Nanoscale Epitaxial Films of Cu 2 O 2-x An attempt to make cubic CuO Wolter Siemons
Selective-Area Atomic Layer Deposition of Copper Nanostructures … · 2014. 10. 15. · Cu 2p...
Transcript of Selective-Area Atomic Layer Deposition of Copper Nanostructures … · 2014. 10. 15. · Cu 2p...
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Selective-Area Atomic Layer
Deposition of Copper
Nanostructures for Direct Electro-
Optical Solar Energy Conversion
Brian Willis
UCONN
Cancun, MX 2014
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Plasmonics
Plasmonic nanoparticles have applications for localized heating,
SERS, nanophotonics, catalysis, sensors and more…
Au, Ag, Cu Nanoparticles interact strongly with visible/ near IR
radiation, tunable by size/shape.
plasmons=collective modes that strongly
concentrate light at the nanoscale.
+ + + +
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E
Quasi-Static Approximation
𝛼 = 4𝜋𝑎3𝜀(𝜔) − 𝜀𝑚𝜀(𝜔) + 2𝜀𝑚
Resonance at Re [𝜀(𝜔)] = -2𝜀𝑚
Van Duyne, Nat. Mater. 2008
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Plasmonic Dimers
Nordlander, Science 2014
Geometric effect concentrates electric fields at nano-gaps
leading to large field enhancements.
EM field induces a voltage drop across the nanoelectrodes.
“hot spots” are created by “plasmonic
dimers”
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Plasmonic Enhancement
Mayer, J. Phys. Conds. Mater. 2009
Theoretical studies predict large enhancements in rectification
ratios for plasmonic resonances.
Lightning rod effect concentrates field at tip = asymmetry.
0.5 nm radius tip
Ag tip
/ is rectification ratio
Geometric Asymmetry
lightning rod
effect
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Optical Rectification
Concept: direct conversion of EM radiation (solar) into DC
power by antenna – coupled high speed diodes to rectify
optical frequency charge waves.
Potential Advantages: low cost fabrication, tunable
absorbance including IR (waste heat), and device integration.
2
2
2
4
1)()(
V
IVVIIVII
photoDCphotoDC
Antenna
DC Filter Load
Diode
No band-gap limitations!
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Literature Data
Ward et al have demonstrated optical
rectification in an electromigration
junction.
- not tuned for plasmonic resonance
- not scalable
- no control of geometry
- isolated nanostructures
Ward, Nat. Nano. v. 5, 732 (2010)
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Rectenna Concept
Electrically connected
Sized for plasmon
resonance in visible
Asymmetric geometry
Nanoscale tunnel gap
Not commercially available!
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Rectenna Challenges
Nanoscale antenna tuned to visible/near-IR
Geometric asymmetry, diode response at low voltage
Electrically contacted, tunneling devices
RC time constant, impedance matching
Extremely fast diodes, 1015 Hz
Diode is most critical need!
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Rectenna Concept
Electrically connected
Asymmetric geometry
Sized for plasmon
resonance in visible
Nanoscale tunnel gap
ALD used to make tunnel diodes
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ALD Metals Review
Choices for ALD Cu, Ag, and Au are limited, non-ideal.
Cu(II)thd2/H2 process is well-behaved for selective growth.
Year Reactants GPC
(nm)
T window
(°C) type
1998 Cu(II)thd2 // H2 0.03-0.04
190-260 thermal
2003 bis(N,N’-diisopropylacetamidinato)diCu(I) // H2 0.01-
0.05 220-300 thermal
2013 1,3-Diisopropyl-imidazolin-2-ylidene Cu(I) hexamethyldisilazide
// H radicals 0.02 250 plasma
2014 Cu(II)(OCHMeCH2NMe3)2 // BH3(NHMe2) 0.013 130-160 thermal
2007 (2,2-dimethylpropionato) Ag(I)triethylphosphine // H radicals 0.12 140 plasma 2010 (hfac)Ag(I)(1,5-COD) // propanol - 110-150 thermal
2011 Ag(I)(fod)(PEt3) // H radicals 0.03 120-150 plasma
2014 (hfac)Ag(I)(PEt3) // HCHO - 170-200 thermal
Au??
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Selective Area Growth
oxide/nitride
metal
seed No growth
Growth
Selective area ALD enables deposition on seeded regions
and eliminates the need for etching.
SA-ALD enables nanostructures.
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Plasmonic Structures
10x10 “Rectangular Dipole”
Rectenna Array, 550 nm x 800
nm spacing
10x10 “Large Triangle” Rectenna Array
500 nm x 500 nm spacing
Electrically isolated structures are used to investigate tuning
of plasmonic response by selective area ALD.
Pd nanostructures
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Resonance Tuning
FDTD simulations predict that resonance red-shifts for
decreasing gap size and for increasing nanostructure size.
Gap size dependence Nanostructure size dependence
bar-shape structures
bar size: 150 x 50 x 35 nm
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Tuning Plasmonics with ALD
Plasmonic dimers have strong electric field enhancement
>103, with fields > 5x108 V/m 1.
“lightening rod” effect further
intensifies electric fields
1Ward et al., Nature Nano v. 5 p. 732 (2010)
FDTD
simulations
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Optical measurements
Focus spot is 3-7 um; sized to the test array.
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Optical Response
Spectroscopy data show red-shift after ALD.
Red-shift &
increased
extinction
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Red-Shifts
Red-shift magnitude tracks ALD thickness/cycles.
In-situ measurements would provide feedback on nanogap.
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Nanoscale Tunnel Junctions by ALD
geometric
asymmetry
Device arrays with 1000’s of tips converge to tunneling similarly
to suspended structures.
Geometric asymmetry contributes to desired diode IV character.
SiO2
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0
1 10-8
2 10-8
3 10-8
4 10-8
5 10-8
0 200 400 600 800 1000
Curr
ent
(A)
Time (seconds)
-0.4 -0.2 0 0.2 0.4
d2I/d
V2 In
ten
sity
Volts (V)
Trapping and IETS detection of Acetic Acid
molecules in nanojunctions
adsorption
IETS
ALD Grown Tunnel Junctions
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HIM Images
< 9 nm < 7 nm
HIM provides some estimates for gap size.
Hang Dong, Rutgers
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Nano-Nucleation Challenges
Growth is sensitive to surface pre-treatment, growth inhibition is
observed without UVO.
ALD metal films are not layer-by-layer, Nanograins form with inherent
surface roughness. How smooth can we achieve? Could be shape
dependence, crystal structure effects.
Without UVO With UVO
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UV/Ozone Sample Pre-treatment
planar films
ALD Cu roughness values are similar to
other reports2, scales with thickness.
UVO enhances/enables growth, removes most C 1s, but residual C
remains even after long time (60 min).
acetate-like
2Winter et al. Chem. Mater. v. 26, p. 3731 (2014)
XPS
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Selectivity
Selectivity is lost at higher temperatures, inverted selectivity?
Selectivity was a mystery; how does Cu grow on oxide?
Annealed in He at 600°C prior to
growth at 220°C High T growth, 230°C
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ALD Fundamentals with in-situ SE
Cu(thd)2 is a solid source precursor. H2 is co-
reactant.
In-situ, real time SE provides saturation curves.
note O
content of
precursor
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SE Growth Trajectory
in-situ SE provides growth “finger-print”, sensitive to
substrate preparation and growth conditions.
extracted GPC ~ 0.04 nm/cycle matches SEM data.
~15/466 = 0.03
delta/cycle
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Original Mechanism
Cu(thd)2 adsorbs to saturation during precursor dose.
Martensson et al, JES, v. 145, p. 2926 (1998)
Martensson’s
mechanism
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SE Growth Signature
SrO ALD
SrO ALD shows characteristic signature for stable precursors
adsorption.
Cu ALD has opposite signature with reversible adsorption.
Cu ALD
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Original Mechanism
Selectivity follows from H2 dissociation requirement.
Original mechanism is NOT consistent with CuI CVD by
disproportionation at 150-200°C, and can’t explain
observed GPC. Martensson et al, JES, v. 145, p. 2926 (1998)
Martensson’s
mechanism
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New Mechanism
New mechanism explains SE signature and is consistent
with reversible adsorption. Also explains high GPC.
Roll of Pd-H2 is now more complex, strong chemisorption.
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SE Growth vs. Temperature
Mechanism not sensitive to temperature. H* must be stable.
Dissociative reversible adsorption is consistent with
thermodynamics if Hads ~ 34 kcal/mol.
Real-time SE vs. Temp Coverage Simulations
CuL2 + 2* CuL* + L*
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QCM Studies
QCM measures mass and thermal effects.
Reaction signal (at 150°C) is consistent with RTSE data,
GPC ~ 0.04 nm/cycle.
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Role of Pd
Temperature and time dependence of Cu XRD and XPS signals indicate
extensive Cu/Pd mixing. Lower temperatures = higher Cu/Pd ratios.
ARXPS shows evidence for Pd enrichment at surface.
Pd explains H chemisorption. What happens if the Pd runs out?
XRD XPS
Pd segregation
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Cu/Pd vs. Cu/Pt
Both Pt & Pd act as seed layers, but Pd mixes more
extensively with Cu.
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GPC on Pd/H*
GPC decreases as Cu/Pd becomes more Cu rich.
Cu/Pd ratio measured as 45:1 for this sample.
Cu ALD on Pd
150C
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Selectivity
Selectivity is evident from in-situ SE data, but how to explain
non-selective growth on SiO2?
No adsorption/
desorption cycles
for SiO2, Si3N4
Martensson et al, JES, v. 145, p. 2926 (1998)
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Non-Selective Growth Mechanism?
C 1s signal is removed by sputtering, but O 1s signal remains. Cu 2p signal
could be Cu0 or CuI. Cu/O ratios are ~1.3 . AES peaks indicates CuI.
CuO/Cu2O growth explains non-selective growth mechanism. Mechanism
requires self-decomposition. H2 not required, but may affect thickness.
For thick Cu films, self-decomposition may contribute to growth.
C 1s Cu 2p O1s
Growth on SiO2 at 205°C
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H2 effect on Non-selective Growth
0
0.5
1
1.5
2
2.5
3
3.5
130 140 150 160 170 180 190 200 210
ExcellentGoodPoor
pH
2 (
To
rr)
Temperature (C)160 C
3 Torr
200 C
1.5
Torr
H2 pressure plays a significant role for
non-selective growth. Why?
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H2 effect on Non-Selective Growth
180 C
3 torr H2
180 C
1.5 torr H2
180 C
0.5 torr H2
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H2 Effect on Selective Growth
0.0 0.5 1.0 1.5 2.0 2.5 3.0
0.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
GP
C(n
m/c
yc
le)
H2 partial pressure (torr)
140C
160C
GPC Measured by QCM in selective window shows no effect
of H2 partial pressure.
No growth without H2.
Cu ALD on Pd Seed Layers
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Summary
ALD Cu is useful for tuning the optical
response of plasmonic materials. ALD
of Ag, Au would be nice.
Selective Area Cu ALD works well and is
useful for tunnel junctions with many
potential applications.
Cu ALD mechanism is partially
understood, Pd, H effects are important.
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Acknowledgements
Co-Pi’s (Physics, Penn State): Darin Zimmerman,
Gary Weisel, James Chen
Students: X. Jiang & J. Qi (UCONN)
R. Wamboldt (Penn State)
Funding: NSF-EECS