Advanced Materials / Nanoscienceand Nanotechnology

(CHEM552/CHEM634)

Oxide nanoelectronics

Andreas Ruedigerruediger@emt.inrs.ca

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