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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

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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*

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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

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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:

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-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”

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-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

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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”

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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:

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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:

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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

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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‾

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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:

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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?

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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

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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?

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“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

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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

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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

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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”

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XRCC polymer

CMOS with support electronicsThin film transistor

substrate

Target: high “value added”to CMOS by integratingmolecular nonvolatile

memory

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Low density prototype tester: