Phase Change Materials and PCRAMPCRAM

L.P. Shi and T. C. ChongData Storage Institute

A*STAR (Agency for Science, Technology and Research)Singapore

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Outline

Phase Change Materials for PCRAM

Physical Limitation and Nano-Phase Change

PCRAM as Universal Memory

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WRITE (“1” t “0”)

Operation of PCRAMWRITE: (“1” to “0”)

mpe

ratu

re

Melting Point

Anneal

Crystalline TemperatureTem

TimeLong Pulse with Low Current

WRITE: (“0” to “1”)( )

Cr stalline Temperat repera

ture

Melting Point

Melt & Quench Crystalline Temperature

Tem

pTime

Short Pulse with High Current

READREAD

erat

ure Melting Point

ORCrystalline Temperature

Tem

p

TimeVery Low Current

OR

Chalcogenide Based Phase Change Materials

Binary Ternary QuaternaryGaSb Ge2Sb2Te5 AgInSbTe

InSb Ge1Sb2Te4 BiGeSbTe

InSe Ge1Sb4Te7 Ge41Sb12Te41Se6

As2S3 InSbTe GeSnSbTe

Sb2Te3 SnSb2Te4 GeSbSeTe

Chalcogenide elements

GeTe GaSeTe Te81Ge15Sb2S2

GeSb Si2Sb2Te5 Ge2Sb2Te5:N Ge

(Low St M t i l M t i l P t

GeTe

crystallization speed, high

stability)

Storage Requirement

Materials Requirement

Material Property

Writability Easy amorphousing Melting point / layer design

Stability Stable amorphous High activation energy

Ge1Sb2Te4

Ge1Sb4Te7

Ge2Sb2Te5

phase

Readability Large s/n ratio High optical constant / high electrical resistance

Erasibility Fast Simple crystalline phase, low

Te SbSb2Te3(High crystallization speed, low stability)

as b y asrecrystallization

S p e c ys a e p ase, oviscosity

Cyclability Stable layer stack Low stresses

U tifi i l t t t i h h t i l ’ ti

Superlattice-like (SLL) Phase Change StructuresUse artificial structures to engineer phase change materials’ properties

Top electrode

Top ElectrodePC1 PC2PC1

Phase change layer

Bottom Electrode

SLL Phase Change Structures

PC1PC2

PC1 PC2

Theoretical consideration: 3.5

Substrate Bottom ElectrodeSubstrate

• Material consideration• Crystallization consideration • Thermal consideration• Electrical consideration 2

2.5

3

nt (m

A)

Bulk GST225Pair SLLSandwiched SLL

1 T C Chong etal “Phase

• Reduce 70% Current

• Increase Speed

PC1, e.g. GeTe

PC2 eg. Sb2Te3

0.5

1

1.5

Cur

ren 1.T.C. Chong, etal. Phase

change random access memory cell with superlatticelike structure”, Applied Physics Letters, Vol. 88, No.12, (2006).

2.T.C. Chong, etal, “Crystalline

PC2

00 10 20 30 40 50

Pulse Width (ns)

g yAmorphous Semiconductor Superlattice” Phys Rev Lett, 100, (2008)

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Scalability of PCRAM

Scalability of PCRAM - the smallest volume of phase change materials that undergoes stable and reversible phase change

Nano-Phase Change - phase change behavior at the nano-scale is different from bulk due to large volume of interfaces: Thickness (size) dependent Interface dominated

Thermal diffusion Interface dominated Capping materials related

Important Factors: R i f h

Melting point

Mass density

Ratio of the atoms on interface/atoms in volume

Mean free pathElectrical

Activation energy Stress

Crystallization resistivity

Band gap

Crystallization temperature

Refractive index

L.P. Shi, T.C. Chong, “Nano-phase change for data storage” J. nanoscience & nanotechnology, 7, 65-93 (2007).

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Physical Limitation vs. Nano-Phase ChangeH ll? Operation WindowPhase ChangeHow small?

lithography

Electrode

How small? Reversible Phase Change

Operation WindowPhase Change

1.E+06

1.E+07

1.E+08

m)

2 nm

500

600

700

re (C

)

Phase Change

ElectrodeInsulator

V1.E+03

1.E+04

1.E+05

Res

ista

nce

(oh

30nm

20nm

10nm

8nm

5nm

3nm

2nm

100

200

300

400

Te

mp

era

tur

Melting point

Crystalization point

effect of diffusion protective layer1 E 08

V

How small? Current

How small? GeTe film, 2 nm GeTe film, 20 nm

Density Diffusion

1.E+02100 120 140 160 180 200 220 240

Temperature (°C)

2nm

00 10 20 30 40 50 60

Thickness (nm)

1.E+05

1.E+06

1.E+07

1.E+08

ohm

)

with GeNx Without GeNx

doping

dopingBit line

Cross-talk

104

105

106

107

108

109

R, a

.u.

109

c

As-grown, ~4.6 g/cm3

250 oC, ~4.8 g/cm3

350 oC, ~4.8 g/cm3

The critical angles c shift after annealed.

103

104

105

106

107

108

109

R, a

.u.

As-grown, ~5.7 g/cm3

250 oC 350 oC, layer thickness decreased

as the peaks shifted to higher angle.4.6 g/cm3 @ 2 nm 5.7 g/cm3 @ 20 nm

1.E+02

1.E+03

1.E+04

100 150 200 250 300 350 400

R (o

Wor

d lin

e

Limitation of PCRAM:

2 4 6

102

103

, deg.

0.18 0.20 0.22

0 2 4

101

102

, deg.

100 150 200 250 300 350 400T(C)

Limitation of PCRAM:~ 5 nm

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PCRAM as Universal Memory

N l tilHigh density ~ 5.7 F2

Non-volatileg y

High speedUniversal Memory

High > 1012

?(current: ~100 ns)

gendurance

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High Speed PCRAMCorrelation between Phase Change

Speed and Size

Amorphous

High Speed PCRAM with 108 cycles

100

1000

ance (kΩ

)

Crystalline

10

Resista

1

1E+03 1E+04 1E+05 1E+06 1E+07 1E+08 1E+09

Number of overwriting cycles

Reset pulse width: 6 nsSet pulse width: 9 ns

W. J. Wang, etal., “Fast phase transitions induced by picosecond electrical pulses on phase change memory cells”, APL, 93, (2008).

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Thank You Thank You

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Nano-scaling Effects on Small Cells

Smaller dimension change the boundary conditions, the material properties due to the nano‐size effect and interface effect, resulting in the changes of the electrical performance

Nano phaseNano phase change

Phase change temperature

Cell structure

Boundary conditions are changed

Melting point

Crystallization stress

Boundary condition

Thermal confinement

Free carrier concentration

Carrier diff i

confinement

New switching mechanisms

diffusion

Phonon diffusion

Switching power Switching speed Life time retention

Bit line 1 Bit line 2 Bit line 3

Bit linePCRAM Introduction

Ch l id

Electrodeword line 1

word line 2

Chalcogenide

Insulator

Wor

d lin

e

word line 3

Electrode

W

PCRAM IntroductionElectrode

Ch l id

Anneal

InsulatorAmorphousCrystalline

Chalcogenide Melt & Quench

ElectrodeAnnealing

yer

e

digital dataRec

ordi

ng L

ayTe

mpe

ratu

re

Melting PointCrystalline

Temperature

V

digital data0 1

At t lli t t

Crystallization time Time

At h t t V• At crystalline state• High free electron density• Low resistance

• At amorphous state• Low free electron density• High resistance

PCRAM IntroductionElectrode

Ch l id

Anneal

InsulatorAmorphousCrystalline

Chalcogenide Melt & Quench

Electrode

yer

e

Melting & Rapid Cooling Annealing

digital dataRec

ordi

ng L

ayTe

mpe

ratu

re

Melting PointCrystalline

Temperature

V

digital data1 0

Crystallization time Time

At t lli t tTh i diff i l V• At crystalline state• High free electron density• Low resistance

• The resistance difference is several orders. Cell read by measuring resistance

High Scalability

Electrode

Phase Change

Electrode

Ch l id

10

A)

Phase Change

Electrode

Insulator

InsulatorAmorphous

Chalcogenide

1

et c

urre

nt (m

AElectrode

0.110 100 1000

Res

e

10 100 1000Contact diameter (nm)

V

V

• Better performance with smaller size: lower current lower power consumption and faster speedV• Better performance with smaller size: lower current, lower power consumption and faster speed.

• Flat topology and lower voltage operation allow to remove scaling barriers

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