AC Thin-film Electroluminescent Devices Research Articles

High efficiency ZnS:Mn ac thin film electroluminescent device structure

A method for improving the luminous efficiency of a ZnS:Mn ac thin film electroluminescent device by modifying the structure to include thin, 100 Å, barrier layers of a Y2O3 is presented, with experimental comparisons of devices showing a fourfold increase in efficiency. The results are discussed in terms of electron tunneling and field redistribution as the mechanisms responsible for the enhancement of efficiency.

Current Density-Voltage Characteristics of AC Thin-Film Electroluminescent Devices with Different Dielectric-Phosphor Interfaces

Four kinds of AC thin-film electroluminescent devices constructed with different insulating layers were prepared to study the I-V characteristics. In this study, we evaluated the I-V characteristics from the charge density-voltage (Q-V) characteristics which are measured using a Sawyer-Tower circuit and a simple series capacitor model. The results of these two methods are very similar to each other. From the I-V characteristics of these samples, a simple conclusion can be drawn: the sample with higher brightness and efficiency has higher current density. The relationships between brightness and transferred charge (B-Q), brightness per unit transferred charge and transferred charge (B/Q-Q), and the differential value of ΔB/ΔQ and transferred charge (ΔB/ΔQ-Q) of these samples have been discussed. From these results, we can conclude that the interface between the low-resistivity insulating layer and the active layer can provide the more efficient electrons for exciting the luminous centers.

Evidence of donor center involvement in ac thin film electroluminescent devices exhibiting memory

Alternating current thin film electroluminescent (EL) devices exhibiting memory with low Mn concentrations have a temperature independent threshold, but a temperature dependent saturation brightness and memory margin. Based upon these observations we argue that EL emission is initiated via the usual tunneling process from interface states, but subsequent characteristics are determined via a second carrier generation process. Shallow donor sites are calculated to exist at a depth of 0.12 eV below the conduction band. These are believed to be involved in the second carrier generation process since brightness-voltage characteristics are associated with Poole–Frenkel emission.

Memory effect in ZnS:Mn AC thin-film electroluminescent devices with low Mn concentration

Electroluminescent (EL) memory is exhibited in ZnS:Mn AC thin-film EL (ACTFEL) devices with Mn concentrations between 0.2 and 0.7 wt.% (+or-0.2%). Maximum hysteresis width is observed for Mn concentrations of 0.3 wt.% (+or-0.2%), compared to 1 wt.% for previous devices exhibiting memory. The phenomenon is seen in Zn-rich, but not in S-rich ZnS:Mn films. Low-field electrical characterization has previously shown the presence of shallow donor sites in Zn-rich ZnS at 0.11 eV below the conduction band. Electron ejection of donors near the interfaces is believed to sustain electroluminescence at voltages below threshold. The trap is thought to be created by an excess of S vacancies (donor sites) over the trapping or recombination centers. >

Phosphor currents in ZnS:Mn ac thin film electroluminescent display devices

Conduction current in the phosphor layer of ZnS:Mn ac thin film electroluminescent (ACTFEL) display device was measured as a function of the amplitude and the rise time of the bipolar voltage pulses: electric field in the phosphor was also determined. It was found that the clamping field of an ac thin film electroluminescent device is not a single-valued device characteristic. The clamping field depended upon the rise time of the applied voltage pulse increasing in value as the rise time became smaller. These data were interpreted in terms of a nonideal breakdown of the insulator-phosphor interface and a time delay involved in the emission of electrons from this interface. To account for the nonideality in the interface breakdown the ac equivalent circuit of the ACTFEL device was modified by including a voltage-dependent resistor in parallel with the phosphor capacitance. The phosphor current measurements, transferred charge measurements and tunneling current calculations indicate that in the nonhysteretic ZnS:Mn devices under investigation, impact ionization in the phosphor layer, if present at all, leads to relatively small carrier multiplication factors.

A model for electroluminescence in SrS:Ce ACTFEL display devices

An analytical model for electroluminescence in SrS:Ce AC thin-film electroluminescent display devices is presented. The model incorporates an exact calculation of the electric field and the effect of activator ionization and bulk traps. Activator ionization is needed to explain several features of luminescence behavior in SrS:Ce devices. These features include the second luminescence peak at the trailing edge of the voltage pulse and the time lag between the luminescence and the applied voltage when the applied voltage consists of bipolar trapezoidal pulses and rectangular pulses of low voltage amplitudes. As a mechanism for the ionization of activators, field-assisted tunneling is shown to be more likely than impact ionization by hot electrons. Physical processes are described in terms of rate equations, and field, current, and luminescence waveforms are calculated for one set of device parameters. The calculated and experimental luminescence waveforms agree. >

Electric field and conduction current in ac thin-film electroluminescent display devices

Current flow through the phosphor layer of ac thin film electroluminescent display devices has been analyzed. It is shown that the traditional capacitive bridge technique for measuring the time dependence of phosphor conduction current [iLP(t)] can yield inaccurate results, especially when the multiplication factor of [(CI + Cp)/CI] is not used, and the phosphor layer has bulk space charge; CI and Cp are insulator and phosphor layer capacitances, respectively. A new method for the measurement of iLP(t) has been developed. It is more accurate and is applicable even when the bulk charge in the phosphor layer is not negligible. The new method was applied to the study of ZnS:Mn ac thin-film electroluminescent (ACTFEL) devices excited by voltage pulses of three different slew rates. iLP(t) measured by the new method was compared against the bridge current (ibridge). Electric field and displacement current in the phosphor were calculated. Transferred charge computed by integrating iLP and ibridge over half cycle was compared against the polarization charge obtained from the Q–V curve of the device.

Effects of insulating layer on the performance of thin film electroluminescent devices

Thin film electroluminescent devices were fabricated with active layer of ZnS:Mn and different insulators viz Sm2O3, Eu2O3, Na3A1F6, MgF2, CeO2 and SiO in MIS and MISIM structure. The threshold voltage for light emission in AC thin film electroluminescent devices of MIS and MISIM structures is found to depend on the dielectric properties of insulating materials. The observed threshold voltage for these devices and its variations for devices with different insulators are explained using the equivalent circuit for the device and the dielectric properties of the insulting material used for the preparation of device. Variation of threshold voltage with operating time is also studied for some of the devices.

Low-voltage driven ZnS:Mn MIS and MISIM thin film electroluminescent devices with Eu2O3 insulator layer

AC thin film electroluminescent devices of MIS and MISIM have been fabricated with a novel dielectric layer of Eu2O3 as an insulator. The threshold voltage for light emission is found to depend strongly on the frequency of excitation source in these devices. These devices are fabricated with an active layer of ZnS:Mn and a novel dielectric layer of Eu2O3 as an insulator. The observed frequency dependence of brightness-voltage characteristics has been explained on the basis of the loss characteristic of the insulator layer. Changes in the threshold voltage and brightness with variation in emitting or insulating film thickness have been investigated in metal-insulator-semiconductor (MIS) structures. It has been found that the decrease in brightness occurring with decreasing ZnS layer thickness can be compensated by an increase in brightness obtained by reducing the insulator thickness. The optimal condition for low threshold voltage and higher stability has been shown to occur when the active layer to insulator thickness ratio lies between one and two.

On the measurement of conduction current in AC thin-film electroluminescent display devices

Estimates are obtained for errors associated with the use of a capacitive bridge method for the measurement of the conduction current-time waveform in AC thin-film electroluminescent display devices. The error is a function of the shape and the slew rate of the excitation voltage pulses and can be quite large. In general, the bridge method underestimates the value of conduction current. An improved technique for measuring the current waveform is outlined. The proposed method is more accurate and more generally applicable.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Diamond-like carbon films for electroluminescent applications

Diamond-like carbon (DLC) films grown by plasma-enhanced chemical vapor deposition are employed for the first time as an active layer in an alternating current (a.c.) thin-film electroluminescent (ACTFEL) device configuration. The ACTFEL device configuration consists of the DLC active layer sandwiched between two insulating layers which are contacted by a reflecting and transparent electrode. A large a.c. waveform with a small duty cycle is applied with a sufficiently large amplitude so that the DLC layer electrically breaks down; the insulators prevent this breakdown from being catastrophic. Electroluminescence arises from radiative recombination of electrons and holes via bandtail-to-bandtail states. This type of luminescence is inherently very broad band and the electroluminescence appears white. We have been able to produce electroluminescent emission well into the UV region of the electromagnetic spectrum so that these devices are potentially useful for visible and UV applications. The brightness and efficiency of our devices are very low to date: 0.5 ft lambert and 0.002 lm W -1.

Effects of Mn effusion cell temperature on the film properties and on the AC electroluminescence of molecular beam deposited ZnS:Mn

The AC thin-film electroluminescent (ACTFEL) devices were prepared by using 1 μm-thick ZnS:Mn luminescent layers formed by molecular beam deposition (MBD). The devices had a structure of glass/In 2O 3/barium-titanate/ZnS:Mn/Al. The EL properties of the device are remarkably dependent on the temperature of a Mn effusion cell ( T Mn) during the ZnS:Mn deposition. Although the crystalline quality of the layers was poor and the surface morphology was rough, a low threshold field ( E th) of 0.82×10 6 V/cm, and a high maximum luminance ( L max) of 4200 cd/m 2 were obtained at 5 kHz sinusoidal operation. The ACEL properties were dependent not only on the crystallographic quality but also on the Mn concentration in the luminescent layer.

A computationally simple model for hysteretic thin-film electroluminescent devices

A temporal model based on the Howard, Sahni, and Alt model (1982) that describes hysteresis, the transient and the steady-state values of the interface state charge, and the trapped bulk charge in an AC thin-film electroluminescent device is developed. A fast and efficient numerical procedure that calculates the steady-state hysteretic state of a ZnS:Mn thin-film device is developed for the above model. Graphs of device current and luminance that result from the above model are given. >

A Study of the Crystallographic and Luminescent Characteristics of ZnS:Mn Films Prepared by an RF Magnetron Sputtering Method for AC Thin-Film Electroluminescent Devices

Characteristics of ZnS:Mn films prepared by an rf sputtering method for electroluminescent devices were examined in detail; the features were clarified by a comparison with films produced by an electron-beam method. It was found that the incident species to the substrate during sputtering were composed of ZnS, Zn2, Zn2S, MnS and ZnMn in addition to Zn and S atoms; in the case of the electron-beam method, they were composed of Zn and S2. We confirmed that preparative, crystallographic, microstructural , chemical, and eventually electroluminescent characteristics in sputtered ZnS:Mn films were different from those in electron-beam films, according to the fundamental difference of the incident species to the substrate, which affected film growth.

New AC thin-film electroluminescent device with frequency-controlled brightness characteristics

A novel thin-film electroluminescent device having a brightness/voltage characteristic depending strongly on the frequency of the excitation voltage is described. It is essentially a ZnS:Mn thin-film EL device of double-insulating-layer structure with a new dielectric layer of frequency-dependent impedance characteristic as the insulator.

Monolithic Multicomponents Ceramic (MMC) Substrate

AbstractNew monolithic multicomponents ceramic (MMC) substrates were made using a glass-ceramic material, low firing high dielectric constant ceramic material, metal oxide resistance materials, piezoelectric ceramic material and metal conductors. In the MMC substrate, many passive components, such as capacitors, resistors and wiring conductors can be included. In addition, designed-space can be formed in the substrate with new designed-space forming technology.The new MMC substrates have the following excellent characteristics. 1)A wide range of capacitors can be formed in the substrate.(10pF˜3μF).2)A wide range of resistors can be formed in the substrate.(10Ω/π∼1MΩ/π).3)A plurality of resistor and capacitor elements can be three-dimensionally constructed in the substrate.4)Designed-space can be formed in the substrate.5)Miniaturization and cost reduction can be accomplished in the substrate.The MMC substrates were applied to voltage controlled crystal oscillators (VCXO), CR active filters, memory cards for personal computers, AC thin film electroluminescent (AC TFEL) devices and drop-on demand ink jet heads.These devices using MMC substrate showed good electrical propeties and achieved miniaturization, multifunction and cost reduction. The new monolithic multicomponent ceramic substrate can be applied to many kinds of hybrid microcircuits and electronic components and devices.

A new multicolor electroluminescent device

A new multicolor ac thin-film electroluminescent device is hereby proposed. This device differs from usual devices in the structure of its active layer, i.e., the additional layer (‘‘gate layer’’) is buried in ZnS active layers. Thus, the structure of the device is a glass substrate: SnOx-Y2O3-ZnS:SmF3-gate layer-ZnS:ErF3-Y2O3-Al. In particular, when the gate layer is a thin SmF3 film (approximately 500 Å) or an extremely thin Al film (approximately 100 Å), the emission color of the device continuously varies between red and green with the driving voltage or frequency.

Efficiency and Saturation in AC Thin Film EL Structures

The maximum achievable brightness of ac thin film electroluminescent devices is determined by the maximum transferable charge and the maximum efficiency of electroluminescence η. The first is controlled by the charge at breakdown of the insulator, whereas the latter is determined by the properties of the semiconductor layer. By a factorization into partial efficiencies η = ηexcηoptηlum characterizing the three independent fundamental processes involved in these structures, (i) impact excitation of centers, (ii) radiative decay of excited centers, and (iii) outcoupling of emitted photons, some insight ino the limiting physical mechanisms is obtained. The efficiency in ZnS : Mn is limited via ηlum by an influence of non-radiative decay channels which increases with the Mn concentration N and the concentration of excited centers N* In the concentration range (N) of practical interest ηexc is shown to be controlled by σMn2+N, where σMn2+ is the impact cross-section of Mn2+. The limiting factor is the cross section which is determined by a very straight-forward evaluation from experimental data to be σMn2+ = 3.7 × 10−16 cm2. The time behaviour of ηexc within the excitation pulse is measured with a submicrosecond resolution and the results give some hints on spatial inhomogeneities — grown-in and operational ones. Numerical estimates of the maximum achievable brightness show that there is not much improvement possible within the ZnS: Mn system using present day insulator technology.

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.

How the ZnS:Mn layer thickness contributes to the performance of AC thin-film EL devices grown by atomic layer epitaxy (ALE)

Luminance, efficiency, threshold voltage, and voltage polarity dependence of the light emission have been investigated as a function of the ZnS:Mn layer thickness in ac thin-film electroluminescent (TFEL) devices grown by Atomic Layer Epitaxy (ALE). The crystallinity of the ZnS:Mn layer was studied by X-ray diffraction. The layer of poor crystallinity is about 35 nm and coincides with the dead layer observed in luminance. Luminance is nearly proportional to the ZnS:Mn layer thickness and might indicate a homogeneous excitation throughout the layer at high voltages.