Name: tuning oxide properties by oxygen vacancy control during growth and annealing
Uploaded: 2023-06-09T15:00:00.000+00:00
Duration: 6 min 44 s
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 interface.</li>
		<li>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.</li>
		<li>Ultrasonicate the substrates for 20 min at 70 &#176;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.</li>
		<li>Ultrasonicate the substrates in a 3:1:16 HCl:HNO3:H2O acidic solution (e.g., 9:3:48 mL) at 70 &#176;C for 20 min in a fume hood to selectively etch SrO due to the basic nature of SrO surface domains, the acidity of TiO2, and the presence of the Sr-hydroxide complexes.</li>
		<li>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 surface24,27. 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.</li>
		<li>Thermally treat the substrates in an atmosphere of 1 bar of oxygen for 1 h at 1,000 &#176;C with a heating and cooling rate of 100 &#176;C/h in a ceramic tube furnace, to relax the substrate surface into a state with low energy.</li>
	</ol>
	</li>
	<li>Deposition of the thin film(s) on the substrate
	<ol>
		<li>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.</li>
		<li>Connect the four corners of the SrTiO3 surface to a chip carrier electrically using, for instance, standard wedge wire bonding with 20 &#181;m-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.</li>
		<li>Place the TiO2-terminated substrate 4.7 cm from the single-crystalline Al2O3 target for a typical deposition of Al2O3 on SrTiO3.</li>
		<li>Start sheet resistance measurements using the Van der Pauw geometry29, if in situ transport measurements are to be performed.</li>
		<li>Heat the substrate to 650 &#176;C at a rate of 15 &#176;C/min or keep the substrate at room temperature.</li>
		<li>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.</li>
		<li>Deposit the desired thickness of &#947;-Al2O3 (typically 0&#8211;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 &#947;-Al2O3 on the part of the substrate using a physical mask.</li>
		<li>Cool down the &#947;-Al2O3/SrTiO3 heterostructure at a rate of 15 &#176;C/min at the deposition pressure without performing an additional annealing step if a high-temperature deposition is done.</li>
		<li>Remove the sample from the deposition chamber and stop the electrical measurements.</li>
		<li>Store the sample in vacuum, nitrogen or, alternatively, at ambient conditions. The sample degradation is slowest when stored in vacuum or nitrogen20.</li>
	</ol>
	</li>
</ol>
2. Controlling properties by thermal annealing
<ol>
	<li>Mount the sample with silver paste on a chip carrier.</li>
	<li>Connect the sample electrically to the chip carrier using, for instance, wedge wire bonding of Al wires in the Van der Pauw geometry29.</li>
	<li>Connect the chip carrier electrically to the measurement equipment, using a connector and wires with&#160;thermally resistant insulation.</li>
	<li>Start the sheet resistance measurements.</li>
	<li>Place the chip carrier equipped with the sample in a closed furnace.</li>
	<li>Flush thoroughly with the gas used for the annealing while checking whether the sample resistance is sensitive to a change in the atmosphere.</li>
	<li>Anneal the sample using the desired annealing profile. Typical annealing temperatures are 50&#8211;250 &#176;C and 100&#8211;350 &#176;C for a-LaAlO3/SrTiO3 and &#947;-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&#8211;400 &#176;C are needed.</li>
	<li>Abort the annealing when a desired change in the sheet resistance has occurred.</li>
	<li>Cool down the sample by ramping down the temperature, or take out the sample.</li>
	<li>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.</li>
</ol>

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The authors have nothing to disclose.

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

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3.0K Views.  Technical University of Denmark. 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.

Video: Tuning Oxide Properties by Oxygen Vacancy Control During Growth and Annealing

Dry Oxidation and Vacuum Annealing Treatments for Tuning the Wetting Properties of Carbon Nanotube Arrays

In this article, we describe a simple method to reversibly tune the wetting properties of vertically aligned carbon nanotube (CNT) arrays. Here, CNT arrays are defined as densely packed multi-walled carbon nanotubes oriented perpendicular to the growth substrate as a result of a growth process by the standard thermal chemical vapor deposition (CVD) technique.1,2 These CNT arrays are then exposed to vacuum annealing treatment to make them more hydrophobic or to dry oxidation treatment to render them more hydrophilic. The hydrophobic CNT arrays can be turned hydrophilic by exposing them to dry oxidation treatment, while the hydrophilic CNT arrays can be turned hydrophobic by exposing them to vacuum annealing treatment. Using a combination of both treatments, CNT arrays can be repeatedly switched between hydrophilic and hydrophobic.2 Therefore, such combination show a very high potential in many industrial and consumer applications, including drug delivery system and high power density supercapacitors.3-5
The key to vary the wettability of CNT arrays is to control the surface concentration of oxygen adsorbates. Basically oxygen adsorbates can be introduced by exposing the CNT arrays to any oxidation treatment. Here we use dry oxidation treatments, such as oxygen plasma and UV/ozone, to functionalize the surface of CNT with oxygenated functional groups. These oxygenated functional groups allow hydrogen bond between the surface of CNT and water molecules to form, rendering the CNT hydrophilic. To turn them hydrophobic, adsorbed oxygen must be removed from the surface of CNT. Here we employ vacuum annealing treatment to induce oxygen desorption process. CNT arrays with extremely low surface concentration of oxygen adsorbates exhibit a superhydrophobic behavior.

This article describes a simple method to fabricate vertically aligned carbon nanotube arrays by CVD and to subsequently tune their wetting properties by exposing them to vacuum annealing or dry oxidation treatment.

In this article, we describe a simple method to reversibly  tune the wetting properties of vertically aligned carbon nanotube (CNT) arrays.  Here, CNT ...

Engineering

Fabrication of Spatially Confined Complex Oxides

Complex materials such as high Tc superconductors, multiferroics, and colossal magnetoresistors have electronic and magnetic properties that arise from the inherent strong electron correlations that reside within them. These materials can also possess electronic phase separation in which regions of vastly different resistive and magnetic behavior can coexist within a single crystal alloy material. By reducing the scale of these materials to length scales at and below the inherent size of the electronic domains, novel behaviors can be exposed. Because of this and the fact that spin-charge-lattice-orbital order parameters each involve correlation lengths, spatially reducing these materials for transport measurements is a critical step in understanding the fundamental physics that drives complex behaviors. These materials also offer great potential to become the next generation of electronic devices 1-3. Thus, the fabrication of low dimensional nano- or micro-structures is extremely important to achieve new functionality. This involves multiple controllable processes from high quality thin film growth to accurate electronic property characterization. Here, we present fabrication protocols of high quality microstructures for complex oxide manganite devices. Detailed descriptions and required equipment of thin film growth, photo-lithography, and wire-bonding are presented.

We describe the use of pulsed laser deposition (PLD), photolithography and wire-bonding techniques to create micrometer scale complex oxides devices. The PLD is utilized to grow epitaxial thin films. Photolithography and wire-bonding techniques are introduced to create practical devices for measurement purposes.

Complex materials such as high Tc superconductors, multiferroics, and colossal magnetoresistors have electronic and magnetic properties that arise from ...

Atomically Defined Templates for Epitaxial Growth of Complex Oxide Thin Films

Atomically defined substrate surfaces are prerequisite for the epitaxial growth of complex oxide thin films. In this protocol, two approaches to obtain such surfaces are described. The first approach is the preparation of single terminated perovskite SrTiO3 (001) and DyScO3 (110) substrates. Wet etching was used to selectively remove one of the two possible surface terminations, while an annealing step was used to increase the smoothness of the surface. The resulting single terminated surfaces allow for the heteroepitaxial growth of perovskite oxide thin films with high crystalline quality and well-defined interfaces between substrate and film. In the second approach, seed layers for epitaxial film growth on arbitrary substrates were created by Langmuir-Blodgett (LB) deposition of nanosheets. As model system Ca2Nb3O10- nanosheets were used, prepared by delamination of their layered parent compound HCa2Nb3O10. A key advantage of creating seed layers with nanosheets is that relatively expensive and size-limited single crystalline substrates can be replaced by virtually any substrate material.

Various procedures are outlined to prepare atomically defined templates for epitaxial growth of complex oxide thin films. Chemical treatments of single crystalline SrTiO3 (001) and DyScO3 (110) substrates were performed to obtain atomically smooth, single terminated surfaces. Ca2 Nb3 O10- nanosheets were used to create atomically defined templates on arbitrary substrates.

Atomically defined substrate surfaces are prerequisite for the epitaxial growth of complex oxide thin films. In this protocol, two approaches to obtain ...

Writing and Low-Temperature Characterization of Oxide Nanostructures

Oxide nanoelectronics is a rapidly growing field which seeks to develop novel materials with multifunctional behavior at nanoscale dimensions. Oxide interfaces exhibit a wide range of properties that can be controlled include conduction, piezoelectric behavior, ferromagnetism, superconductivity and nonlinear optical properties. Recently, methods for controlling these properties at extreme nanoscale dimensions have been discovered and developed. Here are described explicit step-by-step procedures for creating LaAlO3/SrTiO3 nanostructures using a reversible conductive atomic force microscopy technique. The processing steps for creating electrical contacts to the LaAlO3/SrTiO3 interface are first described. Conductive nanostructures are created by applying voltages to a conductive atomic force microscope tip and locally switching the LaAlO3/SrTiO3 interface to a conductive state. A versatile nanolithography toolkit has been developed expressly for the purpose of controlling the atomic force microscope (AFM) tip path and voltage. Then, these nanostructures are placed in a cryostat and transport measurements are performed. The procedures described here should be useful to others wishing to conduct research in oxide nanoelectronics.

Oxide nanostructures provide new opportunities for science and technology. The interfacial conductivity between LaAlO3 and SrTiO3 can be controlled with near-atomic precision using a conductive atomic force microscopy technique. The protocol for creating and measuring conductive nanostructures at LaAlO3/SrTiO3 interfaces is demonstrated.

Oxide nanoelectronics is a rapidly growing field which seeks to develop novel materials with multifunctional behavior at nanoscale dimensions.  Oxide ...

Improved Heterojunction Quality in Cu2O-based Solar Cells Through the Optimization of Atmospheric Pressure Spatial Atomic Layer Deposited Zn1-xMgxO

Atmospheric pressure spatial atomic layer deposition (AP-SALD) was used to deposit n-type ZnO and Zn1-xMgxO thin films onto p-type thermally oxidized Cu2O substrates outside vacuum at low temperature. The performance of photovoltaic devices featuring atmospherically fabricated ZnO/Cu2O heterojunction was dependent on the conditions of AP-SALD film deposition, namely, the substrate temperature and deposition time, as well as on the Cu2O substrate exposure to oxidizing agents prior to and during the ZnO deposition. Superficial Cu2O to CuO oxidation was identified as a limiting factor to heterojunction quality due to recombination at the ZnO/Cu2O interface. Optimization of AP-SALD conditions as well as keeping Cu2O away from air and moisture in order to minimize Cu2O surface oxidation led to improved device performance. A three-fold increase in the open-circuit voltage (up to 0.65 V) and a two-fold increase in the short-circuit current density produced solar cells with a record 2.2% power conversion efficiency (PCE). This PCE is the highest reported for a Zn1-xMgxO/Cu2O heterojunction formed outside vacuum, which highlights atmospheric pressure spatial ALD as a promising technique for inexpensive and scalable fabrication of Cu2O-based photovoltaics.

Improved Heterojunction Quality in Cu2O-based Solar Cells Through the Optimization of Atmospheric Pressure Spatial Atomic Layer Deposited Zn1-xMgxO

Here we present a protocol for synthesizing Zn1-xMgxO/Cu2O heterojunctions in open-air at low temperature via atmospheric pressure spatial atomic layer deposition (AP-SALD) of Zn1-xMgxO on cuprous oxide. Such high quality conformal metal oxides can be grown on a variety of substrates including plastics by this cheap and scalable method.

Atmospheric pressure spatial atomic layer deposition (AP-SALD) was used to deposit n-type ZnO and Zn1-xMgxO thin films onto p-type thermally oxidized Cu2O ...

Epitaxial Growth of Perovskite Strontium Titanate on Germanium via Atomic Layer Deposition

Atomic layer deposition (ALD) is a commercially utilized deposition method for electronic materials. ALD growth of thin films offers thickness control and conformality by taking advantage of self-limiting reactions between vapor-phase precursors and the growing film. Perovskite oxides present potential for next-generation electronic materials, but to-date have mostly been deposited by physical methods. This work outlines a method for depositing SrTiO3 (STO) on germanium using ALD. Germanium has higher carrier mobilities than silicon and therefore offers an alternative semiconductor material with faster device operation. This method takes advantage of the instability of germanium's native oxide by using thermal deoxidation to clean and reconstruct the Ge (001) surface to the 2&#215;1 structure. 2-nm thick, amorphous STO is then deposited by ALD. The STO film is annealed under ultra-high vacuum and crystallizes on the reconstructed Ge surface. Reflection high-energy electron diffraction (RHEED) is used during this annealing step to monitor the STO crystallization. The thin, crystalline layer of STO acts as a template for subsequent growth of STO that is crystalline as-grown, as confirmed by RHEED. In situ X-ray photoelectron spectroscopy is used to verify film stoichiometry before and after the annealing step, as well as after subsequent STO growth. This procedure provides framework for additional perovskite oxides to be deposited on semiconductors via chemical methods in addition to the integration of more sophisticated heterostructures already achievable by physical methods.

This work details the procedures for the growth and characterization of crystalline SrTiO3 directly on germanium substrates by atomic layer deposition. The procedure illustrates the ability of an all-chemical growth method to integrate oxides monolithically onto semiconductors for metal-oxide semiconductor devices.

Atomic layer deposition (ALD) is a commercially utilized deposition method for electronic materials. ALD growth of thin films offers thickness control and ...

Growth and Electrostatic/chemical Properties of Metal/LaAlO3/SrTiO3 Heterostructures

The quasi 2D electron system (q2DES) that forms at the interface between LaAlO3 (LAO) and SrTiO3 (STO) has attracted much attention from the oxide electronics community. One of its hallmark features is the existence of a critical LAO thickness of 4 unit-cells (uc) for interfacial conductivity to emerge. Although electrostatic mechanisms have been proposed in the past to describe the existence of this critical thickness, the importance of chemical defects has been recently accentuated. Here, we describe the growth of metal/LAO/STO heterostructures in an ultra-high vacuum (UHV) cluster system combining pulsed laser deposition (to grow the LAO), magnetron sputtering (to grow the metal) and X-ray photoelectron spectroscopy (XPS). We study step by step the formation and evolution of the q2DES and the chemical interactions that occur between the metal and the LAO/STO. Additionally, magnetotransport experiments elucidate on the transport and electronic properties of the q2DES. This systematic work not only demonstrates a way to study the electrostatic and chemical interplay between the q2DES and its environment, but also unlocks the possibility to couple multifunctional capping layers with the rich physics observed in two-dimensional electron systems, allowing the fabrication of new types of devices.

Growth and Electrostatic/chemical Properties of Metal/LaAlO3/SrTiO3 Heterostructures

We fabricate metal/LaAlO3/SrTiO3 heterostructures using a combination of pulsed laser deposition and in situ magnetron sputtering. Through magnetotransport and in situ X-ray photoelectron spectroscopy experiments, we investigate the interplay between electrostatic and chemical phenomena of the quasi two-dimensional electron gas formed in this system.

The quasi 2D electron system (q2DES) that forms at the interface between LaAlO3 (LAO) and SrTiO3 (STO) has attracted much attention from the oxide ...

Atomic Layer Deposition of Vanadium Dioxide and a Temperature-dependent Optical Model

Vanadium dioxide is a material that has a reversible metal-insulator phase change near 68 &#176;C. To grow VO2 on a wide variety of substrates, with wafer-scale uniformity and angstrom level control of thickness, the method of atomic-layer deposition was chosen. This ALD process enables high-quality, low-temperature (&#8804;150 &#176;C) growth of ultrathin films (100-1000 &#197;) of VO2. For this demonstration, the VO2 films were grown on sapphire substrates. This low temperature growth technique produces mostly amorphous VO2 films. A subsequent anneal in an ultra-high vacuum chamber with a pressure of 7x10-4 Pa of ultra-high purity (99.999%) oxygen produced oriented, polycrystalline VO2 films. The crystallinity, phase, and strain of the VO2 were determined by Raman spectroscopy and X-ray diffraction, while the stoichiometry and impurity levels were determined by X-ray photoelectron spectroscopy, and finally the morphology was determined by atomic force microscopy. These data demonstrate the high-quality of the films grown by this technique. A model was created to fit to the data for VO2 in its metallic and insulating phases in the near infrared spectral region. The permittivity and refractive index of the ALD VO2 agreed well with the other fabrication methods in its insulating phase, but showed a difference in its metallic state. Finally, the analysis of the films&#39; optical properties enabled the creation of a wavelength- and temperature-dependent model of the complex optical refractive index for developing VO2 as a tunable refractive index material.

Thin films (100-1000 &#197;) of vanadium dioxide (VO2) were created by atomic-layer deposition (ALD) on sapphire substrates. Following this, the optical properties were characterized through the metal-insulator transition of VO2. From the measured optical properties, a model was created to describe the tunable refractive index of VO2.

Vanadium dioxide is a material that has a reversible metal-insulator phase change near 68 &#176;C. To grow VO2 on a wide variety of substrates, with ...

Bulk and Thin Film Synthesis of Compositionally Variant Entropy-stabilized Oxides

Here, we present a procedure for the synthesis of bulk and thin film multicomponent (Mg0.25(1-x)CoxNi0.25(1-x)Cu0.25(1-x)Zn0.25(1-x))O (Co variant) and (Mg0.25(1-x)Co0.25(1-x)Ni0.25(1-x)CuxZn0.25(1-x))O (Cu variant) entropy-stabilized oxides. Phase pure and chemically homogeneous (Mg0.25(1-x)CoxNi0.25(1-x)Cu0.25(1-x)Zn0.25(1-x))O (x = 0.20, 0.27, 0.33) and (Mg0.25(1-x)Co0.25(1-x)Ni0.25(1-x)CuxZn0.25(1-x))O (x = 0.11, 0.27) ceramic pellets are synthesized and used in the deposition of ultra-high quality, phase pure, single crystalline thin films of the target stoichiometry. A detailed methodology for the deposition of smooth, chemically homogeneous, entropy-stabilized oxide thin films by pulsed laser deposition on (001)-oriented MgO substrates is described. The phase and crystallinity of bulk and thin film materials are confirmed using X-ray diffraction. Composition and chemical homogeneity are confirmed by X-ray photoelectron spectroscopy and energy dispersive X-ray spectroscopy. The surface topography of thin films is measured with scanning probe microscopy. The synthesis of high quality, single crystalline, entropy-stabilized oxide thin films enables the study of interface, size, strain, and disorder effects on the properties in this new class of highly disordered oxide materials.

The synthesis of high quality bulk and thin film (Mg0.25(1-x)CoxNi0.25(1-x)Cu0.25(1-x)Zn0.25(1-x))O and (Mg0.25(1-x)Co0.25(1-x)Ni0.25(1-x)CuxZn0.25(1-x))O entropy-stabilized oxides is presented.

Here, we present a procedure for the synthesis of bulk and thin film multicomponent (Mg0.25(1-x)CoxNi0.25(1-x)Cu0.25(1-x)Zn0.25(1-x))O (Co variant) and ...

Niobium Oxide Films Deposited by Reactive Sputtering: Effect of Oxygen Flow Rate

Reactive sputtering is a versatile technique used to form compact films with excellent homogeneity. In addition, it allows easy control over deposition parameters such as gas flow rate that results in changes on composition and thus in the film required properties. In this report, reactive sputtering is used to deposit niobium oxide films. A niobium target is used as metal source and different oxygen flow rates to deposit niobium oxide films. The oxygen flow rate was changed from 3 to 10 sccm. The films deposited under low oxygen flow rates show higher electrical conductivity and provide better perovskite solar cells when used as electron transport layer.

Here, we present a protocol for niobium oxide films deposition by reactive sputtering with different oxygen flow rates for use as an electron transport layer in perovskite solar cells.

Reactive sputtering is a versatile technique used to form compact films with excellent homogeneity. In addition, it allows easy control over deposition ...

Tuning Oxide Properties by Oxygen Vacancy Control During Growth and Annealing

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