Electrostatic carrier doping and quantum critical phenomena in transition metal oxides

Inhomogeneous Current
Distribution at Oxide Interface
National Institute of Advanced Industrial Science & Technology (AIST) (Tsukuba, Japan)
Isao H. Inoue
2
Electrostatic carrier doping
Quantum critical
phenomena
Mott transistor Exotic
phonomena
集中講義
「モットトランジスタは実現できるのか？」
本来は金属であるはずの物質が、強い電子相関（電子どうしに働くクーロン斥力）の
ために絶縁体となっている物質を「モット絶縁体」と呼ぶ。モット絶縁体にキャリア
をドープすると、局在していた全てのキャリアがいっせいに動き出して金属になる。
いわゆるモット転移である。このモット転移を利用して、新概念のトランジスタを作
れないだろうか。そのためには何が必要なのか？
本講義では、「強相関」「電界効果ドーピング」という二つのキーワードに関する基
本的な物理を簡単に解説し、それに伴う物理現象を紹介する。そして、現状の半導体
デバイスの問題点と「モットトランジスタ」の可能性について議論したい。
日時：2014年7月15日(火) 10:30-12:00 (90分), 13:15-14:45 (90分), 15:00-16:30 (90分)
同　　16日(水) 10:30-12:00 (90分), 13:15-14:45 (90分), 15:00-16:30 (90分)
　　　　　　　　　　　　　　　　　　　　　　　(講演会)
i.inoue@aist.go.jp http://staff.aist.go.jp/i.inoue/
Seminar @ TITech, 16 July 2014
3
場所：東京工業大学大岡山キャンパス
4
Electrostatic carrier doping
Quantum critical
phenomena
Mott ✔ transistor Exotic
phonomena on the horizon!
Quantum criticality
Temp
Classical
Critical Point
Ordered
State
Quantum
Critical Point
Seminar @ TITech, 16 July 2014 Physical Parameters
(Pressure, Magnetic field,
Carrier number, etc.)
What happens at QCP?
Classical
Critical Point
Quantum
Critical Point
Super
Physical Parameters
5 i.inoue@aist.go.jp http://staff.aist.go.jp/i.inoue/
Temp
Ordered
State
Pressure
UGe2
Super
CeCu2Si2
Temp
Temp
Ferro Antiferro
Pressure
6

Randomness-free method:
quantum critical phenomena is
so vulnerable to disorders
Continuous and reversible control
of electronic states on the verge
of the quantum critical point
7
For QCP study, we need …
Phys. Parameters are always
either Pressure or Mag. Field
CeCu2Si2
AF
Super
UGe2
Ferro
Super
Pressure
Temp
Saxena et al., Nature 406, 587 (2000)
[ Coleman, Nature 406, 580 (2000) ]
Pressure
Yuan et al., Science 302, 2104 (2003)
Temp
8
Electrostatic Carrier Doping
Seminar @ TITech, 16 July 2014 9
Why is electrostatic carrier
doping so difficult?
Defects in transition-metal oxides
TiO2-x Co1-xO, Fe1-xO, Ni1-xO
valence electron
1st electron
ionisation
2nd electron
ionisation
neutral
composite
valence electron
neutral composite
11
Why is electrostatic carrier
doping so difficult?
Transition-Metal Oxides
≈ ionic crystals
(because of the strong electron correlations)
!
!
!
→ Defects form easily under large electric field.

Good examples of
electrostatic carrier doping?
VG = 0V
VG = -2.5V
1010
10 6
Channel Resistivity (Ωcm)
10-2
10-3
10-4
Nd0.5Sm0.5NiO3
220 260
300
Temperature (K)
S. Asanuma et al., APL. 97, 142110 (2010)
VO2
10 2
Channel Resistance (Ω)
100 150 200 250 300
Temperature (K)
M. Nakano et al., Nature. 487, 459 (2012)
Electrochemical reaction…?
Science 339, 1402 (2013)
VO VO2 2
Electron correlation
Electrolytic colouration
16
M. M. Abraham et al.,
J. Solid State Chem. 51, 1 (1984)
MgO single crystal (transparent)
Mn impurity of ~100ppm
2.5mm thick
Pt cathod
+ + + + + + + + + + + + + Pt anode
apply 1.1kV for 2hrs
at 1050ºC
17
-
€
MgO single crystal (transparent)
MnO+ 2e− →Mn(metal)+ 1
2 O2
Electroreduction!
M. M. Abraham et al.,
J. Solid State Chem. 51, 1 (1984)
Mn impurity of ~100ppm
2.5mm thick
Pt cathod
+ + + + + + + + + + + + + Pt anode
apply 1.1kV for 2hrs
at 1050ºC
18
-

Current induced oxidation
J [A/cm2]
instantaneous destruction of Ti film
atomic rearrangement occurs!
oxygen is provided from air:
no barrier is formed in vacuum
nanocracks at grain boundary
T. Schmidt et al.,
Appl. Phys. Lett. 73, 2173 (1998)
108
107
106
J ~ 107A/cm2
Electroxidation!
I. H. Inoue et al.,
Phys. Rev. B77, 035105 (2008)
19
"Redox memory"
RRAM, ReRAM, Memristor, and so on
20
Resistance switching of redox memory
I. H. Inoue et al.,
Phys. Rev. B77, 035105 (2008)
Dielectric breakdown
“Lichtenberg Figure”
http://www.CapturedLightning.com/
An electron accelerator of three million volts blasts
electrons through the acrylic sheet.
It traps the electrons inside.
The electrons will stay trapped for hours, but a
knock with a sharp point opens a path for them to
make a quick escape.
River of electrons
“Lichtenberg Figure”
http://www.CapturedLightning.com/
e
Electrons gather from all parts of
the block, joining up to form
larger and larger streams of
electric current on their way toward
the exit point.
As the charge leaves, it heats up
and damages the plastic along the
branching trails it follows, leaving a
permanent trace of its path
e
e
e
e
Breakdown forms a bush
a point of high local field
(rough electrode, conducting
inclusions, etc.)
H. J. Wiesmann and H. R. Zeller,
J. Appl. Phys. 60, 1770 (1986)
filamentation (rapid
flow of space charge)
amplification, propagation and
multiplication (branching)
24

Breakdown forms a bush
H. A. Fowler, J. E. Devaney, and J. G. Hagedorn,
IEEE Transactions on Dielectrics and Electrical Insulation, 10, 73 (2003)
25
Forming of RRAM
anode (+)
anode (+)
e O2
O2
O2
cathode (-)
O2
electro-reduction
anode (+)
cathode (-)
cathode (-)
25th June 2008 26
€
CoO→
1
2
O2(gas) + Co2+ + 2e−
e e
e e
e
e
e
e e e
e
Open Faucet Closed Faucet
Current density at a faucet of
7x10-10cm2 (φ150nm)
Seminar @ TITech,N a1n6 oJeullye c2t0r1o4nics Days 2008 @ Aachen 15 May 2008
Reset
Jon ~ 1×107 A/cm2
27
Switching of RRAM
Can we apply
large electric field to TMO
without creating oxygen defects?
Transition-Metal Oxides
≈ ionic crystals
(because of the strong electron correlations)
!
!
!
→ Defects form easily under large electric field.
Yes!
Use Parylene to suppress
the defects formation
National Institute of Advanced Industrial Science & Technology (AIST) (Tsukuba, Japan)
Isao H. Inoue Neeraj Kumar Ai Kitou
Protect surface by Parylene
"Biocompatible glass is
coated with protective
substances for anti-migration
and insulating
properties and this is
where the Parylene C
coating comes. ! It also protects the
microchip from natural
substances in the body,
that may penetrate
through micro-cracks
caused by mechanical
damages."
From "moving a pet to Australia" website
Parylene coated
rotors and stators are
used to control the
Canadian arm for
NASA Space Shuttle.
Parylene coated circuit boards provides
excellent resistance to moisture, chemicals,
and mold. Circuit boards for medical
equipment can be steam and gamma
sterilised. Parylene can also prevent
dendrite and tin whisker growth.
From "Paratronix Inc." website
Parylene coating of paper
documents, autographs, and
photos retards the aging process
and protects from moisture, mold,
and chemicals.
30

Protect oxide surface by Parylene
Creation of oxygen vacancies is suppressed. !Channel is kept clean.
conformal coating
P.-J. Chen et al.,
Lab on a Chip 6, 803 (2006)
oxides
Parylene/SrTiO3 FET
SrTiO3
Using Parylene for the gate insulator,
mobility is drastically enhanced.
But carrier density is not large...
32
must be very thin
High-k/Parylene bilayer
to accumulate more carriers
In General, Parylene film is very thick
"Biocompatible glass is
coated with protective
substances for anti-migration
and insulating
properties and this is
where the Parylene C
coating comes. ! It also protects the
microchip from natural
substances in the body,
that may penetrate
through micro-cracks
caused by mechanical
damages."
From "moving a pet to Australia" website
Parylene coated
rotors and stators are
used to control the
Canadian arm for
NASA Space Shuttle.
Parylene coated circuit boards provides
excellent resistance to moisture, chemicals,
and mold. Circuit boards for medical
equipment can be steam and gamma
sterilised. Parylene can also prevent
dendrite and tin whisker growth.
From "Paratronix Inc." website
Parylene coating of paper
documents, autographs, and
photos retards the aging process
and protects from moisture, mold,
and chemicals.
Parylene film
in most of the
literatures are
more than
~1μm thick.
34
High-k (HfO2, Ta2O5, etc.)/Parylene bilayer
Hybrid gate insulator
!
high-k materials (~15 < ε < ~25)
+
Parylene-C (ε=3.2)
Isao Inoue and Hisashi Shima,
Japan Patent Number: 5522688, Date of Patent: 18th April, 2014
Au
HfO2
Al
SrTiO3
Ti
parylene
BF-TEM image
36

SrTiO3
Au
Al
HfO2
Ti
parylene
BF-TEM image
SrTiO3
Al
HfO2
parylene
BF-TEM image
38
Au Ti
HfO2
parylene
Al
SrTiO3
STEM-EDS mapping
39
We are preparing FET devices
using a conventional
photolithography
“Intel 4004 IC”
the original microprocessor
or “computer on a chip.”
Preliminary data of 20μm devices
41
High-k/Parylene/SrTiO3
cleaner interface
continuous doping control

National Institute of Advanced Industrial Science & Technology (AIST) (Japan)
Isao H. Inoue
Nanyang Technological University (Singapore)
Christos Panagopoulos
*also AIST
(now a PhD student in
Cornell University, US)
Azar B. Eyvazov*
CNRS & Université Paris Sud (France)
Pablo Stoliar** Marcelo J. Rozenberg***
**also Universidad Nacional de San Martin, Argentina,
and Université de Nantes, France
***also Universidad de Buenos Aires, Argentina
Unusual I-V curves
Hardly seen in Al2O3/SrTiO3
Al2O3/SrTiO3 has some amount
of carriers from the first
K. Ueno et al., App. Phys. Lett. 83, 1755 (2003)
10-6
44
A. B. Eyvazov et al., Sci. Rep. 3, 1721 (2013)
When increasing VSD
normal
abnormal !
10-6
increasing VSD
for large fixed VG
ΔV/VSD
ISD
increasing VSD
for small fixed VG
ΔV/VSD
ISD
0.3mm/0.8mm = 0.375
0.3mm/0.8mm = 0.375
45
normal
abnormal !
10-6
When increasing VG
not observed
ΔV/VSD
ISD
increasing VG
for any fixed VSD
ΔV/VSD
ISD
0.3mm/0.8mm = 0.375
0.3mm/0.8mm = 0.375
46
Negative Differential Resistance
Proc. Phys. Soc. 82, 954 (1963)
“Nonlinear…”
by E. Scholl,
Cambridge Univ. Press
(2001)
Negative Differential Resistance
increasing VSD
for small fixed VG
field domain!
increasing VG
for any fixed VSD
current path!
ΔV/VSD
ISD
0.3mm/0.8mm = 0.375
ΔV/VSD
ISD
0.3mm/0.8mm = 0.375
48

Numerical simulation: results
49
Comparison of Exp & Calc
Experiment Calculation
10-6
50
Simulation of path formation
51
Schematic picture of channel
52
High-k/Parylene/SrTiO3 FET
cleaner interface
filamentation
53

same S-shape I-V curves were observed
0 . 1 5
0 . 1
0 . 0 5
Hardly seen in Al2O3/SrTiO3
Al2O3/SrTiO3 has some amount
of carriers from the first
290 K
0 0 . 0 0 5 0 . 0 1 0 . 0 1 5
0 . 6
0 . 4
0 . 2
0
V 12 ( V )
280 K
ID ( nA)
0 0 . 0 1 0 . 0 2 0 . 0 3 0 . 0 4
0
V 1 2 ( V )
180 K
220 K
210 K
200 K
I
D ( nA)
€
ΔV ≡V1 −V2
€
ΔV ≡V1 −V2
55
Parylene/SrTiO3 FET
Filamentation at 7K
H. Nakamura et al.,
Appl. Phys. Lett. 89, 133504 (2006)
1011 1012
h/e2=25.8kΩ
i.inoue@aist.go.jp http://staff.aist.go.jp/i.inoue/ 26
Filamentation Occurs
even at 7K!!
Then, what happens
at ultra-low T
superconductivity?
SC at LaAlO3/SrTiO3 interface
nonvolatile JC ~ 100μA/cm
S. Thiel et al., Science 313, 1942 (2006)
TC ~ 200mK
HC2 ~ 0.1T
N. Reyren et al., Science 317,
1196 (2007)
SC at electrolyte/SrTiO3 interface
TC ~ 400mK
HC2 ~ 0.1T
JC ~ 300μA/cm
K. Ueno et al., Nature Materials 7, 855 (2008)
SC of bulk SrTiO3-δ
M. Jourdan et al., Eur. Phys. J. B 33, 25 (2003)
TC ~ 140mK
HC2 ~ 0.3T
JC ~ 100A/cm2
~ 100μA/cm (for t10nm)
Good agreement in the orders with
!
1) SC at LaAlO3/SrTiO3 interface,
2) SC at electrolyte/SrTiO3 interface,
!
and
!
3) our gate-annealed SC
(next slide).

SC in "gate-annealed" parylene/SrTiO3
31
Sample A
Al electrode
0.2
Voltage (mV)
0
SC was seen only after
1.4 1.8 2.2
Temperature (K)
VG threshold is lowered.
Nonvolatile metallic state.
Bulk-like superconductivity
due to oxygen vacancies?
TC ~ 350mK
HC2 ~ 0.1T
prolonged (one-day) application
of large VG
H. Nakamura et al., J. Phys. Soc. Jpn. 78, 083713 (2009).
Oxygen vacancy creation
on SrTiO3
M. Janousch et al., Adv. Mat. 19, 2232 (2007)
0.2 mol% Cr-doped
SrTiO3
By applying 105V/cm
for about 30 min
Pt
Pt
Oxygen vacancies are created,
and distributed in the channel,
and form a metallic path.
All the superconductivity of
SrTiO3 interface shown here
might be caused by oxygen
defects…
Is this the conclusion? Is this the conclusion?
All the superconductivity of
SrTiO3 interface shown here
might be caused by oxygen
defects…
No. we observed another
39
Another “ ” State
Domain formation of doped carrier
and percolation transition
1011 1012
is Fragile
·SC at electrolyte/SrTiO3 interface
·gate-annealed SC
·SC at LaAlO3/SrTiO3 interface
·bulk superconductivity
TC ~ 400mK
HC2 ~ 0.1T
JC ~ 300μA/cm

Summary
high-k/Parylene to
protect surface
zzz
Filamentation

Electrostatic carrier doping and quantum critical phenomena in transition metal oxides

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