Biaxial strain-induced transport property changes in atomically tailored ${\mathrm{SrTiO}}_{3}$-based systems

Biaxial strain-induced transport property changes in atomically tailored ${SrTiO}_{3}$ -based systems

Z. Huang, Z. Q. Liu, M. Yang, S. W. Zeng, A. Annadi, W. M. Lü, X. L. Tan, P. F. Chen, L. Sun, X. Renshaw Wang, Y. L. Zhao, C. J. Li, J. Zhou, K. Han, W. B. Wu, Y. P. Feng, J. M. D. Coey, T. Venkatesan, and Ariando

Phys. Rev. B 90, 125156 – Published 29 September 2014

Abstract

Several metallic ${SrTiO}_{3}$ -based systems, including reduced ${SrTiO}_{3 − δ}$ , Nb-doped ${SrTiO}_{3}$ , and two-dimensional electron gas at the ${LaAlO}_{3} / {SrTiO}_{3} (001)$ interface, have been epitaxially fabricated on various substrates inducing different strain, from −2.98% (compressive) to +0.99% (tensile). For all the ${SrTiO}_{3}$ -based systems, strain reduces conductivity. Tensile strain, however, is much more effective at reducing conductivity compared to compressive strain. Further, carrier mobility is found to be more sensitive to strain than carrier density. Calculations based on density functional theory show that strain can break the cubic symmetry of ${TiO}_{6}$ octahedron, lift the degeneracy of $Ti 3 d$ orbitals, and reduce the number of available states at the bottom of the conduction band to cause low carrier mobility. Our results show the critical features of strain effect on the conducting ${SrTiO}_{3}$ -based systems, and shed some light on strain engineering of these functional materials.

Received 13 April 2014
Revised 18 August 2014

DOI:https://doi.org/10.1103/PhysRevB.90.125156

Authors & Affiliations

Z. Huang¹, Z. Q. Liu^1,2, M. Yang², S. W. Zeng^1,2, A. Annadi^1,2, W. M. Lü^1,3, X. L. Tan⁴, P. F. Chen⁴, L. Sun¹, X. Renshaw Wang^1,2, Y. L. Zhao^1,2, C. J. Li¹, J. Zhou², K. Han¹, W. B. Wu⁴, Y. P. Feng², J. M. D. Coey^1,5, T. Venkatesan^1,2,3, and Ariando^1,2,*

¹NUSNNI-NanoCore, National University of Singapore, 117411 Singapore
²Department of Physics, National University of Singapore, 117542 Singapore
³Department of Electrical and Computer Engineering, National University of Singapore, 117576 Singapore
⁴Hefei National Laboratory for Physical Science at Microscale, University of Science and Technology of China, Hefei 230026, People's Republic of China
⁵School of Physics and CRANN, Trinity College, Dublin 2, Ireland

^*ariando@nus.edu.sg

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Vol. 90, Iss. 12 — 15 September 2014

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Figure 1
(a) Sketches for tetragonal-like ${TiO}_{6}$ octahedral distortion under different strains. The in-plane expansion and out-of-plane shrinkage are expected under tensile strain, while the opposite distortion can be obtained under compressive strain. (b) In situ RHEED oscillation for STO/LSAT heterostructure. (Inset) AFM image for 50-uc STO/LSAT film. (c) XRD $θ - 2 θ$ scans around (002) reflections are shown for 300-uc STO/LSAT and 160-uc NSTO/LSAT samples. (d) RSM around (–103) is shown for LAO (25-uc)/STO(20-uc)/LSAT heterostructure. Note that the units are defined using the absolute lattice constants. For example, for $(h k l) = (- 103)$ , the projection of LSAT (–103) along the [001] direction $Q_{[003]}$ is 0.597 4, and it can be written as $Q_{[003]} = l λ / 2 a_{[001]}$ . Hence the out-of-plane lattice constant for LSAT $a_{[001]} = l λ / 2 Q_{[003]} = (3 \times 1.540 6 Å) / (2 \times 0.597 4) = 3.868 Å$ .
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Figure 2
$R_{S} (T)$ curves for STO in (a)–(d), NSTO in (e)–(h), and 2DEG at the LAO/STO interface (i)–(l), under different strain states from tensile (left) to compressive (right).
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Figure 3
(a) AFM image of 300-uc ${SrTiO}_{3} / {DyScO}_{3}$ films. (b) Height profile along line AB shown in (a). Clearly, the film exhibits a smooth surface where the step height is around 1 uc (∼0.4 nm), and no crack or film discontinuity was found in our AFM measurement.
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Figure 4
Sheet carrier density $n_{S}$ and mobility $μ_{S}$ for 160-uc NSTO on different substrates are shown in (a) and (b), respectively. The blue-dashed line in (a) denotes the value of carrier density calculated based on 0.5 wt% Nb doping in 160-uc NSTO film. And for the 2DEG at an epitaxially grown LAO(15-uc)/STO(12-uc) interface, the sheet carrier density and mobility are also shown in (c) and (d), respectively. The conventional 2DEG with 15-uc LAO film on STO substrate is also plotted for comparison. The blue-dashed line in (c) denotes the value of carrier density required to prevent polar catastrophe at the LAO/STO interface.
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Figure 5
(a) $R_{S} (T)$ curves for STO/LSAT films with different film thicknesses. (b) Room-temperature sheet conductance for STO/LSAT films at different thicknesses, and the dark cyan line is fitted by assuming $t_{D}$ is 45 uc. (Inset) Conductivity as a function of STO/LSAT thickness at 200 K. (c) XRD $θ - 2 θ$ scans around (002) reflections for STO/LSAT films from 60 to 300 uc. (d) RSM around (–103) for 300-uc STO/LSAT films, where the film and substrate share the same projection along [100] orientation, indicating coherent growth. (e) The $a_{[001]} / a_{[100]}$ ratio (indicating the octahedral tetragonal distortion) and angle $α^{'}$ (denoting the octahedral rotation) are shown as a function of STO/LSAT film thickness.
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Figure 6
(a) Lifting $Ti t_{2 g}$ orbital degeneracy by strain-induced ${TiO}_{6}$ distortion. (b) The DFT-calculated band structure of ${SrTiO}_{3}$ under different strain states (+2% tensile strain, 0% no strain, and −2% compressive strain). DFT-calculated DOS of the conduction band for ${SrTiO}_{3}$ in (c) and ${SrTi}_{7 / 8} {Nb}_{1 / 8} O_{3}$ in (d). The energy is denoted with respect to the bottom of the conduction band $E_{CB}$ .
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Physical Review B

covering condensed matter and materials physics

Biaxial strain-induced transport property changes in atomically tailored ${SrTiO}_{3}$ -based systems

Z. Huang, Z. Q. Liu, M. Yang, S. W. Zeng, A. Annadi, W. M. Lü, X. L. Tan, P. F. Chen, L. Sun, X. Renshaw Wang, Y. L. Zhao, C. J. Li, J. Zhou, K. Han, W. B. Wu, Y. P. Feng, J. M. D. Coey, T. Venkatesan, and Ariando

Phys. Rev. B 90, 125156 – Published 29 September 2014

Abstract

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Physical Review B

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Biaxial strain-induced transport property changes in atomically tailored SrTiO3-based systems

Z. Huang, Z. Q. Liu, M. Yang, S. W. Zeng, A. Annadi, W. M. Lü, X. L. Tan, P. F. Chen, L. Sun, X. Renshaw Wang, Y. L. Zhao, C. J. Li, J. Zhou, K. Han, W. B. Wu, Y. P. Feng, J. M. D. Coey, T. Venkatesan, and Ariando

Phys. Rev. B 90, 125156 – Published 29 September 2014

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Biaxial strain-induced transport property changes in atomically tailored ${SrTiO}_{3}$ -based systems