Films minces à base de Si nanostructuré pour des cellules ...

Figure 2.16: Schematic diagram of principle and experimental set-up of an ellipsometer.
tel-00916300, version 1 - 10 Dec 2013
The measurements were done at an angle of incidence Φ xed at 66.2°, between
1.5-4.5 eV with a resolution of 0.01 eV. The unpolarized incident light is linearly
polarized when it passes through the polarizer operated at a xed angle of 45 °. A
quartz modulator xed at normal incidence, phase modulates the linearly polarized
beam and this beam interacts with the sample (Si substrate with the dielectric layer,
in our case). The light after undergoing multiple reections leave the layer under
the same exit angle as the incidence angle and passes through a rotating analyzer
operated at 45°. The analyzer measures the reectance from all the phases and a
photodetector records the intensity of this phase modulated light which is a measure
of Y and D. The data acquistion is done for each sample using ELLI42 software provided
by the seller, in the measured energy range. The optical parameters of the thin
lm are obtained by making a model that ts the measured spectra with the theoretical
ones. Ellipsometry thus is a non contact method to determine the thickness
in contrast to the tedious sample making procedures for microscopic analysis.
Modelling the spectra: Ellipsometry is a model dependent method. The
modelling of the samples is done using a DeltaPsi software provided by Jobin Yvon
[ellipso 1], which estimates the thickness, the optical constants of the sample etc. A
dispersion model of n and k as a function of energy known as the new amorphous
model derived from the Forouhi-Bloomer model [Forouhi 86] is used so as to x
the initial values of the optical constants: n ∞ , w j , G j , w g and f j . The phenomenon
of dispersion depends upon the complex refractive indices of materials which are a
function of various parameters related as follows:
n(E)=n ∞ + B(ω − ω j) + C
(ω − ω j ) 2 + Γ 2 j
Eqn (2.11)
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