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.     

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.