Microdefects in Al2O3 films and interfaces revealed by positron lifetime spectroscopy

Article abstract of DOI:10.1063/1.120278

We have studied microdefects and interfaces of Al₂O₃ films on iron and nickel aluminide substrates using variable-energy positron lifetime spectroscopy. Di-vacancies, vacancy clusters, and microvoids were observed in the oxide scales. Their sizes and distributions were determined by the nature of the process used to synthesize the alumina film, and influenced by the composition of the alloy substrates. For oxide-iron aluminide interfaces, positron lifetimes are longer than those for the alumina layer itself, suggesting a greater defect concentration at such sites.

Full text of DOI:10.1063/1.120278

Microdefects in Al 2 O 3 films and interfaces revealed by positron lifetime spectroscopy
Jun Xu, B. Somieski, L. D. Hulett, B. A. Pint, P. F. Tortorelli, R. Suzuki, and T. Ohdaira
Citation: Applied Physics Letters 71, 3165 (1997); doi: 10.1063/1.120278
View online: http://dx.doi.org/10.1063/1.120278
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/71/21?ver=pdfcov
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Microdefects in Al O films and interfaces revealed by positron lifetime
2
3
spectroscopy
Jun Xu,a) B. Somieski, L. D. Hulett, B. A. Pint, and P. F. Tortorelli
Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6142
R. Suzuki and T. Ohdaira
Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba, Ibaraki 305, Japan
͑
Received 21 July 1997; accepted for publication 26 September 1997͒
We have studied microdefects and interfaces of Al O films on iron and nickel aluminide substrates
2
3
using variable-energy positron lifetime spectroscopy. Di-vacancies, vacancy clusters, and
microvoids were observed in the oxide scales. Their sizes and distributions were determined by the
nature of the process used to synthesize the alumina film, and influenced by the composition of the
alloy substrates. For oxide–iron aluminide interfaces, positron lifetimes are longer than those for the
alumina layer itself, suggesting a greater defect concentration at such sites. © 1997 American
Institute of Physics. ͓S0003-6951͑97͒02547-3͔
Surface oxides are used for various applications that
range from thin films for microelectronics to thick ones that
act as protective coatings for alloys and ceramics. Alumina is
of particular interest for thick-scale protective layers because
it is relatively inert and diffusion-protective. Understanding
the nature of microdefects in oxide layers and at their inter-
faces is important, since these defects can significantly influ-
ence their adherence and ability to act as protective surface
positron beam line at the Electrotechnical Laboratory. De-
tailed descriptions of the positron apparatus appeared in Ref.
9. Positrons were produced by converting gamma rays, gen-
erated by bombardment of 75 MeV linac electrons on a tan-
talum target, into electron–positron pairs and then moderated
in tungsten plates. The positron pulses are stretched in a
Penning trap and rebunched with a high frequency ͑178
MHz͒ and narrow width ͑Ͻ300 ps͒. The positron energy was
varied in the range of 0.25–23 keV by biasing the sample.
Figure 1 shows the positron lifetime spectra for the ther-
mally grown Al O scales on both Fe AlϩY O and ␤-NiAl
1
,2
coatings. Conventional methods for probing these defects
include scanning and transmission microscopy ͑SEM and
3
TEM͒, which are sensitive to larger-scale ͑Ͼa few nm͒
2
3
3
2
3
voids. For detecting smaller defects, which may be the pre-
ϩHf and for the PVD Al O film on Fe AlϩZr. The incident
2
3
3
4
5,6
cursor of larger voids, positron spectroscopy is useful. To
date, no studies of alumina films using positron techniques
have been reported. In this letter, we present positron life-
time studies of alumina films with various positron energies
in the range necessary to enter the alumina scales and reach
their interfaces with metals.
positron energies were 5 keV, corresponding to a mean im-
Alumina layers were synthesized by two different pro-
cesses: ͑1͒ oxidation of iron– and nickel–aluminide sub-
strates for 2 hr at 1200 °C in flowing O and ͑2͒ physical
2
7
vapor deposition ͑PVD͒ on iron aluminide. The thermally
grown scales were prepared on both ͑69.9 at %͒Fe-͑28͒Al-
͑
2͒Cr-͑0.1͒Y O ͑referred to as Fe AlϩY O ͒ and ͑48.95
2 3 3 2 3
at %͒Ni-͑51͒Al-͑0.05͒Hf ͑referred to as ␤-NiAlϩHf͒ sub-
strates, which were polished to 0.3 ␮m and cleaned ultra-
sonically in acetone and methanol baths prior to oxidation.
These two substrates were chosen because ␤-NiAlϩHf has a
relatively
void-free
metal–oxide
interface
while
8
Fe AlϩY O forms large interfacial voids. These films have
3
2
3
a crystalline structure ͑␣͒ with a thickness of about 1 ␮m.
The alumina film deposited on ͑66.9 at %͒Fe-͑28͒Al-͑5͒Cr-
͑
0.1͒Zr ͑referred to as Fe AlϩZr͒ by PVD was amorphous
3
and approximately 5 ␮m thick. While the PVD Al O is
2
3
relatively pure, there is ϳ0.5 wt % Fe or Ni in the thermally
grown ␣-Al O scales on iron– and nickel–aluminides, re-
2
3
spectively.
FIG. 1. Semi-log plots of the counting rates as functions of the arrival time
of annihilation gamma rays for the thermally grown Al O scales on both
Positron lifetime measurements were conducted in a
high-vacuum chamber (10 Torr͒ which is attached to the
2
3
Ϫ8
Fe3AlϩY2O3 ͑open circles͒ and ␤-NiAlϩHf ͑crosses͒, and for the PVD
Al O film on Fe AlϩZr ͑filled circles͒. Inset: Probability density functions
2
3
3
as a function of positron lifetime, resulting from Laplace inversion ͑CON-
TIN͒.
a͒Electronic mail: xuj2@ornl.gov
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34.153.41.27 On: Wed, 26 Nov 2014 15:14:05
1

plantation depth of ϳ0.13 ␮m.10 The positron lifetime spec-
tra were analyzed by Laplace inversion ͑CONTIN͒11 which
deconvolutes the time spectra into the probability density
function ͑pdf͒ as a function of positron lifetime, as shown in
the insert. Total counts of the spectra are limited to be
5
around 5ϫ10 , which may affect the width in the pdf
1
2
peaks. However, the peak positions are reasonably accurate
since they are generally not altered by statistics. For the
␣
-Al O scales on Fe AlϩY O and ␤-NiAlϩHf, the major-
2 3 3 2 3
ity of positrons annihilate around 206 and 194 ps, respec-
tively, with satellite peaks ͑4%͒ of about 1400–1500 ps.
Positron lifetimes in defect-free alumina lattices are
about 140 ps and positrons trapped in aluminum-site vacan-
cies in alumina have lifetimes of about 165 ps.13 Thus, the
lifetimes ͑194–220 ps͒ measured for the scales are estimated
due to mainly di-vacancy sites by a linear extension of the
above two lifetimes, as indicated in Ref. 14 for semiconduc-
tors. Laplace inversion also reveals that the positron lifetimes
are significantly different for the Al O scales on
2
3
Fe AlϩY O ͑206 ps͒ and ␤-NiAlϩHf ͑194 ps͒ even though
3
2
3
they were formed by the same oxidation process. This life-
time difference may be due to the chemical composition of
the scales, particularly the iron and nickel cation impurities.
For the PVD film, the main component of the lifetimes is
around 388 ps, much longer than the lifetimes for the ther-
mally grown scales. The PVD-film lifetime may be due to
the larger interatomic spaces in the amorphous structure,
which are approximately equivalent to the size of a cluster of
about 8 atomic vacancies in the crystalline mode.
FIG. 2. ͑a͒ Positron lifetimes and ͑b͒ related intensities for thermally grown
alumina scale on iron aluminide substrate as a function of positron energy.
For lower energies ͑Ͻ2 keV͒, I , I , I , ␶ and ␶ were determined by fitting
͑POSFIT͒ with ␶1 fixed at the average bulk lifetime ͑202.5 ps͒. For other
energies, all parameters were obtained by fitting.
2
2
3
2
3
Voids are also indicated by appearance of the long life-
time components around 600–1400 ps. TEM images indicate
that the thermally grown Al O scales contain a few percent
voids, generally 20–100 nm in size. This would yield a long
positron lifetime ͑Ͼ5 ns͒ if positronium is formed in these
voids. The observed shorter lifetimes may indicate that pos-
itrons are attached to the inner wall of the voids or that the
voids that trap positrons are much smaller.
scale thickness. Possible causes of increase are as follows:
͑1͒ interfacial dislocations, ͑2͒ larger vacancy clusters, and
͑3͒ the chemical difference around the interface. The differ-
ence between the positron lifetimes for 15 and 20 keV pos-
itrons injected into alumina scales on ␤-NiAlϩHf is not as
large as that observed for Fe3AlϩY2O3. This suggests that
the size of the interfacial vacancy clusters for nickel alu-
2
3
2
Positron lifetime spectra measured as a function of pos-
itron energy in the range 0.25–23 keV, provided depth pro-
files of defects in the scales on Fe AlϩY O . Figures 2͑a͒
3
2
3
and 2͑b͒ show the positron lifetimes and their intensities,
respectively, as functions of positron energy. ␶ represents
2
the surface lifetime. ␶ and ␶ are the short and long com-
1
3
ponents of the bulk lifetime, respectively. The intensity of
the surface component (I ͒ decreases as the energy in-
2
creases, while the intensity of the bulk component (I ) varies
1
inversely. This variation is interpreted with the positron dif-
fusion process in which the low-energy positrons diffuse
back to the surface, while the high-energy positrons are
trapped in the bulk. The diffusion length was estimated to be
about 20 nm, which is much shorter than that of perfect
alumina lattices ͑100 nm͒. The truncated length confirms the
high concentration of microdefects in the scales.
A clear difference was observed between the lifetime
spectra of 15 and 20 keV positrons injected into the alumina
scale on Fe AlϩY O , as shown in Fig. 3. Laplace inversion
3
2
3
reveals that the lifetime for 15 keV peaks around 205 ps,
while 20 keV positrons annihilate around 219 ps. This dif-
ference is believed to be due to the interface since the mean
depth of positron penetration for 20 keV is close to the 1 ␮m
FIG. 3. Probability density functions as a function of positron lifetime for
1
5 and 20 keV positrons ejected into the thermally grown Al O scale on the
2 3
iron–aluminide substrate.
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166 Appl. Phys. Lett., Vol. 71, No. 21, 24 November 1997 Xu et al.
34.153.41.27 On: Wed, 26 Nov 2014 15:14:05
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1

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2
3
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3
2
3
8
␤
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In summary, for the first time, microdefects in Al O
4
5
2
3
films and Al O -metal interfaces have been evaluated using a
2
3
6
7
positron probe. Di-vacancies, vacancy clusters, and voids
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2
3
␣
-Al O and were affected by the type of substrate. For the
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͑
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1
1
0
1
Research is sponsored by Division of Materials Science
of DOE under Contract No. DE-AC05-96OR22464 with
Lockheed Martin Energy Research Corp. The authors ac-
knowledge the support from Science Technology Agency of
Japan and Ion Brown at LBL for making the PVD coating.
BAP was supported by DOE Distinguished Postdoctoral Re-
search Program administrated by ORISE.
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