XPO4(X=Lu, Y)电子结构的第一性原理研究

基于密度泛函理论(DFT)第一性原理对LuPO₄和YPO₄两种磷酸盐晶体的电子结构进行计算模拟.结果表明:LuPO₄的带隙为5.639 eV,YPO₄的带隙为4.884 eV.通过对态密度的分析得知LuPO₄的导带主要贡献来自Lu的5d态电子,费米能级附近价带主要由Lu的4f态和O的2p态电子贡献. YPO₄的导带主要贡献来自Y的4d态电子,价带顶的主要贡献来自于O的2p态电子.通过电荷密度和布局分析得知了材料内部原子间的电荷转移情况,进而对原子间成键情况进行了分析与讨论.     

硅中的过渡元素杂质能级

本文提出了半导体中过渡元素杂质的一个简单模型,用格林函数方法计算了硅中替代和间隙原子产生的杂质能级和波函数。发现两者的性质有很大的差别。替代原子只有当d原子能级V_d低于价带顶时才能产生杂质能级。它的波函数主要是悬键态,当能级靠近导带边时变成正键态。间隙原子只有当V_d高于价带顶时才能产生杂质能级。它的波函数主要是中心原子d态,当能级靠近导带边时变成弱反键态。最后定性地说明了过渡元素杂质能级的化学趋势和一些实验事实。 关键词：     

单层Nb2SiTe4基化合物的带隙异常变化

传统硫族化合物中阳离子相同时,随着阴离子原子序数的增加,价带顶逐渐升高,带隙呈减小趋势.在A₂BX₄基(A=V,Nb,Ta;B=Si,Ge,Sn;X=S,Se,Te)化合物中,观察到随着阴离子原子序数增加,其带隙呈现反常增大的现象.为了探究其带隙异常变化的原因,基于第一性原理计算,对A₂BX₄基化合物的电子结构展开系统地研究,包括能带结构、带边相对位置、轨道间耦合作用以及能带宽度等影响.研究发现,Nb2SiX4基化合物中Nb原子4d轨道能量明显高于阴离子p轨道,其价带顶和导带底主要由Nb原子4d轨道相互作用组成,其带宽主要影响带隙大小.Nb₂SiX₄基化合物的带隙大小通过Nb—Nb和Nb—X键共同作用于Nb原子4d轨道的宽度来控制.当阴离子序数增加时,Nb—Nb键长增加,其相互作用减弱,由Nb原子4d轨道主导的能带变宽,带隙减小;另一方面,Nb—X键长增加又使Nb原子4d带宽变窄,带隙增加,并且Nb—X键长增长占主导,所以带隙最终呈现异常增加的趋势.     

强激光照射对2H-SiC晶体电子特性的影响

使用基于密度泛函微扰理论的第一原理赝势法, 计算了纤锌矿结构2H-SiC晶体在强激光照射下的电子特性, 分析了其能带结构和电子态分布. 计算结果表明: 2H-SiC平衡晶格参数a 和c随电子温度T_e的升高逐渐增大; 电子温度在0–2.25 eV范围内时, 2H-SiC仍然是间接带隙的半导体晶体, 当T_e超过2.25 eV达到2.5 eV以上时, 2H-SiC变为直接带隙的半导体晶体; 随着电子温度升高, 导带底和价带顶向高能量或低能量方向发生了移动, 当电子温度T_e大于3.5 eV以后, 价带顶穿越费米能级; 电子温度T_e在0–2.0 eV变化时, 带隙随电子温度升高而增大; T_e在2.0–3.5 eV范围变化时, 带隙随电子温度升高而快速地减少, 表明2H-SiC晶体的金属性随电子温度T_e的继续升高而增强. 在T_e =0, 5.0 eV 处, 计算了2H-SiC晶体总的电子态密度和分波态密度. 电子结构表明T_e =0 eV 时, 2H-SiC 是一个带隙为2.3 eV的半导体; 在T_e =5.0 eV时, 带隙已经消失而呈现出金属特性, 表明当电子温度升高时晶体的共价键变弱、金属键增强, 晶体经历了一个熔化过程, 过渡到金属状态.     

KDP和尿素晶体电子结构的第一性原理研究

基于第一性原理的平面波超软赝势法对KDP(KH₂PO₄)和尿素(CH₄N₂O)晶体的能带结构、电子态密度、电荷差分密度以及布局分析进行了计算讨论.结果表明:尿素晶体中的C1-O1、C1-N1、N1-H2和N1-H1键都具有共价键特性,带隙值为4.636 eV,价带顶主要由H-1s与N、O的2p态贡献,导带底主要是H-1s与C、N、O的2p态贡献;KDP晶体的H1-O1键具有离子性而P1-O1则具有共价性,带隙宽度为5.713 eV,价带顶主要由O-2p以及P-3p贡献,导带底主要由H-1s、P-3s和3p以及K-4s和3p态贡献.     

宽禁带半导体材料CuYO2的第一性原理研究

本文利用基于第一性原理的广义梯度近似方法分析研究宽禁带半导体材料CuYO₂能带结构、晶格常数和态密度.计算结果表明,CuYO₂的价带区主要由Cu的3d态和O的2p态构成,而导带区主要由Y的3d态组成.在进行+U修正之后,随着U参量的增加,CuYO₂的价带区和导带区发生分裂,导带区中Y的3d主峰向高能区移动导致导带扩大,带隙也随之扩大,当U取值为3 eV时导带底由L点转变为T点,表明+U计算主要修正CuY₂导带从而能较好的改进理论带隙值.     

气体吸附对硅锗异质结纳米线电子结构与光学性质的影响

基于密度泛函理论体系下的广义梯度近似(GGA),采用第一性原理方法探讨了沿[112]晶向的硅锗异质结纳米线作为气体传感器检测CO,CO₂和Cl₂的能力,着重计算了其吸附气体分子前后的吸附能、能带结构与光学性质.几何结构优化计算表明：不同硅锗组分的[112]晶向的硅锗纳米线对CO,CO₂和Cl₂分子的吸附能的绝对值在0.001 eV至1.36 eV之间,其中Si₂₄Ge₃₆H₃₂对CO₂气体的吸附能最大,气敏性能最好.能带结构计算表明：吸附CO和CO₂分子的[112]晶向硅锗纳米线能带的简并度明显减小,带隙变化较小;而吸附Cl₂分子后的价带顶与导带底之间产生了杂质能级使其带隙减小.光学性质计算表明：Si₂₄Ge₃₆H₃₂纳米线吸附CO, CO₂和Cl₂分子后的光学...     

无杂质空位扩散法造成InGaAsP/InP多量子阱结构带隙蓝移规律的研究

采用光荧光谱(PL)和光调制反射谱(PR)的方法,研究了由Si₃N₄、SiO₂电介质盖层引起的无杂质空位(IFVD)诱导的InGaAsP四元化合物半导体多量子阱(MQWs)结构的带隙蓝移。实验中Si₃N₄、SiO₂作为电介质盖层,用来产生空位,再经过快速热退火处理(RTA)。实验结果表明:多量子阱结构带隙蓝移和退火温度、复合盖层的组合有关。带隙蓝移随退火温度的升高而加大。InP、Si₃N₄复合盖层产生的带隙蓝移量大于InP、SiO₂复合盖层。而InGaAs、SiO₂复合盖层产生的带隙蓝移量则大于InGaAs、Si₃N₄复合盖层。同时,光调制反射谱的测试结果与光荧光测试的结果基本一致,因此,PR谱是用于测试带隙变化的另一种方法。     

立方(Ba0.5Sr0.5)TiO3高压诱导带隙变化的第一性原理研究

采用基于密度泛函理论(DFT)的第一性原理计算研究了 (Ba_0.5Sr_0.5)TiO₃ (BST) 晶体在高压下的电子结构及能带变化行为. 研究结果发现,随着压强的增加,BST能带间隙先增加,在压强为55 GPa时达到最大值,然后减小,这些有趣的结果将有助于开发与设计新的BST铁电器件. 进一步地,通过电子态密度和密度分布图的研究分析可知:在低压区域(055 GPa),则是出现的离域现象占主导(电子的离域作用超过键态的作用),从而使带隙减小. 关键词： 钛酸锶钡 第一性原理 高压 能带间隙     

Te掺杂单层MoS2的电子结构与光电性质

采用基于密度泛函理论的第一性原理计算,研究了Te掺杂对单层MoS₂能带结构、电子态密度和光电性质的影响。结果表明,本征单层MoS₂属于直接带隙半导体材料,其禁带宽度为1.64 eV。本征单层MoS₂的价带顶主要由S-3p态电子和Mo-4d态电子构成,而其导带底则主要由Mo-4d态电子和S-3p态电子共同决定;Te掺杂单层MoS₂为间接带隙半导体材料,其禁带宽度为1.47 eV。同时通过Te掺杂,使单层MoS₂的静态介电常数增大,禁带宽度变窄,吸收光谱产生红移,研究结果为单层MoS₂在光电器件方面的应用提供了理论基础。     

Si–Si bond as a deep trap for electrons and holes in silicon nitride

A two-stage model of the capture of electrons and holes in traps in amorphous silicon nitride Si₃N₄ has been proposed. The electronic structure of a “Si–Si bond” intrinsic defect in Si₃N₄ has been calculated in the tight-binding approximation without fitting parameters. The properties of the Si–Si bond such as a giant cross section for capture of electrons and holes and a giant lifetime of trapped carriers have been explained. It has been shown that the Si–Si bond in the neutral state gives shallow levels near the bottom of the conduction band and the top of the valence band, which have a large cross section for capture. The capture of an electron or a hole on this bond is accompanied by the shift of shallow levels by 1.4–1.5 eV to the band gap owing to the polaron effect and a change in the localization region of valence electrons of atoms of the Si–Si bond. The calculations have been proposed with a new method for parameterizing the matrix elements of the tightbinding Hamiltonian taking into account a change in the localization region of valence electrons of an isolated atom incorporated into a solid.     

Benefits of oxygen in CuInSe2 and CuGaSe2 containing Se-rich grain boundaries

Using density functional theory calculation, we show that oxygen (O) exhibits an interesting effect in CuInSe₂ and CuGaSe₂. The Se atoms with dangling bonds in a Se-rich Σ3 (114) grain boundary (GB) create deep gap states due to strong interaction between Se atoms. However, when such a Se atom is substituted by an O atom, the deep gap states can be shifted into valence band, making the site no longer a harmful non-radiative recombination center. We find that O atoms prefer energetically to substitute these Se atoms and induce significant lattice relaxation due to their smaller atomic size and stronger electronegativity, which effectively reduces the anion–anion interaction. Consequently, the deep gap states are shifted to lower energy regions close or even below the top of the valence band.     

Gap states in hydrogenated amorphous silicon: A comparison of photoemission and photoconductivity results

We report photoemission results from which we directly determined the density of states g(E) in the gap of a-Si:H between the top of the valence band E_v and the Fermi level. At 0.4 eV above E_v, g(E) was found to be ≈1×10²⁰ cm^-3 eV^-1 in the undoped film; P-doping increased g(E) in this region whereas annealing reduced it. The photoconductivity-derived optical absorption spectrum matched the shape of the photoemission spectrum, and thus supports the explanation that the photoconductivity shoulder at photon energies in the region of 1.3 eV is due to transitions from localized states above the valence band to the conduction band.     

New Energy Spectra Structure of Polaron in Trans-Polyacetylene

In the weakly coupled electron-phonon systems, the existing theory pointed out that the energy spectra of polaron include four electronic bound states. Our work shows that, due to the non-nearest neighbor hopping interactions, the electron-hole symmetry of the energy band structure implied by SSH model is broken, and the numbers of the bound electronic states are changed. For a negative charged polaron, one new bound state is found near the bottom of conduction band, and the original two bound states below the bottom of the valence band and above the top of the conduction band disappear. For a positive charged polaron, five bound states have been found: one of them is an additional bound state at the top of the conduction band, the others are just the states found in the SSH model. Besides, the energy gap 2Δ is slightly shifted by turning on the long-range hopping interactions.     

Possible effects of oxygen in Te-rich Σ3 (112) grain boundaries in CdTe

Using density functional theory calculation, we show that oxygen (O) exhibits an interesting effect in CdTe. The Te atoms with dangling bonds in a Te-rich rich Σ3 (112) grain boundary (GB) create deep gap states due to strong interaction between Te atoms. However, when such a Te atom is substituted by an O atom, the deep gap states can be shifted toward the valence band, making the site no longer a harmful non-radiative recombination center. We find that O atoms prefer energetically substituting these Te atoms and induce significant lattice relaxation due to their smaller atomic size and stronger electronegativity, which effectively reduces the anion–anion interaction. Consequently, the deep gap states are shifted to lower energy regions close to or even below the top of the valence band.     

Recombination centers in phosphorous doped hydrogenated amorphous silicon

Optical absorption below the mobility gap of a-SiH_x:P films is derived from photoconductivity measurements and interpreted in terms of optical transitions from occupied localized states in the exponential valence band tail and dangling bond states 0.8 eV above the valence band edge to unoccupied free electron conduction band states. Collection efficiency measurements of Schottky barrier structures indicate that P doping introduces centers with large capture cross-section for holes.     

A first-principles calculation on the electronic properties of Si/N-codoped TiO2

To deeply understand the effects of Si/N-codoping on the electronic structures of TiO₂ and confirm their photocatalytic performance, a comparison theoretical study of their energetic and electronic properties was carried out involving single N-doping, single Si-doping and three models of Si/N-codoping based on first-principles. As for N-doped TiO₂, an isolated N 2p state locates above the top of valence band and mixes with O 2p states, resulting in band gap narrowing. However, the unoccupied N 2p state acts as electrons traps to promote the electron-hole recombination. Using Si-doping, the band gap has a decrease of 0.24 eV and the valence band broadens about 0.30 eV. These two factors cause a better performance of photocatalyst. The special Si/N-codoped TiO₂ model with one O atom replaced by a N atom and its adjacent Ti atom replaced by a Si atom, has the smallest defect formation energy in three codoping models, suggesting the model is the most energetic favorable. The calculated energy results also indicate that the Si incorporation increases the N concentration in Si/N-codoped TiO₂. This model obtains the most narrowed band gap of 1.63 eV in comparison with the other two models. The dopant states hybridize with O 2p states, leading to the valence band broadening and then improving the mobility of photo-generated hole; the N 2p states are occupied simultaneously. The significantly narrowed band gap and the absence of recombination center can give a reasonable explanation for the high photocatalytic activity under visible light.     

First-principles energy band calculation for undoped and N-doped InTaO4 with layered wolframite-type structure

The electronic structures of undoped and N-doped InTaO₄ with optimized structures are calculated within the framework of the density functional theory. Calculated lattice constants are in excellent agreement with experimental values, within a difference of 2%. The valence band maximum (VBM) is located near the middle point on the ZD line and the conduction band minimum (CBM) near the middle point on the DX line. This means that InTaO₄ is an indirect-gap material and a minimum theoretical gap between VBM and CBM is ca. 3.7 eV. The valence band in the range from −6.0 to 0 eV mainly consists of O 2p orbitals, where In 4d5s5p and Ta 5d orbitals are slightly hybridized with O 2p orbitals. On the other hand, the conduction band below 5.5 eV is mainly composed of the Ta 5d orbitals and the contributions of In and O orbitals are small. The band gap of N-doped InTaO₄ decreases by 0.3 eV than that of undoped InTaO₄, because new gap states originating from N 2p orbitals appear near the top of the valence band. This result indicates that doping of N atoms into metal oxides is a useful method to develop photocatalysts sensitive to visible light.     

Theoretical study of the electronic structure of the Si3N4(0 0 0 1) surface

Density functional theory has been applied to a study of the electronic structure of the ideally-terminated, relaxed and H-saturated (0 0 0 1) surfaces of β-Si₃N₄ and to that of the bulk material. For the bulk, the lattice constants and atom positions and the valence band density of states are all in good agreement with experimental results. A band gap of 6.7 eV is found which is in fair accord with the experimental value of 5.1-5.3 eV for H-free Si₃N₄. Using a two-dimensionally-periodic slab model, a π-bonding interaction is found between threefold-coordinated Si and twofold-coordinated N atoms in the surface plane leading to π and π* surface-state bands in the gap. A surface-state band derived from s-orbitals is also found in the gap between the upper and lower parts of the valence band. Relaxation results in displacements of surface and first-underlayer atoms and to a stronger π-bonding interaction which increases the π-π* gap. The relaxed surface shows no occupied surface states above the valence band maximum, in agreement with recent photoemission data for a thin Si₃N₄ film. The π* band, however, remains well below the conduction band minimum (but well above the Fermi level). Adsorbing H at all dangling-bond sites on the ideally-terminated surface and then relaxing the surface and first underlayer leads to smaller, but still finite, displacements in comparison to the clean relaxed surface. This surface is more stable, by about 3.67 eV per H, than the clean relaxed surface.     

Direct observation of occupied gap states in amorphous Si1-xGex: H alloys by SXS

SiKβ emission band have been analysed from a-Si_1-xGe_x: H alloys for Ge concentrations varying from 0.05 to 0.60. Localized states with p character are observed in the pseudo-gap near the valence band edge.