Contributions
C ) OXYGEN METALS AND OXIDATION PHENOMENA
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Oxidation of epitaxial yttrium-hydride films
Katharina Theis-Bröhl, Murat Ay, Hartmut Zabel
Institut für Experimentalphysik/Festkörperphysik, Ruhr-Universität Bochum, 44780 Bochum
Elizabeth Grier, The Clarendon Laboratory, Parks Road, Oxford OX1 3PU
Intensity (log)
We report oxidation studies of 300 Å thick epitaxial yttrium-hydride thin films. Y (0001) was
2O3 substrate system by molecular beam
epitaxy. After growth the samples were UHV-transferred from the preparation chamber to a
different chamber with hydrogen loading possibility. Subsequently in-situ H-loading was
performed for 5 min at a hydrogen partial pressure of 20 mbar and at a temperature of 400°C.
After exposing to air, the samples were measured with high resolution X-ray scattering. The
Bragg scattering intensity of an as-grown sample is shown in Fig. 1. It shows a sharp YH2
(111) peak of a mono-crystalline Y dihydride phase and a sharp Y2O3 (222) peak accounting
for an epitaxial Y2O3 component. No pure Y (0001) peak is observed, indicating the complete
hydrogenation of the Y film during vacuum-H-loading.
Close to the Al2O3 peak two Nb peaks occur. On the right side, a strong Nb (110) peak from
the Nb buffer is observed, as expected. Additionally, left from the sapphire peak, a small NbH
lattice-gas-peak occurs. From the occurrence of this peak one can conclude that the hydrogen
already passed through the Y / Nb interface during loading and that hydrogenation of the Nb
buffer has started.
Annealing at 300°C in air has no major effect on the films as can be concluded from the X-ray
radial scans performed at room temperature after annealing for 30 min, 210 min, and 140,5
hours, respectively.
Only a small loss in
intensity of the
Al O (1102)
Y2O3(222)
NbH 2 3
YH2 phase can be
Nb(110)
YH2(111)
observed.
additional
Remarkable
24 h @ 400°C
changes only occur
additional
after increasing the
24 h @ 335°C
annealing
temperature. After
annealing for 24
hours at 335°C the
YH2 (111) peak
slightly shifted to
not annealed
the left and the
Y2O3 (222) peak
intensity increased.
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The NbH lattice2θ
gas-peak intensity
also
increased,
Fig.1: Bragg scans performed at a 300 Å thick Yttrium-hydride thin indicating that by
film. The scans were performed on the as-grown sample and after thermal activation
annealing in air, first at 300° and subsequently at 335°C and 400°C.
more H passes
through the Y / Nb
interface. Further annealing for 24 hours at 400°C leads to a complete oxidation of the
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Yttrium-hydride film as can be concluded from the vanished YH2 (111) peak in the radial
scan. The hydrogen must have been passed over to the Nb film as can be seen from the
increased NbH peak and the almost vanished Nb (110) buffer peak.
Comparing the oxidation behavior of the hydrogenated Y film with that of pure Y films, as
was studied by Ay et al. [1,2] shows, that the oxidation processes are not much different. After
oxidation of a pure Y(0001) for only 5 min in air, the Y(0001) peak almost is vanished. Three
peaks, a strong YH2 (111) peak, an Y2O3 (222) peak and an additional Y (OH)3 peak can be
observed after annealing at 300°C for 5 min.
In contrast, a Y (OH)3 peak does not appear
in the vacuum-loaded Y films during
annealing. This component obviously is
connected to the hydrogenation of the pure Y
film during oxidation in air and can be
avoided by oxidation of a hydride sample.
As the YH2 phase rapidly grows during
annealing of pure Y films on the expense of
the pure Y (0001) phase, further annealing of
both systems can be observed in a similar
manner as the oxidation of hydrogenated
films.
Optical microscopy studies have been
performed on an oxidized yttrium-hydride
thin film. A wide network of ridges can be
observed. These ridges can be explained by
material squeezed out during hydrogenation.
Several groups have observed similar ridgepatterns, i.e. by Grier [3]for trihydride samples
and by van Gogh [4] for unloaded Y (111)
films on CaF2 substrates. In our case, it is
possible
that during in-situ H-loading already
Fig. 3: AFM picture of an oxidized
the
trihydride
phase formed but unloaded
Yttrium-hydride thin film. Line scans were
performed perpendicular to the ridges
showing the height of the ridges of 100150 Å.
back to the dihydride phase under uncapped
vacuum conditions, as described by Grier [3]
This could explain the similar pattern as
observed by Grier for the trihydrate phase.
AFM studies confirm the observation of
ridges. The scan range of 4 µm however, was
50 µm
not high enough for observing the network of
ridges. Line scans were performed across the
ridges as shown in Fig. 3 for line A, and give Fig. 2: Optical microscopy picture of an
a ridge height of about 100 –150 Å.
oxidized Yttrium-hydride thin film. A
In conclusion, oxidation of vacuum-loaded wide network of ridges can be seen
YH2 (111) films in air shows similar coming presumably from squeezed out
characteristics as the oxidation of pure Y material.
(0001) films. From a network of ridges, also
observed in the trihydride phase by Grier, can be concluded that during hydrogen loading of
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the uncapped films the trihydride phase YH3 formed but unloaded back under vacuum
conditions to the dihydrade phase.
References:
[1] Diploma thesis, Ruhr-Universität Bochum (2000).
[2] M. Ay et al., Oxidation of epitaxial Y(0001)-thin films, Annual report 2000.
[3] E. Grier, Ph.D. thesis, Oxford University (2000).
[4] M. van Gogh, Ph.D. thesis, Vrije Universiteit, Amsterdam (2000).
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Contributions
Oxidation of epitaxial Y(0001)-thin films
M. Ay, O. Hellwig, S. Gök, H. W. Becker*, O.H. Seeck**, H. Zabel
Institut für Experimentalphysik Festkörperphysik
*Institut für Experimentalphysik Ionenstrahlphysik
Ruhr-Universität Bochum, D-44780 Bochum
**HASYLAB, Hamburg
Although Y-layers are known to be protective and often used to cover rare earth thin films or
superlattices, their oxidation and corrosion has not been investigated in detail yet [1]. Y is
also well known for taking up large amounts of hydrogen [2]. We study the reactions of
epitaxial Y-films with air at elevated temperatures including the formation of oxide,
hydroxide and hydrogen lattice gas.
The samples have been grown via molecular beam epitaxy (MBE) and were investigated by
different x-ray diffraction methods at room temperature in air. The electron density profile of
the sample has been studied via x-ray reflectivity while the crystalline structure has been
determined by Bragg-scattering. The x-ray measurements were performed at the beamline W1
at HASYLAB in Hamburg. From the Laue oscillations and the FWHM of the Bragg-peaks we
could extract the actual thickness of each layer.
For as prepared samples investigated at ambient conditions, Bragg-scans (Fig. 1a) along the
growth direction exhibit the sapphire substrate and Nb(110) buffer reflection. In addition we
intensity (logarithmic)
Nb(110)
34.77°
Al203
34.04°
Y(0002)
Y2O3(222)
10
15
20
25
2θ [°]
30
35
40
10
intensity (logarithmic)
intensity (logarithmic)
8
YH α
YO x H x
16
18
12 14 16 18 20 22 24 26
22
24
2 θ [°]
Y2O3
26.31°
FWHM:0.227°
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30
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λ
Y-Laue-oszillations
=>300Å
28.67°
30.78°
26
2θ [°]
20
Y(0002)
28.08°
d)
YOxHx
Laue-oszillations
=>23.63Å
Y(0002)
Y 2 O 3 (222)
14
λ
c)
λ =1.3937Å
b)
intesity (logarithmic)
λ=1.3937Å
a)
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2θ [°]
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Figure 1: a) Bragg-scan performed at W1 at HASYLAB b) fitted radial scan shown at Y
Bragg position depicting Y, Y2O3, YOxHx and YH c) and d) details of the indicated areas
of the radial scan
see the Y(0002) peak accompanied by Laue oscillations corresponding to a layer thickness of
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300Å. Furthermore we observe a reflection, consisting of a sharp Y2O3 and a broad YOxHx
component indicating two different Y2O3 / YOxHx thicknesses (Fig. 1b). The broad
component additionally shows Laue oscillations, corresponding to a thickness of 25Å (Fig.
1c) while the thickness of the sharp component could be determined to 310 Å by its FWHM
(Fig. 1d). Thus the larger oxide thickness roughly corresponds to the Y thickness. Samples
kept in air at room temperature seem to be stable in this state.
To continue the oxidation process, the samples were postannealed in air at 300°C respectively
(Fig. 2). Additional x-ray characterisation was always performed at room temperature. The
further oxidation process can be divided into three fundamental steps, occurring in a typical
time scale that depends on the annealing temperature.
1. The Y(0002) peak shifts partially into the Y-H α-phase indicating the formation of a
hydrogen lattice gas coexisting with an YH2 β-phase. The corresponding lattice expansion
in the α-phase leads to a lattice
parameter of 2.93Å while the βY2O3
phase has a lattice parameter of
3.01Å. The Y2O3 Peak remains
unchanged.
YH
λ
intensity (logarithmic)
2
YHx
Y
14.5h
6.
9.5h
3h
5.
4.
0.5h
3.
2. The YH2 peak vanishes while the
intensity of the Y2O3 reflection
increases. The YHx peak remains
unchanged.
3. The Y-hydrogen peak vanishes too,
leaving only the Y2O3 peak, which
shifts to a slightly lower angle i.e.
higher lattice spacing.
The observations lead to the following
conclusion. Whereas for other systems
like Nb, Cr or Ni the oxide grows in a
2.
5min
horizontal front into the film, thus
creating a well defined metal as well as
0min
oxide layer thickness, Y seems to form
1.
oxide domains along defect structures
through the entire film thickness and a
20
30
40 25Å YOxHx surface layer. This growth
model is also confirmed by the oxide
thickness oscillations gained from small
angle x-ray reflectivity. In contrast to
Figure 2: Bragg-scans performed at W1 HASYLAB
other
metals Y does not exhibit an
for different oxidation times at 300°C.
increasing oxide layer thickness with
further annealing. Thus it seems that oxidation does not occur in a well defined front parallel
to the surface. Instead oxide formation starts to nucleate along defect structures normal to the
surface. A reason for that might be the low diffusion barrier for oxygen along such defects.
Once reached the interface to the bottom Nb-layer, oxidation continues laterally in all
directions from the defect.
2θ [°]
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Y-film
H
100
80
O100%=Y2O3
60
40
20
O100%=Nb+O
H100%=Nb+H
H100%=Y2O3+H2
By
employing
RBS-Methods
and
hydrogen-depth-profiling methods at the
sample after 14.5h of oxidation (fig. 2
:state 6) the expansion of the lattice and
subsequent shift to smaller scattering
vector of niobium and yttrium oxide can
be linked to the presence of hydrogen. The
hydrogen concentration in Y2O3 decreases
surface
concentration [%]
Figure 3: Depth profile of oxygen and
hydrogen. The percentage of oxygen and
hydrogen in Yttrium (grey) and niobium
(white) is shown. Where O(100%)=Y2O3,
O(100%)=Nb+O, H(100%)=Y2O3+H2 and
H(100%)=Nb+H denotes.
Nb-buffer
O
0
t [Å ]
from the boundary Y2O3/Nb to the
surface. A fit to the RBS
measurements shows a relative
proportion of 2:3:1, whereas in
niobium the hydrogen concentration
remains constant. The proportion of
oxygen in Nb measured by RBS is
1:100, strongly indicating that also
the shift of the niobium Bragg reflex
is induced by hydrogen. According
to this conjecture a hydrogen oxygen depth profile is shown in fig.
3.
Using these results the following
oxidation model (fig. 4 a-d) can be
established. In fig. 4a the coexistence
of two oxide phases is depicted, a
thin YOxHx cap layer and deeply
entrenched
Y2O3(222)
oxide
reaching the niob interface. The next
picture (fig. 4b) illustrates the
oxidation process after the hydrogen
uptake is completed: oxidation
advances at the YH2/Y2O3 interfaces.
Arrows indicate the flow of oxide
material through conical channels,
which gain their shape by advancing
of the oxidation. This could be
shown by an oxygen depth profile.
Further oxidation leads to the state in
fig. 4c. Transport of material through
imperfections leads to ridges, which
have been verified by AFM
a)
25Å
Y 2O 3(222)
b)
YH2(111)
200Å
Nb(110)+H
a-Al2O3
c)
d)
Figure4 a-d: AFM measurements (5µmx5µm) and
oxidation model during the main steps of oxidation,
explanation: see text.
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measurements. Finally (fig. 4d) the aperture angle of the conical channels increases until the
normal of the oxide growth is parallel to the surface normal and the oxidation of the yttrium is
complete. A hydrogen gradient remains in the yttrium oxide.
We gratefully acknowledge financial support by DFG: SFB 558 (Metall-SubstratWechselwirkung in der heterogenen Katalyse)
Reference
[1] C. F. Majkrzak, J. Kwo, M. Hong, D. Gibbs, C. L. Chien, and J. Bohr, Advances in
Physics 40, 99- 189 (1991)
[2] A. Remhof, Hydrogen in yttrium films, Dissertation, Ruhr-Universität Bochum (2000)
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