Preparation of Preparation of Novel PZTN Thin Film Co-doped Novel PZTN Thin Film Co-doped

Si for High-Density FeRAMSi for High-Density FeRAM

7 Oct. 2004

Seiko Epson Corporation

T. Kijima, Y. Hamada, K. Ohashi, H. Miyazawa, T. Aoyama, E. Natori and T. Shimoda

OutlineOutline

● Objectives　　　■ Development of novel ferroelectric material for high integrated FeRAM● Experimental

■　　　 Nb 20% substitution to PZT with Si 1 mol%　 : PZTN● Discussion

■　　　 First-principles calc. of PZTN● Summary

Present condition of FeRAM Present condition of FeRAM 　　

Ferro. S tructure Des ignrule(um)Accumulation

(bit)Drive voltage

(V)

Fujitsu P ZT 1T1C 0.18 4M～ 3V

Matsushita S BT 2T2C 0.18 1M～ 3V～

TI P ZT 1T1C 0.13 64M～ 1.5V

Samsung P ZT 1T1C 0.25 32M～ 3V

Toshiba P ZT 1T1C 0.2 32M 3V,2.5V

NEC P ZT 2T2C 0.25 0.35～ 3V

OKI S BT 2T2C1T1C 0.25 16M～ 3V

Hynix S BTBLT2T2C1T1C 0.35 4M～ 1.8 3V～

５Ｖ16k０．８２Ｔ２ＣPZTRohm３－５ V64k０．６２Ｔ２ＣSBTMatsushita３ V256ｋ０．５２Ｔ２ＣPZTFujitsu

Mass production level

Research level

Key points and Key points and What we have achievedWhat we have achieved

There are many subjects in reliability for FeRAM.

Material is the key. We have invented a new ferro-electric

material, named PZTN. A big step to High-density FeRAM.

Problems of PZTProblems of PZT

Poor reliabilityLarge leakage current

52/4840/60

30/7020/80

: Initial: Final ; R.T., 25Hrs.

1.E-11

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

1.E-01

1.E+00

-10 -8 -6 -4 -2 0 2 4 6 8 10Voltage (V)

Leak

age

Curr

ent (

A/cm

2)

52/4840/60

30/70

Leakage

0

10

20

30

40

50

60

1.E+00 1.E+02 1.E+04 1.E+06 1.E+08Cycles

2Pr(

uC/c

m2)

＠ 1.8V,50kHz

Fatigue Loss

-100

-80

-60

-40

-20

0

20

40

60

80

-2.4 -1.6 -0.8 0 0.8 1.6 2.4Voltage (V)

Pola

rizat

ion

(uC/

cm2)

Imprint Problem

20/8020/80

Zr/Ti Zr/Ti

Various properties of the Various properties of the element which constitutes PZTelement which constitutes PZT

Valence Value　 Atomic Radius B inding Energy Ionization Potential(Ionic radius ( ))Å ( )Å M- O(kcal/ mol) (eV)

Pb 207.2 +2(1.08),+4(0.78) 1.33 38.8 7.416

Zr 91.224 +4(0.72) 1.6 185 6.84

T i 47.88 +2(0.86),+3,+4(0.61) 1.47 73 6.82

Nb 92.906 +3,+4,+5(0.64) 1.47 177- 189 6.88

O 54.36 - 1,- 2(1.4) 0.61 - 13.618

Name Atomic Weight

Oxygen vacancy takes place!Oxygen vacancy takes place!

Pb： Binding energy of Pb-O is small and easy to evaporate.⇒If Pb falls out, O will escape from the system due to the principle of electric charge neutrality.

PZT forms a Shottkey defects easily.

Band gap narrowing Oxygen ionic conduction

↓Low reliability for FeRAM

Oxygen vacancy

Nb doping to PZTNb doping to PZTNb Ion size is almost same as Ti. Nb5+ may compensate the hole of Pb vacancy with preventing oxygen loss. Strong covalent-bond between Nb and O. good for ferroelectricity⇒

It is difficult to substitute B site in PZT by Nb because of high crystallization temperature.

⇒Nb is only 2.3at% added to PT tetragonal composition. (T.Matsuzaki and H.Funakubo, J. Appl. Phys,86(1999)4559)

Addition of 1%-Si into the PZT+Nbto reduce the crystallization temperature.

OutlineOutline

● Objectives■　　　 Development of novel

ferroelectric material for high integrated FeRAM

● Experimental ■Nb 20% substitution to PZT with Si 1 mol%　 : PZTN● Discussion

■　　　 First-principles calc. of PZTN● Summary

PZT and PZTNPZT and PZTN

Top Electrode ：　 PtBottom Electrode ：　 PtProcess Temperature ：　 650℃Thickness ：　 150nmCrystal Orientation ： (111) pseudo cubicCrystal Structure : Tetragonal

Zr Ti Nb Si

40 60

20 60 20 1 mol%

PZT(40/60)

PZTN(20/60/20)

Sol-Gel MethodSol-Gel Method

0

10000

20000

30000

40000

50000

20 30 40 502θ (deg)

Inten

sity(c

ps)

0

1000

2000

3000

20 30 40 502θ (deg)

Inte

nsity

(cps)

0

1000

2000

3000

20 30 40 502θ (deg)

Intensit

y(cps)

Si = 0% Si = 0.5% Si = 1%

800℃

Pyr

o Pt(1

11)

Pt(1

11)

Pt(1

11)

PZT

(100

)

PZT

(111

)

Pyr

o650℃650℃

Effect of Si Effect of Si on PZTN(110/20/60/20)on PZTN(110/20/60/20)

Without Si, PZTN contains pyrochlore phase, but with 1% of Si, PZTN has perovskite phase only.

P-V hysteresis curvesP-V hysteresis curves

of PZTN capacitorsof PZTN capacitors

- 60

- 40

- 20

0

20

40

60

- 10 - 5 0 5 10Voltage (V)

Pola

rizat

ion

(uC

/cm

2)

- 5 0- 4 0- 3 0- 2 0- 1 0

01 02 03 04 05 0

- 2 - 1 0 1 2V o l t a g e ( V )

Polar

izatio

n (u

C/cm

2)

Leakage current propertyLeakage current property

PZT

PZTN

-10 -8 -6 -4

リ

ー

ク 電 流 密 度

10-3

10-4

10-5

10-6

10-7

10-8

10-9

Leak

age

curr

ent d

ensi

ty (A

/cm

2)

Applied voltage (V)

-2 0 10 8 64 2

Shottky emission mode

F-N mode

0

0.2

0.4

0.6

0.8

1

1.2

1.E+00 1.E+02 1.E+04 1.E+06 1.E+08 1.E+10

Cycles

Pr (N

orm

alize

d)

PZT PZTN

Fatigue PropertyFatigue Property

- 60

- 40

- 20

0

20

40

60

- 3 - 2 - 1 0 1 2 3Vol t age ( V)

Polar

izatio

n (u

C/cm

2)

- 60

- 40

- 20

0

20

40

60

- 3. 3 - 2. 2 - 1. 1 0 1. 1 2. 2 3. 3Vol t age ( V)

Polar

izatio

n (u

C/cm

2)

PZTN

PZTE(Fatigue Pulse) = 150kV/cm

PZTN shows better fatigue propertyeven on Pt electrode.

PZTN

PZT

-3-2-10123

0 10 20 30 40 50 60

Imprinting time (hrs)

Voltag

e (V)

Dynamic Imprint

　 : positive

　 : negative

-100

-50

0

50

100

-4 -3 -2 -1 0 1 2 3 4

Voltage (V)

Pola

rizat

ion

(uC

/cm

2)

Static Imprint

positive

negative-40

-20

0

20

40

0 10 20 30 40 50 60

Retention time (hrs)

Polar

izatio

n (uC

/cm2)

Retention

　 : positive

　 : negative

ReliabilityReliabilityof PZTNof PZTN

125℃

109 cycles R.T. 125℃

initial

final

PZTN realizes lower domain pining.

Nb truly makes a Solid-Solution Nb truly makes a Solid-Solution in PZT even at the amount of 20 in PZT even at the amount of 20 at.% ?at.% ?

24.3atom.%OTiPb Zr+NbUnit

56.411.821.7 10.0atom.% 50.618.17.0

Total

100100

RBS, NRAPZT

PZTN

Unit Pb-4f Nb-3dTi-2pZr-3d Si-2pO-1s TotalXPS

PZTPZTN

atom.%atom.%

100100

21.124.8 5.75.715.324.4 5.7 0.1

48.448.8

The film composition of PZT The film composition of PZT and PZTN Zr:Ti:Nb=and PZTN Zr:Ti:Nb=20:60:2020:60:20

NbTiPb ZrUnit TotalICP53.0atom.%

9.630.350.3 9.8atom.% 38.18.9

100100PZT

PZTN

26.72.3

26.82.0

3-D reciprocal space

Q(φ)

Q(ψ)

Q(2θ)

All reciprocal points are projected on to the single mapby rotating the sample.

～ 1,000 rpm

XRD reciprocal space mapsXRD reciprocal space maps

PZT(20/80) PZTN(Nb:20%)

SiPZT(N)PZT(N)

Pt

0

60

20

40

ψ / d

egre

e

10 20 30 40 50 60

Cu Kα 2θ / degree

10 20 30 40 50 60

Cu Kα 2θ / degree

60

PZTPt

Si-sub.

PZT(N)

XRD reciprocal space mapsXRD reciprocal space maps

J

JJ

J

300

310

320

330

340

0 10 20 30 40

Raman shift / cm-1

Ram

an sh

ift /

cm-1

Inte

nsity

Nb doped / mol%

A1(2TO)

0%

20%

30%

Raman spectra Raman spectra ofof A1(2TO) A1(2TO) frequency frequency atat PZTN systems PZTN systems

5%

A1(2TO)

16OH

16O

206Pb207Pb

208Pb 48Ti

Mass spectra using SIMS Mass spectra using SIMS

for PZTN and PZTfor PZTN and PZT

PZTN

PZT

Depth profile by RBS/NRA Depth profile by RBS/NRA and AES measurementsand AES measurements

PZTN

PZT

Pt

SiO

O

Ti

Zr+NbPbTi

Pt

SiO

O

TiZr

Pb Ti

0

20

40

60

80

100

0.0 20.0 40.0 60.0 80.0

O

PtO

Pb SiTi

Zr+Nb

Ti

0

20

40

60

80

100

0.0 20.0 40.0 60.0 80.0

Pt O

Pb SiO

Ti

Zr Ti

Atom

ic C

once

ntra

tion

/ ato

mic

%

Sputtering Time / min.

RBS/NRA AES

Summary: ExperimentSummary: Experiment

● Novel material PZTN for FeRAM■　　　 Sol-Gel method at 650 ℃

■Nb 20 % with Si 1 mol% ● High reliability 　 　 ■ Good squareness of hysteresis

loop 　　　■ Very low leakage current ■Good fatigue, retention and imprint properties ● Nb is located at B site in ABO3● Sharp depth profile

OutlineOutline

● Objectives■　　　 Development of novel

ferroelectric material for high integrated FeRAM

● Experimental■　　　 Nb 20% substitution to PZT with

Si 1 mol%　 : PZTN ● Discussion　　　■ First-principles calc. of PZTN● Summary

Electronic structure of PZTN systems

Focus on the oxygen vacancy of conventional PZT systems

2. Defect formation energy1. Density of states (DOS)

Aim ：Analyze the PZTN systems using the first-principles calculation.

Nb 20～ 25%

High reliability （ 10-3 lower leakage current → Good Imprint, Retention and Fatigue)

Pb defectO defect

PZTN: Pb (ZrTiNb)O3 Zr, Ti=4+, Nb=5+

Why does PZTN have excellent leakage current?

b

c

a

Method ： Super Cell

super cellABO3 (2x2x2)

Treat defect and atom displacement

A site

First-principles calc. ： Density Functional Theory, Local Density Approx. FLAPW method

Assume cubic and paraelectic stateslattice constant : optimized

Ecut=15Ryk-mesh=(1x1x1), (2x2x2) for Eg

O deficit ： nearest to Pb defect and nearest to Zrby Miyazawa (2003)

Nb is located at B site from Raman analysis. by Aoyama (2003)

PZTN: Pb1-1/8(Zr0.25Ti0.50Nb0.25)O3

Electronic structure of PZT: Density of States (DOS)

PZT: Pb (Zr0.25Ti0.75)O3

Total

Ti site

Zr site

40

30

20

10

0

Tota

l DO

S /e

V s

pin

-20 -15 -10 -5 0 5 10 15

Energy (eV)

Total

1.5

1.0

0.5

0.0

Par

tial D

OS

/eV

spi

n at

om-20 -15 -10 -5 0 5 10 15

Energy (eV)

Ti

1.5

1.0

0.5

0.0

Par

tial D

OS

/eV

spi

n at

om

-20 -15 -10 -5 0 5 10 15

Energy (eV)

Zr

Pb 5dO 2s

Pb 6s

O 2p TM d

Valence band top : O 2p orbitals

Conduction band bottom :

Ti 3d orbitals

DOS of PZT

PZT: Pb (Zr0.25Ti0.75)O3

Pb deficit hole dope

O deficit electron dope

Pb-O deficit Insulator

Ti 3d orbitals

O 2p orbitals

Shottkey defect

=1/8=0.125Pb 1- (Zr0.25Ti0.75)O3-

Impurity states : Zr d orbitals

TM atom which is N.N. to O defect site

O can act as 2- ion.

Pb can act as 2+ ion.

40

30

20

10

0

Tota

l DO

S /e

V s

pin

-20 -15 -10 -5 0 5 10 15

Energy (eV)

TotalPb 5dO 2s

Pb 6s

O 2p TM dTi d

PZT-O

PZT-Pb

PZT

40

30

20

10

0

Tota

l DO

S /e

V s

pin

-20 -15 -10 -5 0 5 10 15

Energy (eV)

PZT-O

40

30

20

10

0

Tota

l DO

S /e

V s

pin

-20 -15 -10 -5 0 5 10 15

Energy (eV)

PZT-Pb

insulator

electron

hole

40

30

20

10

0

Tota

l DO

S /e

V s

pin

-20 -15 -10 -5 0 5 10 15

Energy (eV)

PZT-PbOZr d

insulator

PZT-PbO

PZTN: Pb1-(Zr0.25Ti0.50Nb2)O3

DOS of PZTN

Insulator

Subtract Pb 0.125

Substitute B site Ti 0.25 to Nbelectron dope

Nb can be seen as 5+ ion.Ti 3d, Nb 4d orbitals

O 2p orbitals

Start at PZT: Pb (Zr0.25Ti0.75)O3

=1/8=0.125

insulator

PZT+Nb

PZT－ Pbhole

40

30

20

10

0

Tota

l DO

S /e

V s

pin

-20 -15 -10 -5 0 5 10 15

Energy (eV)

PZT-Pb

PZT

40

30

20

10

0

Tota

l DO

S /e

V s

pin

-20 -15 -10 -5 0 5 10 15

Energy (eV)

TotalPb 5dO 2s

Pb 6s

O 2p TM d

40

30

20

10

0

Tota

l DO

S /e

V s

pin

-20 -15 -10 -5 0 5 10 15

Energy (eV)

PZT+Nb

40

30

20

10

0

Tota

l DO

S (

/eV

spi

n)

-20 -15 -10 -5 0 5 10 15

Energy (eV)

Total

PZT+Nb-Pb

Pb 5dO 2s

Pb 6s

O 2p TM d

insulator

electron

No impurity states

hole dope

Pb can be seen as 2- ion.

PZTN: Pb1-(Zr0.25Ti0.50Nb2)O3

DOS of PZTN

1.5

1.0

0.5

0.0

Par

tial D

OS

/eV

spi

n at

om

-20 -15 -10 -5 0 5 10 15

Energy (eV)

Zr1

40

30

20

10

0

Tota

l DO

S (

/eV

spi

n)

-20 -15 -10 -5 0 5 10 15

Energy (eV)

Total

1.5

1.0

0.5

0.0

Par

tial D

OS

/eV

spi

n at

om

-20 -15 -10 -5 0 5 10 15

Energy (eV)

Nb1

1.5

1.0

0.5

0.0

Par

tial D

OS

/eV

spi

n at

om

-20 -15 -10 -5 0 5 10 15

Energy (eV)

Ti3

Total

Ti site

Nb site

Zr site

Pb 5dO 2s

Pb 6s

O 2p

TM d

=1/8=0.125

Conduction band bottom

Ti 3d and Nb 4d orbitals

J

J

B

- 1 . 6

- 1 . 4

- 1 . 2

- 1

- 0 . 8

- 0 . 6

- 0 . 4

- 0 . 2

0

0 2 4 6 8 1 01 2 1 41 6 1 8 2 0

Ba

nd

g

ap

(

eV

)

;

di

ff

er

en

ce

P b d e f i c i t ( % )

Pb-O deficit and bandgap

PZT-PbO

PZTN

In the case of PZTN, the bandgap is maintained the almost same width as PZT without atom deficit.

In the case of PZT with Pb-O defect, the bandgap is reduced drastically.

Pb (Zr0.２５Ti0.７５ )O3

=1/8

Change of bandgap

Oxygen vacancy lowers the Madelung potential of the nearest neighbor Zr d orbitals.

Electronic states

Substituting B-site 4+ ion by Nb 5+ ion induces electrons at the conduction band bottom, which consists of Nb 4d and Ti 3d orbitals.

PZTN systems : Substitute Ti 2 to Nb and subtract Pb .

Subtracting A-site Pb creates holes at the conduction band maximum.

No need of oxygen vacancy to maintain insulator states.

Keep bandgap wide enough.

In PZTN case, the doped electrons and holes cancel out each other, and the system recovers an insulator states.

Pb-O defect creates impurity states at the conduction band bottom. These states consists of Zr d orbital which is nearest neighbor to the oxygen-defect site.

PZT systems with Pb-O deficit

Narrow the bandgap drastically.

Oxygen vacancy lowers the Madelung potential of Zr d orbitals.

1.E- 10

1.E- 09

1.E- 08

1.E- 07

1.E- 06

1.E- 05

1.E- 04

1.E- 03

- 10 - 5 0 5 10Votlage (V)

Leak

age

Curr

ent (

A/c

m2) PZT

PZTN

Pt PZT－ PbO

Band Diagram at Interface

Pt PZTN

by Hamada (2003)z

Energy

F-N tunnelingShottkey emission (@ large Eg)

Space charge limited current (@ large J)

=0.27eV

= 0.19eVExperiment

Formation energy of Pb deficit in PZTN

Pb (Zr0.25Ti0.50Nb0.25)O3Pb1-0.125(Zr0.25Ti0.50Nb0.25)O3 fcc-Pb+

H = － 0.55eV/ABO3TSHG S ; Entropy due to defects

PZTN with Pb deficit has 0.55eV lower energy than PZTN with no Pb deficit.

Corresponding to amount of Nb addition, the system turns to insulator by losing Pb atom automatically without losing oxygen. （@ thin film fabrication process）

Hypothesis

We need 0.15eV/ ABO3 more to subtract oxygen in order to form Shottky defects.Nb addition has the ability to prevent oxygen deficit even it induces Pb deficit.

Defect formation energy ：PZT

[eV / super cell]

Stoichiometric PZT with no deficit is the most stable states in the ground state.However, due to the high vapor pressure of Pb, we should start the reaction at the states with Pb deficit.

Pb (Zr0.50Ti0.50)O3

Pb (Zr0.50Ti0.50)O 3-1/8

Pb 1-1/8 (Zr0.50Ti0.50)O3-1/8

Pb 1-1/8(Zr0.50Ti0.50)O3-3.77

-0.59

-2.81

-2.22

-0.96Pb-O deficit（ Shottkey deficit）

O deficit

Pb deficit

Cubic、 Paraelectric states

Lattice constant is optimized. metal

insulator

metal

insulator

Under this condition, system may finally wants to go to Pb-O deficit(Shottkey deficit).

Defect formation energy ： PZTN

[eV / super cell]

Cubic, Paraelectric states

Lattice constant is optimized.

The most stable state is the insulator state with Pb defect and without oxygen defect.

-4.41 -1.22

-3.08-3.88Pb1-1/8(Zr0.25Ti0.50Nb0.25)O3

Pb1-2/8(Zr0.25Ti0.50Nb0.25)O3

Pb (Zr0.25Ti0.50Nb0.25)O3 Pb1-1/8(Zr0.25Ti0.50Nb0.25)O3-1/8

Pb1-2/8(Zr0.25Ti0.50Nb0.25)O3-1/8

metal

metal

metal

insulator

insulator

Pb deficit

2Pb deficit Pb and Pb-O deficit

Pb-O deficit

We suppose that this is the state of PZTN which realizes very low leakage current.

System seems to automatically recovers charge neutrality

Pb(TiZrNb)O3

Hypothesis: According to Nb addition,

system automatically recovers charge neutrality by inducing Pb evaporation in order that charge-

neutrality condition maintains.

Pb Zr/Ti Nb O

PbZrTiO3Pb-O deficit : Shottky deficit

Pb ev

apora

tion

Pb Zr/Ti O

Pb ev

apora

tion

O evapolation

bandgap lowering

Nb suppresses oxygen vacancy keep bandgap

Why is Ti rich leaky ?It is still mystery.

band offset lowering leaky

F-N mode

keep band offset low leakage

Shottky mode

Summary: First-principles calc. Why PZTN has excellent leakage current?

Adding Nb(2) with Pb() deficit makes system insulator states.

The study of defect-formation energy indicates that adding Nb (2) with Pb () deficit and without oxygen deficit realizes the most stable states.

In conventional PZT, the Pb-O deficit narrows the bandgap due to the impurity states at the conduction band bottom. This is because that the oxygen deficit lowers the Madelung potential of the nearest-neighbor TM-d orbitals. This reduces the band offset with electrodes and makes the system leaky.

In PZTN systems, adding Nb suppresses the oxygen deficit and it sustains the bandgap wide enough to keep Shottkey leakage mode.

↓ very low leakage current

Why does PZTN have excellent leakage current?

Corresponding to the amount of Nb addition, the system turns to insulator by losing Pb atom automatically without losing oxygen. （ Hypothesis）

b

c

a

Oxygen vacancy and the reliability: Imprint, retention and fatigue

Mobile oxygen ion may also cause the imprint, retention and fatigue.

Injected electrons caused by narrow band offset may induce the fixed charge at oxygen vacancy site in the film and this charge acts as domain pinning sites which cause the imprint.

Oxygen vacancy

Create large space

Oxygen easily move through this path.

Driving force of mobile oxygen ion : Oxygen and Pb vacancy

MD simulation of BaTiO3 : Trajectory of Ion diffusion

BaTiO

4% at O site12% at A site

vacancyBorn-Mayer-Huggins potential

(4x4x4)

Oxygen is the most mobile ion in ABO3 perovskite-type structure.

Plus and minus ion vacancy attracts each other.

MD simulation of BaTiO3 : Ion diffusion trajectory

Oxygen ion diffuses along octahedron network.Oxygen →　 Oxygen siteBa， Ti →　 A and B site

O >> Ba, TiWide space of diffusion path for O in ABO3 system

These defects may tend to accumulate at electrode interfaces.

Imprint, Retention, Fatigue

Because the interface layer has low Pb density.

Become domain pinning site

SummarySummary

We tried to produce the new material for FeRAM.

We substituted 20 at.% Nb at PZT and produced perovskite-type structure of Pb(Zr,Ti,Nb)O3(PZTN) films.

We achieved : ・ Large switching charge and good squareness of hysteresis loop. ・ High reliability with low leakage current.

1% of Si are necessary for single perovskite-type PZTN.

First-principles calculation study

・ Conventional PZT with Pb-O defects has narrow bandwidth.・ Our PZTN, which is supposed to have low O defects due to Nb doping, can maintain bandgap wide enough to suppress the leakage current.