Name: growth and electrostatic/chemical properties of metal/laalo3/srtio3 heterostructures
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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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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">
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	</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 ...

Watch this Scientific Journal Video about Growth and Electrostatic/chemical Properties of Metal/LaAlO3/SrTiO3 Heterostructures at JoVE.com

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 ...

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