This work received support from the ERC Consolidator Grant #615759 &#34;MINT&#34;, the region &#206;le-de-France DIM &#34;Oxymore&#34; (project &#34;NEIMO&#34;) and the ANR project &#34;NOMILOPS&#34;. H.N. was partly supported by the EPSRC-JSPS Core-to-Core Program, JSPS Grant-in-Aid for Scientific Research (B) (#15H03548). A.S. was supported by the Deutsche Forschungsgemeinschaft (HO 53461-1; postdoctoral fellowship to A.S.). D.C.V. thanks the French Ministry of Higher Education and Research and CNRS for financing of his PhD thesis. J.S. thanks the University Paris-Saclay (D&#39;Alembert program) and CNRS for financing his stay at CNRS/Thales.

Note: All 5 steps described in this protocol can be paused and restarted at any time, with the single condition that the sample is kept under high vacuum from step 3.4 through 5.
1. STO(001) Substrate Termination:
<ol>
	<li>Fill an ultrasonic cleaner (with a 40 kHz transducer) with water and heat it to 60 &#176;C. Fill a borosilicate glass beaker with acetone. Independent of the beaker size, be sure to fill it with at least 20% of its maximum volume, to ensure that the subst...

<ol>
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Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Control+of+electronic+conduction+at+an+oxide+heterointerface+using+surface+polar+adsorbates.">Control of electronic conduction at an oxide heterointerface using surface polar adsorbates.</a> Nat. Commun. 2, 494 (2011).</li><li>Scheiderer, P., Pfaff, F., Gabel, J., Kamp, M., Sing, M., Claessen, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Surface-interface+coupling+in+an+oxide+heterostructure:+Impact+of+adsorbates+on+LaAlO3/SrTiO3.">Surface-interface coupling in an oxide heterostructure: Impact of adsorbates on LaAlO3/SrTiO3.</a> Phys. Rev. B. 92 (19), (2015).</li><li>Vaz, D. 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During substrate termination, one should be extremely careful with the submerging time in HF solution. We observed under- and over-etched surfaces by varying just 5 s with regard to the original recipe. Additionally, we observed a dependence between substrate step size and submerging time. For smaller step sizes (less than 100 nm) submerging 30 s might lead to over-etching, even though afterwards the annealing procedure might be sufficient to properly reconstruct the surface. Due to the risks of using HF based acids, we ...

Quasi 2D electron systems (q2DES) have been extensively used as a playground to study a multitude of low-dimensional and quantum phenomena. Starting from the seminal paper on the LaAlO3/SrTiO3 system (LAO/STO)1, a burst of different systems that host new interfacial electronic phases have been created. Combining different materials led to the discovery of q2DESs with additional properties, such as electric-field tunable spin polarization2, extremely high electron mobilities3 or ferroelectricity-coupled phenomena4. Although an immense body of work has been dedicated to unravel the creation and manipulation of these systems, several experiments and techniques have shown contradictory results, even in rather similar conditions. Additionally, the balance between electrostatic and chemical interactions was found to be essential to correctly understand the physics at play5,6,7.
In this article, we thoroughly describe the growth of different metal/LAO/STO heterostructures, using a combination of pulsed laser deposition (PLD) and in situ magnetron sputtering. Then, to understand the effect of different surface conditions in the buried q2DES at the LAO/STO interface, an electronic and chemical study is performed, using transport and electron spectroscopy experiments.
Since multiple methods have been previously used to grow crystalline LAO on STO, the choice of appropriate deposition techniques is a crucial step for the fabrication of high quality oxide heterostructures (in addition to possible cost and time constrains). In PLD, an intense and short laser pulse hits the target of the desired material, which is then ablated and gets deposited on the substrate as a thin film. One of the major advantages of this technique is the ability to reliably transfer the stoichiometry of the target to the film, a key element in order to achieve the desired phase formation. Furthermore, the capability of performing layer-by-layer growth (monitored in real time using reflection high-energy electron diffraction - RHEED) of a vast number of complex oxides, the possibility of having multiple targets inside the chamber at the same time (allowing the growth of different materials without breaking vacuum) and the simplicity of the setup make this technique one of the most effective and versatile.
Yet, other techniques such as molecular beam epitaxy (MBE) allow the growth of even higher quality epitaxial growth. Instead of having a target of a specific material, in MBE each specific element is sublimed towards the substrate, where they react with each other to form well defined atomic layers. Additionally, the absence of highly energetic species and more uniform energy distribution allows the fabrication of extremely sharp interfaces8. This technique is however much more complex than PLD when it comes to the growth of oxides, since it must be performed in ultra-high vacuum conditions (so that the long mean free path is not destroyed) and requires in general a larger investment, cost- and time-wise. Although the growth process used in the first LAO/STO publications was PLD, samples with similar characteristics have been grown by MBE9. It is also worth noting that LAO/STO heterostructures have been grown using sputtering10. Although atomically sharp interfaces were achieved at high temperatures (920 &#176;C) and high oxygen pressures (0.8 mbar), interfacial conductivity was not achieved.
For the growth of the metallic capping layers, we use magnetron sputtering, as it provides a good balance between quality and flexibility. Other chemical vapor deposition based techniques might however be used to achieve similar results.
Lastly, the combination of transport and spectroscopy techniques showed in this article exemplifies a systematic way of probing both electronic and chemical interactions, emphasizing the importance of crosschecking different approaches to fully understand the many features of these types of systems.

<table border="0" cellpadding="0" cellspacing="0" style="width:1433px;" width="1433">
	<colgroup>
		<col />
		<col />
		<col />
		<col />
	</colgroup>
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">Pulsed Laser Deposition</td>
			<td style="width:200px;">SURFACE</td>
			<td style="width:305px;">PLD Workstation + UHV Cluster System</td>
			<td style="width:521px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">KrF Excimer Laser</td>
			<td style="width:200px;">Coherent</td>
			<td style="width:305px;">Compex Pro 201F</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">Reflection High-Energy Electron Diffraction (electron gun)</td>
			<td style="width:200px;">R-Dec Co., Ltd.</td>
			<td style="width:305px;">RDA-003G</td>
			<td>Distributed in Europe by SURFACE.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">Reflection High-Energy Electron Diffraction (CCD camera)</td>
			<td style="width:200px;">k-Space Associates, Inc.</td>
			<td style="width:305px;">kSA 400</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">Variable Laser Beam Attenuator</td>
			<td style="width:200px;">Metrolux</td>
			<td style="width:305px;">ML 2100</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">Excimer Laser Sensor</td>
			<td style="width:200px;">Coherent</td>
			<td style="width:305px;">J-50MUV-248</td>
			<td></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;width:407px;">LaAlO3 target</td>
			<td style="width:200px;">CrysTec</td>
			<td style="width:305px;"></td>
			<td>Single-crystal target</td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;width:407px;">SrTiO3 subtrates</td>
			<td style="width:200px;">CrysTec</td>
			<td style="width:305px;"></td>
			<td>Several different sizes. Possibility to order TiO2 terminated.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">Buffered HF Acid</td>
			<td style="width:200px;">Technic</td>
			<td style="width:305px;">BOE 7:1</td>
			<td>buffered hydrofluoric acid = BOE 7:1 (HF : NH4F = 12.5 : 87.5%) in&#160;VLSI-quality.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">Silver Paste</td>
			<td style="width:200px;">DuPont</td>
			<td style="width:305px;">4929N</td>
			<td>Conductive Silver Composite.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">Ultrasonic Cleaner</td>
			<td style="width:200px;">Bransonic</td>
			<td style="width:305px;">12</td>
			<td>Ultrasonic Cleaning Bath</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">Tube Furnace</td>
			<td style="width:200px;">AET Technologies</td>
			<td style="width:305px;">Heat Treatment Furnace</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">Borosilicate Glass Beaker</td>
			<td style="width:200px;">VWR</td>
			<td>213-1128</td>
			<td>Iow form</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">PTFE Beaker</td>
			<td style="width:200px;">Dynalon</td>
			<td style="width:305px;">PTFE Beaker</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">Substrate holder &#34;dipper&#34;</td>
			<td style="width:200px;">Eberl&#233;</td>
			<td style="width:305px;"></td>
			<td>Custom made dipper</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">Magnetron Sputtering</td>
			<td style="width:200px;">PLASSYS</td>
			<td style="width:305px;">Sputtering system</td>
			<td>5 chambers for targets.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">Metal targets</td>
			<td style="width:200px;">Neyco S.A.</td>
			<td style="width:305px;"></td>
			<td>Purity &#62; 99.9%</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">X-Ray Photoelectron Spectroscopy System</td>
			<td style="width:200px;">Omicron</td>
			<td style="width:305px;">Custom XPS System</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">X-Ray Source</td>
			<td style="width:200px;">Omicron</td>
			<td style="width:305px;">DAR 400</td>
			<td>Twin Anode X-Ray Source.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">Energy Analyser</td>
			<td style="width:200px;">Omicron</td>
			<td style="width:305px;">EA 125</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">Atomic Force Microscopy</td>
			<td style="width:200px;">Bruker</td>
			<td style="width:305px;">Innova AFM</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">Atomic Force Microscopy Probes</td>
			<td style="width:200px;">Olympus</td>
			<td style="width:305px;">OMCL-AC160TS-R3</td>
			<td>Micro Cantilevers</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">Wire bonding</td>
			<td style="width:200px;">Kulicke &#38; Soffa</td>
			<td style="width:305px;">4523AD</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:407px;">PPMS</td>
			<td style="width:200px;">Quantum Design</td>
			<td style="width:305px;">PPMS Dynacool</td>
			<td>9T magnet.</td>
		</tr>
	</tbody>
</table>

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.

The full experimental system used for growth and characterization is shown in Figure 2. Having different setups connected in UHV through a distribution chamber is highly recommended to ensure that the surface of the sample after each growth process is kept pristine. The PLD chamber (Figure 3), magnetron sputtering (Figure 7) and XPS chamber (Figure 8) are also described ...

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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.

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 ...

The overall goal of this procedure is to fabricate and investigate metal lanthanum-aluminate-strontium type heterostructures using a combination of pulsed-laser deposition magnetron sputtering x-ray photoemission spectroscopy and magneto transport experiments. This method can help answer key questions about the interplay between electrostatic and chemical phenomenon of the quasi two-dimensional electron gas formed in these types of systems. The main advantages of this technique are it's simplicity and versatility, which together allow high-quality layer-by-layer growth of multi-power ultra-thin films.First, fill a 40 kilohertz ultra sonic cleaner with water and heat the bath to 60 degrees Celsius. Add to a borosilicate glass beaker sufficient acetone to fill the beaker to at least 20%of it's maximum volume. Place in the beaker an out-of-the-box mix-terminated single-side polished 001 oriented STO single crystal substrate.Sonicate the substrate in acetone for three minutes. Dry the substrate under a stream of nitrogen gas from a blow gun. Repeat the sonication in isopropanol and de-ionized water in sequence.Mount the clean substrate in a polyvinylidene fluoride dipper. Fill another borosilicate glass beaker with flowing de-ionized water. Then, fill about 20%of a similarly sized polytetrafluoroethylene beaker with hydrofluoric acid buffered with ammonium fluoride.Use the dipper to immerse the substrate in the buffered oxide etch for precisely 30 seconds. Immediately transfer the etched substrate to the beaker of flowing de-ionized water. Gently agitate the substrate in the flowing de-ionized water for two minutes.Dry the substrate under a stream of nitrogen gas and anneal the etched substrate in an oxygen environment at 1000 degrees Celsius. Next, gently mechanically polish a single-crystal one-inch LAO target with 180-grit aluminum oxide sandpaper lubricated with isopropanol. Once the surface appears homogenous dry the target under a flow of nitrogen gas.Mount the polished LAO target in a carousel that allows target rotation. Insert the carousel into the load-lock chamber, evacuate the chamber, and transfer the carousel to a pulsed-laser deposition chamber. Once the pulsed-laser deposition chamber pressure reaches 10 to the 9th millibars, supply oxygen to the chamber to achieve an oxygen partial pressure of 2x10 to the 4th millibars then place an excimer laser energy meter in the krypton fluoride laser beam path between the second converging lens and the quartz window.Set the laser to an arbitrary frequency and read the energy on the meter. Select the target rotation, remove the energy meter, and pre-ablate the target at three or four hertz for 20, 000 pulses in order to clean it's surface. Prior to sample preparation, use atomic force microscopy to confirm the termination, morphology, and cleanness of the terminated STO substrate surface.Use the conductive silver paste to fix the terminated STO substrate to a sample holder with the terminated surface face up, ensuring that the substrate is in the center of the holder. Heat the sample holder on a hot plate at 100 degrees Celsius for 10 minutes to evaporate solvent and solidify the paste for optimal thermal conduction. Then, allow the sample holder to cool to room temperature.Insert the sample holder into the load lock chamber and transfer the holder to the x-ray photoelectron spectroscopy chamber. Align the sample with the surface normal parallel to the electron analyzer axis. Move the x-ray gun as close as possible to the sample without allowing mechanical contact between the gun and the sample holder.After ensuring that the chamber is under ultra-high vacuum, turn on the x-ray gun. Acquire a survey spectrum between 12, 000 and 0 electron volts, with a selected step of 0.2 electron volts, a dwell time of 0.2 seconds, a pass energy between 30 and 60 electron volts, and the smallest spot size possible. Acquire a spectra of peaks of interest for later elemental analysis.After initial XPS analysis of the sample, transfer the sample holder to the PLD chamber and position the holder so the substrate faces the LAO target. Supply oxygen to the chamber to achieve an oxygen partial pressure of 2x10 to the 4th millibars. Ramp the sample holder temperature to 730 degrees Celsius at 25 degrees Celsius per minute, then set the reflection high-energy electron diffraction gun to a source voltage of 30 kilovolts, and a current of about 50 microamps.With the sample holder, place 63 millimeters away from the target, align the read electron beam with the substrate surface at a grazing angle between one and three degrees, so that the diffraction spots appear on the phosphor screen. Monitor the spot intensities in real-time via a CCD camera and image-analysis software. Once the reading stabilizes, start the laser.Observe the plume and monitor the read oscillations. Stop the laser at the peak of an oscillation upon achieving the desired thickness. Shut down the read gun, reduce the holder temperature to 500 degrees Celsius, and increase the oxygen partial pressure of the chamber to 0.1 millibars.Once the temperature and pressure are stable, increase the oxygen partial pressure to 300 millibars and anneal the sample at 500 degrees Celsius for 60 minutes. Analyze the LAO/STO sample with XPS. Transfer the sample holder with the LAO/STO substrate to the sputtering chamber under vacuum.Position the holder so that the substrate is about 7cm from the cobalt target. Supply pure argon gas to the sputtering chamber. With the target shutter closed, ramp the current to about 100 milliamps to ignite the plasma.Reduce the current to 80 milliamps and decrease the argon supply to 5.2 sccm, pre-sputter the cobalt target and then open the shutter and deposit cobalt for 25 seconds. Close the shutter to end deposition. For transport experiments, deposit a capping layer of three nanometers of aluminum to prevent oxidation of the underlying metal layer upon exposure to air.Otherwise, analyze the sample with XPS after cobalt deposition. Remove the sample holder from the system and transfer the sample to a transport measurement holder. On the sample stage of an ultrasonic wedge bonding machine, use aluminum or gold wires that are 0.025 millimeters in diameter to wire-bond the four corners of the sample to one channel of the transport measurement holder in hull geometry.Ensure that the wires stick well to the sample's surface. Then, wire bond the four corners to a second channel in vanderpow geometry. Check the contacts with a multimeter.Mount the holder in a transport setup and measure the magneto resistance and hull effect at various temperatures. Magneto transport measurements showed strikingly varied behavior depending on the metal layer deposited. LAO/STO samples capped with reactive metals showed s-shaped hull traces, indicating that an inter-facial quaside two-dimensional electron system had formed.Similar two-unit cell LAO/STO samples capped with noble metals, showed linear hull traces with resistance changes of only a few tens of milliohms over 4 teslas. Consistent with an insulating interface at which only the metal layer was detected. XPS of a tantalum-capped one unit cell LAO/STO sample showed tantalum oxide peaks and partial to total suppression of the metallic tantalum feature, suggesting that oxygen tended to diffuse toward the metal from the perovskite surface.Titanium atoms hosted some of the electrons released to the lattice by the formation of oxygen vacancies at the perovskite surface, changing the titanium valence state from 4 to 3, and forming a quaside 2D electron system. The titanium 3+to titanium 4+intensity ratio was about 20%Changing the electron take-off angle from 0 to 50 degrees had a minimal effect on the titanium 3+intensity, indicating that the quaside 2D electron system extended beyond the maximal XPS probing depth of five nanometers. Once mastered, the full procedure for one sample can be done in about one or two days.While attempting to perform PLD growth, pay special attention to the quality of both the substrates and the targets used. Perform extra analysis if the samples grown do not show the expected properties. Due to it's versatility, this procedure allows the highly-controlled creation and analysis of virtually any type of oxide heterostructures, as well as the possibility to explore a multitude of new interfacial and surface physical effects.

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10.1K Aufrufe.  Université Paris-Saclay. The overall goal of this procedure is to fabricate and investigate metal lanthanum-aluminate-strontium type heterostructures using a combination of pulsed-laser deposition magnetron sputtering x-ray photoemission spectroscopy and magneto transport experiments. This method can help answer key questions about the interplay between electrostatic and chemical phenomenon of the quasi two-dimensional electron gas formed in these types of systems. The main advantages of this technique are it's simpl...