Tuning Oxide Properties by Oxygen Vacancy Control During Growth and Annealing

Summary

Oxide materials show many exotic properties that can be controlled by tuning the oxygen content. Here, we demonstrate the tuning of oxygen content in oxides by varying the pulsed laser deposition parameters and by performing postannealing. As an example, electronic properties of SrTiO₃-based heterostructures are tuned by growth modifications and annealing.

Abstract

Electrical, optical, and magnetic properties of oxide materials can often be controlled by varying the oxygen content. Here we outline two approaches for varying the oxygen content and provide concrete examples for tuning the electrical properties of SrTiO₃-based heterostructures. In the first approach, the oxygen content is controlled by varying the deposition parameters during a pulsed laser deposition. In the second approach, the oxygen content is tuned by subjecting the samples to annealing in oxygen at elevated temperatures after the film growth. The approaches can be used for a wide range of oxides and nonoxide materials where the properties are sensitive to a change in the oxidation state.

The approaches differ significantly from electrostatic gating, which is often used to change the electronic properties of confined electronic systems such as those observed in SrTiO₃-based heterostructures. By controlling the oxygen vacancy concentration, we are able to control the carrier density over many orders of magnitude, even in nonconfined electronic systems. Moreover, properties can be controlled, which are not sensitive to the density of itinerant electrons.

Introduction

The oxygen content plays a vital role in the properties of oxide materials. Oxygen has a high electronegativity and, in the fully ionic limit, attracts two electrons from neighboring cations. These electrons are donated to the lattice when an oxygen vacancy is formed. The electrons can be trapped and form a localized state, or they can become delocalized and capable of conducting a charge current. The localized states are typically located in the band gap between the valence and conduction band with a total angular momentum that can be nonzero^1,2,3. The localized states can, thus, form localized magnetic moments and have a large impact on, for instance, the optical and magnetic properties^1,2,3. If the electrons become delocalized, they contribute to the density of itinerant charge carriers. In addition, if an oxygen vacancy or other defects are formed, the lattice adapts to the defect. The presence of defects can, thus, naturally lead to local strain fields, symmetry breaking, and a modified electronic and ionic transport in oxides.

Controlling the oxygen stoichiometry is, therefore, often key to tune, for instance, the optical, magnetic, and transport properties of oxide materials. A prominent example is that of SrTiO₃ and SrTiO₃-based heterostructures, where the ground state of the material systems is very sensitive to the oxygen content. Undoped SrTiO₃ is a nonmagnetic insulator with a band gap of 3.2 eV; however, by introducing oxygen vacancies, SrTiO₃ changes the state from insulating to metallic conducting with an electron mobility exceeding 10,000 cm²/Vs at 2 K⁴. At low temperatures (T < 450 mK), superconductivity may even be the favored ground state^5,6. Oxygen vacancies in SrTiO₃ have also been found to render it ferromagnetic⁷ and result in an optical transition in the visible spectrum from transparent to opaque². For more than a decade, there has been a large interest in depositing various oxides, such as LaAlO₃, CaZrO₃, and γ-Al₂O₃, on SrTiO₃ and examining the properties arising at the interface^{8,9,10,11,12,13}. In some cases, it turns out that the properties of the interface differ markedly from those observed in the parent materials. An important result of the SrTiO₃-based heterostructures is that the electrons can be confined to the interface, which makes it possible to control the properties related to the density of itinerant electrons using electrostatic gating. In this way, it becomes possible to tune, for instance, the electron mobility^14,15, superconductivity¹¹, electron pairing¹⁶, and magnetic state¹⁷ of the interface, using electric fields.

The formation of the interface also enables a control of the SrTiO₃ chemistry, where the deposition of the top film on SrTiO₃ can be used to induce a redox reaction across the interface^18,19. If an oxide film with a high oxygen affinity is deposited on SrTiO₃, oxygen can transfer from the near-surface parts of SrTiO₃ to the top film, thereby reducing SrTiO₃ and oxidizing the top film (see Figure 1).

Figure 1: Oxygen vacancy formation in SrTiO₃. Schematic illustration of how oxygen vacancies and electrons are formed in the interface-near region of SrTiO₃ during the deposition of a thin film with a high oxygen affinity. Reprinted figure with permission from a study by Chen et al.¹⁸. Copyright 2011 by the American Chemical Society. Please click here to view a larger version of this figure.

In this case, oxygen vacancies and electrons are formed near the interface. This process is expected to be the origin of the conductivity formed during the deposition at the interface between SrTiO₃ and room-temperature-grown metal films or oxides such as amorphous LaAlO₃^18,20 or γ-Al₂O₃^10,21,22,23. Thus, the properties of these SrTiO₃-based interfaces are highly sensitive to the oxygen content at the interface.

Here, we report the use of postdeposition annealing and variations in the pulsed laser deposition parameters to control the properties in oxide materials by tuning the oxygen content. We use γ-Al₂O₃ or amorphous LaAlO₃ deposited on SrTiO₃ at room temperature as examples on how the carrier density, electron mobility, and sheet resistance can be changed by orders of magnitude by controlling the number of oxygen vacancies. The methods offer some benefits beyond those obtained with electrostatic gating, which is typically used to tune the electrical^9,11,14 and in some cases the magnetic^15,17 properties. These benefits include forming a (quasi-)stable final state and avoiding the use of electric fields, which requires electrical contact to the sample and may cause side effects.

In the following, we review general approaches for tuning the properties of oxides by controlling the oxygen content. This is done in two ways, namely, 1) by varying the growth conditions when synthesizing the oxide materials, and 2) by annealing the oxide materials in oxygen. The approaches can be applied to tune a range of properties in many oxide and some monoxide materials. We provide a concrete example on how to tune the carrier density at the interface of SrTiO₃-based heterostructures. Ensure that a high level of cleanliness is exercised to avoid contamination of the samples (e.g., by using gloves, tube furnaces dedicated to SrTiO₃, and nonmagnetic/acid resistant tweezers).

Protocol

1. Controlling properties by varying growth conditions

1. Preparation of high-quality surfaces of SrTiO₃
   1. Purchase mixed terminated SrTiO₃ substrates (e.g., of 5 mm x 5 mm x 0.5 mm in size) with a typical surface angle of 0.05°–0.2° with respect to the (001) crystal planes.
      NOTE: The miscut angle determines the flatness of the surface, which is important for epitaxial growth on the substrate, as well as for the resulting properties at the interface.
   2. Clean the desired number of substrates by ultrasonication in acetone for 5 min and ethanol for 5 min at room temperature in a standard ultrasonicator.
   3. Ultrasonicate the substrates for 20 min at 70 °C in clean water, which dissolves SrO²⁴ or form Sr-hydroxide complexes at surface domains terminated with SrO²⁵, while leaving the chemically stable TiO₂-terminated domains unchanged²⁶.
   4. Ultrasonicate the substrates in a 3:1:16 HCl:HNO₃:H₂O acidic solution (e.g., 9:3:48 mL) at 70 °C for 20 min in a fume hood to selectively etch SrO due to the basic nature of SrO surface domains, the acidity of TiO_2, and the presence of the Sr-hydroxide complexes.
   5. Remove the residual acid from the substrates by ultrasonication in 100 mL of clean water for 5 min at room temperature in a fume hood.
      NOTE: TiO₂-terminated SrTiO₃ can be purchased commercially or prepared in various ways based on the selective etching of SrO on the surface^24,27. The conventional etching in HF also leads to TiO₂-terminated SrTiO₃, but this is avoided here due to safety concerns and a risk of unintentional F-doping of SrTiO₃²⁸.
   6. Thermally treat the substrates in an atmosphere of 1 bar of oxygen for 1 h at 1,000 °C with a heating and cooling rate of 100 °C/h in a ceramic tube furnace, to relax the substrate surface into a state with low energy.
2. Deposition of the thin film(s) on the substrate
   1. Mount the substrates on the heater or a chip carrier, depending on whether in situ transport measurements during the deposition are to be performed.
      NOTE: A silver paste that cures at room temperature can be conveniently used for substrate mounting.
   2. Connect the four corners of the SrTiO₃ surface to a chip carrier electrically using, for instance, standard wedge wire bonding with 20 µm-thick Al wires, if in situ transport measurements are desired. Mount the chip carrier onto a chip carrier holder where wires connect the sample to an electrical measurement setup through a vacuum-compatible connector.
   3. Place the TiO₂-terminated substrate 4.7 cm from the single-crystalline Al₂O₃ target for a typical deposition of Al₂O₃ on SrTiO₃.
   4. Start sheet resistance measurements using the Van der Pauw geometry²⁹, if in situ transport measurements are to be performed.
   5. Heat the substrate to 650 °C at a rate of 15 °C/min or keep the substrate at room temperature.
   6. Prepare for ablating from a single-crystalline Al₂O₃ target in an oxygen pressure of 1 x 10^-5 mbar using, for instance, a nanosecond-pulsed KrF laser with a wavelength of 248 nm, a laser fluence of 3.5 J/cm², and a frequency of 1 Hz. Tune the properties using the oxygen content by using an oxygen deposition pressure in the range of 10^-6 to 10^-1 mbar or by varying other deposition parameters.
   7. Deposit the desired thickness of γ-Al₂O₃ (typically 0–5 unit cells).
      NOTE: This can be determined using, for instance, reflective high-energy electron diffraction (RHEED) oscillations or atomic force microscopy measurements, where the latter is measured as the height difference produced by preventing the deposition of γ-Al₂O₃ on the part of the substrate using a physical mask.
   8. Cool down the γ-Al₂O₃/SrTiO₃ heterostructure at a rate of 15 °C/min at the deposition pressure without performing an additional annealing step if a high-temperature deposition is done.
   9. Remove the sample from the deposition chamber and stop the electrical measurements.
   10. Store the sample in vacuum, nitrogen or, alternatively, at ambient conditions. The sample degradation is slowest when stored in vacuum or nitrogen²⁰.

2. Controlling properties by thermal annealing

1. Mount the sample with silver paste on a chip carrier.
2. Connect the sample electrically to the chip carrier using, for instance, wedge wire bonding of Al wires in the Van der Pauw geometry²⁹.
3. Connect the chip carrier electrically to the measurement equipment, using a connector and wires with thermally resistant insulation.
4. Start the sheet resistance measurements.
5. Place the chip carrier equipped with the sample in a closed furnace.
6. Flush thoroughly with the gas used for the annealing while checking whether the sample resistance is sensitive to a change in the atmosphere.
7. Anneal the sample using the desired annealing profile. Typical annealing temperatures are 50–250 °C and 100–350 °C for a-LaAlO₃/SrTiO₃ and γ-Al₂O₃/SrTiO₃ heterostructures, respectively, depending on the thickness of the top film and the desired rate of oxygen incorporation.
   NOTE: Use more heat-compatible options than Al wires and standard ceramic chip carriers if temperatures above 350–400 °C are needed.
8. Abort the annealing when a desired change in the sheet resistance has occurred.
9. Cool down the sample by ramping down the temperature, or take out the sample.
10. Stop the electrical measurements.
    NOTE: The resistance is generally temperature dependent, which must be taken into account if specific transport properties at a certain temperature are the goal.

Results

Controlling properties by varying growth conditions
Varying the deposition parameters during the deposition of oxides can lead to a large change in the properties, in particular for SrTiO₃-based heterostructures, as shown in Figure 2.

Figure 2: Controlling the transpor...

Discussion

The methods described here rely on using the oxygen content to control oxide properties, and the oxygen partial pressure and operating temperature are, thus, critical parameters. If the total oxidation state of the system is tuned in a way where the system remains in a thermodynamic equilibrium with the surrounding atmosphere (i.e., changed pO₂ at high temperature), the changes can be reversible. However, in the case of SrTiO₃-based heterostructures, interfacial oxygen vacancies are typically formed...

Materials

Name	Company	Catalog Number	Comments
SrTiO3	Crystec		Single crystalline (001) oriented, 0.05-0.2 degree miscut angle
LaAlO3	Shanghai Daheng Optics and Fine Mechanics Co.Ltd.		Single crystalline
Al2O3	Shanghai Daheng Optics and Fine Mechanics Co.Ltd.		Single crystalline
Chemicals and gases	Standard suppliers
Silver paste	SPI Supplies, Structure Probe Inc		05001-AB, High purity silver paint
Ultrasonicator	VWR		USC500D HF45kHz/100W
Wedge wire bonder	Shenzhen Baixiangyuan Science & Technology Co.,Ltd.		HS-853A Aluminum wire bonder
Pulsed laser deposition	Twente Solid State Technologies (TSST)		PLD from TSST with software version V3.0.29, equipped with a 248 nm KrF nanosecond laser (Compex Pro 205 F) from Coherent
Resistance measurement setup	Custom made		Based on the following electrical instruments and custom written software: Keithley 6221 DC and AC current source Keithley 2182A nanovoltmeter Keithley 7001 switch system with a matrix card Keithley 6487 picoammeter
Hall measurements	Cryogenics		Based on the following electrical instruments and custom written software: Keithley 2400 DC current source Keithley 2182A nanovoltmeter Keithley 7001 switch system with a matrix card
Furnace	Custom made		Custom written software control of a FTTF 500/70 tube furnace from Scandia Ovnen AS and a eurotherm 2216e temperature controller