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

investigation. This value is obtained by subtracting instrumental width β inst
from the experimental width β exp after perfoming curve tting using Gaussian
functions on the (111) peak. Thus β = β 2 exp − β 2 inst where:
β inst = (0.15/cosθ) + 0.07 Eqn (2.6)
2.2.3 X- Ray Reectivity (XRR)
Principle
tel-00916300, version 1 - 10 Dec 2013
X-Ray Reectivity is a non-contact technique that enables determination of lm
thicknesses ranging between 2-200 nm with a precision of 10-30 nm. The process
of total reection of X-rays from solid samples with at and smooth surfaces and
interfaces forms the basis of XRR technique. Since the refractive index of X-rays
is slightly lower than 1, a beam of incident X-rays impinging on a at surface can
be totally reected if the angle of incidence remains below a critical angle θ c . This
is called as total external reection. The reectivity decreases at larger angles of
incidence (θ > θ c ). If the surface is not ideally at and possesses some roughness,
the X-rays are scattered and the reected intensity deviates from that predicted by
Fresnel reectivity. The interface roughness gives rise to a progressive damping of
the interference fringes. Thus from an analysis of the reected intensity, we can
extract the electron density prole, the total thickness in a monolayered sample
or sublayer thicknesses in a multilayered sample. This technique does not work
eectively if there is no sucient contrast in the electronic density (in the refractive
indices) of the dierent sublayers in a multilayer lm and yields information only
on the pattern thickness.
Experimental set-up and working
The same apparatus used for XRD is used for XRR with a slight change in the
conguration (Fig. 2.9). As seen from the gure, two modes of measurement are
possible with XRR: (i) Specular reection and (ii) Non-specular reection.
(i) In specular reection, both the X-ray beam and the detector move at an
angle θ from the surface. This is known as the θ − θ measurement. The rotation of
the detector is controlled through a programmable slit. The incident angle moves
between 0-2° in steps of 0.001°. The spectrum of specular reectivity shows two
regions discussed as follows:
Region 1: Where θ is less than the critical angle θ c , there is a total reection on
the sample surface and the detector receives the maximum intensity. The value of
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