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>
	<li>Pavlenko, N., Kopp, T., Tsymbal, E. Y., Sawatzky, G. A., Mannhart, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Magnetic+and+superconducting+phases+at+the+LaAlO3/SrTiO3+interface:+The+role+of+interfacial+Ti+3d+electrons.">Magnetic and superconducting phases at the LaAlO3/SrTiO3 interface: The role of interfacial Ti 3d electrons.</a> Physical Review B. 85 (2), 020407 (2012).</li><li>Sch&#252;tz, P., 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=Microscopic+origin+of+the+mobility+enhancement+at+a+spinel/perovskite+oxide+heterointerface+revealed+by+photoemission+spectroscopy.">Microscopic origin of the mobility enhancement at a spinel/perovskite oxide heterointerface revealed by photoemission spectroscopy.</a> Physical Review B. 96, 161409 (2017).</li><li>Choi, H., Song, J. D., Lee, K. -. 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D., 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=Electric+field+control+of+the+LaAlO3/SrTiO3+interface+ground+state.">Electric field control of the LaAlO3/SrTiO3 interface ground state.</a> Nature. 456 (7222), 624-627 (2008).</li><li>Christensen, D. 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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. V., Smith, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Is+&#947;-Al2O3+polar.">Is &#947;-Al2O3 polar.</a> Applied Surface Science. , 887-890 (2017).</li><li>Gunkel, 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=Thermodynamic+Ground+States+of+Complex+Oxide+Heterointerfaces.">Thermodynamic Ground States of Complex Oxide Heterointerfaces.</a> ACS Applied Materials &amp; Interfaces. 9 (1), 1086-1092 (2017).</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 ...

The method presented here allows for controlling the amount of oxygen vacancies in oxide thin films both during and after the deposition. The main advances of this approach is that electrical and magnetic properties can be tuned by modifying the amount of oxygen vacancies. Oxygen vacancies serve as functional defects in most oxide materials, and the properties of many oxides can therefore be controlled systematically by defect engineering using this approach.Demonstrating the procedure will be Shinhee, Carlos, and Eric, a post-doc and two PhD students from our laboratory. To begin, purchase mixed-terminated strontium titanate substrates with a typical surface angle of 0.05 to 0.2 degrees with respect to the crystal planes. Clean the desired number of substrates by ultrasonication in acetone for five minutes.Then ultrasonicate the substrates for 20 minutes at 70 degrees Celsius in clean water, which dissolves strontium oxide or forms strontium hydroxide complexes at surface domains terminated with strontium oxide, while leaving the chemically stable, titanium dioxide terminated domains unchanged. In the meantime, prepare aqua regia solution by slowly adding hydrochloric acid into water and then adding nitric acid into the solution. Next, ultrasonicate the substrates in an acidic solution containing hydrochloric acid, nitric acid, and water at 70 degrees Celsius for 20 minutes in a fume hood to selectively etch strontium oxide due to the basic nature of strontium oxide surface domains, the acidity of titanium dioxide, and the presence of the strontium hydroxide complexes.Remove the residual acid from the substrates by ultrasonication in 100 milliliters of clean water for five minutes at room temperature in a fume hood. Then bake the substrates in an atmosphere of one bar of oxygen for one hour at 1, 000 degrees Celsius with a heating and cooling rate of 100 degrees Celsius per hour in a ceramic tube furnace to relax the substrate surface into a state with low energy. To deposit a thin film on the substrates, mount the substrates on a heater or chip carrier, depending on whether NC2 transport measurements are to be performed during the deposition.Next, place the titanium dioxide terminated substrate 4.7 centimeters from the single crystalline alumina target for a typical deposition of gamma alumina on strontium titanate at room temperature. Prepare for ablating from a single crystalline alumina target in an oxygen pressure of 10 to the power minus five millibar. Tune the properties using the oxygen content by using an oxygen deposition pressure in the range of 10 to the power minus six to 0.1 millibars, or by varying other deposition parameters.After incubation, observe the substrate for the desired thickness of gamma alumina deposition. Next, remove the sample from the deposition chamber, and stop any electrical measurements. Then store the sample in a vacuum.The sample degradation is slowest when stored in a vacuum or nitrogen. Mount the sample on a chip carrier using silver paste. Then electrically connect the sample to the chip carrier using wedge wire bonding of aluminum wires in the Van der Pauw geometry.Next, place the chip carrier with the sample in a closed furnace. Then using a connector and wires with thermally resistant insulation, electrically connect the chip carrier to the measurement equipment, and start the sheet resistance measurements. Then place the chip carrier equipped with the sample in a closed furnace and flush thoroughly with the gas used for the annealing while checking whether the sample resistance is sensitive to a change in the atmosphere.Anneal the sample using the desired annealing profile, depending on the thickness of the top film and the desired rate of oxygen incorporation. Abort the annealing when a desired change in the sheet resistance has occurred. Using this setup, the development of the sheet resistance in oxide heterostructures such as gamma alumina strontium titanate and lanthanum aluminate strontium titanate can be monitored in-situ during pulsed laser deposition.When the measurement environment is changed by measuring ex-situ or via in-situ oxygen flushing, significant changes in the sheet resistance of the strontium titanate based heterostructures can be observed. In samples where gamma alumina is deposited on strontium titanate, the electron mobility stays largely unchanged at room temperature, but changes dramatically at two Kelvin when the deposition pressure is varied. The properties of oxide heterostructures can also be tuned after deposition using annealing.The final state is determined by the annealing time and annealing temperature and atmosphere. The sheet conductants of heterostructures composed of strontium titanate capped with gamma alumina or amorphous lanthanum aluminate are measured at various annealing temperatures. The fastest decrease in the conductance was observed for the amorphous lanthanum aluminate strontium titanate heterostructures.For the strontium titanate heterostructures, the carrier density is controlled by controlling the annealing and oxygen. Consecutive annealing steps result in a steady decrease of carrier density and a transition from a metallic conducting to an insulating interface. Changing the conducting state in the strontium titanate heterostructure can enable different properties.Here, attempts to write nanowires using a conducting atomic force microscope were not possible before annealing. However, after annealing, conducting lines can be written and erased at the interface. Using this approach, we can systematically change the magnetic and electronic properties of oxide heterostructures, and in this way, study the role of oxygen vacancies in determining these properties.

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Research

JoVE Journal

Chemistry

2.9K vistas.  Technical University of Denmark. El método presentado aquí permite controlar la cantidad de vacantes de oxígeno en películas delgadas de óxido tanto durante como después de la deposición. Los principales avances de este enfoque es que las propiedades eléctricas y magnéticas se pueden ajustar modificando la cantidad de vacantes de oxígeno. Las vacantes de oxígeno sirven como defectos funcionales en la mayoría de los materiales de óxido, y las propiedades de muchos óxidos pueden, por lo tanto, controlarse sistem?...