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本論文は2002年度から2005年度にかけて使用された日本の中学校英語検定教科書に対して、特に語彙とリーダビリティに焦点をあて分析を行ったものである。複数の従来研究によって、中学校教科書に関する基本的な情報はすでに示されており、本研究はそれらを補完し、発展させたものともいえる。まず、語彙については、頻度といった基本情報だけではなく、教科書における学年と英語学習などでも使われる語彙リストのレベルの関係性も調査した。その結果、中学校教科書で使用される語彙では、その学年と語彙リストのレベルの間にほとんど相関がみられないことが明らかとなり、中学英語教育における語彙リストの有効性や問題点が認められた。また、リーダビリティについては、教科書本文のリーダビリティ推定値に加え、それに関わる諸要因についても報告している。, This paper reports on a study of the characteristics of Japanese junior high school English textbooks (from the 2002 to 2005 academic years) with a focus on vocabulary and readability. First, to investigate the vocabulary in English instruction in Japan, we analyze not only basic information such as word frequency but also the relation between grades and the common vocabulary lists in use. The results revealed that there is almost no correlation between grades and the level in vocabulary lists employed for teaching English in Japan. Second, this research provides information on factors related to readability and estimates the readability of the textbooks. Since previous research has already dealt with the fundamental information on textbooks, this research could be significant as an auxiliary and advanced investigation., 研究論文

A selectional restriction specifies what combinations of words are semantically valid in a particular syntactic construction. This is one of the basic and important pieces of knowledge in natural language processing and has been used for syntactic and word sense disambiguation. In the case of acquiring the selectional restriction for many combinations of words from a corpus, it is necessary to estimate whether or not a word combination that is not observed in the corpus satisfies the selectional restriction. This paper proposes a new method for estimating the degree of satisfaction of the selectional restriction for a word combination from a tagged corpus, based on the multiple regression model. The independent variables of this model correspond to modifiers. Unlike a conventional multiple regression analysis, the independent variables are also parameters to be learned. We experiment on estimating the degree of satisfaction of the selectional restriction for Japanese word combinations 〈 noun, postpositional-particle, verb 〉. The experimental results indicate that our method estimates the degree of satisfaction of a word combination not very well observed in the corpus, and that the accuracy of syntactic disambiguation using the co-occurrencies estimated by our method is higher than using co-occurrence probabilities smoothed by previous methods.

母語話者/非母語話者文書を自動的に高精度で判別する技術は, 良質コーパスの作成支援, 非母語話者の文書推敲支援, 第二言語習得に有益な知識獲得などの面で重要な役割を果たす.本論文では, 品詞trigram分布の相違度に基づく母語話者/非母語話者文書判別法を提案する.確率分布の相違度として, ゼロ頻度問題に対処できるSkew Divergenceを用いる.Skew Divergenceはパラメタαを設定する必要があるが, 現在その設定法に定石は存在しない.本論文ではSkew Divergenceのパラメタαの設定法についても議論する.英語科学技術論文を対象にした判別実験の結果, 提案手法の有効性が確認された.

The semantic similarity (or distance) between words is one of the basic knowledge in Natural Language Processing. There have been several previous studies on measuring the similarity (or distance) based on word vectors in a multi-dimensional space. In those studies, high dimensional feature vectors of words are made from words' cooccurrence in a corpus or from reference relation in a dictionary, and then the word vectors are calculated from the feature vectors through the method like principal component analysis. This paper proposes a new placement method of nouns into a multi-dimensional space based on words' cooccurrence in a corpus. The proposed method doesn't use the high dimensional feature vectors of words, but is based on the idea that ``vectors corresponding to nouns which cooccur with a word w in a relation f constitute a group in the multi-dimensional space''. Although the whole meaning of nouns isn't reflected in the word vectors obtained by the pro posed method, the semantic similarity (or distance) between nouns defined with the word vectors is proper for an example-based disambiguation method.

It is revealed that the electroluminescence (EL) spectra of bluish-green luminescent Ce3+ centers can be stabilized by Rb doping in SrS:Ce thin film EL devices prepared by an electron-beam evaporation method. It is also found that Rb doping results in reduction of the current spike of EL devices, indicating suppression of Ce3+–VSr complex center formation. For the transparent SrS:Ce,Rb device, even without postdeposition annealing, the bluish-green EL corresponding to the Commission Internationale de l'Eclairage (CIE) color coordinates of (0.18, 0.34) was obtained with a luminance of 190 cd/m2 under 1 kHz pulse drive at 40 V above the threshold voltage. The luminance of the SrS:Ce,Rb EL device was improved to 220 cd/m2 by annealing at 650°C while maintaining almost the same CIE color coordinates.

Hybrid electroluminescent (EL) devices, which consist of thick BaTiO3 dielectric and thin SrS:Ce phosphor films, have been investigated. The SrS:Ce films were deposited by electron-beam evaporation under H2S gas supply. It is found that by employing thermally decomposed H2S, a reduction of the BaTiO3 dielectric film is suppressed, which enables us to deposit SrS:Ce films even at high substrate temperature, resulting in improved EL characteristics. The hybrid EL device prepared at 570°C with the thermally decomposed H2S supply showed a luminance of 1060 cd/m2 and an efficiency of 1.1 lm/W under 1 kHz pulse drive at 60 V above the threshold voltage with the Commission Internationale de l'Eclairage (CIE) color coordinates of (0.30, 0.55).

We develop a method for generating multiplechoice test for evaluating comprehension of an arbitrary English text and its answer to support more extensive reading in English. Our test consists of one sentence (a correct optional sentence) that is consistent with the selected English text and several sentences (distractor optional sentences) that are inconsistent with this text. Learners select one sentence consistent with the text from multiple optional sentences on reading the text. In our proposed method, the system extracts several important sentences from the text, and replaces at least one word in each of these sentences with its synonym (if possible). One of these sentences is then selected as the correct response, while further changes to the polarities or nouns in the remaining sentences are carried out to generate distractor sentences for the multiplechoice test. Our method has potential to improve the learning effect associated with extensive reading.

Enhancement of emission intensity of rare-earth-ion doped SrTiO 3 by Al addition has been investigated. In the case of Pr 3+ and Tb 3+, addition of 23-mol% Al intensifies emission by more than 200 times. In contrast, the addition of 20 mol% Al intensifies emission at most by three times in the case of other rare-earth ions. The temperature dependence of PL spectra shows that the energy transfer from carriers to Pr 3+ or Tb 3+ ions is much more efficient than that to other rare-earth ions in SrTiO 3. It can be speculated that the energy transfer in SrTiO 3: Pr 3 or Tb 3+ occurs from carriers to Pr 3+ or Tb 3+ ion via 4f-5d transitions, which are much higher in oscillator strength than 4f-4f transitions.

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.

ZnxMg1−xS : Mn thin film electroluminescent (TFEL) devices have been developed in a wide range of compositions, x between 0.16 and 1.0, for use as yellow/green-light-emitting displays. Electroluminescence shows the blue shift alike photoluminescence with the increase in Mg composition. The Zn0.98Mg0.02S : Mn TFEL device driven at 1 kHz has a high luminance of 1056 cd/m2 and an efficiency of 1.2 lm/W. The CIE color coordinates varied from (x, y)=(0.54, 0.45) at x=0.98 to (0.47, 0.52) at x=0.16 with the decrease in Zn composition x in consistence with the peak shift from 585 to 565 nm.

SrS based blue electroluminescent phosphor thin films, such as SrS : Ce and SrS : Cu, have been prepared by hot-wall deposition using alternating source supply. The SrS films deposited at 600°C are formed by columnar polycrystalline grains of 0.5–1 μm, mainly oriented along the [1 0 0] axis. From the average deposition rate, it is deduced that 1 Sr–S layer is deposited for one cycle of Sr and S vapor supply. The SrS : Cu,Cl device shows pure blue EL emission. The luminance and efficiency measured at 1 kHz are 100 cd/m 2 and 0.10 lm/W, respectively. The SrS : Ce,Cl device shows a greenish–blue EL emission; however, the luminance and efficiency are much lower than those for the devices prepared by other deposition methods.

The single-phase CaAl 2 O 4 (CA) : Eu 2+ powder phosphors showing 440 nm blue emission were prepared by two-step firing synthesis. It is expected that CA : Eu 2+ phosphor can be applied to plasma display panels (PDPs), because it is more resistant to luminance degradation by heating process in comparison to BaMgAl 10 O 17 (BAM) : Eu 2+ . However, the initial PL luminance of CA : Eu 2+ excited by 147 nm light remains 50% of that of BAM : Eu 2+ .

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.

In aiming to improve the performance and reliability of infrared absorption type CO2 gas sensors, a high-sensitivity bulk thin-plate-type pyroelectric infrared sensor and an infrared-absorption-type CO2 gas sensor using electrooptic chopping have been fabricated and their performance studied. In a gas sensor combining a conventional bulk-type pyroelectric sensor and a mechanical chopper, a detection sensitivity of 0.029 mV/ppm for a unit concentration is obtained. On the other hand, if the bulk thin-plate-type pyroelectric sensor is used, the sensitivity is 0.12 mV/ppm, which is four times as great. When a bulk thin-plate-type pyroelectric sensor and a PLZT optical shutter are combined, the sensitivity is 0.026 mV/ppm. Hence, a highly reliable CO2 sensor that eliminates the mechanical chopper with a performance equivalent to that of the conventional type can be realized. © 1999 Scripta Technica, Electron Comm Jpn Pt 2, 82(4): 1–8, 1999

Excitation and photoluminescence (PL) spectra of both nominally undoped and Ce 3+ -activated SrGa 2 S 4 thin films have been measured at various temperatures between 10 and 300 K. The undoped film shows a complex emission band which appears to involve emission from different defect states. Each defect center effects the recombination kinetics and luminescence spectra in a different temperature region. At 77 K, recombination is dominated by the highly associated defect centers producing an intense red emission band. At higher temperatures, thermalization effects reduce the net capture rates at the competitive defect centers, and recombination through the defect centers at room temperature produces a broad emission band with the peak in the blue spectral region. Evidence of the interaction and energy exchange between defect centers and Ce 3+ activator states is inferred from the rise and decay time measurements of PL emission in SrGa 2 S 4 :Ce thin films. As a result of thermalization, the efficiency of energy transfer to Ce 3+ ions increases at room temperature but at the expense of introducing a long afterglow into the activator decay. Measurements of temperature dependence of PL intensity suggest that the kinetic model for energy transfer in SrGa 2 S 4 :Ce combines aspects of long-range resonance transfer and indirect thermalization from defect states with subsequent retrapping at the activator sites. In addition, possible mechanisms are discussed which may explain the observed concentration dependence of the Ce 3+ fluorescence decay time. Preliminary results are reported for a thin-film electroluminescent device with a SrGa 2 S 4 :Ce, Li phosphor layer prepared by the low-temperature deposition from binary vapors process.

The effects of firing temperature on the crystallinity and the luminescent properties of BaMgAl 10 O 17 :Eu 2+ phosphor have been investigated over the temperature range 1000-1800°C under preparative conditions which do not use any flux such as AlF 3 . The effects of particle shape and size of the ingredient Al 2 O 3 on the phosphor have also been studied under a preparative condition which has a firing temperature of 1700°C and does not use any flux. A single-phase BaMgAl 10 O 17 :Eu 2+ with sufficient luminescent properties can be obtained over a firing temperature range of 1600-1800°C through formation of the intermediate product BaAl 2 O 4 :Eu 2+ . Furthermore, the particle shape and size of the ingredient Al 2 O 3 determine the particle shape and size of BaMgAl 10 O 17 :Eu 2+ . High-temperature firing in the absence of any flux has provided for the preparation of a single-phase BaMgAl 10 O 17 :Eu 2+ with various particle shapes, such as spherical or platelike, and particle sizes ranging from 2 to 18 μm.

Lectures and teaching, as well as researches, are important social duties of universities. From the viewpoint of importance of lectures, the department of electrical and electronic engineering of Tottori university organizes a committee aiming to improve the members' teaching skill. The committee plans and sponsors "Class Visitation" which is being held monthly by the members of the department. The members open their lectures mutually in order to improve their teaching skill by incorporating good points of other members' lectures. This is an interim report on the "Class Visitation" project. Though the project has just started, some practical results are obtained.

The mechanism of luminance decrease in blue emitting BaMgAl{sub 10}O{sub 17}:Eu{sup 2+} phosphor after annealing has been studied. It was shown experimentally that annealing in air oxidizes BaMgAl{sub 10}O{sub 17}:Eu{sup 2+} and substantially changes the Eu{sup 2+} ion to an Eu{sup 3+} ion. This is also deactivates the Eu{sup 2+} luminescent center, thus decreasing Eu{sup 2+} emission. Furthermore, the compound EuMgAl{sub 11}O{sub 19}, in which the valence of the Eu ion is trivalent, was detected in BaMgAl{sub 10}O{sub 17}:Eu{sup 2+} annealed at temperatures higher than 800 C as a resulting product of oxidation. It was demonstrated that BaMgAl{sub 10}O{sub 17}:Eu{sup 2+} was converted by oxidation into a mixture of the two compounds BaMgAl{sub 10}O{sub 17} and EuMgAl{sub 11}O{sub 19}, and resulting in a further decrease in luminance. This suggests that obstructing the formation of Eu(III)MgAl{sub 11}O{sub 19} during the annealing is essential for alleviating the decrease in luminance. Preparing BaMgAl{sub 10}O{sub 17}:Eu{sup 2+} having both superior crystallinity and completely stoichiometric composition has been found to be effective in alleviating the luminance decrease.

To improve performance characteristics of a pyro-electric infrared sensor, especially voltage responsivity (Rv), noise equivalent power (NEP), and response speed, it is necessary to reduce the heat capacity of an infrared sensing element which is made of Pb(Zr, Ti)O3 (PZT) ceramics. A new design for a sensor whose sensing element is made of a thin bulk PZT ceramic plate mounted on a diaphragm fabricated by the Si micromachining method has been proposed and its prototypes have been tested. The prototype sensor has a PZT ceramic plate which is about 10-μm thick and has a sensing area of 1 × 2 mm2. The new sensor has an Rv value of 36.3 to 37.9 mV/μW which is about five times higher than a pyroelectric sensor with a 100-μm thick PZT bulk ceramic plate. The amount of sensor noise was expected to be reduced by half. However, the amount of sensor noise increased by a factor of two. Because of the increase in the sensor noise, the values of NEP were 1.8 to 3.3 × 10−2 μW, which is a factor of two to three smaller than the NEP value of the bulk sensor. Furthermore, the rise time decreased to 31 to 35 ms, which is 10 to 20 percent shorter than that of the bulk sensor. Thus, the reduction of sensor noise will be an important challenge for the future.

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.

The growth of thin films of ZnS and SrS by the hot-wall evaporation technique has been investigated. By supplying additional sulfur (S) vapor, the deposition rate of ZnS was increased, allowing ZnS films to be grown at substrate temperatures over 400°C. SrS thin films were grown through the chemical reaction of Sr and S vapor with the substrate temperature in the region of 350–500°C. SrS/ZnS multilayered thin films were grown by the sequential deposition of the SrS and ZnS films. The (SrS/ZnS)51 multilayered thin film showed X-ray diffraction lines from both ZnS and SrS layers, even when the thickness of each SrS and ZnS layer is 20 nm. The electroluminescent devices with SrS: Ce/ZnS multilayered thin films showed a greenish blue EL emission.

The electroluminescence (EL) and photoluminescence (PL) characteristics of blue emitting ZnS: Tm3+ phosphors have been studied. The difference of the EL excitation mechanism from that of PL is also discussed. For EL excitation, the emission is concentrated more in the infra-red regions, whereas for PL excitation, blue emission dominates the spectrum. It is found that the PL excitation mechanism of Tm3+ centers is attributable to the non-radiative energy transfer from the ZnS host to the Tm3+ centers. In contrast with PL, the dominant EL excitation mechanism results from the direct impact excitation of the Tm3+ centers by hot electrons rather than energy transfer. The cause for the relative inefficiency of blue emission in EL is that the excitation of the lower lying 3F4 state is dominant compared with that of the higher lying 1G4 state.

The luminescent properties and device characteristics of ZnxMg1−xS : Mn thin films have been investigated by photoluminescence (PL) and electroluminescence (EL). The blue shift of the Mn2+ emission band in PL and EL has been clearly observed in the whole composition range. The chemical shift of the Mn2+ emission band in ZnxMg1−xS has been discussed consistently in the framework of crystal field treatment.

Defect-reduction process in SrTiO 3 by Al addition has been investigated by using high-resolution transmission electron microscope (HRTEM) images, X-ray diffraction (XRD) patterns and photoluminescence (PL) spectra. HRTEM images in SrTiO 3 : Pr 3+ show the reduction of planar faults by Al addition. The planar faults are probably SrO thin layers. In Eu-doped SrTiO 3 reduced by firing in hydrogen atmosphere, Ruddlesden-Popper phases Sr n+1 Ti n O 3n+1 (n = 1, 2,…, ∞) are precipitated in SrTiO 3 , as confirmed by XRD patterns. Sr n+1 Ti n O 3n+1 consists of a SrO planar fault interleaved with SrTiO 3 slabs. With Al addition, the planar faults cannot be easily formed. Instead, strontium aluminates, SrAl 2 O 4 or/and SrAl 12 O 19 , as well as SrTiO 3 are precipitated, as a result of scavenging the SrO planar faults by added Al. The precipitation is confirmed also by PL from Eu 2+ in the strontium aluminates.

Recent advances in the development of thin-film electroluminescent (EL) devices having alkaline-earth sulfide phosphors doped with rare-earth luminescent centers are summarized. The effect of the preparation conditions on the quality of the resultant thin films is discussed. It is shown that the crystallinity of the evaporated thin films can be improved by an increase in substrate temperature, and that oxygen contamination is reduced by sulfur coevaporation. The EL characteristics and the excitation mechanism of these EL devices are discussed on the basis of the band structure of the alkaline-earth sulfides and the energy levels of the luminescent centers. Experimental results on the excitation spectra of photo-induced transferred charge, which give clear evidence for the ionization of the luminescent centers, are presented. The EL characteristics of white EL devices based on novel SrS phosphors are also discussed.

SrS:Cu, Cl thin-film electroluminescent (TFEL) devices have been prepared by hot-wall deposition using a successive supply of source materials. The TFEL devices show blue EL emission with the chromaticity coordinate of (0.19, 0.28). The luminance and the efficiency at 1 kHz drive are 100 cd/m2 and 0.1 lm/W, respectively. EL emission is observed at both the leading and the trailing edge of the applied pulse voltage. The presence of trailing edge emission implies that Cu+ centers are ionized during EL excitation, and then electrons are subsequently captured by ionized Cu2+ centers giving rise to the EL emission.

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.

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.

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.

The authors have succeeded in improving the luminance levels of the blue SrS:Ce,K and white SrS:Ce,K,Eu TFEL (thin-film electroluminescent) devices by post-deposition annealing in an Ar-S atmosphere. It has been shown that the increase in the luminance and efficiency results from the improvement of the crystallinity of the thin films. The luminance level achieved for white TFEL reached the goal of realizing a monochrome white TFEL display. The SrS TFEL device showed a luminance of 900 cd/m/sup 2/ and an efficiency of 0.44 lm/W at 1 kHz drive. >

Blue-emitting Eu2+-doped calcium aluminate phosphor thin films were obtained using the pulsed laser deposition technique with post annealing. As-deposited films were amorphous and showed weak red Eu3+ photoluminescence (PL). By annealing in reducing atmosphere (N2/H2:2% mixed gas) at 950°C for 3 h, the film was crystallized and showed a PL emission band peaking at about 447 nm, which originated from the 4f65d to 4f7 transition of Eu2+ ion. It is considered that the deposited film consists mainly of CaAl2O4 and partly of other binary compounds of the CaO–Al2O3 system. It was determined that the PL intensity of Eu2+ in CaAl2O4 can be controlled by the laser fluence, target-substrate distance and injection gas.

The mechanism of the blue-shift of Mn2+ luminescence in ZnMgS:Mn is studied. Single crystal zincblende Zn1-x MgxS:Mn thin films (x≤0.50) were grown by molecular beam epitaxy on GaAs substrates. The photoluminescence (PL) spectrum of Zn0.50Mg0.50S:Mn peaks at 572 nm, which is 12 nm shorter than the PL peak wavelength of ZnS:Mn. The PL excitation spectrum of Zn1-x MgxS:Mn has five excitation bands due to the energy levels of Mn 3d5 electron configuration. These excitation bands shift to shorter wavelengths with increasing Mg content. The cause of these shifts seems to be a weakening of the crystal field due to an increase in the distance between Mn and S. The ratio of the shift in emission energy to the relative change in lattice constant, ΔE/(Δa /a), is found to be 2.3 eV (23 meV/%). This value is approximately half the value of the Mn emission in ZnS:Mn under high pressure.

Thin films of Ce-activated SrGa 2 S 4 were prepared by combining electron-beam evaporation of Ce-doped SrS with direct evaporation of Ga 2 S 3 . With respect to the stoichiometry of SrGa 2 S 4 films, a Ga 2 S 3 /SrS flux ratio in the 60-100 range was found empirically to give the best results for a substrate temperature of 460°C. Photoluminescence and decay characteristics of SrGa 2 S 4 :Ce films were investigated as a function of Ce concentration. The results indicate that increase in Ce concentration from 0.2 to 6 mol% is accompanied by a marked increase in both the cerium absorption and decay rate, whereas the emission peaks remain unchanged with cerium doping. A possible reason for the observed effects is discussed.

Ce emission in SrS thin films has been investigated in the temperature range of 6-300 K. Films codoped with Cl show a strong thermal quenching around 50 K with a thermal activation energy of 0.3-0.4 eV. The low activation energy indicates the presence of shallow quenching centers. These centers are probably created out of various possible complex centers formed due to pairing of strontium and sulfur vacancies with co-activators and are associated with Ce emission centers. Ce in the presence of Cl has a greater tendency of forming Sr vacancies as compared to that in the presence of excess S. Zn codoping along with K has been found to reduce these vacancies and significantly improve the thermal stability of the luminescent characteristics of SrS:Ce thin films.

Aging characteristics of electron-beam evaporated SrS:Ce thin film electroluminescent devices have been improved by using a SrS:Ce and ZnS mixture source pellet. For the Zn-doped device, luminance is maintained at more than 60% of the initial value and threshold voltage remains constant for more than 1000 hours of aging at 500 Hz drive. A rapid decrease in luminance corresponds to a decrease in dynamic space charge, and is reduced by Zn doping. In contrast, a slow decrease in luminance is attributed to a change in the electron emission characteristics of phosphor-insulator interfaces.

The electrical properties of SrS:Ce thin-film electrolum inescent (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/cm 2 . 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/cm 2 . 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.

Transparent Y 2 O 2 S:Eu 3+ phosphor thin films have been grown by a reactive evaporation method. Y metal vaporized by electron-beam irradiation was allowed to react with SO 2 gas molecules. Eu was simultaneously supplied from a resistively heated crucible containing EuCl 3 powder. By employing a heated W filament and an ac plasma cell to excite (crack) the SO 2 molecules, Y 2 O 2 S:Eu 3+ thin films and thin films containing both Y 2 O 3 and Y 2 O 2 S phases which showed intense red photoluminescence were obtained. These results suggest that the films obtained are crystallized and Eu 3+ ions substitute for Y ions in the Y 2 O 3 and Y 2 O 2 S lattices and hence form luminescent centers.

Significant progress in the performance and commercialization of color thin-film electroluminescent (EL) displays has been achieved. This is due to the remarkable progress made in the performance of existing EL phosphors, development of new phosphor materials, and design of new EL phosphor structures. The physics of materials and luminescent centers of current EL phosphors (ZnS:Mn 2+ , ZnS:RE 3+ , SrS:Ce 3+ , and SrGa 2 S 4 :Ce 3+ ) are reviewed. As potential EL phosphors, metal oxides, yttrium oxides, oxysulfides, fluorides, and mixed-crystal-based phosphors are discussed. Multilayered and superlattice-structured EL phosphors are also discussed. The basic physics of the materials, host lattices, and luminescent centers is important in the development of novel EL phosphors.

The electroluminescent (EL) and cathodoluminescent (CL) characteristics of ITO-ZnS:Cu,Mn(Tb)-Al DC EL thin films were investigated. The experimental results showed that efficient resonant energy transfer takes place from self-activated ZnS and Cu centers to Mn ions resulting in high luminance of ZnS:Mn,Cu cells. EL properties of ZnS:Cu,Tb cells can be explained on the basis of the direct impact excitation model, while the energy transfer mechanism makes a negligible contribution to the excitation process. This accounts for the comparatively low EL luminance of Tb-doped cells.

The quenching mechanism of blue Tm3+ luminescence in Y2O2S phosphors due to residual Pr3+ ions has been studied. Results of photoluminescence and excitation spectra measurements and selective excitation measurements of Y2O2S:Tm3+, Pr3+ phosphors show that there is no energy transfer between Tm3+ and Pr3+ centers. The quenching of the blue Tm3+ emission by Pr3+ ions can be ascribed to the large electron-hole capture cross section of Pr3+ centers.

Y2O2S:Tb phosphor thin films have been grown by using a multi-source deposition (MSD) method. Y2O2S thin films grown by MSD method are highly crystallized with hexagonal structure and strongly oriented to direction. Tb doped Y2O2S thin films show an efficient green photoluminescence. The growth process of Y2O2S:Tb thin films is also discussed.

An (Y2O2S:Tb/ZnS) n multilayered thin-film electroluminescence (TFEL) device has been fabricated by the multisource deposition method. The multilayered TFEL device has shown a maximum luminance of 124 cd/m2 at 5 kHz. It is also seen that the EL characteristics strongly depend on the structure of phosphor layers, and the TFEL device with the Y2O2S:Tb single layer showed nominal EL emission. The excitation mechanism of the (Y2O2S:Tb/ZnS) n multilayered TFEL devices is discussed.