NC STATE UNIVERSITY

Direct observation and characterization of domain-patterned Direct observation and characterization of domain-patterned ferroelectrics by UV Photo-Electron Emission Microscopy ferroelectrics by UV Photo-Electron Emission Microscopy

Woochul Yang, Brian J. Rodriguez, Alexei Gruverman, and Robert J. NemanichDepartment of Physics, and Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695-8202

SummarySummary• We observed polarity contrast of domain-patterned ferroelectrics with UV-PEEM.

• Enhanced emission from negative domains is attributed to differences in electron

affinity.

• The photothreshold for negative domains of LNO and PZT were found to be 4.6eV

and less than 4.3eV, respectively.

Future WorkFuture Work• Compare PEEM of an as-loaded and clean surface of ferroelectrics

• Include screening charge (internal and external) into model

• XPS/UPS to measure surface Fermi level position

• How are these differences manifested in other interfaces (GaN growth on PPLN)

AcknowledgementsAcknowledgementsDuke University Free Electron Laser Laboratory

ONR and AFOSR

PspPsp

Negative domain

Positivedomain

-- --

- -- -- - - -+ + + +

++++

++++

E- - - - - - - -

E

-+ Polarization-induced bound charges+ - Absorbates (accumulated charges)- - Electrons emitted from valence band

PspPsp

-- --

- -- -- - - -+ + + +

++++

++++

- - - -

- - - -

Emission modelEmission model

• Surface dipole induced by adsorbates changes surface electron affinity.• Electron affinity for negative domain is lower and thus emission is more intense.

PEEM contrast of ferroelectricsPEEM contrast of ferroelectrics

Negative domain

Eth = Eg + s - Positive domain

Eth* = Eg + s + *

Evac

Ec

EF

Ev

Psp

Eth

s

Eg

+ ++

++

+

Evac

Ec

EF

Ev

Psp

Eth*s

Eg

*

++

+

++

+

___

___

eff

eff*

Energy band diagram of polar domainsEnergy band diagram of polar domains

• Difference in electron affinity due to the surface dipole causes a PEEM polarity contrast between the positive and negative end domains of ferroelectrics surfaces.

• PZT

- s ~ 3.5eV (Jpn. J. Appl. Phys. 38, 2272 (1999)), Eg ~ 3.4 eV Eth ~ 6.9eV

- PEEM measurement: Eth ~ < 4.3eV, eff = s - = Eth – Eg = < 0.9 eV

Eth* ~ > 6.0eV, eff* = s + * = Eth* – Eg = > 2.6 eV

• LNO

- s ~ 1.1eV (Sov. Phys. Solid State 25, 1990 (1983)), Eg ~ 3.9 eV Eth ~ 5.0 eV

- PEEM measurement: Eth ~ < 4.6eV, eff = s - = Eth – Eg = < 0.7 eV

Eth* ~ > 6.2eV, eff* = s + * = Eth* – Eg = > 2.3 eV

MotivationMotivation

• Precise control of ferroelectric domains has become important as a

new approach to the self-assembly of complex nanostructures.• Direct information about local polarization, charge distribution, and

potential of the ferroelectric surface is necessary to control the

local electronic structures and to influence the chemical reactivity.• UV-FEL PEEM can allow us to image ferroelectric domain

structures with high resolution (~ 10nm) and to obtain local

polarization and surface electronic structures through the variation

in work function on the surface.

GoalsGoals

• PEEM observation of ferroelectrics with polarity patterned domains• Understanding PEEM polarity contrast of ferroelectric materials• PEEM measurement of photothresholds of ferroelectrics to

understand local electronic properties

Sample (-20kV)

Anode (ground)

Channel PlateP-screen

CCD

Objective lens

Intermediate lens

hv

Projective lens

Concept of PEEM • Illuminate sample with UV photons just

above the photoelectron threshold.

• Accelerate photo-electrons with an immersion lens and image the surface with conventional electron optics with high magnification.

Advantages of PEEM • Good surface sensitivity: depth(1-20 nm)

• High spatial resolution: ~10nm

• In situ, real-time characterization of film surfaces

• Non-destructive imaging method

• Measurement of surface work function and electronic structures of materials (UV-FEL PEEM)

What is PEEM?What is PEEM?

Coherent FEL radiationSpontaneous Radiation 4.0-6.3eV

UV-XUV FEL

OK-4 System

e-beam Injection

Laser Mirror

Mirror

1 GeV Duke Storage Ring

Evaporator

CCD

AES

MBE

Lens Column

computer& image processor

et

• hv : 4 – 6.3 eV

• Pa~ 2mW, Pp~ 20W

• : 100 psec • t : 6 nsec

• np : 5 x 1015 photon/sec

• E/E : ~1%

UV-FEL Parameters

FEL-PEEM : PEEM (~10nm resolution) + FEL (tunable, high-intensity, polarized light)

UV-PEEM at Duke FELUV-PEEM at Duke FEL

Measure piezoelectric properties by detection of sample deformation

Positive domain Negative domain

PspPsp

-- --

- -- - ++++

++++

Phase contrast of ferroelectric domain polarity

In phase: Brighter (negative domain)Out of phase: darker (positive domain)

LNO50m

Piezoresponse Force MicroscopyPiezoresponse Force Microscopy

positive domain

negative

domain

10m

PEEM PFM AFM

hv = 4.8eV

• In PEEM, the brightness contrast displays different polar domains.• PFM measurement confirms that the bright regions are negative domains.

Polarity-patterned PbZrTiOPolarity-patterned PbZrTiO33(PZT) thin films(PZT) thin films

150oC 250oC 300oC

The polarity contrast disappears at near the Curie temperature of ~ 300oC

10m hv = 4.9eV

PEEM images of PZT during annealingPEEM images of PZT during annealing

hv=4.3eV 4.8eV 5.0eV

5.2eV 5.5eV

Emission threshold of negative domains is less than 4.3eV

- +

6.3eV

PEEM images of PZT-photon energy scanPEEM images of PZT-photon energy scan

10m

10m

Before etchingBefore etching After etchingAfter etching

PEEM

PFM

AFM

PFM

• The brighter (negative) domains is wider than darker (positive) domains.• Chemical etching (negative domain) and PFM measurement confirm that

the emission from the negative domains is more intense.

+ -

+ -+ -

+-

Polarity patterned LiNbOPolarity patterned LiNbO33 (LNO) crystal (LNO) crystal

hv=4.5eV 4.6eV 4.7eV

5.2eV

Emission threshold of negative domains is less than 4.6eV

5.9eV 6.2eV

PEEM images of LNO - photon energy scanPEEM images of LNO - photon energy scan

Evac

Eg

h

Semiconductor SurfacesMinimum Escape Energy

h = + Eg

Ferroelectric Semiconductor Surfaces: local variations in (electron affinity), Eg

(band gap), band bending, doping density will change the minimum escape energy and lead to PEEM contrast.

PEEM Image Contrast: Photo-threshold differencePEEM Image Contrast: Photo-threshold difference

Eg

*1

2

• Contrast between two regions can be obtained by choosing 2 < hv < 1

• Photo electrons will be emitted from region 2 (bright) but not from region 1 (dark)

10m

2

1