Alternating-current Thin-film Electroluminescent Devices Research Articles

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.

Fabrication and characterization of alternating-current-driven ZnO-based ultraviolet light-emitting diodes

Alternating-current-driven ZnO-based light-emitting diodes (LEDs) with an Au/ZnMgO/ZnO/ZnMgO/GaN/In structure have been fabricated. Both polarity-controlled electroluminescence (EL) and ultraviolet emission were achieved when driven by an alternating current (AC) under a much lower root-mean-square voltage than that of conventional inorganic AC thin-film electroluminescent devices. This ZnO-based LED can be regarded as a series incorporating a metal-insulator-semiconductor diode and a p-i-n diode. The EL mechanisms were discussed in terms of the band diagrams and carrier transport behavior with reference to the semiconductor heterojunction theory.

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.

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.

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.

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.

Chemical Vapor Deposition of ZnS:Mn for Thin-Film Electroluminescent Display Applications

Results are presented from a systematic investigation to design and optimize a low-pressure chemical vapor deposition (CVD) process for manganese-doped zinc sulfide (ZnS:Mn) thin films for electroluminescent (EL) device applications. The CVD process used diethylzinc (DEZ), di-π-cyclopentadienyl manganese (CPMn), and hydrogen sulfide (H2S) as co-reactants and hydrogen (H2) as carrier gas. A design of experiments approach was used to derive functionality curves for the dependence of ZnS:Mn film properties on substrate temperature and flow rates (partial pressures) of DEZ, CPMn, H2S, and H2. Film physical, chemical, structural, and optical properties were examined using Rutherford backscattering spectrometry, dynamic secondary ion mass spectroscopy, x-ray photoelectron spectroscopy, nuclear-reaction analysis, x-ray diffraction, transmission electron microscopy, atomic force microscopy, and scanning electron microscopy. EL measurements were carried out on ZnS:Mn-based dielectric–sulfur–dielectric stacks incorporated into alternating-current thin-film electroluminescent devices. An optimized process window was established for the formation of films with predominantly (0 0 2) orientation, grain size larger than 0.2 μm, and Mn dopant level approximately 0.5 at.%. A brightness of 407 cd/m2 (119 fL) and efficiency of 1.6 lm/W were obtained, as measured at 40 V above threshold voltage and 60 Hz frequency.

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.

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.

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.

Effect of carrier trapping time on performance of alternating-current thin-film electroluminescent devices

In spite of a significant number of studies on memory effect in alternating-current thin-film electroluminescent devices, the phenomenon is still unclear. In the present work we investigate some peculiarities of memory behavior of the devices from the viewpoint of the nonequilibrium carrier trapping time. Applying a combination of unique voltage staircase waveforms, we demonstrate that the degree of memory of the device is determined by the trapping time of the carriers at the insulator–semiconductor interface and the sulfur vacancy (Vs) concentration in the bulk of the active layer. We also show that separating electrons and holes before the pulse arrival can increase the effective field across the phosphor layer.

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).