eV For Amorphous Films Research Articles

Excited state dynamics of nanocrystalline VO2 with white light continuum time resolved spectroscopy

In an attempt to use ultrafast pump probe spectroscopy technique with white light continuum to reveal wavelength dependent dynamics of VO2, bandgap needs to be opened. Therefore, nanostructured amorphous and crystalline VO2 thin films were prepared with pulsed DC magnetron reactive sputtering. The mean diameters of grains were measured as 22±0.1nm and 44±0.1nm for amorphous and crystalline VO2 thin films, respectively. Temperature dependent resistance measurements show that nanocrystalline VO2 thin film exhibit metal insulator phase transition. The films exhibited dual band gaps (2.3eV, <0.6eV for amorphous films and 1.3eV, 1.8eV for crystalline film). Increased band gaps made it possible to perform time resolved transmission and reflection experiments with white light continuum at fluences above and below photo induced phase transition threshold. Although transmission chance due to photo induced phase transition of VO2 in the literature usually takes places at infrared region of the spectrum, transmission chance was observed in visible as low as 630nm in broadband probe spectra. It was observed that measured time scales depend on not only pump fluence but also probe wavelength. Experiments gave the evidence of the long-lived lower energy non-equilibrium state related to the photo induced phase.

BAND ALIGNMENT AND ATOM SEGREGATION OF LaYbO3 FILMS ON SILICON

Ternary rare earth oxides are expected to be more promising high-k dielectric materials than conventional binary rare earth oxides due to higher band gap, higher permittivity and good interfacial stability. In the present study, the band alignment and atom thermal diffusion of LaYbO3 , a new ternary rare earth oxide, are studied by X-ray photoelectron spectrum (XPS) and angle-resolved XPS, respectively. The band gap value for LaYbO3 crystalline film rises to 6.7 eV compared with 6.2 eV for amorphous film. Valence (ΔEv) and conduction band (ΔEc) offset are ΔEv = 3.5 eV, ΔEc = 1.6 eV for the amorphous film and ΔEv = 3.3 eV, ΔEc = 2.3 eV for the crystalline film. From elemental depth profile through high-k layer and silicon substrate, it is shown that La atom tends to diffuse into silicon substrate and piles up at oxide/silicon interface at high annealing temperature ~1000°C.

Structural and optical characterisation of thermally evaporated tungsten trioxide (WO 3) thin films

Thin films of amorphous tungsten trioxide (WO 3) have been thermally evaporated onto quartz substrates held at 350 K. Annealing at 723 K caused the formation of crystalline WO 3. X-ray diffraction (XRD) analysis revealed monoclinic structure with lattice parameters of a = 7.311 ± 0.05 Å, b = 7.603 ± 0.05 Å, c = 7.713 ± 0.05 Å and β = 91.137°. Optical properties were investigated for both the amorphous and annealed films by using spectrophotometric measurement of transmittance and reflectance at normal incidence in the wavelength range of 200–2500 nm. The obtained spectra showed that all the films are transparent above λ = 500 nm and the measured optical constants are independent of the film thickness. Both the refractive index, n, and absorption index, k, were determined for the amorphous and crystalline films. The optical dispersion parameters have been analysed by single oscillator model. The values of oscillator energy ( E o) and dispersion energy ( E d) were found to be 5.95 and 18.18 eV for the amorphous films and 2.92 and 11.13 eV for the crystalline films. Indirect optical transitions with corresponding energy gaps = 3.28 and 3.03 eV for amorphous and crystalline films, respectively, were obtained. The obtained results are compared and discussed.

On the radiative lifetime, quantum yield and fluorescence decay of Alq in thin filmsElectronic supplementary information (ESI) available: Experimental data. See http://www.rsc.org/suppdata/cp/b2/b211264g/

The fluorescence decay of tris(8-hydroxyquinolinato)aluminium(III) (Alq) in thin films and organic solvents was studied as a function of temperature. The variation of the nonradiative rate constant of Alq with temperature follows the Arrhenius equation and the activation energy is 0.074 eV for crystalline film and 0.039 eV for amorphous film. Similarly, the activation energies of Alq in frozen organic solvents were determined to be 0.051 eV (DMSO) and 0.049 eV (ethanol). The radiative lifetime of Alq was determined to be 156 ± 19 ns using the linear correlation of the inverse fluorescence lifetime at 77 K in several organic solvents with the square of the refractive index. The maximum internal fluorescence quantum yield for a thin crystalline and amorphous film of Alq are estimated to be 0.36 and 0.21, respectively.

電子ビーム蒸着法による層状性ＧａＳｅ薄膜作製

The growth of thin GaSe film on fused silica substrate is investigated using the electron beam deposition method. The thin GaSe films are evaluated by several measurements of the x-ray diffraction, the nano-probe microscopy and the temperature dependence of electrical conductivity. It is found that the GaSe films deposited at R.T. and 200°C have amorphous structures, while those at 300-500°C have polycrystalline structures with c-axis perpendicular to the substrate surface. The activation energy of the crystal growth of GaSe is discussed with the temperature dependence of the average lateral grain size from the surface morphologies. It is obtained that the activation energies of the thermally activated charge transport are 0.70 eV for amorphous film and 0.41 eV for crystalline film.

Optical properties of thermally vacuum evaporated thin films

Silver antimony diselenide thin films were prepared by a thermal vacuum evaporation technique onto quartz and glass substrates kept at room temperature . The as-deposited films were amorphous and transformed to a face centred cubic (FCC) polycrystalline nature with the lattice constant on post-deposition annealing above 423 K for one hour in argon atmosphere. The optical constants (the refractive index n, and the absorption index k) of the films were determined for several samples of different thickness (180 nm-270 nm), using spectrophotometric measurements of the transmittance T and reflectance R at normal incidence in the spectral range 500-2500 nm. These constants were also determined for preannealed films, (polycrystalline). The obtained values of both n and k were independent of the film thickness within the above-mentioned thickness range. The refractive index data fitted a single-oscillator model with high-frequency dielectric constants increasing from 13 for the amorphous films to 15 for the crystalline films. It was found that the high-frequency dielectric constant has the same values as the lattice dielectric constant . The analysis of the spectral behaviour of the absorption coefficient in the intrinsic absorption region revealed the existence of an indirect allowed optical transition with energy gap 1.2 eV for amorphous films and 1.03 eV for the crystalline films, respectively.

Structure and optical properties of C 60 thin films

By varying the deposition conditions of C 60 thin films and using X-ray diffraction techniques to study the results, we obtain a continuum of structure types ranging from crystalline to amorphous. By measuring the absorption spectra of these films we find that as the film crystallinity increases the fundamental absorption edge and nearby spectral features shift progressively to longer wavelengths. Using a Tauc-type approximation, we find that the optical bandgap E g = 1.3–1.6 eV for crystalline films and E g = 2.4–2.6 eV for amorphous films. Intermediate values of E g are obtained depending upon the degree of crystallinity of the film.

Properties of tantalum oxide thin films grown by atomic layer deposition

Thin tantalum oxide films were deposited using atomic layer deposition from TaCl 5 and H 2O at temperatures in the range 80–500 °C. The films deposited at temperatures below 300 °C were predominantly amorphous, whereas those grown at higher temperatures were polycrystalline containing the phases TaO 2 and Ta 2O 5. The oxygen to tantalum mass concentration ratio corresponded to that of TaO 2 at all growth temperatures. The optical band gap was close to 4.2 eV for amorphous films and ranged from 3.9 to 4.5 eV for polycrystalline films. The refractive index measured at λ = 550 nm increased from 1.97 to 2.20 with an increase in growth temperature from 80 to 300 °C. The films deposited at 80 °C showed low absorption with absorption coefficients of less than 100 cm −1 in the visible region.