Treated substrate

As perovskite SrTiO₃ is composed of SrO- and TiO₂-alternative. The (100) surface can be terminated by either SrO- or TiO2- domain. The surface physical and chemical properties must be greatly influenced by the surface morphology and the ratio of these two kinds of terminated domains. We can obtain the smooth TiO2 plane by treating the SrTiO3 (100) substrate with a buffered oxide Etch 6:1 (NH4F–HF) solution. Because the HF etching removes Sr more efficiently than Ti, it seems that the etchant mostly attacks the Sr at the step edges, dissolving it and then removing Ti by lift-off, as shown in the processes as the following shown in Figure 3.12.

A SrTiO3(100) substrate of 10x10x0.5 mm (Shinkosha Co., Japan) was first cleaned surface by using a 3-step sonication process in acetone, ethanol, DI water, and drying in a nitrogen stream. The second, the SrTiO3 (100) was treated with a buffered oxide Etch 6:1 (NH4F–HF) solution at room temperature for 7 minute. The treated SrTiO3 was annealed in air at 1100⁰C for 7 hour, and then cooling down to room temperature with cooling rate was 5⁰C/m.

Treated substrate

Annealing(7hours) Etching(BOE 6mins)

SrTiO3(001) Non -treatment

Treated substrate

AO BO2

AO

BOE: Buffered Oxide Etch 6:1

Figure 3.12 Illustration of the substrate treatment processes to obtain the TiO₂- terminated on the surface of STO(100) substrate

To conclude this chapeter, understanding the properties of a material system requires knowledge of the structure of the material, which is determined during the fabrication. All three parts, fabrication, characterization and functionality, require carefully paid attention and sophisticated techniques. It is this sequence - fabrication determines structure which in turn determines properties - that forms the basis of the research in this thesis. Pulsed laser deposition in combination with RHEED monitoring has become a mainstay of oxide thin film fabrication because of the tuneable growth kinetics and the ease of stoichiometric transfer. The use of single-terminated substrates allows for the growth of well- defined structures of complex oxides. Several techniques to characterize these structures, either available in-house or through collaborations with groups world-wide, have been discussed. Finally, techniques to measure the electrical and optical properties are necessary to be able to discuss the relation between these functional properties and the structural features.

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[4] D. B. Chrisey, G. K. Hubler, Pulsed laser deposition of thin films, New York, John Wiley and Sons 1994, 55-85.

[5] D. Bauerle, Laser processing and chemistry, Berlin, Springer-Verlag 1996, 3-62.

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[7] J. Als-Nielsen, D. McMorrow, Elements of Modern X-ray Physics, Wiley 2001; 1st edition, 107-195.

[8] B. Fultz, J. Howe, Transmission electron microscopy and diffractometry of materials Berlin;

New York: Springer 2001, 1-118.

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[10] M. Alexe, A. Gruverman (Eds.), Nanoscale Characterisation of Ferroelectric Materials.

Scanning Probe Microscopy Approach, Springer 2004, 45-56.

[11] J. Tichý, J. Erhart, E. Kittinger, J. Přívratská, Fundamentals of Piezoelectric Sensorics Mechanical: Dielectric, and Thermodynamical Properties of Piezoelectric Materials, Springer;

1st edition 2010, 1-53.

[12] R. C. Jaklevic, J. Lambe, A. H. Silver, J. E. Mercereau, Phys. Rev. Lett. 1964, 12, 159.

[13] B. D. Josephson, Physics Lett. 1962,1, 251.

[14] P. W. Anderson, J. M. Rowell, Phys. Rev. Lett., 1963, 10, 230.

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Chapter 4: Ferroelectric control of the conduction at the LaAlO3/SrTiO3

hetero-interface

4.1 Introduction

Complex oxide hetero-interfaces have emerged as one of the most exciting subjects in condensed-matter due to their unique physical properties and new possibilities for next-generation electronic devices [1, 2]. In the push for practical applications, it is desirable to have the ability to modulate the interface functionalities by external stimulus. In this study, we propose a generic approach by inserting a functional layer to the heterostructure to acquire the non-volatile control of the intriguing properties at oxide interfaces. The LAO/STO interface is served as a model system in which a highly mobile quasi-two dimensional electron gas (2DEG) forms between two band insulators [3,4] , exhibiting 2D superconductivity [5] and unusual magnetotransport properties [6]. Although a modulation of the carrier density and mobility of the LAO/STO interface was achieved by using electric field effect [7–9], it is essential to extend the control concepts to gain nonvolatile and reversible abilities for practical applications. Recently, the nonvolatile modification of the local conduction at the LAO/STO interface has been demonstrated by scanning probe techniques [10–12]. Several possible mechanisms have been proposed to explain this interesting behavior based on the electrostatic effects either attributed to induced ferroelectricity or surface charge [13,14]. In this study, we bring in a ferroelectric Pb(Zr_0.2Ti_0.8)O₃ (PZT) layer nearby the LAO/STO interface. The ferroelectric polarization of PZT layer serves as a control parameter to modulate the 2DEG conducting behaviors. The as-grown polarization (P_up state) leads to charge depletion and consequently a low conduction.

Switching the polarization direction (P_down state) results in a charge accumulation and enhances the conduction at the interface of LAO/STO. The origin of this modulation is attributed to a change in the electronic structure due to the ferroelectric polarization states, evidenced by x-ray photoelectron spectroscopy (XPS) and the cross-sectional scanning tunneling microscopy/spectroscopy (XSTM/S). Control of the conduction at this oxide interface suggests that the concept can be generalized for other oxide systems to design functional interfaces.

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4.2. Experimental methods