A calibration method for accurate prediction of amorphous to nanocrystalline transition from line intensities of optical emission spectrum

To be able to use the simple technique of optical emission spectroscopy (OES) for the prediction of the transition of growth from a-Si to nc-Si via the H_α/Si^? emission ratio, a regime-dependent correction factor is required to relate the measured H_α/Si^? emission ratio to the true flux (to the substrate) ratio of atomic hydrogen to deposited silicon radicals. Through an in-depth study in a very high frequency plasma enhanced chemical vapor deposition process, we obtained that the flux ratio of atomic hydrogen and deposited silicon radicals to the growing surface,Γ^H/Γ^Si, is related to the emission ratio of H_α and Si^?, I_rad^H_α/I_rad^Si*, by the relation, $R_{\text{rad}}\frac{I_{\text{rad}}^{{\text{H}}_{\alpha }}}{I_{\text{rad}}^{{\text{Si}}^{\text{*}}}}/\frac{{\Gamma }^{\text{H}}}{{\Gamma }^{\text{Si}}}=a\left\{{\left(p{d}_{}\right)}^2/k{T}_{\text{gas}}\right\}+b$, where the parameters p (pressure), d (inter-electrode distance) and T_gas (gas temperature) are experimentally obtained quantities and R_rad is the ratio of the rate coefficients for radiation of Si^? and H_α. We obtained the calibration parameters a and b to be 1.9·10^? 21 ± 2·10^? 22 Pa m^? 1 and 5.5 ± 1.9 respectively which is valid in a broad range of power and pressure settings. With these parameters, it is easy to estimate the flux ratio of atomic hydrogen and silicon species at any deposition condition using the OES data and this will allow accurate prediction of the phase transition. According to simulations in the linear low-pressure regime, the amorphous to nanocrystalline phase transformation occurs at the flux ratio Γ^H/Γ^Si = 12, which translates, using the factors a and b, to the required emission ratio.     

Optical properties of amorphous (As0.33S0.67)100−xTex (x = 0, 1, 5 and 10) chalcogenide thin films,photodoped step-by-step with silver

We have analysed in detail the effect of silver-content on the optical properties of Ag-photodoped amorphous (As_0.33S_0.67)_100?xTe_x (with x = 0, 1, 5 and 10 at.%) chalcogenide thin films; the chalcogenide host layers were prepared by vacuum thermal evaporation. Films of composition Ag_y[(As_0.33S_0.67)_100?xTe_x]_100?y, with y ? 18 at.%, were successfully obtained by successively photodissolving about 20- or 40-nm-thick layers of silver. The optical constants (n, k) have been accurately determined by an improved envelope method [J.M. González-Leal, R. Prieto-Alcón, J.A. Angel, D.A. Minkov, E. Márquez, Appl. Opt. 41 (2002) 7300], based on the two envelope curves of the optical-transmission spectrum, obtained at normal incidence. The dispersion of the refractive index of the Ag-photodoped chalcogenide films is analysed in terms of the Wemple–DiDomenico single-effective-oscillator model: $n^2(?\omega )=1-E_{\text{o}}{E}_{\text{d}}/(E_{\text{o}}^2-(?\omega {)}^2)$, where E_o is the single-oscillator energy, and E_d the dispersion energy. We found that the refractive index of the Ag-doped samples strongly increases with the Ag-content, whereas the optical band gap, $E_{\text{g}}^{\text{opt}}$, decreases also notably. For instance, in the particular case of x = 10 at.%, the largest Te-content, $E_{\text{g}}^{\text{opt}}$ decreases from 2.17 down to 1.67 eV. It should also be mentioned that, in the case of the undoped samples, when the Te-concentration increases from zero up to 10 at.%, the value of $E_{\text{g}}^{\text{opt}}$ decreases from 2.49 down to 2.17 eV.     

Optical properties and structure of amorphous films Agx(As0.33S0.67−ySey)100−x

As₃₃S_67−ySe_y, where y = 0, 16.75, 33.5, 50.25 and 67, amorphous thin films were prepared by a vacuum thermal evaporation technique. The films with known silver concentrations and good optical quality were prepared by thermal vacuum evaporation of a silver film on the top of As₃₃S_100−ySe_y films with sequential step-by-step optically- and thermally-induced diffusion and dissolution (OIDD) of silver. The range of silver content was x = 0–25 at.%. The kinetics of OIDD of silver were measured optically by monitoring the change of thickness of the undoped part of the chalcogenide during broadband illumination. Compositions of the reaction products have been determined by scanning electron microscope with energy-dispersive X-ray microanalyser EDS. Optical properties $(T\text{,}n\text{,}{E}_{\text{g}}^{\text{opt}})$ of thin films were measured and/or calculated by the Swanepoel method [R. Swanepoel, J. Phys. E: Sci. Instrum. 16 (1983) 1214]. The refractive index increase with increasing silver and selenium concentration has been shown. The difference of the refractive index (Δn) between undoped and silver doped films was ∼0.4 and between As₃₃S₆₇ and As₃₃Se₆₇ was films ∼0.42. Non-linear indices of refraction were estimated according to Tichy’s formula [H. Ticha, L. Tichy, J. Optoel, Adv. Mat. 4 (2002) 381]. The values of non-linear refractive index grew with increasing silver and selenium content. The difference of optical bandgap, $\mathrm{\Delta }{E}_{\text{g}}^{\text{opt}}$, between undoped As₃₃S₆₇ and fully doped films with Ag and Se was ∼1 eV. Raman spectroscopy showed a decrease in S–S or Se–Se bonds with increasing silver content.     

Correlations between apparent activation energy and thermostability and glass forming ability for Fe based metallic glasses

Apparent activation energies (E_g) of glass transition, glass transition temperature (T_g) and crystallization temperature (T_x) of amorphous alloys of composition Fe_91?xB_xZr₅Nb₄ (FBZN, 5 ? x ? 30 at.%) and Fe_61?xCo_xZr₅B₃₀Nb₄ (FCZBN, 0 ? x ? 15 at.%) were obtained by using differential scanning calorimeter (DSC) measurement and Kissinger equation, and correlations between E_g and T_g, T_x and glass-forming ability (GFA) were studied in the present work. It was found that the T_g and T_x are not independent each other for each glass composition in the two alloy systems, but related by a formula, T_x = αT_g+β, where α and β are constants, and were measured by nonisothermally scanning in the DSC together with the Lasocka’s equation. The E_g was found to be directly proportional to α and β, respectively, and had a correlation with T_x and T_g, $T_{\text{x}}=\frac{E_{\text{g}}-1.09}{3.527}{T}_{\text{g}}-\frac{E_{\text{g}}-4.86}{0.0041}$, indicating that E_g determines linear relationship between T_x and T_g. Supercooled liquid region ΔT_x is used as characterization of GFA of the Fe based metallic glasses and related to E_g and T_g by a formula: $\mathrm{\Delta }{T}_{\text{x}}=E_{\text{g}}(\frac{T_{\text{g}}}{3.527}-234.9)+1185.37-1.309T_{\text{g}}$, indicating that E_g and T_g can characterize GFA of the Fe based metallic glass well.     

Free volume in poly(propylene glycol) and its relationships to spin probe reorientation

In this work we carried out positron annihilation lifetime spectroscopy (PALS) on pure poly(propylene glycol) (PPG 4000) and an electron spin resonance (ESR) study on PPG 4000 doped with the spin probe TEMPO. The spectral and dynamic features of both the microscopic probes such as the ortho-positronium (o-Ps) lifetime, τ₃, the spectral parameter, T_50 G, and the correlation time of spin probe, τ_c, as a function of the temperature were compared. New empirical relationships between the characteristic PALS temperatures ($T_{\text{b}1}^{\text{L}}$ and $T_{\text{b}2}^{\text{L}}$) and the characteristic ESR temperatures ($T_{\text{X}1}^{\text{L}}$ and $T_{\text{X}2}^{\text{L}}$) have been revealed. In addition, these characteristic PALS and ESR temperatures can be connected with the Schönhals and Stickel ($T_{\text{B}}^{\text{SCH}}$, $T_{\text{B}}^{\text{ST}}$) temperatures of the primary α relaxation process of PPG 4000 as obtained by dielectric spectroscopy (DS) studies in the literature. Next, the mean free volume hole sizes of the matrix were related to the spin probe size and to the Arrhenius or the non-Arrhenius motional regimes of TEMPO. Finally, the temperature dependences of the reorientational correlation time have been analysed using the Vogel–Fulcher–Tamman–Hesse (VFTH) equation and its generalized form, i.e., the Bendler–Shlesinger–Fontanella (BSF) one, the latter derived from the defect diffusion model of the dynamics and transport properties. The fitting parameters are then compared with those of the primary α relaxation process.     

Ternary chalcogenides LiBC2 (B = In, Ga; C = S, Se, Te) for mid-IR nonlinear optics

We review on the growth improvement and optical characterization of a new family of ternary lithium-based chalcogenide crystals of generic formula ${\text{LiB}}^{\text{III}}{\text{C}}_2^{\text{VI}}$ (B = In, Ga; C = S, Se, Te) which displays improved thermo-mechanical properties for mid-IR nonlinear optical applications. Some of these compounds are now produced in sufficiently large size, single-domain quality to allow their implementation in optical parametric oscillators.