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:
17LT24@Orlando 16/08/2005
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 )
27LT24@Orlando 16/08/2005
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
30LT24@Orlando 16/08/2005
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
32LT24@Orlando 16/08/2005
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)
35LT24@Orlando 16/08/2005
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)
36LT24@Orlando 16/08/2005
5mV
5nA
VBG = 200V T =1.9K
Near the ohmic-nonlinearboundary, hysteresis appears in
the I-V Curve !!
37LT24@Orlando 16/08/2005
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