Physica B 248 (1998) 48—52
High resolution X-ray scattering investigation
of Pt/LaF /Si(1 1 1) structures
3
S.S. Fanchenko!,*, N. Jedrecy", W. Moritz#, A.A. Nefedov!
! RRC Kurchatov Institute, Kurchatov Sq. 1, 123182, Moscow, Russian Federation
" LURE, CNRS-MRES-CEA, Batiment 209D, Centre Univeritaire, F-91405, Orsay, France
# Humboldt-University, Bunsenstr. 1, D-10117, Berlin, Germany
Abstract
Recently developed high resolution X-ray methods have been used to characterize the structure of Pt/LaF /Si samples.
3
The asymptotic Bragg diffraction is used for the study of the LaF /Si interface and the grazing-incidence X-ray diffraction
3
(GIXD) for the study of the LaF texture. The LaF /Si interface was found to be thin and smooth with the interface
3
3
thickness value 0.55$0.1 nm. The essential extra-broadening of the main peak in diffraction experiments for samples
with the top platinum layer is explained by the small angle scattering of the X-ray beam on the rough Pt film. The
LaF -film domain structure is derived from GIXD data. ( 1998 Elsevier Science B.V. All rights reserved.
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Keywords: X-ray scattering; Pt/LaF /Si samples
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1. Introduction
The quantitative determination of hydrogen fluoride and fluorine in gases is of great importance as
these gases are often used in various industrial
processes (e.g. production of polymers, aluminum
and glass or preparation of the nuclear fuels). At the
same time both gases are toxic even at low concentrations. Therefore, there is a great demand for
sensors for these gases and alarm systems controlling the environment.
The
metal-ionic conductor—semiconductor
(MICS) structure with solid electrolyte LaF Layer
3
is known to be the perspective field-effect sensor as
for oxygen [1], so for both fluorine and hydrogen
* Corresponding author. E-mail: ssf@chat.ru.
fluoride [2]. The interaction of fluorine with the
Pt/LaF /Si structure leads to the change of the
3
electrochemical potential and as a result the shifting of the capacitance—voltage (C—») curves is observed. The stability of this potential depends on
the properties of the three-phase boundary formed
by gas, platinum and lanthanum thrifluoride. Besides, the electronic properties of MICS sensors
depend on the characteristics of Si/LaF interface,
3
while the sensitivity is due to the reaction at a three
phase boundary — gas/Pt/LaF [2]. The structure
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perfection of the ion-sensitive LaF membrane
3
could also influence on the sensor properties. So the
X-ray investigation of the main crystal characteristics of these MICS structures attracts much interest. It is especially interesting due to the fact, that
the lattice parameters of LaF are very close to that
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0921-4526/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 0 2 0 1 - 4
S.S. Fanchenko et al. / Physica B 248 (1998) 48—52
of the hexagonal Si (1 1 1) surface, so that the epitaxial growth of thin LaF films could be enabled
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[3]. It was found recently that rather thick
(+100 nm) LaF films grown at the room temper3
ature are disordered and become crystalline upon
annealing [4]. Besides, the deposition of LaF at
3
high temperatures could lead to the epitaxial
growth of the film with the c-axis of the hexagonal
unit cell parallel to the Si [1 1 1] direction.
2. Experiments and results
2.1. Sample preparation
We have performed several series of measurements for different sets of samples. Semiconductor/ionic conductor structures were produced by
growing lanthanum thrifluoride layer on single crystalline silicon (1 1 1) by the thermal vapor deposition technique. The thickness of the LaF layers
3
varied from 5 to 300 nm, the deposition was performed at substrate temperature of 550°C. The
Pt/LaF interface should be a three phase bound3
ary in order to provide the Nerstian behavior of the
Pt/LaF electrode. This was enabled by DC sput3
tering (10—80 nm) layers of platinum in argon
plasma with following thermal annealing at 360°C
in air. Several LaF /Si samples were left without the
3
top Pt electrodes in order to test the LaF crystal3
line properties.
In the present work we consider the application
of the following methods in the investigation of
Pt/LaF /Si structures: the asymptotic Bragg dif3
fraction (ABD) [5] or crystal truncation rod [6]—
for LaF /Si interface; the grazing-incidence X-ray
3
diffraction (GIXD) [7] — LaF film texture.
3
2.2. ABD experiments
The scheme of the ABD-experiment was given in
Ref. [5]. The triple crystal scheme is used in this
technique and the dependence of diffraction intensity on the analyzer angular position H is investi3
gated at rather large values of the angle a of the
sample deviation from the Bragg position. Usually,
there are three peaks on the triple crystal curve
49
— the diffuse peak, situated near origin; the pseudopeak, associated with the asymptotic diffraction on
the monochromator and situated at H "a, and
3
the so-called main peak, associated with the asymptotic diffraction from the investigated sample and
situated at H "2a. Due to the high resolution
3
achieved in this technique, one can investigate the
coherent diffraction intensity far from the Bragg
position. Experimentally it is done by measuring
the main peak area at different a.
We used Cu K radiation, Si (1 1 1) reflection and
a
Si(1 1 1) crystals as monochromator and analyzer.
The measured triple-crystal curves for Pt/LaF /Si
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sample with LaF layer thickness 240 nm and
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Pt film thickness about 40 nm (curve 1) and for
LaF /Si sample with LaF layer thickness 145 nm
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(curve 2) are depicted in Fig. 1a. The main peak
could be reliably distinguished from the diffuse
background in the both cases and several curves of
the main peak are shown in Fig. 1b. The essential
extra-broadening of main peak curves for two Pt/
LaF /Si samples (dot—dash and dash lines) com3
pared to the LaF /Si curve (solid line) could be
3
explained by the small angle scattering of the X-ray
beam on inhomogeneities of the rough Pt film [8].
In order of simplicity we would consider only
Gaussian height-to-height roughness correlation
function. In that case the double propagation of the
d-shaped beam through the rough layer results in
the following variation of the beam shape:
I(u)"e~2L2n k2s20P5@4*/2hBd(q)#2e~L2n k2s20P5@4*/2hB
](1!e~L2n k2s20P5@4*/2hB) e~0.5q2L2-¸ /J2p
#(1!e~L2n k2s20P5@4*/2hB)2e~q2L2-¸ /Jp,
where q"ku cosH ; k"2p/j, j is the wavelength;
B
H is the Bragg angle; u is the angle of the small
B
scattering in the diffraction plane; ¸ and ¸ are
n
the roughness value and lateral roughness correlation lengths, accordingly; s is the platinum polar0P5
izability.
The estimations for the sample 1 (dot-dash line)
in Fig. 1b have given the following values of parameters: ¸ +¸ +40 nm. Considering that the ton
tal thickness of the Pt film is about 40 nm, one can
conclude, that this film is really an island-like one.
The similar result was obtained for the sample
2 (dash line) too.
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S.S. Fanchenko et al. / Physica B 248 (1998) 48—52
Fig. 2. The relative diffraction intensity (averaged over three
Pt/LaF /Si samples) vs. the angle a of the sample deviation from
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the Bragg position.
The relative intensity curve averaged over three
samples data is shown in Fig. 2. The characteristic
depth of the distorted region ¸ is determined by
$
the angular half-width of that curve D:
Fig. 1. Triple-crystal rocking curves for Pt/LaF /Si samples
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with Pt layer thickness 40 nm, LaF layer thickness 240 nm and
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LaF /Si samples with LaF layer thickness 145 nm. (a) a"60A
3
3
— Pt/LaF /Si structure — curve 1; LaF /Si structure — curve 2.
3
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(b) a"360A — two different samples of Pt/LaF /Si structure
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— dot-dash and dash lines; LaF /Si structure — solid line (only
3
main peaks are shown).
The crystal perfection of the LaF /Si interface
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could be determined from the so-called relative
intensity curve (the ratio of the coherent diffraction
intensity from the investigated crystal to the one
from the ideal crystal) [6]. The distinction of the
relative intensity from unity is associated with the
crystal structure distortions near surface. Due to
the large values of a the kinematic treatment of the
diffraction could be used.
j
¸"
.
$ 4pD cos h
B
The shape of the observed curve is typical for
insulator/silicon interfaces, giving the distorted interface thickness value 0.55$0.1 nm. So, the LaF /Si
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interface structure is characterized by rather small
distortions. The weak diffuse peak on the triplecrystal rocking curve for LaF /Si sample without
3
the top Pt film (Fig. 1, curve 2) indicates the negligible amount of the defects located near LaF /Si
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interface. The shape of the relative intensity curve
for these samples was the same as for the samples
with Pt film. This fact indicates that the LaF /Si
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interface is not influenced by Pt-film deposition.
2.3. Grazing incidence X-ray diffraction
A 300 nm LaF film grown on Si (1 1 1) was
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analyzed by GIXD-experiments. The experiment
was perfomed with the ultra-high-vacuum 6-circle
diffractometer, set on the synchrotron wiggler
beam line DW12 of LURE (Orsay, France). Radiation was aligned using a laser beam, and the
S.S. Fanchenko et al. / Physica B 248 (1998) 48—52
crystallographic alignment was achieved by use of
the Si bulk reflections. The incidence angle was set
to the critical value for total reflection. The specular
intensity detected with a X-ray camera showed that
the surface was homogeneous and quite smooth.
Assuming a (0 0 01) surface of LaF , with the
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azimuthal orientation [1 0 0] LaF //[1 0 11 ] Si
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(labeled A ), we used the reciprocal-space (RS)
lattice (a*, b*, c*) associated to the bulk hexagonal
LaF lattice (a"4.148 A_ , c"7.3554 A_ ). A RS posi3
tion will be denoted (h, k, l ), according to the
momentum transfer q"2p(ha*#kb*#lc*). The
expected reflections from LaF and Si are depicted
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in Fig. 3a in a (11 1 0) Si cross section of the reciprocal space. Two azimuthal orientations, at 180° from
each other along the normal, are possible. However, they cannot be distinguished, the symmetries
in the unit cell leading to a rod (h,k) being the same
as the rod (hM , kM ). The Fig. 3b presents the rods (1, 0)
and (1, 1) from l-scans. The rod (11 , 0) was qualitatively equivalent to the rod (1, 0). According to orientation A , one should observe along rod (1,0)
peaks at l"1 and l"3, the atomic configuration
in the LaF unit cell producing a very weak reflec3
tion at l"2. In the case of the rod (1, 1), as there is
an extinction for all reflections at l odd, one expects
one peak at l"2. The experimental l-scans confirm
the epitaxial relationship, but reveal extra peaks, at
l"0.7, 1.4, 3.6, on the rod (1, 0) (see Fig. 3b), and at
l"3.2 on the rod (1, 1). The integrated intensities
for each l value were derived from H-scans and
corrected by the active sample area, the Lorentz
and the polarization factors. The intensities of the
reflections at integer l values agree with those calculated for the atomic model, known from literature. The correlation length was evaluated as
150 A_ . This limited value for such a thick film
indicates the existence of multiple distorted regions.
In fact the availability of other orientations results
in the limiting of the width of the domains oriented
as A . Due to that fact the extra peaks detected in
1
the (1, 0) l-scan are weakened, so some other l-scans
were performed at (h, k) values close to (1, 0), that is
after slight rotation of the sample along its normal
surface, in order to characterize these extra peaks.
After a 1° rotation from the Bragg position, the
peaks at l"1 and 3, have lost half of their intensity, contrary to the extra peaks which remain
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Fig. 3. (a) Cross section (11 1 0) Si of the reciprocal space, showing the Si reflections (squares) as well as the LaF reflections
3
related to orientation A (full circles). The crosses correspond to
1
extra reflections issued from differently oriented LaF domains.
3
(b) l-scans of (h, k)"(1, 0) and (1, 1).
nearly the same. The latter can even be detected
after a 7° rotation. Thus, the correlation length of
the corresponding domains is rather small, but
from the intensities of the related peaks (of the same
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S.S. Fanchenko et al. / Physica B 248 (1998) 48—52
order of magnitude than the l"1 peak), they must
imply a large portion in the film. The absence of
extra peaks along rod (1, 1) indicates orientations
turned around the [11 1 0] Si axis with respect to
orientation A . A transmission electron diffraction
1
analysis has indicated that the extra spots were
connected by faints lines (see Fig. 3a), which could
correspond to (0 0 0 1) faulted planes of grains
inside the film, turned by 20° around [11 1 0] Si
from the normal Si (1 1 1) orientation.
Acknowledgements
This work was supported by the INTAS within
grant d94-3643 and by special grant d57 of RRC
Kurchatov Institute.
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