Alternating-current Thin-film Electroluminescent 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.

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.

Electroluminescent characteristics of ZnGa2O4:Dy3+ thin film devices fabricated on glass substrates

AbstractAn alternating current thin film electroluminescent (ACTFEL) device was fabricated on commercial glass substrate with dysprosium doped zinc gallate as the active layer. The phosphor layer and the top dielectric layer were deposited using rf magnetron sputtering technique. The devices fabricated with an asymmetric double insulating structure gave a white electroluminescent (EL) emission when driven by a voltage pulse frequency of 1.5 kHz, even without post‐deposition annealing of the active layer. Such an emission resulted from the simultaneous occurrence of multicolor transitions, characteristic of the dopant ion. The exponential dependence of luminance on applied voltage was quite evident from the luminance–voltage (L–V) curves. Devices were also fabricated with a highly crystallized interfacial layer of ZnO in between the substrate and the phosphor layer. Such devices could withstand a wider range of applied voltage, henceforth resulting in enhanced device performance. The chromatic quality of the EL emissions from both the devices were gauged using chromaticity coordinates. magnified imageThe CIE coordinates of fabricated ZnGa2O4:Dy3+ ACTFEL devices (a) without and (b) with ZnO interfacial layer.

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...

Low temperature deposition of SrS:Cu,F ACTFEL device by electron beam evaporation

Abstract Photoluminescence (PL) and electroluminescence (EL) of SrS:Cu,F alternating current thin film electroluminescent (ACTFEL) device prepared by electron beam/thermal multi-source evaporation are presented. The active layer was grown at 380 °C and neither post-deposition annealing nor sulphur co-evaporation was performed. Two bands at 380 and 435 nm were present in the PL spectrum, which are suggested to be due to donor acceptor recombination. EL spectrum consisted of an additional band at 535 nm, which is attributed to Cu + intracenter emission. The device exhibited yellowish white EL emission with chromaticity coordinates x =0.25, y =0.27 and low threshold voltage.

Fabrication and Performance of ACTFEL Display Devices Using Manganese-Doped Zinc Germanate as a Green-Emitting Electroluminescent Layer

Alternating-current thin-film electroluminescent (ACTFEL) display devices fabricated using manganese-doped zinc germanate (Zn2GeO4:Mn) as a green-emitting electroluminescent layer material are described. The ACTFEL display devices were fabricated with a standard bottom emission structure having a multilayer stack of thin films in the metal/semiconductor/insulator/ metal (MSIM) configuration. The device was constructed on a transparent Corning glass substrate through which the emitted EL light passed. The Zn2GeO4:Mn emission layer was synthesized by using a RF magnetron sputter deposition method, followed by post-annealing at 700 C in air ambient for 1 hour. The obtained Zn2GeO4:Mn films were found to be polycrystalline with a rhombohedral crystal structure. A green emission spectrum with a maximum at approximately 538 nm was produced from the fabricated device. The chromaticity color coordinates of the EL emission were measured to be x = 0.308 and y = 0.657. The device demonstrated a sharp increase in the intensity of green EL emission upon increasing the AC peak voltage applied to the device above a threshold of 148 V.

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.

Sputter deposited electroluminescent zinc sulfide thin films doped with rare earths

The effects of deposition conditions on the physical properties and threshold voltages of ZnS alternating current thin film electroluminescent (ACTFEL) phosphors are reported. The ZnS thin films were deposited by rf magnetron cosputtering of both a pure ZnS and a ZnS target doped with either 1.5mol% ErF3, NdF3, or TmF3, and EL peaks in both the visible and near infrared (NIR) were observed. The substrate temperature during deposition was varied between 140 and 190°C, and the highest NIR EL intensity was measured from as-deposited films at 140°C. Energy dispersive x-ray analysis was used to determine the rare earth concentrations between 1.0 and 2.6at.%, and the maximum emission was at ∼0.8–1at.%. The EL intensity decreased at higher concentrations due to poor crystallinity and decreased at lower concentrations due to lower rates of excitation. The threshold voltage and phosphor thickness for these ACTFEL devices both decreased as the deposition temperature was increased due to re-evaporation of the incongruently sputter transported zinc and sulfur. Electrical data confirmed that the optical threshold voltage for both NIR and visible emission were equal to one another and to the electrical breakdown voltage, indicating that at breakdown the energies of ballistic electrons are sufficient to excite both visible and infrared emissions.

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.

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.

Nanolaminated Ta2O5–Al2O3 Insulator Effect on Luminescent and Electrical Properties of Thin-Film Electroluminescent Devices

Three ZnS:Tb alternating-current thin-film electroluminescent (ACTFEL) devices were fabricated with different insulator structures by utilizing nanolaminated Ta2O5–Al2O3 and/or Al2O3. These devices were grown by sequential atomic-layer-deposition processes for bottom insulator, phosphor, and top insulator layers. The nanolaminated ACTFEL device with two Ta2O5–Al2O3 insulators were found to have low onset voltages, 50 V less than that of the Al2O3 device, for inducing conductive charge transfer and concomitant electroluminescence. The devices with the nanolaminated insulator showed more excessive dynamic space charge generation within the phosphor than the Al2O3 device. These insulator-dependent features and various observed electrical responses have been discussed in terms of the difference in the relative dielectric constants of used insulators (7 for Al2O3 and 15 for Ta2O5–Al2O3), and the creation mechanism of dynamic space charge.

Visible and near-infrared alternating-current electroluminescence from sputter-grown GaN thin films doped with Er

A demonstration of visible and near-infrared (NIR) alternating-current electroluminescence from sputter-grown GaN thin films doped with Er is reported. The alternating-current thin-film electroluminescent (ACTFEL) devices were constructed using a standard single-insulating structure, Al/GaN:Er/aluminum–titanium–oxide/indium–tin–oxide/Corning 7059 glass. Visible emissions peaked at 550 and 665 nm as well as NIR emissions centered at 1000 and 1550 nm were observed from the fabricated ACTFEL devices operating at room temperature. The visible and NIR emissions at 550, 665, 1000, and 1550 nm were attributed to the Er3+ 4f–4f intrashell transitions from the 4S3/2, 4F9/2, 4I11/2, and 4I13/2 excited-state levels to the 4I15/2 ground state, respectively. The green 550 nm emission had a larger dI/dV and a higher threshold voltage, Vth than the NIR 1550 nm emission, which could result from the need for higher electron impact energy to impact-excite the Er ion into the higher-energy excited-states for emission of green light.

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.

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.

Control of point defects and space charge in electroluminescent ZnS:Mn thin films

The mechanisms leading to improved brightness, efficiency, and stability of alternating-current thin-film electroluminescent (ACTFEL) ZnS:Mn phosphors have been studied. Previously we have shown that ex situ codoping of the sputter deposited ZnS:Mn active layer with K and Cl results in a 53% improvement in brightness, a 62% improvement in efficiency, and better 100 h accelerated aging stability. In this work, we demonstrate that these improvements result from a 75% increase in excitation efficiency for conduction electrons, combined with a small decrease in both light outcoupling and nonradiative recombination. Electrical properties data were used to determine that there is a reduced amount of static space charge in the codoped films, resulting in a larger average field, increased excitation efficiency, and increased charge multiplication. The reduced space charge is attributed to the addition of charge compensating zinc vacancy–chlorine complexes and isolated chlorine point defects, which are acceptor and donor defects, respectively, and a reduction of zinc vacancy deep hole traps. It is postulated that higher average fields results in sufficient electron multiplication or donor ionization such that current for EL excitation is limited by the phosphor resistance rather than capacitance or density of interface states. The possibility of using these mechanisms to increase the efficiency of other ACTFEL phosphors is discussed.

Zn 2GeO 4:Mn alternating-current thin-film electroluminescent devices

Electrical, electro-optic, temperature, and aging characteristics of green-emitting Zn 2GeO 4:Mn alternating-current thin-film electroluminescent (ACTFEL) devices are presented. The Zn 2GeO 4:Mn phosphor layers are prepared by RF sputtering. A maximum luminous efficiency of 0.45 lm/W and luminance of 105 cd/m 2 at 60 Hz and 40 V above threshold are obtained. Field-ionization of impact-excited Mn 2+ luminescent impurities is found to give rise to positive space charge within the Zn 2GeO 4 phosphor, leading to unusual device behaviors such as conduction current delay, anomalous positive polarity transient luminance annihilation, transferred charge being comprised of mainly relaxation charge, and a decrease in the 60 Hz threshold voltage with increasing temperature. Low temperature aging experiments suggest that aging is at least partially due to hot electron-induced degradation. Although most Zn 2GeO 4:Mn ACTFEL devices exhibit a moderate amount of aging, certain devices are found to exhibit no measurable aging at 1 kHz when aged for 24 h .

Subthreshold voltage‐induced transferred charge measurements of evaporated ZnS:Mn alternating‐current thin‐film electroluminescent devices

Abstract—Subthreshold voltage‐induced transferred charge (VIQ) analysis is proposed as a novel method for assessing the nature of aging in alternating‐current thin‐film electroluminescent (ACTFEL) devices. VIQ analysis involves the application of 20,000 bipolar voltage pulses of variable amplitude and measurement of the subthreshold charge transferred in the phosphor for voltage amplitudes from 0 V to threshold. VIQ experiments provide information related to the physical location, density, energy depth, and capture cross‐section of phosphor traps responsible for ACTFEL device aging. VIQ aging experiments were performed on ZnS:Mn ACTFEL devices whose phosphor layers were deposited by thermal evaporation, atomic layer epitaxy using chlorine as a constituent of the precursor gas [ALE (Cl)], and atomic layer epitaxy with diethyl zinc as a precursor gas [ALE (DEZ)]. The depth of the traps responsible for VIQ in evaporated, ALE (Cl), and ALE (DEZ) ZnS:Mn are estimated to be ∼1.0–1.2, ∼0.3, and ∼0.7–0.9 eV, respectively. It is speculated that the traps responsible for VIQ are due to sulfur vacancies, chlorine, and oxygen, for evaporated, ALE (Cl), and ALE (DEZ), respectively.

Luminescent impurity doping trends in alternating-current thin-film electroluminescent phosphors

The doping properties of three alternating-current thin-film electroluminescent (ACTFEL) phosphor host/luminescent impurity systems, ZnS:Mn, SrS:Ce, and SrS:Cu, are elucidated, and the ACTFEL device implications of these properties are assessed. Mn is isovalent, Ce is a donor, and Cu is an acceptor. Moreover, Ce is readily ionized in SrS, so that it behaves as a double donor. The distinctly different doping nature of these three luminescent impurities leads to dramatically disparate defect and device physics trends. The donor/acceptor nature of Ce/Cu in SrS results in charge neutrality being achieved in SrS:Ce and SrS:Cu via self-compensation-induced vacancy creation; subsequent defect complexing between oppositely charged luminescent impurities and self-compensation-induced vacancies results in more complex ACTFEL device behaviors such as dynamic space charge, trailing-edge emission, charge collapse, color tuning, and electroluminescence (EL) thermal quenching. In contrast, the isovalent nature of ZnS:Mn leads to more ideal ACTFEL device operation. This suggests that the optimal ACTFEL phosphor luminescent impurity is isovalent.

Insulator issues in alternating-current thin-film electroluminescent devices

An n-sheet, state-space model is employed for the assessment of insulator issues of relevance to the operation of alternating-current thin-film electroluminescent (ACTFEL) devices. The model presented herein has been refined by the inclusion of luminance, which is modeled as arising from impact excitation of luminescent impurities. Simulation demonstrates that ACTFEL devices which have large insulator capacitances and no space charge present in the phosphor are optimal in terms of providing maximum luminance and efficiency. A method for estimating the phosphor space charge density from a single transferred charge curve is presented. Simulation indicates that the use of a leaky insulator in an ACTFEL device can result in an increase in luminance, but a decrease in efficiency and that measured charge–voltage and internal charge–phosphor field curves are misleading when one or more of the constituent insulator or phosphor layers is leaky.

Electroluminescence thermal quenching in alternating-current thin-film electroluminescent devices

Electroluminescence (EL) thermal quenching in alternating-current thin-film electroluminescent (ACTFEL) devices refers to a reduction in the luminance with increasing temperature, which is in excess to that of normal thermal quenching and is concomitant with a reduction in the transferred charge. A comparison of thermal quenching trends for ZnS:Mn, SrS:Ce, SrS:Cu,Ag, and multilayer SrS:Cu,Ag/SrS:Ce ACTFEL devices is presented. Respectively, ZnS:Mn; SrS:Cu and SrS:Cu,Ag; and SrS:Ce and multilayer SrS:Cu,Ag/SrS:Ce ACTFEL devices exhibit very little (normal), a large amount (of primarily EL), or a small amount (of primarily normal) thermal quenching. Insertion of one or more interface layers of SrS:Ce significantly reduces the extent of EL thermal quenching in a SrS:Cu,Ag ACTFEL device. Simulation suggests that SrS:Ce interface layers decrease the rate and displace the location of hole creation by band-to-band impact ionization away from the SrS:Cu,Ag layer, where hole trapping at Cu or Ag acceptor sites leads to EL thermal quenching via thermally activated annihilation of positive space charge due to thermionic emission of holes from Cu or Ag acceptor traps.