First-principles calculations of F-, Cl-, and N-related defects of amorphous SiO_2 and their impacts on carrier trapping and proton release

The first-principles calculations based on density functional theory are performed to study F-,Cl-,and N-related defects of amorphous SiO2(a-SiO2) and their impacts on carrier trapping and proton release.The possible geometric configurations of the impurity-related defects,the formation energies,the hole or electron trapping of the neutral defects,and the mechanisms to suppress proton diffusion by doping N are investigated.It is demonstrated by the calculations that the impurity atoms can interact with the oxygen vacancies and result in impurity-related defects.The reactions can be utilized to saturate oxygen vacancies that will cause ionization damage to the semiconducting devices.Moreover,the calculated formation energy indicates that the F-or Cl-related oxygen vacancy defect is a deep hole trap,which can trap holes and prevent them from diffusing to the a-SiO2/Si interface.However,three N-related defects,namely N(2)o-H,N(2)o=O,and N(3)o-Vo,tend to act as shallow hole traps to facilitate hole transportation during device operation.The N(2)o and N(3)o configurations can be negatively charged as deep electron traps during the oxide charge buildup after ionization radiation.In addition,the nudged elastic band(NEB) calculations show that four N-related defects,namely N(2)o,N(2)o-H,N(2)o=O,and N(3)o are capable of capturing protons and preventing them from diffusing to and de-passivating the interface.This research reveals the fundamental properties of the F-,Cl-,and N-related defects in amorphous silica and the details of the reactions of the carrier trapping and proton release.The findings help to understand the microscopic mechanisms that alleviate ionization damage of semiconducting devices by doping a-SiO2.     

Comparison between various finite-size supercell correction schemes for charged defect calculations

We present a comparison of the most common finite-size supercell correction schemes for charged defects in density functional theory calculations. Considered schemes include those proposed by Makov and Payne (MP), Lany and Zunger (LZ), and Freysoldt, Neugebauer, and Van de Walle (FNV). The role of the potential alignment is also assessed. Supercells of various sizes are considered and the corrected formation energies are compared to the values obtained by extrapolation to large supercells. For defects with localized charge distributions, we generally find that the FNV scheme slightly improves upon the LZ one, while the MP scheme generally overcorrects except for point-charge-like defects. We also encountered more complex situations in which the extrapolated values do not coincide. Inspection of the defect electronic structure indicates that this occurs when the defect Kohn–Sham states are degenerate with band-edge states of the host.     

多元半导体光伏材料中晶格缺陷的计算预测

半导体光伏材料的发展在过去60多年中表现出了清晰的多元化趋势. 从20世纪50年代的一元Si太阳能电池, 到20世纪60年代的GaAs和CdTe电池、70年代的CuInSe₂电池、80年代的Cu(In, Ga) Se₂、90年代的Cu₂ZnSnS₄电池, 再到最近的Cu₂ZnSn(S, Se)₄和CH₃NH₃PbI₃电池, 组成光伏半导体的元素种类从一元逐渐增多到五元. 元素种类的增多使得半导体物性调控的自由度增多, 物性更加丰富, 因而能满足光伏等器件应用的需要. 但是, 组分元素种类的增多也导致半导体中晶格点缺陷的种类大幅增加, 可能对其光学、电学性质和光伏性能产生显著影响. 近20年来, 第一性原理计算被广泛应用于半导体中晶格点缺陷的理论预测, 相对于间接的实验手段, 第一性原理计算具有更加直接的、明确的优势, 并且能对各种点缺陷进行快速的研究. 对于缺陷种类众多的多元半导体体系, 第一性原理计算能预测各种点缺陷的微观构型、浓度和跃迁(离化)能级位置, 从而揭示其对光电性质的影响, 发现影响器件性能的关键缺陷. 因而, 相关的计算结果对于实验研究有直接、重要的指导意义. 本文将首先介绍半导体点缺陷研究的第一性原理计算模型和计算流程; 然后, 总结近5年来两类新型光伏半导体材料, 类似闪锌矿结构的Cu₂ZnSn(S, Se)₄半导体和有机-无机杂化的钙钛矿结构CH₃NH₃PbI₃半导体的点缺陷性质; 以这两类体系为例, 介绍多元半导体缺陷性质的独特特征及其对太阳能电池器件性能的影响.     

Al掺杂对HfO2俘获层可靠性影响第一性原理研究

采用基于MS(Materials Studio)软件和密度泛函理论的第一性原理方法, 研究了HfO₂ 俘获层的电荷俘获式存储器(Charge Trapping Memory, CTM)中电荷的保持特性以及耐擦写性. 在对单斜晶HfO₂中四配位氧空位(V_O4) 缺陷和V_O4 与Al替位Hf掺杂的共存缺陷体(Al+V_O4)两种超晶胞模型进行优化之后, 分别计算了其相互作用能、形成能、Bader电荷、态密度以及缺陷俘获能. 相互作用能和形成能的计算结果表明共存缺陷体中当两种缺陷之间的距离为2.216 Å时, 结构最稳定、缺陷最容易形成; 俘获能计算结果表明, 共存缺陷体为双性俘获, 且与V_O4缺陷相比, 俘获能显著增大; Bader电荷分析表明共存缺陷体更有利于电荷保持; 态密度的结果说明共存缺陷体对空穴的局域能影响较强; 计算两种模型擦写电子前后的能量变化表明共存缺陷体的耐擦写性明显得到了改善. 因此在HfO₂俘获层中可以通过加入Al杂质来改善存储器的保持特性和耐擦写性. 本文的研究可为改善CTM数据保持特性和耐擦写性提供一定的理论指导.     

Be,Mg,Mn掺杂CuInO_2形成能的第一性原理研究

研制开发新型的光电材料对促进社会经济发展具有重要的科学意义和实用价值.利用宽禁带CuInO_2铟基材料实现全透明光电材料是目前深入研究的热点.通过基于密度泛函的第一性原理计算方法,本文计算出掺杂元素Mg, Be, Mn在CuInO_2的形成能.计算结果表明,施主类缺陷(如掺杂元素替代Cu原子或进入间隙位置)由于较高的形成能和较深的跃迁能级,很难在CuInO_2材料中出现N型导电;而受主缺陷中,在氧原子化学势极大的情况下, Mg原子替代In能成为CuInO_2理想的受主缺陷.计算结果可为制备性能优异的CuInO_2材料提供指导.     

Geometry and electronic properties of single vacancies in achiral carbon nanotubes

An ideal single vacancy can be formed by removing one carbon atom from a hexagonal network. The vacancy is one of the most important defect structures in carbon nanotubes (CNTs). Vacancies can affect the mechanical, chemical, and electronic properties of CNTs. We have systematically investigated single vacancies and their related point defects for achiral, single-walled carbon nanotubes (SWNTs) using first-principles calculations. The structures around single vacancies undergo reconstruction without constraint, forming ground-stateor metastable-state structures. The 5-1DB and 3DB point defects can be formed in armchair CNTS, while the 5-1DB-P and 5-1DB-T point defects can be formed in zigzag CNTs. The related point defects can transform into each other under certain conditions. The formation energies of armchair CNTs change smoothly with the tube radius, while in the case of the 3DB defect, as the radius get larger, the formation energies tend towards a constant value.     

Stability of the DX- center in GaAs quantum dots

Using a first-principles band structure method, we study how the size of quantum dots affects the stability and transition energy levels of defects in GaAs. We show that, although a negatively charged DX- center is unstable in bulk GaAs:Si with respect to the tetrahedral coordinated Si(-)(Ga), it becomes stable when the dot size is small enough. The critical size of the dot is about 14.5 nm in diameter. The reason for the stabilization is the strong quantum-confinement effect, which increases the formation energy of Si(-)(Ga) more than that of the DX- defect center. Our studies show that defect properties in quantum dots could be significantly different from those in bulk semiconductors.     

Molecular static model of CuInSe2 crystal: Energy properties of some structural defects

A molecular static model of a CuInSe₂ crystal is constructed, and the energies of formation of isolated defects are calculated. An analysis of the interaction between defects shows that, for a high defect concentration (exceeding 1%), the energies of interaction between defects can considerably exceed the values obtained in the continuous medium approximation. It is found that the role of interaction between defects in quantum-chemistry calculations using the model of supercells (periodic defect) is considerably underestimated.     

Defects in silicon: the role of vibrational entropy

Vibrational free energies are calculated from first-principles in the same Si periodic supercells routinely used to perform defect calculations. The specific heat, vibrational entropy, and zero-point energy obtained in defect-free cells are very close to the measured values. The importance of the vibrational part of the free energy is studied in the case of two defect problems: the relative energies of the H₂ and H₂^∗ dimers and the binding energy of a copper pair. In both cases, the vibrational entropy term causes total energy differences to change by about 0.2 eV between 0 and 800 K. We also comment on the rotational entropy in the case of H₂ and the configurational entropy in the case of the Cu pair. These examples illustrate the importance of extending first-principles calculations of defects in semiconductors to include free energy contributions.     

Magnetic and electronic properties of two-dimensional metal-organic frameworks TM_3(C_2NH)_(12)

The ferromagnetism of two-dimensional(2 D) materials has aroused great interest in recent years, which may play an important role in the next-generation magnetic devices. Herein, a series of 2 D transition metal-organic framework materials(TM-NH MOF, TM = Sc–Zn) are designed, and their electronic and magnetic characters are systematically studied by means of first-principles calculations. Their structural stabilities are examined through binding energies and abinitio molecular dynamics simulations. Their optimized lattice constants are correlated to the central TM atoms. These 2 D TM-NH MOF nanosheets exhibit various electronic and magnetic performances owing to the effective charge transfer and interaction between TM atoms and graphene linkers. Interestingly, Ni-and Zn-NH MOFs are nonmagnetic semiconductors(SM) with band gaps of 0.41 e V and 0.61 e V, respectively. Co-and Cu-NH MOFs are bipolar magnetic semiconductors(BMS), while Fe-NH MOF monolayer is a half-semiconductor(HSM). Furthermore, the elastic strain could tune their magnetic behaviors and transformation, which ascribes to the charge redistribution of TM-3 d states. This work predicts several new 2 D magnetic MOF materials, which are promising for applications in spintronics and nanoelectronics.     

Distribution of Topological Defects on Axisymmetric Surface

We propose a general method of determining the distribution of topological defects on axisymmetric surface,and study the distribution of topological defects on biconcave-discoid surface, which is the geometric configuration of red blood cell. There are three most possible cases of the distribution of the topological defects on the biconcave surface:four defects charged with 1/2, two defects charged with 1, or one defect charged with 2. For the four defect charged with 1/2, they sit at the vertices of a square imbedded in the equator of biconcave surface.     

估算等离子体效应对类氢离子束缚态能级能量影响的新公式

在均匀电子气模型下,通过求解Dirac方程,计算了类氢离子束缚态能级能量与等离子体密度的关系,得到了能近似估算能级能量随等离子体密度变化的新拟合公式,该公式同样可以用以估算不同束缚态能级发生压致电离时的临界电子密度.通过与自洽场离子球模型计算结果的比较,作为一种简单和快捷的计算方法,均匀电子汽模型在近似计算束缚态能级能量及压致电离临界电子密度方面是自洽场离子球模型得很好近似方法.     

V+20离子的能量和偶极振子强度

用全实加关联方法计算了类锂V+20离子 1s2nl(l=s,p, d;n≤9) 态的电离势和精细结构.依据单通道量子亏损理论, 确定了这三个Rydberg系列的量子数亏损.用这些作为能量的缓变函数的量子亏损,可以实现对任意高激发态(n≥10)的能量的可靠预言.用在计算能量过程中确定的波函数,计算了V+20离子1s22s-1s2np及1s22p-1s2nd (n≤9)跃迁的振子强度.将这些分立态振子强度与单通道量子亏损理论相结合,得到在电离域附近束缚态间的偶极跃迁振子强度以及束缚态-连续态跃迁的振子强度密度,从而将V+20离子的这一重要光谱特性的理论预言外推到整个能域.     

Probing the out-of-plane distortion of single point defects in atomically thin hexagonal boron nitride at the picometer scale

Crystalline systems often lower their energy by atom displacements from regular high-symmetry lattice sites. We demonstrate that such symmetry lowering distortions can be visualized by ultrahigh resolution transmission electron microscopy even at single point defects. Experimental investigation of structural distortions at the monovacancy defects in suspended bilayers of hexagonal boron nitride (h-BN) accompanied by first-principles calculations reveals a characteristic charge-induced pm symmetry configuration of boron vacancies. This symmetry breaking is caused by interlayer bond reconstruction across the bilayer h-BN at the negatively charged boron vacancy defects and results in local membrane bending at the defect site. This study confirms that boron vacancies are dominantly present in the h-BN membrane.     

缺陷对钠在石墨烯上吸附性能的影响

采用基于密度泛函理论的第一性原理方法,研究了本征石墨烯和缺陷石墨烯吸附钠原子的电荷密度、吸附能、态密度和储存量.结果表明,本征石墨烯中,钠原子的最佳吸附位置为H位,缺陷石墨烯中,钠原子的最佳吸附位置为T_D位.缺陷石墨烯对钠原子的吸附能是-4.423 eV,约为本征石墨烯对钠原子吸附能的2.5倍;钠原子与缺陷石墨烯中的碳原子发生轨道杂化,而与本征石墨烯没有发生轨道杂化现象.缺陷石墨烯能够吸附10个钠原子,与本征石墨烯相比显著提高.因此,缺陷石墨烯有望成为一种潜在的储钠材料.     

非晶态半导体的缺陷态化学键的量子化学研究

用量子化学的半经验自洽场分子轨道方法CNDO/2研究硫属玻璃中的各种缺陷态,包括带正电荷、负电荷和中性的三配位硫原子簇的化学键。从原子簇能量、原子净电荷、键级函数(双原子能量)等方面作了比较,取得了与Kastner相一致的结果,验证了K-A-F的换价对(VAP)理论。探讨了孤对电子对于缺陷态化学键的影响,证实了孤对电子在硫属玻璃中的重要作用。 关键词：     

Accurate first-principles detailed-balance determination of auger recombination and impact ionization rates in semiconductors

The technologically important prediction of Auger recombination lifetimes in semiconductors is addressed by means of a fully first-principles formalism, based on precise energy bands and wave functions provided by the full-potential linearized augmented plane wave code. The minority carrier Auger lifetime is determined by two related approaches: (i) a direct evaluation within Fermi's golden rule, and (ii) an indirect evaluation, based on a detailed balance formulation combining Auger recombination and its inverse process, impact ionization, in a unified framework. Lifetimes determined with the direct and indirect methods show excellent consistency between them (i) for n-doped GaAs and (ii) with measured values for GaAs and InGaAs. This indicates the computational formalism as a new sensitive tool for use in materials performance optimization.     

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-principles study of solute diffusion in Ni_3Al

Using first-principles calculations in combination with Wagner–Schottky and kinetic Monte Carlo methods, the diffusion behaviors of solutes via various vacancy-mediated diffusion mechanisms in L1_2 γ-Ni_3Al were investigated. The formation energies of the point defects and the migration energies for solutes were calculated. Adding alloying elements can decrease the defect-formation energies of Ni_(Al), increase the defect-formation energies of Al_(Ni), and have little effect on the formation energy of V_(Ni). The migration energies of solutes are related with the site preference and the diffusion mechanism. The diffusion coefficients of Ni, Al, and solutes were calculated, and the concentration of antisite defects plays a crucial role in the elemental diffusion.     

KDP晶体本征中性点缺陷的第一性研究

 用第一性原理研究了KH2PO4(KDP)晶体中性本征点缺陷的形成能并计算了常温下点缺陷的浓度。计算得到中性填隙氢原子的形成能为2.05 eV，进而得到298 K下的浓度约为1.21×10^-17 mol/L。由于填隙氢原子在带隙中形成缺陷能级，并使能隙降低了2.6 eV, 因此消除填隙氢原子有利于提高晶体在355 nm附近的激光损伤阈值。计算得到的氧间隙、氧空位、钾空位和氢空位的形成能分别为0.60、5.25、6.50 和6.58 eV，常温下它们在晶体中也以较高的浓度存在。钾空位使晶胞体积增大约3.2%，并可能提高晶体电导率，从而降低光损伤阈值。P取代K的反位结构缺陷形成能尽管较低(4.1 eV), 但由于晶体生长溶液中P是以PO4四面体的形式存在，故此点缺陷的存在几率很小。