Theoretical investigation of the conduction and valence band offsets of GaAs1−x Nx/GaAs1−yNy heterointerfaces

Theoretical investigations of the conduction band offset (CBO) and valence band offset (VBO) of the relaxed and pseudo-morphically strained GaAs_1−xN_x/GaAs_1−yN_y heterointerfaces at various nitrogen concentrations (x and y) within the range 0-0.05 and along the [0 0 1] direction are performed by means of the model-solid theory combined with the empirical pseudopotential method under the virtual crystal approximation that takes into account the effects of the compositional disorder. It has been found that for y < x, the CBO and VBO have negative and positive signs, respectively, whereas the reverse is seen when y > x. The band gap of the GaAs_1−xN_x over layer falls completely inside the band gap of the substrate GaAs_1−yN_y and thus the alignment is of type I (straddling) for y < x. When y > x, the alignment remains of type I but in this case it is the band gap of the substrate GaAs_1−yN_y which is fully inside the band gap of the GaAs_1−xN_x over layer. Besides the CBO, the VBO and the relaxed/strained band gap of two particular cases: GaAs_1−xN_x/GaAs and GaAs_1−xN_x/GaAs_0.98N_0.02 heterointerfaces have been determined.     

Coexistence of type-I and type-II band alignment in Ga(Sb,P)/GaP heterostructures with pseudomorphic self-assembled quantum dots

Band alignment of heterostructures with pseudomorphic GaSb_{1 ? x} P _x /GaP self-assembled quantum dots (SAQDs) lying on a wetting layer was studied. Coexistence of type-I and type-II band alignment was found within the same heterostructure. Wetting layer has band alignment of type-I with the lowest electronic state belonging to the X _XY valley of GaSb_{1 ? x} P _x conduction band, in contrast to SAQDs, which have band alignment of type-II, independently of the ternary alloy composition x. It is shown that type-I-type-II transition is a result of GaP matrix deformation around the SAQD.     

First-principles study of optical properties in Ca-doped ZnO alloys

Various electronic and optical properties of Zn_1?x Ca _x O ternary alloys of wurtzite structure are calculated using a first-principles approach based on the framework of the generalized gradient approximation to density-functional theory. In particular, on-site Coulomb interactions are introduced, which can reasonably well predict the electronic properties and band gaps of the Zn_1?x Ca _x O (0??x??0.25) system. The imaginary part of the calculated dielectric function indicates that the optical transition between O 2p states in the valence band and Zn 4s states in the conduction band shifts to the high-energy range as the Ca concentration increases. The calculated band gap shows a significant increase with increasing Ca concentration. Therefore, Zn_1?x Ca _x O ternary alloys may be a potential candidate alloy for optoelectronic materials, and especially for light-emitters and detectors.     

Optoelectronic properties of GaAs1−xPx alloys under the influence of temperature and pressure

This work is concerned with the dependence of the electronic energy band structures for GaAs_1−xP_x alloys on temperature and pressure that is based on local empirical pseudo-potential method. The band structures of GaAs_1−xP_x alloys were calculated in the virtual crystal approximation using the EPM which incorporates compositional disorder as an effective potential.     

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.     

Electronic structures of zigzag AlN, GaN nanoribbons and AlxGa1−xN nanoribbon heterojunctions: First-principles study

The electronic and structural properties of zigzag aluminum nitride (AlN), gallium nitride (GaN) nanoribbons and Al_xGa_1−xN nanoribbon heterojunctions are investigated using the first-principles calculations. Both AlN and GaN ribbons are found to be semiconductor with an indirect band gap, which decreases monotonically with the increased ribbon width, and approaching to the gaps of their infinite two dimensional graphitic-like monolayer structures, respectively. Furthermore, the band gap of Al_xGa_1−xN nanoribbon heterojunctions is closely related to Al (and/or Ga) concentrations. The Al_xGa_1−xN nanoribbon of width n=8 shows a continuously band gap varying from about 2.2 eV-3.1 eV as x increases from 0 to 1. The large ranged tunable band gaps in such a quasi one dimension structure may open up new opportunities for these AlN/GaN based materials in future optoelectronic devices.     

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.     

AlN, GaN, AlxGa1 − xN nanotubes and GaN/AlxGa1 − xN nanotube heterojunctions

Semiconductor optoelectronic devices based on GaN and on InGaN or AlGaN alloys and superlattices can operate in a wide range of wavelengths, from far infrared to near ultraviolet region. The efficiency of these devices could be enhanced by shrinking the size and increasing the density of the semiconductor components. Nanostructured materials are natural candidates to fulfill these requirements. Here we use the density functional theory to study the electronic and structural properties of (10,0) GaN, AlN, Al_xGa_1 − xN nanotubes and GaN/Al_xGa_1 − xN heterojunctions, 0<x<1. The Al_xGa_1 − xN nanotubes exhibit direct band gaps for the whole range of Al compositions, with band gaps varying from 3.45 to 4.85 eV, and a negative band gap bowing coefficient of −0.14 eV. The GaN/Al_xGa_1 − xN nanotube heterojunctions show a type-I band alignment, with the valence band offsets showing a non-linear dependence with the Al content in the nanotube alloy. The results show the possibility of engineering the band gaps and band offsets of these III-nitrides nanotubes by alloying on the cation sites.     

Geometric and electronic structure of epitaxial NbxTi1−xO2 on TiO2(110)

We have investigated the detailed geometric and electronic structure of MBE-grown Nb_xTi_1−xO₂ on TiO₂(110) by means of high-resolution transmission electron microscopy, X-ray photoelectron diffraction, ultraviolet and X-ray photoemission and electron energy loss spectroscopy. We find no measurable change in the NbO bond length relative to that for TiO bonds in TiO₂ in the dilute limit (x = 0.05), and that the epitaxial layers remain strained and coherent with the substrate for x ⩽ ≈ 0.3. However, significant dislocation generation occurs for x > ≈ 0.3. Nb substitution for Ti in the lattice introduces an additional valence electron per atom. The resulting density of states falls in the valence band region, but no new state density occurs in the either the band gap or conduction band. This result is in contrast to what occurs in the very dilute limit (parts per thousand), where Nb electrons occupy a shallow donor level near the conduction band minimum. Based on the electron counting rule, the extra Nb electrons form a non-bonding band which is degenerate with the valence band. The significance of these results for enhanced thermal and photochemistry on Nb_xTi_1−xO₂ surfaces vis a vis TiO₂ is discussed.     

Alloying induced shift between excitation and luminescence of the nitrogen bound exciton in GaPxAs1−x alloys

Photoluminescence and luminescence excitation spectra have been performed on epitaxial layers of nitrogen doped GaP_xAs_1?x alloys (x > 0.85). The main luminescence excitation band A appears above the photoluminescence band N_x. The composition dependence of this energy shift suggests an alloying energy shift. The origin of this new effect would be the thermalization of the bound exciton population to the lower energy states of the A absorption band which reflects the density of states broadening due to As-P disorder around N atoms     

Electronic band structure and optical absorption of nanotubular zinc oxide doped with Iron, Cobalt, or Copper

The absorption spectra of the precursor-derived solid solutions Zn_{1 ? x} M _x O (M = Fe, Co, Cu) with a tubular morphology of aggregates have been investigated in the ultraviolet and visible regions. The maximum metal concentration x in the Zn_{1 ? x} M _x O solid solutions is 0.075 for iron, 0.2 for cobalt, and 0.1 for copper. It has been found that the optical absorption and the band gap of the Zn_{1 ? x} M _x O compounds depend on the type of dopant. The obtained experimental data have been interpreted using the results of the performed ab initio calculations of the electronic band structure and optical absorption.     

Optical properties of MgxZn1−xO nanowire photonic crystals

We investigate the optical properties of two-dimensional periodic arrays of well-aligned Mg_xZn_1−xO nanowires, i.e., Mg_xZn_1−xO nanowire photonic crystals. The nanowire photonic crystal can exhibit a photonic band gap in the visible range. As the mole fraction of Mg, x, increases, the edge frequencies of the band gap increase and the band gap size decreases. The characteristics of relative band gap and vacant point defect mode are also studied with varying x. From the finite-difference time-domain simulations, we show that the light extraction from nanowires can be controlled by varying the distance between optically excited nanowires and a waveguide, and the mole fraction of Mg. Controlling the light extraction from nanostructures can be useful in the implementation of nanoscale light emitting devices.     

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     

Photoemission study of Pd Zr and Cu Zr alloys

The XPS valence bands and core levels of the alloys Pd_1?xZr_x (0<x<1) and Cu_1?xZr_x (0<x<1) have been measured. The alloys prepared by coevaporation are crystalline — but their valence band spectra are close to those of the metallic glasses of the same compositions. The large valence band and core level shifts observed for Pd can be explained by a simple theory, not necessitating the postulation of a new type of bonding in these systems.     

X-ray spectroscopic study of the valence band structure of Cd1−xMnxTe, x = 0.6

The valence band structure of Cd_1?xMn_xTe (x = 0.6) was studied by soft X-ray emission spectroscopy using a 11.5 m concave grating grazing incidence spectrometer in conjunction with a high power rotating anode, which provided the radiation for flourescence excitation. The position of the Mn 3d⁵ states within the energy band system could be identified unambiguously; they form a band near the top of the CdTe valence band. The results are discussed with regard to recently published photoemission data.     

Influence of indium content on the optical, electrical and crystallization kinetics of Se100−xInxthin films deposited by flash evaporation technique

Thin films of Se _100−xIn_x (x=10, 20 and 30 at%) have been prepared by the flash evaporation technique. The effect of the indium content on optical band gap of the Se_100−x In_x films has been investigated by the optical characterization. The optical band gap values of the Se_100−x In_x thin films were determined and are found to decrease with increasing indium content. This indium content changes the width of localized states in the optical band gaps of the thin films. It was found that the optical band gap, E_g, of the Se_100−x In_x films changes from 1.78 to 1.37 eV with increasing indium content from 10 to 30 at%, while the width of localized states in optical band gap changes from 375 to 342 meV. The temperature dependence of the dark electrical conductivity were studied in the temperature range 303-433 K and revealed two activation energies providing two electrical conduction mechanisms. The activation energy of the Se_100−x In_x films in the high temperature region changes from 0.49 to 0.32 eV with increasing indium content from 10 to 30 at%, while the hopping activation energy in the lower temperature region changes from 0.17 to 0.22 meV. The change in the electrical conductivity with time during the amorphous-to-crystalline transformation is recorded for amorphous Se_100−xIn_x films at two points of isothermal temperatures 370 and 400 K. The formal crystallization theory of Avrami has been used to calculate the kinetic parameters of crystallization.     

Study on the electronic structure and optical properties of different Al constituent Ga1−xAlxAs

Using quantum mechanics GASTEP software package based on the first principle density function theory, the electronic structure and optical properties of Ga_1−xAl_xAs at different Al constituent are calculated. Result shows that with the increase of Al constituent, the band gap of Ga_1−xAl_xAs increases and varies from direct band gap to indirect band gap; the absorption band edge and the absorption peak move to high-energy side; the static reflectivity decreases. With the increasing of the incident photon energy, Ga_1−xAl_xAs shows metal reflective properties in certain energy range. With the increasing of Al constituent, static dielectric constant decreases and the intersection of dielectric function and the x-axis move towards high-energy side; the peak of energy loss function move to low-energy side and the peak value reduces.     

Interband emission energy in a dilute nitride quaternary semiconductor quantum dot for longer wavelength applications

Excitonic properties are studied in a strained Ga_1−xIn_xN_yAs_1−y/GaAs cylindrical quantum dot. The optimum condition for the desired band alignment for emitting wavelength 1.55 µm is investigated using band anticrossing model and the model solid theory. The band gap and the band discontinuities of a Ga_1−xIn_xN_yAs_1−y/GaAs quantum dot on GaAs are computed with the geometrical confinement effect. The binding energy of the exciton, the oscillator strength and its radiative life time for the optimum condition are found taking into account the spatial confinement effect. The effects of geometrical confinement and the nitrogen incorporation on the interband emission energy are brought out. The result shows that the desired band alignment for emitting wavelength 1.55 µm is achieved for the inclusion of alloy contents, y=0.0554% and x=0.339% in Ga_1−xIn_xN_yAs_1−y/GaAs quantum dot. And the incorporation of nitrogen and indium shows the red-shift and the geometrical confinement shows the blue-shift. And it can be applied for fibre optical communication networks.     

First-principles study on the electronic and optical properties of SnxGe1 − x

The electronic and optical properties of the direct band gap alloys Sn_xGe_1 − x (x = 0.000, 0.042, 0.083, 0.125, 0.167, and 0.208) have been studied by using the generalized gradient approximation in the framework of the density functional theory. The calculated lattice constants obey Vergard's law. The band structures show that the alloys have direct band gap and the band gaps can be tunable by Sn contents. The optical properties of the Sn_xGe_1 − x alloys with the physical quantities such as the complex dielectric function, the energy-loss function and the static dielectric constant, respectively, are shown to support the potential application of infrared devices in the future.     

Superconducting properties of PbxSn1−xTe

Superconducting transition temperatures and critical magnetic field curves on low carrier concentration Pb_xSn_1?xTe samples where 0.25 ?x?0.55 are reported. These data are interpreted in terms of a changing band structure from that of SnTe to one which is similar to that of GeTe. The importance of carriers (holes) in secondary maxima of the valence band is emphasized.