Ab initio molecular dynamics simulations on structural change of supercooled liquid Si at different temperatures from 1700 to 1100 K

Using ab initio molecular dynamics simulations, the local atomic structure and electronic properties of supercooled liquid Si (l-Si) at different temperatures from 1700 to 1100 K were studied. Our calculated coordination numbers present no obvious change in the temperature range investigated. Our results indicate that the structure of supercooled l-Si may be well described as a combined local atomic configuration of white-tin and diamond type structures. Upon cooling from 1700 to 1100 K, the tetrahedral white-tin type ordering collapses gradually toward the tetrahedral diamond-type structure. No drastic change behavior is observed in our work.     

Structural features of liquid metallic glass former

The structural parameters of Au₇₅Si₂₅ alloy, pure Au, Al₈₈Si₁₂, Cu₈₇Sn₁₃, In₉₈Al₂ and Al₉₃Ni₇ alloys have been measured by X-ray diffractometer. It is found that there are subpeaks in the pair correlation functions in liquid Au₇₅Si₂₅ alloy. The addition of Si in liquid Au results in a decrease in both the correlation radius and the coordination number of the nearest atomic neighbors. The Au₇₅Si₂₅ alloy nearest atomic distance has a more stable dependence on temperature compared to In₉₈Al₂, Al₉₃Ni₇ and Cu₈₇Sn₁₃ alloys. The atomic density change of liquid Au₇₅Si₂₅ alloy is less dependent on temperature than the liquid Au and Al₈₈Si₁₂ alloy. The liquid metallic good glass former Au₇₅Si₂₅ alloy possesses a more stable liquid structure than that of poor glass formers, indicating the essential of the fragility of the superheated melts.     

Theoretical study on electronic and optical properties of Zn-doped In<Subscript>0.25</Subscript>Ga<Subscript>0.75</Subscript>As photocathodes

First-principles calculations for intrinsic and Zn-doped In_0.25Ga_0.75As are performed based on density functional theory to study the influence of Zn doping on electronic and optical properties. The band structure, density of state, Mulliken population, dielectric function, complex refractive index, absorption coefficient and reflectivity of In_0.25Ga_0.75As are calculated. Results show that the Fermi levels of two Zn-doped models enter into the valence bands and Zn atom is more easily to replace In atom than Ga atom. The lattice constant of In_0.2344Ga_0.75Zn_0.0156As reduced after optimization, while that of In_0.25Ga_0.7344Zn_0.0156As increased to the contrary. The Mulliken bond population shows that the doping Zn atoms can enlarge the strength of In–As and Ga–As polar covalent bonds. Furthermore, the calculated absorption coefficient and reflectivity are used to characterize the performance of photoemission, indicates that the photoresponses of Zn-doped models are better than that of the intrinsic.     

Room temperature growth of InxGa1−xN thin films by mixed source modified activated reactive evaporation

Polycrystalline In_xGa_1−xN thin films were prepared by mixed source modified activated reactive evaporation (MARE) technique. The films were deposited at room temperature on glass substrates without any buffer layer. All the films crystallize in the hexagonal wurtzite structure. The indium concentration calculated from XRD peak shift using Vegard's law was found to be varying from 2% to 92%. The band gap varies from 1.72 eV to 3.2 eV for different indium compositions. The indium rich films have higher refractive indices as compared to the gallium rich films. The near infra-red absorption decreases with gallium incorporation into InN lattice which is mainly due to decrease in the free carrier concentration in the alloy system. This fact is further supported from Hall effect measurements. MARE turns out to be a promising technique to grow In_xGa_1−xN films over the entire composition range at room temperature.     

Exciton absorption,photoluminescence and band structure of N-Free and N-DOPED In1−xGaxP

Optical absorption data on In_1?xGa_xP are reported showing the first observation of the exciton absorption peak in a III-V ternary alloy. This allows the most accurate determination to date of both the composition dependence of the Γ band gap and the position of the Γ ? X crossover (x_c = 0.72, 77°K; x_c = 0.73, 300°K). Absorptio and photoluminescence spectra are compared for both direct and indirect In_1?xGa_xP. In addition, absorption, photoluminescence, and luminescence lifetimes of In_1?xGa_xP:N are examined. These data suggest that the A-line is perturbed by alloy disorder and forms a broad luminescence band for x ? 0.94. The observed Stokes shift for the N-trap in the alloy is characteristic of an impurity strongly coupled to the lattice. The temperature dependence of the N luminescence band is found to be well described by the conventional configuration-coordinate model for phonon-coupled impurity luminescence. Implications of these results for light emitting devices are mentioned.     

Thermodynamics of type A1−xBxC1−yDy III–V quaternary solid solutions

An improved theory based on the pair approximation is given for the thermodynamic properties of type A_1?xB_xC_1?yD_yIII–V quaternary solid solutions. The theory takes into account the quasi-chemical nature of the nearest neighbor pair distribution, which has been neglected or inadequately treated in previous work. With a suitable thermodynamic treatment, a quasi-chemical equilibrium relation for the nearest neighbor pair distribution is derived. Numerical calculations using available thermodynamic data show that the distribution will deviate appreciably from random mixing values in the In_1?xGa_xP_1?yAs_y and Al_1?xGa_xAs_1?ySb_y systems due to a short-range clustering effect of nearest neighbor pairs with strong bond energy, whereas the deviation will be small in the Al_1?xGa_xP_1?yAS_y system. The expressions for the chemical potentials and the activity coefficients of the binary compound components are given. These are readily applicable to phase diagram calculations.     

EXAFS study of the atomic structure of amorphous Tb20Fe80

The atomic structure of amorphous Tb₂₀Fe₈₀ thin films has been studied by Extended X-ray Absorption Fine Structure (EXAFS) of both FeK and TbL _III absorption edges. The local site geometry around Fe atoms shows predominantly Fe nearest neighbors with an Fe-Fe distance distribution centered on 2.50±0.02 Å and a coordination number of 9.1±1. In contrast, the radial structure function (RSF) obtained at the Tb edge is broad and asymmetric. The peak in the RSF corresponds to a Tb-Fe near neighbor distance of 2.94±0.1 Å with no evidence for Tb-Tb nearest neighbor coordination. The width and the shape of the RSF suggest that the Tb-Fe atomic environment is anisotropic and strained probably as a consequence of the growth process. This distorted atomic environment is suggested to be responsible for the magnetic anisotropy in these alloys. Thermal annealing at 200 °C leads to reduction inK _u. We propose that this results from reordering of the Tb local environment such that the average structural anisotropy in the distribution is reduced. EXAFS data show that annealing at 400°C causes precipitation of bcc polycrystalline Fe. The addition of 7 at.% Au to the alloy prevents this recrystallization and preserves the amorphous state but does not prevent the structural relaxation which reducesK _u at lower temperatures.     

Nanomechanical characterizations of InGaN thin films

In_xGa_1−xN thin films with In concentration ranging from 25 to 34 at.% were deposited on sapphire substrate by metal-organic chemical vapor deposition (MOCVD). Crystalline structure and surface morphology of the deposited films were studied by using X-ray diffraction (XRD) and atomic force microscopy (AFM). Hardness, Young's modulus and creep resistance were measured using a nanoindenter. Among the deposited films, In_0.25Ga_0.75N film exhibits a larger grain size and a higher surface roughness. Results indicate that hardness decreases slightly with increasing In concentration in the In_xGa_1−xN films ranged from 16.6 ± 1.1 to 16.1 ± 0.7 GPa and, Young's modulus for the In_0.25Ga_0.75N, In_0.3Ga_0.7N and In_0.34Ga_0.66N films are 375.8 ± 23.1, 322.4 ± 13.5 and 373.9 ± 28.6 GPa, respectively. In addition, the time-dependent nanoindentation creep experiments are presented in this article.     

Built-in-polarization and thermal conductivity of InxGa1-xN/GaN heterostructures

The effect of built-in-polarization (BIP) field on thermal properties of In_xGa_1−xN/GaN heterostructure has been investigated. The thermal conductivity k of In_xGa_1−xN alloy has been estimated using Callaway's formula including the BIP field for In content x = 0, 0.1, 0.3, 0.5 and 0.9. This study reports that irrespective of In content, the room temperature k of In_xGa_1−xN/GaN heterostructure is enhanced by BIP field. The result predicts the existence of a characteristic temperature T_p at which both thermal conductivities (including and excluding BIP field) show a crossover. This gives signature of pyroelectric nature of In_xGa_1−xN alloy which arises due to variation of polarization with temperature indicating that thermal conductivity measurement can reveal pyroelectric nature. The pyroelectric transition temperature of In_xGa_1−xN alloy has been predicted for various x. The composition dependent nature of room temperature k for x = 0.1 and 0.5 are in line with prior experimental studies. The result can be used to minimize the self heating effect in In_xGa_1−xN/GaN heterostructures.     

Electronic structure of GaxIn1-xAsySb1-y quaternary alloy by recursion method

Summary  This work reports the electronic structure of GaInAsSb quaternary alloy by recursion method. A five-orbital sp₃s^* per atom model was used in the tight-binding representation of the Hamiltonian. The local density of states (LDOS), integrated density of states (IDOS) and structural energy (ST.E) were calculated for Ga, In, As and Sb sites in Ga_0.5 In_0.5 As_0.5 Sb_0.5 and GaInAsSb lattice matched to GaAs and the same alloy lattice matched to GaSb. There are 216 atoms in our cluster arranged in a zincblend structure. The results are in good agreement with available information about the alloy.     

FA(Ga+,In+,Tl+) tunable laser activity and interaction of halogen atoms (F,Cl,Br,I,At) at the (001) surface of KCl crystal: ab initio calculations

An attempt has been made to examine F_A(Ga⁺,In⁺,Tl⁺) tunable laser activity and adsorptivity of halogen atoms (F,Cl,Br,I,At) at the (0 0 1) surface of KCl crystal using an embedded cluster model, CIS and density functional theory calculations with effective core potentials. The ion clusters were embedded in a simulated Coulomb field that closely approximates the Madelung field at the host surface. The nearest neighbor ions to the defect site were then allowed to relax to equilibrium. Based on the calculated strength of electron–phonon coupling and Stokes-shifted optical transition bands, The F_A(Tl⁺) center was found to be the most laser active in agreement with the experimental observation that the optical emissions of F_A(In⁺) and F_A(Ga⁺) centers were strongly quenched. The disappearance of the anisotropy and np splitting observed in the absorption of F_A(Ga⁺,In⁺,Tl⁺) centers were monotonically increasing functions of the size of the impurity cation. The F_A(Ga⁺,In⁺,Tl⁺) defect formation energies followed the order F_A(Ga⁺)>F_A(In⁺)>F_A(Tl⁺). The Glasner–Tompkins empirical relationship between the principal optical absorption of F centers in solids and the fundamental absorption of the host crystal was generalized to include the positive ion species. As far as the adsorptivity of the halogen atoms is concerned, the F and F_A(In⁺,Tl⁺) centers were found to change the nature of adsorption from physical adsorption to chemical adsorption. The adsorption energies were monotonically increasing functions of the electronegativity of the halogen and the amount of charge transferred from the defect-free surface. The calculated adsorption energies were explainable in terms of the electron affinity, the effective nuclear charge and the electrostatic potentials at the surface. The spin pairing mechanism played the dominant role in the course of adsorbate–substrate interactions and the KCl defect-free surface can be made semiconducting by F or F_A(In⁺,Tl⁺) surface imperfections.     

Band structure enhancement of indirect transitions

Indirect and direct transitions in Ga_xIn_1?xP alloys has been studied in modulation spectroscopy, between 10 and 300 K. Deviation from simple indirect absorption has been studied and shown to be consistant with a model which takes into account band structure enhancement of the absorption coefficient at alloy composition values where direct and indirect edges occur, at nearly the same energy. For the first time we observe structures of Δε₁ corresponding to band structure enhanced indirect transitions.     

X-ray investigation of the interface structure of free standing InAs nanowires grown on GaAs [\bar{1}\bar {1}\bar{1}]_{\mathrm{B}}

The heteroepitaxial growth process of InAs nanowires (NW) on GaAs $[\bar{1}\bar{1}\bar{1}]_{\mathrm{B}}$ substrate was investigated by X-ray grazing-incidence diffraction using synchrotron radiation. For crystal growth we applied the vapor–liquid–solid (VLS) growth mechanism via gold seeds. The general sample structure was extracted from various electron microscopic and X-ray diffraction experiments. We found a closed Ga _x In_1?x As graduated alloy layer at the substrate to NW interface which was formed in the initial stage of VLS growth with a Au–Ga–In liquid alloy. With ongoing growth time a transition from this VLS layer growth to the conventional VLS NW growth was observed. The structural properties of both VLS grown crystal types were examined. Furthermore, we discuss the VLS layer growth process.     

Theoretical investigation of electronic structure and optical response in relation to the transport properties of Ga1−xInxN (x = 0, 0.25, 0.50, 0.75)

The state-of-the-art all-electron FLPAW method and the BoltzTrap software package based on semi-classical theory were adopted to explore the electronic structure and the optical and thermoelectric properties of Ga_1−xIn_xN. Ga_1−xIn_xN is predicted to be a direct band gap material for all values of x. Moreover, the band gap varies between 2.99 eV and 1.95 eV as x changes. Optical parameters such as the dielectric constant, absorption coefficient, reflectivity and refractive index are calculated and discussed in detail. The doping of In plays an important role in the modulation of the optical constants. The static dielectric constant ɛ(0) of Ga_1−xIn_xN was calculated as 3.95, 3.99, 3.99 and 4.03 at x = 0.00, 0.25, 0.50 and 0.75, respectively. The static refractive index is 2.0 for pure Ga_1−xIn_xN at x = 0.00. The thermal properties varied greatly as x fluctuated. The ternary alloy has large values for the Seebeck coefficient and figure of merit at high temperatures and is thus suitable for thermoelectric applications. Pure Ga_1−xIn_xN at x = 0 exhibited ZT = 0.80 at room temperature, and at higher temperatures, the thermal conductivity decreased with increased In doping.     

Chemical and structural properties of ternary post-transition metal oxide thin films: InZnO,InGaO and GaZnO

The chemical states of ternary post-transition metal oxide thin films of InGaO, GaZnO and InZnO were investigated using X-ray photoelectron spectroscopy. Detailed binding energy (BE) analyses revealed certain evolution in chemistry in the ternary oxides compared to the reference binary oxides of In₂O₃, ZnO, or Ga₂O₃. In particular, O 1s BEs were changed with the compositions, which suggests that the charge transfer (CT) between In³⁺/Ga³⁺/Zn²⁺ and O²⁻ ions is significant. Results of extended X-ray absorption fine structure analyses further showed that the first shell coordination (cation–O bond) is roughly maintained even though the ternary oxide films were structurally disordered. This implies that the CT process via O²⁻ ions can influence the charge reconstructions in the ternary oxide systems.     

The evolution of covalent bonding structures of liquid In–Sn alloys

The short-range orders of liquid In_xSn_100−x binary alloys have been investigated along the liquidus based on the experimental results obtained using X-ray diffraction. The covalent bonding structures characterized by the shoulder on the high-Q side of the first peak of S(Q) for all the In_xSn_100−x melts are detected. With increasing Sn content, there is a rather continuous change of the local structure possibly related to the presence of nearly covalent bonds in the liquid alloy. The peculiar temperature dependence of the viscosity is assigned to the evolution of tetrahedral units. The observed correlation between liquid structures and peculiar viscosity behaviors provides new insight into nucleation phenomena from microscopic viewpoint.     

First principles study on electronic and optical properties of Al<Subscript>x</Subscript>Ga<Subscript>1−x</Subscript>N and In<Subscript>y</Subscript>Ga<Subscript>1−y</Subscript>N

The band structure, density of states of Al_xGa_1?xN and In_yGa_1?yN was performed by the first-principles method within the local density approximation. The calculated energy gaps of the AlN, Al_0.5Ga_0.5N, GaN, In_0.5Ga_0.5N and InN were 5.48, 4.23, 3.137, 1.274 and 0.504 eV, which were in agreement with the experimental result. The dielectric functions, absorption coefficient and loss function were calculated based on Kramers–Kronig relations. Further more, the relationships between electronic structure and optical properties were investigated theoretically. For Al_xGa_1?xN and In_yGa_1?yN materials, the micromechanism of the optical properties were explained.     

Al和Sn液态结构的温度变化特性

通过对Al和Sn的液态X射线衍射数据的分析，发现Al和Sn的液体结构随温度的变化都存在突变.Al的突变发生在1050—1250 ℃的温度区间内，Sn有两个突变点，一个在800℃左右，另一个在1200℃附近.随着温度的升高，两种液态金属的平均最近邻原子间离r₁都呈现减小的趋势.在短程尺度上Sn的液体结构类似于α-Sn的结构.对相关半径r_c的物理意义进行了探讨. 关键词：     

NixPt1−x/Rh(111): A stable surface alloy with enhanced magnetic moments

We have performed ab initio density functional theory calculations to investigate the miscibility and magnetic properties of pseudomorphically grown monolayers of Ni_xPt_1?x surface alloys on a Rh(111) substrate. We find that the formation of this alloy is energetically favored over phase-segregated forms, and its magnetic moment is also enhanced. A significant contribution to this enhanced magnetic moment is found to come from the induced moments on the otherwise non-magnetic elements Pt and Rh. A low concentration of Ni gives rise to a high magnetic moment per Ni atom. We find that a low effective coordination and a high non-spin-polarized density of states at the Fermi level are responsible for these enhanced moments.     

Effect of lattice constant on band-gap energy and optimization and stabilization of high-temperature In x Ga1?x N quantum-dot lasers

We analyze the effect of the lattice constant on the band-gap energy of In _x Ga_1?x N and optimize the structure of the device with a separate-confinement heterostructure. To vary the lattice constants, we change the In molar fraction, which permits us to investigate a wide range of the band gap of the active material employed in diode lasers. In _x Ga_1?x N is a promising active material for high-performance 1.55???m quantum-dot lasers due to its excellent band-gap-energy stability with respect to temperature variations. The band gap of In _x Ga_1?x N decreases from 3.4 to 0.7?eV, and the necessary band gap can be achieved by changing the lattice parameters depending on the device application. It has been found that In_0.86Ga_0.14N can be a promising material for emitting light at a wavelength of 1.55???m.