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

Summary

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.

Abstract

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.

Introduction

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 b....

Protocol

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:

1. Fill an ultrasonic cleaner (with a 40 kHz transducer) with water and heat it to 60 °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 substrates are well submerged.
   1. Place an out of the box mix-terminated single-side polished (001)-oriented STO single-crystal substrate (55 mm

Results

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 .......

Discussion

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 .......

Materials

List of materials used in this article

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