Amorphous solid Devices

Nano & Flexible

Device Materials Lab.

Amorphous materials for electronic devices

DevicesAmorphous solid

• Flexible & Transparent applications

• Low temperature process- no need of crystallization Polymeric glass

• Bio-applications• High thermodynamic solubility- no crystalline lattice to break Polymeric glass

• Large-area applications• Large area uniformity- no grain boundary

Oxide glass / Ionic glass

• Phase change memory (PcRAM), Optical storage

• Electrical/optical contrast with crystalline Chalcogenide glass

Phase Transformation In Materials

Nano & Flexible


Amorphous semiconductor for electronics

Storage Devices

- Contrast of resistance between amorphous and crystalline

Amorphous Ge-Sb-Te (α-GST)…

• Phase-change Random Access Memories (PcRAM)

Issues: Reliability of amorphous electronic devices due to

structural instability of amorphous semiconductors.

Large-area Electronics

- Low temperature process and large-area uniformity (no GB)

Amorphous Si, Amorphous In-Ga-Zn-O (α-IGZO)…

• Displays

• Solar cells

Nano & Flexible


Amorphous In-Ga-Zn-O (a-IGZO) TFT

High carrier mobility (5 ~ 50 cm2/V∙s)

Transparency (wide bandgap)

Low temperature process

H. Hosono, Handbook of Transparent Conductors (2010)

High structural stability of a-IGZO

Large size and charge mismatch between

the ions leads to inherent high structural

stability of amorphous materials.

A-IGZO: Ionic bonding system

T. Kamiya et al., NPG Asia Mater. (2010)

Nano & Flexible

Device Materials Lab.4

Operation Principle of PcRAM

Phase-Change Random Access Memory

• Resistance difference between

crystalline and amorphous phase

• Phase change by joule heating.

BEC

Top Electrode

Dielectric

GST

Heater

Top Electrode

Contact

Bottom Electrode

Contact

V+

Reset Pulse

(> Tm)

Set Pulse

( > Tc)

Po

we

r

Time

Po

we

r

Time

Joule Heating

Using

electrical current

(106 ~ 107 A/cm2 )

Set

Low Resistance

(Ordered)

Crystalline

High Resistance

(Disordered)

Reset

Amorphous

Nano & Flexible


Chalcogenide Materials

Chalcogenides: alloys with at least one Group VI element

PcRAM uses a Ge-Sb-Te alloy as phase change material.

Can exist in either of two stable states.

Ge2Sb2Te5 is most widely used due to its nature of phase stability

and rapid crystallization.

Also used in CD-RW and DVD-RW applications .

GeTeSb2Te3

Pseudo Binary GeTe-Sb2Te3

Ge

2S

b2Te

5

Abrikosov et al.

Periodic Table

Te

Sb Ge

Ge2Sb2Te5

Nano & Flexible


Endurance issues in PRAM

Reset

Set

High electrical current(~ 107 A/cm2 )

Kim et al., IRPS (2005)

Reset-stuck

Set-stuck

Material degradation in phase change volume → Operational failure

Crystalline

Amorphous

Cu

rre

nt

Time

Cu

rre

nt

Time

melting

melting

Reset-Stuck: fixed at high R

- Void formation

at the BEC interface

- Interfacial Delamination

Compositional change

of phase change material

during operations

Set-Stuck: fixed at low R

Nano & Flexible


Composition change in PcRAM during operation

S. O. Ryu et al. (ETRI)

e-

J. Electrochem. Soc., (2006)

Sb-rich

J. B. Park et al. (Samsung) S. M. Yoon et al. (ETRI)

J. Electrochem. Soc., (2007)

• Nonstoichiometric phase in the

active regions

• Thermal interdiffusion of Sb and

Te

• Sb-rich / Te-deficient composition • Sb-excess GST

• Ge segregated to the

boundary region where it

then was oxidized

• Sb-rich / Ge-deficient GST

in the active region

Sb-rich / Te-deficient Sb-excess Sb-rich / Ge-deficient

Appl. Surf. Sci., (2007)

Stoichiometric change of multi-element alloy is induced by

flux difference in electromigration

Nano & Flexible


PcRAM characteristics: SET speed vs. Retention

High resistance“1”

Amorphous state

Low resistance“0”

Crystalline state

Crystallization

SET

RESET

- Prediction of behaviors in a wide range of time (10-9 to 109 sec.).- Contradictory requirement for crystallization kinetics.

• Issues

Crystalline

SET speed

Retention loss

ns μs days yrsms sec ~ hrs decades

Switching regime

Reliability regime

Amorphous

10-9

10-6

10-3

100

103

106

109

400

600

800

Tem

per

atu

re (

K)

Time (sec)

D. Lencer et al., Advanced

Materials, 23, 2030 (2011).

Nano & Flexible


Amorphous materials vs. device characteristics

Tm

Tm

Temperature

Sp

ec

ific

vo

lum

e, E

ne

rgy

Crystalline

Temperature

Sp

ec

ific

vo

lum

e, E

ne

rgy

Amorphous

Temperature

Vis

co

sit

yTg Tm

Tg

Temperature

Vis

co

sit

y

Tm

Tm

→ Retention

→ SET speed

Amorphous phase stability at low temperature

• Crystallization temperature (Tx)

• Fragility (m)

Low viscosity at high temperature

• Super-cooled liquid region (Tx - Tg)

Tx

Tx

Super-

cooled liquid

Nano & Flexible


Volume Change during the Heating

Temperature

Specific Volume

1. Glass transition: Volume expansion (Compressive stress side)

2. Crystallization: Volume shrinkage (Tensile stress side)

Liquid

Undercooled liquid

Glass

Crystal

Tg

TmOver heated

Tc

21

Set-state

Reset-state

Phase change stress

Nano & Flexible


Supercooled Liquid Region

-150

-100

-50

0

50

TcN 10 at.%

Str

ess

Ch

an

ge

(M

Pa)

Tg

Tc

-150

-100

-50

0

50

Un-doped

Tc

0 50 100 150 200 250 300 350-200

-150

-100

-50

0

50

N 15 at.%

Temperature (Degrees Celcius)

Tc

Tg

Difference between Tg and Tc:

Supercooled liquid region (ΔT)

0 5 10 15

0

20

40

60

80

100

120

T

(D

eg

ree

s C

elc

ius)

Nitrogen Contents (at.%)

N-doping increase supercooled region

=> N-doped GST has superior glass-

forming ability (GFA)

Nano & Flexible


Structure-properties map for PcRAM characteristics

0 100 200 300 400 500 600

50

100

150

200

250

10

yr

data

rete

nti

on

(oC

)

SET time (ns)

Al-doping

C-doping

N-doping

Pure

Bi-doping

SET speed – Super-cooled liquid region (Tx - Tg)

Data retention - Crystallization temperature (Tx)

0 30 60 9090

120

150

180

210

240

270

300

Cry

sta

llizatio

n tem

pera

ture

, T

x (

oC

)

Super-cooled liquid region, Tx - T

g

N 5.3

N 10

N 1.8

0 30 60 9090

120

150

180

210

240

270

300

Cry

sta

lliza

tio

n te

mp

era

ture

, T

x (

oC

)


g

Al 2.3Al 4.4Al 8.5

Al 1.5

0 30 60 9090

120

150

180

210

240

270

300

Cry

sta

lliza

tio

n te

mp

era

ture

, T

x (

oC

)


g

Bi 3.7Bi 6.6

Bi 1.8

Bi 9.6

0 30 60 9090

120

150

180

210

240

270

300

Cry

sta

llizatio

n tem

pera

ture

, T

x (

oC

)


g

C 1.3

C 4.3

C 7.0

C 10

0 30 60 9090

120

150

180

210

240

270

300

Cry

sta

llizatio

n tem

pera

ture

, T

x (

oC

)


g

Pure

Fast transformer

Lo

ng la

stin

g

Acta Materialia (2015)

Nano & Flexible


Origin of fast crystallization of a-Ge2Sb2Te5

Fragile nature (m ~ 90) ofa-Ge2Sb2Te5

leads to “fast transforming” characteristics.

C.A. Angell, J. Non-Cryst. Solids (1988)M. Wuttig et al., Nat. Mater., 11 (2012) J. Orava et al., Nat. Mater., 11 (2012)

RT

ET exp)( 0

Strong GlassArrhenius behavior

• α-SiO2, α -InGaZnO etc.

• Stability against crystallization

in covalent bonds

• Fragility, m ~ 16

0

00 exp)(

TT

TDT

Fragile GlassNon-Arrhenius behavior

• Polymer, Ge2Sb2Te5 etc.

• Extremely rapid crystallization

Vogel-Fulcher-Tammann

in organic, ionic bonds

• Fragility, m » 16

Nano & Flexible


Doping Effects on Fragility

• Fragility (m)

0 2 4 6 8 1010

20

30

40

50

60

Frag

ility

, m

Dopant concentration (at.%)

CN

Al

Bi

Arrhenius (Strong)

• Substitutional doping (Al, Bi)

: Fragile to Strong transition

• Interstitial doping (C, N)

: maintained as Fragile

→ Consistent results with changes in Tx - Tg

C.A. Angell, J. Non-Cryst. Solids (1988)

gTg RT

E

TTd

dm

g

10ln)/(

log

CRT

E

g

ln

→ Heating rate variation : 1.0 - 20 °C/min

Amorphous solid Devices

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