Superior nanoscale passive oxide layers synthesized under photon irradiation for environmental protection

Article abstract of DOI:10.1063/1.2952282

We report on synthesis and functional properties of ultrathin oxide layers synthesized on metal surfaces by room temperature photon irradiation. We show that the impedance of a passive aluminum oxide film synthesized under ultraviolet photon irradiation is an order of magnitude larger than that of native oxide in a 0.5M NaCl solution. Further, the structure and impedance of existing native oxide layers can be dramatically improved by minutes-long exposure to photon irradiation. Depth profiling studies with x-ray photoelectron spectroscopy shows that chlorine uptake in UV-synthesized oxides, compared to that of native oxides, is reduced which can contribute to the improvement in corrosion resistance. The results are of significance to synthesis of ultrathin passive layers on metal and alloy structures for environmental protection.

Full text of DOI:10.1063/1.2952282

Superior nanoscale passive oxide layers synthesized under photon
irradiation for environmental protection
Chia-Lin Chang, Mark H. Engelhard, and Shriram Ramanathan
Citation: Appl. Phys. Lett. 92, 263103 (2008); doi: 10.1063/1.2952282
View online: http://dx.doi.org/10.1063/1.2952282
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APPLIED PHYSICS LETTERS 92, 263103 ͑2008͒
Superior nanoscale passive oxide layers synthesized under photon
irradiation for environmental protection
Chia-Lin Chang,1,a͒ Mark H. Engelhard,2 and Shriram Ramanathan1
1Harvard School of Engineering and Applied Sciences, Harvard University, Cambridge,
Massachusetts 02138, USA
2W.R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory,
Richland, Washington 99352, USA
͑Received 25 January 2008; accepted 6 June 2008; published online 1 July 2008͒
We report on synthesis and functional properties of ultrathin oxide layers synthesized on metal
surfaces by room temperature photon irradiation. We show that the impedance of a passive
aluminum oxide film synthesized under ultraviolet photon irradiation is an order of magnitude larger
than that of native oxide in a 0.5M NaCl solution. Further, the structure and impedance of existing
native oxide layers can be dramatically improved by minutes-long exposure to photon irradiation.
Depth profiling studies with x-ray photoelectron spectroscopy shows that chlorine uptake in
UV-synthesized oxides, compared to that of native oxides, is reduced which can contribute to the
improvement in corrosion resistance. The results are of significance to synthesis of ultrathin passive
layers on metal and alloy structures for environmental protection. © 2008 American Institute of
Physics. ͓DOI: 10.1063/1.2952282͔
Ultrathin metal oxide films are of great significance to
various technologies including electronics, information stor-
age, and environmental protection. The nature of defects
present not only influences the dielectric properties, but also
impact passivity against corrosive environments that inhibit
underlying metals’ collapse. Aluminum is among the most
important metals for several engineering technologies. In
aqueous environment, corrosion pit initiation on aluminum
typically requires aggressive halogen ions, such as Cl−.
Transport of chloride ions through the oxide film via ͑a͒
oxygen vacancies,1,2 ͑b͒ water channels,3 and ͑c͒ localized
oxide film thinning or dissolution4–6 are possible mecha-
nisms of chloride entry. These are dependent on the defect
structure ͑for example, nonstoichiometry͒ of the passive ox-
ide films present on the metallic surface. Nonstoichiometric
alumina, for example, can have much smaller bandgap com-
pared to that of bulk stoichiometric alumina ϳ8–9 eV.7 This
is believed to be due to defects, i.e., higher localized states8
and reducing the number of localized states is expected to
have improved resistance to corrosion.9 In addition, chlorine
implantation studies10 have indicated that the incorporation
of Cl− in passive oxide films can induce pit nucleation in-
stead of surface adsorption controlled pit nucleation. Follow-
ing pit initiation, chloride ions at oxide/metal interface stabi-
lize the pit growth, assist metal dissolution, and result in
collapse. Creation of artificial passive oxide layers by expo-
sure to ozone11 and electron enhanced oxidation12 of alumi-
num have been found to provide improved corrosion resis-
tance due to the improved passive oxide film structure
against NaCl solution.
indicate that rate of oxygen incorporation can be enhanced
compared to natural oxidation,13–15 and also that the quality
of oxide films is dramatically improved. Mechanisms leading
to the improvement in oxide quality have been ascribed to
improvement in film density18 and reduced oxygen vacancies
due to creation of activated oxygen species that are highly
reactive.16,17 In this article, a systematic comparison of elec-
trochemical properties of Al2O3 films grown with and with-
out UV exposure is presented. We show that the exposure of
UV photons on both pristine metal surfaces during oxide
growth, as well as on pre-existing native oxides, can signifi-
cantly improve corrosion resistance of the passive oxide
films. This unique approach is scalable, does not require
high-vacuum technology, can be adapted to large structures,
such as ships or aircraft, and can improve quality of existing
oxide films on a metal or alloy surface.
Aluminum thin films were deposited on ultralow-
resistivity ͑100͒ silicon wafers by sputtering from an alumi-
num target and the thickness of the aluminum films was
nearly 147 nm. Every silicon wafer, prior to aluminum depo-
sition, was dipped in dilute hydrofluoric acid for 2 min, then
rinsed in de-ionized water and dried in nitrogen to remove
the native oxide and enable good electrical contact between
the aluminum film and substrate. Following aluminum depo-
sition, the sample was transferred into the load lock for pho-
toactivated oxidation. An UV photon source has been custom
designed and built into the load lock to perform oxidation
studies under controlled atmospheres and as a function of
temperature. Various photon exposures, ranging from
1 to 120 min were performed at room temperature. More-
over, natural oxidation ͑i.e., no exposure to UV light͒ at
room temperature with identical oxygen pressures and times
was performed on reference samples.
In this letter, we demonstrate that photon irradiation dur-
ing oxide growth or on existing native oxides can result in
significant improvement in the corrosion resistance of ultra-
thin alumina films. The effect of UV photons during oxida-
tion of metals as well as on ultrathin oxide synthesis has
been studied theoretically and experimentally.13–18 Reports
The electrochemical corrosion test cell was filled with
0.5M NaCl solution and included three electrodes: a refer-
ence electrode, a counter electrode ͑graphite rod͒, and a
working electrode ͑147 nm aluminum on silicon wafer͒.
Prior to electrochemical impedance spectroscopy ͑EIS͒ mea-
surements, Porthole™ sticking mask was attached on each
a͒
Electronic mail: clchang@fas.harvard.edu.
0003-6951/2008/92͑26͒/263103/3/$23.00
92, 263103-1
© 2008 American Institute of Physics
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263103-2
Chang, Engelhard, and Ramanathan
Appl. Phys. Lett. 92, 263103 ͑2008͒
FIG. 1. Electrochemical impedance spectra of aluminum oxidized under UV
photons and natural oxidation as a function of oxidation time.
sample for an exact 3 cm2 circular opening to face the 0.5M
NaCl solution electrolyte. The electrolyte was deaerated in
99.99% nitrogen gas for 1 h before the measurements. EIS
spectra on multiple samples were acquired at open circuit
potential to minimize the effect of chemistry on the oxide
film in the solution using our Solartron EIS system in the
frequency range of 100 kHz to 0.01 Hz. The amplitude of
the applied alternating current potential was 40 mV for all
measurements. All measurements were performed on mul-
tiple samples to ensure reproducible results. The x-ray pho-
toelectron spectroscopy ͑XPS͒ measurements were obtained
with Physical Electronics Quantum 2000 Scanning ESCA
Microprobe. Ar+ ion sputtering was used on both natural
oxide and UV oxide to determine the spatial uptake distribu-
tion of chloride ions as a function of depth. The sputter rate
was calibrated using known thicknesses of Al2O3 films.
EIS spectra for various oxide synthesis conditions are
shown in Fig. 1. The impedance of the cell can be described
well by the three element equivalent circuit model.11 The
resistance is not significantly enhanced by an increase in oxi-
dation time up to 120 min for natural oxidation conditions,
as shown in Fig. 1 suggesting that the passive layer quality
formed by natural oxidation is not influenced by longer oxy-
gen exposures at room temperature.
Strikingly different behavior is revealed when aluminum
is oxidized under photon irradiation at room temperature.
From Fig. 1, the impedance of Al2O3 grown under UV irra-
diation for 1 h is enhanced nearly three times, compared to
the native oxide. With an additional hour ͑for a total of
120 min͒ of UV oxidation, the EIS spectra reveal a dramatic
increase in resistance, which is approximately 15 times
larger than native oxide. Because 120 min natural and UV
oxides have nearly the same thickness ͑as determined from
cross-sectional transmission electron microscopy͒, it is likely
that UV exposure enables formation of denser oxide, with
reduced oxygen point defects and greater resistance to ion
migration. A similar increase in density of oxide grown un-
der UV irradiation has been reported for SiO2 as well.15 In
addition, linear polarization measurements reveal that the
UV-grown oxide exhibits nearly two orders of magnitude
reduction in leakage current compared to native oxide in the
passive region and the pitting potential for the UV oxide is
FIG. 2. ͑a͒ Nyquist plot of Z vs Z for UV light compensated native oxide
Ј
Љ
as a function of exposure time. ͑b͒ Plot of extracted resistance from ͑a͒ as a
function of UV exposure time on native oxide.
0.155 V higher than the native oxide suggesting superior
film stability toward pitting.
Since UV-grown oxide is a superior corrosive-resistant
layer compared to native-grown oxide, we have explored UV
exposure on native oxides to determine if their properties can
be improved. Following natural oxidation process, UV expo-
sure was performed at room temperature for times ranging
from 1 to 120 min to monitor the kinetics of UV light com-
pensation on existing natural oxides. A Nyquist plot of Z
Љ
versus Z for UV exposure on the native oxide reveals the
Ј
time evolution of impedance, as is shown in Fig. 2͑a͒. The
outermost semicircle representing the Nyquist plot for a
120 min, UV-grown oxide with resistance of 0.15 M ⍀ is
included for reference. The increase in radii of semicircles
indicates the impedance increases as the UV exposure time
increases, indicating an enhancement in oxide quality result-
ing from photon irradiation. Figure 2͑b͒ shows the evolution
of resistance of UV-compensated native oxides. A saturation
stage occurs around 20 min UV exposure time, and does not
change even up to 120 min. The resistance reaches
ϳ98 000 ⍀ saturation value after ϳ20 min of UV compen-
sation time.
To further understand structural differences between
native- and UV-grown oxides, we examined the depth distri-
bution of chloride-ion uptake using high energy resolution
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263103-3
Chang, Engelhard, and Ramanathan
Appl. Phys. Lett. 92, 263103 ͑2008͒
the UV-grown oxide is more stoichiometric than the native
oxide and the results will be discussed in detail elsewhere.
In summary, we have found that passive oxide layers
grown on aluminum films under UV irradiation possess sig-
nificantly superior corrosion-resistance properties compared
to those of native oxides. The method is simple, inexpensive,
does not require high vacuum, and is highly scalable for
applications ranging from nanoscale to large-scale structures.
Further, UV irradiation has been shown to enhance the qual-
ity of existing native oxides on an aluminum surface. The
thickness of the passive layer can be controlled by tempera-
ture and oxygen partial pressure. We anticipate this approach
to be of great benefit to the synthesis of superior corrosion
resistant coatings in a variety of technological applications.
C. L. C. and S. R. acknowledge funding from the Office
of Naval Research for financial support. The XPS experi-
ments were performed at the W. R. Wiley Environmental
Molecular Sciences Laboratory, a national scientific user fa-
cility sponsored by the Department of Energy’s Office of
Biological and Environmental Research and located at Pa-
cific Northwest National Laboratory.
FIG. 3. Uptake behavior of chlorine ions by natural oxide and UV grown
oxide normalized to XPS spectra ratio of chlorine ions before and after
subsequent argon ion sputter increment treatments.
XPS. Oxide films ͑both native-oxide grown and UV grown͒
soaked in 0.5M NaCl for 6 h at room temperature at open
circuit conditions were used for the XPS measurements. A
series of XPS Cl 2p spectra were acquired from both
samples before and after argon sputter etching. After oxide
film removal by argon ions at increments of ϳ0.2 nm, the
spatial distribution of chlorine incorporated into the oxide
differed between the native- and UV-synthesized oxides. The
normalized XPS ratio of chlorine 2p spectra, defined as the
amount of chlorine remaining in the sample divided by the
amount of chlorine on as-received samples, is plotted in
Fig. 3.
After removal of the first oxide layer ͑ϳ0.23 nm͒, 87%
of the chlorine remained in the native oxide, whereas only
53% of the chlorine was detected in the UV oxide ͑Fig. 3͒.
This notable drop indicates that after a 6 h immersion with-
out polarization in 0.5M NaCl and at room temperature, the
UV-grown oxide can effectively block ϳ34% more chlorine
than the natural-grown oxide at the near-surface region. At a
depth of 1.8 nm, only 4% of the chlorine is found in the
UV-grown oxide, compared to ϳ19% of the chlorine in natu-
ral oxide. This result suggests that in accordance with the
high resistance revealed from EIS spectra, the Al2O3 struc-
ture resulting from UV oxidation is a superior oxide which
may reduce the possibility for pitting initiation. Analysis of
the binding energy shift in the Al 2p peaks also indicated that
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