Thin-film Electroluminescent Structures Research Articles

Light-emitting devices using micromachined Si-tip mirror arrays

In this paper we introduce a new structure and process scheme to generate visible light by the combination of Si-tip reflectors with thin film electroluminescent structure and report preliminary results of light emission observed from the structure. As a result, 3600 reflectors were formed within one pixel and these picture elements create a well-concentrated visible light around the point reflectors under a bipolar pulse excitation. Even though the absolute luminance level is not very high, this result is very interesting because the front emission observed by a viewer originated intrinsically from the edge emission. The new devices suggested will contribute to the development of a high-bright micro-display with the integrated drivers.

Mobility of sodium in SrS based thin film electroluminescent structures grown by atomic layer epitaxy

Mobility of sodium in SrS-based electroluminescent thin film stacks grown by atomic layer epitaxy was studied in the temperature range 600–850 °C using ion implantation for doping and nuclear resonance broadening for profiling of sodium. The crystallinity and orientation of SrS and SrS:Ce films were studied using wide angle x-ray scattering (WAXS). It was found that sodium diffuses rapidly to the interfaces between SrS and upper and lower Al2O3 insulators. These insulators act effectively as diffusion barriers for sodium up to a temperature of about 700 °C. In the case of SrS:Ce films, even after annealing at high temperatures for long times, the Na concentration in the bulk of the film remained in the range 0.4–0.6 at. % which is about two to three times the concentration of Ce. WAXS measurements revealed the preferred [111] orientation of SrS and SrS:Ce films with an average size of crystallites ⩾120 nm. It turned out that annealing for a few hours at about 700–750 °C was needed to recover the implantation damage and restore the crystallinity of SrS to its original level.

Modeling of the time characteristics of luminance in thin-film electroluminescent structures

The pattern of luminance waves in thin-film electroluminescent structures was modeled and the duration of transient lighting processes in these systems was determined. A principle of the indicator display control is formulated which provides for a maximum luminance of the light-emitting devices.

Thin-film electroluminescent structures on substrates with rough surfaces

It is shown that the use of substrates with inner rough and outer diffusely scattering surfaces in film electroluminescent structures makes it possible to substantially increase the coefficient of radiation extraction (by a factor of 1.3–2.5) and the brightness of luminescence (by a factor of 1.2–3.5) as compared to the structure on a smooth surface, which can be employed to enhance the brightness of individual colors of the luminescence of electroluminescent emitters. The results of the investigations indicate a substantial decrease in the constant of the time of brightness buildup (by a factor of 6–8 for MDSDM structures and by a factor of ∼3.8 for MDSCM structures) and the appearance of two segments of brightness decay for MDSMD structures with different constant decay times in going from the structure on a smooth substrate to the structures built on substrates with an inner rough surface. Unlike this, MDSCM structures have two segments of brightness decay on both a smooth substrate and a rough substrate.

Effect of ultraviolet irradiation on the luminescence and optical properties of ZnS: Mn films

Changes have been observed and investigated in the electrooptic properties of ZnS: Mn films used in thin-film electroluminescent structures as a result of irradiation by ultraviolet pulses with energies per pulse much smaller than the threshold energy of laser annealing. It is found that in disordered ZnS: Mn films processes of defect generation are important even for below-threshold energies of the ultraviolet radiation pulses, and can facilitate the effective diffusion and activation of Mn atoms in the ZnS lattice. It is shown that short-time ultraviolet processing of thin-film electroluminescent structures improves their characteristics and, by making the preparation technology simpler and cheaper, allows structures with detector characteristics to be fabricated on low melting-point substrates.

Enhancement of the characteristics of thin-film electromluminescent structures based on ZnS:Mn films after low-power laser irradiation

It is shown that short-term exposure of thin-film electroluminescent structures based on ZnS:Mn films to ultraviolet pulses with energy much lower than the threshold energies for laser annealing of ZnS:Mn films can enhance the brightness and efficiency of the electroluminescence, increase the slope of the brightness-voltage characteristics, improve the operating stability, and increase the electrical safety margin of the structures. It is established that the improvement in the properties of structures is caused by the photostimulation of below-threshold defect annealing processes and impurity diffusion, which lead to redistribution of the Mn ions in the ZnS film and reduce the concentration of shallow states which influence the structural characteristics.

Complementary analysis of ALE-grown SrS based thin film electroluminescent structures with ion beam methods

Ion beam analysis techniques available at low energy accelerators (<10 MV) were utilised in the complementary analysis of thin film electroluminescent (TFEL) structures based on SrS phosphor material. The methods used include backscattering (BS), nuclear resonance broadening (NRB), deuteron induced reactions, elastic recoil detection, particle induced X-ray emission, etc. These methods are characterised by their non-destructive nature, good reproducibility and quantitativity. They were used to determine the depth profiles of the main components in a multilayered structure and also the amounts of impurities or dopants such as hydrogen, carbon, oxygen, fluorine, sodium, manganese and cerium. The capability and accuracy of these methods are discussed in the frame of examples and the analysis of measured spectra.

Thin-film electroluminescent structures on substrates with a diffuse-scattering emitting surface

Results are obtained to demonstrate the significant brightness enhancement (by a factor of 3.5–5.6) of the output of thin-film ac electroluminescent emitters on glass substrates with a diffuse-scattering emitting surface as compared with the conventional designs of such emitters on “smooth” glass substrates. In principle, this advantage can help to solve the problem of insufficient brightness of electroluminescent emitters for certain light colors, creating a promising outlook for the fabrication of full-color, thin-film electroluminescent panel displays with a high data capacity.

Structural and electroluminescent characteristics of sputtered SrS:Ce thin films by rapid thermal process

Thin film electroluminescent (EL) structure of Y 2O 3/SrS:Ce/BaTiO 3 has been deposited by rf-magnetron reactive sputtering and subsequently submitted to various thermal treatments. The effects of rapid thermal process (RTP) on the structural and EL characteristics of SrS:Ce thin films have been investigated, and compared with the results obtained by the conventional furnace. After the post-deposition annealing, the SrS film has the tendency to recrystallize preferentially in the (200) plane. The higher RTP temperature also results in the larger grain size of SrS. Auger measurements show that the RTP method can overcome the nonstoichiometry and interdiffusion problems encountered in the furnace-treated sample. It was found that the EL peaks gradually recovered to the band transitions of Ce 3+ ions (475 and 530 nm) in the SrS crystal field as the RTP temperature increased from 650 to 850°C. The threshold voltage can be further reduced to 150 V and the brightness increases eight times as much as the optimum furnace-treated device. The improvements in EL performance are due to the facts that the RTP can enhance the crystallinity of SrS, alleviate oxygen-induced defect centers from interdiffusion, activate Ce 3+ ions in phosphor, and yield a blue shift in emission colors.

Investigation of thin-film electroluminescent structures on rough subsrates

Results of experimental investigations of the volt-brightness characteristics, frequency dependences of brightness, and the directional radiation pattern of electroluminescent MSDM, MSCM, and MSDCM emitters, where M stands for the first transparent and second nontransparent electrodes, S is a semiconductor, D is a thin-film dielectric, and C is a silicone-based composite liquid dielectric with a powdered segnetoelectric filler, developed on conventional “smooth” and rough glass substrates are presented. It is shown that electroluminescent structures on rough surfaces have a brightness approximately two times higher than that of similar structures developed on a “smooth” substrate.

Trapping levels of carriers in thin-film electroluminescent structures based on ZnS

Trapping levels of carriers in thin-film electroluminescent structures based on ZnS

Thin-film electroluminescent emitters based on calcium sulfide doped with cerium

Thin-film electroluminescent structures (TFELS) based on calcium sulfide doped with cerium have long evoked interest, especially in recent years. Such systems feature a higher quantum yield compared with traditional zinc-sulfide doped with manganese due to the screened state of the shells within which radiative transitions take place. The CaS-Ce luminophor does not have a great tendency toward saturation, in contrast ZnS-Mn, which seems to be attributable to the similar chemical properties of the cations of the basic type AIIa and ions of rare-earth elements of type AIIIa. Moreover, their ionic difference is small. The band gap of calcium sulfide is equal to 3.4-4.4 eV, which permits one to apply an electric field of the high strength necessary for exciting electroluminescence. However, in their durability and electroluminescence yield these structures are three-four times inferior to zinc-sulfide systems. In our opinion, such a short TFELS lifetime is explained by a lack of understanding of the mechanisms involved in current passage and electroluminescence in the literature. Proceeding from these prerequisites, in the present work we carried out investigations of the voltage-current (VCC) and voltage-brightness (VBC) characteristics of CaS-Ce.

Properties of zinc sulfide luminophores in thin-film structures

Thin-film electroluminescent (TEL) structures of the metal-dielectric-luminophor-dielectric-transparent electrode type are the most promising devices for the creation of emission sources. In order to elucidate the processes of energy conversion into emission in luminophor layers, it is necessary to investigate the properties of luminescent films and the physical phenomena that occur in thin-film systems. However, experimental studies of such systems require the development of new methods and measurement tools, since the luminophor in these systems is electrically insulated from the electrodes. In connection with this, the authors conducted theoretical and experimental studies of the nonequilibrium, electronic processes in TEL structures based on zinc-sulfide luminophors.

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.

Energy dissipation in thin-film electroluminescent structures

Energy dissipation in thin-film electroluminescent structures

Forward recoiling spectroscopy analysis of thin film electroluminescent structures

Forward recoiling spectroscopy has been used to profile hydrogen concentrations in multilayer thin film electroluminescent (TFEL) structures grown by atomic layer epitaxy. The reversible current-voltage and brightness-voltage characteristics of an a.c. TFEL device were characterized. The hydrogen content in the dielectric/ZnS:Mn/dielectric structure was observed to be less than 0.5 at.%, near the detection limit. The analysis revealed, however, that the hydrogen content changed slightly depending on the treatment of the device.

Effect of a phosphosilicate glass layer on the electro-optical performance of thin film electroluminescent structures

The electro-optical performance of different thin film electroluminescent structures was studied by changing the composition and the deposition method of the lower dielectric adjacent to the atomic layer epitaxy (ALE) grown ZnS:Mn. Al 2O 3 deposited by r.f. sputtering, Al χ Ti γ O z grown by ALE, and the double dielectric of their combination, resulted in a soft threshold for the electron injection. A drastic improvement was observed in the characteristics when a phosphosilicate glass (PSG) layer was substituted for the lower dielectric layers or when an Al 2O 3/PSG or Al χ ,Ti γ O z /PSG double structure was used as a lower dielectric.

Ion beam analysis of multilayer structures grown with atomic layer epitaxy (ALE)

Numerous ion beam analysis techniques, e.g. Rutherford backscattering (RBS), proton induced γ-ray emission (PIGE), nuclear resonance broadening (NRB), proton induced X-ray emission (PIXE), and forward recoiling spectroscopy (FRES) were applied to analyse thin film electroluminescent multilayer structures. These methods are characterized by non-destructive nature, good reproducibility and quantitativity. Good depth resolution is achieved with RBS and NRB, and to a limited extent with FRES, whereas PIGE and PIXE can probe only average concentrations of the surface layer. In general, most elements including hydrogen can be detected with these methods. The analysis of the multilayer structure Al x Ti y O z /ZnS:Mn/Al x Ti y O z grown with atomic layer epitaxy (ALE) revealed a high purity with minor amounts of contaminants such as hydrogen, sodium andd nickel.

Properties and performance of TFEL structures

The performance of AC thin film electroluminescent structures is discussed in terms of visual appearance, power consumption and reliability. The thin film deposition technique is of fundamental importance in order to obtain a reliable display with a uniform emission. To reduce power consumption in a matrix display, it is essential to minimize the modulation voltage by having a stable luminance versus voltage characteristic with a steep rise in luminance for voltages above threshold for the onset of the emission. Diffuse reflectance increases with the ZnS : Mn layer thickness suggesting that a rather thin light emitting layer is optimal in displays intended to be used in strong illumination.

ZnS blue emission probing for the study of AC thin film electroluminescent structures

The broad-band “blue emission” (BE) characteristic of ZnS electroluminescence (EL) is studied. Time-resolved behaviour of BE, Mn emission and conduction current are compared. Time-averaged BE measurements are also reported, to provide a direct probe of the contribution of the dielectric layers to the power dissipation in the EL structure. Also, a comparative study of the respective efficiencies of the electrons injected into ZnS for BE and Mn excitations was carried out for various field values in ZnS, which yields new information on the energy distribution of the hot electrons.