Influence of x‐ray irradiation on doped europium in BaMgAl10O17 studied by x‐ray absorption fine structure and x‐ray diffraction

Abstract— We studied the influence of x‐ray irradiation on the doped europium ion in BaMgAl₁₀O₁₇ (BAM) by x‐ray absorption fine structure measurement and x‐ray diffraction. We found that x‐ray irradiation promoted oxidation of doped europium and fading emissions. However, no difference was found in x‐ray diffraction patterns before and after x‐ray irradiation. We concluded that the fading of BAM after x‐ray irradiation was mainly caused by oxidation of the doped europium in the BAM.     

Effect on deoxidation of doped europium in oxidized BaMgAl10O17

Abstract— We studied the effect of the deoxidation of oxidized blue‐light‐emitting europium‐doped BaMgAl₁₀O₁₇ (BAM) phosphor in a plasma‐display panel by making photoluminescence and powder X‐ray diffraction measurements and by X‐ray absorption fine structure analysis. Photoluminescence spectra, powder X‐ray diffraction, and X‐ray absorption near‐edge structure, and extended X‐ray absorption fine structure spectra of BAM at the europium L₃‐edge suggest that the oxidized BAM annealed in air is revived by heating in a H₂/N₂ (5% H₂) reducing atmosphere above 500°C.     

Host emission from BaMgAl10O17 and SrMgAl10O17 phosphor: Effects of temperature and defect level

Abstract— Understanding the mechanism of blue‐light emission in Eu‐doped BAM phosphor as well as its sensitive degradation is required because this is a very important material in fluorescent lamps and plasma‐display panels. In this study, both theoretical and experimental investigations on the host emissions in BaMgAl¹⁰O¹⁷ and SrMgAl¹⁰O¹⁷ were performed. Host emissions from BaMgAl¹⁰O¹⁷ and SrMgAl¹⁰O¹⁷ by photoluminescence and thermoluminescence spectra were observed. Photoluminescence spectra suggested that the host emission from SrMgAl¹⁰O¹⁷ was easily quenched by thermal vibrations. The thermoluminescence spectra showed the existence of shallow and deep defect levels in BaMgAl¹⁰O¹⁷ and SrMgAl¹⁰O¹⁷ phosphors. It was shown that SrMgAl¹⁰O¹⁷ and its conduction plane could undergo degradation during irradiation of vacuum‐ultra‐violet (VUV) lights based on the calculated energy of formation of an oxygen vacancy. Moreover, the structural defects, such as oxygen vacancies, would cause localizing levels in the upper level in the valence band and in theconduction band. The results suggest the contribution of the host emission to the energy transfer to the Eu atoms would not be significant and the oxygen vacancies would act as the traps for excited carriers.     

Quantum chemistry and QSPR study on relationship between crystal structure and emission wavelength of Eu2+‐doped phosphors

Abstract— The relationship between crystal structures and emission properties has been computationally investigated for Eu²⁺‐doped phosphors. The electronic structure of the Eu²⁺‐doped BaMgAl¹⁰O¹⁷ phosphor was analyzed by using the quantum chemistry method. The different effects of O and Ba atoms on the Eu 5d states were determined. The presence of O and Ba atoms increases and decreases the energy level of the Eu 5d orbital by forming anti‐bonding and bonding interactions, respectively. According to the electronic‐structure analysis, the structure index that represents the local geometrical information of the Eu atom was defined. The relationship between the crystal structures and the emission wavelengths of the 1 6 Eu²⁺‐doped oxide phosphors were studied by using the quantitative structure‐property relationship (QSPR). The QSPR model suggested that the both O and alkaline‐earth atoms around the Eu atom are of importance in the determination of the emission wavelength. The interaction between the Eu and the nearest O atoms make the Eu²⁺ emission wavelength short. On the other hand, the interaction from the alkaline‐earth atoms around the Eu atom lengthens the Eu²⁺emission wavelength. This evaluation method is useful in selecting the host material that indicates a desirable emission wavelength of the Eu²⁺‐doped phosphors.     

Phosphors for plasma‐display panels: Demands and achieved performance

Almost two‐thirds of the discharge cells in plasma‐display panels (PDPs) are covered with phosphors. Beyond the efficient conversion of vacuum UV photons into visible light, the phosphor layer serves as a reflective mirror transporting light in the desired viewing direction. The quantum efficiency of state‐of‐the‐art PDP phosphors is, at its upper limit, 80–95%. Today's improved blue‐emitting BaMgAl₁₀O₁₇:Eu (BAM) phosphor still deteriorates during panel processing and operation, resulting in a loss of efficiency and color purity. A reduction in the phosphor particle size below 2 μm is suited to ease panel manufacturing and to improve light output.     

Study of the effect of water on thermal and operating degradation of BaMgAl10O17: Eu2+ (BAM) blue phosphor

Abstract— From the adsorption and desorption characteristics of water, we showed that water can intercalate into BaMgAl₁₀O₁₇: Eu²⁺ blue phosphor. ESR, XANES, and XPS analyses confirmed that oxidation by water causes thermal degradation of BAM. We also demonstrated that intercalated water accelerates luminance degradation under VUV irradiation and showed oxidation of Eu²⁺ during panel operation by means of μ‐XPS. We concluded that the cause of thermal and operating degradation of BAM is the oxidation of Eu²⁺ due to water.     

Characterization of a SAG‐InGaN/AlGaN LED by mixed‐source HVPE with a multi‐sliding boat system

Abstract— The selective area growth (SAG) of a InGaN/AlGaN light‐emitting diode (LED) is performed by using mixed‐source hydride vapor‐phase epitaxy (HVPE) with a multi‐sliding boat system. The SAG‐InGaN/AlGaN LED consists of a Si‐doped AlGaN cladding layer, an InGaN active layer, a Mg‐doped AlGaN cladding layer, and a Mg‐doped GaN capping layer. The carrier concentration of the n‐type Al_xGa_1?xN (x ～ 16%) cladding layer depends on the amount of poly‐Si placed in the Al‐Ga source. The carrier concentration is varied from 2.0 × 10¹⁶ to 1.1 × 10¹⁷ cm^?3. Electroluminescence (EL) characteristics show an emission peak wavelength at 426 nm with a full width at half‐maximum (FWHM) of approximately 0.47 eV at 20 mA. It was found that the mixed‐source HVPE method with a multi‐sliding boat system is a candidate growth method for III‐nitride LEDs.     

A study on the chromaticity shifts of blue phosphor for color plasma displays

The dependency of the chromaticity shifts on the concentration of Eu²⁺ doped in BaMgAl₁₀O₁₇ (BAM) was investigated under heat‐treatment and vacuum ultraviolet (VUV) irradiation. The Eu²⁺ ions in BAM show an asymmetrical broad emission band with a maximum at ～452 nm under excitation of VUV light at room temperature, showing that multiple crystalline cationic sites exist in the host. It was found that the chromaticity shifts greatly decrease with increasing heat‐treatment temperature. Regardless of the Eu²⁺ concentration, the chromaticity shifts caused by heat‐treatment are greater than that caused by VUV irradiation. Compared with conventional BAM, a solid solution of BAM with barium aluminate as a powder and film was also studied, and very few chromacity shifts were observed. It is suggested that the distribution of Eu²⁺ ions in different sites in a BAM lattice results in different chromaticity coordinates. By increasing the Eu²⁺ concentration in BAM, or under heat‐treatment and VUV irradiation, the emission band shifts towards longer wavelengths.     

High‐efficiency luminescent sources fabricated in mesoporous anodic alumina by sol‐gel synthesis

Abstract— This paper summarizes our recent results on the synthesis and investigation of photoluminescence (PL) from lanthanide‐doped microporous xerogel solids mesoscopically confined in porous anodic alumina (PAA). It was demonstrated, for Tb‐doped samples, that the PL intensity is strongly enhanced in comparison to thin xerogel films processed onto flat surfaces and increased with the thickness of the PAA layer. It was revealed for both Tb‐ and Eu‐doped PAA‐based structures that maximum emission is achieved at a excitation wavelength near 285 nm for the employed TiO₂ and Al2O₃ xerogels. Strong Eu‐ and Tb‐related PL visible to the naked eye was demonstrated, and a method for the fabrication of luminescent images based on anodizing, photolithography, and sol‐gel processes is proposed.     

Effect on electrical performance of titanium co‐sputtered IZO active‐channel thin‐film transistor

We investigated the electrical performance of Ti–IZO active‐channel layer thin‐film transistors (TFTs) using a radio frequency (RF) magnetron co‐sputtering system to co‐sputter IZO and Ti targets. The samples were fabricated by changing the RF gun power of the IZO. The other parameters such as the RF gun power of the Ti target, oxygen partial pressure [O₂/(Ar + O₂)], and initial and process pressure of the chamber were unchanged. Unlike the sample sputtered only with IZO, the thin films of the Ti–IZO samples could control the oxygen vacancy because Ti reacts with the oxygen in the IZO. Therefore, Ti–IZO thin films can suppress the carrier concentration and thus have an effect on the electrical performance of TFTs.     

Identification of charge‐transfer transition between Gd3+ to O2− in gadolinium‐containing oxide phosphors in VUV region

Abstract— The broad bands at around 155 nm for GdAl₃(BO₃)₄:Eu, at 184 nm for Ca₄GdO(BO₃)₃:Eu, at 183 nm for Gd₂SiO₅:Eu, and at 170 nm for GdAlO₃:Eu were observed. These bands were assigned to the charge‐transfer (CT) transition of Gd³⁺‐O^2?. In the excitation spectrum of (Gd,Y)BO₃:Eu, a broadened excitation band was observed in VUV region. It could be considered that this band was composed of two bands at about 160 and 166 nm. The preceding band was assigned to the BO₃ group absorption. The later one at about 166 nm could be assigned to the CT transition of Gd³⁺‐O^2?, according to the result of GdAl₃(BO₃)₄:Eu, Ca₄GdO(BO₃)₃:Eu, Gd₂SiO₅:Eu, and GdAlO₃:Eu. The excitation spectra overlapped between the CT transition of Gd³⁺‐O^2? and BO₃ groups absorption. It caused the emission of Eu³⁺ to take place effectively in the trivalent europium‐doped (Gd,Y)BO₃ host lattice under 147‐nm excitation.     

Development of a new blue CaMgSi2O6:Eu2+ phosphor for PDPs

Abstract— A blue‐light‐emitting Eu²⁺‐doped CaMgSi₂O₆ phosphor having a long lifetime for a plasma‐display panel (PDP) was developed. The CaMgSi₂O₆:Eu²⁺(CMS:Eu) phosphors show no luminance degradation during the baking process, and an equivalent photoluminescence peak intensity compared to that of the conventional blue‐phosphor BaMgAl₁₀O₁₇:Eu²⁺ (BAM) after baking. CMS: Eu shows a poor luminescent characteristic for the Xe excimer band excitation due to the lack of absorption. To introduce the absorption center for the Xe excimer band, we performed Gd‐codoping of CMS: Eu as a sensitizer and found a new excitation band around 172 nm, which originated from Gd³⁺. The test PDPs panels using synthesized CMS: Eu phosphor and CMS: Eu, Gd phosphor were examined to investigate the luminescent and aging characteristics of a Xe‐discharge excitation source. The CMS: Eu panel shows an emission peak intensity comparable to that of the BAM panel (i.e., a comparable stimuli L/CIEy, 93% of BAM), while the CMS: Eu, Gd panel shows poorer blue emission intensity compared to the BAM panel (up to 53% of total stimuli of BAM). The CMS: Eu panel and the CMS: Eu, Gd panel show less luminance degradation than the BAM panel under the aging test, and the panel retains 90% of its luminance after 300 hours of driving. It was found that CMS: Eu appears to be a candidate for a new blue PDP phosphor because of its longevity in a Xe‐discharge plasma environment.     

Properties and cathodoluminescence of Y2O2S:Eu thin films

Abstract— Europium‐activated yttrium oxysulfide thin films were fabricated by electron‐beam deposition of Y₂O₂S:Eu with consequent annealing in an H₂S/Ar gas mixture. Transformations in film composition and surface morphology as well as corresponding changes in the optical, photoluminescent, and cathodoluminescent properties were studied and will be discussed. It is shown that 248‐nm laser irradiation of heat‐treated films improves cathodoluminescence by as much as 30%, supposedly due to annealing. The developed phosphor films have a luminance comparable to that of annealed europium‐activated yttrium oxide films and improved color properties (CIE color coordinates: x = 0.624, y = 0.329), and are suitable for display and optoelectronics applications.     

Single‐source dual‐layer amorphous IGZO thin‐film transistors for display and circuit applications

In this study, the authors report on high‐quality amorphous indium–gallium–zinc oxide thin‐film transistors (TFTs) based on a single‐source dual‐layer concept processed at temperatures down to 150°C. The dual‐layer concept allows the precise control of local charge carrier densities by varying the O₂/Ar gas ratio during sputtering for the bottom and top layers. Therefore, extensive annealing steps after the deposition can be avoided. In addition, the dual‐layer concept is more robust against variation of the oxygen flow in the deposition chamber. The charge carrier density in the TFT channel is namely adjusted by varying the thickness of the two layers whereby the oxygen concentration during deposition is switched only between no oxygen for the bottom layer and very high concentration for the top layer. The dual‐layer TFTs are more stable under bias conditions in comparison with single‐layer TFTs processed at low temperatures. Finally, the applicability of this dual‐layer concept in logic circuitry such as 19‐stage ring oscillators and a TFT backplane on polyethylene naphthalate foil containing a quarter video graphics array active‐matrix organic light‐emitting diode display demonstrator is proven.     

Lanthanide‐doped sol‐gel‐derived films fabricated in mesoporous anodic alumina

Abstract— Porous anodic alumina films with controllable morphology were fabricated under varied anodizing conditions and examined by scanning electron microscopy and photoluminescence analysis. Erbium‐, terbium‐, and europium‐doped xerogel films were formed inside the porous layer. Strong visible and infrared photoluminescence from the erbium‐doped xerogel/porous anodic alumina structure was observed. An electroluminescent cell based on xerogel/porous anodic alumina structure was also developed, with terbium‐ and europium‐related electroluminescence investigated.     

Letter: Solution‐processed flexible zinc‐tin oxide thin‐film transistors on ultra‐thin polyimide substrates

In this letter, solution‐processed flexible zinc‐tin oxide (Z_0.35T_0.65O_1.7) thin‐film transistors with electrochemically oxidized gate insulators (AlOx:Nd) fabricated on ultra‐thin (30 µm) polyimide substrates are presented. The AlOx:Nd insulators exhibited wonderful stability under bending and excellent insulating properties with low leakage current, high dielectric constant, and high breakdown field. The device exhibited a mobility of 3.9 cm²/V · s after annealing at 300 °C. In addition, the flexible device was able to maintain the electricity performance under various degrees of bending, which was attributed to the ultra‐thin polyimide substrate.     

Asymmetric source/drain offset structure for reduced leakage current in polycrystalline‐silicon thin‐film transistors

Abstract— An asymmetric source/drain offset structured (AOS) polycrystalline‐silicon (poly‐Si) thin‐film transistor (TFT) has ben developed by employing alternating magnetic‐field‐enhanced rapid thermal annealing (AMFERTA). The realized AOS poly‐Si TFT, with long drain‐side offset length L_Off1 and short source‐side offset length L_Off2, considerably suppresses leakage current without sacrificing ON‐current. The offset regions of the AOS TFT are naturally lightly doped due to the diffusion of n⁺ ions by AMFERTA crystallization. The fabrication process of the AOS TFT does not require any additional offset mask step or doping process. Experimental results show that the leakage current is considerably suppressed when the drain‐side offset length L_Off1 is larger than 1.25 μm.     

Orange‐red upconversion luminescence of Sm3+‐doped ZnO‐B2O3‐SiO2 glass by infrared femtosecond laser irradiation

Abstract— Near‐infrared‐to‐visible upconversion luminescence was observed in Sm³⁺‐doped ZnO‐B₂O₃‐SiO₂ glass under femtosecond laser irradiation. The luminescence spectra show that the upconversion luminescence originates from ⁴G^5/2 to ⁶H^j/2 (j = 5, 7, 9) transition of Sm³⁺. The dependence of the fluorescence intensity of Sm³⁺ on the pump power indicates that a two‐photon absorption process is dominant in the conversion of infrared radiation to the visible luminescence. The analysis of the upconversion mechanism reveals that the simultaneous absorption of two infrared photons produces the population of upper excited states, which leads to the characteristic orange‐red emission of Sm³⁺. A three‐dimensional display is demonstrated based on the multiphoton absorption upconversion luminescence.     

Flexible color LCD panel driven by low‐voltage‐operation organic TFT

Abstract— A flexible color LCD panel driven by organic TFTs (OTFTs) was successfully demonstrated. A pentacene OTFT with an anodized Ta₂O₅ gate insulator, which can be operated at low voltage, was developed. In order to improve the electrical performance of the OTFT, the gate insulator was surface treated by processes such as O₂ plasma, UV light irradiation, and hexamethyldisilane treatments. The fabricated OTFT exhibited a mobility of 0.3 cm²/V‐sec and a current on/off ratio of 10⁷ with a low operating drain voltage of ?5 V. A fast‐response‐time flexible ferroelectric LCD, which contains polymer networks and walls, was integrated with the OTFTs by using a lamination and a printing technique. As a result, color images were achieved on the fabricated panel by using a field‐sequential‐color method at a low driving voltage of less than 15 V_pp.     

Rare‐earth materials for use in the dark

Abstract— Recently, it was found that some materials doped with rare‐earth ions show bright and long‐lasting phosphorescence. They do not include radioactive elements and can be safely used as luminous paints for use in the dark. Some of them are better than the traditional zinc sulfide doped with copper (ZnS:Cu). The most important rare‐earth materials with long‐lasting phosphorescence are aluminates such as alkaline‐earth aluminates MAl₂O₄:Eu²⁺, Dy³⁺ (M = Sr, Ca) and garnets Y₃Ga₅O₁₂:Tb³⁺, Gd₃Ga₅O₁₂:Tb³⁺, Cd₃Al₂Ge₃O₁₂:Tb³⁺, Cd₃M₂Ge₃O₁₂:Pr³⁺ (M = Al, Ge), Y₃Al_5?xGa_xO₁₂:Ce³⁺ (x = 3, 3.5). Some oxides such as InBO₃:Tb³⁺, Ba₂SiO₄:Dy³⁺ also show long‐lasting phosphorescence properties. Other sulfide materials include ZnS:Eu, Ca_xSr_1?x S:Bi, Tm, Cu or Ca_xSr_1?xS:Eu. Alkaline‐earth aluminates MAl₂O₄:Eu²⁺ (M = Mg, Ca, Sr, Ba) codoped with RE³⁺ (RE = Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) were synthesized by using homogeneous precipitation method.