Alexei O. OrlovDepartment of Electrical Engineering

University of Notre Dame, IN, USA

Temperature dependence of

locked mode in a Single-Electron Latch

Notre Dame research team

• Experiment: – Dr. Ravi Kummamuru– Prof. Greg Snider– Prof. Gary Bernstein

• Theory– Mo Liu – Prof. Craig Lent

• Supported by DARPA, NSF, ONR, and W. Keck Foundation

Outline of presentation

Introduction Power Gain in nanodevices Clocked single-electron devices

Bistability for memoryExperiment and simulations

Temperature dependence of bistability and hysteresis loop size

Summary and conclusions

Problems shrinking the current-switch

Electromechanicalrelay

Vacuum tubes Solid-state transistors CMOS IC

New idea

Valve shrinks also – hard to get good on/off

Current becomes small -

resistance becomes high Hard to turn next switchCharge becomes quantized

Power dissipation threatens to melt the chip!

Quantum Dots

How to make a power amplifier using quantum wells?

0 1

0

en

erg

y

xClock

Small Input Applied

Clock Applied

Input Removed

but Information is preserved!

0

Keyes and Landauer, IBM Journal of Res. Dev. 14, 152, 1970

Quantum-dot Cellular Automata

A cell with 4 dots

Tunneling between dots

Polarization P = +1Bit value “1”

2 extra electrons

Polarization P = -1Bit value “0”

Neighboring cells tend to align.Coulomb coupling

Current switch Charge configuration

Old Paradigm New Paradigm

Clocking for single-electron logic:Quantum-dot Cellular Automata and

Parametrons

Clocked QCA : Lent et al., Physics and Computation Conference, Nov. 1994

Parametron: Likharev and Korotkov, Science 273, 763, 1996

Metallic or molecular dots (parametron): Clocking achieved by modulating energy of third state directly

P= +1 P= –1 Null State

Semiconductor dots (QCA):Clocking achieved by modulating barriers between dots

NanoDevices Group

1st evaporation2nd evaporation

Resulting Pattern

Oxidation

Metal “dot” fabrication process

• Aluminum Tunnel junction technology combining E beam lithography with a suspended mask technique and double angle evaporation

• Oxide layer between two layers of Aluminum forms tunnel junctions.

Ultra-sensitive electrometers for QCA

Sub-electron charge detection is needed Single-electron transistors are the best choice

SET electrometers can detect «1% of elementary charge.

GD GE

VG VE

dot electrometer

Single-Electron Latch: a Building Block Layout And Measurement Setup

+VIN

+VIN

+VIN

~

A

Vg

SEM Micrograph of SE latch

MTJ

MTJ D3

D1

D2

+VIN

+VIN

+VIN

1m

Electrometer

MTJ=multiple tunnel junction

The third, middle dot acts as an adjustable barrier for tunneling

(0,0,0) neutral

Animated three-dot SE latch operation

+

(0,0,0) (0,-1,1) switch to “1”

-VCLK-VIN +VINVCLK=0

(0,-1,1) storage of “1” (0,0,0) (0,-1,1) back to neutral

D1 D3

D2

-VIN=0 +VIN=0

Clock signal >> Input signal Clock supplies energy, input defines direction of switching Three states of SE latch: “0” , “1” and “neutral”

Bit can be detected

Experiment: Single-Electron Latch in Action

Weak input signal sets the direction of switching Clock drives the switching

Bistable Switch + Inverter demonstrated Memory Function demonstrated

D1

D2

D3

E1

+VIN

-VIN

VCLK

Latch

SET electrometer

-6

-3

0

-0.5

0.0

0.5

VC

LK (

mV

)V

IN

+ (m

V)

0 2 4 6 8 10

-0.2

0.0

0.2

VD

1 (m

V)

Time (sec)

Switch to “1”

Hold “1”

Switch to “neutral”

Switch to “0”

Hold “0”

Switch to “neutral”

Input

Clock“High

”

T=100 mK

How temperature affects bistability?

-5 0

-0.5

0.0

0.5

Vdo

t (a.

u.)

VIN

(mV)

region suitable for latch operationBinary “1”

Binary “0”

• 2 level switch with memory = there must be a Hysteresis

• SEL operates fine @ T=100 mK

• Charging energy consideration EC≥10 kT , EC =0.8 meV (9.3 K)

• What is the highest operating temperature?

• Zero K calculations were performed before – Korotkov et al. (1998)– Toth et al. (1999)

-0.5

0.0

0.5

Sweeping input bias

0 0 0

EC

-eV0

eV

EC

EC

EC

-eV

0

eV

EC

EC-eV

0

eV

EC

EC

-e2Cin 0 e2Cin

EC

2.5 50VIN(mV)

VD1(mV)Equilibrium Border

VCLK=0VIN- VIN

+

D1

EC

-000

EC

-eV0

eV

EC

EC

EC

-eV

0

eV

EC

EC-eV

0

eV

EC

EC

-e2Cin 0 e2Cin

EC

Assume Coulomb barrier is the same for hops between adjacent dots

How bistabile behavior scales with temperature?

• Thermal energy surmounts Coulomb barrier

• Hysteresis loop shrinks and then disappears

EC

e×(-V)

0

e×(+V)

EC

kT

kT

kT

-0.5

0.0

0.5

-0.5

0.0

0.5

-6 -4 -2 0 2 4 6-0.5

0.0

0.5

VD

OT (

mV

)V

DO

T (m

V)

VIN

(mV)

VD

OT (

mV

)

Hysteresis loop change with Temperature

T=90 mK

T=160 mK

T=320 mK

Calculations performed using time dependent master equation

for orthodox theory of Coulomb blockade

Bistability area vs kT• Relative loop size V/V0

• Calculations represent ensemble averaging = averaging over multiple scans

• At T >300 mK no bistability is observed

• Bistability disappears for kT~W/30, where W is Coulomb barrier

• At T=>0 (V/V0 )>1, it means that system becomes multistable

0 100 200 3000.0

0.5

1.0

V/V

0

T (mK)

-5 0 5

0.0

0.5

Vdo

t (a.

u.)

VIN

(mV)

V0

V

Summary & Conclusions Temperature dependence of bistable switching in

Single-Electron Latch is studied experimentally Theoretical calculations using time-dependent

master equation are performed Hysteresis loop size vs temperature is studied Bistability disappears as kT reaches EC/30 For 300K operation W~30 kT≈1 eV The real world applications can be implemented

using “molecular assembly line” once technology becomes available

Vc

Metal-dot Single-Electron Latch Molecular Single-Electron Latch

Measured and calculated charging diagrams

• Charging diagram is a 3D plot (gray scale map) of dot potential vs input and clock bias

• White is positive, black is negative

• Calculated data are superimposed with measured

-4 -2 0 2 4

(1,0,-1)

(-1,1,0)(0,1,-1)

(0,-1,1)(1,-1,0)

(0,0,0)

VIN

(mV)

-4 -2 0 2 4-8

-4

0

4

8

V IN (m V)V IN ( m V )

V IN ( m V ) V IN (m V )

VC

(m

V)

VC

(m

V)

VC

(m

V)

VC

(m

V)

B A

DC

[0,0,0]

[0,-1,1] [1,-1,0]

(a ) ( b)

(c) ( d)

P Q P Q

-0.2

0.0

0.2

0 1 2 3 4 5 6 7

-0.2

0.0

0.2

VD

1 (m

V)

VD

3 (m

V)

Time (sec)

-6

0

VC

LO

CK (m

V)

-1.0-0.50.00.51.0

VIN (

mV

)

Single-Electron Latch in Action

• Two electrometers are used

• Both are connected to end dots