Oxide Nanoelectronics
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Transcript of Oxide Nanoelectronics
Advanced Materials / Nanoscienceand Nanotechnology
(CHEM552/CHEM634)
Oxide nanoelectronics
Andreas [email protected]
Overview
• Oxides- composition- structure- electric properties
• Applications- free charges- bound charges- intermediate scenarios
• Nanoscale characterization
Literature
R. Waser (editor) Rabe, Ahn, Triscone(editors)
Why oxides?
• Si is a one-component system and has dominated the last decades more than any other material
• Oxides are more complex, chemically and structurally so they offer more degrees of freedom but they are much harder to control
• Their applications range from superconductors to insulators, from emitters to sensors and from static to THz or even optic components
Why nanoelectronics?
• The present generation of Si-based processors is fundamentally limited by heat dissipation (twice as much as a hot plate)
• New materials required to change existing paradigms: cheaper, lower consumption and adaptable to novel circuit designs (architectures)
• Integrated functionality
Bound charge systems
Applications
Applications
Free charge systems
Examples
• TiO metallic, TiO2 insulator
• SrTiO3: insulator, SrTiO3:Nb (0.5 %wt) metallic and superconducting below 900 mK
• BaTiO3: insulator, ferroelectric
• BaxSr1-xTiO3 (BST): tuneable dielectric
• SrRuO3: metallic
• YBa2Cu3O7-d : superconductor high Tc : 93K
Composition
• Small compositional variations can dramatically modify the properties
- intrinsic defects are isolated only in small concentrations, for moderate and high concentrations, they have a tendency to aggregate and to form clusters, chains or planes
- extrinsic defects (dopants) are often significant in concentrations of only a few ppm
Structure
anatase rutile brookite
Strong performance variation in photocatalytic properties
http://www.davidonindustries.com/
Structure (BaTiO3)
Electric properties
• Chemically tuneable properties such as permittivity and conductivity
• Most general description by impedance (resistance and capacitance)
• In general strongly dependent on temperature and frequency
Ohm’s law
ji=sijEj
sij=qemijN
ji =: current density [A/m2]sij: conductivity [1/Wm]Ej: electric field [V/m]q: charge number (integer of + or -)e: elementary charge [C]mij: mobilityN: number of charges
Capacitive behaviour
• C=ee0A/d
Please forget that you ever heard of a dielectric “constant”!!!
BaTiO3
BaxSr1-xTiO3
Leaky capacitors (free carriers in a bound-carrier system)
K.S. Seol et al. Appl. Phys. Lett., 85, 2325 (2004)Comment : A. Rüdiger, Appl. Phys. Lett., 86, 256101 (2005)
Maxwell-Wagner effect
Frequency [Hz]
200 nm
Alternation of conducting and insulating layers has zero DCconductivity but extremely high AC conductivity (together with a high dielectric permitttivity)
How to modify the conductivity?
Conductivity and defects (intrinsic)
Oxygen sensor at high temperatures
Conductivity and defects (extrinsic)
Example: local conductivity in SrTiO3
K. Szot et al. Nature Materials (2006)
Bistable electrochemical switches
Bistable resistive switches (memristor hype)
Bistable resistive switches (integration)
Current line width 20 nm
Bistable resistive switches (model)
The nature of the conductive state is the best indication forthe mechanism:1) semiconducting favors TiO2-x
2) metallic would be in agreement with pure TiFor bipolar switching, we observesemiconducting characteristics forunipolar (fuse-antifuse) we obtainmetallic conduction
Bistable resistive switches (speed)
10 ns pulse width, 3 ns rise time
Bound charges
Classification
dielectric32/32
point-groups
piezo-electric20/32
hjk=dijkEi
Î3dijk=dijk=0
pyro-electric10/32
DPi=giDT
polar axis
Pi=e0cijEj+e0cijkEjEk+…
ferro-electric
Structure-polarization relationshipin perovskites
Ti4+
O2-PBa2+
Ti4+
O2-
Ba2+
Ti4+
O2-
Ba2+
Ti4+
O2-
Ba2+
≈0.4 nm
perovskite: BaTiO3, PbTiO3, Pb(ZrxTi1-x)O3
Conventional detection
Sawyer-Tower circuit
Phase transitions
Domains
Electrostatics of the depolarization fieldcounterbalanced by domain wall energy
FeRAM or FeHDD
courtesy of D.J. Jung, Samsung Y. Cho et al., Appl.Phys.Lett., 87, 232907 (2005)
P
Challenges: scaling of displacement charges
FerroFET
Challenges: retention and scaling
Superparaelectric limit
• In analogy to the superparamagnetic limit there is a critical size below which the polarization irrevocably ceases
• This limit is of high industrial relevance as it indicates the ultimate physical limit for integration of ferroelectric devices
• Different from ferromagnets, this limit strongly depends on the system, i.e. electrodes and the material
Ferroelectrics goes bananas
P-E loops of leaky dielectrics look almost like ferroelectric hysteresis loopsBa2NaNb5O15 (nicknamed banana) compared to a real bananaJ.F. Scott “Ferroelectrics go bananas”, Journal of Physics: Condensed Matter (2007)
Piezoelectrics• Di=dijkTjk (direct piezoelectric effect)
• hij=dkijEk (converse piezoelectric effect)
Highly efficient electromechanical energy conversion: energy harvesting
Pyroelectrics
• DPi=giDT
Motion detection, thermal imagingmost sensitive working point: close to phase transitions at the price of a narrow temperature range of operation
Total dielectric displacement
• Di=e0(eijEj+dijkTjk+giDT+Psi)
• Surface charge density given by:
1) induced polarization (external field)2) piezoelectric effect (stress)
• 3) pyroelectric effect (temperature)
• 4) permanent polarization (internal field)
Coupling to the environment
Multiferroics
Coexistence of ferroelectric, ferroelastic or magnetic ordering: multiferroicIf ferroelectricity and magnetism are coupled: magnetoelectric -> sensors
Intermediate case (photoexcited carriers)
• Photorefractive effect: light is used to modify the refractive index profile via charge transfer:
holographic high density data storage and optical transistors
• BULK-photovoltaic effect: photocurrents without need for interface engineering
The photorefractive effect
Source: K.Buse, University of Bonn
Challenge: low mobility and low carrier concentration
Bulk-photovoltaic effect
• Charge separation after electron-hole creation (band-band illumination) by the internal polarization
• Low fabrication effort
challenges:
• High internal resistance of the current source
• Large bandgap for most ferroelectrics or limited penetration depth
Nanoscale characterization techniques
• Piezoresponse force microscopy to monitor the local polar properties (>10 nm)
• Conductive AFM to measure local conductivity(>3 nm)
• Tip-enhanced Raman spectroscopy for composition and structure (>30 nm)
Piezoresponse Force Microscopy (PFM)
z
x
y
b
cd
a D
z
V
1 lock-in
2 lock-in
low-pass
feedback
t-b
l-r
topography
Lateral
vertical
piezo unit
lase
r
ferroelectricbottom electrode
reference
w
Detection scheme: contact mode
vertical lateral
Converse piezo-electric effect:hjk =dijkEi
Assumption: Electric field has only z-component
Adapted from: A. Rüdiger et al., Applied Physics A (80), 1247, 2005L.M. Eng et al., Adv. In Solid State Physics. 41, 287-298 (2001)
P
U
jk
Vpm
kijjiji xEdxx
/
D h
PFM Amplitude and Phase
XR
Y
t
Amp ExcitationResponse I180o shiftResponse II
Assumption: Linear response
AE
AIAII
Advantage of X and Y: Higher bandwidth
For complete characterization:- Amplitude and Phase- In-plane and Out-of-plane response- 4 inputs required
restriction to commercially available tools
Out-
of-
pla
ne
In-p
lane
Conductive atomic force microscopy
Tip-enhanced Raman spectroscopy
• Raman spectroscopy provides a vibrationalfingerprint of a material and is sensitive to phase transitions (LST-relation)
• In order to achieve a lateral resolution below the diffraction limit of light, we use a tip-enhanced configuration
Best lateral resolution today 15 nm FWHM, single molecule sensitivity
Future of oxide nanoelectronics• Nanoelectromechanical systems (NEMS)
- actors, emitters, and sensors
• Non-volatile memories
• Chemical sensors
• Electrode materials in chemically aggressive environments
• Scientific challenges on the local surface control of functional properties