Electrochimica Acta 55 (2010) 7884–7891
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Combinatorial investigation of Hf–Ta thin films and their anodic oxides
Andrei Ionut Mardare a,b , Alfred Ludwig c , Alan Savan c , Andreas Dirk Wieck c , Achim Walter Hassel a,b,∗
a
Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Str. 1, 40237 Düsseldorf, Germany
Institute for Chemical Technology of Inorganic Materials, Johannes Kepler University Linz, 4040 Linz, Austria
c
Ruhr-Universität Bochum, 44780 Bochum, Germany
b
a r t i c l e
i n f o
Article history:
Received 31 October 2009
Received in revised form 23 March 2010
Accepted 24 March 2010
Available online 31 March 2010
Keywords:
Combinatorial libraries
High-throughput experimentation
Scanning droplet cell
Anodic oxide film
a b s t r a c t
A co-sputtering technique was used for the fabrication of a thin film combinatorial library (Hf–21 at.%
Ta to 91 at.% Ta) based on alloying of Hf and Ta. The microstructure and crystallography of individual
metallic alloy compositions were analyzed using SEM and XRD mapping, respectively. Three different
zones of microstructure were identified within the range of alloys, going from hexagonal to tetragonal
through an intermediate amorphous region. The local oxidation of Hf–Ta parent metal alloys at different
compositions was investigated in steps of 1 at.% using an automated scanning droplet cell in the confined
droplet mode. Potentiodynamic anodisation cycles combined with in situ impedance spectroscopy provide basic knowledge regarding the oxide formation and corresponding electrical properties. Dielectric
constants were mapped for the entire composition range and XPS depth profiles allowed investigation
of the oxide compositions.
© 2010 Elsevier Ltd. All rights reserved.
1. Introduction
Tantalum and Hafnium are both valve metals with high melting
points and remarkable mechanical properties. They are currently
used for the fabrication of superalloys, in turbines and for nuclear
reactor components [1]. The corrosion properties of Ta were shown
to be improved by the addition of Hf, for challenging applications
such as nuclear fuel element cladding or liquid alkali metal containment [2]. Ta also has good biocompatibility and is a candidate
for use in orthopaedic implants [3]. Hf may similarly be attractive
for bio-medical applications [4]. The use of alloys of these metals to achieve further enhanced anticorrosion properties is thus
suggested. Even though the Hf–Ta system is characterized by complete miscibility at high temperatures in the liquid state, in the solid
state at lower temperatures a large miscibility gap with a monotectoid reaction is present [5]. Nevertheless, this does not exclude
the possibility of obtaining metastable or even amorphous phases
by co-deposition techniques onto a low-temperature substrate for
some alloy compositions. With respect to the high melting points
of the metals, the tendency to form undercooled solid solutions is
very high [6].
Semiconducting metal oxides are commercially used as gas sensors, because they have good thermal, mechanical and chemical
stability and an appropriate variation in electrical conductivity
∗ Corresponding author at: Institute for Chemical Technology of Inorganic Materials, Johannes Kepler University Linz, Altenberger Str. 69, 4040 Linz, Austria.
Tel.: +43 732 2468 8704; fax: +43 732 2468 8905.
E-mail address: [email protected] (A.W. Hassel).
0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.03.066
with the gas composition of the surrounding atmosphere. HfO2 is a
promising material for use as a carbon monoxide sensor for monitoring the completeness of combustion in fuel gas, important for
optimizing safety, efficiency and fuel consumption [7].
The oxides Ta2 O5 and HfO2 are used in electronics in the fabrication of capacitors and high power resistors either in the form of
oxidized metal powder or sputtered thin films [8]. The mixture of Hf
and Ta in the oxidized state has been studied as a high-k gate dielectric and Hf-doped Ta2 O5 with a thickness of less than 2 nm was
successfully used for the fabrication of metal-oxide-semiconductor
transistors [9,10]. HfTaO was used as a buffer layer in ferroelectricgate field effect transistors (FETs) for very high density devices,
because of improved thermal stability with silicon and higher crystallization temperature compared to pure HfO2 [11]. A detailed
knowledge of properties such as dielectric constant, leakage current density and breakdown field strength, as well as chemical and
thermal stability with respect to microelectronic manufacturing
processes, is critical for their industrial application.
On both metals, anodic oxides can be grown with a breakdown
field strength identical to the oxide formation field strengths or
the reciprocal film formation factor [12]. The anodic oxidation of
hafnium has been reported to produce a composite layer structure
which is strongly influenced by the electrolyte [13]. This is a good
starting point to study the anodisation kinetics and the properties
of the resulting oxides of various Ta/Hf alloys. One more reason
is that their low dissipation factors were already demonstrated in
anodised Hf/Ta multilayered structures [14].
Only a systematic study of the entire range of the binary alloys
can reveal whether or not there are compositions with superior
properties to those of the pure metals [15] or the bulk alloy com-
A.I. Mardare et al. / Electrochimica Acta 55 (2010) 7884–7891
positions that were investigated so far. For instance, Hf has been
identified as one of the most promising elements for improving the
oxidation resistance of Ta, while Hf with 20 wt.% Ta has been found
to have excellent high temperature oxidation resistance [16].
These studies indicate the value of the combinatorial approach
to investigate the entire range of possible alloys. In particular,
searching for an optimum composition or looking for threshold
properties requires a method that is more efficient than the time
consuming one by one preparation of alloys [17]. Therefore the
combinatorial materials science technique is used here. Thin film
libraries with continuous composition gradients, i.e. a composition
spread, were fabricated and characterized.
The work presented here complements two previous studies in
the Hf–Ti and the Ta–Ti systems [18,19]. In those studies, increasing
Ti content significantly affected the formation and properties of
semiconducting oxides.
2. Experimental
2.1. Hf–Ta combinatorial library fabrication
Hf–Ta thin film samples with a regularly varying composition
were obtained using a co-sputtering technique in an ultra-high
vacuum system (DCA, Finland) [20]. Two 101.6 mm pure element
targets, aimed at the center of the substrate, were placed at an angle
of 144◦ with respect to each other. Both Hf and Ta were sputtered
in the RF mode from high purity targets (99.995%, Kaistar R&D)
using a power of 200 W, which led to deposition rates of 0.96 nm s−1
and 0.73 nm s−1 , respectively. Three thermally oxidized Si wafers
with diameters of 100 mm were sequentially used as substrates for
the depositions in order to obtain a Hf–Ta combinatorial library
with a total compositional spread ranging from 20 at.% Ta to 91 at.%
Ta. For all depositions, the base pressure of the chamber was less
than 2 × 10−6 Pa. The depositions were carried out at room temperature in Ar atmosphere with a pressure of 6.66 × 10−1 Pa. The
target-substrate distances were approximately 19 cm, leading to
the formation of wedge-shaped concentration gradients of each
metal in the deposited thin films. The total thickness of the films
was approximately 300 nm at the wafer center. Additionally, pure
Hf and Ta thin films were separately deposited, using the same
conditions, as reference materials in this combinatorial library.
After the fabrication of the Hf–Ta films, energy dispersive X-ray
spectroscopy (EDX) was used for mapping the element concentrations across the wafers. In Fig. 1 the EDX mappings are presented
for all samples. The concentration gradient direction is imposed
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by the co-deposition geometry position of the targets. A precise
measurement of the concentration gradient allowed an accurate
identification of Hf–Ta alloys in the combinatorial library. More
details regarding the fabrication and high-throughput characterization of thin film materials libraries can be found elsewhere [20].
2.2. Description of the scanning droplet cell
A scanning droplet cell (SDC) constructed in an acrylic block with
a 3-electrode configuration was used for the growth and characterization of the anodic oxides on the surface of the Hf–Ta films [21]. A
borosilicate glass capillary with a diameter of 2.5 mm, used as the
outer body of the cell, was made using a puller (PC-10, Narishige)
and a tip with an outer diameter of 200 ␮m was obtained using a
micro-grinder (EG-400, Narishige). The reference electrode used
was a capillary-based ␮-AuHg/Hg2 (CH3 COO)2 /NaCH3 COO reference electrode, having an 100 ␮m tip diameter [22]. A 1 mm wide
Au band, wrapped around the reference electrode capillary was
used as a counter electrode and they were inserted together into
the main capillary body. More details regarding the reference electrode and cell fabrication can be found in [23]. A silicone sealing
gasket was fabricated at the tip of the cell by immersing it into liquid silicone followed by drying in flowing nitrogen. For ensuring a
reproducible wetted surface (i.e. working electrode) on the Hf–Ta
alloys, the tip of the cell was pressed against the investigated surface with a predefined force, producing an elastic deformation of
the seal. In this way, any electrolyte-air contact was avoided and the
wetted area on the sample surface had a very high reproducibility,
with errors smaller than 1%, as previously shown on pure Hf films
[24].
2.3. Hardware description and measurement details
A computer-controlled, micro-syringe pump (Micro 4, World
Precision Instruments) combined with a 100 ␮l syringe was used
for dosing the electrolyte from the cell in order to wet the investigated spot. The electrical contact to the metallic surface was
achieved using a W needle in hard contact with the sample. In order
to investigate the composition gradient of the metallic alloy along
the surface of the sample wafers, an automated, high-throughput
measurement approach was applied. The tip of the microelectrochemical cell was systematically moved across the sample surface
by a computer-controlled XYZ translation stage, resulting in a 3D
scanner. Using an in-house developed control and data acquisition
software written with LabView, high-throughput experimentation
Fig. 1. EDX concentration mappings of the Hf–Ta combinatorial library.
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Fig. 2. SEM images of the Hf–Ta combinatorial library at different concentrations together with the surface of pure Hf and Ta as references.
was achieved. Details about the software can be found elsewhere
[23]. A force sensor (KD45 2N, ME-Messsysteme) combined with a
lock-in amplifier (EG&G 7265) was used in the automated control
of the force applied for pressing the tip of the microelectrochemical
cell against the sample. The microstructure of the Hf–Ta samples
was characterized by SEM and 1◦ grazing incidence X-ray diffraction (GIXRD) at different locations with compositions identified
by EDX mapping. The surface was locally anodised in an acetate
buffer electrolyte (pH 6.0) prepared from p.a. chemicals and deionized water using a potentiostat (Solartron Schlumberger 1287).
The surface of the Hf–Ta samples was scanned in steps of 1 at.%
composition difference and small oxide spots were grown locally.
The anodisations were carried out potentiodynamically at a potential scan rate of 100 mV s−1 . The starting potential for anodisations
was 0 V (SHE). Cyclic voltammograms having the upper potential
limit between 1 V and 10 V were sequentially recorded in 1 V steps
for each investigated region. Before each cyclic voltammogram, the
impedance of the already-formed oxide layer was measured at high
(1 kHz) and low (0.1 Hz) frequency using a frequency response analyzer (S5720C, NF Electronic Instruments) with a 10 mV AC voltage
stimulus in order to determine the dielectric constant and electric
resistivity of the anodic oxides in situ during their growth. X-ray
photoelectron spectroscopy (XPS) was used for chemical analysis
of the Hf–Ta library at different compositions.
3. Results and discussion
3.1. Microstructure of Hf–Ta thin film alloys
The microstructure of the Hf–Ta thin film alloys was investigated by SEM as seen in Fig. 2. The surface images of pure Hf and
Ta thin films deposited in similar conditions were added as references. The surface of Hf thin films shows two distinct types of
grains. Small grains, less than 100 nm in diameter form a compact
base on the surface with bigger triangular shaped grains forming
uniformly distributed islands on top of the small-grain layer. The
Hf grain structure is strongly modified upon addition of 20 at.%
Ta and several compositional zones can be identified. In the first
zone, small spherical grains form a compact surface and the grain
diameters continuously grow with increasing Ta concentration. At
Hf–35 at.% Ta the surface microstructure shows an elongation of the
grains, most probably due to coalescence of neighbouring grains
arising from the minimizing of the surface energy. The elongated
grains start to align themselves at Hf–40 at.% Ta, and at a composi-
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Fig. 3. XRD spectra measured at various concentrations on the Ta–Hf library.
tion of 45 at.% Ta the almost complete disappearance of the round
grains marks the beginning of a second compositional zone.
In the range between 45 at.% Ta and 70 at.% Ta, all alloys in the
second zone have similar surface characteristics. These appear as
aligned elongated grains, sometimes longer than 200 nm, which
form a smooth surface with well-defined grain boundaries. With
further increase of the Ta content, at 75 at.% Ta the granular structure of the surface is re-established in the third compositional zone
and two distinct types of grains are again observable. The structure
evolves with the increase of Ta concentration and at Hf–90 at.% Ta
the microstructure resembles the surface of pure Ta, restoring the
elemental properties.
The crystallographic analysis of the Hf–Ta thin film metallic
alloys by XRD investigations, is presented in Fig. 3. In the first compositional zone, defined by the microstructure analysis in Fig. 2,
crystallographic structures similar to pure Hf are observed. With
the increase of the Ta concentration up to 40 at.%, the hexagonal
structure of pure Hf can still be observed, with the main peak (1 0 3)
slightly shifted toward higher angles. The beginning of the second zone at Hf–45 at.% Ta is marked by a sudden change in the
crystallographic structure. The main (1 0 3) peak characteristic to
the hexagonal symmetry is replaced by another peak positioned
at approximately 37◦ , which can probably be attributed to diffraction from the (1 0 1) plane. The increase of Ta content in the second
zone leads to a broadening of the (1 0 1) peak between 60 at.% Ta
and 70 at.% Ta which suggests a certain degree of amorphisation of
the thin film alloys. This amorphisation can be due to a screening
of atoms with different atomic radius which are co-deposited on
the substrate, combined with the relatively low deposition temperature applied here (limited surface mobility of the adatoms).
Once the third compositional zone is reached at Hf–75 at.% Ta,
the tetragonal symmetry characteristic of pure Hf is observed,
together with the reappearance of the grains in the SEM images
(see Fig. 2). The increase in Ta content of Hf–Ta alloys in the third
compositional zone results in a stabile tetragonal structure and
therefore no significant change is observed in the crystallographic
properties.
3.2. Potentiodynamic oxide formation and characterization
The Ta–Hf thin film alloys were anodised using the SDC in
a potentiodynamic regime (100 mV s−1 ) and several anodisation
series with a maximum oxide formation potential of 10 V are shown
in Fig. 4(a). A typical valve metal behaviour is observed which is
characterized by current suppression when the potential starts to
decrease in the backward scan of each CV loop. The overshoot which
is characteristic to low oxide formation potentials is present in all
cases of the first CV steps up to 5 V. The current density plateaus,
characterizing the anodic oxide formation, are better observed
at higher anodisation potentials and increased plateau values are
observed in the amorphisation zone.
During the first CV, when the first oxide step is grown, the current onset potential (zero current potential) directly characterizes
the thickness of the naturally grown oxide. The current increase
shows that the electric field inside the natural oxide became strong
enough for initiating the ion hopping mechanism of anodic oxide
formation. The zero current potentials (Ei=0 ) and the open circuit
potentials (OCP) were mapped on the surface of the Hf–Ta combinatorial alloys and the results are summarized in Fig. 4(b). The
values measured on pure Hf and Ta thin films were added as references. Both curves are fitted using multi-peak Lorentzian functions.
Small concentrations of Ta initially lead to the formation of a peak
at approximately Hf–25 at.% Ta which slightly decays with increasing Ta concentration. In the amorphous compositional zone, the
zero current potential has a reasonably stable value around 0.65 V
and a second peak is observed in the last compositional region. The
curve also shows two minima, each marking the beginning of a new
compositional zone. The evolution of the OCP on the surface is more
uniform. Several significant minima are observed, similar to those
seen in the zero current potential mapping. In this case though,
two minima possibly defining the three compositional regions are
slightly shifted toward lower Ta concentrations while a third minimum can be observed around Hf–55 at.% Ta. This corresponds to
the composition where the strongest (1 0 3) peak could be observed
in the XRD measurements from Fig. 3.
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Fig. 4. (a) Cyclic voltammograms recorded during the potentiodynamic oxide
growth on Hf–Ta alloys at different concentrations. (b) Zero current potentials and
OCP of the Hf–Ta combinatorial library surface.
For selected compositions, the cyclic voltammograms recorded
during anodisation are shown in Fig. 4. After exceeding the former oxidation potential, a steep increase is observed until reaching
the plateau current density iox . This plateau current density is a
direct consequence of the stoichiometric oxide growth without side
reactions. Eq. (2) describes that the product of iox and the oxide specific constant Kox which can be calculated from the molar mass and
density of the oxide and its valency is equal to the product of the
oxide formation factor k and the potential scan rate v. This equation
has been rearranged to allow a direct determination of k from the
known or measured values.
k = iox Kox v−1
(2)
For calculating the density of the Hf–Ta alloys, a mixed matter
model was used that assumes a linear transition between the densities of the pure oxide HfO2 and Ta2 O5 (9.68 g cm−3 and 8.10 g cm−3
respectively).
The kinetic hindrance due to the formation of a space charge
layer that gives rise to the current overshoot at the beginning of
each cycle is not further considered here since it is only compensating the delayed oxide formation. In this way the system is
regaining the lost current and the conditions in the plateau current
are restored in exact agreement with the Cabrera Mott theory [25].
When increasing the maximum limit of the anodisation potentials,
the opposite effect is observed which causes a further increasing
gap between subsequent scans. Also in this case the plateau current
Fig. 5. Impedance (a) and phase (b) measured by EIS at various concentrations on
the Hf–Ta library.
was evaluated in which oxide growth and further potential increase
yield a stationary equilibrium. A broad overview of theories of oxide
formation can be found in [26].
Before electrochemical oxidation of the Hf–Ta alloys using the
automated SDC was performed, the electrochemical impedance
spectra (EIS) of a thin oxide layer grown at several compositions
uniformly spread along the compositional gradient were investigated. The results of this investigation are summarized in Fig. 5.
The evolution of both impedance (a) and phase (b) measured
at different frequencies show the typical behaviour of an RC-R
equivalent circuit, independent of the parent metal concentrations.
The phase information also shows that the maximum frequency
where the behaviour of the system appears mainly capacitive
(due to a phase shift to −90◦ ) is 1 kHz. This coincides with the
value chosen for single frequency measurements used for capacitive characterization of the anodic oxides. Also, this frequency
is convenient since it reduces the total measurement time. At
low frequencies, (0.1 Hz), the impedance characterizes mainly the
oxide resistance according to the RC-R model. Single frequency
measurements at 0.1 Hz allow a direct calculation of the oxide
resistance.
The low and high frequency impedance measurements performed after each step of anodic oxide thickness increase, allowed
a determination of the electrical resistance (R) and capacitance (C)
of the oxides. For each investigated spot on the surface of the Hf–Ta
A.I. Mardare et al. / Electrochimica Acta 55 (2010) 7884–7891
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Fig. 7. (a) Oxide formation factors, dielectric constants and (b) electrical resistivities
for the anodic oxides grown on the Hf–Ta combinatorial library.
Fig. 6. Potential dependent inverse capacitance (a) and resistance (b) of the anodic
oxides grown at various concentrations on the Ta–Hf compositional spread.
combinatorial samples, the dependence of the inverse capacitance
and resistance of the growing oxide on the anodisation potential was investigated. Some of these results are shown for various
concentrations in Fig. 6(a) and (b). Since the anodisation potentials are proportional to the anodic oxide thicknesses, the slopes
of the inverse capacitance and resistance curves directly allow
the calculation of the dielectric constant and electrical resistivity,
respectively. Such curves were automatically recorded for every
investigated composition in the combinatorial Hf–Ta library with
a resolution of 1 at.%. Automatic data handling and fitting software
was used to map the electrical properties at different concentrations.
From each current density plateau measured at each anodisation spot, the oxide formation factor (k), which represents the
proportionality factor correlating the oxide thickness with the
anodisation potential, was mapped along the concentration gradient. The results are plotted in Fig. 7(a), together with the dielectric
constant measured in situ during the anodic oxide growth. The values measured on anodised Hf and Ta are given as references and
the dotted lines suggest the evolution of the mappings in the compositional areas not covered by the Hf–Ta compositional spread
samples. At low Ta concentrations of approximately 20 at.%, both
the oxide formation potential and the dielectric constant show
slight deviations from the reference points. With the increase of
Ta content, both curves show an almost linear increase until the
threshold of the amorphous parent alloys formation is reached
between Hf–40 at.% Ta and Hf–45 at.% Ta. Once the amorphisation
of the Hf–Ta alloys starts, both curves show strong decays until
plateaus can be observed for Ta concentrations higher than 60 at.%.
At the end of the compositional spread in the third zone, the values
slightly increase toward the reference values measured on pure Ta.
The evolution of the electrical resistivity of the oxides anodically grown on the Hf–Ta thin film library has a different shape,
as shown in Fig. 7(b). Starting at low Ta contents, the resistivity
initially decreases with the increase in the Ta concentration. A minimum of approximately 1.5 × 1011 cm is reached at the beginning
of the amorphous alloy compositional zone, where the oxide has
a higher dielectric constant. After this threshold, the increase of
Ta in the alloys leads to an increase in the oxide’s resistivity and
a peak is observed at the end of the amorphous region, close to
the threshold of the third compositional zone where the tetragonal
symmetry starts to dominate the alloys’ structure (see Fig. 3). In the
last part of the compositional range, where the amorphisation ends
and the crystalline structure becomes stable, the electrical resistivity settles in a small plateau of approximately 5 × 1011 cm for Ta
concentrations higher than 80 at.%.
3.3. Surface analytical investigations
In order to characterize the formation of anodic oxide on the
surface of the Hf–Ta alloys, XPS depths profiles for the qualitative
analysis of the mixed oxide were recorded. The XPS spectra of the
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Table 1
Metal concentration ratios for the parent metal alloys (Me), anodic oxides (Ox) and
their individual variations with respect to the metal concentrations (Me;Ox ).
HfMe :TaMe (at.%)
HfOx :TaOx (at.%)
Me;Ox Hf (%)
Me;Ox Ta (%)
75:25
45:55
25:75
15:85
88.5:11.5
57.1:42.9
35.8:64.2
23.1:76.9
+18.0
+26.8
+43.2
+54.2
−54.1
−21.9
−14.4
−9.6
for the amorphisation of the parent alloys, can be attributed more
to the oxidation of Hf rather than to the Ta oxidation.
For a quantitative evaluation of the oxidized species in the mixed
anodic oxides, the XPS spectra measured on the surface of the
anodic oxides at different concentrations were integrated and the
results are summarized in Table 1. Both oxides are always present
on the surface and their composition can be followed in the second column of Table 1. The mixed anodic oxide shows deviations
from the compositions of the parent metal alloys and in the right
part of Table 1, the deviations are given with respect to the corresponding metal concentrations. Even though the transport number
of Hf (0.05) is much smaller than the transport number of Ta (0.24)
[27,28], HfO2 shows an enrichment on the surface of the anodic
oxide as compared to the Hf content in the parent alloys. At the
same time, less Ta2 O5 is present in the mixed oxide than the corresponding Ta amount in the metallic films. These results suggest
the formation of a Hf-rich region at the metal/oxide interface which
would promote Hf oxidation while inhibiting Ta oxidation. A similar
idea can be found in previous studies which describe the formation
of a bidimensional Hf film on the surface of Hf–Ta alloys prior to
the oxidation [29].
At low Ta concentrations (Hf–25 at.% Ta), the amount of oxidized
Hf is 18% higher than the amount of Hf in the metallic state of the
parent alloy. With the increase of Ta concentration in the metallic
library, this deviation further increases until approximately 54%,
where more oxidized Hf can be observed in the anodic oxide grown
at Hf–85 at.% Ta. The deviations of the Ta2 O5 amount found in the
mixed anodic oxide, as reported for the metal alloy compositions,
decreases with increasing Ta concentration. A Ta concentration of
25 at.% in the metallic state produced a deficiency of almost 54 at.%
in the oxide, while 85 at.% metallic Ta resulted in only approximately 10 at.% less oxidized Ta.
4. Conclusions
Fig. 8. XPS depth profiling spectra of the anodic oxides grown on the Hf–Ta combinatorial library at different compositions.
oxides, grown at several different parent metal concentrations are
shown in Fig. 8. The depth scale is given on the right part of each
plot as calibrated to the SiO2 .
For all the presented cases, both HfO2 and Ta2 O5 are always
simultaneously present on the surface of the oxides which suggests
the formation of a mixed oxide. The intensities of the XPS peaks vary
with the parent metal concentrations and with the depth. Both Hf
and Ta have a similar behaviour. The oxide peaks measured on the
surface decrease in intensity with depth and the presence of metallic Hf and Ta starts to be observable at depths of approximately
10 nm and 5 nm on the SiO2 scale, respectively. Partially, this can
be an artefact due to a possible reduction of the oxide thickness
during the Ar sputtering, since metallic and oxide peaks coexist at
these depths. For a parent metal composition close to the middle
of the compositional spread, such as Hf–55 at.% Ta, a simultaneous
presence of both HfO2 and Ta2 O5 is observed for depths as high
as 20 nm on the SiO2 scale. In all spectra, the presence of HfO2 is
observed in all depths. This suggests that the high oxide formation
factors described in Fig. 7(a), close to the compositional threshold
In conclusion, the Hf–Ta thin film combinatorial library was
investigated and three different compositional zones, including an
amorphous region, could be identified. The synthesis and characterization of the alloys’ anodic oxide using an automated SDC led
to a mapping of electrical properties of the mixed oxides. Whereas
the onset potential of oxide formation and the OCP were found
to vary only a little with composition, distinct maxima are found
for the film formation factor and the relative permittivity number. This maximum is found at a composition near 40 at.% Ta which
coincides with the transition from crystalline to amorphous parent
metal. One possible explanation for the increased film formation
factor could be a roughening of the surface which, however is not
conclusively observed in the SEM. XPS depth profiling showed the
oxidation’s dynamic and information about the oxides’ compositions could be obtained and discussed. Surprisingly, more Hf in
the oxidized state could be detected on the surface of the mixed
oxides than expected from the literature values of the ionic transport numbers. These observations are explained by a cooperative
growth process in which both Hf and Ta atoms are supporting each
other concerning the anion to cation transport number but at the
same time competing with each other for a higher share of the
cation transport number.
A.I. Mardare et al. / Electrochimica Acta 55 (2010) 7884–7891
Acknowledgements
A.I. Mardare acknowledges IMPRS SurMat for financial support
through a fellowship. Caesar Institute in Bonn is acknowledged for
the co-depositions of the Hf–Ta thin films.
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