PHOTO-POLARONS AND RELATED
PHENOMENA IN QUANTUM PARAELECTRIC
PEROVSKITE-LIKE OXIDES
V.A. Trepakov1,2, A. Gubaev3, S.E. Kapphan3, P. Galinetto4,
F. Rossella4, P.A. Markovin2, P.P. Syrnikov2 and L. Jastrabik1
1
Institute of Physics AS CR, Prague, Czech Republic
2 A.F. Ioffe Physical-Technical Institute, Russia
3 FB Physik, University of Osnabrueck, Germany
4 Physics Dept. “A. Volta”, University of Pavia, Italy
SpbRussia
Dipartimento di
Fisica “A. Volta”
UniPv – Pavia Italy
Outline
Introduction:
Charge carriers doping effects; role and nature of photo-carriers
localized states;
“Photoinduced structural transitions”.
“Giant low-T photo-dielectricity” in Incipient Ferroelectrics
Main Part
UV-light induced IR absorption in SrTiO3 and KTaO3
Dilute KTN :
UV-light induced IR absorption
Nb4+ photo-polarons in diluted KTN
Photoconductivity and visible luminescence
….conclusions
Milano 2006, July 13th
P. Galinetto DFAV Uni of Pavia ITALY
Charge carriers doping effects; role and nature of photo-carriers localized states
Phys. Rev. 137, A1575–A1582 (1965)
Some Transport Properties of Oxygen-Deficient Single-Crystal
Potassium Tantalate (KTaO3)
S. H. Wemple
The photo charge carriers dinamics, the localization
processes and the nature of the carriers localized states
Phys. Rev. 180, 547–556 (1969)
at low T in incipient FEElectron
are ”old”
problems
Scattering in Perovskite-Oxide Ferroelectric Semiconductors
S. H. Wemple, M. DiDomenico, Jr., and A. Jayaraman
…but there is a lack of optical experiments and reliable data on high
purity controlled crystals in order to elucidate these phenomena.
Recently...
•
•
•
•
•
Hasegawa T., Shiraii M., Tanaka K.: Localizing nature of photo-excited states in
SrTiO3. J. Lumin. 87-89, 1217-1219 (2000)
Hasegawa, K. Tanaka K.: Photo-induced polaron states in strontium titanate. J. of
Lumin. 94-95, 15-18 (2001)
Kapphan S.E., Gubaev A.I., Vikhnin V.S.: UV light-induced absorption in pure
KTaO3 at low temperature. Phys. Stat. Sol. (c) 2, 128-131 (2005)
Laguta V., Glinchuk M., Bykov I., Cremoma A., Galinetto P., Giulotto E., Jastrabik L.,
Rosa J.: Light-induced defects in KTaO3. J. Appl. Phys. 93, 6956-6054 (2003)
Maiwald M., Schirmer O.F.: O- dynamic Jahn-Teller polarons in KTaO3. Europhys.
Lett. 64, 776-782 (2003)
Milano 2006, July 13th
P. Galinetto DFAV Uni of Pavia ITALY
Charge carriers doping effects; role and nature of photo-carriers localized states
the broad absorption bands peaking at 1.23 eV, 1.53 eV and 2.36 eV have been reported
for reduced KTO samples at LT. Additional UV-illumination in the band-gap region at
LT gave rise to broad light induced absorption bands in the same 1-2 eV region, more
pronounced for as grown and oxidized specimens. This IR light-induced effect has been
tentatively attributed to different sorts of the O- hole polaronic centers, which where
Maiwald
Schirmer
O- dynamic Jahn-Teller polarons in KTaO3. Europhys.
revealedM.,
in recent
EPRO.F.:
experiments.
Lett. 64, 776-782 (2003)
Laguta et al.: Light-induced defects in KTaO3. J. Appl. Phys. 93, 6956-6054 (2003)
Kapphan S.E et al. Phys. Stat. Sol. (c) 2, 128-131 (2005)
Milano 2006, July 13th
P. Galinetto DFAV Uni of Pavia ITALY
Charge carriers doping effects; role and nature of photo-carriers localized states
O- reorienting hole polaron
with dipole moment
O- role in electronic photoconductivity
enhancement
STO, KTO, STO:Ca, KTO:Li
Photo-Induced Phase Transitions
Photoinduced LOCAL and GLOBAL Phase Transitions (PhIPT)
Schematic illustration of photoinduced
phase transitions:
- hidden multi-stability of the ground
state
- proliferation of optical induced states
Relaxation of optical excited states results in
collective ordering motions of large number of
atoms and electrons, appearance of macroscopic
domains, new structural and electronic orders
M. Ueta, H. Kanzaki, K. Hanamura, Y. Kobayashi, E. Toyozawa, Excitonic proceses in solids (1986);
K. Nasu, Relaxation of excited states and PhIPT (1997)
Photo-Induced Phase Transitions
Ferroelectrics: cooperative interactions electron-dipole
PhIPT are facilitated in systems with the hidden multi-stability,
and quasi/degenerated ground state, close to stability limit, e.g.,
in the system close to structural phase transitions (PT).
STO, KTO at the low-T, nearly quantum mechanical
stability limit, are namely such materials.
Milano 2006, July 13th
P. Galinetto DFAV Uni of Pavia ITALY
Giant low-T photo-dielectricity in Incipient Ferroelectrics
Recent observations of giant UV
light induced:
-enhancement of the dielectric
constant (“critical behavior” or
“giant photo-dielectricity”),
- unconventional Debye-type
dispersion in STO and KTO
have been tentatively connected with
possible PhIPT.
Giant low-T photo-dielectricity” in Incipient Ferroelectrics
Observations of gigantic PhI
dielectricity in STO, KTO but not in
SrTi18O3 allowed to propose that
this phenomenon is inherent to QPE
in the region of dominating
quantum statistic but not in FE,
although this effect was found later
in incipient ferroelectric CaTiO3 too.
•T. Hasegava, K. Tanaka, ICL 2002
•Hasegava, S. Mouri, Y. Yamada, K. Tanaka, J. Phys. Soc. Jpn. 72 (2003) 41.
•I. Katayama, Y. Ichikawa, K. Tanaka, Phys. Rev. B 67 (2003) 100102(R).
•M. Takesada, T. Yagi, M. Itoh, S-y Koshihara, J. Phys. Soc. Jpn. 72 (2003) 37.
•T. Ishikawa, M. Itoh, M. Kurita, H. Shimoda, M. Takesada, T. Yagi, S. Koshihara,
Ferroelectrics 298 (2004) 141.
•M. Takesada, T. Yagi, M. Itoh, T. Ishikawa, S. Koshihara, Ferroelectrics 298 (2004) 317.
The mechanisms suggested:
-Space charge effects – no, due to absence in STO-18 and dc-dependence.
-FE transition with photo-induced polar clusters.
-Photoinduced insulator-to-metal transition – no, weak light intensity
-Important role of Quantum Paraelectricity due to effect absence in STO-18
Role and nature of photo-carriers localized states
+ Photoinduced structural
transitions” + “Giant low-T photo-dielectricity” in Incipient Ferroelectrics
Carrier’s localization is essential for the comprehension
of the gigantic PhI dielectric effects and PIPD
Polarons and their role in structural transformations, dielectric and optical
properties in ABO3 perovskites are actual and intensively discussed problems
nowadays.
Whereas hole O- polarons have already been established in
KTO
[1,2]
BaTiO3 [3]
LiNbO3 [4]
Electronic polarons were not proven experimentally neither in STO nor in KTO
Small electronic Nb4+ polarons are known for LNO [4] and nonisovalent Nb
doping plays a crucial role in the Ti3+ Jahn-Teller electronic polaron formation for
BTO [5]
1. V. Laguta et al. J. Appl. Phys. 93, 6956 (2003).
2. M. Maiwald, O. F. Schirmer, Europhys. Lett. 64, 776782 (2003).
3. F. Schirmer et al. Phys. Stat. Sol. (c) 2, 124 (2005)
4. O. Schirmer, D. von der Linde, Appl. Phys. Lett. 33, 35 (1978).
5 S. Lenjer et al. Phys. Rev. B 66, 165106-1 (2002).
Milano 2006, July 13th
P. Galinetto DFAV Uni of Pavia ITALY
Studies of polaronic problem for KTN are practically absent, although
the PhI Nb4+ polaronic concept has been used as the possible origin of
laser-induced transient gratings in four-wave-mixing experiments for
KTN
-Liu H., Powell R.C., Boatner L.A., . J. Appl. Phys. 70, 20-28 (1991)
-Liu H., Powell R.C., Boatner L.A., Phys. Rev. B 44, 2461-2469 (1991)
But KTN is very suitable object to study polaronic
problems and their role in structural transformations in
ABO3 perovskites.
We believed to get evidences of Nb4+ polarons in KTN
•in the UV light-induced NIR optical absorption
• in PC vs T
• in photo-luminescence
Milano 2006, July 13th
P. Galinetto DFAV Uni of Pavia ITALY
The transparent colorless high purity
KTN single crystals with Nb content
x = 0, 0.004, 0.007, 0.012, 0.07 were
grown and used;
Samples were prepared as optically
polished parallelepipeds oriented along
<100> cubic axes.
The specimens’ conductivity at RT
was in the order of 10-13 -1cm-1
EXPERIMENTAL DETAILS
ABS in IR-Vis Range Fourier spectrometer (Bruker IFS120HR, 200-32000 cm-1), with L-He cryostat
(Leybold, 1.3–600 K) and XBO high pressure lamp with band pass filter (300–400 nm) for PhI Abs.
PC measurements: sandwich Ag/KTN/Ag structures (Ag electrodes) in the temperature region 15 – 300 K
using an electctrometer (Keithley Model 6517) and XBO lamp light filtered by monochromator. The current
dc- driving field was about 70 kV/m.
PL spectra were measured by a photon counting system with SPEX1402 double grating monochromator with a
cooled RCA31034 GaAs photomultiplier and closed cycle cryostat (Leybold), XBO 300W/HTP lamp and
SPEX Minimate monochromator or/and the variable bandpass filters as the excitation source.
RESULTS
All KTN and KTO samples at RT display
a) fundamental absorption in the UV (> 3.25 eV for KTN and >3.5 eV for KTO
b) a weak absorption in the FIR region (< 0.2 eV  6) at ~ 3500 cm-1 due to OH
centers incorporated during crystal growth.
T=RT
KTO:
1:R133
2:N131
3:R132.13
4:R132.12
5:R132.18
6:R132.15
8
7
-1
 [cm ]
6
5
4
3
1
2
4
2
5
3
1
6
3440
3450
3460
3470
3480
3490
3500
-1
wavenumber [cm ]
Milano 2006, July 13th
P. Galinetto DFAV Uni of Pavia ITALY
• No new absorption bands were observed at RT under interband UV-light
excitation.
BUT AT LOW TEMPERATURES:
KTaO3 and SrTiO3
only absorption related with O- photo-hole polarons
2.5
0.5eV
0,05
1.6
0.77eV
1.25
0.99eV
1
1.24eV
1.0
STO, T~47 K
KaTaO3
Optical Density
Optical Density
0,04
0,03
0,02
0.8
0.6
0.4
0.2
0,01
T = 1.3 K
9200; 1.09; 1.14eV
0.0
0,00
4000
6000
8000
10000
12000
-1
Wavenumber [cm ]
14000
16000
3000 4000 5000 6000 7000 8000 9000 10000 11000
-1
Wavenumber, cm
KTN-0.4 and KTN-0.7.
2.5
0.5eV
Nb concentration below
the QMDL xcr = 0.008.
KTN-0.7
5600; 1.79; 0.69eV
Optical Density
Both compositions are
cubic paraelectric down
to lowest T.
1,0
1.6
0.77eV
1.25
0.99eV
1
1.24eV
KTO
0,8
0,6
0,4
T = 1.3 K
0,2
9200; 1.09; 1.14eV
KTN-0.4
0,0
3000
4000
5000
6000 7000 8000 9000 10000 11000
-1
Wavenumber, cm
UV-light induced NIR absorption in
KTO, KTN-0.4 and KTN-0.7
KTN-0.4- PhI absorption is very close to that for nominally pure KTO.
-The rather pronounced band peaking at ~ 9200 cm-1 (1.14 eV) - O- hole polaron;
- a very faint band in the ~ 5000 – 6000 cm-1 region.
KTN-0.7 - Emergence of a wide light-induced NIR absorption band at 5600 cm-1
(0.69 eV) with half-width ~ 0.5 eV at 1.3 K
KTN-0.7
UV-light induces a wide NIR absorption band peaking at ~ 5600 cm-1 (0.69
eV) with half-width ~ 0/5 eV at 1.3 K.
14.08.2005, PC-487: KTO-0.7
300-400 nm pumping light.
0,8
ODL-I
0,7
UV light-induced IR absorption increases
and saturates approx. 60 s after UV light
switching on and does not decay in the
1.4K
0,6
dark at least for hour.
0,5
0,4
0,3
2000
4000
6000
8000
10000
12000
-1
Wavenumber [cm ]
The position of absorption maximum agrees well with electronic polarons energies in:
- theory: 0.76 eV for Nb4+ [Eglitis, Kotomin, Borstel 2005]
- in TiO2 (0.82eV) and BaTiO3 (0.6 eV) [Firsov 1975, Bursian 1973 ]
-0.8 eV in KNbO3 [pulsed electronic beam excitation Grigor’eva, Millers, SSC 2004]
Large bandwidth evidences strong electronic-lattice coupling.
We suggest that this absorption is connected with Nb4+ electronic
polarons
KTN-1.2:
displays only one cubic-rhombohedral FE PT at ~20 K.
1.6
0.77 eV
2.5
0.5 eV
16.6K
6000
530 Hz
2,0
1,8
1,6
Optical Density
Permittivity, real
5000
4000
3000
2000
1,4
1,2
1,0
0,8
1.25
0.99 eV
1
1.24 eV
-1
6390 cm ; 1.54 ; 0.8 eV
1.3K
20K
30K
40K
45K
50K
55K
60K
70K
80K
KTN-1.2
0,6
0,4
1000
302
0,2
0,0
0
0
50
100
150
200
Temperature, K
250
300
3000
4000
5000
6000
7000
Wavenumber cm
8000
9000 10000
-1
UV light-induced absorption increases with Nb concentration and absorption
maximum shifts closer to energies that for Nb4+ polarons in KNbO3.
The UV light-induced absorption in the paraelectric cubic phase:
-increases with T up to ~ 40 K,
- afterward decreases, and
- practically vanishes at ~ 80 K
KTN-7: Three C-T-Rhombic-Rhombohedral Fe PT take place
PC-406: KTN-7PhD
6171 cm
0,35
-1
0,30
0,25
OD
0,20
0,15
0,10
0,05
0,00
2000
3000
4000
5000
6000
7000
Wavenumber cm
8000
9000
10000 11000
-1
-Character of the UV light-induced NIR absorption appears to be very similar.
-PhI absorption maximum locates at 6171 cm-1 (1.62, 0.76 eV) for 1.3 K, which can be
related with changes in the lattice constant due to Nb doping and correlation effects
- A similar behavior is shown in the T -dependence of the UV light-induced absorption for
different Nb concentrations
The optical absorption spectra can be described by the conventional
model of polaron formation at Nb ions in close Nb pairs.
Configurations curves for two-site model
 4Ea
Absorption maximum corresponds to the “interband” optical Frank-Condon O  O’
transition at ~ 0.7 eV and polaronic activation energy ¼E ~175 meV (~ 2000 K).
0
 LO  55meV
T 0
2
Ea 0
width of polaronic absorption band 8
2
At low
characteristic energy of LO phonons
This gives ~ 0.5 – 0.7 eV
The presence of Nb pairs is not a strong assumption.
KTN-0.4: the quantity of Nb pairs is small.
UV light-induced absorption only faintly pronounced as a weak shoulder in the
5000 – 6000 cm-1 region.
KTN-0.7 the concentration of Nb pairs increases,
as does the respective electron polaron absorption.
KTN-1.2: The light-induced absorption increases appreciably.
But further on, for KTN-7 this increasing appears to be rather weak.
This fact can be understood if in the polaron optical absorption only
pairs of neighboring Nb ions play the main role and it is not
necessary to form infinite clusters of the nearest neighbor Nb ions.
Milano 2006, July 13th
P. Galinetto DFAV Uni of Pavia ITALY
However, absence of the infinite Nb clusters and Nb4+ polarons
formation should strongly decrease the static conductance and
photoconductivity at low temperatures.
Photocurrent, A
Photocurrent, A
1E-6
1E-7
1E-8
1E-9
pure KTO
1E-10
1E-9
1E-10
KTN-1.2
KTN-1.2
1E-11
KTN-0.4
0
50
100
150
200
250
300
1E-11
KTN-0.4
Temperature, K
0
50
100
150
200
250
300
Temperature, K
-Nb- admixture depress PhC at all temperatures in respect to KTaO3
-PhC is depressed strongly at lowest T.
-PhC temperature maximum appears to be in the T-region, where UV-light
induced IR absorption decreases strongly.
Estimation of the traps depth
KTN – 1.2
-20
e
experimental data
linear fit
-21
e
current (A)
Y = A + B1 * (1/kT)
B1 = 0.02799 eV
-22
e
-23
e
Good agreement with activation energy for
UV-induced IR absorption
-24
e
-25
e
-26
e
0
100
200
300
400
500
600
700
Estimation of the decay-time constant
-1
1/kT (eV )
160
6.00E-008
140
120
 (s)
current (A)
4.00E-008
2.00E-008
100
80
60
40
0.00E+000
1000
2000
time (s)
Milano 2006, July 13th
10
20
30
40
50
60
T (K)
P. Galinetto DFAV Uni of Pavia ITALY
These are reasonable arguments for assumption of:
-Nb-pair polarons formation;
-strong charge localization at the lowest temperatures just in the
dominant quantum fluctuations region.
This suggestion is supported also by:
dielectric permittivity enhancement (9.46 MHz) under UV light;
2.75eV
1.65eV
1.91eV
2.25eV
80000
The emission intensity smoothly
decays
with temperature practically vanishing
90 K
80 K
70 K
60 K
50 K
45 K
40 K
30 K
20 K
10 K
Counts
strong increasing of the VIS luminescence decay time just below the T above
60000
at ~100
K. UV- light induced
which
absorption decreases
(KTO -  ~ ms, in KTN-1.2 increases below 40 K up to 10 s at 10 K, QFR)
40000
This coincidence strongly supports the suggestion that the localization and
following release of
the photo-induced charge carriers occurs mainly by Nb
20000
constituting.
400
450
500
550
600
650
Wavelength, nm
700
750
Temperature dependence modeling.
Low temperatures:
-The light-induced absorption increases with T.
-The nature of this effect is not clear and needs further studies. Theory is not developed.
-It only can be assumed that photo-carriers generation, transport, trap transformation
processes and changing in oscillator strength can play here a decisive role.
High temperatures:
The strong decay above 40 K yields from fitting an estimate of the value
for the optical absorption thermal quenching activation energy ~ 36 meV.
for PC quenching a trap depth ~ 28 meV
Milano 2006, July 13th
P. Galinetto DFAV Uni of Pavia ITALY
CONCLUSIONS:
We report on detailed and systematic stuides of the low-temperature UV-light induced NIR abs,
PhC and VIS-PL in KTaO3 and SrTiO3 high polarizable perovskite like oxides as well as in
series of KTa1-xNbxO3 single crystals (x=0, 0.004, 0.007, 0.012, 0.07) in which depending on Nb
concetration behaviour of the incipent ferroelectrics and low-T ferroelectrics can be realized
(modelled).
UV-induced NIR ABS in KTO and STO  localized optical hole polarons
UV-induced NIR ABS in dilute KTN 
Results are considered as evidence for the formation of the small electronic
Nb4+ polarons at low temperatures:
a) No photochromic effect was found at RT in all Nb containing species as well as in nominally pure KTaO 3.
b) The emergence of the UV-light induced wide absorption bands in the NIR region with maxima at 0.69–0.8 eV at
low temperatures is treated s a manifestation of the localization of photo-induced electrons and the formation of
Nb4+ small electron polarons in close-neighbor Nb-Nb pair centers.
c) Position and width of the light-induced NIR absorption bands is in good agreement with theory of small polarons.
Nb doping suppresses the photoconductivity in KTN at low temperatures.
This agrees well with the close-neighbors Nb4+-Nb5+ pair centers model,
considering Nb5+ ions as effective traps for photo-electrons at low T.
The effective trapping of charge carries is supported also by the,
characteristic for ABO3 oxides, “green” photoluminescence study. The
strong increasing of the luminescence decay time from millisecond up to
about 10
seconds was found in KTN.
In the medium temperature region UV-light induced NIR absorption begins to decrease and
then vanishes (~ at 40 K and 80 K respectively for KTN with x=0.012), due to the presence of a
quenching level or thermal activation of the electron from the Nb4+ polaron into CB.
The results evidence localization of UV excited charges carriers as electronic
polarons (Nb4+) at the low T for diluted KTN.
Subjects under inquiry (in progress):
• Photo-EPR
• UV-light induced effects in Raman light scattering;
• UV-light induced effects in thermo-optical properties;
• Dielectric spectroscopy: low-f and THz (THz: TOs or
plasma frequency?)
Obtained low-T optical hole and electronic polarons formation can lead to
strong increasing of measurable dielectric permittivity (giant dielectriciry)
under UV-light action at low T, as it was reported by Japanies.
But this effects is rather atributed to polaronic migration contribution of
excited charge carriers (polarons) than ferroelectric phase transition.
At least our UV-ligh Raman experiments gave no evidences of UV light
effect on soft TO mode or appearance of UV-light induced polar region.
UV-light induced Raman light scattering: KTN-0.4
2TA
8K
514nm
300mW
counts
200
100
irradiated 25 minutes
Hg Lamp filtered with UG1 (200-400 nm)
before irradiation
20
40
60
80
100
120
140
160
-1
Raman (cm )
Thank you for your attention
Giant low-T photo-dielectricity” in Incipient Ferroelectrics
Photocarriers are promptly charge separated and localized as small polarons
T Hasegava, M. Shirai, K. Tanaka, J. Lumn.87-82 (2000) 12.
Small polarons accompanied by strong lattice distortions are origin of PhI
dipoles and PhIPD
PhIPD interaction [J(n) <0]. Finite lifetime
Prevents PhIPD from long-range ordering
Schematic picture of PhIPD.
Photocarriers recombines radiatively with
mean~ 1ms and PhIPD vanish.
Milano 2006, July 13th
P. Galinetto DFAV Uni of Pavia ITALY