Transcript photo gating - Lehigh University

Photo-Electronic Modulation of Nafion-gated Single-Walled Carbon
Nanotube FETs
Happiness Munedzimwe3 , S. V. Rotkin1,2
1Physics
Department 2Center for Advanced Materials and Nanotechnology 3Integrated Business and Engineering Department Lehigh University, Bethlehem, PA.
INTRODUCTION
Ionomers have recently begun to be seriously considered as plausible replacements for silicon back gate in SWNT FETs. Optoelectronic characterization of
SWNT on Si FETs is a powerful tool to investigate processes at the Si/SiO2 interface. Marcus et al(1) have demonstrated that for such systems with a back gate
geometry photoelectric modulation of the SWNT drain current (Id) is mostly due to charge trapping at the Si/SiO2 interface and the gating events from that
charge alteration. Our work investigates photo gating of a SWNT Film FET (a TFT topology) arrangement with Nafion-117 ionomer as a substrate and the
gate spacer and shows the predominance of direct nanotube responses in such systems as opposed to ambient events in the former. Hence we show that
ionomer gating can be used for accurate characterization of nanotube electronics and with optimization of the TFT design in practical devices
BACKGROUND
METHOD
Nanotube Geometry and Electronic Properties
Chiral vector: [2]
Ch = na1 + ma2 ≡ (n, m)
(n, m are integers, 0 ≤ |m| ≤
n)
We use a back-gated film SWNT TFT geometry with Nafion-117 ionomer substrate for
characterizing gating performance and the trapping processes at the Nafion/ CNT layer
interface.
Both electrostatic gating and photo gating are employed, with similar signal
profiles.
PREPARATION
Rule: n - m = 3i
⇒ Metallic if i is integer
⇒ Semiconductor if i is noninteger
[3]
Sonication:of
Dropper
Bucky Paper in
Deposition
DichloroMethane onto Nafion
at > 60°C
Any batch of SWNTs
contains metallic, semimetallic and semiconducting tubes because
of chiral statistics
SEM Channel Close-up:
Nanotube Forest
RESULTS
0.000032
ELECTROSTATIC GATING
[17,0]
zig-zag
0.000030
Vth = 1-3 V
Vbi = 40-70mV
Id
0.000028
P-channel behavior
1 Conducting: 2 Semiconducting
0.000026
0.000024
-
6
2.́ 10-
However, for both in-situ grown and deposited SWNT
selection, separation and reorientation are crucial for single
SWNT electronics but are quite difficult.
 Major Obstacle
 SWNT films with random tubes show p-type conductivity
due to ambient O2 and n-type conductivity with
functionalization
0
-
6
2.´ 10-
Funding Acknowledgements . This work was partially
supported by DoD-ARL (Grant W911NF-07-2-0064) under the LehighArmy Research Lab. Cooperative Agreement and by the American
Chemical Society Petroleum Research Fund (Grant 46870-G10).
- ´ 10-6
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6
3.´ 10-
2
2. ´ 10-6
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4
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2
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0
2
4
3. ´ 10-
4
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2
-
0
2
4
Vg
PHOTO GATING
NAFION 117 SUBSTRATE [4,5]
Nafion is a per fluorinated ionomer where
X- is a sulfonic or carboxylic functional group
and M+ is a metal cation (See Diagram)
 Transport via mobile cation-water clusters
 Major cation is H+
µ of Na+ in Nafion ~ 0.3e-07 m2V-1s-1 vs.
0.5e-07 m2V-1s-1 (2) in plain aqueous solution
=> low mobility
Permittivity of 4<εr< 20
6
-
10
6
1.´ 10-
0
Gating effect reduced by;
•electrostatic shielding of NT layers away from the
back gate
•mixed chirality of nanotubes
0
1.´
-
Vg
2.´ 10-6
-
Single SWNT electronics research is promising with
commercial and near commercial applications
-2
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-4
6
1.́ 10-
-Ig
MOTIVATION FOR SWNT FILM ELECTRONICS
500nm
Systematic of Final Bulk CNT FET Topology
Band profiles of nanotubes
show a direct band gap in
66% of nanotubes in a
batch.
[10,10]
arm chair
Desiccation
then Metallization
IR Illumination under a drain bias
of 10 mV
Rise time ~ 200s
Fall time ~ 100s
Peak Ig= 18 nA, Id=44 uA
Photo gating and
photocurrent generation
usually distinguished by
time scale of response
(s to min vs. fs to ns
respectively)
Charge trapping
dynamics in Nafion-117
slows down both effects
ZERO BIAS ILLUMINATION
IR illumination under Zero Bias conditions
-Peak current generated n SWNT channel = 20uA
Zero bias conditions give Nafion is also photoactive
the magnitude of the
-Peak photocurrent in Nafion substrate = 18 nA
photocurrent
-Gating effects present from generated Igs current
CONCLUSION: Photo modulating SWNT FETs based on Nafion differentiates increased
channel conductivity due to excitonic absorption and photo-generation of charge carriers from
real photo-gating which features charge redistribution in the substrate/gate interface which then
indirectly changes the channel conductivity.
CITATIONS
1. M. S. Marcus, J. M. Simmons, O. M. Castellini, R. J. Hamers, M. A. Eriksson. Photo gating Carbon Nanotube Transistors J. Appl. Phys.
100 (2006)
2. M.S. & G. Dresselhaus, P. Avouris,Carbon Nanotubes, Springer (2001)
3. Image From Royal Chemistry Society www.rsc.org/ej/CC/2001/b102348a/b102348a-f2.gif
4.Nafion < http//www.psrc.usm.edu/mauritz/nafion.html >
5 T. Okada et al. Electrochimica Acta 43, Iss. 24 (Aug. 1998) p. 3741-3747