Materials-286K 15th December, 2014

Correlations between structure and transport in BaTiO3

Santosh RaghavanMaterials Department, University of California, Santa Barbara

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BaTiO3

A. Von Hippel, Reviews of Modern Physics, 22, 221 (1950)B. Sarkar et al, J. Phys D: App Phys, 45, 505304 (2012)

• Ferroelectric perovskite at RT (TC = 120 °C)

• Used in capacitors and piezo applications

d

http://www.mdpi.com/sensors/sensors-10-01935/article_deploy/html/images/sensors-10-01935f2-1024.png

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Local Ti distortions toward <111> octahedral faces

Q. Zhang, T. Cagin and W. A. Goddard, PNAS, 103, 14695 (2006)

BaTiO3

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Local Ti distortions toward <111> octahedral faces

Q. Zhang, T. Cagin and W. A. Goddard, PNAS, 103, 14695 (2006)

Vacancy formation

Carrier transport mechanism

Metal – Insulator transition

BaTiO3

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1. Vacancy formation in Sr1-xBaxTiO3

H.M. Chan, M.P. Harmer, D. M. Smyth, J. Am. Ceram. Soc., 69, 507 (1986)D. Makovec, Z. Samardzija, U. Delalut and D. Kolar, J. Am. Ceram. Soc., 78, 2193 (1995)L. Wang, Y. Sakka, Y. Shao, G.A. Botton, T. Kolodiazhnyi, J. Am. Ceram. Soc., 93, 2903 (2010)

In SrTiO3,

La + SrTiO3 LaSr + VSr’’ + TiTi+ 3OO + 1e-

A-site substitution A-site vacancy formation!

A-site substitution B-site vacancy formation!But easier to dope the B-site with Nb instead…

In Sr1-xBaxTiO3, both A-site and B-site vacancies!

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1. Vacancy formation in Sr1-xBaxTiO3

VTi’’’’ – Most unlikely, highly

energetic, major perturbation to the lattice

Why does it form instead of VBa’’?

Reason Ti <111> displacement, Ti—O hybridisation

Off-center Ti covalently bonds to Oxygen Stabilises VTi

’’’’

Excess charge can be transferred to nearby partial covalent Ti—O bonds

L. Wang, Y. Sakka, Y. Shao, G.A. Botton, T. Kolodiazhnyi, J. Am. Ceram. Soc., 93, 2903 (2010)

28% La-doped Sr1-xBaxTiO3

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2. BaTiO3 – Polaron hopping or Band transport?

T. Holstein, Annals of Physics, 8, 325 (1959)J.P. Boyeaux and F.M. Michel-Calendini, J. Phys. C: Solid State Phys.,12, 545 (1979)

In BaTiO3, experimentally measured μHall and μdrift are comparable!

Holstein’s small polaron (SP)model

μho – SP hoppingμb – SP band transportμH – SP Hall transport

Measured μHall and μdrift are supposed to be different

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2. BaTiO3 – Polaron hopping or Band transport?

T. Kolodiazhnyi, A. Pedric, N. Niewczas, C. Bridges, A. Safa-Sefat and J.E. Greedan, PRB. 68, 085205 (2003)D. Emin, PRL, 25, 1751 (1970)

Temperature – dependant mobility values in BaTiO3 from literature all over the map…

• Most recent Y-doped BaTiO3 samples showed mobility as high as 12 cm2/Vs at 120K! – Large polarons?

• Measured μHall and μdrift are supposed to be different but are comparable

• Band transport with a very heavy mass and phonon scattering?

• Correlated small polaron hopping theory can account for comparable μHall and μdrift?

Ba0.996Y0.004TiO3

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2. BaTiO3 – Polaron hopping or Band transport?

• No last word in transport mechanisms even in the easier SrTiO3 system

• Debate on exact BaTiO3 transport mechanisms expected to continue…

A. Verma, A.P. Kajdos, T.A. Cain, S. Stemmer and D. Jena, PRL, 112, 216601 (2014)

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3. BaTiO3 – Metal-Insulator-Transition

A. Spinelli, M. A. Torija, C. Liu, C. Jan, C. Leighton, PRB, 81, 155110 (2010)

In SrTiO3, La + SrTiO3 LaSr + VSr’’ + TiTi+ 3OO + 1e-

SrTiO3 SrSr + VO’’+ TiTi + 2OO + ½O2 + 2e-

Mott criterion for MIT at nc ~ 4×1010cm-3

Compensated carriers Mott criterion for MIT at nc ~ 1015cm-3

For SrTiO3

Mott criterion for MIT at nc ~ 1×1020cm-3

For BaTiO3

nc1/3aH ~ 0.25

SrTiO3-δ

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3. BaTiO3 – Metal-Insulator-Transition

T. Kolodiazhnyi, PRB, 78, 045107 (2008)

In BaTiO3, BaTiO3 BaBa + VO’’+ TiTi + 2OO + ½O2 + 2e-

Mott criterion for MIT at nc ~ 4×1010cm-3

Compensated carriers Mott criterion for MIT at nc ~ 1015cm-3

For SrTiO3

Mott criterion for MIT at nc ~ 1×1020cm-3

For BaTiO3

BaTiO3-δ

nc1/3aH ~ 0.25

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3. BaTiO3-δ – Ferroelectric metal?

T. Kolodiazhnyi, M. Tachibana, H. Kawaji, J. Hwang and E. Takayama-Muromachi, PRL, 104, 147602 (2010)

Low symmetry phases seen even in metallic BaTiO3-δ

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3. BaTiO3-δ – Ferroelectric metal?

T. Kolodiazhnyi, M. Tachibana, H. Kawaji, J. Hwang and E. Takayama-Muromachi, PRL, 104, 147602 (2010)

• Low symmetry phases seen in metallic BaTiO3-δ

• Tc (CT) reduces as [VO’’] increases

• Change in slope of Tc near the MIT

• Itinerant electrons seem to screen the destabilising effect of VO’’ to preserve low symmetry phases?

• Slope says n > nc (1.9×1021 cm-3 ) needed to destroy the lower symmetry states

• Not possible to achieve nc with VO’’

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3. BaTiO3-δ – Ferroelectric metal?

• n ~ 2×1021 cm-3

• Sample retains long range cubic symmetry at all temperatures

• Local Ti <111> displacements still persist in the sample at all temperatures!!

Highly Nb doped BaTiO3

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3. BaTiO3-δ – Ferroelectric metal?

• n ~ 2×1021 cm-3

• Sample retains long range cubic symmetry at all temperatures

• Local Ti <111> displacements still persist in the sample at all temperatures!!

Highly Nb doped BaTiO3

K. Page, T. kolodiazhnyi, T. Proffen, A.K.Cheetham and R. Seshadri, PRL, 101, 205502 (2008)

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3. BaTiO3-δ – Ferroelectric metal?

I-K. Jeong, S. Lee, S-Y Jeong, C. J. Won, N. Hur and A. Llobet, PRB, 84, 064125 (2011)

• PDF still shows 2 Ti-O distances, albeit reduced

Mixture of 40% tetragonal FE phase and 60% metallic cubic phases

Claim no co-existence of metallicity and ferroelectricity?

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3. MIT due to e- doping or donor-lattice distortion?

Y. Iwazaki, T. Suzuki, Y. Mizuno and S. Tsuneyuki, PRB, 86, 214103 (2012)

DFT says pure electron doping can cause CT transition to vanish!

Claim local Ti <111> displacements vanish as well by pure electron doping

Calculated nc ~ 1.36×1021 cm-3

Donors like Nb or VO’’ only accelerate the disappearance of the CT transition

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R+3Ti+3O-23

Ba+2Ti+4O-23

Integration of BaTiO3 with RTiO3/SrTiO3 interfaces

A. Ohtomo and H. Y. Hwang, Nature, 427, 423 (2004)R. Ohtsuka, M. Matvejeff, K. Nishio, R. Takahashi, and M. Lippmaa, Appl. Phys. Lett., 96,192111 (2010)P. Moetakef, T. A. Cain, D. G. Ouellette, J. Y. Zhang, D. O. Klenov, A. Janotti, C. G. Van de Walle, S. Rajan, S. J. Allen, and S. Stemmer, Appl. Phys. Lett. 99 (2011) 232116

LSAT

5 nm SmTiO3

20 nm SrTiO3

x nm BaTiO3

???Ba2+O2-

Ba2+O2-

Study of the BaTiO3 – RTiO3 interface

• Will there still be a 2DEG at the interface?

• What will happen to the insulating BaTiO3 films?

• Similar interface between SrZrO3 and SmTiO3 has no 2DEG!

• Start by inserting thin BaTiO3 layers in between SrTiO3 and SmTiO3

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Integration of BaTiO3 with RTiO3/SrTiO3

SmTiO3

SrTiO3

BaTiO3

[100] [110]

[100] [110]

[100] [110]

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Integration of BaTiO3 with RTiO3/SrTiO3

LSAT

5 nm SmTiO3

20 nm SrTiO3

x BaO layers

1 BaO2 BaO3 BaO4 BaO5 BaO8 BaO14 BaO

• Hall coefficient indicates presence of space charge layer with ~3×1014 cm-2 of electrons similar to RTiO3/SrTiO3

• Increasing BaTiO3 film thickness, the films switch from metallic to insulating behaviour at 5 BaO (or 4 u.c. BaTiO3)

17 nm BaTiO3 / 5 nm SmTiO3

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Integration of BaTiO3 with RTiO3/SrTiO3

LSAT

5 nm SmTiO3

20 nm SrTiO3

x BaO layers

1 BaO2 BaO3 BaO4 BaO5 BaO8 BaO14 BaO

17 nm BaTiO3 / 5 nm SmTiO3

• If all the electrons are in the BaTiO3, the critical carrier concentration (remote doping) for metallic BaTiO3 is n3D ~ 1.2×1021 cm-3 (for 5 BaO)

T. Kolodiazhnyi et al., PRL 104,147602 (2010)V. Fritsch et al., PRB 64, 045113 (2001)

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Integration of BaTiO3 with RTiO3/SrTiO3

LSAT

5 nm SmTiO3

20 nm SrTiO3

x BaO layers

1 BaO2 BaO3 BaO4 BaO5 BaO8 BaO14 BaO

• Mobility of the space charge layer shows a sudden drop around 3 to 5 BaO layers

• Possible that BaTiO3 switches from tetragonal to cubic around 4 u.c. due to pure electron doping?

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Summary

Q. Zhang, T. Cagin and W. A. Goddard, PNAS, 103, 14695 (2006)

Vacancy formation

Carrier transport mechanism

Metal – Insulator transition

BaTiO3