AC Thin-film Electroluminescent Devices 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.

CuBr blue light emitting electroluminescent thin film devices

AbstractVisible blue cathodoluminescence (CL), photoluminescence (PL), x‐ray excited optical luminescence (XEOL) and electroluminescence (EL) via Zf free excitonic emission have been obtained from CuBr thin films deposited on glass, Cr coated glass, GaAs and Si substrates. In addition to the Zf emission several peaks corresponding to Cu+ emissions via 3d94s ↔ d10 transitions were observed on the AC EL spectrum at 2.10, 2.7, 3.03, 3.07 and 3.80 eV. X‐ray diffraction (XRD) measurements confirmed that the vacuum evaporated CuBr thin films grow preferentially with a (111) orientation irrespective of the substrate. While the AC voltage source was found to have no detrimental effects on CuBr thin films, cathodic deposition of Cu metal via electrolytic decomposition was observed under steady state DC voltage stresses. The results from the various luminescence experiments suggest that CuBr thin films may be used as a phosphor layer in blue emitting AC thin film electroluminescent devices (ACTFELD) (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The Formation of Eu2 + Clusters in Saturated Red Ca0.5Sr0.5S : Eu Electroluminescent Devices

A high-luminance and high-color-purity inorganic phosphor has been developed for the red component in ac thin-film electroluminescent devices using electron-beam evaporation. The influence of the Eu concentration on the luminescence and response characteristic of the devices was studied. A high Eu concentration causes not only concentration quenching of photoluminescence and electroluminescence (EL) emission, but also increases the light-emitting response time of the EL devices upon the application of an electric field. Decay characteristics of the thin films point out the creation of nonradiative decay channels due to the formation the clusters at high Eu concentrations. The slow light-emitting response in EL devices was attributed to the presence of traps generated by these clusters. To understand the nature of these traps, the frequency and temperature dependence of the response time was studied.

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.

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.

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.

Characterization of BaTiO3 thin films on p-Si

Polycrystalline BaTiO3 thin films grown on p-Si (100) substrates by RF sputtering and subjected to a post-thermal annealing at 700°C, were characterized for potential applications as cladding insulators in AC thin-film electroluminescent devices. Building such a device requires the study of an insulator/semiconductor and an insulator/phosphor interface. The study of the BaTiO3 surface in the present work provides information on the creation of defects due to the deposition process. Rutherford backscattering spectroscopy and X-ray photoelectron spectroscopy were used to investigate the surface and bulk created defects. They showed the existence of Ba and Ti oxides formed on the surface probably due to the post thermal annealing. These oxides may be responsible for the creation of interface states between the BaTiO3 film and the subsequent deposited ZnS films in order to build an AC thin-film electroluminescent device.

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 ...

Color Control in Sulfide Phosphors: Turning up the Light for Electroluminescent Displays

Manipulation of the defect chemistry in ZnS:Mn and SrS:Cu bulk and thin-film phosphors has resulted in specific and controllable emission colors across the entire visible spectrum. Saturated red emission is observed at exceptionally low threshold voltages in ac thin-film electroluminescent devices, while unsurpassed levels of luminous efficiency are observed for devices operating at saturated green wavelengths.

Activator ionization and deep level ionization in SrS:Ce ac thin-film electroluminescent devices

This report focuses on the ionization mechanism of dynamic space charge formation during ac thin-film electroluminescent device operation. Laser excitation under operating condition is used to identify the ionization mechanism. It is concluded, that Ce3+ ionization and ionization of other deep levels are forming the dynamic space charge. The charge emission process is impact excitation in combination with tunnel emission. The Ce3+ excited state is more than 0.23 eV below the conduction band and the ground state of other deep levels is more than 3.6 eV below the conduction band.

Kinetics and aging in atomic layer epitaxy ZnS:Mn ac thin-film electroluminescent devices

The kinetics of the aging of atomic layer epitaxy ac thin-film electroluminescent devices was studied. In a first series of experiments, we aged devices at different temperatures from 50 to 190 °C, and measured the steady-state transferred charge versus voltage characteristics. From monitoring Q145 V, the charge transferred at 145 V, we could trace the relationship between Q145 V and the aging time. The aging process was found to be temperature dependent, and we could deduce an activation energy of 0.34 eV. In a second series of experiments, devices were aged 16 h at room temperature and subsequently heat treated at different temperatures from 250 to 450 °C. Monitoring again Q145 V, we found that the devices recover from aging following the relationship −krecot=ln[Q145 V/Q145 V(t=0)], where t is the heat treatment time. The recovery rate constant kreco was found to have an activation energy of 1.3 eV. In a last series of experiments we found the aging rate to be proportional with the transferred charge. Possibly aging is a process of defect creation at the interface near the substrate. For this creation thermal energy and the energy of the accelerated electrons are needed. The defects can be annihilated by heating the device above 350 °C.

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.

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.

Use of Di‐π‐cyclopentadienyl Manganese as a Dopant Source for ZnS in Metallorganic Chemical Vapor Deposition

We report the successful use of di‐π‐cyclopentadienyl manganese as a manganese dopant for ZnS films, fabricated by low‐pressure metallorganic chemical vapor deposition onto an indium‐tin oxide coated glass substrate. The substrate temperature was 225°C. The concentration of manganese in the films was studied as a function of a bubbler temperature. The crystallinity and morphology of the films were examined by x‐ray diffraction and atomic force microscopy. The grain size was 43 nm, as evaluated by the Debye‐Scherrer relation. ZnS:Mn ac thin film electroluminescent devices with a double insulating layer structure were prepared. The luminance exceeded 1500 cd/m2, which is higher than the previously reported value of 680 cd/m2, employing the same Mn source.

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.

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.

Electron ejection processes at insulator-semiconductor interfaces in ACTFEL display devices

Luminance, conduction current and threshold voltage of ZnS:Mn ac thin film electroluminescent display devices were measured as functions of device temperature (10 K-300 K) and risetime of the excitation voltage pulse (2 /spl mu/s-50 /spl mu/s). Results provided insight into the electron ejection mechanism at the insulator-phosphor interfaces. It was found that the distribution of interface state electrons at the beginning of the excitation voltage pulse varied substantially with device temperature. Pure tunneling is thought to be the dominant electron ejection mechanism at the beginning of the voltage pulse while phonon-assisted tunneling is responsible for altering the interface electron distribution during the interval between the pulses. A delay of several microseconds was observed in the build up of the transferred charge. It is attributed to the relatively small population of electrons available at the insulator-phosphor interface. >

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>