Thin-film Electroluminescent Devices Research Articles

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.

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.

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.

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.

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.

ＺｎＳ：Ｍｎ発光層の発光中心に対するスパッタ雰囲気中Ｈ２ガスの影響

A very small amount of H2 gas was introduced in the sputtering environment to get high quality ZnS : Mn active layer for thin-film electroluminescent device. The effect of H2 gas concentration on the ZnS host material and the luminescent center in the active layer was examined.Electron spin resonance (ESR) investigations were performed to examine the Mn2+ cohesive state. The peak intensity of isolated single Mn2+ increased when the partial pressure of H2 was increased up to about 2.7 × 10-6Pa. And when the amount of H2 was increased further, isolated single Mn2+ signal decreased and crystallinity decreased because of the decrease in S of the ZnS host material.

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.

Implement of a CCD spectrometer as experimental set-up to evaluate the emission of an electroluminescent device using the Taguchi methodology

Taguchi methodology was applied to study the influence of the electrical field variables on the emission yield and spectral response of a standard ZnS-based thin film electroluminescent device (TFEL). A measurement set-up based on a CCD spectrometer was designed and different TFEL working regimes were applied. As a result of the statistical analysis the optimal operation conditions were established and the influence of the external measurement factors were minimized.

PL and EL properties of Tm-activated vanadium oxide-based phosphor thin films

The preparation of high-luminance blue emitting Tm-activated multicomponent oxide phosphor thin films is described. The phosphor thin films were deposited on thick BaTiO 3 ceramic sheets by r.f. magnetron sputtering using powder targets. A very high photoluminescence (PL) intensity in blue emission could be observed in postannealed Tm-activated gadolinium oxide–vanadium oxide Gd 0.5–V 0.5–O:Tm thin films prepared using Tm 2O 3-doped (Gd 2O 3) 0.5–(V 2O 5) 0.5 targets (V 2O 5 content of 50 mol.%). It should be noted that thin films of this phosphor postannealed at 1100 °C in air exhibited higher PL intensity than those from commercially available blue BaMgAl 14O 23:Eu (BAM) phosphor powder and Gd 0.5–V 0.5–O:Tm powders. In addition, a Gd 0.5–V 0.5–O:Tm thin-film electroluminescent device exhibited a blue emission that was the same as the PL emission.

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.

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.

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.

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.

Ballistic electrons in GaAs and ZnS

Ballistic electron scattering and transport in the compound semiconductors GaAs and ZnS are simulated by a Monte Carlo program based on the electron interaction with polar-optical and acoustic phonons, intervalley and interband scattering as well as impact ionization. These scattering processes are considered for higher electron energies and proved in a multiple energy band model of the materials in crystalline fcc-zinkblende structure. The energy band structure and the related density of states are calculated by the empirical pseudopotential method. The scattering processes were considered as intra- and inter-valley transitions within and between nonparabolic bands and in terms of effective mass tensors. A first check of the scattering and transport simulation is applied to electron drift velocity and mobility calculations demonstrating the well-known Gunn effect in these materials. Then, the high electric field transport is aimed at the avalanching process as well as at vacuum emission experiments as a direct spectroscopy of ballistic electrons. MC simulations as well as experimental vacuum emission spectroscopy show the presence of high energy electrons up to 6.5 eV in ZnS supporting the excitation mechanisms in thin film electroluminescence devices.

Oxide thin-film electroluminescent devices and materials

This report reviews newly developed oxide phosphors shown to be promising as the emitting layer of thin-film electroluminescent (TFEL) devices. Since the first report of a high-luminance TFEL device using a Mn-activated Zn 2SiO 4 (Zn 2SiO 4:Mn) phosphor, high-luminance multicolor-emitting TFEL devices have been fabricated using various oxide phosphors activated with Eu or Mn. In addition, many oxides that consist of binary and ternary compounds and multicomponent oxides have been developed and shown to be promising as a host material for TFEL phosphors. This report focuses on Mn-activated Y 2O 3-based oxide phosphors: a binary compound and various ternary compounds and multicomponent oxides, composed of Y 2O 3 in combination with another binary compound such as Ga 2O 3 or GeO 2. TFEL devices of which every constituent was an oxide material were fabricated using oxide phosphor thin films deposited by r.f. magnetron sputtering, pulsed laser deposition or a sol–gel process. High luminances and luminous efficiencies comparable to those of TFEL devices using ZnS:Mn sulfide phosphor were realized using Mn-activated Y 2O 3-based oxide phosphors. Luminances above 7000 cd/m 2 (1 kHz-driving voltage) and luminous efficiencies of approximately 10 lm/W (60 Hz driving voltage) were obtained in yellow emitting TFEL devices fabricated using a Y 2O 3:Mn, a ((Y 2O 3) 0.6–(GeO 2) 0.4):Mn or a ((Y 2O 3) 0.5–(Ga 2O 3) 0.5):Mn thin film and driven by an ac sinusoidal wave voltage. Also, a high luminance above 1000 cd/m 2 for green emission was obtained in a ((Y 2O 3) 0.3–(Ga 2O 3) 0.7):Mn TFEL device driven at 1 kHz.

Improved output coupling efficiency of a ZnS:Mn thin-film electroluminescent device with addition of a two-dimensional SiO2 corrugated substrate

We present a study of the effects of adding a two-dimensional (2-D) SiO2 corrugated substrate onto a ZnS:Mn thin-film electroluminescence (EL) device on its EL properties. Two-step irradiated hologram lithography and reactive ion etching were used to fabricate a 2-D square-lattice-dotted pattern of SiO2 on the glass substrate. The addition of this corrugation leads to a 220% enhancement in the EL efficiency at 40 V above the threshold voltage under a 500-Hz sinusoidal excitation. This improvement in the output coupling efficiency is due to the diffraction scattering produced by the 2-D SiO2 periodic corrugations.

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.