Ce Thin Film Electroluminescent Devices Research Articles

Rb添加によるSrS:Ce薄膜EL素子の青色純度とEL動作の安定性の改善(材料・デバイス連合研究会)

Rb添加によるSrS:Ce薄膜EL素子の青色純度とEL動作の安定性の改善(材料・デバイス連合研究会)

Effects of rapid thermal annealing on electron beam evaporated SrS : Ce thin film electroluminescent devices made in H 2S ambient

Possible device structures for using rapid thermal annealing (RTA) for alternating current thin film electroluminescent (ACTFEL) devices have been evaluated. SrS : Ce ACTFEL structures have been made on a highly doped silicon substrate, which is used as supporting electrode. RTA was performed on these EL devices and the effect of different annealing temperature, annealing time and of ZnS buffer layers was investigated. Besides an increase in total brightness, a change in crystallinity and in device characteristics was seen. Time resolved optical measurements could be explained by assuming the role of interface states. Composition of the layers and diffusion phenomena have been investigated, using X-ray photo-electron spectroscopy. At annealing temperatures above 700°C, the Ce ion becomes mobile. The possible physical processes behind these effects have been discussed.

Influence of the electrical excitation pulse shape on the electro-optical response in SrS:Ce3+ electroluminescent devices

The electro-optical properties of SrS:Ce thin film electroluminescent devices prepared by atomic layer epitaxy have been studied by varying the shape of the electric excitation pulse. The different electric and optical responses have been explained by considering two sets of states at each interface. The shallower set of states will be populated by the electrons originating from the deeper states when the electric field is turned on. Relaxation of the carriers from the shallower states towards the deeper ones modifies the relative intensity of the different optical responses according to the shape of the excitation pulse.

Study of dynamic space charge in SrS:Ce thin-film electroluminescent devices by tunable laser excitation

The origin of the dynamic space charge in SrS:Ce ac-driven thin-film electroluminescent devices is studied using photoinduced current measurements. Field assisted photoionization of Ce3+ centers and of intrinsic electron traps is observed. The results show that about 50% of the dynamic space charge in SrS:Ce originates from cerium ionization. The remaining 50% results from the ionization of crystal imperfections.

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.

Numerical simulation of charge transfer and light emission in SrS:Ce thin-film electroluminescent devices

When an AC voltage is applied to a thin-film electroluminescent device with an SrS:Ce phosphor layer, the current flowing through the phosphor layer and the emitted light can be complex functions of time. In order to explain the observed current and light waveforms, a numerical model has been constructed for the electrical and optical behavior of such a device. This model is based on the tunneling of electrons from the phosphor-insulator interface, multiplication of the electron current and creation of positive space charge in the high field region, recombination of electrons with the positive charges in the low-field region, and a back-flow of electrons from the anodic interface when the field there changes sign. The rather simple model is able to simulate a number of features observed in current and light characteristics quite well. It could serve as a reference for the interpretation of new experimental results.

Role of space charge in SrS:Ce thin-film electroluminescent devices studied by the photoirradiation effect

The role of space charge in SrS:Ce thin-film electroluminescent devices was studied by photoirradiation effects on capacitance–voltage and charge–voltage curves. Unique electrical and optical characteristics, such as relaxation of phosphor field and leading and trailing edge emission, are well interpreted by the model based on the formation of uniform space charge. The density of ionized traps, which cause positive space charge, was estimated to be 5.6×1016 cm−3. The polarization charge of 0.94 μC/cm2 due to the space charge is more than half of the total polarization charge.

Improvement of crystallographic and electroluminescent characteristics of SrS:Ce thin film devices by post-deposition annealing in Ar-S atmosphere

Post-deposition annealing in an Ar-S atmosphere at atmospheric pressure has been demonstrated to improve crystallographic properties and electroluminescent (EL) characteristics of SrS:Ce thin film EL devices. Crystallinity and degree of the orientation of the SrS:Ce thin films with polycrystalline columnar grains are improved by the annealing. It seems that recrystallization takes place between SrS grains. For the annealed devices, charge generation in the SrS:Ce layer is suppressed and relaxation of the phosphor field is decreased. As a result, an instantaneous EL efficiency at the leading edge emission is enhanced. The EL luminance and the average EL efficiency are increased. The annealed SrS:Ce thin-film EL device showed a luminance of 800 cd/m2 and an efficiency of 0.42 lm/W at 1 kHz drive.

Space charge and light generation in SrS:Ce thin film electroluminescent devices

Transient light and charge-voltage measurements on SrS and SrS:Ce thin film electroluminescent devices have been carried out with triangular voltage bursts after illumination. The observations indicate that the trailing edge emission is due to electrons which are emitted from the anodic interface as soon as the field there changes sign and recombine effectively with ionized Ce atoms. From charge-voltage measurements the space charge in the phosphor layer is estimated to be two times higher for the Ce doped sample. Due to this sample charge the field in the phosphor layer of the SrS:Ce devices decreases dramatically from the cathodic toward the anodic interface. If the field at the anodic interface is close to zero, efficient light emission will arise from recombination of electrons with ionized Ce atoms in this region.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The influence of Se-coevaporation on the emission spectra of CaS:Eu and SrS:Ce thin film electroluminescent devices

In order to reduce the number of sulphur vacancies in evaporated CaS:Eu and SrS:Ce electroluminescent thin films, Se-coevaporation can be performed. This process not only improves the brightness of the devices, but also induces considerable changes in the emission spectra. The shift of the electroluminescent spectra upon coevaporation was checked against semi-empirical molecular cluster energy calculations. In the case of SrS:Ce, these spectral changes are seen to be entirely due to the effect of electron-phonon interactions, while for CaS:Eu, only crystal field effects play a role.

Relaxation of phosphor field in SrS:Ce thin film electroluminescent devices

Capacitance-applied voltage and internal charge-phosphor field characteristics of SrS:Ce thin film electroluminescent devices have been studied. The average phosphor field of the SrS:Ce films reaches the maximum of 1.3 MV/cm and relaxes to 0.7 MV/cm, although the applied voltage increases. The turn-on voltage of the SrS:Ce thin film electroluminescent device decreases nonlinearly with increase of conduction charge and depends on the slew rate dV/dt. A large spike current is observed and is thought to correspond to the charge generation in the SrS:Ce layer, which results in the space charge, and causes a relaxation of phosphor field in the SrS:Ce films.

SrS:Ce Thin-Film Electroluminescent Devices Fabricated by Post-Annealing Technique and Their Electrical Properties

A postannealing technique has been used to make double-insulating SrS:Ce thin film electroluminescent (EL) devices. This technique has the effect of lowering the substrate temperature during SrS:Ce deposition, and also of improving the luminance and emission color with blueshift. Using this technique, the controllability of thin film processing and reproducibility of the devices have also been improved. A suitable annealing temperature is about 720°C. The device has a luminance of 320 cd/m2 and improved color coordinates, x=0.18 and y=0.35, using a 1-kHz drive frequency. The devices have also been evaluated as to EL properties and film crystallinity. In particular, a novel way of directly measuring the transient electric field across the active layer has been proposed.

Effects of Substrate Temperature during Phosphor Layer Deposition on Luminance of SrS:Ce Blue-Green-Emitting Thin-Film Electroluminescent Devices

In order to improve the luminance of blue-green-emitting SrS:Ce thin-film electroluminescent devices, the effects of substrate temperature (350-550°C) during phosphor layer deposition were studied. Two deposition procedures were used: sulfur coevaporation and sulfurization of evaporation sources. The luminance increased with increasing substrate temperature in both cases. As the substrate temperature increased, oxygen content decreased and sulfur content remained almost constant for coevaporation; however, sulfur content increased while oxygen content remained low when the sulfurization technique was used. These changes led to improvement in the crystallinity of the phosphor layers. Also, sulfur and oxygen contents in phosphor layers deposited at 550°C by both procedures were almost the same, and as a result, the luminance levels were almost the same.