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Metal-Molecule-GaAs Devices Using Redox-Active Self Assembled
Monolayers
2007 ISDRS
aRand Jean, bBin Xi, bTong Ren and aDavid B. Janes
aSchool of Electrical & Computer Engineering, Purdue University
b Department of Chemistry, Purdue University
2007 ISDRS
Intro. and Summary
Motivation for Metal/Redox-active molecule /Semiconductor Devices
Choice of Molecules
IV Characteristics and Proposed Model
Device Fabrication
Conclusion
1
Modeling Parameters
2007 ISDRS
Motivation for Redox-Active Structures
2
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
*
2007 ISDRS
Device Fabrication
3
1. GaAs with ohmic back contact
2. SiO2 deposition and patterning
3. Molecular deposition
4. Au deposition and patterning
Final measured structures
Low energy, non destructive deposition of Au
Au Deposition Comparison
2007 ISDRS4
• As metal penetration increases.• Conductivity in molecular
samples decreases.
m/z, positive ions230.80 230.85 230.90 230.95 231.00 231.05 231.10
inte
nsi
ty (a
.u.)
0
1
2
3
4
5
77K 300 K
Ar backfill 77K 77K 300 K 300 K
Ar backfill Ar backfill
Au/ODT/GaAs
AuSH2+
PeakAuSH2
+
Peak
AsSC9H16+
PeakAsSC9H16
+
Peak
300 K
77 K
Ar backfill
•R. M. Metzger, T. Xu, and I. R. Peterson, J. Phys. Chem. B 105, 7280 (2001).
2007 ISDRS
Molecules Deposited
5
H
1. 2.3.
Diruthenium (III) tetra-2-anilinopyridinate-2-(Trimethylsilyl)ethyl-4-(ethynyl)phenyl Sulfide (Ru-complex)
16 Å Coverage: 5e11 cm-2 Dissolved in tetrahydrofuran (THF) (0.5mM)
2-(Trimethylsilyl)ethyl-4-phenyl Sulfide (Ligand)
14Å
Dissolved in THF (0.5mM)
SS SS SS S SSS
Octadecanethiol (ODT)
23 Å
Coverage: 4.5e14 cm-3
Dissolved in ethanol (0.5mM)
2007 ISDRS
Molecules Deposited Cont’d
6
4.
STM of Mixed Monolayer of Ru complex in C11 alkanethiol matrix
Amy Szuchmacher Blum, Tong Ren, et. al., JACS VOL. 127, NO. 28, 2005
Mixed Monolayer of Ru complex and Ligand
• Ligand used to prop-up the larger bulkier Ru complex.
• Reasonable packing density.
2007 ISDRS7
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
0.4
2.0
5.0
4.2
2.6
CV Analysis
EHOMO
ELUMO(1)
ELUMO(2)
Emol
Au EF
-2.5-2-1.5-1-0.500.51
-1/-2
+1/00/-1
E(V), vs Ag/AgCl
Ren et. al. J Orgn. Chem. 690, 4734, 2005.
Energies of Molecular Levels (C-V)
Cyclic voltammogram of Ru molecule
2007 ISDRS
I-V Characteristics
8
General increase in current in molecular devices over metal- semiconductor device (control)
Highest current density from loosely packed redox-active Ru complex.
2007 ISDRS
Proposed Model
9
Metal Semiconductor Metal SemiconductorMolecule
TFE
TE
TFE
ODT
Ligand
Ru
Ev
Ef
Ec
Thermionic Emission
Thermionic Field Emission
φ1
φ2
Φ1 > Φ2
2007 ISDRS10
Thermionic &Thermionic-Field Emission comparison
2007 ISDRS
Thermionic &Thermionic-Field Emission comparison Cont’d
11
Thermionic &Thermionic-Field Emission comparison Cont’d
2007 ISDRS12
Modeling Parameters
2007 ISDRS13
0.759
0.755
0.635
0.490
0.794
Barrier Height
(eV)
0.9441.82ODT
1.721.86Mixed Monolayer
0.03343.36Ligand
6.933.01Ru complex
0.03742.05Control
Saturation Current Density (A/cm)
nMol. Layer
Decrease of barrier height with introduction of molecular layer
High values of ideality factor; most prominent in loosely and moderately packed Ru complex and Ligand
2007 ISDRS
Conclusion
14
Increase in Current Density in Metal-Mol-Semi conductor structures over Metal-Semiconductor structures. I-V relationship can be modeled on TE and TFE equations. Possible Mechanisms:
ODT: modest barrier height, low D(Ef)
Ru: low barrier height, large D(Ef)
Acknowledgements:
Collaborators: Jiewen Ying, Bin Xi, Adina Scott and Patrick Carpenter
Funding: NASA INAC and NSF NIRT
