Atomic Layer Deposition of Cerium Oxide Thin-films

Report
1
Tunabiliy and Electrical Measurements
of Atomic Layer Deposited Yttria
Doped Cerium Oxide for Fuel Cell
Applications
Russell Sparks
Carnegie Mellon University
Department of Chemical Engineering
Jorge Rossero*, Gregory Jursich#, Alan Zdunek*, Christos G. Takoudis#,*
University of Illinois at Chicago
Departments of Bioengineering# and Chemical Engineering*
8/1/2013
2
Applications and Advantages of Solid Oxide
Fuel Cells (SOFCs)
Advantages
Applications
• SOFCs could become
• Stationary electrical
clean replacements for
fossil fuel
• SOFCs do not produce
NOx, SOx, or hydrocarbon
emissions
• Reduced CO2 emissions
• Fuel flexibility-SOFCs can
use alternative fuels such
as H2
power generation
• Replacement for car
batteries
N. Perdikaris, K. D. Panopoulos, P. Hofmann, S. Spyrakis and E. Kakaras,
International Journal of Hydrogen Energy, 2010, 35, 2446-2456.
J. Kupecki, J. Milewski and J. Jewulski, Central European Journal of
Chemistry, 2013, 11, 5, 664-671.
3
SOFC Background
• Current SOFCs require operating
temperatures >800 °C
• Reducing this to 500-600 °C
would greatly increase SOFC
utility
• CeO2 increases O2- ion
conductivity at lower
temperatures by creating oxygen
vacancies in the electrolyte
• Several physical and chemical
methods exist to deposit CeO2
and Ceria-based materials
(YDC)
Schematics of a planar SOFC
4
Yttrium doped Cerium (YDC)
• Ce4+ in the electrolyte tends to reduce to Ce3+ in the
anode, increasing the electric conductivity and causing the
cell to short circuit
• Adding Y to Ce film tends to stabilize Ce4+ ions and allow
higher O2- conductivity through vacancies in the lattice
structure
• Research indicates optimal Y concentration to be 20-34 %
Z. Fan, C. Chao, F. Hossein-Babaeiaand, F. B. Prinz, J. Mater. Chem., 2011, 21, 10903.
Z. Li, T. Mori, D. Ou, F. Ye, G. J. Auchterlonie, J. Zou and J. Drennan, The Journal of
Physical Chemistry, 2012, 116, 5435-5443.
5
Atomic Layer Deposition (ALD)
• Gaseous ligand precursor is
pulsed over Si substrate
• Metal ion reacts with –OH
on substrate
• Reaction is self-limited by
amount of –OH on
substrate surface
• Oxidizing gas is pulsed to
react with metal ions
• –OH tails present for next
ALD cycle
ALD Process Schematic*
*J. Päiväsaari, Inorganic Chemistry Publication Series, Helsinki University of
Technology, 2006.
6
ALD
• Tris[isopropyl-cyclopentadienyl]cerium (Ce(iPrCp)3) and
Tris[isopropyl-cyclopentadienyl]yttria (Y(iPrCp)3)
precursors and water are used to deposit CeO2 and Y2O3
onto Si substrates
• Water reacts selectively with metal-ligand bond in ALD
precursors
• Common ligand Ce(thd)4 only reacts with O3
thd = 2,2,6,6-tetramethyl-3,5-hepadionate
7
Experimental Setup*
Ice bath
Hot wall reactor
* P. Majumder, et al., Journal of The Electrochemical
Society, vol. 155, pp. G152-G158, 2008.
8
Electrical Resistivity
• Ce+4 has high ionic
•
•
•
•
conductivity, Ce+3 has less
ionic conductivity
Electric resistance inversely
proportional to ionic
resistance
Film surface resistivity
measured by four-terminal
sensing
Resistivity measured by labbuilt sensor
Sensor consists of 4 Pt
electrical leads resting on a
nonconductive surface to
measure resistance
9
Electrical Resistivity (cont.)
• LCR instrument was calibrated to determine best
operating conditions
• >2 V and 100-200 Hz settings were found to give most
precise resistance readings
• Wide range of YDC, Si, and CeO2 samples tested
• Thickness: 6-28 nm
• Weight added on top of sample: 0-50 g
• Voltage Range: 0.20 V-5 V
• Frequency Range: 10 Hz-100,000 Hz
• 2 V DC bias added to overcome effects of frequency
• 20, 30 %Y content in YDC films
• Annealed and non-annealed YDC films
10
Sheet Resistance (MΩ-Sq)
Sheet Resistance of 20% YDC Films with
Added Weight
9
• Extra weight may
8
cause wire leads to rub
through film
• Substrate resistance
instead of film
resistance measured
• Negative readings
caused by resistances
too high to result in
readings
7
6
5
11 nm
4
22 nm
28 nm
3
2
1
0
0
10
20
Added Weight (g)
Samples were analyzed with 75 Hz and 2V.
11
Effects of Annealing 20 %Y YDC Sample
Sheet Resistance
(MΩ-Sq)
10
8
1 V Non-Annealed
6
2 V Non-Annealed
4
3 V Non-Annealed
2
4 V Non-Annealed
0
5 V Non-Annealed
0
100
200
300
400
500
Sheet Resistance
(MΩ-Sq)
Frequency (Hz)
10
8
1 V Annealed
6
2 V Annealed
4
3 V Annealed
2
4 V Annealed
0
0
100
200
300
Frequency (Hz)
400
500
5 V Annealed
12
Sheet Resistance
(MΩ-Sq)
Sheet Resistance vs. Frequency of YDC
Films
10
8
1V 20% Y YDC
6
2V 20% Y YDC
4
3V 20% Y YDC
2
4V 20% Y YDC
0
0
50
100
150
200
250
300
350
5V 20% Y YDC
Sheet Resistance (MΩSq)
Frequency (Hz)
10
8
1V 30% Y YDC
6
2V 30% Y YDC
4
3V 30% Y YDC
2
4V 30% Y YDC
5V 30% Y YDC
0
0
50
100
150
200
Frequency (Hz)
250
300
350
13
Sheet Resistance
(MΩ-Sq)
Effects of 2 V DC Bias for 20% YDC Films
10
8
1V Without Bias
6
2V Without Bias
4
3V Without Bias
2
4V Without Bias
0
0
100
200
300
5V Without Bias
400
Sheet Resistance (MΩSq)
Frequency (Hz)
10
8
6
1 V With Bias
4
2V With Bias
3V With Bias
2
4V With Bias
0
0
50
100
150
200
250
Frequency (Hz)
300
350
400
450
14
YDC Film Stoichiometric Tunability
0.8
0.7
Y-atom ratio Y/(Y+Ce)
0.6
0.5
0.4
y = 1.00x + 0.062
R² = 0.9994
0.3
0.2
0.1
0
0
0.1
0.2
0.3
0.4
0.5
ALD CYCLE RATIO Y/(Y+Ce)
0.6
0.7
0.8
15
XPS Background
• XPS (X-Ray photoelectron spectroscopy) measures
kinetic energy change of spectral emissions
• Sensitivity analysis calculates the atomic percent
composition of each component element by comparing
peak areas
• XPS can determine Ce4+/ Ce3+ and Ce/Y ratios based on
their respective peak sizes
16
XPS Results for 20 % Y YDC film
• 23% Ce3+ in this sample.
• V0, U0 and V’, U’ indicate
Ce3+, other peaks for
Ce4+.
• Peak sizes analyzed
using Pfau-Schierbaum
method
Binding Energy
I.D
880.32
Vo
882.21
V
884.75
V'
888.55
V"
898.2
V"'
899.2
Uo
900.81
U
903.14
U'
907.13
U"
17
Conclusions
• Yttria doped cerium oxide films were successfully
•
•
•
•
•
deposited via ALD using Ce(iPrCp)3, Y(iPrCp)3 and water
XPS analysis shows that by increasing the ALD cycle ratio
(Y:Ce) the concentration of Yttrium was linearly increased
in the film
Annealed and non-annealed resistances are close and
have same order of magnitude
Higher Y concentration had little effect on measured
resistance
3 V and 150 Hz produce most accurate resistance results
DC bias unnecessary
18
Recommendations for Future
Experiments
• Produce resistance probe station capable of taking
resistance measurements up to 800 °C
• Repeat calibration measurements at high temperature
• Goal: Determine precise YDC doping for maximum
electrical resistance
19
Acknowledgments
• I would like to gratefully acknowledge the financial support
provided by:
• EEC-NSF Grant #1062943
• CBET-NSF Grant #1346282
• I would like to gratefully acknowledge the material support
provided by:
• Advanced Materials Research Lab, University of Illinois at Chicago
for use of laboratory facilities
• Air-Liquide USA for providing precursors
20
References
• M. Fanciulli and G. Scarel (Eds.): Rare Earth Oxide Thin
•
•
•
•
•
Films, Topics Appl. Physics, 106, 15–32 (2007) ©
Springer-Verlag Berlin Heidelberg 2007.
M. Coll, J. Gazquez, A. Palau, M. Varela, X. Obradors and
T. Puig, Chem. Mater. 2012, 24, 3732−3737.
W. Kim, M. Kim, W. J. Maeng, J. Gatineau,d, V. Pallem, C.
Dussarrat, A. Noori, D. Thompson, S. Chu and H. Kima,
Journal of The Electrochemical Society, 158 (8) G169G172 (2011).
Z. Fan, C. Chao, F. Hossein-Babaeiaand and F. B. Prinz,
J. Mater. Chem., 2011, 21, 10903.
P. Gao, Z. Wang, W. Fu, Z. Liao, K. Liu, W. Wang,
X. Bai and E. Wang, Micron, 2010, 41, 301-305.
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Atomic Percent Comparison (PfauSchierbaum Analysis)
•
22
Atomic Percent Comparison
•

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