Name: tuning oxide properties by oxygen vacancy control during growth and annealing
Uploaded: 2023-06-09T15:00:00
Description: Watch this Scientific Journal Video about Tuning Oxide Properties by Oxygen Vacancy Control During Growth and Annealing at JoVE.com

The authors thank J. Geyti from the Technical University of Denmark for his technical assistance. F. Trier acknowledges support by research grant VKR023371 (SPINOX) from VILLUM FONDEN. D. V. Christensen acknowledges the support of Novo Nordisk Foundation NERD Programme: New Exploratory Research and Discovery, Superior Grant NNF21OC0068015.

1. Controlling properties by varying growth conditions
<ol>
	<li>Preparation of high-quality surfaces of SrTiO3
	<ol>
		<li>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&#176;&#8211;0.2&#176; 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 ...

<ol>
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Z., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=On+the+origin+of+metallic+conductivity+at+the+interface+of+LaAlO3/SrTiO3.">On the origin of metallic conductivity at the interface of LaAlO3/SrTiO3.</a> Applied Surface Science. 258 (23), 9242-9245 (2012).</li><li>Trier, F., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Degradation+of+the+interfacial+conductivity+in+LaAlO3/SrTiO3+heterostructures+during+storage+at+controlled+environments.">Degradation of the interfacial conductivity in LaAlO3/SrTiO3 heterostructures during storage at controlled environments.</a> Solid State Ionics. 230, 12-15 (2013).</li><li>Christensen, D. 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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...

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 nonzero1,2,3. The localized states can, thus, form localized magnetic moments and have a large impact on, for instance, the optical and magnetic properties1,2,3. 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 &#60; 450 mK), superconductivity may even be the favored ground state5,6. 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 &#947;-Al2O3, on SrTiO3 and examining the properties arising at the interface8,9,10,11,12,13. 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 mobility14,15, 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 interface18,19. 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).
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/58737/58737fig01v2.jpg" /> 
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. <a href="https://www.jove.com/files/ftp_upload/58737/58737fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
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 LaAlO318,20 or &#947;-Al2O310,21,22,23. 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 &#947;-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 electrical9,11,14&#160;and in some cases the magnetic15,17&#160;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).

<table border="0" cellpadding="0" cellspacing="0" style="width:1260px;" width="1260">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">SrTiO3</td>
			<td style="width:264px;">Crystec</td>
			<td style="width:264px;"></td>
			<td style="width:516px;">Single crystalline (001) oriented, 0.05-0.2 degree miscut angle</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">LaAlO3</td>
			<td style="width:264px;">Shanghai Daheng Optics and Fine Mechanics Co.Ltd.</td>
			<td style="width:264px;"></td>
			<td>Single crystalline</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Al2O3</td>
			<td style="width:264px;">Shanghai Daheng Optics and Fine Mechanics Co.Ltd.</td>
			<td style="width:264px;"></td>
			<td>Single crystalline</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Chemicals and gases</td>
			<td style="width:264px;">Standard suppliers</td>
			<td style="width:264px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Silver paste</td>
			<td style="width:264px;">SPI Supplies, Structure Probe Inc</td>
			<td style="width:264px;"></td>
			<td>05001-AB, High purity silver paint</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Ultrasonicator</td>
			<td style="width:264px;">VWR</td>
			<td style="width:264px;"></td>
			<td>USC500D HF45kHz/100W</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Wedge wire bonder</td>
			<td style="width:264px;">Shenzhen Baixiangyuan Science &#38; Technology Co.,Ltd.</td>
			<td style="width:264px;"></td>
			<td>HS-853A Aluminum wire bonder</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Pulsed laser deposition</td>
			<td style="width:264px;">Twente Solid State Technologies (TSST)</td>
			<td style="width:264px;"></td>
			<td style="width:516px;">PLD from TSST with software version V3.0.29, equipped with a 248 nm KrF 
			nanosecond laser (Compex Pro 205 F) from Coherent</td>
		</tr>
		<tr height="105">
			<td height="105" style="height:105px;width:216px;">Resistance measurement setup</td>
			<td style="width:264px;">Custom made</td>
			<td style="width:264px;"></td>
			<td style="width:516px;">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</td>
		</tr>
		<tr height="84">
			<td height="84" style="height:84px;width:216px;">Hall measurements</td>
			<td style="width:264px;">Cryogenics</td>
			<td style="width:264px;"></td>
			<td style="width:516px;">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</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Furnace</td>
			<td style="width:264px;">Custom made</td>
			<td style="width:264px;"></td>
			<td style="width:516px;">Custom written software control of a FTTF 500/70 tube furnace from Scandia Ovnen AS and a eurotherm 2216e temperature controller</td>
		</tr>
	</tbody>
</table>

tuning oxide properties by oxygen vacancy control during growth and annealing

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.

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.
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/58737/58737fig2v2.jpg" /> 
Figure 2: Controlling the transpor...

Watch this Scientific Journal Video about Tuning Oxide Properties by Oxygen Vacancy Control During Growth and Annealing at JoVE.com

Tuning Oxide Properties by Oxygen Vacancy Control During Growth and Annealing

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.

Electrical, optical, and magnetic properties of oxide materials can often be controlled by varying the oxygen content. Here we outline two approaches for ...

Research

JoVE Journal

Chemistry

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