用第一性原理研究K空位对KDP晶体激光损伤的影响

 用基于密度泛函理论及超软赝势的第一性原理研究了KH­­­₂PO₄(KDP)晶体中K空位的电子结构、形成能及驰豫构型。讨论了K空位形成后电荷密度的重新分布、相应的电子态密度和能带结构等性质。计算得到中性K空位的形成能为6.5 eV, 远小于间隙K原子点缺陷形成能13.07 eV。K空位的存在使晶胞体积增大, 分别沿结晶学轴a方向增大近0.8%, b方向增大近0.87%, c方向增大近1.2%，同时使与之配位的8个氧原子发生较大位移，使这8个氧形成的空腔体积增大近3.2%。空腔体积的增大不仅促进了各种点缺陷的扩散迁移，而且有利于其它杂质原子的填隙。K原子迁移率的增大会引起离子电导率的增大，因而会降低KDP的激光损伤阈值，因此从这个方面讲，K空位的存在是不利的。但是如果能从实验上（如热退火）利用K空位所造成的扩散通道排出或改善缺陷结构，则可提高KDP晶体的光学质量。     

金红石型TiO2点缺陷性质的第一性原理研究

本文运用基于局域密度泛函和赝势的第一性原理方法研究了金红石相TiO2点缺陷的电子性质,结果表明氧空位缺陷使晶体的费米能量升高,在能隙中没有产生杂质能级.钛空位缺陷使晶体的费米能量降低,并在价带顶部产生了一个杂质能级,与价带顶能量相差约0.4 eV.本文还计算了金红石相TiO2在具有氧空位和钛空位点缺陷情况下的键长变化、态密度和电荷布居状况.     

PbWO4晶体中的填隙氧及其对晶体光学性质的影响

运用GULP计算软件模拟计算了PbWO₄(PWO)晶体中不同位置的填隙氧原子点缺陷的生成能，计算结果表明：当填隙氧原子存在于(WO₄)^2-的周围时，填隙氧原子点缺陷的生成能最低；进一步运用基于密度泛函理论的全数值自洽DV-Xα方法计算了包含填隙氧原子的PWO晶体的态密度，计算结果表明：当填隙氧处在(WO₄)^2-的周围时，容易与(WO₄)^2-上的一个或两个氧离子相互作用形成分子离子O₂^2-或O₃^4-，通过分析这些计算结果，认为PWO晶体中350 nm吸收带的出现很可能与晶体中的氧分子离子有关.     

KDP晶体中点缺陷Na取代K的电子结构研究

 对磷酸二氢钾(KDP)晶体中Na取代K点缺陷的几何结构及电子结构进行了第一性研究。计算的形成能约为0.46 eV，因此在KDP晶体中此类缺陷比较容易形成。Na取代K以后没有在带隙中形成缺陷态，但在价带中引入两个占据态。它们分别位于费米面以下49 eV和21.5 eV处，这两个占据态分别由Na原子的s和p轨道形成。相对于K来说，由于它们位于价带深处，具有很低的能量，因此Na在KDP中比K稳定。Na在KDP晶体中与周围氧原子的重叠布居仅为0.09, 故它不与主体原子发生共价作用，仅以静电库仑力影响周围原子，此缺陷周围晶格仅发生微小畸变。     

单层缺陷碲烯电子结构与光学性质的第一性原理研究

碲烯是性质优异的新型二维半导体材料,研究缺陷碲烯的电子结构有助于理解载流子掺杂、散射等效应,对其在电子和光电器件中的应用有重要意义.本文采用基于密度泛函理论的第一性原理计算,研究了常见点缺陷对单层β相碲烯电子结构和光学性质的影响,包括单空位、双空位及Stone Wales缺陷.研究发现,单层β相碲烯中单空位、双空位和Stone Wales缺陷的形成能在0.83—2.06 eV范围,低于石墨烯、硅烯、磷烯和砷烯中对应缺陷的形成能,说明实验上单层β相碲烯中容易形成点缺陷.点缺陷出现后,单层β相碲烯带隙宽度少有变化,禁带中出现依赖于缺陷类型的局域能级,部分双空位缺陷和Stone Wales缺陷使其由直接带隙变为间接带隙.同时我们发现,单层β相碲烯的光吸收性质与缺陷类型密切相关.部分缺陷能增大其静态介电常数,使它的复介电函数的实部、虚部及吸收系数在0—3 eV能量范围内出现新的峰,增强它在低能区的光响应、极化能力及光吸收.本文研究可为碲烯在电子和光电子器件中的应用提供理论参考.     

六方AlN本征缺陷的第一性原理研究

用基于密度泛函理论平面波赝势法首先对六方AlN本征点缺陷(氮空位、铝空位、氮替代铝、铝替代氮、氮间隙、铝间隙)存在时的晶格结构进行优化，得到其稳定结构；然后通过各缺陷形成能的计算可得知其在生长过程中形成的难易程度；最后从态密度的角度对各种本征点缺陷引起的缺陷能级及电子占据情况进行了分析.发现除氮空位外其他本征缺陷在带隙中形成的能级都很深，要得到n型或p型AlN必须要引入外来杂质.计算得到的本征缺陷能级对于分析AlN的一些非带边辐射机理有重要帮助. 关键词： 六方AlN 形成能 缺陷能级 态密度     

钨中氢同位素热脱附实验的速率理论模拟研究

本文采用基于速率理论的模拟方法研究钨材料中氢同位素氘的热脱附谱. 热脱附数据来源于520 K下受等离子体辐照的多晶钨, 入射离子能量为40 eV, 剂量为1× 10²⁶ D/m². 通过调节速率理论中的俘获能、俘获率等参数, 最终获得与实验相符合的热脱附拟合谱. 拟合结果表明, 钨中俘获的氘存在于三种俘获态, 俘获能分别为1.14 eV, 1.40 eV和1.70 eV, 相应脱附温度峰值为500 K, 600 K和730 K. 这三个俘获能分别应对应于第一原理计算得到的空位俘获第3–5个氢原子的俘获能(含零点振动能修正)、空位俘获第1–2个氢原子的俘获能, 空位团簇对氢原子的俘获能. 模拟结果表明, 在本辐照实验条件下, 钨中空位及空位团簇是氘在钨中的主要俘获态.     

点缺陷对单层MoS2电子结构及光学性质的影响研究

采用基于第一性原理的贋势平面波方法,对不同类型点缺陷单层MoS2电子结构、能带结构、态密度和光学性质进行计算。计算结果表明：单层MoS2属于直接带隙半导体,禁带宽度为1.749ev,V-Mo缺陷的存在使得MoS2转化为间接带隙Eg=0.671eV的p型半导体,V-S缺陷MoS2的带隙变窄为Eg=0.974eV,S-Mo缺陷的存在使得MoS2转化为间接带隙Eg=0.482eV; Mo-S缺陷形成Eg=0.969eV直接带隙半导体,费米能级上移靠近价带。 费米能级附近的电子态密度主要由Mo的4d态和s的3p态电子贡献。光学性质计算表明：空位缺陷对MoS2的光学性质影响最为显著,可以增大MoS2的静态介电常数、折射率n0和反射率,降低吸收系数和能量损失。     

氘化对KH2PO4晶体微观缺陷影响的正电子湮没研究

采用传统降温法从不同程度氘化(x=0, 0.51, 0.85)的生长溶液中生长氘化KH₂PO₄(KDP) 晶体, 利用正电子湮没技术(正电子寿命谱和多普勒展宽谱)、结合X射线衍射谱(XRD) 结构分析, 对KDP晶体氘化生长的微观缺陷进行了研究, 讨论了氘化程度对晶体内部微观结构特性、缺陷类型和浓度的影响. XRD结果显示晶胞参数a, b值随氘含量的增加而增加, c值无明显变化; 正电子寿命谱结果发现随着氘化浓度的提高, KDP晶体内部中性填隙缺陷以及氧缺陷不断增加, 引起晶体晶格畸变; 氢空位、K空位、杂质替位缺陷不断发生缔合反应形成复合缺陷, 缺陷浓度不断减少; 团簇、微空洞等大尺寸缺陷也在不断发生聚合反应, 缺陷浓度表现为不断减少. 多普勒实验结果表明随着氘化程度的提升, 晶体内部各类缺陷表现为同步变化. 实验结果表明, KDP晶体在低浓度氘化生长(50%以内)下缺陷反应较弱, 而在高浓度氘化(50%以上)下的缺陷反应显著增强.     

Cu离子注入单晶TiO_2微结构及光学性质的模拟研究

TiO_2是一种新型的第三代半导体材料,具有重要的应用价值.Cu离子掺杂单晶金红石TiO_2,可以改善TiO_2对光谱的响应范围,提高转化效率.本文利用第一性原理分别研究了Cu离子填隙、Cu替代Ti、氧空位、钛空位以及含有复合缺陷时金红石TiO_2结构及其相应光学性质的变化.结果表明,金红石的价带顶主要由O 2p轨道贡献,导带底主要由Ti 3d轨道贡献;掺杂Cu离子后会在能隙中产生两条新的杂质能级;Ti空位使得晶体费米能量降低,在价带顶产生新能级;O空位使得费米能量升高,在导带底产生新能级,表现出n型半导体性质.通过对含有复合缺陷的晶体电子结构的分析,得到同时含有O空位和Cu填隙时对晶体在可见光范围的吸收影响最大.     

Formation energies and electronic structures of native point defects in potassium dihydrogen phosphate

The formation energies and the equilibrium concentration of vacancies,interstitial H,K,P,O and antisite structural defects with P and K in KH 2 PO 4 (KDP) crystals are investigated by ab initio total-energy calculations.The formation energy of interstitial H is calculated to be about 2.06 eV and we suggest that it may be the dominant defect in KDP crystal.The formation energy of an O vacancy (5.25 eV) is much higher than that of interstitial O (0.60 eV).Optical absorption centres can be induced by defects of O vacancies,interstitial O and interstitial H.We suggest that these defects may be responsible for the lowering of the damage threshold of the KDP.A K vacancy defect may increase the ionic conductivity and therefore the laser-induced damage threshold decreases.     

Effect of S Substitution for P Point Defects in KDP Crystals： First-Principles Study

The electronic structure and geometric distribution of phosphor replaced by sulfur in potassium dihydrogen phosphate （KDP） are investigated by first-principles calculations. The point defect narrows down the energy gap to about 4.9eV, corresponding to a two-photon absorption of 355nm after correction. This can explain the decrease of the laser damage resistance in KDP crystals. Moreover, the defects twist the crystal structure and weaken bonds, especially the O-H bonds, so these bonds may be the first sites to crack under laser irradiation.     

Defect stability in thorium monocarbide:An ab initio study

The elastic properties and point defects of thorium monocarbide(ThC) have been studied by means of density functional theory based on the projector-augmented-wave method. The calculated electronic and elastic properties of ThC are in good agreement with experimental data and previous theoretical results. Five types of point defects have been considered in our study, including the vacancy defect, interstitial defect, antisite defect, schottky defect, and composition-conserving defect. Among these defects, the carbon vacancy defect has the lowest formation energy of 0.29 eV. The second most stable defect(0.49 eV) is one of composition-conserving defects in which one carbon is removed to another carbon site forming a C2 dimer. In addition, we also discuss several kinds of carbon interstitial defects, and predict that the carbon trimer configuration may be a transition state for a carbon dimer diffusion in Th C.     

六方氮化硼单层中一种(CN)3VB缺陷的第一性原理计算

二维六方氮化硼(hBN)的点缺陷最近被发现可以实现室温下的单光子发射,而成为近年的研究热点.尽管其具有重要的基础和应用研究意义,hBN中发光缺陷的原子结构起源仍然存在争议.本文采用基于密度泛函理论的第一性原理计算,研究hBN单层中一种B空位附近3个N原子被C替代的缺陷(CN)3VB.在hBN的B空位处,3个N原子各自带一个在平面内的悬挂键及相应的未配对电子,而通过C替换可以消除未配对的电子.系统研究了(CN)3VB缺陷的几何结构、电子结构以及光学性质,结果表明,缺陷可以由一个对称的亚稳态经过原子结构弛豫变成1个非对称的、3个C原子连在一起的基态结构.缺陷的形成在hBN中引入了一些由缺陷悬挂σ键及重构的π键贡献的局域缺陷态.这些缺陷态可以导致能量阈值在2.58 eV附近的可见光内部跃迁.本文的工作有助于进一步理解hBN中点缺陷的构成及光学性质,为实验上探讨发光点缺陷的原子结构起源及其性质提供理论依据.     

Electronic Structure and Optical Property Calculation of an Oxygen Vacancy in NH_4H_2PO_4 Crystals

The electronic structure of perfect ammonium dihydrogen phosphate(ADP) and defective ADP with an oxygen(O) vacancy are calculated by screened-exchange hybrid density functional HSE06. The optimized structural parameters of the defective ADP crystal are analyzed. The PO_4 tetrahedron with an O vacancy is distorted and its symmetry is broken. The band gap of the defective ADP with an O vacancy is about 1.5 eV lower than the perfect ADP, which is due to the new O vacancy defect states near the valence band maximum. Moreover, more peaks appear in the low-energy region(lower than 6 eV) in the curves of the linear optical properties for the defective ADP. The results indicate that the O vacancy will significantly influence the laser damage performance of ADP crystals.     

Molecular dynamics simulation of defects production in the vicinity of voids

We report on the results of computer simulation of point defect production near voids in crystalline Cu at primary knock-on atom (PKA) kinetic energies ranging from 5 to 1000?eV. The PKA energy dependence of numbers of created defects are revealed. The threshold energy for a stable vacancy formation is found to be much smaller than that for an interstitial atom, which results in a biased formation of vacancies in the void proximity in the whole investigated range of PKA energies. Dissolution of small voids by subthreshold irradiation is simulated. The impact of considered radiation effects on kinetics of radiation damage is discussed.     

Effect of Ba in KDP crystal on the wavelength dependence of laser-induced damage

Ba impurity in potassium dihydrogen phosphate(KDP) is studied with the first-principle simulation method. The relaxed configurations and density of the states of KDP crystal with Ba impurity are calculated. We find that Ba can generate a K vacancy and an interstitial O-H unit for charge compensation. The band gap of KDP crystal narrowed down to about 3.9 eV,which is consistent with the experimental data from previously reported studies and indicates that Ba may be a source of low-damage threshold.     

Investigation of radiation defects in Fe-Cr alloy

Abstract

The kinetics of radiation defect accumulation and subsequent recovery during/after electron irradiation below 273 K, at 323 K and 373 K were investigated for the Fe-15.7 at.% Cr using positron annihilation measurements at room temperature. Formation of vacancy clusters was observed at all of the irradiation temperatures. The formation of clusters and kinetics of their accumulation point to mobility of vacancies at least at room temperature. The cluster rearrangement and variations in the cluster configuration take place during annealing.     

Ab initio study of oxygen point defects on tungsten trioxide surface

The gas response of tungsten trioxide (WO3) based sensors strongly depends on the surface properties. Reconstructed surfaces and oxygen point defects at the surface of the monoclinic WO3 are studied using a self-consistent scheme based on first-principle. The oxygen vacancy is found to be the predominant defect independently of the oxygen partial pressure. Indeed, under rich oxygen atmosphere the formation enthalpies are found to be 1.45 eV in LDA (1.28 eV in GGA) for the oxygen vacancy instead of 2.70 eV (2.42 eV) for the oxygen adatom. When the oxygen partial pressure is lowered, the oxygen vacancy formation enthalpy decreases and becomes exothermic under very O-poor condition (? 1.65 eV in LDA and ? 1.36 eV in GGA). On the other hand, the formation enthalpy of an oxygen adatom rises. Finally, the oxygen vacancy formation acts as a n-doping by introducing negative charge carriers at the bottom of the conduction band. All these results can be very helpful in order to explain the electrical resistivity measurements.