PDP放电单元Si掺杂MgO保护层二次电子发射系数理论研究

采用基于密度泛函理论的原子轨道展开方法的第一性原理计算了Mg1-xSixO体系的电子结构、用SIESTA软件包计算了Mg1-xSixO体系的晶胞结构,体系基态总能,确定了MgO的最优晶格常数。同时,计算了Mg1-xSixO的能带结构、态密度、分波态密度等。针对采用氖氙混合气体的PDP放电单元,分析了Si掺杂对MgO晶体的电子结构以及氖离子和氙离子二次电子发射系数的影响。结果表明,掺入微量的Si(掺杂浓度小于0.06)可提高MgO保护层氖离子和氙离子的二次电子发射系数,其中氙离子的二次电子发射系数的提高尤为显著。同时由分析结果表明,Si掺杂量存在最优化值为0.0185。     

MgO表面的第一性原理研究

MgO薄膜被认为是等离子体显示器件中介质保护膜的最理想材料。本文基于第一性原理的密度泛函理论,结合格林函数G和动力学屏蔽库仑相互作用W纠正和Bethe-Salpeter方程的求解,研究了MgO不同晶面取向表面结构的电子特性和光谱特性。在分析不同晶面取向表面能带结构、态密度分布的基础上,估算了相应的二次电子发射系数。结果表明:(100)、(110)和(111)三种晶面取向,(100)禁带宽度最小,二次电子发射系数最大。而(111)晶面吸附氢原子后,禁带宽度显著减小,二次电子发射系数优于(100)晶面,是高Xe、高气压工作气体下等离子体显示器介质保护层材料MgO的优选取向。激子光谱计算结果表明,MgO表面有强激子效应,有利于外电子发射。合理选择MgO晶面取向,可有效提高等离子体显示器放电效率、降低其着火电压,减小Jitter时间。     

LaNi4.5Mn0.5储氢合金及氢化物的电子结构

基于密度泛函理论,采用总能量计算方法与结合超软赝势平面波函数方法,对LaNi4.5Mn0.5储氢合金及其氢化物的晶体几何结构进行了优化,计算了其相应的总体能量、晶体结构、能带结构、及态密度分布等,从理论上给出了其结构参数及性质.结果表明,锰取代3g位后合金晶胞略有膨胀,并伴随着晶体稳定性变差.LaNi4.5Mn0.5合金中EF附近的态密度贡献最主要来自La的p电子,以及Ni和Mn原子的d电子.     

β-CaSiO3晶体的电子结构及光学性质

基于第一性原理的平面波赝势方法(PWP)的局域密度近似(LDA)/广义梯度近似(GGA)计算了β-Ca-SiO3的几何结构、能带结构、态密度和光学性质。其晶胞参数优化结果与实验相比,LDA/GGA的相对误差为-3.62%/1.91%。对优化后的β-CaSiO3晶体进行能带结构分析表明,β-CaSiO3晶体为间接带隙结构,禁带宽度Eg(LDA)=5.53eV,Eg(GGA)=5.18eV。对态密度图及Mulliken电荷分布的分析表明,Ca的d轨道有电子分布,即Ca的s、p、d轨道均参与了成键。β-CaSiO3晶体中Ca与SiO3基团之间形成的化学键主要是离子键,而Si与O之间的化学键是共价键。     

一个新的测量二次电子发射系数的实验方法

本文提出了测量二次电子发射系数的一种新方法-三枪连续脉冲法。这种方法可以用来测定各种金属、介质、半导体、晶体等材料的全能量范围二次电子、发射系数δ。介绍了三枪连续脉冲的原理和实验装置。并给出了用三枪连续脉冲法测量DKDP电光晶体的全能量范围二次电子发射系数。结果表明DKDP晶体的二次电子发射系数与一般介质的二次电子发射系数差不多。还给出了以镍为基底和以DKDP晶体为基底的发射介质膜〔ZnS〕~5+〔MgF_2〕~4的二次电子发射系数。结果表明,基底不同,反射介质膜的电子发射系数也不同。对相同基底的反射介质膜〔ZnS〕~5+〔MgF_2〕~4的二次电子发射系数测量表明它既不同于ZnS也不同于MgF_2的二次电子发射系数。     

SrTiO3材料中氧空位的密度泛函理论

计算了SrTiO3-δ（δ=0，δ=0．125）体系电子结构，分析了氧空位对SrTiO3晶体的价键结构、能带、态密度、分波态密度、差分电荷密度的影响。所有计算都是基于密度泛函理论（DFT）框架下的第一性原理平面波超软赝势方法。计算结果表明：当氧空位浓度δ=0．125时，空位在母体化合物SrTiO3中引入了大量的传导电子，费米能级进入导带，体系显示金属型导电性。由于空位掺杂，导带底附近的态密度发生了畸变，刚性能带模型不再适合描述SrTiO2.875体系。同时，在导带底附近距离费米能级0．3eV处引入了空位能级，这和实验测得的SrTiO3材料内中性氧空位的电离能约为0．4eV相符。此外，Mulliken布局分析、分波态密度和差分电荷密度分析表明，该空位能级主要由与其最近邻的两个Ti原子的3d电子态贡献，并且由该空位引入的导电电子大部分都局域在空位最近邻的两个Ti原子周围。最后，计算了三种典型平衡条件下SrTiO3晶体内中性氧空位的形成能。     

离子轰击MgO薄膜二次电子发射的研究

氧化镁因其二次电子发射系数高、抗溅射能力强等优异的性能,广泛应用于平板显示器等电子器件中,其二次电子发射性能有重要的研究价值。介绍了离子轰击下氧化镁薄膜发射的二次电子的典型测量装置及相关研究结论,总结了离子轰击下氧化镁薄膜二次电子的发射特性,同时对离子轰击的材料产生的二次电子发射的研究提出了建议。脉冲中和法比较适用于离子轰击下的氧化镁薄膜的二次电子发射的测量;不同晶面的MgO薄膜的二次电子发射系数不同,（111）晶面最高;低能离子入射情况下,二次电子的能量分布与离子类型无关。     

基于GW+BSE第一性原理PDP保护层材料Mg_(1-x)Zn_xO激子光谱研究

Mg O薄膜被认为是等离子体显示器件中介质保护膜的最理想材料。本文基于密度泛函理论的第一性原理,研究了掺锌Mg O保护层材料的特性。通过计算不同掺锌浓度下Mg1-xZnxO复合材料的电子结构,分析相应的能带结构和波态密度分布,并在此基础上通过求解Bethe-Salpeter方程计算相应的动态介电函数,以及激子光谱,激子谐振强度和激子束缚能。结果表明:随着掺锌浓度的增加,Mg1-xZnxO复合材料的禁带宽度和激子束缚能均逐渐减小,激子光谱发生红移。说明Mg O掺杂锌后一方面有利于提高保护层材料的二次电子发射效率降低其着火电压;另一方面有利于提高外逸电子发射,减小等离子体显示器件的寻址jitter,从而增强放电的稳定性。     

第一原理研究LuPO4中氧空位的性质

磷酸结构的晶体在掺杂二价阳离子后容易形成产生焦磷酸结构(P₂O₇) ^4-, 这种含有焦磷酸结构的氧化物材料十分适合做质子导体、燃料电池、气体传感器以及陶瓷膜等。本文利用第一性原理研究了LuPO₄晶体中氧空位的结构性质, 结果显示当氧空位带二价正电时, 会引发氧空位周围原子奇特的畸变, 形成焦磷酸结构。为了解释这种结构畸变的机理, 本文利用过渡态搜索计算了结构变化过程中势能面的变化情况, 正一价氧空位形成焦磷酸 结构需要越过2.4 eV的势垒, 而正二价氧空位形成焦磷酸结构则不需要越过任何势垒, 因此很容易形成焦磷酸 结构。最后给出氧空位不同带电态的晶格结构、电子态密度以及电荷密度分布等基本物理性质, 氧空位处于正二价态结构下, 氧空位附近的P原子与O原子成键, 又由于O原子有较强的电负性, P的s轨道电子向O的p轨道转 移。P的s、p轨道在禁带中出现了与总态密度对应的缺陷能级, 结果表明带正二价氧空位的晶体性质发生了明显变化。     

伴有二次电子发射的磁化等离子体鞘层结构特性

建立了包括电子、离子以及器壁发射二次电子的磁化等离子体鞘层流体模型,采用四阶龙格库塔法数值研究了伴有二次电子发射的磁鞘结构特性。模拟结果显示二次电子发射对于弱磁等离子体鞘层中的离子密度影响较大,而对于磁场较强的等离子体鞘层,鞘层中离子密度分布主要由磁场来决定。磁场的存在可以促进器壁电子的发射,磁场的增加或二次电子发射系数的增加都将使得鞘层厚度的减小,同时将导致沉积到器壁的离子动能流发生变化,从而直接影响器壁材料的性能。     

Characterization of C12A7 thin films deposited by spray pyrolysis

In this work, conductive C12A7 thin films were deposited by spray pyrolysis method onto glass substrates. The films, structural, optical and electrical properties were investigated as a function of the spray number. X-rays diffraction showed that the deposited films were polycrystalline with a preferential orientation along the (310) planes. Raman spectroscopy confirmed the C12A7 phase and revealed the superoxide radical \( {\text{O}}_{2}^{ - } \) presence. The C12A7 films, optical transmission varied between 57 and 75 % as a function of the spray number. A constant band energy (4.14 eV), determined from UV–visible spectra, was attributed to the electrons transition from the valence band to the occupied cage level. According to the photoluminescence (PL) spectroscopy, two main emission peaks at 1.55 and 2.81 eV were respectively attributed to the formation of the “F⁺-like centers” and the electron transitions from the occupied cage level to the framework conduction band. Another emission peak at about 2.27 eV was attributed to the cages oxygen vacancies defects. The electrical resistivity variation between 10^?4 and 1.36 Ω cm was correlated to the in cages oxygen vacancies produced during films deposition.     

Effect of oxygen pressure on the structural and optical properties of ZnO thin films on Si (111) by PLD

ZnO thin films were grown on Si (111) substrates by pulsed laser deposition (PLD) at various oxygen pressures in order to investigate the structural and optical properties of the films. The optical properties of the films were studied by photoluminescence spectra using a 325 nm He-Cd laser. The structural and morphological properties of the films were investigated by XRD and AFM measurements, respectively. The results suggest that films grown at 20 Pa and 50 Pa have excellent UV emission and high-quality crystallinity. The research of PL spectra indicates that UV emission is due to excitonic combination, the green band is due to the replacing of Zn in the crystal lattice for O and the blue band is due to the O vacancies.     

Measurement of valence band structure in arbitrary dielectric films

A new way of measuring the band structure of various dielectric materials using the secondary electron emission from Auger neutralization of ions is introduced. The first example of this measurement scheme is the magnesium oxide (MgO) films with respect to the application of the films in the display industries. The density of state in the valence bands of MgO film and MgO film with a functional layer (FL) deposited over a dielectric surface reveals that the density peak of film with a FL is considerably less than that of film, thereby indicating a better performance of MgO film with functional layer in display devices. The second example of the measurement is the boron-zinc oxide (BZO) films with respect to the application of the films to the development of solar cells. The measurement of density of state in BZO film suggests that a high concentration of boron impurity in BZO films may enhance the transition of electrons and holes through the band gap from the valence to the conduction band in zinc oxide crystals; thereby improving the conductivity of the film. Secondary electron emission by the Auger neutralization of ions is highly instrumental for the determination of the density of states in the valence band of dielectric materials.     

Structural and optical properties of La-doped ZnO films prepared by magnetron sputtering

La-doped ZnO films were prepared by RF magnetron sputtering using different composition powder compacted targets (0, 1, 2, 3 and 5 at.%). All films show a preferred c-axis growth orientation. Furthermore, the (002) diffraction peak shifts to a small angle and the full-width at half-maximum augments with increasing La concentration up to 2 at.%, which indicate that a small quantity of La atoms are incorporated into the ZnO lattice. The average transmittance in the visible range is over 80%, and a blue shift of the absorption edge is observed. With increasing La concentration, the band gap of ZnO films evaluated by the linear fitting linearly increases from 3.270 to 3.326 eV. In the photoluminescence spectra, a strong violet emission peak and a weak green emission band can be observed. The former is due to the electron transition between the defect energy levels, associated with the interfacial traps existing at the ZnO grain boundaries, and valence band. The latter could be ascribed to crystal defects related to oxygen vacancies.     

Evidence for a crucial role played by oxygen vacancies in LaMnO3 resistive switching memories

LaMnO(3) (LMO) films are deposited on SrTiO(3):Nb (0.8 wt%) substrates under various oxygen pressures to obtain different concentrations of oxygen vacancies in the films. The results of X-ray diffraction verify that with a decrease of the oxygen pressure, the c-axis lattice constant of the LMO films becomes larger, owing to an increase of the oxygen vacancies. Aberration-corrected annular-bright-field scanning transmission electron microscopy with atomic resolution and sensitivity for light elements is used, which clearly shows that the number of oxygen vacancies increases with the decrease of oxygen pressure during fabrication. Correspondingly, the resistive switching property becomes more pronounced with more oxygen vacancies in the LMO films. Furthermore, a numerical model based on the modification of the interface property induced by the migration of oxygen vacancies in these structures is proposed to elucidate the underlying physical origins. The calculated results are in good agreement with the experimental data, which reveal from a theoretical point of view that the migration of oxygen vacancies and the variation of the Schottky barrier at the interface with applied bias dominate the resistive switching characteristic. It is promising that the resistive switching property in perovskite oxides can be manipulated by controlling the oxygen vacancies during fabrication or later annealing in an oxygen atmosphere.     

Influence of oxygen partial pressure on the structure and photoluminescence of direct current reactive magnetron sputtering ZnO thin films

The effects of oxygen partial pressure on the structure and photoluminescence (PL) of ZnO films were studied. The films were prepared by direct current (DC) reactive magnetron sputtering with various oxygen concentrations at room temperature. With increasing oxygen ratio, the structure of films changes from zinc and zinc oxide phases, single-phase ZnO, to the (002) orientation, and the mechanical stresses exhibit from tensile stress to compressive stress. Films deposited at higher oxygen pressure show weaker emission intensities, which may result from the decrease of the oxygen vacancies and zinc interstitials in the film. This indicates that the emission in ZnO film originates from the oxygen vacancy and zinc interstitial-related defects. From optical transmittance spectra of ZnO films, the plasma edge shifts towards the shorter wavelength with the improvement of film stoichiometry.     

Influence of hydrogen-doped MgO thin films on the discharge characteristics in plasma display panels

In order to improve the discharge characteristics of Magnesium Oxide (MgO) thin films, hydrogen was doped to MgO thin films using an ion plating technique. Changes in the surface morphology, crystal orientation, optical properties, secondary electron emission coefficient, and defect states were studied with increasing hydrogen flow rates during the growth using field emission scanning electron microscope, X-ray diffraction, ellipsometry, γ-focused ion beam, and photoluminescence analysis. The change in firing voltage and delay time in plasma display panels (PDP) with the hydrogen-doped MgO thin films were also investigated. The results indicated that optimal hydrogen doping conditions can affect the surface structure and defect states: resulting in a significant reduction in the firing voltage and delay time of the PDP.     

Structures and properties of titania thin films annealed under different atmosphere

The effects of crystallinity, phase and oxygen vacancies on optical and photocatalytic properties of titania (TiO₂) thin films were systematically studied. The as-deposited amorphous titania films were prepared by reactive sputtering titanium metal targets in argon–oxygen plasma at 100 °C and subsequently annealed at various temperatures of 400–800 °C in air, vacuum and H₂ atmosphere. The results indicate that in general the crystallinity of the annealed films is enhanced with the increasing annealing temperature. At the same temperature, the H₂ annealed films achieve better crystallinity but containing more oxygen vacancies than the films annealed in air and in vacuum. In H₂ or in vacuum, the concentration of oxygen vacancies in the annealed films increases with increasing temperature, while in air it remains constant. Oxygen vacancies in titania film not only facilitate phase transformation but also lower the band gap of titania, and make the film visible-light responsive. Photocatalytic properties of the TiO₂ films were characterized in UV and visible light irradiation by following the Ag reduction and degradation of methylene blue. The films annealed at 600–700 °C in H₂ possess the best film crystallinity and the proper concentration of oxygen vacancies and exhibit the best photocatalytic performance under both UV and visible light.     

Thermopower and optical studies on undoped and manganese doped indium tin oxide films

Thermopower measurements in the range of 300-650 K along with room temperature optical absorption and electrical resistivity studies have been performed on indium tin oxide (ITO) and manganese doped indium tin oxide (Mn:ITO) thin films grown by reactive DC sputtering. The thermopower of the films measured in Ar ambient displayed irreversible changes attributable to oxygen loss. Thermopower, carrier concentration and resistivity of the films have been found to depend on oxygen vacancies and Mn concentration. The observations have been substantiated with optical absorption and room temperature electrical resistivity results. It has also been observed that band gap tuning in these films is possible by the introduction of Mn as well as oxygen vacancies.     

Influences of oxygen pressure on optical properties and interband electronic transitions in multiferroic bismuth ferrite nanocrystalline films grown by pulsed laser deposition

Bismuth ferrite (BiFeO(3)) nanocrystalline films with the crystalline size of 27-40 nm have been grown on c-sapphire substrates under various oxygen pressures of 1 × 10(-4) to 1 Pa by pulsed laser deposition. The X-ray diffraction spectra show that the films are polycrystalline and present the pure rhombohedral phase. It was found that the Raman-active phonon mode E(TO1) shifts towards a higher energy side from 74 to 76 cm(-1) with increasing oxygen pressure, indicating a larger tensile stress in the films deposited at higher oxygen pressure. The X-ray photoelectron spectroscopy analysis suggests that the concentrations of both Fe(2+) ions and oxygen vacancies in the BiFeO(3) films increase with decreasing oxygen pressure. Moreover, the dielectric functions in the photon energy range of 0.47-6.5 eV have been extracted by fitting the transmittance spectra with the Tauc-Lorentz dispersion model. From the transmittance spectra, the fundamental absorption edge is observed to present a redshift trend with increasing the temperature from 8 to 300 K. Note that the optical band gap (E(g)) decreases with increasing the temperature due to the electron-phonon interactions associated with the interatomic distance in the BiFeO(3) films. However, the E(g) decreases from 2.88 to 2.78 eV with decreasing oxygen pressure at 8 K, which can be attributed to the increment of oxygen vacancies leading to the formation of some impurity states between the valence and conduction band. It can be concluded that the oxygen pressure during the film fabrication has the significant effects on microstructure, optical properties, and electronic band structure modification of the BiFeO(3) films.