Determining Optical Constants for
ThO2 Thin Films Sputtered Under
Different Bias Voltages
from 1.2 to 6.5 eV by
Spectroscopic Ellipsometry
William R. Evans
Brigham Young University
Utah Academy 2006 – Snow College
7 April 2006
Overview
• Our goal is a better understanding of the
optical properties of materials in the EUV.
• The material we have been
studying most recently is
ThO2.
EUV Astronomy
The Earth’s magnetosphere in the EUV
• My project was to see if we could increase the density
of our films by depositing at higher bias voltages.
• The films were studied with visible
and near UV spectroscopic
ellipsometry.
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Our Results: n
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n not related to Bias Voltage or
Thickness
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What we Stumbled upon:
Absorption
alpha*d vs E
ThO2 050429 -- 0 V -- 10.468 nm
ThO2 050520 -- 68 V -- 52.617 nm
ThO2 050503 -- 50 V -- 8.906 nm
ThO2 050527 -- 0 V -- 50.423 nm
ThO2 050604 -- 64 V -- 6.589 nm
ThO2 050818 -- 65 V -- 539.281 nm
ThO2 050604-2 -- 0 V -- 334.591 nm
3
2.5
alpha*d
2
1.5
1
0.5
0
3.5
4
4.5
5.5
5
6
6.5
7
E (eV)
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Band Gaps – The Chemistry
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Band Gaps – The Physics
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Our Results for the Band Gap
alpha^2 vs E
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Linear (ThO2 050818 -- 2nd fit -- 65 V -- 539.281 nm)
20
18
16
14
alpha^2
• This measurement
for the band gap of
ThO2 came out to
be about 5.92 eV.
• Our average was
6.11 ± 0.15 eV.
y = 52.483x - 311.26
R2 = 0.9263
ThO2 050818 -- 2nd fit -- 65 V -- 539.281 nm
12
10
8
6
4
2
0
5.9
5.95
6
6.05
6.1
6.15
6.2
6.25
E (eV)
8
6.3
Comparing to the Literature
• In reviewing the literature,
there seems to be a
couple of different band
gaps that people detect:
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Why the Differences?
• Essentially, we find two camps regarding
the band gap of ThO2. One camp
measuring about 5.9 eV, and the other
camp measuring about 4 eV.
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Another Look at the Data:
alpha*d vs E
• Important to note:
ThO2 050503 -- 50 V -- 8.906 nm
ThO2 050527 -- 0 V -- 50.423 nm
ThO2 050604 -- 64 V -- 6.589 nm
ThO2 050818 -- 65 V -- 539.281 nm
ThO2 050604-2 -- 0 V -- 334.591 nm
3
2.5
2
alpha*d
– There is a narrow
absorption feature
at about 6.2 eV, with
full width half max of
about 0.4 eV.
ThO2 050429 -- 0 V -- 10.468 nm
ThO2 050520 -- 68 V -- 52.617 nm
1.5
1
0.5
0
3.5
• Also:
4
4.5
5
5.5
6
6.5
E (eV)
– We find that the main thin film paper reporting a band
gap of about 4 eV is Mahmoud, who deposited his films
by spray pyrolysis onto glass.
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However...
• It would be quite easy to say “You are not
responsible for other people’s data,” or “He
was obviously using an inferior technique
and/or samples.”
• Yet the second statement is anything but
humble regarding our own imperfect
techniques, and the first thoroughly begs
the question as to what’s going on here.
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However...
• Not only that, but
Mahmoud’s
techniques appear
thoroughly valid!
• Whatever it is he is
measuring really does
appear to have a
band gap of 3.8 eV!!!
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However...
• One thing we might note is that
Mahmoud’s samples were prepared
by spray pyrolysis onto glass.
• Depending on the quality of the
glass, this could effectively dope the
samples introducing extra electrons
into the films.
• ThO2 is known to be sensitive to
oxygen doping due to its fluorite structure.
• When annealed in oxygen ThO2 is known to turn opaque.
However, when annealed in hydrogen or vacuum, it
becomes transparent again.
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What we think might be going on...
• Rivas-Silva, et. al. report an energy structure as follows:
• We note some important features:
– The energy spectrum includes a
thin middle energy band due to
the 5f electrons in Th. This band
has a width of about 0.4 eV, the
same as the absorption feature
that we detected.
– The middle band is in almost the
right place if we interpret our
absorption feature as a jump from
the valence band into the middle
band.
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What we think might be going on...
• If the middle band were centered at
about -9.8 eV in stead of -11.8 eV, the
~6 eV band gap reported in the
majority of the thin film sources would
be explained as a jump from the
valence band to the middle band.
• Also, if the conduction band started at
about -6 eV in stead of about -7 eV,
the ~4 eV band gap reported by
Mahmoud and others could be
explained by a transition from the
middle band, which had some
electrons in it due to mild doping,
transitioning into the conduction band.
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“Conclusions”
• First of all, we have shown that reactive sputtering
cannot be expected to significantly affect the optical
constants of ThO2 thin films.
• Secondly, exactly what is going on with the band
gap of ThO2 is still not really understood.
– It appears that there are two fundamental band gaps in
ThO2, but more research is needed.
– One thing that we might try is creating an intentionally
doped sample of ThO2 and measuring it between 1.0 and
10.0 eV to see if we could observe all three hypothesized
absorption features.
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Acknowledgements
• Dr. Allred
• Dr. Turley
• The BYU EUV Thin Film Optics Group, past and
present
• BYU Department of Physics and Astronomy,
BYU Office of Research and Creative Activities,
and Rocky Mountain NASA Space Grant
Consortium for support and funding
• Kristin Evans
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