1
Bikas Das, Nikola Pekas, Rajesh Pillai, Bryan Szeto, Richard McCreeryUniversity of Alberta
National Institute for NanotechnologyEdmonton, Alberta, Canada
Yiliang WuXerox Research Centre of Canada
Redox-gated Molecular Memory Devices based on Dynamic Doping of Polythiophene
2
Today’s solid state memory (>$100 billion annually):
Dynamic Random AccessMemory (DRAM)
Static RAM
Flash (cameras, USB stick)
write/erase cell retention cycle life speed size
< 10 nsec 1T1C* 65 msec >1015
< 1 nsec 6T long with >1015
power on
1 µsec- 10 msec 1T > 10 yrs 103 - 105
No “universal” memory- we need different types for different needs
T = transistor, C = capacitor*
3
Cell Phones
IEEE Nanotech Magazine, December 2009
• enabled mobile electronics (cell phones, MP3 players, etc)• ~$65 billion/year (not counting disk drives)• 16 billion units (i.e. chips) sold in 2009
Nonvolatile solid state Memory Devices:
Scifinder hits: “nonvolatile memory”: 31,800 “resistance memory”: 9,900 “conductance switching”: 10,560
“Alternative” nonvolatile memory in ITRS roadmap:
Phase Change RAM
Magnetic RAM
Polymer memory
Fuse/antifuse
Molecular memory
Nanomechanical
4
ethyl viologen ClO4 + polyethylene oxide
read
write/erase
SiO2
SD
G
PQT
EVpolythiophene
PQT from Xerox Canada
S
S
S
S
C12H25
H25C12
nS
S
S
S
C12H25
H25C12
n
more to scale:
1000 nm
25 nm
D S
initial (neutral) σ ≈ 10-7 S/cm
eˉ
PQT+ “polaron” σ ≈ 10 S/cm
+
-
“Redox gated” molecular memory:
5
-2 -1 0 1 2-10
0
10
20
30
40
50
VSG (V)
PEOStart
EV(ClO4 )2 +PEO
I SG, µ
A-
G
S DPQT
+
V SG
-2 -1 0 1 2
-0.04
-0.02
0.00
0.02
0.04
I S-D
(mA)
VS-D (V)
S D -PQT+
VS-D
-2 -1 0 1 2
-0.01
0.00
0.01
0.02
I S-G
(mA)
VS-G (V)
S D
-
PQT
+
PEO
G
Substrate
VS-G
PEO or PEO/EV
NHE
PQT+/PQT
EV+2/EV+
0.0
+0.76
Eo’ , V
-0.45
VSG
S
G
VSG drives a redox reaction:
PQT + EV+2 → PQT+ + EV+
“write”
“erase”
6
-1.0 -0.5 0.0 0.5 1.0
-20
-10
0
10
20
I S
-D(n
A)
VS-D
(V)
Initial
read
write/erase
SiO2
SD
G
= -2VAfter VSG
+
-I S
D (
0.5
V)
-3 “erase” pulse
…
“ON” state
“OFF” state
+3 “write” pulse
…
1.E-10
1.E-09
1.E-08
1.E-07
1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
0 100 200 300 400 500time (s)
ON/OFF > 104
PQT
EV
= +2VAfter VSG
7
0 40 80 120 160 20010-9
10-7
10-5
10-3
I SD(0
.5V
), A
mp
s
Time(s)
+ Write
- Erase
0 2000 4000 6000 8000
10-9
10-7
10-5
10-3
I SD(0
.5V
), A
mp
s
Time(s)
100 W/R/E/R cycles
ACS Appl. Mater. Interfaces 2013, 5, 11052.
Note:
• resistance readout• low energy W/E• nondestructive “read”• should scale well• potentially longer cycle life than “flash”
8
σ ~ 10 S/cm
σ ~ 10-8 S/cm
800 1000 1200 1400 1600 1800
PQT(neutral)
Raman shift, cm-1 (vs. 780 nm)
1460 cm-1
PQT+
(polaron)
1405 cm-1
Raman permits monitoring of PQT+ formation in working memory device
Raman spectroelectrochemistry of PQT:
9
800 1000 1200 1400 1600 1800
σ ~ 10 S/cm
σ ~ 10-8 S/cmPQT
PQT+
Drain, initial
750 1000 1250 1500 1750
Raman shift (cm-1
)
Ram
an in
ten
sity
(a.
u.)
Source, VSG = +2
*
*Source, VSG = -2
Drain, VSG = +2
Drain, VSG = -2
140
5 cm
-1
146
0 cm
-1
1405 cm-11460 cm-1
S D
-
PQT
+
EV(ClO4 )2 +PEO
G
Substrate
VS-G
Raman laser
VSG pulses are “switching” polymer
between high and lowconductance states
..top view
Source, initial1460
Solid state spectroelectrochemistry:
10
VS-G
S
D
G
D
S
+
-
(a)
Initial
neutral
1600 1500 1400 1300
Drain
Source
(under)Gate
VSG = +2V
polaron
1600 1500 1400 1300
Drain
Source
(under)Gate
GateVSD = -2V
neutral
1600 1500 1400 1300
Raman shift (cm -1)
Drain
Source
(under)Gate
• polaron generation mediates conductance and “memory”
• polaron is produced in entire source/channel/drain region
J. Am. Chem Soc. 2012, 134, 14869
11
SD
VSG
-
+
NP+ P+ P+A¯ A¯ NN
P+NNNN
N
A¯ EV+ A‾EV+2
SD
VSG25 nm
1000 nm
-
+
P+P+ P+ P+
EV+ A‾A¯ A¯
Ne¯
Polythiophene is not only a redox polymer but also a conducting polymer, so electron transport “laterally” is quite efficient.
e¯A¯A¯
N N
NN
NNN
EVA2
SD
VSG
-
+
P+ P+ P+A¯ A¯
P+P+P+
P+
P+P+
A¯
A¯A¯
A¯ A¯
EV+ A‾EV+ A‾EV+ A‾
A‾ = ClO4‾
12
Read
W/E
Now, monitor S-D current during +3 V S-G “write” pulse
0 V
-3 V
+0.5 V
~1 µmPEO-viologen
PQT
SD30 nm
A closer look at dynamics, using a circuit equivalent to a bi-potentiostat:
13
I SD
(m
A)ISD (right axis)
0
1
2
3
4
I SG
(µ
A)
Time (s)
(b)
ISG (left axis)
0
1
2
3
4
0 0.5 1 1.5 2
G
SD
Read
W/E
0 V
-3 V
+0.5 V
RC “charging”
current generating conducting polaron
SD conduction(“readout”)
note 1000x“gain”
P+P+P+
• P+ generation
P+
• P+ propagation
+ + + +
• charging (RC)
- - --
Which one(s) control speed?
14
Time (s)
I SG
µ(A
)
(a)
I SG
(µ
A)
An interesting effectof atmosphere:
vacuum(right scale)
-5
-4
-3
-2
-1
0
1
2
3
4
-80
-60
-40
-20
0
20
40
60
0 2 4 6 8
ACN vapor (left scale)
air (right scale)
ISG, “write”
Time (s)
I SD
(m
A)
ACN vapor
air
vacuum
(b)
ISD, “read”
0
1
2
3
4
5
0 2 4 6 8
RC charging?ionic mobility?propagation speed?
RC (msec)
Ambient 0.18
Vacuum 0.037
ACN vapor 0.081
all < 1 msec, can’t be theproblem
G
SD
R
W/E
0 V
-3 V
+0.5 V
15
G
Read
W/E
0 V
-3 V
+0.5 V
PEO-viologen
SD
Propagation of polarons into channel:
+++++++++
Propagation is essential, could it be the slow step?
16
“Propagation” experiment:
SD
GiSG
iSD
iSG “write”
iSD “read”,10 µm gap
-10 0 10 20 30 40
propagation delay
-10 0 10 20 30 40
iSD “read”,1 µm gap
iSD “read”,10 µm gap
+3 V
17Canadian ECS- 23
18Canadian ECS- 24
19Canadian ECS- 25
20Canadian ECS- 26
21Canadian ECS- 27
22Canadian ECS- 28
23Canadian ECS- 29
24Canadian ECS- 30
25
PEO + ClO4ˉ
-----
close-up of S electrode:
-
+3 V++
• “W” pulse generates polarons
• anions move toward polarons, compensate + charge
• polarons “fill” the Source region
+++
+ + ++ --- • polarons propagate throughout
PQT layer, anions continue to migrate from PEO
++ +
+
- -
--
the bad news: iR losses in PEO layer reduce “write” current, thus requiring ~ 500 msec to “fill” source region.
the good news: there are solid electrolytes with conductivity >1000x higher than PEO-ClO4, which should reduce “write” time to 10 - 100 μsec. We can also make the electrolyte at least 10x thinner
26
Some progress:
G
SD
drop cast PEO-EV
1-2 μmG
SD
spin coated PEO-EV
~0.3 μm
PEO-EV Drop Cast
PEO-EV Spin Coated
“write” pulse
PEO-EV Drop Cast
PEO-EV Spin Coated
“write” pulse
27
0 1 2 3 4 t, msec
26 oC
57 oC
47 oC
ISG, “write”
~100 X higher“write” currentthan drop cast,ambient
acetronitrile vapour:
Note: effective “write”speed of ~2 msecinstead of ~500 msec
0 1 2 3 4 t, msec
26 oC
57 oC
47 oC
ISD, “read”
28
XRCC polymer
CMOS with support electronicsThin film transistor
substrate
Target: high “value added”to CMOS by integratingmolecular nonvolatile
memory
29
Low density prototype tester:
Top Related