Optical Characterization of H-Free a-Si Layers Grown by rf-Magnetron Sputtering by Inverse Synthesis Using Matlab: Tauc–Lorentz–Urbach Parameterization

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 roomtemperature

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, Dd,

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 RFMSa-

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 theWemple–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.