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

Figure 2.9: Schematic diagram of XRR set-up and the two modes of measurement.
tel-00916300, version 1 - 10 Dec 2013
this critical angle gives access to the index of refraction of the surface layer by the
law of Snell-Descarte (cosθ c =n).
Region 2: When θ is greater than θ c , a part of the incident beam is transmitted
into the layer where it undergoes multiple reections leading to the formation of
interference. Since the substrate is optically denser than the lm, a phase dierence
occurs at the lm surface and the substrate. The interference fringes lead to maxima
and minima in the spectra. The dierence between two successive maxima in the
reectivity spectra yields information on the thickness (d) of the lm or the sublayers
forming the lm from the following formula,
d=
λ
2 √ sin 2 θ k+1 −sin 2 θ k
Eqn (2.7)
where λ= 1.54Å and θ k+1 and θ k represent the position of two successive maxima
in the reectivity spectra. Similar calculations are done using a computer simulation
in the laboratory that allows estimation of the sample thickness. For θ > θ c , the
surface roughness causes a sharp decrease in the reected intensity and the other
roughnesses below the surface (example: at the interfaces) reduces the amplitude of
the reected peaks. If the contrast in the electronic densities of the sublayers are
high enough, then their individual thicknesses can be estimated from the resulting
XRR spectra as mentioned above. In the case of our samples, we can estimate
only the pattern thickness. A typical XRR spectra obtained from a 14 patterned
SRSO/SRSN ML grown with 4 nm SRSO and 5 nm SRSN of similar refractive indices
is shown in gure 2.10 from which the total thickness and the pattern thickness
(4 nm+5 nm) is estimated to a high degree of accuracy. From equation 2.7 it can
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