. The inverted, full-stack structures could be flexed in tension and compression without mechanical failure or deterioration of optical output, and flex radii of a few centimeters are routinely achieved. Thus, the feasibility of a flexible, inorganic, large area electroluminescent emitter has been demonstrated. Compared to the Al2O3 devices, the numbers for structures employing BaTa2O6 represent a 29% increase in flux density at 980 nm, a 36% increase at 1540 nm, an 18% increase in conduction charge, a 10% increase in phosphor field and a 14% increase in threshold voltage. Our interpretation of the data is as follows: in the case of BaTa2O6, the interface states from which electrons are sourced into the phosphor are deeper, requiring a higher voltage for field emission. As a consequence, electrons are injected into the phosphor when the phosphor field is higher, resulting in better excitation efficiency and improved optical generation. The increase in conduction charge is interpreted as a higher density of interface states, which appear to have a narrower energy distribution based on the more abrupt threshold behavior observed.

P-225L: Late-News Poster: A Systematic Study of SrS:Ce ACTFEL Devices for Full-Color Display Application

This paper is providing a systematic performance study of SrS: Ce blue light emitting alternating current thin film electroluminescent (ACTFEL) devices for full-color display application in practice. This study includes devices' electrical and optical behaviors on carrier transportation, energy transform efficiency and emission spectra, all of which are based on practical full-color display application. The electroluminescence mechanism is also discussed as well as the crystalline structure of SrS: Ce phosphor layer in the process of device fabrication. The experiment results and measurement analysis are very important and helpful to understand the key role of SrS: Ce blue emitting devices to fulfill the requirements of full-color display application.

8.3: Flexible Infrared Emitting ZnS:ErF[sub 3] Alternating Current Thin Film Electroluminescent Devices

We have developed ZnS:ErF3 electroluminescent thin films by pulsed laser ablation (PLD), which have been incorporated into flexible, inorganic alternating current thin film electroluminescent devices (ACTFELDs). The ZnS:ErF3 phosphor emits green at 550nm (4S3/24I15/2), and infrared at 980nm (2F7/24I11/2) and 1540nm (4I13/24I15/2). The fabricated devices could be flexed in tension and compression without mechanical failure or deterioration of optical output, and flex radii of a few centimeters are routinely achieved. Thus, the feasibility of an inorganic, flexible, electroluminescent device with both visible and infrared emission has been demonstrated. For 700nm thick phosphor films the threshold voltage was typically 140V. At 40V above threshold the radiant flux at 980nm was typically in the 7μW/cm2 range, and at 1540nm it was 23 μW/cm2. The corresponding phosphor field and conduction charge at 40 V above threshold was 2.1MV/cm and 5.23μC/cm2 respectively. Flexible devices were fabricated using molybdenum, tungsten and steel foils as substrates.

Characteristics of a.c. electroluminescence in thin film ZnS : Mn display devices

Thin film electroluminescence (TFEL) displays are complex optoelectronic devices with challenging material requirement. The electrical characteristics of ZnS : Mn alternating current thin film electroluminescent devices grown by electron beam evaporation are assessed as function of voltage frequency and Mn concentration. The threshold voltage increases with increasing frequency of the applied voltage and saturation value of electroluminescence (EL) brightness increases linearly with the frequency of applied voltage. The EL brightness increases linearly with the frequency of the applied voltage. The EL brightness was found optimum for a particular activator concentration. A model is proposed and an expression is derived for the EL brightness, which is able to explain the dependence of the EL brightness on different parameters.

XPS and electroluminescence studies on SrS 1− xSe x and ZnS 1− xSe x thin films deposited by atomic layer deposition technique

SrS 1− x Se x and ZnS 1− x Se x thin films were deposited by the atomic layer deposition (ALD) technique using elemental selenium as the Se source, thus avoiding use of H 2Se or organometallic selenium compounds. X-ray diffraction (XRD) analysis showed that the films were solid solutions and X-ray photoelectron spectroscopy (XPS) data showed that the surface of both ZnS 1− x Se x and SrS 1− x Se x were covered with an oxide and carbon-containing contaminants from exposure to air. The oxidation of SrS 1− x Se x extended into the film and peak shifts from sulfate were found on the surface. Luminance measurements showed that emission intensity of the ZnS 1− x Se x :Mn alternating current thin film electroluminescent (ACTFEL) devices at fixed voltage was almost the same as that of the ZnS:Mn device, while emission intensity of the SrS 1− x Se x :Ce devices decreased markedly as compared to the SrS:Ce device. Emission colors of the devices were altered only slightly due to selenium addition.

Improved brightness and efficiency in Ce codoped ZnS:TbOF electroluminescent thin films

ZnS:TbOF alternating current thin film electroluminescent devices codoped with Ce by rf magnetron sputtering were shown to be brighter than Tb singly doped films. This brightness improvement is shown to results predominantly from increased radiative efficiency, with a secondary improvement in excitation efficiency. At 20 V above threshold for electroluminescence, V20, the radiative efficiency of Ce codoped films is improved ≈130%. The nonlinear exponential decay and improved efficiency of electroluminescent emission for Ce codoped films is attributed to energy transfer from Ce+3 to Tb+3 luminescent centers. The improved excitation efficiency is attributed to reduced space charge.

Addition of He to Ar during sputter deposition of electroluminescent ZnS:TbOF thin films

ZnS:TbOF alternating current thin film electroluminescent (ACTFEL) devices were sputter deposited from a single pressed powder ZnS:TbOF (1.5 mol % of Tb) target using a Ar+He mixed gas. The He gas concentration was changed from 0% to 70% and deposited films were annealed at 500 °C for 60 min under a N2 gas atmosphere. The x-ray diffraction peak full width half maximum (FWHM) of the cubic (111)/hexagonal (0002) plane in as-deposited films decreases from 0.55° for 100% Ar, to 0.39° for 30% Ar+70% He. The film deposition rate decreased about 25% as the He percentage increases from 0% to 70%. In spite of the improved crystallinity as indicated by the decreased FWHM data, the brightness of alternating current thin film electroluminescent devices (ACTFELDs) decreases from 64 cd/m2 for a pure Ar atmosphere to 45–54 cd/m2 at 60%–70% He. This brightness decrease is attributed to an increased dielectric constant (from 12 for pure Ar to 17 at 70% He) and decreased root mean square surface roughness (from 6.7 nm for pure Ar to 4.3 nm at 70% He) of films sputter deposited with high He.

Microstructure and electroluminescence of ZnS:Mn doped with KCl

The microstructure and electroluminescence (EL) characteristics of ZnS:Mn doped with KCl for alternating current thin film electroluminescent devices deposited by a RF sputter system from ZnS and Mn targets are investigated. Transmission electron microscopy (TEM), X-ray diffractometry, energy dispersive spectrometry (EDS), secondary ion mass spectrometry, EL brightness and luminous efficiency (η) measurements were carried out. The as-deposited films have a 100 nm layer of equiaxed grains at the insulator/phosphor interface and columnar grains in the rest of the film. K is uniformly distributed through the 1 μm ZnS film after 5 min anneals at T⩾600 °C, while Cl requires an anneal at T⩾700 °C before diffusion results in a uniform concentration. Upon annealing at 700 °C for 5 min, the equiaxed grains were consumed by growth of the columnar grains, and growth of the columnar grain diameter is enhanced by the presence of KCl. The mixed cubic and hexagonal atomic stacking observed in as-deposited films became more cubic upon annealing with KCl co-dopants. EDS data from plan-view TEM samples indicate that K did not segregate to the grain boundaries. Considering the large ionic radius of K + relative to Zn 2+ (1.51 vs. 0.74 Å), and the small temperature dependence of K diffusion into ZnS, it was postulated that K + diffuses by an interstitial mechanism. In addition, it occupies an interstitial site in the cubic structure where it acts as a donor, similar to substitutional Cl −. ZnS:Mn co-doped with KCl shows significant improvement in EL brightness and luminous efficiency. It was concluded that the improved EL properties result from both improved crystallinity and modification of point defects and space charge in the ZnS:Mn, K, Cl films.

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).

Improved electroluminescence of ZnS:Mn thin films by codoping with potassium chloride

Alternating current thin film electroluminescent devices have been fabricated using sputter-deposited ZnS:Mn with and without codoped potassium chloride via both in situ and ex situ methods. In situ codoping proved to be difficult due to a memory effect in the deposition chamber. Samples codoped with potassium chloride via an ex situ diffusion method exhibited improvements in brightness of up to 70% (572 vs 337 cd/m2) and efficiency of up to 60% (1.95 vs 1.25 lm/W) over noncodoped samples. The threshold voltage increased by ≈5% (160 vs 168 V), and the brightness-versus-voltage curve stabilized more rapidly for the devices. Several possible mechanisms to explain these effects are discussed. While modest microstructural changes contribute to the improvements, changes in point defects which lead to modification of the space charge in the devices appears to be the dominant mechanism.

Improved brightness, efficiency, and stability of sputter deposited alternating current thin film electroluminescent ZnS:Mn by codoping with potassium chloride

Electroluminescent ZnS:Mn thin films have been codoped with KCl via an ex situ deposition and diffusion process. The brightness, efficiency, and stability of sputter deposited ZnS:Mn alternating current thin film electroluminescent devices have been greatly improved over untreated devices. Potassium chloride doping is shown to have a modest fluxing effect on the grain size of ZnS:Mn films, particularly in the region of the film where nucleation occurs. However, improved electroluminescence is postulated to be strongly influenced by doping control of electric fields and injected 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.