(La,Y)掺杂AlN的电子结构和光学性质的第一性原理研究

采用基于密度泛函理论的平面波超软赝势方法对纯AlN、(La,Y)单掺杂以及La-Y共掺杂AlN 超胞进行几何结构优化,计算了稀土元素(La,Y)掺杂前后体系的能带结构、态密度和光学性质。结果表明:未掺杂的AlN是直接带隙半导体,带隙值为E_g=4.237 eV,在费米能级附近,态密度主要由Al-3p、N-2s电子轨道贡献电子,光吸收概率大,能量损失较大;掺杂后使得能带结构性质改变,带隙值降低,能带曲线变密集,总态密度整体下移;在光学性质中,稀土元素掺杂后均提高了静态介电常数、光吸收性能,增强了折射率和反射率,减小了电子吸收光子概率及能量损失;其中La-Y共掺体系变化得较为明显。     

应力对单层GaTe能带结构和迁移率影响的第一性原理研究

基于密度泛函理论的第一性原理计算分析了单层碲化镓(GaTe)能带结构和载流子迁移率随外加应力的变化.计算发现:载流子有效质量和迁移率随K点位置而不同,并且形变势对载流子迁移率影响很大;在单层GaTe的b轴或ab双轴上施加合适的压应力对能带结构的影响较为明显,可使其从间接带隙转变为直接带隙,并且带隙随着压应力的增加而逐渐减小;此外,施加ab双轴压应力更能提高载流子迁移率.因此,单层GaTe在应力调控下可作为制备高性能微纳电子器件的一种有前途的候选材料.     

Ni掺杂ZnS的第一性原理计算

采用基于密度泛函理论(DFT)赝势平面波方法计算了ZnS体系Ni掺杂前后的能带结构、态密度和光吸收系数曲线。结果表明:纯ZnS的能带结构是直接带隙,态密度显示属离子性较强而共价键较弱的混合键半导体材料。掺Ni的ZnS禁带宽度随掺杂量增加逐渐减小,能带简并度增大,且向低能方向移动;在价带顶出现杂质能级,说明是p型掺杂。纯ZnS在3.9 eV以下无吸收,红外透过率较高。掺Ni后吸收边红移,且在低能端(绿光区)出现新的吸收峰。     

ZnO晶体Al杂质与Zn间隙共存的复合缺陷研究

利用第一性原理,研究了ZnO晶体Al杂质与Zn间隙共存的复合缺陷(AlZnZni)电子结构和光学性质.计算结果显示,AlZnZni复合缺陷的形成能为-3.180 eV,较低的形成能表明这种缺陷容易形成;复合缺陷使ZnO晶体的能带整体下移,带隙减小,价带区域展宽;电子能带结构的变化对ZnO的光学性质在低能产生了重要影响,主要表现在:介电函数虚部往低能方向移动且强度显著增强,使得ZnO表现出简并半导体的特性;复合缺陷导致ZnO晶体光学吸收边缘产生了红移,可见光区的吸收系数增大,透过率降低.     

二维正方晶格钨-硅橡胶声子晶体的带隙特性研究

本文利用平面波展开法和有限元法计算了二维正方晶格钨-硅橡胶声子晶体的能带结构,分析了弹性波在嵌有周期性排列的相同孔径的圆柱形钨块声子晶体结构中的传播特性.对能带结构中出现的特殊现象,例如狄拉克点以及狄拉克点附近的简并态等进行了分析.给出了三维能带图以及不同模式下的主要变形情况,发现在基体与散射体结合的部位容易出现应力集中的现象.同时讨论了复合材料的不同参数对带隙的影响,发现随着密度比(散射体密度与基体密度的比值)的增大,第一完全带隙的起始频率逐渐增加,但是截止频率变化较小,带宽逐渐减小.相对于杨氏模量比,密度比对带隙的影响更大.本文的结果将对实际的工程应用提供一定的理论指导.     

单空位缺陷扶手椅型MoS2纳米带结构与电子性质研究

基于密度泛函理论(DFT)的第一性原理计算方法,研究了单硫空位缺陷扶手椅型二硫化钼纳米带(AMoS2NR)的结构与电子性质.结果表明,优化的AMoS2NR纳米带边缘上Mo原子较S原子向纳米带内侧收缩;引入空位缺陷后,边缘上Mo原子向纳米带内侧收缩加剧,稳定性降低;空位缺陷纳米带相比完整纳米带,带隙减小;同时,空位缺陷处原子部分态密度降低,相应的能带线分布稀疏.     

ZnMO(M=Cd,Mg)纳米管发光性能的第一性原理研究

采用热蒸发法制备得到掺Cd量为3.3at%的ZnO纳米管,室温光致发光谱(PL)显示,由于Cd的掺入,ZnCdO纳米管的紫外近带边发射(UV NBE)从纯ZnO的3.26 eV红移到3.20 eV附近。应用基于密度泛函理论研究锯齿型(9,0)ZnMO(M=Cd,Mg)单壁纳米管的电子结构。分析发现Cd掺杂纳米管与薄膜相似,能隙随掺杂量增加逐渐减小,出现红移;而Mg掺杂纳米管则不同,能带变化没有规律性。     

基于边界元法的非完好界面三角晶格声子晶体带隙计算

采用边界元法研究了具有非完好界面条件的二维三角晶格声子晶体的带隙特性.结合Bloch周期原理,针对二组元三角晶格固-固体系声子晶体推导了含非完好界面条件的边界元特征值方程.基于该方程,计算了含有不同截面散射体(圆形截面、椭圆截面、正方截面)的能带结构,讨论了晶格对称性对能带结构的影响;并且分析了圆形截面散射体填充比的变化对带隙位置及宽度的影响.通过与其它计算方法的结果比较,说明边界元法可以有效准确地计算具有不同界面条件和不同散射体形状的声子晶体的能带结构.而且,非完好界面条件的声子晶体可以在低频打开完全带隙,尤其圆形截面最为明显.     

铁磁态Co基层状氧化物Ca2Co2O5电子状态的研究

采用平面波超软赝势法研究了低自旋状态Co基典型层状氧化物Ca2Co2O5的电子结构.结果表明,层状Co基氧化物Ca2Co2O5呈金属型能带结构,具有明显的各向异性,能带中含有5个子能带,其中费米能附近的能带数量较多,呈较宽的带状分布.自旋向上的电子形成半导体型能带,带隙宽度为0.37 eV,自旋向下的电子形成金属型能带.系统内部的电子对态密度贡献程度依s,p,d依次增大.Ca的电子形成的态密度对费米能附近的值贡献非常小,Co的d态电子对两层态密度贡献较大,其对系统电性能起着重要作用,且系统Co-O之间存在着较强的相互作用.Ca-Co-O层中的各态电子对态密度的贡献大于Co-O层中的各态电子对态密度的贡献.     

具有完全带隙的异质结构声子晶体的声波导特性

利用正方形和三角形结构声子晶体具有范围较大完全带隙的特性,设计了新型正方形晶格和三角形晶格构成的声子晶体异质结.采用平面波展开和超原胞相结合的方法研究了该异质结的能带结构特性.给出异质结的结构和相应的能带图,分析了异质结界面传导模,研究两侧结构作横向拉开和侧向滑移时对传导模的影响,讨论这些结构的实际可行性.计算结果表明,没有任何晶格移动,该结构异质结在完全带隙中就有传导模存在,界面两侧发生相对移动时,带隙内传导模的数量及位置都发生变化.     

Free space crystallography and the volume term of Gibbs energy in heptagon patterns

Gaps or free space are studied in heptagon patterns. Geometrical features of 2-D nets are investigated. The volume terms of Gibbs energy in corresponding 3-D lattices is estimated. The importance of the research of gaps and space filling is confirmed by the study of the FSCSC (free space crystallography of strange crystals) row of three heptagon patterns with the interesting number (per unit cell) sequences (heptagons: 4, 6, 8; gaps: 2, 3, 4) represented here.     

Spatially periodic reorientation of liquid crystal director at planar Freedericksz transition

Conditions for the formation of a spatially periodic structure in a liquid crystal cell at the threshold planar-planar director reorientation in a dc electric field are considered. The dependences of the threshold and the wave number of the arising structure on (i) the polar and azimuthal anchoring energy at the surface of the cell substrates, (ii) the ratio r of the elastic constants of the liquid crystal, and (iii) the value of the flexoelectric parameter v are calculated. It is shown that the range of the parameters r and v corresponding to the spatially periodic director reorientation depends strongly on the anchoring energy. The range of allowable values of the parameter r narrows with a decrease in the azimuthal anchoring energy and expands with a decrease in the polar anchoring energy, while the range of allowable values of the parameter v widens with decreasing azimuthal anchoring energy and depends nonmonotonically on the polar anchoring energy.     

Effect of annealing temperature on the structural and optical properties of amorphous Sb2Te2Se thin films

Thin films of Sb₂Te₂Se were prepared by conventional thermal evaporation of the presynthesized material on Corning glass substrates. The chemical composition of the samples was determined by means of energy‐dispersive X‐ray spectrometry. X‐ray diffraction studies on the as‐deposited and annealed films revealed an amorphous‐to‐crystalline phase transition. The as‐deposited and annealed films at T _a = 323 and 373 K are amorphous, while those annealed at T _a= 423 and 473 K are crystalline with a single‐phase of a rhombohedral crystalline structure as that of the source material. The unit‐cell lattice parameters were determined and compared with the reported data. The optical constants (n , k ) of the investigated films were determined from the transmittance and reflectance data at normal incidence in the spectral range 400–2500 nm. The analysis of the absorption spectra revealed non‐direct energy gaps, characterizing the amorphous films, while the crystalline films exhibited direct energy gaps. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

类正方阿基米德格子声子晶体的声学能带结构特性

利用平面波展开法计算了类正方阿基米德格子水/水银声子晶体能带结构。结果表明:与正方晶格水/水银声子晶体对比,单胞含有4个"原子"类正方阿基米德格子声子晶体存在各向同性带隙和高频带隙;在低频范围内,讨论归一化半径对类正方阿基米德格子和正方晶格水/水银声子晶体带隙相对宽度的影响,并比较它们带隙相对宽度,选择合适归一化半径值,这些类型声子晶体能够得到最宽的带隙。     

Magnonic excitations versus three‐dimensional structural periodicity in magnetic composites

The study deals with the spin wave spectrum in magnetic macrostructure (composed of two ferromagnetic materials) showing a 3D periodicity; the structure considered consists of spherical ferromagnetic grains disposed in the nodes of a simple cubic crystal lattice and embedded in a matrix with different ferromagnetic properties. It is shown that the magnonic spectrum of this composite structure exhibits frequency regions forbidden for magnon propagation, and the energy gaps are sensitive to the exchange contrast between the constituent materials as well as to the magnetization contrast. The widths of the respective magnonic gaps are studied as functions of parameters characterizing the magnetic structure. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical absorption of glassy semiconductors of the GeSbSe system

The optical transmittance of chalcogenide glasses Ge₂SbSe₇ (I), Ge₃SbSe₆ (II), GeSb₂Se₇ (III) and GeSbSe₃ (IV) was studied in the near infrared spectral region, 0.7–25 μm. The longwavelength tail of the absorption edge can be described by Urbach's rule. At higher absorption levels the absorption coefficient K depends quadratically on the energy of incident radiation. The temperature dependence of the absorption edge is discussed and the optical gaps 1.77 eV (I), 1.67 eV (II), 1.66 eV (III), 1.57 eV (IV) together with the corresponding temperature coefficients are also determined. The studied glasses are quite transparent in the 600–5000 cm^?1 wavenumber range.     

h-BN型超晶格等离子体光子晶体能带特性研究

六方氮化硼(h-BN)晶格结构是一种类六方对称复式超晶格结构。具有h-BN晶格构型的光子晶体以其宽光子带隙特点受到国内外学者的广泛关注。本文利用不同尺度低压气体放电管与Al₂O₃介质棒周期性排列，构建了新型h-BN型超晶格等离子体光子晶体，实现其空间结构和等离子体参数的动态调控。利用微波透射谱对比研究了h-BN型超晶格与简单三角晶格等离子体光子晶体禁带位置、宽度和数目。分析了放电电流、介质棒阵列数对不同频段光子带隙的影响，以及电磁波入射角度对电磁传输特性的影响。结果表明：等离子体的引入不仅能够形成新的光子带隙，而且可以选择性地使部分禁带位置发生移动；相对于简单三角晶格，h-BN型超晶格等离子体光子晶体呈现出更多光子带隙；Al₂O₃介质棒阵列数对等离子体光子晶体禁带位置、宽度和数目均具有重要影响。电磁波入射角度变化越大，电磁传输特性差别越显著，透射谱相关性越差。本文所设计的新型h-BN型超晶格等离子体光子晶体为制作可调谐光子晶体提供了新的思路，在微波和太赫兹波控制领域具有潜在应用价值。     

Effect of temperature and isomorphic atom substitution on optical absorption edge of TlInS2xSe2(1‐x) mixed crystals (0.25 ≤ x ≤ 1)

The optical properties of the TlInS_2xSe_2(1‐x)mixed crystals (0.25 ≤ x ≤ 1) have been investigated through the transmission and reflection measurements in the wavelength range of 400–1100 nm. The optical indirect band gap energies were determined by means of the analysis of the absorption data. It was found that the energy band gaps decrease with the increase of selenium atoms content in the TlInS_2xSe_2(1‐x)mixed crystals. The transmission measurements carried out in the temperature range of 10–300 K revealed that the rates of change of the indirect band gaps with temperature are γ = –9.2×10^–4 eV/K, –6.1×10^–4 eV/K, –4.7×10^–4 eV/K and –5.6×10^–4 eV/K for TlInS₂, TlInS_1.5Se_0.5, TlInSSe and TlInS_0.5Se_1.5 crystals, respectively. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Annealing temperatures induced optical constant variations of methyl violet 2B thin films manufactured by the spin coating technique

Spin coating technique has been successfully applied to deposit uniform methyl violet 2B (MV2B) thin films. X-ray diffraction and Fourier-transform infrared techniques were used to study the crystal and molecular structure of MV2B. The optical properties of the films have been studied by spectrophotometer measurements of transmittance and reflectance at normal incidence of light in the spectral range of 200–2500 nm. The absorption and refractive indices are independent of the film thickness. The absorption parameters such as molar extinction coefficient, oscillator strength and electric dipole strength have been reported before and after annealing. The type of electronic transition is indirect allowed transition with onset energy gap of 1.82 eV and optical energy gap of 3.65 eV. Annealing temperatures reduce structure disorder, remove trap level, increase values of the onset and optical energy gaps and decrease refractive index. The single oscillator model has been applied for calculating the dispersion parameters. The oscillator energy, the dispersion energy, the high frequency dielectric constant, the lattice dielectric constant and the ratio of free charge carriers' concentration to its effective mass were evaluated before and after annealing. The dielectric properties of the films were also determined.     

Role of annealing environment and partial pressure on structure and optical performance of TiO2 thin films fabricated by rf sputter method

Influences of the different annealing ambient (in air, 1 bar, 2 bar, 3 bar and 4 bar oxygen partial pressure) on the titanium dioxide (TiO₂) thin films deposited on soda lime glass by standard radio frequency (rf) magnetron reactive sputtering method at 100 watt were investigated by means of X–ray diffractometer (XRD), ultra violet spectrometer (UV–vis), and Scanning Electron Microscopy (SEM). It was found that either optical properties or energy band gaps of the films enhanced with increase in the oxygen partial pressure up to 3 bar. The energy band gaps of the films (except for the film annealed in 4 bar oxygen partial pressure) became larger than the film annealed in atmospheric pressure. The best transmission was observed for the thin film annealed in 3 bar oxygen partial pressure. Moreover, not only was grain–like structure found to be more dominant than dot–like structure but also growth of anatase phase was observed instead of that of the rutile phase with increasing oxygen partial pressure up to 3 bar. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)