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