氮对金刚石缺陷发光的影响

利用低温显微荧光光谱研究了IIa型、Ib型、Ia型金刚石的缺陷发光性质. 研究发现, 随着氮含量增加, 间隙原子及空位逐渐被氮原子所束缚, 从而使得GR1中心、533.5 nm及580 nm中心等本征缺陷发光减弱, 而氮-空位复合缺陷(NV中心)及523.7 nm中心等氮相关缺陷发光增强. 高温退火后, 间隙原子与空位可以自由移动, IIa型金刚石中出现了NV⁰中心, Ib型金刚石中只剩下了NV中心, Ia型金刚石中氮原子之间发生团聚, 出现了H3中心及N3中心. 另外, 氮作为施主原子, 有利于负电荷缺陷的形成, 如3H 中心、NV^- 中心.     

氮掺杂金刚石{100}晶面的缺陷发光特性

氮是金刚石中最常见的杂质之一, 其对金刚石的缺陷发光具有重要的影响. 氮可以与金刚石中的本征缺陷形成复合缺陷. 本文首先利用阴极射线发光照片(CL)对一个高温高压合成的氮掺杂金刚石进行表征, 发现{100}晶面为蓝色, 然后利用透射电子显微镜(TEM)对该晶面进行电子辐照及后续退火处理, 以引入本征点缺陷进而形成含氮的复合缺陷, 并利用低温光致发光光谱(PL光谱)表征其缺陷发光特性, 发现该晶面主要以氮-空位复合缺陷(NV中心)发光为主, 并伴随着较弱的503 nm发光. 关键词： 金刚石 缺陷 发光     

金刚石集群NV色心的光谱特征及浓度定量分析

金刚石NV-色心具有优越的光致发光特性,可实现高灵敏度物理量探测。其中,NV-色心的浓度是影响其宏观领域物理量探测灵敏度的重要因素之一。分析了金刚石在NV-色心制备过程中产生的发光缺陷,研究了不同的电子注入剂量与NV-色心浓度的关系。首先,对金刚石进行电子辐照并高温真空退火,制备了NV-色心;然后,利用拉曼光谱仪测试了金刚石在电子辐照前、电子辐照后及退火后三个阶段中的荧光光谱,分析了金刚石在NV-色心制备过程中的光谱特性;最后,对生成的NV-色心的浓度进行了估算,并探究了不同电子注入剂量对NV-色心浓度的影响规律。结果表明,金刚石经电子注入后生成了524.7,541.1,578和648.1 nm发光中心。其中,HPHT合成金刚石经电子注入后普遍存在524.7 nm中心。电子注入后的金刚石经高温(≥800 ℃)真空(≥10-7 Pa)退火后,空位自由移动,不稳定的缺陷消失,当空位靠近氮原子时被束缚而形成氮空位色心。对于氮含量100 ppm的金刚石,当电子注入产生的空位含量小于120 ppm(2.1×1019 cm-3)时,NV-色心浓度与电子注入生成空位的含量的关系符合Boltzmann分布。该研究为利用氮含量100 ppm的金刚石实现定量NV-色心浓度的制备提供了参考依据,为NV-色心在宏观物理量精密测量的应用奠定了基础。     

掺杂B,Cr,Mo,Ti,W,Zr后金刚石中正电子湮灭寿命计算

金属基金刚石复合材料被广泛应用和研究,但金刚石表面预处理所导致的空位、掺杂等缺陷对金属基与金刚石界面性能有很大影响.尽管透射电子显微镜和能谱分析等技术已用于缺陷检测,但这些方法存在局限性.通过计算金刚石中正电子湮灭寿命,可以准确评估金刚石的界面缺陷.本文利用第一性原理计算方法,采用多种正电子湮灭算法和增强因子,计算了金刚石理想晶体、单空位、掺杂B,Cr,Mo,Ti,W和Zr后的正电子湮灭寿命.结果显示,在采用局域密度泛函时,结合Boronski&Nieminen算法以及随机相位近似限制作为湮灭增强因子时,计算得到的正电子湮灭寿命与文献的实验结果更吻合.同时,金刚石中B和Cr的掺杂使正电子湮灭寿命从由单空位119.87 ps增加为148.57 ps和156.82 ps.总体来说,金刚石中的空位和掺杂原子缺陷都会引起正电子湮灭寿命的变化.这些发现为理解和优化金刚石界面提供了有价值的理论依据.     

CVD金刚石膜的结构分析

利用x射线广角衍射和低角掠入射散射谱、正电子湮没谱、定性分析软件和Positronfit程序，研究了生长在Si（100）基底上的金刚石膜微结构.研究发现，在样品邻近基底区域为纳米 多晶结构，具有弱的［111］织构；在邻近表面区域为微米多晶结构，具有强的［220］织构 .金刚石膜样品有空位、空位团和空洞3种缺陷，其中主要缺陷是大约10个空位形成的空位团 . 关键词： 金刚石膜 化学气相沉积 x射线掠入射 正电子湮没谱     

金刚石氮-空位色心的原子自旋声子耦合机理

金刚石氮-空位色心结构因在量子精密测量领域的高灵敏度优势而备受关注.本文引入耦合声子场对氮-空位色心原子自旋进行共振调控,以提高氮-空位色心的自旋跃迁效率.首先,基于波函数和晶格的点阵位移矢量关系,分析了声子与晶格能量交互作用,研究了基于声子共振调控的氮-空位色心的自旋跃迁机理,建立了基于应变诱导的能量转移声子-自旋交互耦合激发模型.其次,基于氮-空位色心晶格振动理论,引入满足布洛赫定理的系数矩阵,建立了不同轴向氮-空位色心第一布里渊区特征区域的声子谱模型.同时,基于德拜模型,考虑热膨胀效应,解析该声子共振系统的声子热平衡性质,并对其比热模型进行研究.最后,基于分子动力学仿真软件CASTEP和密度泛函理论进行第一性原理研究,构建了声子模式下不同轴向氮-空位色心的结构优化模型,并分析了其结构特性、声子特性和热力学特性.研究结果表明,系统声子模式的演化依赖于氮-空位的占位,声子模式强化伴随着热力学熵的降低.含氮-空位色心金刚石的共价键较纯净无缺陷金刚石更弱,热力学性质更不稳定.含氮-空位色心金刚石的声子主共振频段处于THz量级,次共振频率约为[800,1200]MHz.根据次共振频段设计叉指宽度为1.5μm的声表面波共振机构,其中心频率约为930 MHz.在该声子共振调控参数条件下,声子共振调控方法可有效增大氮-空位色心的自旋跃迁概率,实现氮-空位色心原子自旋操控效率的提高.     

金红石TiO_2本征缺陷磁性的第一性原理计算

基于密度泛函理论的平面波超软赝势方法模拟计算了金红石相TiO_2的四种本征缺陷(氧空位、钛空位、钛间隙缺陷、氧间隙缺陷)和两种复合缺陷(氧空位与氧间隙复合缺陷、钛空位与钛间隙复合缺陷)的铁磁特性.结合态密度、电子分布及晶体结构变化分析可知,四种本征缺陷均会在系统内引入缺陷态.氧空位、钛间隙缺陷使费米面升高,引起自旋极化,引入磁矩分别为1.62μB与3.91μB;钛空位的缺陷态处于价带顶,使费米面进入价带,表现出明显的p型半导体特性,引入磁矩约为2.47μB;氧间隙缺陷引入缺陷态但仍然处于自旋对称状态,费米面略微下降;氧空位与氧间隙复合缺陷使费米面上升的程度比单个氧空位时大,模拟的超晶胞保持了氧空位的铁磁特性,大小为1.63μB;钛空位与钛间隙复合缺陷以反铁磁方式耦合,但超晶胞仍具有一定的铁磁特性.     

固态单自旋量子控制研究进展

在量子物理领域的研究中,量子控制是必不可少的.精确高效的量子控制,是利用量子系统进行实验研究的前提,也是量子计算、量子传感等应用的基础.金刚石氮-空位色心作为固态自旋体系在室温下相干时间长,可用光学方法实现初始化和读出,通过微波射频场能实现普适的量子控制,是研究量子物理的优秀实验平台.本文从量子控制出发介绍金刚石氮-空位色心体系在量子物理领域取得的代表性成果,主要讨论了1)金刚石氮-空位色心的物理性质和量子控制原理, 2)氮-空位色心的退相干机制, 3)单自旋量子控制的相关应用及最近的研究进展.     

氮掺杂的金刚石磁性研究

采用第一性原理计算方法研究金刚石中由空位或者N掺杂引起的磁特性. 发现-1价和-2 价的碳空位能分别产生3 μ_B和2μ_B的磁矩; -2价的碳空位能够引发长程有序的铁磁耦合状态, 而-1价的碳空位更倾向于反铁磁耦合.掺杂N元素能有效地控制空位的荷电状态及相应的磁相互作用, 这一结果为在金刚石中实现非过渡族金属掺杂的铁磁性提供了一条新的路径. 关键词： 第一性原理计算 氮掺杂 金刚石 磁性     

掺杂金刚石薄膜的缺陷研究

利用多普勒增宽谱和电子顺磁共振研究了掺硼和掺硫金刚石薄膜的缺陷状态.多普勒增宽谱的结果表明，不同杂质元素掺杂的金刚石薄膜，其中使正电子湮没的缺陷种类是相同的；正电子与不同杂质元素硼、硫之间的相互作用不明显；少量硼可使金刚石膜中的空位浓度减少.EPR结果表明，各掺杂样品的顺磁信号主要来自于金刚石的碳悬键. 关键词： 金刚石 掺杂 多普勒增宽谱 电子顺磁共振     

Study of vacancy on diamond (1 0 0) (2 × 1) surface from first-principles

Structure and energy related properties of neutral and charged vacancies on relaxed diamond (1 0 0) (2 × 1) surface were investigated by means of density functional theory. Calculations indicate that the diffusion of a single vacancy from the top surface layer to the second layer is not energetically favored. Analysis of energies in charged system shows that neutral state is most stable on diamond (1 0 0) (2 × 1) surface. The multiplicity of possible states can exist on diamond (1 0 0) surface in dependence on the surface Fermi level, which supports that surface diffusion of a vacancy is mediated by the change of vacancy charge states. Analysis of density of states shows surface vacancy can be effectively measured by photoelectricity technology.     

Quantum Monte Carlo study of the optical and diffusive properties of the vacancy defect in diamond

Fixed-node diffusion quantum Monte Carlo (DMC) calculations of the ground and excited state energetics of the neutral vacancy defect in diamond are reported. The multiplet structure of the defect is modeled using guiding wave functions of the Slater-Jastrow type with symmetrized multideterminant Slater parts. For the ground state we obtain the 1E state in agreement with experiment. The calculated energy of the lowest dipole allowed transition is consistent with the experimentally observed GR1 band, which has long been identified with the neutral vacancy in diamond, although no previous first-principles ab initio calculation of this transition exists. The calculated multiplet splitting of over 2 eV indicates the importance of a proper treatment of electron exchange and correlation in this system. DMC calculations of the formation and migration energy of the vacancy defect are presented.     

Explanation of atomic displacement around lattice vacancies in diamond based on electron delocalization

The relationship between unpaired electron delocalization and nearest-neighbor atomic relaxations in the vacancies of diamond has been determined in order to understand the microscopic reason behind the neighboring atomic relaxation. The Density Functional Theory (DFT) cluster method is applied to calculate the single-electron wavefunction of the vacancy in different charge states. Depending on the charge and spin state of the vacancies, at outward relaxations, 84-90% of the unpaired electron densities are localized on the first neighboring atoms. The calculated spin localizations on the first neighboring atoms in the ground state of the negatively charged vacancy and in the spin quintet excited state of the neutral vacancy are in good agreement with Electron Paramagnetic Resonance (EPR) measurements. The calculated spin localization of the positively charged vacancy contrasts with the tentative assignment of the NIRIM-3 EPR signal to this center in (p-type) semiconductor diamond. The sign of the lattice relaxation in the diamond vacancy is explained based on the effect of electron delocalization on nearest-neighbor ion-ion screening, and also its effect on the bond length of neighboring atoms.     

Oscillator strength calculations in color centers of diamond and the role of spin

A generalized Hubbard model based on a molecular approach is used to calculate many electron wavefunctions of diamond vacancies. We have calculated the oscillator strength of the dipole transition rates from the ground states of the neutral and negatively charged vacancies. The ratio of the oscillator strengths is in very good quantitative agreement with the reported optical spectroscopic data. Electronic configurations in the ground and dipole allowed excited states are presented. With the proposed picture, the much larger oscillator strength of the negatively charged vacancy with respect to other experimentally investigated color centers N-V, H3, N3 and H4 is explained.Received: 9 March 2004, Published online: 23 July 2004PACS: 61.72.Bb Theories and models of crystal defects - 61.72.Ji Point defects (vacancies, interstitials, color centers, etc.) and defect clusters - 71.55.-i Impurity and defect levels     

First-principles study of defect-induced magnetism in carbon

We have studied the role of defects on the magnetic properties of carbon materials using first-principles density functional methods. We show that, while the total magnetization decreases both for diamond and graphite with increase in vacancy density, the magnetization decreases more rapidly for graphitic structures. The presence of nitrogen nearby a vacancy is shown to produce larger macroscopic magnetic signals as compared to a standalone carbon vacancy. The results indicate the possibility of tuning magnetization in carbon by controlled defect generation and doping.     

First principle study of the behavior of helium in plutonium dioxide

The incorporation and solution of helium in plutonium dioxide have been investigated based on density functional theory. The GGA and GGA + U approximations were used with the projector-augmented-wave method. Several defects that are likely to accommodate the incorporation of helium in PuO₂, such as oxygen vacancy, plutonium vacancy, divacancy and Schotty defects were considered in this work. With GGA approach, the lowest incorporation energy corresponds to neutral trivacancy, followed by divacancy and plutonium vacancy, while the GGA + U scheme gave us that oxygen vacancy is the most favorable incorporation site for He. Both SP-GGA and SP-GGA + U methods obtained a same conclusion that the most favorable solution site for He is oxygen vacancy, interstitial site and plutonium vacancy for under-, perfect and over-stoichiometry, respectively. Additionally, the concentrations of the point defects and the solution energy of He for the different incorporation sites as a function of the stoichiometry were also obtained based on the point-defect model.     

Electron paramagnetic resonance detection of the giant concentration of nitrogen vacancy defects in sintered detonation nanodiamonds

A giant concentration of nitrogen vacancy defects has been revealed by the electron paramagnetic resonance (EPR) method in a detonation nanodiamond sintered at high pressure and temperature. A high coherence of the electron spins at room temperature has been observed and the angular dependences of the EPR spectra indicate the complete orientation of the diamond system.     

Electronic configuration of iron in diamond

CNDO molecular orbital calculations have been performed for Fe on both substitutional and interstitial sites in diamond, as well as Fe adjacent to a vacancy. Different charge states of the defects have also been studied, this dramatically influencing the relative occupancies of the Fe(3d) and Fe(4s) shells. It is this effect which is particularly sensitive to the environment of the Fe atom that in fact influences the Mössbauer spectrum. Conjectures as to the origin of the Mössbauer spectrum of Fe in diamond are made in the light of the present calculations.     

空位在金刚石近(001)表面扩散的分子动力学模拟

用分子动力学方法模拟了空位在金刚石近(001)表面的扩散过程,研究了温度对空位扩散的影响.结果表明,当温度为1000K左右时,位于近表面第二层上的空位开始向表面运动;当温度在1400—2000K时,空位完全扩散到表面.这与实验结果和其他计算结果符合得很好.同时发现,温度为1400—1800K时,空位的扩散经历了两次迁移运动,其分别对应了均方位移图中的两个极大值.在不施加任何约束的条件下得到了空位的动态扩散路径,空位在金刚石近(001)表面的扩散势垒约为042eV.并探讨了一定温度下空位数目增多及其不同排列 关键词： 金刚石 空位 扩散 分子动力学     

The anion vacancy as shallow exciton trap in AgBr

Abstract

Luminescence and luminescence excitation measurements are employed to investigate a recently discovered new exciton line. Based on the intensity behaviour in thermally treated samples in comparison to calculated concentrations of Schottky defects, it is attributed to exciton recombination at a neutral donor formed by a Br^? vacancy plus a trapped electron. To explain the electronic structure of the exciton, a tetragonal distortion around the vacancy has to be assumed.