Correlated Electron Research Center (　　　　　)National Institute ofAdvanced Industrial Science and Technology ( )

AIST Tsukuba

Electrostatic carrier doping toperovskite transition-metal oxides.

2LT24@Orlando 16/08/2005

Basic Researchof phase control

Development offunctional device

Isao H. Inoue, H. Nakamura*, H. Takagi*,Y. Tomioka, Y. Abe, Y. Takahashi,

T. Hasegawa, H. Yamada, M. Kawasaki#, T. Yamada, H. Sato, H. Akoh, Y. Tokura*

also at *Univ. of Tokyo, and #Tohoku Univ.

Collaboration in CERC

3LT24@Orlando 16/08/2005

(A,B)TiO3, (A,B)VO3, (A,B)MnO3,(A,B)CoO3, (A,B)NiO3, (A,B)RuO3,(A,B)2CuO4, (A,B)2RuO4, Fe3O4,TiO2, Ti2O3, Ti4O7, VO2, V2O3, … …

Mission“Doping to Single Crystals”

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Electrostatic Carrier Doping

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Oxide single crystal

Source Drain (Al)

Gate insulator

Channel

Potential probe

GateL 200mm

W 400mm

•No impurities •Continuous/precise carrier density control.

Electrostatic Carrier Doping

6LT24@Orlando 16/08/2005

Rubrene Single Crystal

Example of Organic MaterialsC. D. Frisbie et al., (Univ. Minnesota)

However,only a few examples in TM oxides

SrTiO3 Single CrystalStep&Terrace Surface

Our WorkK. Ueno et al., (CERC, AIST)

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50nm20nm

on/off ~ 100 µFE ~ 0.01

on/off ~ 10000 µFE ~ 0.4

K. Ueno et al., APL83, 1755(2003); K. Ueno et al., APL84,3726 (2004)

Two of a Few Examples

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Electric Breakdown

Gate-Ins/Sample Interface

Barriers at Electrodes

Three Challengesin the Electrostatic Doping

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Ahn, Triscone & Mannhart,Nature424, 1015 (2003)

Electric Breakdown

extremely largeelectric field to

dope an amount ofcarriers

gate insulator withhigh breakdownstrength1013–1015cm-2

Challenge –I–

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600µm

Stochastic phenomenon

20µm × 20µm may avoidthe breakdownsometimes.Kazunori Ueno, Thesis, Univ. Tokyo (2004)

Electric Breakdown

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Au/Al2O3(140nm)/Al(10 nm)20 µm × 20 µm.

!

Q

eS= "

"0

e

#

$ %

&

' ( EG

) 0.5 *1014 [cm-2]

Ahn, Triscone& Mannhart,Nature424,1015 (2003)

10MV/cm with Al2O3

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Kind to the Interface

dielectric constant　3.15thickness　1 µmelectric breakdown　2～3 MV/cm

Parylene-C as Gate Insulator

120℃ 700℃ room temp.Easy to Deposit

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Metal Ins. Sample

Poisson

Poisson-Schrödinger

The peak dependson m and ε

ε

m♥

I H Inoue, Semicon. Sci. Technol. 20, S112 (2005)

Challenge –II–Gate-Ins/Sample Interface

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εSTO /m*=10 εSTO /m*=1000Ga

te E

lect

ric

Field

(MV/

cm)

T

ε

50K 100K0

20000

80V

100000V ε vs T

of SrTiO3

Room temperature Low temperature

Distance from surface (nm)Distance from surface (nm)

Au/Al2O3/SrTiO3Ga

te E

lect

ric

Field

(MV/

cm)

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εSTO /m*=10 εSTO /m*=1000

Gate

Elect

ric

Field

(MV/

cm)

Gate

Elect

ric

Field

(MV/

cm)

T

ε

50K 100K0

20000

80V

100000V ε vs T

of SrTiO3

Room temperature Low temperature

Distance from surface (nm)Distance from surface (nm)

Au/Al2O3/SrTiO3

16LT24@Orlando 16/08/2005

because we are trying todope into INSULATORS

Barriers at ElectrodesChallenge –III–

V2 V1

AA

A

VG

VD

ID

Quandary:Schottky-like

barriers appearespecially at lowtemperatures

Quandary:

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Barrier Width ∝ 1/n 1/2

n = 1015 cm–3

n = 1017 cm–3

n = 1018 cm–3

Metal | channel

Morecarriers arenecessary tobe doped

Barrier Formation at Low T

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4-Probe Measurement

V2 V1

AA

A

VG

VD

ID

!

"V #V1$V

2

1.0

0.5

0

ID (

10

-9 A

)

6420VD ( V )

(a) VG=6V

VG=5V

VG=4V

VG=3V

0.80.40

!V ( V )

(b)

Al2O3 / SrTiO3

300K

Alumina/SrTiO3

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4-Probe Measurement

V2 V1

AA

A

VG

VD

ID

!

"V #V1$V

2

30

20

10

0

ID (

10

-9 A

)

100500

VG ( V )

VD=5V

VD=1V

(c)

0.80.40

!V ( V )

Parylene / SrTiO3

300K

(d)

Parylene/SrTiO3

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4-Probe Measurement

!

Rcnt"VD#$V

ID

Channel Resistance

Contact Resistance

!

VD

ID

=L

WRS

+ Rcnt

V2 V1

AA

A

VG

VD

ID

L W

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4-Probe Measurement

!

Rcnt"VD#$V

ID

Channel Resistance

Contact Resistance

103

104

105

106

107

108

109

1010

R!

, R

cnt (

k!

) Rcnt alumina

R!

alumina

Rcnt parylene

R!

parylene

10-5

10-4

10-3

10-2

10-1

100

101

µ eff

(

cm2/

Vs

)

2.01.00

n! ( 1012

cm-2

)

µeff

parylene

µeff

alumina

!

µeff =1

enSRS

!

VD

ID

=L

WRS

+ Rcnt

!

nS

="ins(E

G#E

th)

e

22LT24@Orlando 16/08/2005

V2 V1

AA

A

VG

VD

ID

I D (A

)

Negative DifferentialResistance

VD = 1 V

!

"V

!

"V #V1$V

2

Parylene/SrTiO3

Hardly seen in Al2O3/SrTiO3

Al2O3/SrTiO3 has some amount of carriers from the first

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E. Shöl: Nonequilibrium PhaseTransitions in Semiconductors(Springer Verlag, Berlin,1987)

N-type NDR S-type NDRdue to domains ofdifferent field strength

due to filaments withdifferent current density

Two Types of NDR

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Filamentary ConductionI D

(A)

!

"V

Parylene/SrTiO3

AA

A

VG

VD

ID

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Further Carrier Injection

SrTiO3 (0.5mm)

SrZrO3

( 8Å )

2µm

back gateIBG

VBG

sourcedrain

VBG

VD

injection, or tunneling

diffusion,drift,or hot electrontransport

Back Gate through SrTiO3

• Aluminum S/D electrodes

• L=100µm, W=20µm

source

ID

drain

VD

Barriers

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No field effectVD (10-3V)

I D (

10-9

A)

160KR

(

103 h

/e2 )

Field effect !!

100K

VD (10-3V)I D

(10

-9A

)R

(

h/e

2 )

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Ohmic I-V

60K

VD (10-3V)

I D (

10-9

A)

R

( h

/e2 )

Nonlinear I-V

30K

VD (10-3V)I D

(10

-9A

)R

(

h/e

2 )

28LT24@Orlando 16/08/2005

-20V < VBG < 60V

VD (10-3V)

I D (

10-9

A)

R

( h

/e2 )

60V < VBG

VD (10-3V)I D

(10

-9A

)R

(

h/e

2 )R ~ h/e2

2.1K

29LT24@Orlando 16/08/2005

SemiconductingMedium

MetalElectrode

S D

Channel

R

(h

/e2)

Si MOSFETM. P. Sarachik, S.V. Kravchenko Proc. Natl. Acad. Sci. 96, 5900 (1999).

T (K)

10

1

0.1

1

10S D

Semiconducting Medium

Channel MetalElectrode

Non-linear Region

V

I

tunnel or hoppinge

2D Metallic Domain

MIT@R□~500series resistance ofa few hundreds ofmetallic domains.

Linear Region

V

I

R ~ h/e2n = 1015 cm–3

n = 1017 cm–3

n = 1018 cm–3

Metal | channel

Barrier

Schottkey Barrierformation due to adecrease ofcarrier densities

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Carrier density

Tem

p

Isotop

e

exchang

e

23K

0.3K

30%

100%

1019cm-3 1020cm-3

Unexplored Region

Super-conducting

Ferro-

electric

Under Investigation

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Al2O3 target

SrTiO3 S-T : 12 cmpower：100 Wpure Ar ̃15 mTorr

Ar plasmaAl2O3

Al2O3 film properties：

dielectric constant　8.5～9thickness　100 nmdielectric breakdown　3～6 MV/cm

Magnetron Sputtering

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Capping Layer

gate Al2O3 insulator

On the top of a step andterrace surface of SrTiO3

SrTiO3

Capping Layer(10Å LaAlO3 or SrZrO3 )

source drain

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True Channel Mobility

measured contact

-1

!

µ =1

Cins

"

"VG

TRUE

slope =

TRUEHamadani & Natelson, JAP95, 1227 (2004)

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NonlinearityPoor Mobility carriers pile up, with some tendency

to diffuse back to contact.

Image Charge carriers also feel image chargeattraction from metal electrode.

!

RsVG,T( ) ~

1

µ VG,T( )exp

"B

kBT

#

$ %

&

' (

!

ID~ V

Dexp

" VD

kBT

#

$ % %

&

' ( (

Hamadani & Natelson, JAP95, 1227 (2004)

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5mV

5nA

VBG = 200V T =1.9K

Near the ohmic-nonlinearboundary, hysteresis appears in

the I-V Curve !!

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Bistable Resistance Switchonly at the nonlinear-ohmic boundary

-200

-100

0

100

200

I ou

t (

10

-9 A

)

-3 -2 -1 0 1 2 3

Vread ( 10-3

V )

(1): initial state(2): +5.0mA & 12msec pulse after (1),

(3): -5.5mA & 60msec pulse after (2)

(4): +5.0mA & 12msec pulse after (3)

0.1μA2mV

VBG = 200V T =1.9K High R State

~60ms~-5mA

pulseLow R State

~10ms

~5mA

pulseLow R State

High R State

Low R State

Resistance RAM

pulse

I. H. Inoue et al., JPN Patent Appl. #2003-295641