Thin-film Electroluminescent Research Articles

Electroluminescent Oxide Phosphor Thin Films Prepared by a Sol-gel Process

ABSTRACTGa2O3:Mn and (Ga2O3-SnO2):Eu thin films have been prepared by a sol-gel process. High luminance emissions were obtained in TFEL devices: green using a Ga2O3:Mn and red using a Ga2O3:Eu or a SnO2:Eu thin-film emitting layer. These sol-gel prepared devices were produced without using any vacuum processes and always exhibited higher luminances than equivalent devices prepared by r.f. magnetron sputtering: 40, 309 and above 1000 cd/m2 using SnO2:Eu, Ga2O3:Eu and Ga2O3:Mn phosphor thin films, respectively, in TFEL devices prepared by the sol-gel process and driven at 1kHz.

Growth and characterization of ZnSe on Si by atomic layer epitaxy

High-quality undoped ZnSe films on n-type (1 0 0)-silicon substrate were obtained by atomic layer epitaxy (ALE), in which dimethylzinc (DMZn) and hydrogen selenide (H 2Se) were used as the reactants. Key growth steps are described. Growth parameters were varied to obtain films of higher quality. It was found that ZnSe monolayer growth rate is independent of substrate temperature (150–225°C) and the mole fractions of both DMZn and H 2Se. ZnSe epilayer characteristics were investigated by XRD, PL and electrical measurement. A single intense diffraction line was observed at 65.8°, which is slightly smaller than the standard value of bulk ZnSe crystal, confirming that the epitaxial layer is a (1 0 0)-oriented single-crystalline layer. PL spectra, obtained at 7 and 300 K, showed a strong and dominant peak emission at near-band-edge emission of 2.81 eV (4412 Å). Schottky diodes were fabricated from the undoped ZnSe layer and the electrical properties were measured at room temperature. From the current–voltage ( I– V) characteristics, a high reverse breakdown voltage (>40 V) and an extremely low cut-in voltage of 0.6–0.8 V were obtained. On the basis of the observed ZnSe/Si epitaxial film properties, the material is suitable for fabrication of ZnSe-based blue-light-emitting diodes (LEDs) and for application in direct-current thin-film electroluminescence (DCTFEL).

Green Emission from Er-Doped AlN Thin Films Prepared by RF Magnetron Sputtering

ABSTRACTThin films of Er doped AlN, ∼ 200 nm thick, were grown on indium tin oxide/aluminum titanium oxide/glass substrates using RF magnetron sputtering in a pure nitrogen atmosphere. To optically activate Er all films were subject to post-deposition annealing in flowing nitrogen atmosphere at atmospheric pressure at temperatures between 1023-1223 K for 10-60 minutes. The visible cathodoluminescence (CL) in the green was detected at both 11 K and 300K. The strongest CL peaks were observed at 558 nm and 537 nm (11 K), which correspond to the transitions from 4S3/2 and 2H11/2 to the 4I15/2 ground level. Electroluminescence (EL) studies of AlN:Er alternating-current thin-film electroluminescent (ACTFEL) devices were performed at 300 K. The turn-on voltage was found to be around 80-100 V for our ACTFEL devices. The intensity of the EL emission rapidly increases with the voltage increasing in the investigated range of 110-130 V.

Thin-Film Electroluminescent Device Physics Modeling

Previous and current approaches to alternating current thin-film electroluminescent (ACTFEL) device physics modeling are reviewed. The primary goal of the modeling efforts of interest to this review is to accurately simulate the electrical properties of two-terminal ACTFEL test structures when subjected to realistic applied voltage waveforms. A comprehensive review of previous ACTFEL modeling efforts is presented. Additionally, two modeling methodologies that are believed to hold the most promise for accurate ACTFEL device modeling are presented. These methodologies involve state-space simulation of a discretized phosphor and SPICE simulation.

Lanthanide doping in ZnS and SrS thin-film electroluminescent devices

The relative high field transport efficiency and short wavelength electroluminescence (EL) potential of the phosphors ZnS and SrS for alternating-current thin-film electroluminescent (ACTFEL) device flat-panel display applications are assessed via a comparison of the EL spectra of ZnS and SrS ACTFEL devices prepared in a very similar manner and doped with the same lanthanide luminescent impurities: Dy, Er, Ho, Tb, and Tm. For all of the lanthanide luminescent impurities studied, it is found that the the higher energy EL peaks are much more intense for SrS than for ZnS, even though the average phosphor field in SrS is smaller than in ZnS. These observations show SrS to be a superior high-field electron transport material compared to ZnS. All of the ZnS EL spectra show a dramatic cut off in their EL intensities at about 440–460 nm; this suggests that ZnS is not an appropriate phosphor for blue light emission since its electron distribution does not appear to be adequately heated to efficiently excite blue luminescent impurities.

AlN:TbF3 Electroluminescence Thin Film Prepared by Radio-Frequency Magnetron Sputtering

High quality AlN thin films doped with TbF3 were prepared by rf magnetron reaction sputtering. High purity Al metal and TbF3 were used as target materials with N2 and Ar as sputtering gases. The influence of preparation conditions on the photoluminescence brightness was studied, and the electroluminescence was found to have a similar dependence on the concentration of TbF3 and substrate temperature. The characteristic emission of Tb3+ ions was obtained in an AlN:TbF3 alternating current thin film electroluminescence device prepared with 600° C substrate temperature and 4.0 mol% concentration of TbF3.

Investigation on composition and structure of ZnS : Mn evaporation films with sulfur supply

The heat treatment technique in sulfur-saturated vapor for ZnS : Mn thin films deposited by thermal evaporation is described in this paper. Results of Auger electron-spectroscopy (AES) and X-ray diffraction (XRD) analyses and the measurement of the energy gap show that thin films made in this way have better stoichiometric proportion and hexagonal system to meet the need of electroluminescent thin films.

Grain growth in thin-film strontium sulfide electroluminescent phosphors

A comprehensive study of grain growth in strontium sulfide (SrS) thin-film electroluminescent (TFEL) phosphors is reported. It is suggested that the grain growth during annealing is influenced by the migration of the dopant ion from the grain boundary into a lattice position. The difference in the free energy of formation between SrS and the sulfur compounds of the dopants changes the activation energy for the grain boundary movement and promotes the grain growth. This understanding has successfully been used to explain the grain growth phenomena observed in SrS TFEL phosphors with different dopants, such as Cu, Ag, Mn, and Ga.

Alkali metal coactivators in SrS: Cu,F thin-film electroluminescent devices

A unique approach for obtaining bright and efficient saturated green phosphors for alternating-current thin-film electroluminescent (ACTFEL) device applications is presented. The approach involves color-shifting blue SrS:Cu,F ACTFEL phosphors into the green region of the spectrum via the incorporation of alkali metal ions into the SrS lattice. Alkali metals are incorporated into SrS:Cu,F phosphors by using LiF, NaF, KF, RbF, or CsF coactivators. The best result to date is obtained by using a KF coactivator and results in a saturated green brightness and efficiency of 52.7 cd/m2 and 0.973 lm/W (at a frequency of 60 Hz and an overvoltage of 40 V). In addition to providing a color shift, the alkali-metal fluorides improve the overall performance of the ACTFEL device by increasing the magnitude of the electric field and its uniformity across the phosphor through suppression of positive space charge.

Display Applications of Rare-Earth-Doped Materials

The human eye places remarkably stringent requirements on the devices we use to illuminate objects or generate images. Exceedingly small deviations in color or contrast from what we consider natural are easily judged by the brain to be fake. Such cognition drives consumer practice, so great efforts have been made for over a century to synthesize emissive materials that match the response functions associated with the human perception of color. This is an extremely difficult task, given the diverse range of considerations, some of which include whether (1) the display is viewed under artificial light or natural sunlight, (2) the images are stationary or moving, and (3) the rendering of depth in a two-dimensional image is believable.Established technologies including cathode-ray tubes (CRTs), vacuum fluorescent displays (VFDs), lamps, and x-ray phosphors have made possible a wide variety of display and imaging devices. However, continued advances are required to increase brightness, contrast, color purity, resolution, lifetime, and viewing angle while still lessening the cost, weight, volume, and power consumption. Mature or emerging technologies that address these issues include thin-film electroluminescent (TFEL) displays, liquid-crystal displays (LCDs),8 field-emission displays (FEDs),9 and plasma displays (PDs).10-12 Each of these technologies uses luminescent materials consisting typically of an activator from which light is emitted and a host for low concentrations of the activator (typically >1% activator). The requirements of the host and activator are discussed in a later section. The luminescent material can exhibit either a narrow emission spectrum, useful for color displays, or a broadband emission, which can extend into multiple colors. In addition, with multiple activator/host combinations, a luminescent material can emit several colors and even white light. While LCDs are light valves, which may be used in a reflective mode and therefore do not require a luminescent material, low-light situations require a backlight generated by a luminescent material. Many of the most versatile, efficient activators are rare-earth (RE) elements, for reasons that will be discussed. The ability of RE ions to emit red, green, and blue light make them well suited for application in visible-display technologies. This article reviews dopant and host material systems, excitation mechanisms, and the factors that limit the achievable luminescent intensity and efficiency. Device configurations for modern displays are discussed, as are materials and structures for next-generation technologies. Since each display technology has different performance and operational requirements, only the basic characteristics will be discussed here to enable an appreciation of emission from RE activators. References to the literature are supplied to further direct the reader to more in-depth discussions.

An X-Ray Photoelectron Spectroscopy Study of Elements' Chemical States in SrGa2S4:Ce Blue Electroluminescent Thin Films

SrGa2S4:Ce blue electroluminescent thin films with a (440) single orientation have been prepared by a multisource deposition method using a molecular beam epitaxy (MBE) apparatus. The elements' chemical states and the doped oxygen effects on them have been analyzed by X-ray photoelectron spectroscopy (XPS). A small variation in the elements' chemical state with depth indicates that the growth of the thin film is stable. Sr ions exhibit no chemical shift due to a Sr–O bond, but Ga ions exhibit a chemical shift due to a Ga–S bond and a larger shift due to a Ga–O bond, indicating that the oxygen ion does not substitute for a S site in which the S ion bonds with the Sr ion. No Ce4+ ions have been detected and no oxidation of Ce3+ ions occurs even if some of the S ligands of Ce3+ are replaced by doped oxygen ions. The trivalent state of Ce is stable in the SrGa2S4 crystal.

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.

An Analysis of Field Dependent Capture of Electrons by Ionized Activators and their Effects on Luminance in SrS Based Blue Emitting ACTFEL Display Devices

AbstractA new model for understanding luminance‐time characteristics of blue‐emitting SrS based a.c. thin film electroluminescent (ACTFEL) display devices was developed. It included (a) field‐dependence of the capture of conduction electrons by ionized activators and (b) field‐dependence of the relaxation of activators. Experiments involving a “dented” voltage waveforms were performed to evaluate the theoretical model.

A Novel Blue Thin Film Electroluminescent Phosphor Gd3Ga5O12:Ag

A new blue thin film electroluminescence (TFEL) phosphor Gd3Ga5O12:Ag is prepared by electron beam evaporation. Photoluminescence and Electroluminescence of Gd3Ga5O12:Ag film were investigated. The luminance of TFEL device excited by an alternating-current sinusoidal voltage with frequency of 1000 Hz is about 2 cd/m2.

Roughness of ZnS:Pr,Ce/Ta/sub 2/O/sub 5/ interface and its effects on electrical performance of alternating current thin-film electroluminescent devices

Roughness effects of neighboring dielectrics on electrical characteristics of thin-film electroluminescent devices were investigated in order to improve the understanding of physics for the devices. Atomic force microscopy analysis reveal that thicker bottom layer of Ta/sub 2/O/sub 5/ shows rougher surface resulting in the rougher surface of ZnS:Pr,Ce layer. It can be easily seen that the dc leakage current increases rapidly with increase of surface roughness. Furthermore, it is notable that the initiation field of Poole-Frenkel current conduction is lowered by increasing surface roughness of Ta/sub 2/O/sub 5/ thin film. Internal charge-phosphor field (Q/sub int/-F/sub p/) analysis and capacitance-ac voltage (C-V) analysis for ITO-Ta/sub 2/O/sub 5/-ZnS:Pr,Ce-Al and ITO-Ta/sub 2/O/sub 5/-ZnS:Pr,Ce-Ta/sub 2/O/sub 5/-Al show that the steady state phosphor field is smaller and C-V curve in transition region is less steep with increase of root-mean-square roughness between lower dielectric and phosphor layer in the alternating current thin-film electroluminescent (ACTFEL) devices. Therefore, we conclude that interface roughness is one of the physical factors to change the electrical performance of ACTFEL device.

Orange Electroluminescence from a Divalent Europium Complex

Divalent molecular lanthanide complexes are shown to offer promise as tunable light emitting materials in thin film electroluminescent displays for the first time in this study. Bright orange luminescence is obtained from thin film EL device structures containing bis[tris(dimethylpyrazolyl)borate]europium(II) (1), which has a high quantum efficiency in the solid state. The effect of device structure upon device efficiency and light purity is discussed and an optimum EL device structure for 1 detailed.

Oxide Phosphor Green EL on Glass Substrates

AbstractA new thin film EL device based on green emitting oxide phosphor Zn2Si.5Ge.5O4:Mn has been realized on glass and quartz substrates. The device relies on thin film dielectrics which are sputter deposited. Maximum processing temperature for the device is 700°C and excellent brightness of over 180 cd/m2 at 60 Hz may be achieved at 50 V above threshold. Typical device structures and their performance will be described.

Improved blue luminescence in Ag-codoped SrS:Ce thin films made by atomic layer epitaxy and ion implantation

Silver ions were implanted into SrS:Ce3+ thin films grown by atomic layer epitaxy (ALE) to examine the effects of Ag+ codoping on the luminescence of SrS:Ce3+ used in thin-film electroluminescent (TFEL) devices. High-temperature annealing (800 °C) of the standard ALE SrS:Ce prepared at 510 °C caused a pronounced blueshift of the photoluminescence (PL) emission, but the intensity was decreased. Codoping by ion implantation of Ag+ and high-temperature annealing (800 °C) caused no further blueshift but the PL intensity was more than double that of the nonimplanted samples. The implanted SrS:Ce, Ag also exhibited the longest decay value (SN=22 ns at 520 nm with 420 nm excitation) reported for ALE SrS:Ce thin films. The improved PL properties show that codoping with Ag+ can be advantageous in ALE SrS:Ce3+ thin films used for electroluminescent multicolor and full-color flat panel displays. The results also suggest the potential of the ion implantation technique in TFEL device preparation.

Growth and luminescence of Zn xMg 1− xS : Mn ternary compound crystal films

Solid solutions of crystalline Zn x Mg 1− x S thin films have been developed in the wide composition x between 0.13 and 1.0 for the use as short-wavelength-light-emitting devices. The crystal structure, optical and luminescent properties and device characteristics of Zn x Mg 1− x S thin films have been investigated by X-ray diffraction, optical absorption, transmission electron microscope, photoluminescence, and electroluminescence. It was revealed that a zincblende phase Zn x Mg 1− x S crystal could be formed and the lattice constant was determined and also the optical band gap energy was evaluated. Photo- and electro-luminescent properties of Zn x Mg 1− x S : Mn have been investigated for the application as an electroluminescent thin film. The blue shift of the Mn emission band with the increase of Mg composition has been determined. Photoluminescent intensity has been also enhanced with the increase of Mg content. The origin of the blue shift of Mn emission in Zn x Mg 1− x S have been discussed in the framework of crystal field in relation with the composition dependence of the lattice parameter. The blue shift of Mn emission is ascribed to the change of lattice parameter which increase the bond length between Mn 2+ ion and ligand S atom with the increase of Mg content in Zn x Mg 1− x S. Present status of electroluminescent capability has been also described.

The Electrical Properties of Europium Complex Thin Film EL Devices Prepared by Vacuum Evaporation Method

Electroluminescent (EL) and I-V characteristics of Eu (TTA)3 (phen) layers with various thickness were investigated. The red EL spectrum emitted from the Eu (TTA)3 (phen) layer is almost the same as the PL spectrum. Based on the I-V characteristics, it was found that the turn-on voltage is 9V and current density was 0.01 A/cm at a dc operation voltage of 9V. The observed electrical transport phenomena were explained within a trapped-charge-limited current model describing I-V characteristics.