ACTFEL Devices Research Articles

High Field Electroluminescence Characteristics of Pr$^{3+}$-Doped ZnS/TiO$_{2}$

This paper reports the fabrication, operation, and applications of rare-earth (RE) ion praseodymium (Pr <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^{3+}$</tex></formula> )-doped ZnS/TiO <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$_{2}$</tex> </formula> core-shell nanoparticle-based thin-film electroluminescent (ACTFEL) devices. Pr <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$^{3+}$</tex></formula> :ZnS/TiO <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$_{2}$</tex></formula> ACTFEL devices showed white light emission due to impact excitation of the Pr <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^{3+}$</tex></formula> ions by hot carriers followed by radiative RE relaxation under the high field of alternating current. The preparation of Pr <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^{3+}$</tex></formula> :ZnS/TiO <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$_{2}$</tex> </formula> core-shell nanoparticles was done by the colloidal synthesis method. To observe the effect of the TiO <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$_{2}$</tex></formula> shell layer on electroluminescence (EL) brightness and properties of Pr <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^{3+}$</tex></formula> :ZnS, three types of molar ratios 1:7:7, 9:5:5, and 1:14:14 of Ti:Zn:S were taken in the synthesis process, and to be employed for these three different thin-film EL devices for high-field EL measurements. Each EL device consists of glass substrate/ITO (indium-tin oxide)/TiO <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$_{2}$</tex></formula> coated Pr <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX"> $^{3+}$</tex></formula> -doped ZnS nanocrystalline emission layer/Al (aluminum). The brightness characteristics of Pr <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^{3+}$</tex></formula> :ZnS/TiO <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$_{2}$</tex> </formula> TFELDs were evaluated as a function of bias and frequency. Brightness–voltage ( B–V) measurements at 600 Hz showed the maximum efficiency of the Pr <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^{3+}$</tex></formula> :ZnS/TiO <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$_{2}$</tex> </formula> EL device at molar ratios 1:14:14 of Ti:Zn:S. To observe the frequency-dependent characteristics, the EL device was tested at various frequencies. The measurements showed the decrease in the threshold voltage of the EL device with increased frequencies. High-field voltage–current (V–I) characteristics also showed the frequency-dependent onset voltage and current density. It showed the minimum turn-on voltage and maximum current density at a higher frequency of 1000 Hz.

Electroluminescence from Tm-doped GaN deposited by radio-frequency planar magnetron sputtering

Room-temperature blue and near-infrared (NIR) electroluminescence ac (EL) has been demonstrated from Tm-doped GaN thin films prepared by rf planar magnetron sputter deposition. Blue and NIR EL emission peaks at ∼475 and ∼800 nm, respectively, were observed from the Tm3+ 4f intrashell transitions from the G41 and F43 excited states to the H63 ground state, respectively. The threshold voltage for the 475 nm blue emission was consistently 11 to 19 V higher than that for the 800 nm NIR emission. The EL intensity ratio of the 475 to 800 nm emission (I475/I800) was increased ∼900% with a 67% increase in the applied ac voltage from 120 to 200 V. These results indicate that the excitation process of EL from GaN:Tm ACTFEL devices is dominated by impact excitation of the Tm3+ luminescent centers by direct interaction between hot electrons and the ground state 4f electrons.

The study of the low‐field behavior of optically generated charges in thin‐film electroluminescent devices

Abstract—The measurement of photo‐stimulated currents (PSC) or thermally stimulated currents (TSC) in ACTFEL devices is sometimes used as a method to identify energy levels of trap centers in the phosphor layer. These methods are based on the fact that optically or thermally generated charges are able to move in the polarization field in the phosphor layer of an EL device, which remains after charging the device with a voltage pulse. Similar information about space charge can be obtained by the measurement of loss factor and capacitance of uncharged EL devices under illumination during a low ac‐voltage excitation. This new method avoids one of the major disadvantages of the other methods, namely, the continuous change of the field situation throughout the measurement. The measurement of low‐voltage photo‐stimulated currents (LVPSC) creates an identical and reproducible field situation at each wavelength, which makes interpretation more reliable. Moreover, this new method allows the extraction of information about the low‐field conduction behavior of EL thin films.

High-field transport in AC thin film electroluminescent devices: theory and experiment

We present a combined experimental and theoretical analysis of the high field transport in ZnS:Mn ACTFEL devices. The theory consists in a first principles calculation of the impact ionization (II) coefficient and then this parameter is used in a drift-diffusion modeling of the carrier transport in such devices in the presence of a high AC field. By comparing the results obtained with our II rate with those from other models previously used important differences for the transferred charge occur. From the experiments we extract the time dependence of the dissipative current.

Poster Session:Photo- and Electroluminescence in Zinc Germanate and Zinc Silicate Powders and ACTFEL Devices(1. 6th STDP概要報告)

Poster Session:Photo- and Electroluminescence in Zinc Germanate and Zinc Silicate Powders and ACTFEL Devices(1. 6th STDP概要報告)

Growth, characterization and modeling of alternating-current thin-film electroluminescent devices

This article seeks to put together the most important and updated information and to present a unified, systematic and concise overview of the classification, physical principles and methods of the ACTFEL device growth, characterization and modeling. The review takes a comprehensive look at the evolution of the equivalent schemes of ACTFEL devices from the early purely interfacial model to the most recent models enabling simulation of the space charge formation. Comparison of experimental and computer simulated characteristics of the ACTFEL stack may bring a better understanding of the possibilities of charge storage in various regions of the device and help in finding how this may impact device performance. Some peculiarities of the SPICE simulation of the devices are also discussed. Main peculiarities of the characterization techniques are described and their role in the understanding of the device operation is presented. Some important features of the conventional characterization techniques, such as VIQ, PDP, L-V, Q-V, C-V, Q max-V max , and Q p-F int measurements in identifying carrier traps, their energy position, and some other main device parameters are emphasized. An important role of the destructive methods of analysis, such as the scanning election microscopy, secondary electron contrast profiling, and SIMS measurements of ACTFEL devices as complimentary tools to the non-destructive characterization techniques is demonstrated. The paper addresses the well-known as well as “non-traditional” theoretical and experimental techniques of ACTFEL device analysis. The understanding of the ACTFEL devices can be greatly improved by using the innovative analogy approach of the analysis, some aspects of which were originally developed in Luxell and converted to a characterization technique.

34.3: Modeling Of Optoelectronic Processes In SrS:Cu ACTFEL Display Devices

AbstractFrom the results of measurements involving transient response and dented voltage waveforms, a new model for luminance and current in SrS:Cu ACTFEL devices is developed. It includes the the backflow of electrons form the anodic interface states, field dependent capture of electrons by ionized activators and the effect of bulk phosphor dipoles.

Bound exciton luminescence in Te-doped SrS

This article describes the synthesis and characterization of SrS:Te powders which were found to have two high energy emission bands under photoluminescent excitation. To understand the radiative recombination mechanism, photoluminescence emission spectra were measured as a function of temperature. A simple configuration coordinate model was used to explain the emission band broadening and luminescence quenching that was observed with increasing temperature. Luminescence decay measurements were also performed, and the empirically determined transition rates were compared to theoretical calculations. Finally, SrS:Te ACTFEL devices were fabricated and tested under electroluminescence excitation.

An Improved Understanding on the Relationship between Interface Roughness of ZnS:Pr,Ce Phosphor‐Ta2O5 Insulating Film and Electrical Characteristics of TFEL Devices

ABSTRACTInterface roughness effects in ITO‐Ta2O5‐ZnS:Pr,Ce‐Ta2O5‐Al structures were investigated as a function of thickness of dielectric layer. Internal charge‐phosphor field (Qint‐Fp) analysis and capacitance‐voltage analysis(C‐V) showed that the steady state phosphor field is more smaller and C‐V curve in transition region less steeper with increasing root‐mean‐square‐roughness between dielectric and phosphor layer. The experimental results demonstrate that interface roughness is one of the physical factors to change the electrical performance of ACTFEL devices.

Analysis of image retention in alternating-current thin-film electro- luminescent devices

Image retention can limit the upper operational frequency of ZnS-based alternating-current thin-®lm electro-luminescent (ACTFEL) devices. We studied the impact of indium-tin-oxide (ITO) electrodes pro®le, ZnS deposition conditions, the moisture content of the source material, and the drive conditions on image retention. Layer-by-layer etching and scanning electron microscopy (SEM) crosssectioning techniques were used for the investigation of this phenomenon. The failure was found to start at the column electrode edges of the pixel. Some peculiarities of the image-retention phenomenon are discussed, on the basis of an equivalent electrical scheme of the device and via crystalline properties of the active layer. The conditions for producing displays with a reduced occurrence of image retention are also discussed. ZnS:Mn-based alternating-current thin-®lm electroluminescent devices (ACTFEL) are being widely applied as high-contrast, lightweight, and rugged displays. Additional signi®cant advantages of these devices are their wide viewing angle, suf®ciently low power consumption, the capability to operate in wide temperature ranges, long life, and potentially high legibility in almost all ambient light conditions [1]. Many reports exist in the literature on the growth techniques of ACTFEL devices and the different ways to improve the ef®ciency of the structure [2, 3]. Much less attention has been paid to the appearance of destructive defects and aging phenomena, such as image retention or differential aging, which limit the operating potential of the device and ultimately hold back the exploitation of the displays. The available information is limited mostly by studies on domain electro-luminescence [4], localized destructive breakdown phenomenon [5], and the effect of moisture presence on some characteristics of the devices [6]. The appearance of the differential aging is usually explained by improper driving methods [7]. In this letter, we concentrate on the failure analysis of ZnS:Mn based ACTFEL devices, and present some important results that expand the understanding of image retention in ACTFEL devices. The typical ACTFEL structure, consisting of the glass substrate, indium-tin-oxide patterned layer, 6000 AE ZnS:Mn active layer sandwiched between two dielectrics, back aluminum electrode and packaging oxide layer, was used for this study. The dielectrics are typically yttrium oxide (Y2O3), aluminum oxide (Al2O3), and silicon oxi-nitride (SiON). The in uence of growth conditions, such as growth rate and substrate temperature Tsub, as well as the ITO shoulders pro®le and ZnS moisture content, were analyzed to clarify the origin of image retention. Before environmental and electrical testing, each structure was stabilized by a burn-in procedure. The optical and SEM studies of cross-sectioned and layer-by-layer etched samples were used as basic failure and structural analysis techniques in this series of experiments. The layer-by-layer etching technique was used to clarify the position of the damage region in the stack. All reactants were properly prepared to attack only each individual layer, without touching the adjacent one. The image retention can be described as undesirable and unrecoverable changes in driven areas of the device, leading to the appearance of the remaining image in non-energized displays (Fig. 1). It was found that the image retention can be brought on by aggressive drive conditions, such as high voltage (V . 180 V for device with threshold voltage 130 V), square wave-form pulse, and=or high frequency (.1 kHz). The ®rst two drive parameters result in the current overshooting at the onset of the pulse. The high frequency of the addressing signal leads to additional stress on the insulator= semiconductor interfaces (ISI), as well as extra heat energy release. Fig. 2a shows the typical ACTFEL ITO electrode pro®le. We suggest that in the case of a steep pro®le, electrode edges are too sharp to provide a uniform thickness distribution of the phosphor layer. As a result, the ZnS layer becomes thinner along the column edges. This results in a higher electric ®eld

The study of aging mechanism in ZnS:Mn thin-film electroluminescent devices grown by MOCVD

Evidence from DLTS and FTIR measurements strongly supports the assertion that the degradation mechanism of ZnS:Mn ACTFEL devices is mainly due to the deep electron trap, E t, which comes from the Mn activators reacting with surface water molecules. The photoluminescence measurements reveal that the Mn-related E t trap behaves like a nonradiative center. As a result, poor brightness characteristics including lower brightness and a higher threshold voltage were obtained when samples become aged.

The study of crystallinity of ZnS:TbOF thin films and green thin‐film electroluminescent devices with different stacked insulating layers

Abstract— In order to improve the characteristics of thin‐film electroluminescent (TFEL) devices, the dependence of crystallinity on deposition conditions of ZnS:TbOF thin films deposited by rf‐magnetron sputtering have been studied. The optimal deposition conditions obtained are: rf power density, 1.54 W/cm2; substrate temperature, 175°C; pressure, 5 mTorr; and sputtering gas, pure Ar. Several types of ZnS:TbOF green ACTFEL devices with different stacked insulating‐layer structures were prepared and compared. The ACTFEL device obtained with the superior structure is ITO/SiO2/Ta2O5/ZnS:TbOF/SiO2/Ta2O5/Al. This result suggests that a high‐resistivity dielectric can be used as the second insulating layer adjacent to the phosphor layer and can improve the luminous characteristics of ACTFEL devices in this study.

High‐performance alkaline‐earth thiogallate blue‐emitting ACTFEL devices

Abstract— Significant improvement in the luminescent performance of SrxCa1−xGa2S4: Ce (0 &lt; × &lt; 1) EL devices was achieved by optimizing the Sr/Ca ratio and oxygen doping. The key factor for improvement is oxygen doping of thiogallate thin films. Microstructural analysis from x‐ray diffraction and transmission electron microscopy showed that oxygen doping increased crystal grain size and induced a preferred orientation along (602) planes. Photoluminescence studies showed the luminescence decay time increased with oxygen doping, indicating the suppression of non‐radiative energy transitions at structural defects, e.g., sulfur vacancies, due to improved crystalline quality of the thiogallate thin films. The emission color blue shift in oxygen‐doped Sr‐Ca mixed thiogallate was attributed to the preferential substitution of Ce3+ for Sr2+ rather than Ca2+ due to a better match of ion size and a possible ordering effect from lattice strain introduced by oxygen incorporation. The threshold‐field reduction and steeper turn‐on near threshold in oxygen‐doped devices was considered to be a result of the introduction of shallow trapping states at the insulator/phosphor interface and the elimination of bulk trapping states by oxygen doping.

Aging of ZnS:Mn ALE ac thin‐film electroluminescent devices

Abstract— The aging behavior of ZnS:Mn ACTFEL devices, driven with asymmetrical voltage waveforms, was analyzed. Samples were aged with asymmetrical waveforms, so a time‐averaged electric field exists in the phosphor layer during operation. Transient measurements were used to determine the voltage polarity dependence of the aging process. While with symmetrical aging and waveforms which cause a positive time‐averaged field the positive polarity is mainly affected, a negative time‐averaged field affects both polarities. A simple model to explain these measurements is proposed.

A method for evaluating the frequency characteristics of ac thin film electroluminescent devices

Although there are some obvious variances in both electrical and optical characteristics describing the frequency dependence in ACTFEL devices, a method for the quantitative analysis of frequency characteristics is still lacking. This paper presents a method for including the frequency dependence. We assume that there is a phase shift between the applied voltage and the equivalent current emitted from the phosphor-insulator interface. Combining the continuity equations and the tunnel current formula, we have obtained the voltage in the phosphor layer, the phase shift between the applied voltage and the tunnel current, the average current and the total amount of space charge in the phosphor layer at varying applied voltages and at different frequencies. Some physical parameters of the phosphor layer can be obtained by fitting the theoretical model proposed to the experimental results in the range of 500 Hz to 2000 Hz. The results are satisfactory.

Studies of a high frequency hysteresis phenomenon in ZnS:Mn actfel devices

A high frequency hysteresis in brightness has been discovered in all ZnS:Mn devices with suitable insulator layers regardless of Mn concentration. This phenomenon can be explained by the interface electron source model along with the relaxation process of electrons. It is estimated from the critical frequency that within the polarization field the relaxation time of trapped electrons at the I-S interface is on the order of 10 -4 second.

Comparison of ZnS: Mn AC TFEL devices prepared by manganese diffusion and coevaporation

The diffusion of Mn into ZnS from a thin Mn layers as diffusion source, was investigated as an alternative fabrication method for ZnS: Mn AC TFEL devices. Different device properties are obtained for devices diffused from the top or from the bottom of the ZnS layer. A comparison with coevaporated devices show the same dependence of brightness and efficiency from doping concentration. A maximum brightness of about 800 cd/m2 at 5 kHz square wave excitation has been achieved proving the feasibility of this method. However, this value is nearly one order of magnitude lower than the best reported results in literature for coevaporated devices. Further experiments have been carried out using TbF3 as dopant. A maximum brightness of about 4000 cd/m2 at the same excitation have been achieved.

TbF 3 complex centre in ZnS ACTFEL devices

From a comparison of the properties of ZnS: TbF 3 and Zn: Tb 3+ electroluminescence (EL) devices, we relate the better efficiency of the complex centre TbF 3 to a longer decay time, which corresponds to the fact that TbF 3 is less coupled to the matrix than Tb 3+. A brightness of 1000 cd m -2 and an efficiency of 0.23 lm/W are obtained, at 5 kHz, for an optical concentration of 3 mol%. This maximum efficiency, as well as the excitation and radiative efficiencies are discussed.

Study of highly concentrated ZnS:Mn ACTFEL devices

The electroluminescence (EL) of ZnS:Mn (ACTFEL) ac driven thin film electroluminescence devices shows that for Mn concentrations beyond 1 mol%, the characteristic yellow emission of the Mn2+ center gradually decreases, when red (R) and infrared (IR) emissions appear successively. Comparative studies of the temperature quenching of these emissions, and their excitation spectra in PL show energy transfer from excited Mn2+ to the centers associated with the R and IR bands. A discussion of the results leads to point out that the deexcitation process of Mn2+ centers can be related to a percolation problem. The nature of the R and IR centers is also discussed. Les spectres d'electroluminescence des dispositifs (ACTFEL) de ZnS:Mn montrent que lorsque la concentration de manganese croǐt a partir de 1%, l'emission caracteristique de Mn2+ diminue progressivement d'intensite, alors qu'apparaissent successivement des bandes d'emission rouge et infrarouge. L'etude comparative des declins de ces emissions B differentes temperatures, et celle de leur spectre d'excitation en photoluminescence, signalent l'existence de transferts d'energie des Mn2+ excites vers les centres responsables des bandes rouge et infrarouge. Une analyse plus poussee des resultats obtenus conduit a interpreter ceux-ci dans le cadre d'un modele de percolation. La nature des centres rouge et infrarouge est discutee.

Mechanisms of the negative-resistance characteristics in AC thin-film electroluminescent devices

A model describing the mechanism responsible for generating the negative-resistance phenomenon found in hysteretic ZnS:Mn ACTFEL devices is presented. This model, which is based on simple physics intuitions rather than elaborate mathematical and numerical analyses, outlines several essential device parameters that are necessary to induce the negative-resistance effect.