The surface potential of a multidomain BaTiO3 (001) surface was imaged in the presence and absence of ultraviolet (UV) illumination. The UV radiation induces a decrease in the surface potential contrast between c+ and c− domains with a time constant of a few seconds due to redistribution of photocarriers and screening. A slower process of recovery was observed after illumination. In addition, scanning a conducting atomic force microscopy tip in contact under UV illumination destabilizes some ferroelectric domains

Bonnell, Dawn A

Nikiforov, Maxim P

Shao, Rui

English

Kosmopolis

University of Pennsylvania
ScholarlyCommons
Departmental Papers (MSE) Department of Materials Science & Engineering
September 2006
Photo-induced Charge Dynamics on BaTIO3
(001) Surface Characterized by Surface Probe
Microscopy
Rui Shao
University of Pennsylvania
Maxim P. Nikiforov
University of Pennsylvania
Dawn A. Bonnell
University of Pennsylvania, bonnell@lrsm.upenn.edu
Follow this and additional works at: http://repository.upenn.edu/mse_papers
Postprint version. Published in Applied Physics Letters, Volume 89, Issue 11, 2006, 112904-1-3.
Publisher URL: http://dx.doi.org/10.1063/1.2348776
This paper is posted at ScholarlyCommons. http://repository.upenn.edu/mse_papers/108
For more information, please contact libraryrepository@pobox.upenn.edu.
Recommended Citation
Shao, R., Nikiforov, M. P., & Bonnell, D. A. (2006). Photo-induced Charge Dynamics on BaTIO3 (001) Surface Characterized by
Surface Probe Microscopy. Retrieved from http://repository.upenn.edu/mse_papers/108
Photo-induced Charge Dynamics on BaTIO3 (001) Surface
Characterized by Surface Probe Microscopy
Abstract
The surface potential of a multidomain BaTiO3 (001) surface was imaged in the presence and absence of
ultraviolet (UV) illumination. The UV radiation induces a decrease in the surface potential contrast between
c+ and c− domains with a time constant of a few seconds due to redistribution of photocarriers and screening.
A slower process of recovery was observed after illumination. In addition, scanning a conducting atomic force
microscopy tip in contact under UV illumination destabilizes some ferroelectric domains.
Comments
Postprint version. Published in Applied Physics Letters, Volume 89, Issue 11, 2006, 112904-1-3.
Publisher URL: http://dx.doi.org/10.1063/1.2348776
This journal article is available at ScholarlyCommons: http://repository.upenn.edu/mse_papers/108
Photoinduced charge dynamics on BaTiO3 „001… surface characterized
by scanning probe microscopy
Rui Shao, Maxim P. Nikiforov, and Dawn A. Bonnella
Department of Materials Science and Engineering, University of Pennsylvania, 3231 Walnut St.,
Philadelphia, Pennsylvania 19104
Received 4 May 2006; accepted 26 July 2006; published online 11 September 2006
The surface potential of a multidomain BaTiO3 001 surface was imaged in the presence and
absence of ultraviolet UV illumination. The UV radiation induces a decrease in the surface
potential contrast between c+ and c− domains with a time constant of a few seconds due to
redistribution of photocarriers and screening. A slower process of recovery was observed after
illumination. In addition, scanning a conducting atomic force microscopy tip in contact under UV
illumination destabilizes some ferroelectric domains. © 2006 American Institute of Physics.
DOI: 10.1063/1.2348776
Super-band-gap light illumination on a semiconducting
ferroelectric surface induces various interesting charge ef-
fects related to the spontaneous polarization.1 It is well
known that photoexcitation causes increased surface conduc-
tivity, domain imprint, and changes in dielectric constant and
piezoelectric coefficient. The underlying mechanism is the
redistribution of photoexcited charge carriers in a ferroelec-
tric domain driven by the depolarization field to form intrin-
sic screening layers. Consistent with this picture is the re-
ported domain orientation-dependent local photochemical
reactivity on ferroelectric surfaces, i.e., reduction occurs on
out-of-the-plane polarized domains and oxidation on into-
the-plane polarized domains.2 This discovery led to domain
controlled photocatalysis and opened a pathway to the di-
rected assembly of nanoparticles with engineered domain
structures.3,4 Understanding the charge dynamics is critical
for applications of ferroelectric materials in electro-optical
storage and sensor devices. Extensive experimental studies
on the photoinduced charge effects using the macroscopic
parallel-plate capacitor configurations5 have been done. Cur-
rently several electrical scanning probe techniques with na-
nometer spatial resolution and high sensitivity to electric
fields at this length scale make direct observation of photo-
carriers on individual domains possible, which is the subject
of this letter. Local imaging by a scanning probe provides
information about an electrode-free ferroelectric surface;
thus screening charge is from either intrinsic charge and/or
adsorbates rather than from carriers in electrodes as in the
case of parallel-plate capacitor measurements.
Scanning probe imaging of domains and measurement of
local hysteresis loops in ferroelectrics have been carried out
routinely by many researchers.6 Only Gruverman et al. re-
ported the UV light-induced shift in hysteresis loops and
domain pinning using piezoresponse force microscopy
PFM.7 PFM, a powerful imaging tool, differentiates do-
mains based on the contrast in the phase shift of the cantile-
ver oscillation. To detect the relative amount of surface
charge on domains, however, scanning surface potential mi-
croscopy SSPM also referred to as scanning Kelvin probe
microscopy8 is used in this study. SSPM is implemented in
noncontact mode, in which case charge transfer between tip
and surface is minimal. PFM, in contrast, is in contact mode
with a conductive tip as an electrode. This difference may
lead to different effects on a ferroelectric surface under UV
light, which is also investigated in this work.
A BaTiO3 single crystal 551 mm3, TC=130 °C,
MTI Corp. with an epi-polished 001 surface was heated on
a hot plate to 150 °C above the Curie temperature and then
cooled to room temperature on a copper plate. Surface cor-
rugations due to ferroelectric domains are visible under re-
flected light. Atomic force microscopy AFM topography,
SSPM, and PFM measurements were performed on a com-
mercial Dimension 3100 AFM system with a gold-coated
silicon cantilever with spring constant k7 N/m. In SSPM,
the topography is acquired by intermittent contact mode with
the tip oscillating at resonance, f0=202.3 kHz. For subse-
quent PFM imaging of the same location, the same AFM tip
was used and the AFM control switched to contact mode. A
5 V p.p., 50 kHz ac signal was applied to the tip. The first
harmonic amplitude and phase shift of the induced piezo-
electric sample oscillation are detected by a lock-in amplifier.
The domains with different orientations can be distinguished
from the contrast in the phase image. UV illumination was
carried out by fixing an 11SC-1 pencil style UV lamp pri-
mary line of 254 nm, 4.5 mW/cm2 at 2.54 cm within 2 cm
of the BaTiO3 sample with the angle between the lamp and
the surface normal less than 30° to ensure sufficient intensity.
Shown in Fig. 1a is the topography image of the
BaTiO3 001 surface with corrugations corresponding to the
tilting of adjacent a and c domains. a and c domains are
identified by the shape of the domain walls in the SSPM and
aElectronic mail: bonnell@lrsm.upenn.edu
FIG. 1. Color online a AFM topography of a BaTiO3 001 surface that
shows three corrugations due to a-c domain walls. z scale is 200 nm. b
The surface potential image of the same area as shown in a shows c
domains with curved domain walls. z scale is 0.25 V c The surface poten-
tial image when the UV light is on. z scale is 0.25 V.
APPLIED PHYSICS LETTERS 89, 112904 2006
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PFM images Figs. 1b and 1d: 90° a-c domain walls
intersect the 001 surface along straight lines while 180° c
domain walls intersect 001 plane along curves. Since the
surface of a c+ domain terminates with positive polarization
charge and that of a c− domain with negative polarization
charge, curved regions with higher potential should be c+
domains, as will be further discussed later. The surface po-
tential contrast is the largest between c+ and c− domains and
we denote this value as Vc+,c−. The measured Vc+,c− in this
experiment is 150–200 mV.
When the sample was illuminated, a significant and
gradual decrease in Vc+,c− to 10 mV was observed within a
few seconds Fig. 1c. When the UV is switched off,
Vc+,c− gradually increases back to the original value at a
slower rate. Clearly the surface potential contrast is strongly
influenced by UV illumination, indicating that screening of
the polarization bound charge on the domain surfaces is in-
volved. By scanning repeatedly along a single line, indicated
in Fig. 1b, at a rate of 0.5 Hz, the kinetics of the surface
charge relaxations in the presence and absence of light were
monitored Fig. 2. The potential contrast changes  in
both processes are exponentially dependent on time with an
associated time constant, i.e., Vc+,c−=A+B exp−t /. The
time constants extracted from the data are on=6.5 s for
light-on processes and off=40.1 s for light-off processes.
Since the electrons and holes are generated at the same lo-
cation and are equal in number, they do not cause the poten-
tial contrast change upon generation. The exponential decays
must originate from the spatial redistribution and retrapping
of photocarriers. Because of the large cross section of pho-
toexcitation in the UV range, photogenerated electron-hole
pairs initially only exist in the surface layer. In a c+ domain,
which has a polarization vector out of the surface plane, the
internal polarization field drives positive charge holes away
into the bulk while retaining electrons near the surface. Some
redistributed photocarriers are trapped in vacant trap states.
Thus a space-charge field opposing the polarization field in-
creases until the space-charge field equals the polarization
field or the rate of electron-hole pair generation equals that of
recombination. When the light is switched off, the trapped
carriers are thermally activated and diffuse along the concen-
tration gradient to eventually recombine. With carrier mobil-
ity in BaTiO3 on the order of 1 cm
2/ V s, the vertical length
of a domain about 10 m, and the polarization field about
10 kV/cm, the time of carrier drift is extremely short. There-
fore, the light-on and light-off processes are limited by the
rate of photoexcitation and that of thermal excitation, respec-
tively. Relaxation time constants are given by =0 /,
where  is the dielectric constant of BaTiO3 and  is the
conductivity. So the ratio between light conductivity and
dark conductivity is L /D=6.1.
Some previous studies considered extrinsic screening on
ferroelectric surfaces, in which case determination of c+ and
c− domains from the contrast in a SSPM image is
complicated.9,10 Depending on the amount of adsorbate, the
sign of Vc+,c− can be either positive when the surface is
partially screened or negative when the surface is completely
screened. In the case where physisorbed charge completely
screens the surface, the very low desorption energies
1 kJ/mol Ref. 11 of ambient adsorbates would result in
immediate desorption upon light illumination, leading to a
reversal of the sign of Vc+,c−. This is, however, not ob-
served in the experiment. In addition, the dynamics of extrin-
sic adsorption has a time constant 30–100 times longer than
the ones measured here. Thus, screening by adsorbates does
not play a dominant role in our experiment.
Although the domain potential contrast variations due to
charge redistribution were observed on UV illumination,
change in the domain structure did not occur. This confirms
that illumination did not reorient the ferroelectric domains.
Furthermore the domain structure did not change during
SSPM imaging for a variety of imaging conditions lift
height, drive amplitude, etc.. Subsequently, PFM imaging at
the same location was done in the absence of UV light
Fig. 3a. The domain pattern matches that imaged by
SSPM, except for a reversed contrast. Note that, in principle,
domain orientation is determined from PFM phase; ideal pi-
ezoelectric coupling results in a phase shift between c+ to c−
domains of +180°. For the images in Fig. 3, the contrast in
the phase image is less than 4°, which indicates that electro-
static coupling plays a larger role. Also note that the phase
contrast fluctuates slightly and slowly Fig. 3, which cannot
be easily attributed to piezoelectric properties. This indicates
that the magnitude of the phase shift in PFM is a critical
parameter for image interpretation and should always be re-
ported. Nevertheless, the PFM imaged domain pattern is suf-
ficiently clear to examine domain stability.
FIG. 2. Time dependence of surface potential contrast between c+ and c−
domains immediately after UV light is switched a on and b off.
FIG. 3. Color online a PFM phase image of domains in BaTiO3. b–
d Consecutive images showing the motion of 180° domain walls during
UV illumination as imaged with PFM. The arrows indicate the slow scan
direction. The scan rate is 2 min and 50 s per frame. z scale is 4°. All images
are 5050 m2 in size.
112904-2 Shao, Nikiforov, and Bonnell Appl. Phys. Lett. 89, 112904 2006
Downloaded 09 Nov 2006 to 130.91.116.168. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
When the UV light is switched on during imaging in
contact mode, rapid 180° domain wall motion is visible in
consecutive PFM images Figs. 3b–3d. The image acqui-
sition time is 2.8 min. The domain wall velocity increases
with the curvature, indicating a clear tendency for coarsen-
ing. The conducting AFM tip, in contact with the surface, has
destabilized the domain pattern. It is known that the surface
conductivity of a ferroelectric is enhanced by super-band-gap
light and, as discussed above, a screening layer of photocar-
riers exists on the surface that stabilizes the domain structure.
The AFM tip with only a small amplitude high frequency ac
voltage is effectively a grounding electrode. Thus the most
likely cause of the destabilization is the removal of screening
charge.
The curvature of a domain wall is 1 /R, where R is the
local radius of curvature. For simplicity, we consider the sta-
bility of a small cylindrical c+ domain with radius R embed-
ded in a large c− domain, or vice versa. Stability of a domain
without screening is established, when 2ECP+dw+dp0,
where EC is the resistance to depolarization generally de-
noted as coercive field, P is the polarization vector, dw is
the domain wall stress due to wall energy 	, and dp is the
stress due to depolarization field.12 A lower bound for R of a
stable domain is given by R
4	ECP. Screening charge re-
duces P by sc which is the surface charge density, and thus
the lower bound for R decreases. So c domains with a crucial
value of curvature are stable if screened but will shrink if
screening charge is removed by the AFM cantilever.
An alternative explanation is that the imaging voltage in
PFM destabilizes domain polarization. To clarify this, we
scanned the surface under UV radiation without applying an
ac bias. Contact induced domain wall motion was still ob-
served. It should be noted that in the absence of UV illumi-
nation, PFM images acquired consecutively do not exhibit
any change in domain structure. In addition, no change in
90° domain walls was found, which indicates that, although
c domains can reverse polarization upon charge removal,
they do not easily transform into a domains because of the
large energy associated with crystallographic strain.13
The above observation implies that PFM implemented in
contact mode with a conducting tip strongly interacts with
the ferroelectric surface, which can alter the properties of the
sample. As a result, PFM imaging on the evolution of do-
main structures in semiconducting ferroelectric surfaces
needs to take into account the possibility of any charge trans-
fer induced destabilization. A clear understanding of charge
transfer between the cantilever and the surface is necessary
to quantify PFM on semiconducting ferroelectrics.
In summary, SSPM was used to directly observe photo-
carrier screening on a BaTiO3 001 surface. Surface poten-
tial contrast of c domains is significantly reduced by UV
illumination and recovers after the illumination is switched
off. Both relaxation processes are found to be characteristic
of charging and discharging due to photocarriers. While do-
main structure does not change under UV during SSPM im-
aging, a rapid domain wall motion takes place during PFM
scanning which is attributed to domain destabilization due to
the removal of the screening photocarriers by the conducting
AFM tip.
The authors are grateful for insightful discussions with
Sergei V. Kalinin at Oak Ridge National Laboratory. This
work was supported by NSF Grant No. DMR 03-04531 and
utilized facilities supported in part by NSF Grant No. DMR
96-32596.
1V. M. Fridkin, Photoferroelectrics Consultants Bureau, New York, 1980.
2J. L. Giocondi and G. S. Rohrer, J. Phys. Chem. B 105, 8275 2001.
3S. V. Kalinin, D. A. Bonnell, T. Alvarez, X. Lei, Z. Hu, R. Shao, and J. H.
Ferris, Adv. Mater. Weinheim, Ger. 16, 795 2004.
4X. Lei, D. Li, R. Shao, and D. A. Bonnell, J. Mater. Res. 20, 712 2005.
5D. Dimos, W. L. Warren, M. B. Sinclair, B. A. Tuttle, and R. W. Schwartz,
J. Appl. Phys. 76, 4305 1994; A. L. Kholkin, and N. Setter, Appl. Phys.
Lett. 71, 2854 1997.
6A. L. Gruverman, J. Hatano, and H. Tokumoto, Jpn. J. Appl. Phys., Part 1
36, 227 1997; L. M. Eng, M. Friedrich, J. Fousek, and P. Günter,
J. Vac. Sci. Technol. B 14, 1191 1996.
7A. Gruverman, B. J. Rodriguez, R. J. Nemanich, and A. I. Kingon,
J. Appl. Phys. 92, 2734 2002.
8J. M. R. Weaver and David W. Abraham, J. Vac. Sci. Technol. B 9, 1559
1991.
9S. V. Kalinin, and D. A. Bonnell, Phys. Rev. B 63, 125411 2001.
10J. E. Spanier, A. M. Kolpak, J. J. Urban, I. Grinberg, L. Ouyang, W. S.
Yun, A. M. Rappe, and H. Park, Nano Lett. 6, 735 2006.
11S. V. Kalinin, C. Y. Johnson, and D. A. Bonnell, J. Appl. Phys. 91, 3816
2002.
12X. Li, A. Mamchik, and I.-W. Chen, Appl. Phys. Lett. 79, 809 2001; B.
Wang, and C. H. Woo, J. Appl. Phys. 94, 610 2003.
13B. A. Strukov and A. P. Levanyuk, Ferroelectric Phenomena in Crystals:
Physical Foundations Springer-Verlag, Berlin, 1988.
112904-3 Shao, Nikiforov, and Bonnell Appl. Phys. Lett. 89, 112904 2006
Downloaded 09 Nov 2006 to 130.91.116.168. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp


Photo-induced Charge Dynamics on BaTIO\u3csub\u3e3\u3c/sub\u3e (001) Surface Characterized by Surface Probe Microscopy

ScholarlyCommons@Penn

University of PennsylvaniaScholarlyCommonsDepartmental Papers (MSE) Department of Materials Science & EngineeringSeptember 2006Photo-induced Charge Dynamics on BaTIO3(001) Surface Characterized by Surface ProbeMicroscopyRui ShaoUniversity of PennsylvaniaMaxim P. NikiforovUniversity of PennsylvaniaDawn A. BonnellUniversity of Pennsylvania, bonnell@lrsm.upenn.eduFollow this and additional works at: http://repository.upenn.edu/mse_papersPostprint version. Published in Applied Physics Letters, Volume 89, Issue 11, 2006, 112904-1-3.Publisher URL: http://dx.doi.org/10.1063/1.2348776This paper is posted at ScholarlyCommons. http://repository.upenn.edu/mse_papers/108For more information, please contact libraryrepository@pobox.upenn.edu.Recommended CitationShao, R., Nikiforov, M. P., & Bonnell, D. A. (2006). Photo-induced Charge Dynamics on BaTIO3 (001) Surface Characterized bySurface Probe Microscopy. Retrieved from http://repository.upenn.edu/mse_papers/108Photo-induced Charge Dynamics on BaTIO3 (001) SurfaceCharacterized by Surface Probe MicroscopyAbstractThe surface potential of a multidomain BaTiO3 (001) surface was imaged in the presence and absence ofultraviolet (UV) illumination. The UV radiation induces a decrease in the surface potential contrast betweenc+ and c− domains with a time constant of a few seconds due to redistribution of photocarriers and screening.A slower process of recovery was observed after illumination. In addition, scanning a conducting atomic forcemicroscopy tip in contact under UV illumination destabilizes some ferroelectric domains.CommentsPostprint version. Published in Applied Physics Letters, Volume 89, Issue 11, 2006, 112904-1-3.Publisher URL: http://dx.doi.org/10.1063/1.2348776This journal article is available at ScholarlyCommons: http://repository.upenn.edu/mse_papers/108Photoinduced charge dynamics on BaTiO3 „001… surface characterizedby scanning probe microscopyRui Shao, Maxim P. Nikiforov, and Dawn A. BonnellaDepartment of Materials Science and Engineering, University of Pennsylvania, 3231 Walnut St.,Philadelphia, Pennsylvania 19104Received 4 May 2006; accepted 26 July 2006; published online 11 September 2006The surface potential of a multidomain BaTiO3 001 surface was imaged in the presence andabsence of ultraviolet UV illumination. The UV radiation induces a decrease in the surfacepotential contrast between c+ and c− domains with a time constant of a few seconds due toredistribution of photocarriers and screening. A slower process of recovery was observed afterillumination. In addition, scanning a conducting atomic force microscopy tip in contact under UVillumination destabilizes some ferroelectric domains. © 2006 American Institute of Physics.DOI: 10.1063/1.2348776Super-band-gap light illumination on a semiconductingferroelectric surface induces various interesting charge ef-fects related to the spontaneous polarization.1 It is wellknown that photoexcitation causes increased surface conduc-tivity, domain imprint, and changes in dielectric constant andpiezoelectric coefficient. The underlying mechanism is theredistribution of photoexcited charge carriers in a ferroelec-tric domain driven by the depolarization field to form intrin-sic screening layers. Consistent with this picture is the re-ported domain orientation-dependent local photochemicalreactivity on ferroelectric surfaces, i.e., reduction occurs onout-of-the-plane polarized domains and oxidation on into-the-plane polarized domains.2 This discovery led to domaincontrolled photocatalysis and opened a pathway to the di-rected assembly of nanoparticles with engineered domainstructures.3,4 Understanding the charge dynamics is criticalfor applications of ferroelectric materials in electro-opticalstorage and sensor devices. Extensive experimental studieson the photoinduced charge effects using the macroscopicparallel-plate capacitor configurations5 have been done. Cur-rently several electrical scanning probe techniques with na-nometer spatial resolution and high sensitivity to electricfields at this length scale make direct observation of photo-carriers on individual domains possible, which is the subjectof this letter. Local imaging by a scanning probe providesinformation about an electrode-free ferroelectric surface;thus screening charge is from either intrinsic charge and/oradsorbates rather than from carriers in electrodes as in thecase of parallel-plate capacitor measurements.Scanning probe imaging of domains and measurement oflocal hysteresis loops in ferroelectrics have been carried outroutinely by many researchers.6 Only Gruverman et al. re-ported the UV light-induced shift in hysteresis loops anddomain pinning using piezoresponse force microscopyPFM.7 PFM, a powerful imaging tool, differentiates do-mains based on the contrast in the phase shift of the cantile-ver oscillation. To detect the relative amount of surfacecharge on domains, however, scanning surface potential mi-croscopy SSPM also referred to as scanning Kelvin probemicroscopy8 is used in this study. SSPM is implemented innoncontact mode, in which case charge transfer between tipand surface is minimal. PFM, in contrast, is in contact modewith a conductive tip as an electrode. This difference maylead to different effects on a ferroelectric surface under UVlight, which is also investigated in this work.A BaTiO3 single crystal 551 mm3, TC=130 °C,MTI Corp. with an epi-polished 001 surface was heated ona hot plate to 150 °C above the Curie temperature and thencooled to room temperature on a copper plate. Surface cor-rugations due to ferroelectric domains are visible under re-flected light. Atomic force microscopy AFM topography,SSPM, and PFM measurements were performed on a com-mercial Dimension 3100 AFM system with a gold-coatedsilicon cantilever with spring constant k7 N/m. In SSPM,the topography is acquired by intermittent contact mode withthe tip oscillating at resonance, f0=202.3 kHz. For subse-quent PFM imaging of the same location, the same AFM tipwas used and the AFM control switched to contact mode. A5 V p.p., 50 kHz ac signal was applied to the tip. The firstharmonic amplitude and phase shift of the induced piezo-electric sample oscillation are detected by a lock-in amplifier.The domains with different orientations can be distinguishedfrom the contrast in the phase image. UV illumination wascarried out by fixing an 11SC-1 pencil style UV lamp pri-mary line of 254 nm, 4.5 mW/cm2 at 2.54 cm within 2 cmof the BaTiO3 sample with the angle between the lamp andthe surface normal less than 30° to ensure sufficient intensity.Shown in Fig. 1a is the topography image of theBaTiO3 001 surface with corrugations corresponding to thetilting of adjacent a and c domains. a and c domains areidentified by the shape of the domain walls in the SSPM andaElectronic mail: bonnell@lrsm.upenn.eduFIG. 1. Color online a AFM topography of a BaTiO3 001 surface thatshows three corrugations due to a-c domain walls. z scale is 200 nm. bThe surface potential image of the same area as shown in a shows cdomains with curved domain walls. z scale is 0.25 V c The surface poten-tial image when the UV light is on. z scale is 0.25 V.APPLIED PHYSICS LETTERS 89, 112904 20060003-6951/2006/8911/112904/3/$23.00 © 2006 American Institute of Physics89, 112904-1Downloaded 09 Nov 2006 to 130.91.116.168. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jspPFM images Figs. 1b and 1d: 90° a-c domain wallsintersect the 001 surface along straight lines while 180° cdomain walls intersect 001 plane along curves. Since thesurface of a c+ domain terminates with positive polarizationcharge and that of a c− domain with negative polarizationcharge, curved regions with higher potential should be c+domains, as will be further discussed later. The surface po-tential contrast is the largest between c+ and c− domains andwe denote this value as Vc+,c−. The measured Vc+,c− in thisexperiment is 150–200 mV.When the sample was illuminated, a significant andgradual decrease in Vc+,c− to 10 mV was observed within afew seconds Fig. 1c. When the UV is switched off,Vc+,c− gradually increases back to the original value at aslower rate. Clearly the surface potential contrast is stronglyinfluenced by UV illumination, indicating that screening ofthe polarization bound charge on the domain surfaces is in-volved. By scanning repeatedly along a single line, indicatedin Fig. 1b, at a rate of 0.5 Hz, the kinetics of the surfacecharge relaxations in the presence and absence of light weremonitored Fig. 2. The potential contrast changes  inboth processes are exponentially dependent on time with anassociated time constant, i.e., Vc+,c−=A+B exp−t /. Thetime constants extracted from the data are on=6.5 s forlight-on processes and off=40.1 s for light-off processes.Since the electrons and holes are generated at the same lo-cation and are equal in number, they do not cause the poten-tial contrast change upon generation. The exponential decaysmust originate from the spatial redistribution and retrappingof photocarriers. Because of the large cross section of pho-toexcitation in the UV range, photogenerated electron-holepairs initially only exist in the surface layer. In a c+ domain,which has a polarization vector out of the surface plane, theinternal polarization field drives positive charge holes awayinto the bulk while retaining electrons near the surface. Someredistributed photocarriers are trapped in vacant trap states.Thus a space-charge field opposing the polarization field in-creases until the space-charge field equals the polarizationfield or the rate of electron-hole pair generation equals that ofrecombination. When the light is switched off, the trappedcarriers are thermally activated and diffuse along the concen-tration gradient to eventually recombine. With carrier mobil-ity in BaTiO3 on the order of 1 cm2/ V s, the vertical lengthof a domain about 10 m, and the polarization field about10 kV/cm, the time of carrier drift is extremely short. There-fore, the light-on and light-off processes are limited by therate of photoexcitation and that of thermal excitation, respec-tively. Relaxation time constants are given by =0 /,where  is the dielectric constant of BaTiO3 and  is theconductivity. So the ratio between light conductivity anddark conductivity is L /D=6.1.Some previous studies considered extrinsic screening onferroelectric surfaces, in which case determination of c+ andc− domains from the contrast in a SSPM image iscomplicated.9,10 Depending on the amount of adsorbate, thesign of Vc+,c− can be either positive when the surface ispartially screened or negative when the surface is completelyscreened. In the case where physisorbed charge completelyscreens the surface, the very low desorption energies1 kJ/mol Ref. 11 of ambient adsorbates would result inimmediate desorption upon light illumination, leading to areversal of the sign of Vc+,c−. This is, however, not ob-served in the experiment. In addition, the dynamics of extrin-sic adsorption has a time constant 30–100 times longer thanthe ones measured here. Thus, screening by adsorbates doesnot play a dominant role in our experiment.Although the domain potential contrast variations due tocharge redistribution were observed on UV illumination,change in the domain structure did not occur. This confirmsthat illumination did not reorient the ferroelectric domains.Furthermore the domain structure did not change duringSSPM imaging for a variety of imaging conditions liftheight, drive amplitude, etc.. Subsequently, PFM imaging atthe same location was done in the absence of UV lightFig. 3a. The domain pattern matches that imaged bySSPM, except for a reversed contrast. Note that, in principle,domain orientation is determined from PFM phase; ideal pi-ezoelectric coupling results in a phase shift between c+ to c−domains of +180°. For the images in Fig. 3, the contrast inthe phase image is less than 4°, which indicates that electro-static coupling plays a larger role. Also note that the phasecontrast fluctuates slightly and slowly Fig. 3, which cannotbe easily attributed to piezoelectric properties. This indicatesthat the magnitude of the phase shift in PFM is a criticalparameter for image interpretation and should always be re-ported. Nevertheless, the PFM imaged domain pattern is suf-ficiently clear to examine domain stability.FIG. 2. Time dependence of surface potential contrast between c+ and c−domains immediately after UV light is switched a on and b off.FIG. 3. Color online a PFM phase image of domains in BaTiO3. b–d Consecutive images showing the motion of 180° domain walls duringUV illumination as imaged with PFM. The arrows indicate the slow scandirection. The scan rate is 2 min and 50 s per frame. z scale is 4°. All imagesare 5050 m2 in size.112904-2 Shao, Nikiforov, and Bonnell Appl. Phys. Lett. 89, 112904 2006Downloaded 09 Nov 2006 to 130.91.116.168. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jspWhen the UV light is switched on during imaging incontact mode, rapid 180° domain wall motion is visible inconsecutive PFM images Figs. 3b–3d. The image acqui-sition time is 2.8 min. The domain wall velocity increaseswith the curvature, indicating a clear tendency for coarsen-ing. The conducting AFM tip, in contact with the surface, hasdestabilized the domain pattern. It is known that the surfaceconductivity of a ferroelectric is enhanced by super-band-gaplight and, as discussed above, a screening layer of photocar-riers exists on the surface that stabilizes the domain structure.The AFM tip with only a small amplitude high frequency acvoltage is effectively a grounding electrode. Thus the mostlikely cause of the destabilization is the removal of screeningcharge.The curvature of a domain wall is 1 /R, where R is thelocal radius of curvature. For simplicity, we consider the sta-bility of a small cylindrical c+ domain with radius R embed-ded in a large c− domain, or vice versa. Stability of a domainwithout screening is established, when 2ECP+dw+dp0,where EC is the resistance to depolarization generally de-noted as coercive field, P is the polarization vector, dw isthe domain wall stress due to wall energy 	, and dp is thestress due to depolarization field.12 A lower bound for R of astable domain is given by R4	ECP. Screening charge re-duces P by sc which is the surface charge density, and thusthe lower bound for R decreases. So c domains with a crucialvalue of curvature are stable if screened but will shrink ifscreening charge is removed by the AFM cantilever.An alternative explanation is that the imaging voltage inPFM destabilizes domain polarization. To clarify this, wescanned the surface under UV radiation without applying anac bias. Contact induced domain wall motion was still ob-served. It should be noted that in the absence of UV illumi-nation, PFM images acquired consecutively do not exhibitany change in domain structure. In addition, no change in90° domain walls was found, which indicates that, althoughc domains can reverse polarization upon charge removal,they do not easily transform into a domains because of thelarge energy associated with crystallographic strain.13The above observation implies that PFM implemented incontact mode with a conducting tip strongly interacts withthe ferroelectric surface, which can alter the properties of thesample. As a result, PFM imaging on the evolution of do-main structures in semiconducting ferroelectric surfacesneeds to take into account the possibility of any charge trans-fer induced destabilization. A clear understanding of chargetransfer between the cantilever and the surface is necessaryto quantify PFM on semiconducting ferroelectrics.In summary, SSPM was used to directly observe photo-carrier screening on a BaTiO3 001 surface. Surface poten-tial contrast of c domains is significantly reduced by UVillumination and recovers after the illumination is switchedoff. Both relaxation processes are found to be characteristicof charging and discharging due to photocarriers. While do-main structure does not change under UV during SSPM im-aging, a rapid domain wall motion takes place during PFMscanning which is attributed to domain destabilization due tothe removal of the screening photocarriers by the conductingAFM tip.The authors are grateful for insightful discussions withSergei V. Kalinin at Oak Ridge National Laboratory. Thiswork was supported by NSF Grant No. DMR 03-04531 andutilized facilities supported in part by NSF Grant No. DMR96-32596.1V. M. Fridkin, Photoferroelectrics Consultants Bureau, New York, 1980.2J. L. Giocondi and G. S. Rohrer, J. Phys. Chem. B 105, 8275 2001.3S. V. Kalinin, D. A. Bonnell, T. Alvarez, X. Lei, Z. Hu, R. Shao, and J. H.Ferris, Adv. Mater. Weinheim, Ger. 16, 795 2004.4X. Lei, D. Li, R. Shao, and D. A. Bonnell, J. Mater. Res. 20, 712 2005.5D. Dimos, W. L. Warren, M. B. Sinclair, B. A. Tuttle, and R. W. Schwartz,J. Appl. Phys. 76, 4305 1994; A. L. Kholkin, and N. Setter, Appl. Phys.Lett. 71, 2854 1997.6A. L. Gruverman, J. Hatano, and H. Tokumoto, Jpn. J. Appl. Phys., Part 136, 227 1997; L. M. Eng, M. Friedrich, J. Fousek, and P. Günter,J. Vac. Sci. Technol. B 14, 1191 1996.7A. Gruverman, B. J. Rodriguez, R. J. Nemanich, and A. I. Kingon,J. Appl. Phys. 92, 2734 2002.8J. M. R. Weaver and David W. Abraham, J. Vac. Sci. Technol. B 9, 15591991.9S. V. Kalinin, and D. A. Bonnell, Phys. Rev. B 63, 125411 2001.10J. E. Spanier, A. M. Kolpak, J. J. Urban, I. Grinberg, L. Ouyang, W. S.Yun, A. M. Rappe, and H. Park, Nano Lett. 6, 735 2006.11S. V. Kalinin, C. Y. Johnson, and D. A. Bonnell, J. Appl. Phys. 91, 38162002.12X. Li, A. Mamchik, and I.-W. Chen, Appl. Phys. Lett. 79, 809 2001; B.Wang, and C. H. Woo, J. Appl. Phys. 94, 610 2003.13B. A. Strukov and A. P. Levanyuk, Ferroelectric Phenomena in Crystals:Physical Foundations Springer-Verlag, Berlin, 1988.112904-3 Shao, Nikiforov, and Bonnell Appl. Phys. Lett. 89, 112904 2006Downloaded 09 Nov 2006 to 130.91.116.168. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

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Photo-induced Charge Dynamics on BaTIO\u3csub\u3e3\u3c/sub\u3e (001) Surface Characterized by Surface Probe Microscopy

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