Thin-film Electroluminescent Research Articles

Influence of the growth conditions on the properties of CaS:Eu electroluminescent thin films

A detailed study of the structural, compositional and optical properties of electron beam deposited CaS:Eu electroluminescent devices is presented. Specific attention is paid to the influence of H 2S addition — applied for the first time — and Se coevaporation during the phosphor film deposition. Upon addition of H 2S or Se, the crystallinity of the layers improves, the oxygen contamination decreases and the surface roughness increases. Finally, the electroluminescent characteristics are correlated with the results of the structural analysis.

Volatile Metal β‐Diketonates: ALE and CVD precursors for electroluminescent device thin films

AbstractThe use of volatile β‐diketonate chelates as precursors for the deposition of thin films for electroluminescent devices is reviewed. Alternating current thin film electroluminescent (ACTFEL) devices consist of an emitting layer sandwiched between two dielectric layers, together with conducting and buffer layers. Besides various physical deposition techniques, the commonly applied methods for preparing thin films are chemical vapor deposition (CVD) and its particular variant atomic layer epitaxy (ALE). Alkaline earth β‐diketonates are used as precursors for deposition of the semiconducting alkaline earth sulfide and thiogallate films that provide the matrix in emissive layers, while β‐diketonates of lanthanides and a few other metals (Mn, Na, K) are precursors for dopants and codopants producing colors. β‐Diketonate precursors can also be used for the preparation of dielectric alkaline earth titanate and oxide layers. Current research on thin film electroluminescent materials is focused on improving the blue color produced by cerium doping of alkaline earth sulfide and thiogallate matrices. The synthesis and properties of β‐diketonate chelates and the growth and characterization of the thin films obtained with them are presented and discussed. This review of precursors for TFEL materials is also relevant for other materials, including oxide superconductors.

Preparation of CaGa 2S 4:Ce TFEL devices with inter-layer reaction

Ce 3+-doped alkaline earth thiogallates with blue emission have been intensively investigated to fabricate full-color thin film electroluminescence (TFEL) devices, with sputtering, atom layer epitaxy (ALE) and molecular beam epitaxy (MBE) being the methods used. In this paper, a new way of preparing CaGa 2S 4:Ce TFEL devices, based on inter-layer reaction is first reported. CaS:Ce and Ga 2S 3 were deposited by electron beam evaporation, layer by layer, and CaGa 2S 4:Ce thin film was formed by post processing. The results of X-ray diffraction, scanning transmission electron microscopy (STEM) microphotographs and energy dispersive X-ray spectroscopy (EDX) show that CaGa 2S 4:Ce thin film with good crystallinity and stoichiometry were obtained. Photoluminescence (PL) and electroluminescence (EL) of these devices were investigated.

MATERIALS FOR FULL-COLOR ELECTROLUMINESCENT DISPLAYS

▪ Abstract New thin-film electroluminescent (EL) materials with blue and broadband emissions recently have been developed that meet the phosphor requirements for commercial multicolor and full-color EL display products. Improvements have been achieved in phosphor luminance, luminous efficiency, emission color, and aging characteristics. This paper reviews the latest advancements in red, green, and blue thin-film phosphors for patterned phosphor EL display applications, and broadband emission phosphors for the filtered white or color-by-white EL display approach. Focus is placed on the new EL materials and the composition modifications that have improved the performance of well-known EL materials.

ELECTRICAL CHARACTERIZATION OF THIN-FILM ELECTROLUMINESCENT DEVICES

▪ Abstract Electrical characterization methods for the analysis of alternating current thin-film electroluminescent (ACTFEL) devices are reviewed. Particular emphasis is devoted to electrical characterization techniques because ACTFEL devices are electro-optic display devices whose performance is to a large extent determined by their electrical properties. A systematic procedure for ACTFEL electrical assessment is described. The utility of transient charge, voltage, current, and phosphor field analysis is explained. Steady-state electrical characterization methods discussed in this review include charge-voltage (Q-V), capacitance-voltage (C-V), internal charge-phosphor field (Q-Fp), and maximum charge-maximum applied voltage (Qmax-Vmax) analysis. These electrical characterization methods are illustrated by reviewing relevant results obtained from the analysis of evaporated ZnS:Mn and atomic layer epitaxy (ALE) SrS:Ce ACTFEL devices.

Ce-Activated SrS Thin Film Electroluminescent Devices Fabricated by Multi Source Deposition Using Ga2S3 Precursor

For the first time, cerium-doped strontium sulfide (SrS:Ce) thin film electroluminescent (TFEL) devices have been fabricated by multi source deposition using Ga2S3 compound. Ga2S3 is able to provide elemental sulfur without leaving Ga in the phosphor films. This new precursor is excellent for controlling the sulfur supply, due to its low vapor pressure. Highly crystalline SrS thin films have been deposited with preferentially (200)-oriented. Full width at half maximum of the X-ray diffraction peak is 0.22°. The composition of the thin film is close to the stoichiometry, and the element Ga was not detected in the film. The TFEL device shows a luminance level of 160 cd/m2 at 40 V above the threshold voltage, and the CIE color coordinates are x=0.30 and y=0.51.

Characterization of the electrical properties of SrS:Ce thin‐film electroluminescent devices

Abstract— The electrical properties of SrS:Ce thin‐film electroluminescent (TFEL) devices were studied. Photo‐irradiation effects on capacitance‐voltage and charge‐voltage characteristics of SrS:Ce TFEL devices were measured and compared with those of ZnS: Mn devices. For the SrS:Ce device, the polarization charge due to the positive space charge was estimated to be 0.94 μC/cm2. This polarization charge was remarkably reduced by photo‐irradiation. For the ZnS: Mn TFEL device, the polarization charge due to the positive space charge was 0.66 μC/cm2. However, the reduction of the polarization charge by photo‐irradiation was small compared to that of the SrS:Ce device. The unique electrical and optical characteristics observed in the SrS:Ce TFEL devices such as relaxation of phosphor field and leading‐ and trailing‐edge emissions are considered as functions of the large amount of space charge and also due to the electron emission from electron traps and electron recapture by the traps.

ZnGa 2O 4 as host material for multicolor-emitting phosphor layer of electroluminescent devices

ZnGa 2O 4 is described as an excellent host material for multicolor-emitting thin-film electroluminescent (TFEL) device phosphors. High luminance multicolor emissions were realized in TFEL devices using a Mn-, Cr- or rare earth-activated ZnGa 2O 4 phosphor.

Dependence on Ce Concentration of Blue Emission and Crystallographic Properties in SrGa2S4:Ce Electroluminescent Thin Films Grown by Molecular Beam Epitaxy

The Ce-doping dependence of blue emission properties has been investigated for SrGa2S4:Ce electroluminescent (EL) thin films grown by molecular beam epitaxy (MBE). EL spectra, EL luminance, photoluminescence (PL) spectra, X-ray diffraction (XRD) intensity, XRD rocking curves, spacing of lattice planes, EL and PL decay times were measured in samples with Ce-doping concentrations ranging from 0.87 mol% to 8.37 mol%. EL luminance reached a maximum at 2.40 mol%. In particular, EL chromaticity remained within the pure blue region even with higher Ce-doping concentrations.

CL and EL properties of blue-emitting SrGa 2S 4:Ce thin films prepared by multi-source deposition

SrGa 2S 4:Ce films for blue-emitting thin-film EL devices have been prepared by multi-source deposition. It is found that when Ga 2S 3 evaporates from the source, it decomposes into GaS and S 2. Therefore, reactions between Sr, GaS and S 2 should be considered in the deposition process. It was found that the formation of SrGa 2S 4, SrS or GaS depends strongly on the substrate temperature and Ga 2S 3 Sr . The SrGa 2S 4:Ce film showing blue luminescence was deposited at a substrate temperature of 450 °C and the flux ratio > 40.

Electroluminescence in thin solid films of closely packed CdS nanocrystals

Thin solid films of closely packed CdS nanocrystals with sizes between 1.5 and 1.8 nm have been prepared and sandwiched between indium–tin–oxide (ITO) and silver contacts. At room temperature, these CdS nanocrystals start to emit light visible to the unaided eye at a bias above 15 V. The spectral characteristics are voltage controlled and tunable between 1.8 eV and 2.8 eV with increasing bias from 20 to 40 V. The current–voltage dependence indicates hopping conductivity of the carriers injected. At high voltage the electroluminescence and photoluminescence spectra are similar showing a strong red shift of the luminescence maximum with respect to the absorption peak at 3.75 eV which we explain by an effective radiative recombination through deep traps in the interface region. Consequently, the tunability of electroluminescence is attributed to excitation of different deep traps by electron transport across neighbouring CdS nanocrystals.

Phosphor efficiency and deposition temperature in ZnS:Mn A.C. thin film electroluminescence display devices

ZnS:Mn A.C. thin film electroluminescent (ACTFEL) display devices were fabricated at various phosphor deposition temperatures ( T s). The phosphor layer of all devices was formed by electron beam evaporation from ZnS:Mn powder with 1.2 wt.% Mn content. The amount of Mn actually present in the film was found to increase with T s while the luminous efficiency of ACTFEL devices also increased with increasing T s. Increased phosphor efficiency was attributed to the higher Mn content and to the presence of larger phosphor fields in the higher deposition temperature devices. Crystallite perfection as measured by the full width at half maximum of X-ray diffraction peaks remained constant up to 300 °C, but decreased for higher T s values. © 1997 Elsevier Science S.A.

Deep Traps and Mechanism of Brightness Degradation in Mn-doped ZnS Thin-Film Electroluminescent Devices Grown by Metal-Organic Chemical Vapor Deposition

Evidence from deep level transient spectroscopy (DLTS) and Fourier transform infrared spectroscopy (FTIR) measurements strongly supported the assertion that the degradation mechanism of ZnS:Mn AC thin-film electroluminescent (ACTFEL) devices is mainly due to the deep electron trap, E t, which comes from the Mn activators reacting with water molecules. The photoluminescence measurements revealed that the Mn-related E t trap behaves like a nonradiative center. Furthermore, the X-ray diffraction experiments indicate that the crystallinity of ZnS:Mn phosphor films were destroyed in some degree by the E t trap. As a result, poor brightness characteristics including lower brightness and higher threshold voltage were obtained when samples become aged.

ZnF 2: Gd thin film electroluminescent device

ZnF 2:Gd thin-film electroluminescent (EL) devices are known to emit intense ultraviolet (UV) light of a spectrum peaking at 311.5 nm, due to the 6P 7/2 8S 7/2 transition of Gd 3+ ion. The UV emission is utilized as the excitation source for blue-emitting phosphors, realizing blue-emitting EL PL hybrid device. Through a systematic optimization of UV emitting EL device as well as the phosphor materials, a maximum luminance of 10 cd/m 2 and a maximum efficiency of 0.015 lm/W have been achieved to the EL PL hybrid device. Full color emission devices using red, green and blue phosphors are also reported.

Intraband luminescence spectra of insulators and semiconductors excited by pulsed electron beams or electric fields

The features of the intraband luminescence spectrum of wide-gap insulators (KI, KBr, CsCl, etc.) and semiconductors (GaP, CdS, α-SiC, and ZnS) are studied in the temperature interval 80–760 K. The spectra of the intraband luminescence are compared with the spectra of the pre-breakdown electroluminescence of GaP and α-SiC surface-barrier diode structures and of a ZnS thin-film electroluminescence indicator. In alkali halide crystals the short-wavelength boundary hνm of the intraband luminescence is less than the band gap Eg and is governed by complex excitonic processes. In semiconductors with indirect transitions hνm>Eg. The differences in the spectra of the intraband luminescence and the intraband pre-breakdown electroluminescence can be explained by differences in the distributions of the hot charge carriers over levels of the allowed bands and in the maximum energies of the carriers involved in the formation of the spectra.

Possibilities and limitations of blue electroluminescence in CaS: thin films

While CaS:Pb is already well known as an efficient UV/blue emitting photoluminescent phosphor, only preliminary results on its thin film electroluminescence are available. Thin film electroluminescent devices with a CaS:Pb active layer were deposited by electron beam evaporation. It was found that both the activator concentration and the substrate temperature during deposition strongly influence the optical characteristics. X-ray photoelectron spectroscopy was used to determine the composition of the layers, and the spectral characteristics were compared with results from photoluminescence.

Low-voltage thin-film electroluminescent devices with low-resistivity stacked insulators

The characteristics and mechanism of low-voltage-driven thin-film electroluminescent (TFEL) devices with low-resistivity (10(6)-10(7)-ohms cm) SiO(2)/Ta(2)O(5) and Al(2)O(3)/Ta(2)O(5) stacked insulating films have been studied. At 50-Hz sinusoidal wave voltage excitation, the threshold voltage of devices with a ZnS:Mn emitting layer is below 40 V, and the brightness and luminous efficiency are above 1000 cd/m(2) and 4 lm/W, respectively, with 60 V voltage. The characteristics of brightness versus voltage (B-V) curves, integrated charge versus voltage (Q-V) figures, and luminous efficiency versus voltage (eta-V) characteristics are different from conventional devices. The study of a special semiconductor layer--a thin probe-doped layer located at a different part of the pure ZnS layer--has proved that the excitation efficiency is not homogeneous across the emitting layer in this kind of device, and its value decreases from the anode toward the cathode, which is opposite of that made with TFEL devices with high-resistivity insulators. By offering a model of space-charge-limited current, the mechanism of low-voltage-driven thin-film electroluminescence, its optoelectronic characteristics, and the distribution characteristics of excitation efficiency across the emitting layer can be thoroughly explained.

The Characteristics of SrS:Ce Thin Film Electroluminescent Devices

SrS:Ce thin films were fabricated by electron beam evaporation under several conditions such as substrate temperature, deposition rate and sulphur pressure during evaporation. The relationship between the quality of phosphor layers and the luminance of EL (electroluminescent) devices investigated. The bigger dead layer of SrS:Ce phosphor layer produced the larger spike current when bipolar trapezoidal voltage pulses were loaded on the SrS:Ce TFEL (thin film electroluminescent) devices. The low crystallinity of the phosphor bulk layer reduced the spike current. The magnitude of spike current is limitted by a balance between the number of hot electrons injected into the phosphor layer from the phosphor-insulator interface and the number of hot electrons captured by defect levels in phosphor layer. The luminance of EL devices is determined by the effective number of hot electrons and the crystallinity of phosphor layer.

Transient and AC Electroluminescence in Pyridylene Copolymers of PPV

We present transient and AC electroluminescence (EL) measurements on thin films of a newly synthesized copolymer of PPV where pyridine units were incorporated into the polymer backbone (co(PyPV)), and compare it to the EL of thin films of PPV prepared by the method of self-assembly and by spin casting. Transient EL under voltage pulses of co(PyPV) indicates that positive charge carriers have lower mobility than in PPV. Under pulses and AC square wave voltage we have observed a new type of prompt transient EL at the switch-off of the voltage. The transient EL effects are explained by the presence of non-bonding electrons in the polymer chain of co(PyPV). Co(PyPV) based LEDs have improved EL yield compared to PPV. Operations with sine AC voltage mode up to 10 kHz show better stability as compared to DC, without loss in efficiency.

Red–green–blue light emission from a thin film electroluminescence device based on parahexaphenyl

For colored flat panel displays electroluminescent organic semiconductors are very promising. A novel means is described for the realization of red, green, and blue (RGB) dots based on all all‐organic color transformation technique. The construction of the parahexylphenyl‐based electroluminescence device and its combination with suitable matrix dye layers and dielectric mirrors and filters are outlined. The simplicity of the production of the device and the tunability of the comitted colors make this technique technologically attractive.