Anatase TiO2- a model system for large polaron transport - ACS

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Functional Inorganic Materials and Devices

Anatase TiO2- a model system for large polaron transport Bixing Yan, Dongyang Wan, Xiao Chi, Changjian Li, Mallikarjuna Rao Motapothula, Sonu Hooda, Ping Yang, Zhen Huang, Shengwei Zeng, Akash Gadekar Ramesh, Stephen J. Pennycook, Andrivo Rusydi, * Ariando, Jens Martin, and Thirumalai Venkatesan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11643 • Publication Date (Web): 17 Oct 2018 Downloaded from http://pubs.acs.org on October 17, 2018

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ACS Applied Materials & Interfaces

Anatase TiO2- a model system for large polaron transport Bixing Yan , Dongyang Wan , Xiao Chi , Changjian Li , Mallikarjuna Rao Motapothula , 1,2

1,2*

2,3

4

1

Sonu Hooda , Ping Yang , Zhen Huang , Shengwei Zeng , Akash Gadekar Ramesh , Stephen 1

3

1

1

1,2

John Pennycook , Andrivo Rusydi , Ariando , Jens Martin , Thirumalai Venkatesan 4

1

2

3

1,2,3

1,2*

2,6*

1,2,4,5,7*

NUSNNI-NanoCore, National University of Singapore, Singapore 117411, Singapore.

Department of Physics, National University of Singapore, Singapore 117551, Singapore.

Singapore Synchrotron Light Source, National University of Singapore, Singapore 117603, Singapore.

4

Department of Material Science and Engineering, National University of Singapore, Singapore 117575, Singapore.

5

NUS Graduate School for Integrative Sciences and Engineering, National University of Singapore, Singapore 117456, Singapore.

6

Center for Advanced 2D Material, National University of Singapore, Singapore 117551, Singapore.

7

Department of Electrical and Computer Engineering, National University of Singapore, Singapore 117583, Singapore.

KEYWORDS: Anatase TiO2, Electron-phonon interaction, Large Polaron, Disorder, Electronic transport, Magnetoresistance

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ABSTRACT: Large polarons have been of significant recent technological interest as they screen and protect electrons from point scattering centers. Anatase TiO2 is a model system for studying large polarons as they can be studied systematically over a wide range of temperature and carrier density. The electronic and magneto transport properties of reduced anatase TiO2 epitaxial thin films are analyzed considering various polaronic effects. Unexpectedly, with increasing carrier concentration, the mobility increases, which rarely happens in common metallic systems. We find that the screening of the electron-phonon (e-ph) coupling by excess carriers is necessary to explain this unusual dependence. We also find that the magnetoresistance (MR) could be decomposed into a linear and a quadratic component, separately characterizing the carrier transport and trapping as a function of temperature respectively. The various transport behaviors could be organized into a single phase diagram which clarifies the evolution of large polaron in this material.

I. INTRODUCTION: Large polarons have become very important as they screen charges from local defects and this has been exploited in a number of important materials systems- a recent example being the organic perovskites1-3. There have been a number of important recent publications exploring the role of polarons in transport in a number of organic complexes4-10. So an understanding of large polaron behaviour is important in the designing of materials for engineering the transport properties. Since organic systems are not easy to fabricate in the form of single crystalline thin films (ideal for transport studies) we see anatase TiO2 as a model system for this study. We can grow extremely high quality crystals of these films doped over a large range of carrier concentration and study them over a wide temperature range. Anatase titanium dioxide (TiO2), besides its well explored photocatalysis application, has spurred research interests in memristors11, spintronic devices12-14and transparent conducting oxides15-17. Stoichiometric


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anatase TiO2 is an insulator with a band gap of 3.2 eV. With intrinsic or extrinsic dopants, conductive TiO2 typically has carrier concentration above 1017 cm-3 and resistivity between 1 to 10-4 Ω⋅cm11-24. Further, anatase TiO2 shares similar photovoltaic applications with typical polaronic organic materials, and its conductive behavior also depends upon multiple mechanisms common to the organic crystal systems. Early on, scanning tunneling microscopy (STM)18 and scanning electron microscopy(SEM)19 revealed that anatase TiO2 is a disordered system and its transport properties can be tuned by impurity clusters20-21. Also, from angle resolved photoemission spectroscopy (ARPES)22, resonant inelastic x-ray scattering (RIXS)23 and thermoelectric measurement24, it is known that electrons in anatase TiO2 are coupled with phonons, forming the quasiparticle “large polaron” which is an intermediate state between fully localized small polaron and mobile free electron. Usually, since the strength of this electron-phonon (e-ph) coupling is moderate, the large polaron can be easily tuned to different states like dissociation25-26, melting27-28 and crystallization29. However, as of today there has been no systematic study of polaron behavior with temperature and carrier density because generally the data from STM, ARPES and RIXS are analyzable only at low temperatures. With transport measurements these limitations do not exist and by tuning the film carrier density and a complete study as a function of temperature one can elucidate the full phase diagram of large polarons in this material system. II. FABRICATION METHODS: Anatase TiO2 thin films were grown on single crystal (100)-oriented LaAlO3 substrates by pulsed laser deposition with a thickness of 92nm (∼100 monolayers). All samples were fabricated at 800°C under 3×10-3 mTorr oxygen partial pressure. The laser energy and frequency were kept at 1.33J/cm2 and 2 Hz. To study the intrinsic properties of anatase TiO2,



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we choose to introduce electrons through oxygen vacancy by vacuum thermal annealing. We annealed all the samples at the same temperature (900°C) for the same duration (3 hours) while doping level is controlled by the variation in oxygen partial pressure (9×10-8 to 3×10-3 mTorr) during the anneal. This process ensures that the only change is in the carrier density and very little change in the anatase crystal structure. The diffusion length of oxygen ions at 900°C ( 𝑙 = 𝐷𝑡) is 600nm, where D is the oxygen diffusion coefficient30 and t is the annealing time. Since the diffusion length is much larger than the film thickness (∼90nm), we expect a homogenous carrier concentration in the films. III. RESULTS A. Crystal Quality

The X-ray diffraction (CuKα1 ray) patterns of the TiO2 films are shown in Fig. 1(a). The sharp TiO2 (004) peaks indicate the well-maintained anatase structure which is also supported by the scanning transmission electron microscopy (STEM) as shown in Fig. 1(c)-(d). A small diffusion of the Bragg peak is shown in Fig. 1(b), indicating the lattice to be slightly deformed after reduction. The distortion could be attributed to two possible mechanisms: (1) the oxygen vacancies can be viewed as flaws in the lattice frame, broadening the Bragg peak; (2) the eph interaction could deform the lattice, causing a diffusive Bragg peak31. The first mechanism is acceptable since all the FWHM (γ) of reduced samples are larger than the oxygen rich sample (γ0), γ/γ0>1. Nevertheless, it cannot explain the lattice relaxation at ∼3×1019 cm-3 which was reported as the critical concentration for the screening of e-ph coupling by excess carriers22. Thus, the relaxation beyond 3×1019 cm-3 could be attributed to the second mechanism: when the electron-lattice coupling is screened, the lattice deformation will relax.


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Figure 1. (a) X-ray diffraction spectra of as-grown and reduced TiO2 films on LAO substrate. (b) Normalized intensity of X-ray scattering around the (004) Bragg reflection. The FWHM ratio of the reduced to the oxygen-rich sample is shown in the inset of (b). (c) STEM image of epitaxial anatase TiO2 (n=2×1020cm-3) which agrees with the schematic of anatase TiO2 shown in (d)

B. Electrical transport measurement

The transport properties of the anatase TiO2 were measured by the Physical Property Measurement System (Quantum Design).



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Figure 2. (a)Temperature dependence of resistivity in anatase TiO2 thin film. (b) Carrier concentration and mobility from Hall measurement. In 3-300K range, higher carrier concentration leads to a higher mobility as shown in (c). (d) Calculated effective mass of the large polaron. Example of modelled resistivity for n=7×1018cm-3 using variable range hopping and large polaron model is inserted (For other doping levels, see supplementary Fig. S1(b)).

The resistivity of the samples varied from 10-4 to 10-1 Ω⋅cm [Fig. 2(a)], covering the range of most reports11-24. The Hall mobility and carrier concentration are presented in Fig. 2(b). Strikingly, the mobility increases with increasing carrier concentration [Fig. 2(c)]. There are two mechanisms depending on the temperature. At low temperatures (

Anatase TiO2- a model system for large polaron transport - ACS

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