passivation of ga as surface with sa ms final
DESCRIPTION
Conference presentation of surface characterization techniques used in my researchTRANSCRIPT
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Passivation of GaAs Surface using SAM of Redox-Active Ruthenium-based OrganicMolecules in a Matrix of Non Redox Active OPE1 Molecules
*Rand K. Jean, *Dmitry Zemlianov, *David B. Janes, **Bin Xi, and **Tong Ren
*Birck Nanotechnology Center, Purdue University
**Department of Chemistry, Purdue University
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Outline Motivation for use of molecules; redox-active and spin
selective Molecular Deposition Surface Analysis:
Cyclic Voltammetry (CV) Fourier Transform Infra-red Spectroscopy (FTIR) X-ray Photoemission Spectroscopy (XPS)
Conclusion
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Metal
Semiconductor
Molecule
Metal Molecule Semiconductor Metal Redox-Active Molecule Semiconductor
Molecular functionality; possible use in sensing, memory etc.
CV indicate E-levels close to Ef
Non-resonant tunneling
E-levels 1-3 eV apart
Resonant tunneling
Molecular level close to Ef Molecules in net charge state
LUMO
LUMO
HOMO
HOMO
Ef Ef
Ev
Ec
Ev
Ec
-2.5-2-1.5-1-0.500.51
-1/-2
+1/00/-1
E(V), vs Ag/AgCl
* Cyclic Voltammogram of Redox active Molecule
*
Motivation for Redox-active Structures
T. Ren et. al Journal of Organometallic Chemistry 690 (2005) 4734-4739
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Spin Properties
Increased spin selectivity; spin valve effect
Possible use in spin-based transistor, and memory based device
L. Bogani and W. Wernsdorfer, Nature Materials , Vol. 7, MARCH 2008
*
*
Molecular Spin Valve MR Effect
Spin Transistor
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Ru Redox Properties
Ef(Au) = 5.1
0.13
0.5
5.23
4.6
GV Analysis
Evac
0.13
0.5
4.97
5.6
ENHE= 4.43
EAg/AgCl= 0.197
+
-
CV0~4.63
0.4
0.9
2.0
5.0
3.73
2.6
CV Analysis
EHOMO
ELUMO(1)
ELUMO(2)
Emol
Au EF (5.1)
-2.5-2-1.5-1-0.500.51
-1/-2
+1/00/-1
E(V), vs Ag/AgClCyclic voltammogram of Ru molecule
Molecular functionality
Possible use in sensing, memory etc.
CV indicate E-levels close to Au Ef
Ren et. al. J Orgn. Chem. 690, 4734, 2005.
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Other Molecular properties Ru complex
Possibility of net spin in Ru-Ru core Thiol end group allows easy attachment to substrates
OPE1 Morphologically similar to Ru anchor end, thus forming good
undergrowth to fully passivate semiconductor surface
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Molecules Deposited on (100) GaAs
H
1. 2.3.
Diruthenium (III) tetra-2-anilinopyridinate-2-(Trimethylsilyl)ethyl-4-(ethynyl)phenyl Sulfide (Ru-complex)
Length: 16 Å Coverage: ? 8 hrs in 0.5mM in tetrahydrofuran (THF), 60 C
2-(Trimethylsilyl)ethyl-4-phenlSulfide (OPE1)
Length: 14Å
Coverage: 3.04 e14 cm-2
1-24 hrs hrs in 0.5mM in
THF, 60 C
SS SS SS S SSS
Octadecanethiol (ODT)
Length: 23 Å
Coverage: 4.5e14 cm-2
8 hrs in 0.5mM in
ethanol, 60 C
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“Size Matters”
HGa GaAs
Ga GaAs
Ga GaAs
Ga GaAs
Ga GaAs
Ga GaAs
Ga GaAs
GaAs
HGa GaAs
Ga GaAs
Ga GaAs
GaAs
Ru Complex: Maximum density limited by size of molecule
Leaves relatively high density of unterminated surface atoms (and defect states)
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Simple Aromatic
HGa GaAs
Ga GaAs
Ga GaAs
Ga GaAs
Ga GaAs
Ga GaAs
Ga GaAs
GaAs
HGa GaAs
Ga GaAs
Ga GaAs
GaAs
Maximum packing density ~ 75% of a monolayer for simple alkanes, slightly lower for small aromatics.
Local reconstruction of surface allows relatively low interface state density
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Mixed Monolayer Deposition
OPE1 deposition, 3 hours
1-24 hrs 0.5 mM Ru in THF solution, 60 C
TBAF cleaves off protective end group, exchange interaction of OPE1 and Ru complex molecules to form Mixed Monolayer,
Mixed Monolayer has more coverage of surface than Ru SAM
4.
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Characterization Techniques
• Fourier Transform Infrared Spectroscopy (FTIR)
Vibrational modes of molecule
• Cyclic Voltammetry with ferrocene solution –
Molecular layer blocks redox process of ferrocene
• X-Ray Photoemission Spectroscopy – surface chemistry and relative surface coverage
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FTIR Characterization of Ru InsertionC=C IR signature (1506 cm-1) with increase in Ru depostion
time
1.00E-04
1.00E-03
1.00E-02
0 5 10 15 20 25 30
Time (hours)
IR a
bso
rban
ce
C=C presence is 9x greater in Ru than in OPE1; Ru insertion monitored by increase in C=C symmetric stretching signature
Rapid insertion of Ru complex molecules for first 6 hours
Saturation after 6 hours; decrease in wavenumber indicates ordering of SAM
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CV Measurement Set-up
P. Carpenter, Metal/Molecule/Semiconductor Devices, MS thesis Purdue University 2005
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Insulation Properties of SAMs
• Scan rate at 100 mv/s
• Current limit of measurement system was 1e-9A; OPE1 2 hr to 18 hr registered sub nano-ampere currents were given values of zero
-4.00E-05
-2.00E-05
0.00E+00
2.00E-05
4.00E-05
6.00E-05
8.00E-05
1.00E-04
-1 -0.5 0 0.5 1
Voltage (V) vs. Ag/AgCl
Cu
rren
t (A
)
Control
ODT
OPE1 1 hr
OPE1 2hr
OPE1 3hr
OPE1 6 hr
OPE1 18 hr
OPE1 42 hr
OPE1 1 hr indicative of incomplete monolayer; growth stage of pattern
OPE1 2 hr to 18 hr completely insulates the substrate; saturation part of growth pattern.
OPE1 42 hr indicates monolayer is leaky again, indicating decline part of growth pattern
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XPS Wide Scans of SAMs
C and S shows presence of monolayers
Ga and As are the substrate signatures
O indicates the presence of an oxide layer
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As 3d Composition
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As 3d Composition
% Concentration
SAM As 3d 5/2 As 3d 5/2 As-S As-O
OPE1 1hr 80.27 15.25 4.48 -
OPE1 3hr 81.92 14.13 3.95 -
OPE1 6 hr 83.13 13.70 3.57 -
OPE1 24 hr 81.49 13.95 3.25 1.31
ODT 79.68 14.80 5.52 -
Ru 77.43 12.95 2.65 6.97
MM 70.47 12.57 3.37 13.59
O signature only present in OPE1 24 hr, Ru and Mixed Monolayer sample
Optimum deposition time for OPE1 should be less than 24 hours.
Strong As-S presence in SAMs
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Ga 3d Composition
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Ga 3d Composition
% Concentration
SAM Ga 3d 5/2 Ga 3d 3/2 Ga-S Ga-O Ga-S/Ga-O
OPE1 1 hr 81.97 11.61 - - 6.42
OPE1 3 hr 82.06 8.73 7.59 1.62 -
OPE1 6 hr 84.65 11.13 - - 4.22
OPE1 24 hr 85.24 10.45 - - 4.31
ODT 86.66 10.48 2.86 - -
Ru 68.70 26.51 - - 4.79
MM 70.90 26.76 - - 2.34
GaO/GaS signature decreases overall with deposition time
OPE1 3 hr has small GaO signature; ODT has no GaO signature
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Percentage Composition of SAMs* TOA = 0 ˚
ODT most passivating SAM, Ru is least passivating
Optimal deposition time for OPE1 is around 3 hours
Mixed Monolayer shows increase in O, S and C signatures over Ru
SAM Percentage Composition (%)
O N C S As Ga
OPE1 1hr 9 - 32.3 2.1 26.8 29.8
OPE1 3 hr 7.9 - 34.1 2.3 26.9 28.8
OPE1 6 hr 9 - 33.5 1.9 26.3 29.3
OPE1 24 hr
8.2 - 34.9 2.1 26.2 28.6
ODT 3.1 - 51.5 2.2 21.1 22.1
Ru 14.1 5.7 32.2 0.87 22.13 25
Mixed Monolayer
16.2 7.8 33.9 1.2 19 21.9
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Dependence of Elemental Conc. with TOA, OPE1 and MM
OPE1 3 hr Mixed Monolayer
C increase with TOA shows upright orientation for OPE1 3 hr and Mixed monolayer
O signature is inconclusive in OPE1 but decreasing in Mixed monolayer
All other species are at substrate-molecule interface as expected
0
5
10
15
20
25
30
0.40.50.60.70.80.911.1
cos TOA
atom
ic c
once
ntra
tion
%
0
10
20
30
40
50
60
C a
tom
ic c
once
ntra
tion
%
O
S
As
Ga
C
0
5
10
15
20
25
0.40.50.60.70.80.911.1
cos TOA
ato
mic
co
nce
ntr
atio
n %
0
10
20
30
40
50
60
C a
tom
ic c
on
cen
trat
ion
%
O
N
S
As
Ga
C
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Dependence of Elemental Conc. with TOA, ODT and Ru
ODT Ru
C signature increasing with TOA shows upright orientation for ODT and Ru
N increasing with TOA in Ru, indicative of upright orientation
All other species are at substrate molecule interface
0
5
10
15
20
25
0.40.50.60.70.80.911.1
cos TOA
atom
ic c
once
ntra
tion
%
0
10
20
30
40
50
60
70
80
C a
tom
ic c
once
ntra
tion
%
O
S
As
Ga
C
0
5
10
15
20
25
30
0.40.50.60.70.80.911.1
cos TOA
Ato
mic
co
nce
ntr
atio
n %
0
10
20
30
40
50
60
C A
tom
ic c
on
cen
trat
ion
% O
N
S
As
Ga
C
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Percentage Coverage on GaAsSAM Deposition Time (hrs) Coverage (%)
ODT 8 68
Ru 24 24.8
Mixed Monolayer 32
(24 hr OPE1 + 8 hr Ru) 34.8
OPE1 1 hr 1 49.2
OPE1 3 hr 3 56.3
OPE1 6 hr 6 45.4
OPE1 24 hr 24 53.1
ODT has best coverage on GaAs substrate while Ru has least coverage
OPE1 3 hrs has optimal coverage and thus deposition time
Mixed Monolayer increases coverage from solely Ru by 10%
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Conclusion OPE1 undergoes a growth saturation and decline pattern of
SAM coverage with 3 hours being the optimal deposition time
Ru insertion into OPE1 matrix also exhibits a saturation curve, where optimal deposition time is around 6 hours
The heterogeneous SAM or mixed monolayer improves coverage by 10% over the Ru SAM
The most passivating SAM was ODT followed by OPE1 (at 3 hours), then Mixed Monolayer and lastly Ru
Funding: National Science Foundation