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1.
We study the dependence of absorption wavelength on the well width in the (CdS/ZnSe)/BeTe super-lattices(SL). With well-width reduction, the wavelength decreases from 1.795 to 1.57 μm. Structural properties, strain state and interface composition are determined via XRD measurement. A (CdS/ZnSe)/BexMg1−xTe structure is prepared and XRD reveals the average lattice constant match to GaAs substrate. TEM reveals that numerous stacking faults exist in the (CdS/ZnSe)/BeTe structure, and stacking faults are completely suppressed in (CdS/ZnSe)/BexMg1−xTe SLs. Intersubband transition down to 1.535–1.55 μm have been observed in SLs.  相似文献   

2.
The structural, electronic and thermodynamic properties of the SrS1–xOx ternary mixed crystals have been studied using the ab initio full potential linearized augmented plane wave (FP-LAPW) method within density functional theory. The effect of composition on lattice parameter, bulk modulus and band gap was investigated. The lattice constants from Vegard's law and the bulk modulus from linear concentration dependence were observed for the alloys. The microscopic origins of the gap bowing were explained by using the approach of Zunger and co-workers. The thermodynamic stability of these alloys was investigated by calculating the excess enthalpy of mixing ΔHm as well as the phase diagram. In addition to FP-LAPW method, the composition dependence of the refractive index and the dielectric constant was studied by different models.  相似文献   

3.
An accurate ab initio full potential linear muffin-tin orbital method has been used to investigate the structural, electronic and optical properties of BP, BAs and their (BP)n/(BAs)n superlattices (SLs). The exchange-correlation potential is treated with the local density approximation of Perdew and Wang (LDA-PW). The calculated structural properties of BP and BAs compounds are in good agreement with available experimental and theoretical data. It is found that BP, BAs and their alloys exhibit an indirect fundamental band gap. The fundamental band gap decreases with increasing the number of monolayer n. The optical properties show that the static dielectric constant significantly decreases in superlattices compared to their binary compounds.  相似文献   

4.
We present a review on recent study of the type I to type II transition in short-period superlattices (SLs) of GaAs/AlAs by means of cyclotron resonance (CR) in pulsed high magnetic fields. The behavior of CR varies depending on the thickness of the GaAs and AlAs layers. In CR of (GaAs)n/(AlAs)n, the resonance peak at the X minima was observed in the type II regime for n smaller than 14, whereas the resonance at the Γ point was observed for n>15. We estimated electron masses on X and Γ point in the SLs by using the empirical sp3 tight-binding method including second-nearest-neighbor interaction. These calculations have shown good agreement with the experimental results. Moreover, it was found that the angular dependence of the CR peak position does not obey the simple cosine dependence due to the subband mixing in high magnetic fields. From the angular dependence in the SLs, the longitudinal and transverses electron masses of AlAs at the X point were deduced to be mt=0.21m0 and ml=1.04m0, respectively.  相似文献   

5.
Using first-principles calculations, we investigated the structural and electronic properties of two binaries: ZnO in wurtzite structure and CdO in wurtzite and rock-salt structures. In addition several compositions with various ordered structures of ZnxCd1−xO alloys were studied within the theory of order–disorder transformation. The full potential linearized augmented plane wave method was used and the d orbitals of Zn and Cd were included in the valence bands. In this investigation of alloying ZnO with CdO, the fundamental band-gap of the alloys is shown to be direct and to decrease versus the Cd composition.  相似文献   

6.
In this work, we present a second nearest neighbour sp3s* semi-empirical tight-binding theory to calculate the electronic band structure of heterostructures based on group III-N binary semiconductors and their ternaries. The model Hamiltonian includes the second nearest neighbour (2nn) interactions, the spin–orbit splitting and the nonlinear variations of the atomic energy levels and the bond length with ternary mole fraction. Using this sp3s* tight-binding approach, we investigated the electronic band structure of Al1−xGaxN/GaN and In1−xGaxN/GaN heterostructures as a function of composition and interface strain for the entire composition range (0≤x≤1). There is an excellent agreement between the model predictions and experiment for the principal bandgaps at Γ, L and X symmetry points of the Brillouin zone for AlN, GaN and InN binaries and Al1−xGaxN and In1−xGaxN ternaries. The model predicts that the composition effects on the valence band offsets is linear, but on the conduction band offsets is nonlinear and large when the interface strain and deformation potential is large.  相似文献   

7.
Full-potential linearized augmented plane wave method (FP-LAPW) within density functional theory has been used to calculate structural, electronic and optical properties of Ca1−xSrxS, an alkali earth chalcogenide, with varying compositional parameter x in the range 0<x<1. Whereas the structural properties are discussed in terms of charge transfer between the two cations, calculated electronic band structure and density of states have been analyzed in terms of contribution from the S p, Ca d and Sr d states. Furthermore, we have calculated some optical properties such as real and imaginary parts of dielectric constant, ε(ω), and the calculated results have been discussed in comparison with the existing experimental data and other theoretical calculations.  相似文献   

8.
A pseudopotential formalism within the virtual crystal approximation in which the effects of composition disorder are involved is applied to the GaxIn1−xAsyP1−y quaternary alloys in conditions of lattice matching to GaAs, InP and ZnSe substrates so as to predict their energy band gaps. Very good agreement is obtained between the calculated values and the available experimental data for the alloy lattice matched to InP and GaAs. The alloy is found to be a direct-gap semiconductor for all y compositions whatever the lattice matching to the substrates of interest. The (ΓΓ) band-gap ranges and the ionicity character are found to depend considerably on the particular lattice-matched substrates suggesting therefore that, for an appropriate choice of y and the substrate, GaxIn1−xAsyP1−y could provide more diverse opportunities to obtain desired band gaps, which opens up the possibility of discovering new electronic devices with special features and properties.  相似文献   

9.
The electronic and magnetotransport properties of conduction electrons in the grain boundary interface of p-type Hg1−xCdxTe bicrystals are investigated. The results clearly demonstrate the existence of a two-dimensional degenerate n-type inversion layer in the vicinity of the grain boundary. Hydrostatic pressure up to 103 MPa is used to characterize the properties of the two-dimensional electron gas in the inversion layer. At atmospheric pressure three series of quantum oscillations are revealled, indicating that tthree electric subbands are occupied. From quantum oscilations of the magnetoresistivity the characteristics parameters of the electric subbands (subband populations nsi, subband energies EF−Ei, effective electron masses m*ci) and their pressure dependences are established. A strong decrease of the carrier concentration in the inversion layer and of the corresponding subband population is observed when pressure is applied A simple theoretical model based on the triangular-well approximation and taking into account the pressure dependence of the energy band structure of Hg1−xCdxTe is use to calculate the energy band diagram of the quantum well and the pressure dependence of the subband parameters.  相似文献   

10.
The Bi1−xAxFe1−xTixO3 (A—Ca, Sr, Pb, Ba) and BiFe1−xTixO3+δ systems have been studied using X-ray, neutron powder diffraction and magnetization measurements in a magnetic field up to 14 T. It was found that all Bi1−xAxFe1−xTixO3 solid solutions are rhombohedral up to x=0.3. In the case of BiFe1−xTixO3+δ the rhombohedral distortion preserved up to x=0.11. A homogeneous weakly ferromagnetic state was found for Bi1−xCaxFe1−xTixO3 (0.15≤x≤0.25) and BiFe1−xTixO3+δ (0.06≤x≤0.11), probably due to magnetoelectric interactions, whereas Bi1−xAxFe1−xTixO3 (A—Sr, Pb, Ba) compounds above doping level x>0.1 seem to be collinear antiferromagnets.  相似文献   

11.
In metals with strong electronic correlations such as heavy-fermion systems or itinerant-electron magnets it is possible to change from a magnetically ordered to a nonmagnetic groundstate by variation of an external parameter such as composition or pressure. In principle a transition between these groundstates can occur at zero temperature. In case of a continuous transition quantum fluctuations take the role of thermal fluctuations in finite-temperature transitions. The abundance of low-lying magnetic excitations leads in the vicinity of the quantum critical point to unusual behavior of thermodynamic and transport properties at low temperatures T not envisioned by the classical Fermi-liquid behavior that is observed even in strongly correlated electron systems away from the quantum phase transition. We discuss in detail a few examples of this ‘non-Fermi-liquid behavior', viz., CeCu6−xAux, Ce1−xLaxRu2Si2, Ce7Ni3, CeCu2Si2 and CeCu2Ge2, CePd2Si2, and UCu1−xPdx. In CeCu6−xAux the very unusual low-T behavior of the linear specific-heat coefficient C/T−ln(T/T0) and of the resistivity ΔρT can be attributed to quasi-two-dimensional fluctuations as determined from inelastic neutron scattering. The systems CeCu2Ge2 and CePd2Si2 are particuarly interesting since here the magnetic order which is suppressed under hydrostatic pressure gives way to superconductivity, suggesting that spin fluctuations mediate the formation of Cooper pairs at least in the latter system.  相似文献   

12.
We have succeeded in obtaining high critical electric fields from AlGaN layers using the p-InGaN/i-AlxGa1−xN/n-AlxGa1−xN (x=0–0.22) vertical conducting diodes grown on n-SiC substrates by low-pressure metalorganic vapor phase epitaxy (MOVPE). The breakdown voltage (VB) increases with increasing Al composition of the AlGaN layer. The corresponding critical electric fields are calculated to be 2.4 MV/cm for GaN and 3.5 MV/cm for Al0.22Ga0.78N. The critical electric field is proportional to the bandgap energy to a power of 2.5. This bandgap energy dependence is much stronger than that in the empirical expression proposed by Sze and Gibbons. The figure of merit, , increases with increasing Al composition, indicating the AlGaN-based pin diodes are promising for high-power and high-temperature electronic device applications.  相似文献   

13.
The structure and magnetic properties of LaFe13−xSix and Co-substituted LaFe11.8−xCoxSi1.2 alloys prepared by melt spinning, as well as of LaFe11.57Si1.43Hx hydrides prepared by reactive milling are investigated. The hysteresis in the temperature- and field-induced phase transitions is significantly reduced as compared with conventional bulk alloys, which makes these materials very attractive for magnetic refrigerant applications. The unusual combination of features characteristic of first- and second-order phase transitions in the La(Fe,Si)13-based compounds is discussed on the basis of density-functional electronic structure calculations.  相似文献   

14.
Electronic structures of ionic liquids formed by 1-buthyl-3-alkylimidazolium ion [Cnmim]+ (n = 4 and 8) with various inorganic and organic anions have been investigated by ultraviolet photoemission, X-ray photoemission, inverse photoemission and soft X-ray emission spectroscopies (SXES). The comparison of the calculated density of states with the observed spectra revealed that the molecular orbital energies of these ionic liquids are significantly affected by the electrostatic Madelung potential among the ions. The SXES results clearly show that the both highest occupied and lowest unoccupied states of [C4mim]+PF6 are derived from the cation as a result of strong Madelung potential. On the other hand, the SXES results show the valence electronic structures of ionic liquids with larger anion molecules, [Cnmim]+Tf2N and [Cnmim]+OTf are contributed from the both cation and anion.  相似文献   

15.
Atomic and electronic structure modification of a metal-Cd1−xMnxTe interface is achieved using selective etching of the Cd1−xMnxTe surface (x=0, 0.34) and Cd adsorption. It is revealed that Te, TeO2, Mn3O4, and CdTeO3 are formed at the Cd1−xMnxTe surface etched in Br2 solution. Te and Cd1−xMnxTe produce TeCd1−xMnxTe heterojunctions, the salient features of which are nearly symmetric nonlinear I-V characteristics. At the Cd1−xMnxTe surface with adsorbed Cd, CdTe might form, resulting in a CdTe-Cd1−xMnxTe heterojunction. The metal-CdTe-Cd1−xMnxTe microstructure is characterized by a nonlinear dependence of current on voltage and rectifying behaviour. The results obtained give deep insight into electronic processes in metal-Cd1−xMnxTe microstructures.  相似文献   

16.
The effect of Zn and Ti on the magnetic, power loss and structural properties of Li0.5ZnxTixMn0.05Fe2.45−2xO4 ferrites (x=0.0 to 0.30 in step of 0.05)+0.5 wt% Bi2O3, prepared by standard ceramic technique, has been investigated. Complex permeability (μ*=μ′−″) has been analyzed at room temperature in frequency range from 1 to 103 MHz. It was found an enhancement in permeability with Ti and Zn concentration in Li0.5ZnxTixMn0.05Fe2.45−2xO4 and exhibits the maximum value 106 for x=0.20 sample. Complex permeability of these ferrites exhibits stable frequency response up to 7 MHz beyond which the real part decreases sharply and imaginary part increases to have a peak at the relaxation frequency. Power loss measurements have been carried out in induction condition (B=10 mT) in frequency range of 50 kHz to 3 MHz. Power loss has been found to be quite low with the substitution of Ti and Zn in lithium ferrite.  相似文献   

17.
Thin films of amorphous Se100−xSbx (x=5,10 and 20 at%) system are deposited on a silicon substrate at room temperature (300 K) by thermal evaporation technique. The optical constant such as refractive index (n) has been determined by a method based on the envelope curves of the optical transmission spectrum at normal incidence by a Swanpoel method. The oscillator energy (Eo), dispersion energy (Ed) and other parameters have been determined by the Wemple–DiDomenico method. The absorption coefficient (α) has been determined from the reflectivity and transmitivity spectrum in the range 300–2500 nm. The optical-absorption data indicate that the absorption mechanism is a non-direct transition. We found that the optical band gap, Egopt, decreases from 1.66±0.01 to 1.35±0.01 eV with increase Sb content.  相似文献   

18.
SiOx films (1<x<2), 0.5 μm thick, have been elaborated by electron-gun evaporation. A thermal annealing of these films induced a phase separation leading to the formation of Si nanocrystals embedded in a SiO2 matrix. These films have been studied by infrared spectroscopic ellipsometry and by X-ray photoelectron spectroscopy (XPS). The effective dielectric function of the thin films has been extracted in the 600–5000 cm−1 range which allowed us to deduce the dielectric function of the matrix surrounding the Si-nc. A study of the Transverse Optical (TO) vibration mode has revealed the presence of SiOx into the matrix. Before XPS measurements, the films have been etched in fluorhydric acid to remove the superficial SiO2 layer formed during air exposure. The Si 2p core-level emission has been recorded. The decomposition of the Si 2p peak into contributions of the usual five tetrahedrons Si-(Si4−nOn) (n=0–4) has also revealed the presence of a SiOx phase. Consistency between infra-red and XPS results is discussed.  相似文献   

19.
The compositional dependence of the optical constants, the refractive index n, and the absorption index k, of the AsSe1−xTex thin films with 0<x<1.0 were determined in the spectral range of 400–2500 nm. The maximum value of the refractive index n, is shifted toward the long wavelength by increasing the Te content in the examined system. The values of the forbidden energy gap of the system have been determined and were correlated with the type and the amount of chemical bonds formed by the increasing Te content in the AsSe1−xTex glassy system. The value of the dispersion energy Ed exhibits low value at the composition containing the same atomic percent of Se and Te.  相似文献   

20.
We have investigated the relation among ρT characteristics, superconductivity, annealing conditions and the crystallinity of polycrystalline (In2O3)1−x–(ZnO)x films. We annealed as-grown amorphous films in air by changing annealing temperature and time. It is found that the films annealed at 200 °C or 300 °C for a time over 0.5 h shows the superconductivity. Transition temperature Tc and the carrier density n are Tc < 3.3 K and n ≈ 1025–1026 m−3, respectively. Investigations for films with x = 0.01 annealed at 200 °C have revealed that the Tc, n and crystallinity depend systematically on annealing time. Further, we consider that there is a suitable annealing time for sharp resistive transition because the transition width becomes wider with longer annealing times. We studied the upper critical magnetic field Hc2(T) for the film with different annealing time. From the slope of dHc2/dT for all films, we have obtained the resistivity ρ dependence of the coherence length ξ(0) at T = 0 K.  相似文献   

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