Thin-film Electroluminescent Research Articles

The influence of source powder composition on the electroluminescence of Ca 1− xSr xS:Eu thin films

CaS:Eu has been studied as a red emitter for AC thin film electroluminescent (ACTFEL) devices in the beginning of the 1990s. This phosphor has not yet been used for practical display purposes because of the limited luminance and the slow response of the devices. Part of the CaS:Eu emission spectrum is situated at wavelengths longer than 650 nm where the eye sensitivity is very low. The partial replacement of the Ca-ions by Sr-ions causes a shift of the Eu emission to shorter wavelengths and thus an increase in the apparent luminous efficiency. To obtain a high luminance and a good temporal response, very homogeneous, crystalline and pure Ca 1− x Sr x S:Eu thin films were found to be necessary. Several powder preparation techniques were applied to mix CaS and SrS powders on a microscopic scale, in order to achieve a single Ca 1− x Sr x S:Eu phase. The overall phase distribution was determined by θ−2 θ X-ray diffraction (XRD) measurements and by assessing the photoluminescent properties of the Eu-activated powders. Scanning electron microscopy (SEM) images were useful in the determination of the grain size and the porosity of the material. The combination with energy-dispersive X-ray analysis (EDX) allowed mapping of the microscopic distribution of Ca and Sr. A single-phase of Ca 1− x Sr x S:Eu could be obtained using wet chemical synthesis, which considerably improved the luminance of the corresponding ACTFEL devices.

Hot hole excitation of EuGa2S4 electroluminescent thin films

This letter describes which carrier type (electrons or holes) is dominant for EuGa2S4 thin film electroluminescent (TFEL) excitation. The transient EL wave form was measured on the EuGa2S4 TFEL devices having a single insulating thin film, which emit pure green EL due to the 5d–4f transition of Eu2+ ions. An asymmetric EL wave form was observed under a rectangular applied voltage. The EL occurs at the positively biased side of the EuGa2S4 thin film. This indicates that the green EL occurs because of hot hole excitation. Hot electrons are known to be dominant carriers to excite the luminescent centers in TFEL devices. Hot holes are also effective carriers for TFEL excitation.

Increased luminance of ZnS/Mn thin-film electroluminescent displays due to Ag Co-doping

A method of improving the luminance of zinc sulfide doped with manganese (ZnS/Mn) thin-film electroluminescent (TFEL) devices is suggested. The addition of small amounts of silver is found to increase the device luminance. It is suggested that silver reduces Mn clustering effects, thereby resulting in fewer nearest neighbor Mn re-absorption interactions and allowing higher Mn doping concentrations. A luminance increase of up to two and a half times from ZnS/Mn,Ag devices compared to ZnS/Mn is observed. The same magnitude of the luminance increase is demonstrated on ZnS/Ho,Ag samples. Silver was found to cause no change in the crystallite size of the phosphor layer and no additional degradation of the device.

Electroluminescence of europium-doped gallium oxide thin films

Electroluminescence (EL) was observed in Ga 2O 3:Eu thin films prepared by using spray pyrolysis of aqueous solutions. The strongest line was associated with the 5D 0→ 7F 2 electronic transition for Eu 3+. The red emission of the films had chromaticity coordinates of x=0.643 and y=0.356 with a dominant wavelength of 605 nm and a color purity of 99%. The luminance changed with the crystallinity and a quenching effect occurred. The spray pyrolysis process, which eliminates high vacuum apparatus, may be more attractive from the point of view of large-scale and large-area production of electroluminescent devices.

P‐82: Sphere‐Supported Thin Film Electroluminescent Technology

AbstractSphere‐Supported Thin Film Electroluminescence (SSTFEL) constitutes a new, flexible EL technology based on thin film inorganic phosphors. Flexibility is realized by a novel structure comprising a polypropylene‐BaTiO3 composite sheet. A functional, flexible SSTFEL device emitted bright green EL through the same mechanism as conventional thin film EL devices.

36.1: Invited Paper: Progress of Rare‐Earth‐Doped Alkaline‐Earth Sulfide EL Devices

AbstractThe progress of color thin‐film electroluminescent (TFEL) devices based on rare‐earth doped alkaline‐earth sulfide phosphors is reviewed, as a memorial to Prof. Shosaku Tanaka of Tottori University passed away on Sep. 15, 2003. In this paper, his main works, i.e., the basic characterization of rare‐earth doped alkaline‐earth sulfide EL phosphors, the development of white‐light emitting EL phosphor materials, and the full‐color TFEL displays using the combination of white EL and RGB color filters, are introduced.

An Analytical Model for Electron Transport and Luminance in SrS:Cu,Ag ACTFEL Display Devices

An analytical model is developed to explain electron transport and luminance mechanisms in SrS:Cu,Ag ac thin film electroluminescent (ACTFEL) display devices. The model includes shallow and deep interface states, bulk traps, and impact excitation and ionization of activators. Mathematical expressions describing optoelectronic processes in the phosphor layer and interface states are written and numerical solutions for field, current and luminance are obtained. Results of calculations are compared with experimental data. The model is able to simulate the dominant features of the experimental luminance and current waveforms.

Influence of a two-dimensional SiO2 nanorod structure on the extraction efficiency of ZnS:Mn thin-film electroluminescent devices

To improve light extraction from thin-film electroluminescent (TFEL) devices, we have introduced a two-dimensional (2-D) SiO2 nanorod pattern into the glass substrate of a TFEL. This periodic modulation converts the guided waves in the high refractive index ZnS:Mn phosphor layers into leaked external waves. We used the finite-difference time-domain method to optimize the structural parameters of the 2-D nanorod pattern. Using a nanorod pattern with a depth of 300 nm, a lattice constant of 600 nm, and a radius of 180 nm, the extraction efficiency of the TFEL is theoretically predicted to increase by a factor of more than 4.8. We compare the calculated efficiency and that of the actual TFEL structure, with a view to decreasing diameter of nanorod.

Characterization of AlON-TiON Stacked Insulators For ZnS:Mn Thin Film Electroluminescent Devices

The use of TiON films by plasma-enhanced atomic layer deposition as insulators in thin film electroluminescent (TFEL) devices was examined and compared with that of TiO 2 by ALD. A ZnS:Mn TFEL device using AlON-TiON stacked insulator (ATON) exhibited the figure of merit of 12 μC/cm 2 . The luminance of the TFEL device with an ATON insulator was twice as high as that with a conventional Al 2 O 3 insulator. This high performance can be attributed not only to high dielectric constant of the ATON film, but the improved quality of the interface of the phosphor and insulator, as well.

Atomic layer deposition of thin films of ZnSe—structural and optical characterization

Thin films of sphalerite-type ZnSe were grown by atomic layer deposition (ALD) from elemental Zn and Se precursors. These films, grown on various substrates, show bright blue ‘edge’ emission accompanied by donor–acceptor pair emissions in the blue, green and red spectral regions. Red, green and blue emissions mixed together give a white color, with a color temperature between 2400 and 4500 K depending on a layer thickness and temperature. ZnSe grown by ALD is in consequence a promising material for the fabrication of semiconductor-based white light emitting thin film electroluminescence displays.

Preparation of AlN:Mn films by metalorganic chemical vapor deposition for thin film electroluminescent devices

AlN:Mn films with Mn concentrations ranging from 2 × 1018 to 1 × 1021 cm−3 have been prepared by metalorganic chemical vapor deposition using bismethylcyclopentadienyl-manganese as a Mn source. The films grown at 1050 °C were polycrystals, whereas those grown at substrate temperatures (Ts) lower than 800 °C were mixture of polycrystalline and amorphous AlN, with the ratio of the latter phase increasing with decreasing Ts. The films showed red–orange photoluminescence based on the transitions of d-electrons in Mn4+ ions at room temperature. The intensity of the Mn-related emission increased with decreasing Ts. The Ts dependence of PL was discussed in conection with the structual properties of the films. The electroluminescence (EL) properties were also investigated by fabricating thin film EL devices on glass substrates. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

Nanorheological approach for characterization of electroluminescent polymer thin films

Shear-modulated scanning force microscopy (SM-SFM) is introduced as a nanorheological characterization method for the determination of optoelectronic properties of semiconducting polymer thin films (<100 nm). In this letter, the photoluminescence quantum efficiency of poly(p-phenylenevinylene) (PPV) was directly correlated to the glass transition property by SM-SFM. Conformational changes and chain packing were discussed as a function of the conversion temperature of the soluble PPV precursor. Compared to the bulk, very low glass transition temperature values in the range of 65 to 85 °C were found, which imply an increased molecular mobility in thin films of conjugated polymers.

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.

Enhanced blue emission from Tm-doped AlxGa1−xN electroluminescent thin films

Electroluminescent (EL) emission from Tm-doped AlxGa1−xN (AlxGa1−xN:Tm) has been observed with various Al compositions (0⩽x⩽1). AlxGa1−xN:Tm thin films were grown by molecular beam epitaxy with in situ doping of Tm. At lower Al composition (x<0.15), blue emission at 478 nm dominates, corresponding to the Tm G41→H63 transition. For x>0.15, however, a second blue emission peak was observed at 465 nm, becoming dominant with increasing Al composition. The 465 nm emission is attributed to the higher level Tm transition D21→F43, which was not observed in GaN:Tm. Blue EL emission from Tm was enhanced with Al content in the films. The ratio of EL intensity at blue (465 nm plus 478 nm) to infrared (801 nm) wavelengths increased monotonically with Al composition, from ∼2 for GaN:Tm to ∼30 for AlN:Tm.

High-luminance thin-film electroluminescent devices using Y 2O 3:Mn phosphor

Mn-activated yttria (Y 2O 3:Mn) phosphor thin films, consisting of monoclinic Y 2O 3 host material, were prepared by r.f. magnetron sputtering. High-luminance yellow photoluminescence and electroluminescence emissions were obtained using Y 2O 3:Mn thin films prepared with a Mn content of 2 at.% at a substrate temperature of approximately 350 °C and postannealed at approximately 1020 °C in an Ar atmosphere. The obtainable electroluminescent (EL) characteristics of Y 2O 3:Mn thin-film EL (TFEL) devices were mainly related to the crystallinity of the thin-film emitting layers used in the devices. The obtained maximum luminance and luminous efficiency increased as the crystallinity of the monoclinic Y 2O 3:Mn thin-film emitting layers was improved. A luminance of 5110 cd/m 2 was obtained in a 1 kHz-driven TFEL device fabricated with monoclinic Y 2O 3:Mn phosphor thin film prepared under the optimized conditions.

Excitonic singlet-triplet ratios in molecular and polymeric organic materials

A simple technique employing reverse bias measurements of photoluminescent efficiency is described to determine the excitonic singlet-triplet formation statistics of electroluminescent organic thin films. Using this method, the singlet fractions in thin films of two organic emissive materials commonly used in organic light emitting devices, tris(8-hydroxyquinoline) aluminum $({\mathrm{Alq}}_{3})$ and poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), are found to be $(20\ifmmode\pm\else\textpm\fi{}1)%$ and $(20\ifmmode\pm\else\textpm\fi{}4)%,$ respectively. Results are confirmed using a sensitive synchronous detection scheme. We discuss other measurements and the current understanding of exciton formation statistics in polymeric and small molecular weight organic electroluminescent materials.

Effects of photoassisted deposition and/or photoassisted annealing on characteristics of ZnS:Mn thin-film electroluminescent devices

Some essential photoinduced effects on the characteristics of ZnS:Mn electron-beam evaporated films and thin-film electroluminescent (TFEL) devices based on them have been revealed when Hg-lamp irradiation and quartz halogen lamp illumination are used during deposition and postannealing, respectively. The photoassisted deposition with the irradiation of the ZnS:Mn flux or a substrate results in some improvement of the crystal structure of the films, an enhancement of the intensity of photoluminescence and electroluminescence (EL), an increase of the threshold voltage of EL and the luminous efficiency, a lesser change and the quicker stabilization of EL characteristics during short-term forced aging. The use of the photoassisted rapid annealing instead of a usual long-term annealing in a vacuum furnace causes some enlargement of the crystallites and the improvement of performance of the TFEL devices. Both photoassisted processes used lead to a significant reduction of the impurity photocurrent in the photodepolarization spectra. The above results indicate that the following photoinduced changes take place in the ZnS:Mn film: a more densely packed lattice, an increase of the energy depth of the free-electron source, a decrease of the number of pores and defects, particularly, the defects responsible for the radiationless processes and the instability of the EL characteristics in the course of the device operation. Possible mechanisms of the photoinduced effects are discussed.

Infrared emission from zinc sulfide: Rare-earth doped thin films

Infrared (IR) electroluminescent (EL) thin film phosphors were radio frequency magnetron sputter deposited by cosputtering of an undoped ZnS target together with ZnS: 1.5 mole % ErF3 or ZnS: 1.5 mole % NdF3 targets. The ZnS:ErF3 and ZnS:NdF3 thin film phosphors were annealed in a N2 ambient at temperatures ranging from 350 to 475 °C for 1 h to increase radiance. The maximum EL radiance observed was 28 μW/cm2 at 1550 nm for ZnS:ErF3, and 26 μW/cm2 at 910 nm and 15 μW/cm2 at 1060 nm for ZnS:NdF3 (at 40 V above the threshold voltage) after a 425 °C, 1 h anneal in nitrogen. For anneals above 425 °C visible emission increased, while near infrared (NIR) emission from both ZnS:ErF3 and ZnS:NdF3 was either constant or decreased. For ZnS:ErF3, the 1550 nm NIR peak decreased, but the 990 nm peak remained constant in intensity. The crystallinity of ZnS was improved by annealing, and these results are consistent with the postulate that residual defects limit the acceleration of “hot” electrons for anneals at ⩽425 °C. Under these conditions, hot electrons only have sufficient energy to excite Er+3 into the lower lying I413/2 excited state which leads to 1550 nm NIR emission. With increasing annealing temperatures, hot electrons can excite from the I415/2 ground state into higher energy excited states (e.g., the F47/2 state for 990 nm emission). The NIR emissions from ZnS:NdF3 at 910 and 1060 nm originate from the same excited state and both peaks exhibited maximum NIR intensities after annealing at 425 °C. While the emission spectra from Er were independent of annealing temperature, peak shifts were observed for Nd. These shifts were discussed in terms of the nephelauxetic effect and hybridization of the 5d–4f orbitals.

Sputter deposited GaN doped erbium thin films: Photoluminescence and 1550 nm infrared electroluminescence

Erbium doped gallium nitride (GaN) thin films were deposited on Si substrates by reactive rf magnetron cosputtering of a commercial GaN target, together with a metallic erbium target. Nitrogen was employed as the reactive sputtering gas. The gallium nitride doped erbium (GaN:Er) films thus obtained, exhibited the characteristic 525, 540, 660, and 1550 nm photoluminescence emission associated with the Er+3 ion 4f–4f intraband transitions. In addition, 1550 nm IR electroluminescence (EL) emission was observed from the sputter deposited GaN:Er phosphor films. The EL device was an inverted half-stack ac thin-film EL device structure. The 1550 nm EL emission is consistent with impact excitation and subsequent I13/24→4I15/2 radiative relaxation of Er+3 ions. Impact excitation requires conduction electrons with sufficient energy from electrical field acceleration, to excite the transition.