Abstract
Several, nearly-1-µm-thick, pure, unhydrogenated amorphous-silicon (a-Si) thin layers were grown at high rates by non-equilibrium rf-magnetron Ar-plasma sputtering (RFMS) onto room-temperature low-cost glass substrates. A new approach is employed for the optical characterization of the thin-layer samples, which is based on some new formulae for the normal-incidence transmission of such a samples and on the adoption of the inverse-synthesis method, by using a devised Matlab GUI environment. The so-far existing limiting value of the thickness-non-uniformity parameter, Δd, when optically characterizing wedge-shaped layers, has been suppressed with the introduction of the appropriate corrections in the expression of transmittance. The optical responses of the H-free RFMS-a-Si thin films investigated, were successfully parameterized using a single, Kramers–Krönig (KK)-consistent, Tauc–Lorentz oscillator model, with the inclusion in the model of the Urbach tail (TLUC), in the present case of non-hydrogenated a-Si films. We have also employed the Wemple–DiDomenico (WDD) single-oscillator model to calculate the two WDD dispersion parameters, dispersion energy, Ed, and oscillator energy, Eso. The amorphous-to-crystalline mass-density ratio in the expression for Ed suggested by Wemple and DiDomenico is the key factor in understanding the refractive index behavior of the a-Si layers under study. The value of the porosity for the specific rf-magnetron sputtering deposition conditions employed in this work, with an Ar-pressure of ~4.4 Pa, is found to be approximately 21%. Additionally, it must be concluded that the adopted TLUC parameterization is highly accurate for the evaluation of the UV/visible/NIR transmittance measurements, on the H-free a-Si investigated. Finally, the performed experiments are needed to have more confidence of quick and accurate optical-characterizations techniques, in order to find new applications of a-Si layers in optics and optoelectronics.
Highlights
Amorphous silicon (a-Si) still remains at the center of attention of the modern amorphous/glassy solid-state physics community for two main reasons
The as-measured transmittance spectra for the rf-magnetron Ar-plasma sputtering (RFMS)-a-Si samples studied were all analyzed by the Matlab-coded program ‘AdjustTransIS’, and in Table 1 all the best-fit parameters belonging to the TLUC model used, obtained from two representative spectra, are listed
The complex refractive index and dielectric constant of the RFMS-a-Si layers studied, grown onto glass substrates, were very accurately calculated as a function of the vacuum wavelength/photon energy, by using inverse synthesis in a Matlab GUI environment, which is based upon the use of the as-measured transmission spectrum
Summary
Amorphous silicon (a-Si) still remains at the center of attention of the modern amorphous/glassy solid-state physics community for two main reasons. This disordered material, in its hydrogenated and doped forms, is technologically very important. The second reason, which is of great interest to condensed matter physics theory and computer simulation scientists, is that a-Si is one of the simplest and archetypal systems readily available, in order to be tested new theoretical and simulation techniques, developed for non-crystalline materials in general. We have chosen rf magnetron sputtering in order to prepare the a-Si layers, because is a low-cost technique which allows us to reasonably control, up to a point, the deposition parameters
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