Oxide Nanoelectronics
Chang‐Beom Eom
Department of Materials Science and Engineering
University of Wisconsin‐Madison
Supported by
National Science Foundation (NIRT, FRG)
Office of Naval Research
Army Research Office
AFOSR
DOE/BES
IARPA
DARPA
David & Lucile Packard Fellowship
Multifunctionality of Complex Oxides
MRAMs
FRAMs
Ferroelectrics
Ultrasound imaging
Superconductors
Ferromagnetics
Actuators
Medical imaging
LEDs
Maglev trains
Piezoelectrics
Complex oxides
Semiconductors
Transparent
conductors
Optical
waveguides
Nonlinear
optics
Dielectrics
Metallic
conductors
Gate oxides
Flat panel displays
Fuel cells
What is unique about Multifunctional Oxides?
•
•
•
•
Strong anisotropy
Variable oxygen stoichiometry
Controllable by cation doping
Novel Hybrid devices with
many multifunctional oxides
Cu
O
Cu
Cu
O
Silicon
Cu
YBa2Cu3O7
Various forms of Multifunctional Oxides
1. Ceramics
• easy to prepare
• quickest way to search for new materials
• intrinsic properties get masked by grain boundaries
2. Single Crystals
• intrinsic properties can be studied
• hard to obtain large samples with uniform properties
• large sample dimensions
3. Thin Films
• well defined dimensions (thickness, lateral dimensions)
• device applications (multilayered heterostructures)
• orientation controllable
• artificial structures can be made
Not all films are the same!
Polycrystalline Thin Film
Epitaxial Thin Film
Enhancement of ferroelectric transition temperature in
strained BaTiO3 thin films
c
4.15
Tc (50nm BaTiO3/DyScO3)
Lattice Constant (Å)
4.10
a
a
Epitaxial BaTiO3
GdScO3 or DyScO3
substrate
Tc (100nm BaTiO3/GdScO3)
4.05
4.00
Tc (Single Crystal)
3.95
3.90
0
100 200 300 400 500 600 700 800
Temperature (°C)
K.J. Choi et al. Science, 306, 1005 (2004)
Superior Jc of Co-doped Ba122 film on SrTiO3(BaTiO3)/LSAT
10
7
 Critical Current density (Jc at ~5K)
Ba122 on BTO/LSAT
6
Ba122 on STO/LSAT
LSAT
2
Jc (A/cm )
10
Ba122 film
STO(BTO)
10
10
5
4
Ba122 bulk single crystal
(Yamamoto et al., Appl. Phys. Lett., 2008)
Sr122 on bare LSAT
(Hiramatsu et al., Appl. Phys. Express, 2008)
★
Sr122 on bare LAO
(Choi et al., Appl. Phys. Lett.,2009)
Ba(Sr)122 film
(La,Sr)(Al,Ta)O3  LSAT
Ba122 on bare LSAT
10
3
0
2
4
6
8
10
12
14
or LaAlO3  LAO
Magnetic Field (T)
S. Lee, Nature Materials 9, 397 (2010)
4
Why we need atomic layer control?
• Nanoscale Control of Interfaces and Defects
• Uniformity of Barrier Layers
Field Effect Devices
SQUID
Coated Conductors
RI
RI
Top Electrode
~ nm
Bottom Electrode
Substrate
Magnetic Tunnel Junction
Ferroelectric Memories
P
(µC/cm2)
Voltage (V)
Quasi-ideal surface of SrTiO3 substrate
SrTiO3(001)
AFM as received:
mixed termination
Ti
Sr
O
250 nm
500 nm
SrO termination plane
TiO2 termination plane
Etching in BHF
M. Kawasaki, Science, 266, 1540 (1994).
[G. Koster et al., APL, 73 , 2920, (1998).]
Perfect TiO2-terminated SrTiO3 substrate
Thermodynamic stability diagrams
800
600 400
10
T [oC]
25
200
Log (PO2/Torr)
0
-10
V2O5
-20
VO2
-30
V2O3
-40
Cooling
Curve
VO
-50
V
-60
5
10
15
20
25
30
10000/T [1/K]
High PO2 is needed to stabilize oxide phases
35
How to make atomic flux? Laser Ablation
Sputtering
Target
Ar+ ion
Atoms
RHEED Intensity and Pattern
• Intensity: growth kinetics
• Pattern: surface structures
RHEED pattern
thin reciprocal
lattice rods
and morphologies
3D islands
Ideal smooth
Ewald
sphere
screen
Polycrystal
RHEED intensity oscillation
Real smooth
broad reciprocal
lattice rods
Laser MBE system with in situ High Pressure RHEED
Phase transition of SrRuO3
RHEED works up to 1 Torr O2
load lock
laser-beam
substrate holder
heater
phosphor
screen +
camera
Electron-gun
Ø 1 mm
o
0-3
filament
Ø 0.5 mm
850 oC
target holder
100C
SrTiO3
50 mTorr PO2
I/I0
-1
<10Pa
<100 Pa
to main pump
650C
O,Ar,Ne,He
-4
<5×10Pa
`
2
Rijnders et al. Appl Phys. Lett. 70 (1997) 1888
Time (s)
Atomic layer controlled growth of SrRuO3/SiTiO3/SrRuO3 trilayers
SrRuO3
SrTiO3
34 pulses
SrRuO3 bottom layer
SrRuO3
400 nm
SrTiO3 substrate
6 unit cell SrTiO3 barrier layer
22 pulses
200 nm
SrRuO3 top layer
RHEED intensity oscillation
C.B. Eom et al. Science, 258, 1799 (1992).
400 nm
TEM images of SrRuO3/SrTiO3/SrRuO3 trilayers
SrRuO3 top
[110]o
SrRuO3
SrRuO3 bottom
[111]o
[111]oSrRuO3
SrTiO3
[001]
RuO2
TiO2
[110]
[110]SrTiO
3
SrRuO3
SrTiO3 substrate
SrTiO3 barrier
SrO
6ML
TiO2
RuO2
SrO
Sharp interface structure
Single domain structure
TEM: W. Tian and .X. Pan, Univ. of Michigan
Ferroelectric tunnel junctions in nonvolatile memory and logic devices.
A. Gruverman et al. Nano Letters, 9, 3539 (2009)
Ferroelectric Tunnel Junctions
Intensity (percent)
100
Start
80
1
60
8
10 ML
4 5 6 7
9
2
3
40
20
SrRuO3
End
0
40
80
120
160
Elap sed Time (seconds)
200
BaTiO3
SrRuO3
I‐V curves for two opposite polarization directions
Mechanical Writing of Ferroelectric Polarization
H. Lu et al. Science, 336, 59 (2012) Strongly Correlated 2DEG
“Metallic and insulating oxide interfaces controlled by
electronic correlations
H. W. Jang, et al, Science , 331, 886 (2011)
22
B
Scan direction
LaO
OK
Ti L2,3
Intensity (arb. unit)
A
L2
1 nm
L3
455
460
465
470
LaO
SmO
1 nm
1 nm
540
550
Ti L2,3
TiO2/LaO
TiO2/SmO
Intensity (arb. unit)
D
530
Energy loss (eV)
Energy loss (eV)
C
520
450
460
470
Energy loss (eV)
480
E.Y. Tsymbal Giant Piezoelecticity for
Hyper-Active MEMS
• (S.H. Baek et al. Science, 334, 958 (2011)
• Medical Imaging
• Sensing
• Actuation
• Piezotronics
• Energy Harvesting
Ultrasound Medical Imaging
Ceramic PZT
transducer array
At 1 MHz
(1960)
GE Medical
20x20
first 2-D array
(currently
256x256 = 65,536
subdiced elements)
S.W. Smith, Duke Univ.
(2003)
5MHz, intra-cardiac Catheter 2D Array
7 Fr, 112 Elements
Side Scanning
12 Fr, 64 Elements
Side Scanning
Ultrasound used in virtually every medical specialty:
• Obstetrics, Cardiac, Abdominal Rad. (2-10 MHz)
• Endoscopy (5-15 MHz)
• Intravascular, Skin, Eye (10-50 MHz)
• Ultrasound Microscope, Blood cells (100-200 MHz)
Better Resolution and Deeper penetration are desirable.
(Giant Piezoelectric Materials !!!)
Hyper-Active NEMS
MEMS spatial light modulator for maskless lithograohy (V. Aksyuk, Bell Labs)
10 million electrostatically actuated mirrors
Major challenges are emerging as MEMS move to smaller size and
require increased integration density with faster and larger relative
motion range.
A force achieved by electrostatic actuator at 100 V requires only
0.01 V in a hyper-active NEMS. force achieved by the intrinsic
electrostatic
Bulk single crystal relaxor ferroelectrics (Pb(Mg1/3Nb2/3)‐
PbTiO3 (PMN‐PT) and Pb(Mg1/3Zn2/3)‐PbTiO3 (PZN‐PT) ).
(10 times higher piezo response than PZT ceramics, (d33) = 1500‐2500 pm/V )
S‐E. Park, T.R. Shrout J. Appl. Phys. 82, 1804 (1997).
• k33 > 90%, • d33 > 2000 pC/N, • strain up to 1.7%
Field Induced Phase Transition in (001) PZN‐8% PT Single Crystal
S‐E. Park, T.R. Shrout , J. Appl. Phys. 82, 1804 (1997).
SrTiO3 on (001) Silicon
STO
Si
2 nm
MBE growth by D.G. Schlom, Penn State, TEM by X.Q. Pan, Michigan
Cross Sectional TEM images of PMN-PT on silicon
Rocking
Curve
FWHM
PMN-PT
PMN-PT
0.23
SrRuO3
0.58
SrTiO3
100 nm
Si
2 nm
SrRuO3
TEM by X.Q. Pan, Michigan
o
0.32 (bulk PMN-PT single crystal)
o
o
Effective Transverse Piezoelectric Coefficient (-e31)
30
Epitaxial PMN-PTon Si
(This work)
-e31 (C/m2)
25
20
Best PZT Film on Si
15
10
5
0
0.0
PMN-PT on Si (Previous work)
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Thickness (m)
Measured by S. Trolier-McKinstry, Penn State
PMN‐PT Active Cantilevers
Pt
PMN‐PT
Si
SrRuO3
SrTiO3
Cantilever W5‐2‐7
35 µm long, 3.2 µm wide
400
Height (µm)
0.2
0.0
‐0.2
‐0.4
Cantilever clamping point
‐0.6
Resonance frequency (kHz)
400
2.8V
Resonance frequency (kHz)
0.4
Length:35µm
350
300
350
300
250
200
150
250
100
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1000 / length2 (1/µm2)
200
150
0V
‐0.8
0
0.02
0.04
Length (mm)
0.375 µm / V deflection
Resonant frequencies, 100s of kHz.
100
30
35
40
45
Cantilever length (µm)
50
1.2
MEMS performance
• PMN‐PT piezoelectric properties unaffected by processing
• Simulations match experimental data using bulk PMN‐PT parameters
Pt (60nm)
PMN-PT (270nm)
SrRuO3 (100nm)
SrTiO3 (13nm)
Si (substrate)
1.0
0.8
0.6
∆Z (μm)
Tip displacement (µm)
0.8
Modeled PMN‐PT cantilever
0.0
0.4
0.2
0.0
0.0
1.5
3.0
Voltage (V)
0.4
0.2
0.6
Actual PMN‐PT cantilever
Modeled electrostatic cantilever
100
101
Voltage (V)
102
Inter‐digitated
symmetric Electrode 40
Effective Polarization (µC/cm2)
Effective Polarization (µC/cm2)
Pt / PMN‐PT / SrRuO3
Sandwich type Electrode 30
20
10
0
-10
-20
-30
-40
-200
-100
0
100
200
Effective Electric Field (kV/cm)
Unipolar Operation 40
30
20
10
0
-10
-20
-30
-40
-200
-100
0
100
200
Effective Electric Field (kV/cm)
Wide Bandwidth Piezoelectric Micro Energy Harvester Based on Nonlinear Resonance
Power density 2 W/cm3
A. Hajati and S.G. Kim, APL, 99, 083105 (2011)
The maximum extractable power : 45μW (based on PZT: d33 =110 pm/V).
Power‐density: 2 W/cm3 (PZT volume: 4×5݉݉×4݉݉×265݊݉ ≈ 0.021݉݉3)
PMN‐PT: >200 W/cm2 (d33 = 1200 pm/V) Pextractable
1
2
 ce    
2
45μW
PPZT  VPZT
A. Hajati and S.G. Kim, APL, 99, 083105 (2011)
2
2
2
EPZT
SPZT
d33
fex
 PZT
Piezotronics
Saturation in CMOS Computer Speed and Performance
•
More transistors can be put on a chip
(“Moore’s Law”)
Moore’s Law
BUT
• CMOS clock speed has not increased
since 2003
(Compute performance has also slowed)
•
Reason for clock speed saturation is
that Voltage has stopped scaling.
Attempting to increase speed at
constant VDD results in economically
unacceptable power dissipation.
• Challenge is to find a “new switch”
operable at low voltage.
D.M. Newns, IBM
From jai-on-asp.blogspot.com
Low power and fast digital switching technology
A gate voltage on a piezoelectric (PE) applies pressure to a
piezoresistive (PR) material which induces a insulator-metal transition,
turning on the current through sense.
Isense
Insulatormetal
transition
SmSe
Insulator
pressure
Metal
Vgate
Piezoelectronic Transistor (PET)
D.M. Newns, IBM
MRAM Operating at Ultra low voltrage
• Ultrahigh storage capacity of up to 88 Gb/in2
• Ultralow power dissipation as low as 0.16 fJ/bit • Room temperature high‐speed operation below 10 ns.
J.M. Hu et al, Nature Communications, 2, 553 (2011) Summary
1. Oxide materials has a great potential for novel oxide electronics and discovering new solid state phenomena.
2. Strain, domain and interfacial engineering by heteroepitaxy is a general means for achieving extraordinary physical properties in thin films.
3. Oxide electronics is just beginning. There are much challenges and opportunities.
Collaborators
S.H. Baek, J. Park, D.A. Felker, D.M. Kim, R. R. Das, R. Blick, M.S. Rzchowski University of Wisconsin‐Madison
V. Aksyuk
National Institute of Standards and Technology, Gaithersburg
V. Vaithyanathan, J. Lettieri, N. B. Gharb, S. Trolier‐Mckinstry, The Pennsylvania State University D.G. Schlom
Cornell University
V. Nagarajan, R. Ramesh
University of California Berkeley
Y. B. Chen, H. P. Sun, X.Q. Pan
University of Michigan
S.K. Streiffer
Argonne National Laboratory