Ac Thin Film Electroluminescent Research Articles

Review Article: Atomic layer deposition of optoelectronic materials

Optoelectronic materials can source, detect, and control light wavelengths ranging from gamma and x rays to ultraviolet, visible, and infrared regions. Optoelectronic devices are usually systems that transduce electricity to optical signal or vice versa. Optoelectronic devices include many modern necessities such as lamps, displays, lasers, solar cells, and various photodetectors. Some important research topics in the field of optoelectronics materials are development of new materials, new technologies for fabricating materials, and design of device structures. Atomic layer deposition (ALD) is a technology that was developed in the early 1970s for manufacturing high-quality luminescent and dielectric films to be used in AC-driven thin film electroluminescent (TFEL) displays. Monochromic yellow-black displays based on a ZnS:Mn luminescent layer have been manufactured industrially using ALD since the mid-1980s. Multicolor displays (green-yellow-red) were successfully realized by filtering the broad emission band of ZnS:Mn or adding another luminescent material, e.g., green-emitting ZnS:Tb or SrS:Ce. However, applicable full-color AC TFEL devices could not be developed because of the lack of an efficient deep blue-emitting phosphor. Currently, the most promising application area in TFEL displays is transparent displays, which are commonly used in various vehicles. In the mid-1980s, epitaxial III-V semiconductors were studied using ALD. It was shown that manufacturing real epitaxial [atomic layer epitaxy (ALE)] films is possible for different III (Al, Ga, In) and V (N, P, As) materials. The advantages of ALE processing compared to more traditional metalorganic chemical vapor deposition or molecular beam epitaxy methods have remained low, however, and ALE is not used on a large scale. Research continues to be carried out using ALE, especially with nitride films. Thin film solar cells have continuously received attention in ALD research. ALD films may be used as both an absorber (CdTe, SnS) and a passivation [In2S3, Zn(O,S)] material. However, in the solar cell field, the real industrial-level use is in passivation of silicon cells. Thin ALD Al2O3 film effectively passivates all types of silicon cells and improves their efficiency. Transition metal dichalcogenides are emerging 2D materials that have potential uses as channel materials in field-effect transistors, as well as phototransistors and other optoelectronic devices. The problem with achieving large-scale use of these 2D materials is the lack of a scalable, low-temperature process for fabricating high-quality, large-area films. ALD is proposed as a solution for these limitations. This review covers all of these ALD applications in detail.

Electrical and Optical Characteristics of Bi4Ti3O12 Thin Film on Indium–Tin-Oxide/Glass

Thin films of bismuth titanate are deposited on indium–tin-oxide (ITO)/glass substrates by RF magnetron sputtering for 60 min at room temperature using a Bi4Ti3O12+4 wt % Bi2O3 ceramic target, and they are annealed at various temperatures ranging from 600 to 725 °C by a rapid thermal processing for 10 min. The experimental results indicate that the Bi4Ti3O12 films annealed at 675–700 °C exhibit superior electrical characteristics compared to those of other ferroelectric materials such as BaTiO3, PbTiO3, and SrTiO3 films, including a higher dielectric constant, a higher polarization charge density, and a more stable leakage current density. In addition, a high optical transmittance of the film was obtained at an annealing temperature of 650 °C. The results suggest that Bi4Ti3O12 thin film is a candidate worthy of consideration as the insulating layer material of AC thin film electroluminescence devices.

The influence of source powder composition on the electroluminescence of Ca 1− xSr xS:Eu thin films

CaS:Eu has been studied as a red emitter for AC thin film electroluminescent (ACTFEL) devices in the beginning of the 1990s. This phosphor has not yet been used for practical display purposes because of the limited luminance and the slow response of the devices. Part of the CaS:Eu emission spectrum is situated at wavelengths longer than 650 nm where the eye sensitivity is very low. The partial replacement of the Ca-ions by Sr-ions causes a shift of the Eu emission to shorter wavelengths and thus an increase in the apparent luminous efficiency. To obtain a high luminance and a good temporal response, very homogeneous, crystalline and pure Ca 1− x Sr x S:Eu thin films were found to be necessary. Several powder preparation techniques were applied to mix CaS and SrS powders on a microscopic scale, in order to achieve a single Ca 1− x Sr x S:Eu phase. The overall phase distribution was determined by θ−2 θ X-ray diffraction (XRD) measurements and by assessing the photoluminescent properties of the Eu-activated powders. Scanning electron microscopy (SEM) images were useful in the determination of the grain size and the porosity of the material. The combination with energy-dispersive X-ray analysis (EDX) allowed mapping of the microscopic distribution of Ca and Sr. A single-phase of Ca 1− x Sr x S:Eu could be obtained using wet chemical synthesis, which considerably improved the luminance of the corresponding ACTFEL devices.

An Analytical Model for Electron Transport and Luminance in SrS:Cu,Ag ACTFEL Display Devices

An analytical model is developed to explain electron transport and luminance mechanisms in SrS:Cu,Ag ac thin film electroluminescent (ACTFEL) display devices. The model includes shallow and deep interface states, bulk traps, and impact excitation and ionization of activators. Mathematical expressions describing optoelectronic processes in the phosphor layer and interface states are written and numerical solutions for field, current and luminance are obtained. Results of calculations are compared with experimental data. The model is able to simulate the dominant features of the experimental luminance and current waveforms.

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.

Electron energy distribution rebuilding at the insulator–semiconductor interface in AC thin film electroluminescent and field emission display devices

Rebuilding the electron distribution at insulator–semiconductor interface in electronic devices by the addition of a metal interlayer is proposed. Electron emission from the insulator–semiconductor interface is investigated by measuring the transmission currents and threshold voltage of Mo/Ta 2O 5/(Metal)/ZnS/Au thin film devices when triangular voltage waveform is applied. When a metal interlayer is added, the transmission current curve does not exhibit the typical shoulders of insulator–semiconductor interface. This behavior reveals that a metal interlayer can restrict the electron energy distribution to a certain energy level. In addition, for devices with a metal interlayer, the threshold voltage for electrons to inject into the semiconductor layer decreases considerably.

Dynamic space charge mechanism in SrS:Ce ac thin-film electroluminescent devices

Dynamic space charge in SrS:Ce ac thin-film electroluminescent (ACTFEL) devices is not a SrS host property. In SrS:Ce ACTFEL devices, a lack of charge injection from interface states leads to space charge formation and to dynamic space charge formation. The dynamic space charge is formed by the impact ionization of Ce and of Ce induced deep level traps. The crystal field symmetry of the Ce3+ site determines its ionization probability. Ce3+ sites of cubic crystal field symmetry have a lower ionization probability than Ce3+ sites with axial crystal field symmetry. A prerequisite for dynamic space charge, i.e., short lived space charge, is the annihilation of space charge. The charge responsible for space charge annihilation originates from trap levels with an activation energy of about 9 meV. The temperature dependence of the annihilation of the space charge is responsible for the observed temperature dependence of the dynamic space charge formation, the conduction charge, and the quantum efficiency.

Growth and characterization of Zn1-xMgxS thin films for electroluminescent applications

\(Zn_{1 - x} Mg_x S\):Mn thin films (x=0–0.30) were prepared by thermal co-evaporation of ZnS, Mg and Mn. The structural investigation shows the solid solution is formed in the Mg composition range x=0–0.25 and phase segregation occurs at higher Mg composition (x > 0.25). The optical band gap shows an increase with increase of Mg composition. The electroluminescent emission corresponding to the transition \(Mn^{2 + }\) ion shows a blue shift with increase of Mg composition in the film. \(Zn_{1 - X} Mg_x S\):Mn films could be used as an active layer in a.c. thin film electroluminescent (ACTFEL) devices for obtaining green emission color.

Modeling of electron transport and luminance in SrS:Cu,Ag ACTFEL display devices

Transient and steady state responses of luminance and current in SrS:Cu,Ag ac thin film electroluminescent (ACTPEL) display devices were measured and analyzed. The results indicated the importance of bulk space charge, which was attributed to ionized activators in the bulk phosphor layer. For dented voltage pulse excitation, four luminance peaks were observed. These were labeled as the leading edge (LE) peak, dent peak, beginning of trailing edge (BTE) peak and the middle of trailing edge (MTE) peak. Ionized activators play an important role not only in affecting the net phosphor field but also in the device luminance during the trailing as well as the leading edges of the voltage pulse. It is shown that the dent peak and the BTE are due to a bulk recombination process instead of the backflow from the anodic interface. The luminance behavior of the device can be understood in terms of the bulk dipoles in the phosphor layer. A bulk dipole consists of a positively charged ionized Cu activator and a daughter trap (negatively charged) in its vicinity. A physical model for various optoelectronic processes is presented. The model can explain experimental data in a qualitative fashion.

Characterisation of the BaTiO 3/p-Si interface and applications

Barium titanate (BaTiO 3), because of its high dielectric constant ( ε r), has proven to be a very promising candidate for use as dielectric layer in ac thin film electroluminescent (ACTFEL) devices and for use in thin film hybrid and integrated circuits. In the present work, BaTiO 3 films were deposited on p-Si (100) substrates by rf-magnetron sputtering at a base temperature of 200°C. The electronic properties of the BaTiO 3/p-Si interface were examined by means of admittance spectroscopy on metal–insulator–semiconductor (MIS) devices fabricated by thermal evaporation of Al. The density of interface states ( D it) was calculated by both the capacitive and the conductive response of the traps; values of the order of 10 12 eV −1 cm −2 were obtained for the D it and values of 10 −5 s were calculated for the relevant time constants of the traps. These values, together with the dielectric constant of the films ranging between 40 and 60, show that the deposited films were suitable for use as cladding insulators in ACTFEL devices.

Properties of barium titanate (BaTiO3) thin films grown on silicon by rf magnetron sputtering

Abstract Thin films of BaTiO3 were deposited on p-Si substrates by rf magnetron sputtering in order to investigate their suitability for use in ac thin film electroluminescent (ACTFEL) devices and dynamic RAM (DRAM) applications. Post-growth annealing at 700°C and the subseauent deposition of Al contacts resulted in the creation of Al/BaTiO3/p-Si metal-insulator-semiconductor devices. The electronic and structural properties of the films were examined by admittance spectroscopy, current-voltage and transient current measurements, and X-ray diffraction (XRD) characterization. Analysis of the XRD spectra showed the polycrystalline nature of the films but also the presence of an amorphous phase. The electrical measurements revealed a high dielectric constant, around 60, a charge storage capacity exceeding 3 μC cm−2 and a total charge trapped inside the oxide of around 50 nC cm−2 while the density of traps at the BaTiO3/p-Si interface was found to be as high as 1 × 1012 cm−2 eV−1. These results indicate that ...

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.

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.

Current crowding and phosphor efficacy in ac thin‐film electroluminescent display devices

Abstract— The variation of luminance with transferred charge in ZnS:Mn ac thin‐film electroluminescent (ACTFEL) display devices was analyzed to determine the roles played by the preferred current paths (filamentary conduction) and manganese clustering in limiting the phosphor efficiency. Luminance‐charge (L‐Q) characteristics were measured in ZnS:Mn ACTFEL devices of varying Mn concentrations. L‐Q characteristics showed a reduction in slope at higher values of Q. Also, the value of Q where the slope changed varied with the Mn/Zn ratio. These effects were attributed to the existence of preferred current paths in the phosphor layer where hot‐electron flow clustered.

Etching of AC thin film electroluminescent devices

Realization of high definition ac thin film electroluminescent (ACTFEL) devices require fine control of geometric definition. Suitable etching processes need to be defined to address the requirements of both isotropic and anisotropic etching profiles, with good etching rates and high selectivity to underlying Si features. Three different etching processes are analyzed and compared with reference to these parameters. Wet chemical processing provides high etch rates and high selectivities for the ACTFEL materials. Using phosphoric acid leads to an infinite selectivity to all ACTFEL materials. Unfortunately, wet chemical etching has poor control for step profiles. Ion beam etching and sputter etching, however can produce both isotropic and anisotropic patterns, but are comparatively slow and suffer from poor selectivity to the Si based materials. Although individually no method is ideal, for full ZnS:Mn ACTFEL device fabrication these results show that a combination of two of these etching processes will provide the necessary geometry's for very high definition structures.

Electroluminescence in thin films

The three most important technologies for future display applications where electroluminescence (EL) in thin films is used for light generation are discussed. These technologies are firstly, Alq- and PPV-based organic light emitting diodes (LED); secondly, inorganic a-SiC LED and thirdly, ac-driven thin film electroluminescent devices based on II–VI compounds. The last of these is of the high-field EL-type whereas the first two utilise low-field injection type EL. The basis principles and the current status of the respective technologies are summarised. Recent developments in ac thin film electroluminescence for full colour display applications are emphasised.

TFEL optoelectronic integrated circuit on Si

A unique optoelectronic integrated circuit fabricated with AC thin film electroluminescent (TFEL) devices directly onto the drain of an Si DMOS transistor is demonstrated. DMOS switching controls the high voltage ratio between the TFEL device and DMOS transistor. Active matrix addressing for electroluminescent devices is demonstrated using CMOS circuitry.

Enhancing the brightness of thin-film electroluminescent displays by improving the emission process

AC thin film electroluminescent (ACTFEL) devices emit light laterally in addition to conventional surface emission. Because of the waveguiding in ACTFEL devices, lateral emission is stronger than surface emission. However, the former is ignored for displays. This note demonstrates that mirrors can be fabricated into substrates to reflect upward the lateral emission, which can then be summed with surface emission for displays. Use of standard silicon and thin film technology has demonstrated that the reflected lateral emission is a factor of three brighter than conventional surface emission. hence the effective efficiency of out-coupling the light has been improved for an ACTFEL device.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Electron energy distribution at the insulator-semiconductor interface in AC thin film electroluminescent display devices

Electron emission from insulator-semiconductor interface in ZnS:Mn ac thin film electroluminescent (ACTFEL) display devices was investigated by studying the current and field waveforms. A new technique for measuring the interface electron energy distribution at insulator-semiconductor interfaces was developed. The technique involves the measurement of tunnel current transients and can be used to study the interface electron energy distribution between any insulator-semiconductor pair with which an ac thin film test structure can be fabricated. It was applied to a ZnS:Mn display device at two temperatures of 10 K and 300 K and to a metal interface device structure.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>