ZnO高掺杂Ga的浓度对导电性能和红移效应影响的第一性原理研究

采用基于密度泛函理论框架下的第一性原理平面波超软赝势方法,在相同环境条件下建立了浓度不同的由Ga原子取代Zn原子的Zn_1-xGa_xO模型.对低温高掺杂Ga原子的Zn_1-xGa_xO半导体的能带结构、态密度和吸收光谱进行了计算.结果表明:Ga原子浓度越大,进入导带的相对电子数越多,但是电子迁移率反而减小.通过对掺杂和未掺杂ZnO的电导率以及最小间隙带宽度分别进行了比较 关键词： ZnO高掺杂Ga 电导率 红移 第一性原理     

Features of the Percolation Scheme of Vibrational Spectrum Reconstruction in the Ga1 – xAl x P Alloy

Specific features of the properties of Ga–P lattice vibrations have been investigated using the percolation model of a mixed Ga_{1 – x}Al _x P crystal (alloy) with zero lattice mismatch between binary components of the alloy. In contrast to other two-mode alloy systems, in Ga_{1 – x}Al _x P a percolation splitting of δ ~ 13 cm^–1 is observed for the low-frequency mode of GaP-like vibrations. An additional GaP mode (one of the percolation doublet components) split from the fundamental mode is observed for the GaP-rich alloy, which coincides in frequency with the gap corresponding to the zero density of one-phonon states of the GaP crystal. The vibrational spectrum of impurity Al in the GaP crystal has been calculated using the theory of crystal lattice dynamics. Upon substitution of lighter Al for the Ga atom, the calculated spectrum includes, along with the local mode, a singularity near the gap with the zero density of phonon states of the GaP crystal, which coincides with the mode observed experimentally at a frequency of 378 cm^–1 in the Ga_{1 – x}Al _x P (x < 0.4) alloy.

     

First-principles study of the effect of heavy Ni doping on the electronic structure and absorption spectrum of wurtzite ZnO

The band structures, densities of states and absorption spectra of pure ZnO and two heavily Ni doped supercells of Zn_0.9722Ni_0.0278O and Zn_0.9583Ni_0.0417O have been investigated using the first-principles plane-wave ultrasoft pseudopotential method based on the density functional theory. The calculated results showed that the band gap is narrowed by Ni doping in ZnO; this, is because the conduction band undergoes a greater shift toward the low-energy region than the valence band and because heavier doping concentrations lead to, narrower band gaps. Moreover, the optical absorption edge exhibits a redshift due to the narrowing of the band gap. Heavier doping concentrations leads to more significant redshifts, which is in agreement with the experimental results.     

Optically detected electron nuclear double resonance of anion antisite defects in GaP

Anion antisite defects in semi-insulating GaP single crystals (doped with group V atoms) are investigated with optically detected electron paramagnetic resonance and electron nuclear double resonance (ODEPR and ODENDOR) ODEPR and ODENDOR are measured as microwave- and microwave- and rf-induced changes of the magnetic circular dichroism of the optical absorption (MCDA), respectively. In GaP: V two P antisite defects are found in about equal concentration which within ENDOR precision have tetrahedral site symmetry with respect to the nearest P ligands. The two P_GaP₄ defects have only slightly differentg-factors and³¹P superhyperfine interactions, but differ by one order of magnitude in their spin-lattice relaxation times. Their optical properties are also different from those of the P_GaP₄ defects reported earlier in GaP: Cr. It is concluded that there is no experimental prove yet. that any of the P_GaP₄ defects hitherto observed in GaP in an isolated P_GaP₄ defect which is not associated with another defect to form an antisite related defect complex.     

Theory of band gap bowing of disordered substitutional II–VI and III–V semiconductor alloys

For a wide class of technologically relevant compound III?CV and II?CVI semiconductor materials AC and BC mixed crystals (alloys) of the type A _x B_1?x C can be realized. As the electronic properties like the bulk band gap vary continuously with x, any band gap in between that of the pure AC and BC systems can be obtained by choosing the appropriate concentration x, granted that the respective ratio is miscible and thermodynamically stable. In most cases the band gap does not vary linearly with x, but a pronounced bowing behavior as a function of the concentration is observed. In this paper we show that the electronic properties of such A _x B_1?x C semiconductors and, in particular, the band gap bowing can well be described and understood starting from empirical tight-binding models for the pure AC and BC systems. The electronic properties of the A _x B_1?x C system can be described by choosing the tight-binding parameters of the AC or BC system with probabilities x and 1 ? x, respectively. We demonstrate this by exact diagonalization of finite but large supercells and by means of calculations within the established coherent potential approximation (CPA) We apply this treatment to the II?CVI system Cd _x Zn_1?x Se, to the III?CV system In _x Ga_1?x As and to the III-nitride system Ga _x Al_1?x N.     

SiO2 nanoparticles doped nematic liquid crystal system: An experimental investigation on optical and dielectric properties

The aim of this work is to investigate the effect of silica (SiO₂) nanoparticles (NPs) on optical and dielectric properties of BBEA nematic liquid crystal (NLC). For optical analysis the photoluminescence (PL) and UV-absorbance experiments have been performed. The doped system is showing enhancement in the intensity of photoluminescence with varying concentration of nanoparticles. A red shift is observed in the emission spectra of NLC doped with silica nanoparticles. The PL emission peak of NLC is observed at 377.3 nm which is shifted to 379.7 nm in the presence of silica nanoparticles. We have also observed the enhancement in the value of UV absorption for silica doped systems in comparison to the pure system. Energy band gap of pure and doped systems has been calculated and it is found that the energy band gap is decreasing with concentration which is a promising result of this study. The dielectric parameters of the pure and doped NLC systems were carried out as a function of frequency and temperature. Different dielectric parameters such as relative permittivity, loss factor and dielectric conductivity have been measured. The pure and silica nanoparticles doped systems has shown decreased value of dielectric permittivity and loss factor at lower frequency region and at higher frequency regions these values became constant. The value of relative permittivity also decreases with concentration. The increased value of a.c. conductivity for doped systems can be utilized in device designing. Moreover, the temperature dependence of the birefringence (Δn) was determined from the transmitted intensity of light for pure and doped systems and the improvement in its value for both composites has been observed. Improved value of birefringence has pronounced applications in optical devices.     

Some optical properties of (ZnS)−(GaP) alloys and their interpretation

Optical measurements on the single crystals of the pseudobinary (ZnS)?(GaP) alloy were carried out. The band gap energy decreased more rapidly with increase in GaP concentration than that reported previously. The analysis of the absorption spectra for the crystals of up to 70 mole % GaP indicated a direct transition characteristic, and that the band gap becomes nearly equal to that of pure GaP at about 30 mole % GaP. The photoluminescence spectra observed at liquid nitrogen temperature could first be resolved into three kinds of emission band with Gaussian distribution. The peak energy of these bands were found to be independent of the band gap variation. Thus the observed peak energy shift with alloy composition was attributed to the variation of the emission intensity of each band. The band gap shrinkage and the origin of the photoluminescence spectra on the basis of a molecular orbital method were discussed.     

Structural and electrical studies on Zn2+ doped LiCoPO4

Zinc doped LiCoPO₄ was prepared by two step solid state reaction method. XRD studies ensure the formation of phase pure LiCoPO₄ with olivine phase orthorhombic structure with p_nma phase group. 2θ value of XRD peaks of LiCoPO₄ is found to be shifted upon Zn²⁺ doping. The conductivity of pure LiCoPO₄ is noticed to be increased by one order upon doping of Zn²⁺. The scaling behavior of Z″ indicates that the ion dynamical process are temperature independent for both pure and Zn²⁺ doped LiCoPO₄. The temperature dependence of conductivity for both pure and Zn²⁺ doped sample obeys Arrhenius law of conduction.     

Effect of indium incorporation on the optical properties of spray pyrolyzed Cd0.22Zn0.78S thin films

In this study, effect of indium incorporation on the optical properties is investigated for the spray pyrolyzed onto glass substrates at 275°C substrate temperature undoped and indium doped Cd_0.22Zn_0.78S thin films. The average optical transmittance of all the films was over 77% in the wavelength range between 450 and 800 nm. The optical band gap energies of the thin films have been investigated by the measurement of the optical absorbance as a function of wavelength. The optical absorption studies reveal that the transitions are direct band gaps of 3.02 and 3.05 eV for undoped and doped indium Cd_0.22Zn_0.78S thin films, respectively. The Urbach tail parameter and optical constants such as refractive index, extinction coefficient, and dielectric constants were calculated for these films. The dispersion parameters such as single-oscillator energy and dispersive energy were discussed in terms oft he single-oscillator Wemple—DiDomenico model.     

Survey of intermediate band material candidates

A systematic ab initio study, using the local spin density approximation, of the electronic properties of Ga_xP_yM compounds, where M is a transition metal substituting Ga or P atoms in a GaP host semiconductor lattice is presented. This study is oriented towards the early identification of intermediate band materials of recent interest as new photovoltaic materials to exceed the efficiency of single gap and even tandems of two solar cells. M=Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn have been explored as transition metals and Sc, V, Cr, and Fe in Ga₃₂P₃₁M and Cr in Ga₃₁P₃₂M have exhibited the desired intermediate band.     

Ga/N高共掺浓度对ZnO导电性能和红移影响的第一性原理研究

采用密度泛函理论框架下的第一性原理平面波超软赝势方法, 建立了未掺杂ZnO单胞和两种不同浓度的Ga/N高共掺ZnO超胞模型, 分别进行了几何结构优化、总态密度分布和能带分布的计算. 研究表明, ZnO高共掺Ga/N的条件下, Ga/N高共掺浓度越大, 导电性能越弱, 并且高掺杂后高能区红移效应显著, 计算得到的结果与实验结果的变化趋势一致. 关键词： Ga/N高共掺ZnO 电导率 红移 第一性原理     

Bandgap engineering of GaxZn1–xO nanowire arrays for wavelength‐tunable light‐emitting diodes

Wavelength‐tunable light‐emitting diodes (LEDs) of Ga_xZn_1–xO nanowire arrays are demonstrated by a simple modified chemical vapor deposition heteroepitaxial growth on p‐GaN substrate. As a gallium atom has similar electronegativity and ion radius to a zinc atom, high‐level Ga‐doped Ga_xZn_1–xO nanowire arrays have been fabricated. As the x value gradually increases from 0 to 0.66, the near‐band‐edge emission peak of Ga_xZn_1–xO nanowires shows a significant shift from 378 nm (3.28 eV) to 418 nm (2.96 eV) in room‐temperature photoluminescence (PL) measurement. Importantly, the electroluminescence (EL) emission of Ga_xZn_1–xO nanowire arrays LED continuously shifts with a wider range (∼100 nm), from the ultraviolet (382 nm) to the visible (480 nm) spectral region. The presented work demonstrates the possibility of bandgap engineering of low‐dimensional ZnO nanowires by gallium doping and the potential application for wavelength‐tunable LEDs.     

Spin-polarised DFT study of cobalt-doped gallium clusters

The structural, electronic and magnetic properties of small gallium clusters doped with Cobalt have been studied using spin-polarised density functional theory. The binding energy per atom, second-order differences of total energies and fragmentation energies of equilibrium geometries of the host Ga_n+1 and doped Ga_nCo (n = 1–12) clusters are computed. Doped clusters are found to be more stable than pure Ga clusters; Ga₃Co, Ga₅Co and Ga₈Co clusters are exceptionally stable. Doping with Co changes the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO–LUMO) gap, and also affects the magnetic moments of clusters.     

Indirect band gap investigation of orthorhombic single crystals of sulfur

Single crystals of pure and selenium doped orthorhombic sulfur crystal were prepared from carbon disulphide solution. They were investigated at two temperatures (6°C and 85°C) in an optical absorption apparatus. The optical absorption increased with temperature and it was found to be higher for Se doped crystals within the same range of temperatures. The indirect allowed band gap transitions and assisting phonons were observed. Their values were found to shift toward lower energy with increasing temperatures. They are also lower for doped samples. At 6°C, the indirect allowed band gaps are 2.61 eV and 2.56 eV for pure and selenium-doped crystals, respectively. At 85°C, they are 2.44 eV and 2.40 eV for pure and doped crystals, respectively. The phonons associated with the optical transitions could be attributed to the stretching band of S₈ rings.     

Tight-binding studies of the electronic band structure of GaAlN and GaInN alloys

The semi-empirical tight-binding (TB) approach with an sp³s^* orbital basis is used to investigate the electronic band structure of cubic Ga_1-xAl_xN and Ga_1-xIn_xN alloys. The spin–orbit splitting in the Hamiltonian and first- and second-neighbour interactions are considered to explore the gap evolution as a function of the concentration x. The TB parameters used in the studies were obtained within the virtual crystal approximation using those of the binary compounds AlN, GaN, and InN. The binary parameters are calculated by applying an empirical TB Hamiltonian taking into account first- and second-neighbour interactions. A direct to indirect gap transition is obtained for Ga_1-xAl_xN at x=0.60. In contrast, the Ga_1-xIn_xN alloys exhibit direct-gap characteristics as a consequence of being formed by the direct-gap binary compounds GaN and InN. PACS 71.20.Nr; 71.23.-k; 71.55.Eq     

Electronic and optical properties of pure and Mo doped anatase TiO2 using GGA and GGA+U calculations

Comparative GGA and GGA+U calculations for pure and Mo doped anatase TiO₂ are performed based on first principle theory, whose results show that GGA+U calculation provide more reliable results as compared to the experimental findings. The direct band gap nature of the anatase TiO₂ is confirmed, both by using GGA and GGA+U calculations. Mo doping in anatase TiO₂ narrows the band gap of TiO₂ by introducing Mo 4d states below the conduction band minimum. Significant reduction of the band gap of anatase TiO₂ is found with increasing Mo doping concentration due to the introduction of widely distributed Mo 4d states below the conduction band minimum. The increase in the width of the conduction band with increasing doping concentration shows enhancement in the conductivity which may be helpful in increasing electron–hole pairs separation and consequently decreases the carrier recombination. The Mo doped anatase TiO₂ exhibits the n-type characteristic due to the shifting of Fermi level from the top of the valence band to the bottom of the conduction band. Furthermore, a shift in the absorption edge towards visible light region is apparent from the absorption spectrum which will enhance its photocatalytic activity. All the doped models have depicted visible light absorption and the absorption peaks shift towards higher energies in the visible region with increasing doping concentration. Our results describe the way to tailor the band gap of anatase TiO₂ by changing Mo doping concentration. The Mo doped anatase TiO₂ will be a very useful photocatalyst with enhanced visible light photocatalytic activity.     

First principles calculations of electronic and optical properties of Mo and C co-doped anatase TiO2

Using the first principles calculations, the electronic and optical properties of C, Mo and Mo-C-doped anatase TiO₂ are studied. For the Mo mono-doped TiO₂, the band gap reduces little, and the largest perturbation occurs at the CBM of TiO₂. C mono-doping suppresses the effective band gap, but the partially occupied subbands in the gap probably also serve as the recombination centers for electrons and holes. Therefore, the Mo-C co-doping is investigated for the charge compensation consideration. We discuss six doped configurations and find that the total energy of the system is increased with increasing distance of C and Mo. It is found that co-doped configurations with C nearest to Mo possess the lowest total energy. Then, we focus on discussing three possible Mo-C adjacent co-doped configurations. The subbands mainly induced by C-2p states in the band gap become fully occupied because the Mo atom contributes sufficient electrons to C anion for compensation. At the same time, the effective band gap is narrowed about 0.9 eV and the perturbation at the CBM occurred in Mo mono-doped TiO₂ disappears, which means the band edges of doped system still straddle the redox potentials of water. Furthermore, the optical properties of the compensated Mo-C adjacent co-doped TiO₂ and pure TiO₂ are calculated. The optical absorption edges of the Mo-C co-doped TiO₂ shift towards the visible light region.     

Cu掺杂Ga2O3薄膜的光学性能

采用射频磁控溅射和N₂气氛退火处理制备了多晶Ga₂O₃薄膜和Cu掺杂Ga₂O₃薄膜.用X射线衍射仪、紫外-可见分光光度计、荧光光谱仪对Ga₂O₃薄膜和Cu掺杂Ga₂O₃薄膜的结构和光学性能进行了表征.结果表明,Cu掺杂后Ga₂O₃薄膜的结晶质量变差,透过率明显降低,吸收率增加,光学带隙减小.本征Ga₂O₃薄膜在紫外、蓝光和绿光出现了发光带,Cu掺杂后紫外和蓝光发射增强,且在475 nm 处出现了一个新的发光峰.     

Ab initio study of phonon dispersion and thermodynamic properties of pure and doped pyrites

Pyrites (FeS₂) are solid minerals that are found abundantly in Nigeria and are easy to prepare in laboratories. In this work, FeS₂ is studied extensively in its pure state as well as when iron is substitutionally doped with zinc and calcium at concentrations of 0, 0.25, 0.5, 0.75 and 1. Using density functional theory, the eectronic, dynamic and thermodynamic properties were calculated. The results revealed that the lattice parameters and bulk modulus increases with increasing concentration and the obtained values are in agreement with available experimental and theoretical values. Though pyrite, when doped with zinc, obeys Vegard’s law, doping with calcium revealed pronounced deviation from this law. The calculated band structures showed that FeS₂ has an indirect band gap whose size decreases after introducing zinc while doping with calcium increases the band gap. The phonon dispersion of the end members FeS₂ and ZnS₂ indicate that the systems are dynamically stable while CaS₂ is dynamically unstate. Also, the thermodynamic properties of the pure and doped pyrites were calculated and the ranges of temperature at which the lattice and electronic degrees of freedom contribute to the specific heat capacity are presented.     

Deep defect levels in diode structures based on gallium phosphide

We have studied the characteristic features of defect formation and generation of deep levels that control degradation processes in GaP diodes. We have shown that degradation of GaP diodes is mainly connected with the presence of defects in the space charge region of the p-n junction which are V_Ga-donor complexes, antistructural P_Ga defects, and also intrinsic V_P structural defects with high density of localized states. We have established an interconnection between the degradation rate and the features of the energy spectrum of deep levels in GaP diodes, allowing us to use analysis of tunneling spectra for diagnostics of p-n junction quality and prediction of GaP diode degradation. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 1, pp. 78–81, January–February, 2006.