Structural studies on Si-N gradient thin layers
by grazing incidence X-ray diffraction
Malgorzata Karolus*
In this paper, the application of combined method – the powerful tool for structure analysis – used for the structure determination
of non-ordered gradient Si-N thin films is presented. The method is based on the combination of well-known methods, like radial
distribution function (RDF), pair distribution function (PDF) analyses and the Rietveld refinement. The values of radii and number
of coordination spheres, obtained from RDF and PDF analyses, were compared to the theoretical values calculated for typical
crystalline structures. Then, the most probable model of the hypothetical ‘unit cell’ of the analysed material might be chosen
for the Rietveld refinement. In case of the gradient materials and thin films, the structure analysis is more complicated because
the experiment has to be prepared in grazing incident X-ray diffraction geometry. The X-ray diffraction patterns collected for
different angles of the incident beam show different shape, what indicate different kind of order in layers. Finally, it was stated
that the Si-N layers are nonhomogenous, and their structure depends on the penetration depth. Moreover, there was a possibility
to create the complete structure model of whole gradient Si-N thin film. Copyright © 2014 John Wiley & Sons, Ltd.
Keywords: thin films; GIXD geometry; RDF; PDF analysis; Rietveld refinement; combined method – structural analysis tool
Introduction
In this work, the new method of the amorphous and nanocrystal-
line materials structure analysis called combined method
[1]
used
for creation of the complete structure model of gradient Si-N thin
film is presented. The X-ray experiment of thin films made with
using the classical Bragg–Brentano geometry gives only the
information of substrate. Moreover, the materials with low order
(amorphous or nanocrystalline) give as the results of the X-ray
measurement, a diffuse scattering without the well defined
diffraction lines. In such case, the analysis is much more compli-
cated. So, it was decided to use combined method based on the
radial distribution function (RDF),
[2]
pair distribution function
(PDF)
[3]
analyses and the Rietveld refinement
[4]
for structure
studying of the non-ordered Si-N thin film.
Originally, combined method was tested on different kind of
materials. In structure analysis of the magnetic amorphous Fe-
B-Nb alloys (obtained by melt-spinning),
[5]
the relaxed
amorphous phase responsible for improving the magnetic
properties was defined. In case of the nanocrystalline Ni-Mo
electrodeposited alloys,
[1,6]
the verification of method results
was performed by comparing the values obtained by classical
methods: unit cell parameters (Nelson–Riley extrapolation),
crystallite sizes, lattice strains (Williamson–Hall theory) and by
combined method.
Materials
As the material, the non-ordered Si-N gradient thin layers were
chosen. The Si-N thin layers have been deposited by means of
a reactive magnetron technique from pure Si targets onto the
surface of thin pure monocrystalline Si wafers in the atmosphere
of Ar + N2. The thickness and surface of searching layers were
0.1 μm and 10 cm2, respectively. Earlier studies performed using
the grazing incident X-ray diffraction (GIXD) geometry for
different incident angles
[7,8]
were indicated that the Si-N layers
are nonhomogenous and their X-ray diffraction patterns depend
on the penetration depth calculated from the equation:
Z ¼
ln 1 0:05 ð Þ
μ
1
sinα
þ
1
sin 2θα ð Þ
h i
where Z is penetration depth, μ is linear absorption coefficient, α
is incident angle, and 2θ is Bragg angle.
[9]
Finally, it was decided
to study structure of thin film layers by using combined method.
Methods
The X-ray diffraction patterns were measured using the GIXD
technique by the PANalytical X’Pert Diffractometer PW 3040/60
type with the molybdenum radiations (Kα = 0.7107 Å) and mono-
chromator on the diffracted beam. The GIXD made for different
incident angles (α = 0.25°, 0.5°, 1°, 2°) gives the opportunity of
the sample homogeneity analysis and determining the structure
dependently on the penetration depth.
[7]
The structure of thin film characterized by the diffused scattering
was studied by combining method,
[1]
i.e. the RDF analysis was
performed using the R. Diduszko software, (Warsow, Poland)
[2]
the
PDF analysis was performed using the V. Petkov software (Dept.
Physics, Central Michigan University, US) (Rad-gtk 1.0)
[3]
and the
Rietveld refinement – using the DBWS-98 software (Dept. Physics,
Central Michigan University, US) of R. Young.
[4]
The following and
guiding a Rietveld refinement is usually performed in terms of agree-
ment of r values.
[10]
The weighted-profile r value, r
wp
, is defined as
* Correspondence to: M. Karolus, Institute of Material Science, University of
Silesia - Katowice, ul. 75 Pulku Piechoty 1A, 41-500 Chorzów, Poland.
E-mail: karolus@us.edu.pl
Institute of Material Science, University of Silesia - Katowice, ul. 75 Pulku
Piechoty 1A41-500Chorzów, Poland
Surf. Interface Anal. 2014, 46, 1068–1070 Copyright © 2014 John Wiley & Sons, Ltd.
ECASIA special issue paper
Received: 1 September 2013 Revised: 21 December 2013 Accepted: 24 February 2014 Published online in Wiley Online Library: 28 March 2014
(wileyonlinelibrary.com) DOI 10.1002/sia.5487
1068