Gap States of ZnO Thin Films by New Methods:Optical Spectroscopy,Optical Conductivity and Optical Dispersion Energy

The optical reflectance and transmittance spectra in the wavelength range of 300–2500 nm are used to compute the absorption coefficient of zinc oxide films annealed at different post-annealing temperatures 400, 500 and 600°C.The values of the cross point between the curves of the real and imaginary parts of the optical conductivity ɑ_1 and ɑ_1 with energy axis of films exhibit values that correspond to optical gaps and are about 3.25–3.3 eV. The maxima of peaks in plots dR/dλ and dT/dλ versus wavelength of films exhibit optical gaps at about 3.12–3.25 eV.The values of the fundamental indirect band gap obtained from the Tauc model are at about 3.14–3.2 eV. It can be seen that films annealed at 600°C have the minimum indirect optical band gap at about 3.15 eV. The films annealed at 600°C have Urbach's energy minimum of 1.38 eV and hence have minimum disorder. The dispersion energy d of films annealed at 500°C has the minimum value of 43 eV.     

Effect of Microstructure of TiO2 Thin Films on Optical Band Gap Energy

TiO2 coatings are prepared on fused silica with conventional electron beam evaporation deposition. After annealed at different temperatures for four hours, the spectra and XRD patterns of TiO2 thin film are obtained. XRD patterns reveal that only anatase phase can be observed in TiO2 coatings regardless of the different annealing temperatures, and with the increasing annealing temperature, the grain size gradually increases. The relationship between the energy gap and microstructure of anatase is determined and discussed. The quantum confinement effect is observed that with the increasing grain size of TiO2 thin film, the band gap energy shifts from 3.4 eV to3.21 eV. Moreover, other possible influence of the TiO2 thin-film microstructure, such as surface roughness and thin film absorption, on band gap energy is also expected.     

Calculation of the transfer integrals in Wannier representation for a planar trans-polyacetylene chain

In this paper,the electronic transfer integrals,the energy gap,and the bandwidth of a planar trans-polyacetylene chain are calculated in Wannier representation,in which a combination of the wave function of hydrogen-like atoms is used to stand for the Wannier function.When the effective nuclear charge number Z=2.125 and the distortion amplitude of the carbon sites u=0.0038 nm,the nearest,next,and third neighborhopping energies obtained are -3.224 78 eV,-2.388 61 eV,0.148 14 eV,0.006 65 eV,and 0.00650 eV,respectively.The energy bandwidth and gap corresponding to these values are Wd=11.19 eV and Eg=1.70 eV,respectively.These results coincide with the experimental values.     

Reconsideration of Anelastic Relaxation Peaks in Pure Aluminium

The temperature dependence of internal friction （ T- tan φ curves） in pure aluminium （Al） is measured at sixteen different frequencies. Based on T- tan φ curves, the frequency dependence of internal friction （f -tan φ curves） is also obtained by the interpolation method. An internal friction peak is observed in both the T - tan φ curves and the f - tan φ curves. The activation energy of the peak in the f - tan φ curves is found to be 2.08 ± 0.02 eV and compared to the value of 1.60 ± 0.04 eV in the T - tan φ curves. It is suggested that the change of relaxation strength with temperature should be considered when one calculates the activation energy of the peak in T- tan φ curves.     

Electronic and optical properties of CdS/CdZnS nanocrystals

Cd1-x ZnxS nanocrystals are prepared by a co-precipitation method with different atomic fractions of Zn.The texture,structural transformation and optical properties with increasing x value in Cd1-x ZnxS are studied with scanning electron microscopy,electron diffraction patterning,and absorption spectra respectively.Quantum confinement in a strained CdS/Cd1-xZnxS related nanodot with various Zn content values is investigated theoretically.Binding energies on exciton bound CdS/CdxZn1-xS quantum dot are computed,with consideration of the internal electric field induced by the spontaneous and piezoelectric polarizations,and thereby the interband emission energy is calculated as a function of the dot radius.The optical band gap from the UV absorption spectrum is compared with the interband emission energy computed theoretically.Our results show that the average diameter of composite nanoparticles ranges from 3 nm to 6 nm.The X-ray diffraction pattern shows that all the peaks shift towards the higher diffracting angles with an increase in Zn content.The lattice constant gradually decreases as the Zn content increases.The strong absorption edge shifts towards the lower wavelength region and hence the band gap of the films increases as the Zn content increases.The values of the absorption edge are found to shift towards the shorter wave length region and hence the direct band gap energy varies from 2.5 eV for the CdS film and 3.5 eV for the ZnS film.Our numerical results are in good agreement with the experimental results.     

Structural and band tail state photoluminescence properties of amorphous SiC films with different amounts of carbon

Amorphous silicon carbide films are deposited by the plasma enhanced chemical vapour deposition technique,and optical emissions from the near-infrared to the visible are obtained.The optical band gap of the films increases from 1.91 eV to 2.92 eV by increasing the carbon content,and the photoluminescence(PL) peak shifts from 1.51 eV to 2.16 eV.The band tail state PL mechanism is confirmed by analysing the optical band gap,PL intensity,the Stocks shift of the PL,and the Urbach energy of the film.The PL decay times of the samples are in the nanosecond scale,and the dependence of the PL lifetime on the emission energy also supports that the optical emission is related to the radiative recombination in the band tail state.     

First-principles study of the electronic structure and optical properties of cubic Perovskite NaMgF_3

The structural, electronic, and optical properties of cubic perovskite NaMgF3 are calculated by plane-wave pseudopo- tential density functional theory. The calculated lattice constant a0, bulk modulus B0, and the derivative of bulk modulus B~ are 3.872/~, 78.2 GPa, and 3.97, respectively. The results are in good agreement with the available experimental and theo- retical values. The electronic structure shows that cubic NaMgF3 is an indirect insulator with a wide forbidden band gap of Eg = 5.90 eV. The contribution of the different bands is analyzed by total and partial density of states curves. Population analysis of NaMgF3 indicates that there is strong ionic bonding in the MgF2 unit, and a mixture of ionic and weak covalent bonding in the NaF unit. Calculations of dielectric function, absorption coefficient, refractive index, electronic energy loss spectroscopy, optical reflectivity, and conductivity are also performed in the energy range 0 to 70 eV.     

Uniaxial stress influence on lattice, band gap and optical properties of n-type ZnO： first-principles calculations

The lattice,the band gap and the optical properties of n-type ZnO under uniaxial stress are investigated by firstprinciples calculations.The results show that the lattice constants change linearly with stress.Band gaps are broadened linearly as the uniaxial compressive stress increases.The change of band gap for n-type ZnO comes mainly from the contribution of stress in the c-axis direction,and the reason for band gap of n-type ZnO changing with stress is also explained.The calculated results of optical properties reveal that the imaginary part of the dielectric function decreases with the increase of uniaxial compressive stress at low energy.However,when the energy is higher than 4.0 eV,the imaginary part of the dielectric function increases with the increase of stress and a blueshift appears.There are two peaks in the absorption spectrum in an energy range of 4.0-13.0 eV.The stress coefficient of the band gap of n-type ZnO is larger than that of pure ZnO,which supplies the theoretical reference value for the modulation of the band gap of doped ZnO.     

First Principles Study of Half Metallic Properties of VSb Surface and VSb/GaSb （001） Interface

The structural, electronic, and magnetic properties of VSb in zincblende, and NiAs phases, VSb （001） film surfaces and its interfaces with GaSh （001） have been investigated within the framework of the density functional theory using the FPLAPW＋lo approach. The NiAs structure is more stable than the ZB phase, ZB VSb is found to a half-metallic ferromagnetic. The V-terminated surfaces retain the half-metallic character, while the half-metallicity is destroyed for Sb-terminated surfaces due to surface states, which originate from p electrons. The phase diagram obtained through the ab-initio atomistic thermodynamics shows that the formation energy of ZB VSb is about 0.1 Ryd. The half-metallicity character is also preserved at VSb/GaSb （001） interface. The conduction band minimum （CBM） of VSb in the minority spin case lies about 0.47 eV above that of GaSb, suggesting that the majority spin can be injected into GaSb without being flipped to the conduction bands of the minority spin.     

First-principles calculations of optical properties of Mg2Pb

Based on the density functional theory(DFT),the electronic structures and optical properties of Mg2Pb are calculated by using the local density approximation(LDA)and plane wave pseudo-potential method.The calculation results show that the indirect band gap width of Mg2Pb is 0.02796 eV.The optical properties of Mg2Pb have isotropic characteristics,the static dielectric function of Mg2Pb is1(0)=10.33 and the refractive index is n0=3.5075.The maximum absorption coefficient is 4.8060×105cm 1.The absorption in the photon energy range of 25–40 eV approaches to zero,shows the optical colorless and transparent behaviors.     

Compositional and Structural Properties of TiO2-xNx Thin Films Deposited by Radio-Frequency Magnetron Sputtering

TiO2-xNx thin films are deposited onto Si（100） and quartz substrates by arf magnetron sputtering method using a titanium metal disc as a target in Ar, N2, and 02 atmospheres. The substrate temperature is kept at 300℃. The O2 and Ar gas flow rates are kept to be constants and the N gas flow rate is varied. TiO2-xNx films with different N contents are characterized by x-ray diffraction and x-ray photoelectron spectroscopy. The results indicate that the TiO2-xNx thin films can be obtained at 13% N and 15% N contents in the film, and the films with mixed TiO2 and TiN crystal can be obtained at 13% N and 15% N contents in the film. In terms of the results of x-ray photoelectron spectroscopy, N ls of β-N （396 eV） is the main component in the TiO2-xNx thin films. Because the energy level of β-N is positioned above the valence-band maximum of TiO2, an effective optical-energy gap decreases from 2.8 eV （for pure TiO2 film deposited by the same rf sputtering system） to 2.3 eV, which is verified by the optical-absorption spectra.     

Thermodynamic behaviour of Rashba quantum dot in the presence of magnetic field

The thermodynamic properties of an In Sb quantum dot have been investigated in the presence of Rashba spin–orbit interaction and a static magnetic field. The energy spectrum and wave-functions for the system are obtained by solving the Schrodinger wave-equation analytically. These energy levels are employed to calculate the specific heat, entropy,magnetization and susceptibility of the quantum dot system using canonical formalism. It is observed that the system is susceptible to maximum heat absorption at a particular value of magnetic field which depends on the Rashba coupling parameter as well as the temperature. The variation of specific heat shows a Schottky-like anomaly in the low temperature limit and rapidly converges to the value of 2kB with the further increase in temperature. The entropy of the quantum dot is found to be inversely proportional to the magnetic field but has a direct variation with temperature. The substantial effect of Rashba spin–orbit interaction on the magnetic properties of quantum dot is observed at low values of magnetic field and temperature.     

Structural and electronic properties of Fe-doped BaTiO3 and SrTiO3

We have performed first principles calculations of Fe-doped BaTiO3 and SrTiO3. Dopant formation energy, structure distortion, band structure and density of states have been computed. The dopant formation energy is found to be 6.8eV and 6.5eV for Fe-doped BaTiO3 and SrTiO3 respectively. The distances between Fe impurity and its nearest O atoms and between Fe atom and Ba or Sr atoms are smaller than those of the corresponding undoped bulk systems. The Fe defect energy band is obtained, which mainly originates from Fe 3d electrons. The band gap is still an indirect one after Fe doping for both BaTiO3 and SrWiO3, but the gap changes from Γ-R point to Γ-X point.     

PHOTOCONDUCTIVITY OF Cd0.7Mn0.3Se

The photoconductivity spectra of semimagnetic semiconductor Cd0.7Mn0.3Se have been measured at 50-300 K and in the wavelength range of 5500 ? to 8000 ?. It is found that the energy gap of Cd0.7Mn0.3Se changes with the temperature linearly, and the temperature coefficient (dEg/dT) is about -7×10-4 eV/K. A photoconductivity peak which is related to Mn2+ is also found. This peak is located around 1.85 eV, nearly unshifted with variation of the temperature. The possible transition mechanisms in Cd1-xMnxSe have been discussed in the light of group theory and crystal field theory, and the results are in good agreement with experiments.     

Tunable band gap and optical properties of surface functionalized Sc_2C monolayer

Using the density functional theory, we have investigated the electronic and optical properties of two-dimensional Sc_2C monolayer with OH, F, or O chemical groups. The electronic structures reveal that the functionalized Sc_2C monolayers are semiconductors with a band gap of 0.44–1.55 eV. The band gap dependent optical parameters, like dielectric function, absorption coefficients, reflectivity, loss function, and refraction index were also calculated for photon energy up to 20 eV. At the low-energy region, each optical parameter shifts to red, and the peak increases obviously with the increase of the energy gap. Consequently, Sc_2C monolayer with a tunable band gap by changing the type of surface chemical groups is a promising 2D material for optoelectronic devices.     

Infrared-transmission spectra and hydrogen content of hydrogenated amorphous silicon

The hydrogenated amorphous silicon (a-Si:H) thin film has excellent optical char-acteristics, such as broad optical energy gap (about 1.67 eV), high photosensitivity (the ratio of photoconductivity to dark conductivity s p/s d is about 105—106 in order) andcheapness. It is a suitable material for the solar energy cell. Because the hydrogen con-tent and silicon-hydrogen bonding configurations in a-Si:H significantly affect its optical and electrical characteristics[1,2], the determination of…     

Multi-reference configuration-interaction calculations on multiply charged ions of carbon monosulfide

The potential energy curves for neutrals and multiply charged ions of carbon monosulfide are computed with highly correlated multi-reference configuration interaction wavefunctions.The correlations of inner-shell electrons with the scalar relativistic effects are included in the present computations.The spectroscopic constants,dissociation energies,ionization energies for ground and low-lying excited states together with corresponding electronic configurations of ions are obtained,and a good agreement between the present work and existing experiments is found.No theoretical evidence is found for the adiabatically stable CSq+(q>2) ions according to the present ab initio calculations.The calculated values for 1st-6th ionization energies are 11.25,32.66,64.82,106.25,159.75,and 224.64 eV,respectively.The kinetic energy release data of fragments are provided by the present work for further experimental comparisons.     

High Gain Lateral Semi-Insulating GaAs Photoconductive Switch Triggered by 1064nm Laser Pulses

We report on the experimental results of a lateral semi-insulating GaAs photoconductive switch,with a gap of 8 mm between two electrodes,triggered by 1064 nm laser pulses at a wavelength beyond the GaAs absorption edge.Both the linear and nonlinear modes of the switch were observed when it was triggered by light pulses with an energy of 1.9 mJ and a pulse width of 60 ns,and operated at high voltages of 3 and 5 kV.The results show that when the semi-insulating GaAs photoconductive switch operates under an electrical field of 4.37kV/cm,it will enter into the linear mode first,and then the switch will undergo the nonlinear mode (lock-on ) after a delay of about 20-100ns.It is worth noticing that the delay time under high light energy is longer than that in the low optical energy.The non-intrinsic absorption mechanism is discussed.EL2 deep level defects and double-photon absorption in GaAs may play a key part in the absorption process.     

Simultaneous low extinction and high local field enhancement in Ag nanocubes

We theoretically investigate surface plasmon resonance properties in Au and Ag cubic nanoparticles and find a novel plasmonic mode that exhibits simultaneous low extinction and high local field enhancement properties. We analyse this mode from different aspects by looking at the distribution patterns of local field intensity, energy flux, absorption and charge density. We find that in the mode the polarized charge is highly densified in a very limited volume around the corner of the nanocube and results in very strong local field enhancement. Perturbations of the incident energy flux and light absorption are also strongly localized in this small volume of the corner region, leading to both low absorption and low scattering cross section. As a result, the extinction is low for the mode. Metal nanoparticles involving such peculiar modes may be useful for constructing nonlinear compound materials with low linear absorption and high nonlinearity.     

Morphologies of epitaxial islands on a lattice-misfitted substrate

Under certain growth conditions for systems with a film/substrate lattice misfit, the deposited material is known to aggregate into island-like shapes. We have obtained an analytical expression of the total free energy, which consists of strain energy, surface energy and interfacial energy of a coherent island/substrate system, and the change of equilibrium aspect ratio versus the volume of the island and the misfit of lattices in the system, which provides a broad perspective on island behaviour. These then were used to study the equilibrium shapes of the system. The results show that in order to minimize the total free energy, a coherent island will have a particular height-to-width aspect ratio, called equilibrium aspect ratio, that is a function of the island volume and misfit. The aspect ratio is increased with increasing island volume at a fixed misfit, and with increasing misfit strain between the island and substrate at a fixed island volume. Moreover, the effect of misfit dislocation on the equilibrium shape of the island is also examined. The results obtained are in good agreement with experiment of observations and thus can serve as a basis for interpreting the experiments.