Electronic Transport
in Molecular Scale Devices
Takhee Lee,
Tae-Wook Kim, Hyunwook Song, Gunuk Wang
Department of Materials Science and Engineering
Gwangju Institute of Science and Technology (GIST), Korea
1
The 3rd Korea-US Nano Forum, Seoul, April 3-4, 2006
Molecular NanoE
anoElectronics
Laboratory
I. Electronic Transport of Micro & Nano
Via-hole Structures for Molecular
Electronic Devices
Tae-Wook Kim, Gunuk Wang, Hyunwook Song
2
The 3rd Korea-US Nano Forum, Seoul, April 3-4, 2006
Molecular NanoE
anoElectronics
Laboratory
Formation of Alkanethiol Self-Assembled Monolayers
Standard alkanethiol molecule
• Octanethiol (C8)
- CH3(CH2)7SH / length = 13.3Å
• Dodecanethiol (C12)
- CH3(CH2)11SH / length = 18.2Å
C8
C12
C16
S
C
H
• Hexadecanethiol (C16) - CH3(CH2)15SH / length = 23.2Å
Conjugated molecule: Oligo(phenylene ethynylene) (OPE)
Self-assembled monolayer (SAM)
OPE provided by
Dr. Changjin Lee
(KRICT)
• A spontaneous chemisorption process
RS-H + Au → RS-Au + 0.5 H2
• Chemical bond formation of functional end groups of molecules with the
substrate surface
• Intermolecular interactions between the backbones
Reed & Tour, Am. Sci. June 2000
3
The 3rd Korea-US Nano Forum, Seoul, April 3-4, 2006
Molecular NanoE
anoElectronics
Laboratory
Fabrication of Planar-type Via-Hole Molecular Devices
Design and Fabrication of molecular electronic devices
Mask pattern
Unit
Die
Devices
Optical lithography: ~ 2 µm
Plan: E-beam lithography: ~ 50 nm
Schematic of device
Au
SiO
Ti/Au
Silicon wafer
190 µm
2.5 mm
1 cm
One unit Contain 16 dies
Each die contains 20 devices
Total devices are 320 in a unit
SEM / AFM / Optical images
2 µm
4” wafer
(Collaboration with
ETRI, Dr. Hyoyoung
Lee)
1 cm piece
Die
Devices
AFM image
of a device
TW Kim, HW Song, GW Wang
After Top electrode
4
The 3rd Korea-US Nano Forum, Seoul, April 3-4, 2006
Molecular NanoE
anoElectronics
Laboratory
Yield of Microscale Via-Hole Molecular Devices
-2
10
1. Yield of molecular devices ?
2. How to define working molecular devices ?
-3
10
C8
Device yield (micro-via hole molecular devices)
# of
fabricated
device
Fab.
failure
Working
short
13440
392
11744
1103
100%
2.9%
87.4%
8.2%
Open
C8
C12
C16
84
57
60
Current [ A ]
-4
10
-5
10
C12
-6
10
C16
-7
10
201
1.5%
-8
10
Simmons tunneling fitting results for all working devices
-1.0
-0.5
0.0
0.5
1.0
Voltage [ V ]
Alkanethiol J at 1 V (A/cm2)
C8
78000±46000
C12
2000±400
C16
5.2±4.7
ФB (eV)
1.29±0.49
1.26±0.08
2.67±0.28
α
0.76±0.09
0.72±0.04
0.52±0.04
β (Å-1)
0.87±0.16
0.83±0.04
0.87±0.05
Representative devices
determined by
Gaussian fitting of J !
5
The 3rd Korea-US Nano Forum, Seoul, April 3-4, 2006
Molecular NanoE
anoElectronics
Laboratory
Determination of Working Molecular Devices
Log (J) for C8
Log (J) for C12
Log (J) for C16
16
<x> = 4.92
σ = 0.30
28
3σ
24
20
16
12
10
8
6
4
4
2
-2
-1
0
1
2
3
4
5
6
7
0
2.0
8
2.5
3.0
3.5
4.0
4.5
y = f (x)
8
6
4
5.0
0
2
-1
0
1
2
3
Log10 (Current Density @ 1 V (A/cm2))
35
( x −< x > ) 2
30
2σ 2
# of Device
1
f ( x) =
⋅e
2πσ
10
Summary of Log(J)
Gaussian fitting ( 3σ: 99.7 % )
−
12
2
Log10 (Current Density @ 1 V (A/cm ))
Log10 (Current Density @ 1 V (A/cm ))
2
<x> = 0.56
σ = 1.21
14
12
8
0
<x> = 3.06
σ = 0.35
14
# of Device
# of Device
16
# of Device
32
C8
C12
C16
C8
25
20
15
C12
C16
10
5
m − 3σ
m
99.7%
m + 3σ
0
-1
0
1
2
3
4
5
6
Log10 (Current Density @ 1 V (A/cm ))
2
The 3rd Korea-US Nano Forum, Seoul, April 3-4, 2006
6
Molecular NanoE
anoElectronics
Laboratory
II. Molecular Electronic Junctions
Studied by Conducting Atomic Force
Microscopy
Hyunwook Song
7
The 3rd Korea-US Nano Forum, Seoul, April 3-4, 2006
Molecular NanoE
anoElectronics
Laboratory
Charge Transport Study by CAFM
Au(20nm)/Cr(20nm)
coated AFM Probe
I
HP 4145B
Au
Physical
contact area
probe
C8
OPE
Chemisorbed
Au
PSIA, XE-100 model
ambient AFM
The 3rd Korea-US Nano Forum, Seoul, April 3-4, 2006
8
Molecular NanoE
anoElectronics
Laboratory
Force-and Length-Dependent Tunneling
J @ 1 V (1 to 30 nN)
I (nA)
1
0.1
0.01
1 nN
5 nN
10 nN
20 nN
30 nN
1E-3
1E-4
-1.0
-0.5
0.0
0.5
1.0
Current density (A/cm2)
I(V) for C12 (1 to 30 nN)
10
5
10
4
10
3
10
2
10
1
10
0
10
-1
12
C12
C16
C8
2 nN
5 nN
10 nN
15 nN
20 nN
25 nN
30 nN
14
1.11×10
8
8
1.37×10
8
1.45×10
8
1.30×10
7
8.03×10
7.63×10
7
1.25×10
8
16
18
20
22
24
Length (Å)
V (V)
• Larger current with larger loading force
• J: C8 > C12 > C16
• Error bar: sample-to-sample variation
30 nN
25 nN
20 nN
15 nN
10 nN
5 nN
2 nN
1 nN
Loading force Jo (A/cm2)
8
1 nN
1.25×10
Exponential dependence
R ∝ exp (βd)
9
The 3rd Korea-US Nano Forum, Seoul, April 3-4, 2006
Molecular NanoE
anoElectronics
Laboratory
Through-bond vs. Through-space
Tunneling distance in “through space” = d – dcctanθ + dcc
dcc
dcctanθ
θ
CAFM Tip
(dt- δ)
Au
d
JACS, 119, 11910 (1997)
I t = I 0 exp(− β tb LM ) +
I0
LM cosθ
dcc
∑
N =1
ns !
exp( − β tb ( LM − Nd cc tan θ )) × exp(− β ts Nd cc )
(ns − N )! N !
J. Phys. Chem. B, 106, 7469 (2002)
ƒ The dominant transport mechanism of alkanethiol SAM: “through-bond” tunneling
ƒ When tilted considerably, chain-to-chain coupling: “through-space tunneling” appears
10
The 3rd Korea-US Nano Forum, Seoul, April 3-4, 2006
Molecular NanoE
anoElectronics
Laboratory
C8: Thru-bond + Thru-space (Single hopping: N = 1)
-6
-8
70
Tilt angle (degree)
ln (J/Jo)
-4
45
94
1.2
1.0
60
0.8
50
0
-10
Net force (nN)
62
78
10
20
30
Applied loading force (nN)
0.6
Separation between contacts (nm)
-2
N=2
N=1
Experiment
-12
-14
20
C8
30
40
50
60
70
Tilt angle θ (degree)
N~1 (N ≤ LMcos θ/dcc)
11
The 3rd Korea-US Nano Forum, Seoul, April 3-4, 2006
Molecular NanoE
anoElectronics
Laboratory
C12: Thru-bond + Thru-space (Double hopping: N = 2)
-4
-10
-12
Tilt angle (degree)
ln (J/Jo)
-8
60
Net force (nN)
83
66
99
1.6
1.4
50
40
0
1.2
10
20
30
Applied loading force (nN)
N=2
Separation between contacts (nm)
-6
50
N=1
-14
-16
-18
20
30
40
50
60
70
Tilt angle θ (degree)
N~2 (N ≤ LMcos θ/dcc)
12
The 3rd Korea-US Nano Forum, Seoul, April 3-4, 2006
Molecular NanoE
anoElectronics
Laboratory
C16: Thru-bond + Thru-space (Triple hopping: N = 3)
-4
-12
Tilt angle (degree)
ln (J/Jo)
-8
55
2.1
2.0
1.9
50
1.8
45
0
-16
105
1.7
10
20
30
Applied loading force (nN)
1.6
N=3
Separation between contacts (nm)
Net force (nN)
73
89
56
N=2
N=1
-20
C16
-24
20
30
40
50
60
70
Tilt angle θ (degree)
N~3 (N ≤ LMcos θ/dcc)
13
The 3rd Korea-US Nano Forum, Seoul, April 3-4, 2006
Molecular NanoE
anoElectronics
Laboratory
Conclusions
• Electronic transport study by CAFM
- Tip-loading force effects on molecular structural properties
- Thru-bond vs. thru-space transport
• Fabrication and characterization of micro-via hole molecular
devices
- Detail yield study ~ 1.5 % (out of > 13,000 devices measured)
• Electronic properties of various nanoscale building blocks
14
The 3rd Korea-US Nano Forum, Seoul, April 3-4, 2006
Molecular NanoE
anoElectronics
Laboratory