Method Article

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

DOI:

10.3791/56951

⸱

February 8th, 2018

 ,  ,  ,  ,  ,  ,  , 
1Unité Mixte de Physique CNRS/Thales, Université Paris-Saclay, 2Max Planck Institut für Mikrostrukturphysik, 3Department of Applied Physics, Tohoku University, 4Instituto de Magnetismo Aplicado, Universidad Complutense de Madrid

In This Article

Summary

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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

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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

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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 °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.

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Protocol

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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 mm2 in lateral size, 0.5 mm in thickness, miscut angle between 0.01° and 0.02°) inside the borosilicate glass beaker.
    2. Sonicate the substrate in acetone for 3 min. Dry the substrate using a nitrogen blow gun with an operating pressure of about 5 bar.
  2. Repeat the procedure of step 1.1 but using isopropanol and then deionized water.
  3. Place the clean substrate in a sample holder made of polyvinylidene fluoride, PVDF, with a "dipper" shape (Figure 1a). Fill a second borosilicate glass beaker (Figure 1b) with running deionized water.
    Note: The beaker should be big enough so that the sample holder fits in it.
  4. Place the substrate in the sample holder.
  5. Wearing appropriate protection, fill a beaker (Figure 1b), typically made of Polytetrafluoroethylene, PTFE, to about 20% of its maximum volume, with a buffered hydrofluoric (HF) solution (HF:NH4F = 1:7). Use approximately the same size beaker as the one used in step 1.3.
  6. Submerge the sample holder in HF for exactly 30 s and immediately move it into the deionized running water to stop any subsequent chemical reactions. Agitate it lightly.
  7. After 2 min, remove the sample holder from the deionized water. Take out the substrate and dry it with a nitrogen blow gun.
    Note: More details can be found within Kawasaki's recipe11.
    Caution: The HF solution used is highly corrosive and poisonous. Always carry out the manipulation and disposal of used HF solutions in appropriate working environments. Symptoms of poisoning after contact with a body part might start to be visible up to one day after exposure and may not cause any pain in the first few hours. It is also possible to use an alternative termination process based on a HCl-HNO3 acidic solution12 or an acid-free termination recipe13.
  8. Insert the substrate in a tube furnace (Figure 1c) at 20 °C. Set the furnace partial pressure to approximately 1 atm of oxygen. Ramp the temperature to 1000 °C at a rate of 20 °C/min. Anneal the substrates for 3 hours at 1000 °C. After the 3 hours, let the sample cool down to 20 °C. Remove the substrate. Close the oxygen source.
  9. Repeat step 1.1 to remove subsequent surface contaminations promoted during annealing.

2. Preparation of the single-crystal LAO target:

  1. Mechanically polish a single-crystal LAO target (1-inch diameter) gently using sandpaper and isopropanol solution as a lubricant. Dry it using a nitrogen blow gun.
  2. Mount the target in a carousel.
    Note: Be sure that the carousel allows target rotation.
  3. Insert the carousel in the loadlock chamber (Figure 2). Let the target degas in vacuum, while continuously pumping the chamber (until it reaches a pressure in the 10-8 mbar range). Transfer the carousel to the PLD chamber (Figure 3a and 3b). Wait until the base pressure is in the 10-9 mbar range.
    Note: In the absence of a loadlock chamber and in vacuo transfer system, the typical waiting time and base pressure can be severely affected.
  4. Inspect the laser energy using an excimer laser energy meter. To do this, use a rectangular slit (6 mm x 16 mm) and an external attenuator right after the laser source to modulate the shape and energy of the beam (Figure 4a and 4b). Place the energy meter in the path of the laser beam, between the second converging lens and the quartz window. Then, shoot the laser at an arbitrary frequency and read the energy using the energy meter.
  5. Set the energy to be the same (or marginally higher) as the one used during the growth (step 3.12).
    Note: Absolute values of the laser energy might vary depending on the geometry of the setup. However, for LAO target ablation, use a laser fluence of about 1 J/cm2 (fluence = energy/spot area). Also, use a pulsed KrF excimer laser of wavelength λ=248 nm, with a characteristic pulse duration of 25 ns and operated at a minimum of 21 kV (for improved pulse-to-pulse reproducibility).
  6. Rotate the LAO target at about 10 rpm (using the rotation platform of the carrousel, where the target is mounted).
    1. Adapt the target rotation speed to the spot size and laser repetition rate to avoid two consecutive overlapping shots, potentially leading to some local overheating or melting of the target and subsequent off-stoichiometry. A visual description is provided in Figure 4c.
  7. Insert oxygen in the chamber until an oxygen partial pressure of 2x10-4 mbar is achieved. Remove the energy meter. Pre-ablate the LAO target at 3 or 4 Hz for 20000 pulses.
    Note: The laser should be set up so that the angle between the beam and the target is 45° (Figure 4d). This relatively long ablation of the LAO single-target was found to have a determinant role in LAO/STO sample-to-sample reproducibility.

3. PLD growth:

  1. Perform an atomic force microscopy (AFM) scan of the previously terminated STO substrate surface to verify termination, morphology and cleanness (Figure 5).
  2. Using silver paste, glue the substrate, with the terminated surface pointing upwards, to a sample holder. Although the orientation of the substrate is not relevant, be sure that it is placed in the center of the holder (Figure 6a).
  3. Heat it up to about 100 °C for 10 min so that the solvent evaporates and the paste solidifies (for optimum thermal conduction). Let the sample holder cool down.
  4. Insert the sample holder inside the loadlock. Using the arm within the cluster, transfer the sample holder to the XPS chamber to analyze the oxygen, carbon and titanium peaks (refer to step 5 for more details).
  5. Transfer the sample holder to the PLD chamber, with the substrate facing down towards the LAO target (Figure 6b).
  6. Insert oxygen inside the chamber to reach an oxygen partial pressure of 2x10-4 mbar. Raise the temperature of the sample holder to 730 °C (at 25 °C/min).
  7. Using reflective high energy electron diffraction (RHEED), align the electron beam at grazing angle (between 1° and 3°) with the substrate surface so that the diffraction spots are observed on the phosphor screen. Monitor in real-time the intensity of each spot using a CCD camera and image analysis software. Use a source voltage of 30 kV and current of 40 µA.
  8. Place the sample holder 63 mm away from the target.
    Note: The target-to-substrate distance might require some degree of optimization depending on the geometry of the PLD setup used.
  9. Shoot the laser in order to calibrate the energy so that it matches about 1 J/cm2 (in the same fashion as in step 2.4). Again, use a rectangular slit (6 mm x 16 mm) and an external attenuator right after the laser exit to modulate the shape and energy of the beam (Figure 4a and 4b).
  10. Set the laser frequency to 1 Hz. Stop shooting the laser and remove the energy meter.
  11. Start the rotation of the LAO target (same way as in step 2.6). Initiate the RHEED oscillations reading. Wait until it stabilizes.
  12. Start shooting the laser. Observe the plume (Figure 6c) and the RHEED oscillations (Figure 6d). Stop the laser at the peak of one of the oscillation depending on the desired thickness.
    Note: Remember that each oscillation represents one unit cell (uc) grown. For the purpose of this experiment, grow 1 and 2 uc for transport and spectroscopic experiments, respectively.
  13. After the growth is finished, shutdown the RHEED gun and proceed to the post-annealing step.
    Note: The post-annealing step is done right after the growth is finished.
    1. To start the post-annealing, increase the oxygen partial pressure in the chamber from 2x10-4 mbar (growth pressure) to 1x10-1 mbar and decrease the temperature of the sample holder from 730 °C (growth temperature) to 500 °C.
    2. After the temperature and pressure are stabilized, introduce a static oxygen partial pressure of about 300 mbar, while keeping the sample holder temperature at 500 °C. Leave the sample in these conditions for 60 min.
  14. Cool down the sample at 25 °C/min while keeping it in the same oxygen partial pressure until it reaches room temperature.
  15. Transfer the sample to the XPS chamber to investigate possible valence changes in the titanium peak or the relative La/Al concentration (refer to step 5 for more details).
  16. To ensure that the LAO surface is kept pristine, transfer the sample in vacuo to the sputtering chamber (Figure 7a), which is kept at all times at a pressure in the range of 10-8 mbar.
    Note: Performing these experiments ex situ will cause the accumulation of carbon and water on the surface which ultimately leads to altered results.

4. Magnetron Sputtering of Metallic Overlayers:

Note: Depending on the desired metal, parameters such as Ar pressures, deposition currents and target-to-substrate distances might vary slightly. It is advised to optimize each deposition process depending on the geometry of the sputtering setup used. The following procedure describes the deposition of 3 nm of Co.

  1. Place the sample with the surface facing down towards the target.
  2. Insert pure Ar inside the sputtering chamber to achieve an atmosphere of about 4.5x10-4 mbar (about 100 sccm).
  3. Position the substrate (LAO/STO) about 7 cm away from the Co target.
  4. With the shutter closed, ramp the current up to about 100 mA (36 W) so that the plasma is ignited.
  5. With a stable plasma (Figure 7b), lower the current to 80 mA (deposition current) as well as the inflow of Ar to 5.2 sccm. Ensure that the plasma stays stable.
  6. Pre-sputter the Co target for about 5 min to remove any oxidized layer that may have formed at its surface.
  7. With the sample at room temperature, open the shutter and deposit for 25 s. Close the shutter to conclude the deposition.
    1. For transport experiments, deposit a subsequent capping layer of about 3 nm of Al (whose surface passivates, forming an AlOx protective layer upon exposure to air) to prevent oxidation of the underlying metallic layer.
      Note: The growth rate is not measured directly inside the chamber. In order to do this, grow various samples with different deposition times while using the same parameters. Then, measure the thickness of each sample using X-ray reflectometry. Do this procedure once for each metallic target used.
  8. Ramp the current down to zero, close the Ar source and pump the chamber.
  9. Transfer once more the sample holder to the XPS chamber (Figure 8a) to inspect possible valence change at the Ti 2p level as well as a possible oxidation at the metal/LAO interface (refer to step 5 for more information).

5. In Situ X-ray Photoelectron Spectroscopy:

  1. Place the sample with the surface normal aligned parallel to the electron analyzer axis (Figure 8b).
  2. Approach the X-ray gun as close as possible to the sample (avoid mechanical contact between the end of the gun and the sample holder to prevent damage) and turn it on.
    1. In this experiment, use an Mg Kα source with an excitation energy of 1253.6 eV. Set the filament to achieve an emission current of 20 mA at an anode voltage of 15 kV. Regarding the analyzer electron-optics, choose an entrance slit of 2 mm of diameter and an exit slit with rectangular shape of 5x11 mm.
      Note: Refer to the manual of the XPS setup used for information regarding maximum emission currents and anode voltages. Also, the size of entrance and exit slips might be different for other specific setups. If the analyzer has different specifications, choose the slits in a way to avoid too high intensity in the electron counting unit.
  3. After turning the X-ray gun on, ensure that the chamber is in ultra-high vacuum conditions (10-10 mbar range). Collect the survey spectra (between 0 and 1200 eV binding energy) with a selected step of 0.05 eV, a dwell time of 0.5 s, a pass energy between 30 and 60 eV and an adequate lens mode to achieve the smallest spot size possible. Adjust the values depending on the resolution intended.
    1. Locate the position of the relevant peaks (Figure 8c). For improved statistics, measure each peak several times and average the spectra collected.
  4. Analyze the spectra using adequate XPS processing software.
    1. In order to identify the electrons from a given transition, define an energy range that comprises the peak to analyze.
    2. Create an appropriate background curve (normally a Shirley background14) and subtract it from the original data.
    3. Using bibliographic references15, locate the possible peaks that compose the measured peak. Pay special attention to tabulated distances and relative intensities for different peaks.
      Note: A more in depth look at the XPS data collected is provided in the "Representative Results" section as well as in Ref.7.

6. Magnetotransport Experiments:

  1. Using an ultrasonic wedge-bonding machine, wire-bond the metal/LAO/STO sample with Al or Au wires to contact the buried interface ( Figure 9a).
    Note: Select an appropriate wedge-to-sample distance, force and time, depending on the setup used and the type of transport measurement holder.
  2. Use an 8-wire geometry (4 in van der Pauw -channel 1- and 4 in Hall geometry -channel 2-). To do this, start by contacting one of the channels of the transport measurement holder to the four corners of the sample in van der Pauw geometry. Then, contact a second channel to the contacts previously done in the sample (Figure 9b).
  3. Check if contacts are good by measuring the resistance with a multimeter. To ensure that the sample is uniform, verify that the resistance measured in different directions is roughly the same, so that the van der Pauw R100≈R010 condition is satisfied.
    Note: If R100 and R010 are significantly different, the van der Pauw measurement should be performed in both directions (following Ref.16). Previous studies report strong anisotropic electric transport properties in LAO/STO17.
  4. Mount the holder in a transport setup.
    1. Measure the resistance (channel 1) down to 2 K.
    2. At low temperature, measure sequentially the magnetoresistance (channel 1) and Hall effect (channel 2) by sweeping an external and perpendicular magnetic field (from -9 to 9 T), sourcing a current of typically 10 to 100 µA for metal/LAO/STO samples.
    3. Repeat step 6.4.2. for 5 K, 10 K, 50 K, 100 K, 200 K and 300 K, in order to observe the magnetoresistance evolution with temperature.

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Results

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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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Discussion

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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 ...

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Disclosures

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The authors have nothing to disclose.

Acknowledgements

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This work received support from the ERC Consolidator Grant #615759 "MINT", the region Île-de-France DIM "Oxymore" (project "NEIMO") and the ANR project "NOMILOPS". 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'Alembert program) and CNRS for financing his stay at CNRS/Thales.

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Materials

List of materials used in this article
NameCompanyCatalog NumberComments
Pulsed Laser DepositionSURFACEPLD Workstation + UHV Cluster System
KrF Excimer LaserCoherentCompex Pro 201F
Reflection High-Energy Electron Diffraction (electron gun)R-Dec Co., Ltd.RDA-003GDistributed in Europe by SURFACE.
Reflection High-Energy Electron Diffraction (CCD camera)k-Space Associates, Inc.kSA 400
Variable Laser Beam AttenuatorMetroluxML 2100
Excimer Laser SensorCoherentJ-50MUV-248
LaAlO3 targetCrysTecSingle-crystal target
SrTiO3 subtratesCrysTecSeveral different sizes. Possibility to order TiO2 terminated.
Buffered HF AcidTechnicBOE 7:1buffered hydrofluoric acid = BOE 7:1 (HF : NH4F = 12.5 : 87.5%) in VLSI-quality.
Silver PasteDuPont4929NConductive Silver Composite.
Ultrasonic CleanerBransonic12Ultrasonic Cleaning Bath
Tube FurnaceAET TechnologiesHeat Treatment Furnace
Borosilicate Glass BeakerVWR213-1128Iow form
PTFE BeakerDynalonPTFE Beaker
Substrate holder "dipper"EberléCustom made dipper
Magnetron SputteringPLASSYSSputtering system5 chambers for targets.
Metal targetsNeyco S.A.Purity > 99.9%
X-Ray Photoelectron Spectroscopy SystemOmicronCustom XPS System
X-Ray SourceOmicronDAR 400Twin Anode X-Ray Source.
Energy AnalyserOmicronEA 125
Atomic Force MicroscopyBrukerInnova AFM
Atomic Force Microscopy ProbesOlympusOMCL-AC160TS-R3Micro Cantilevers
Wire bondingKulicke & Soffa4523AD
PPMSQuantum DesignPPMS Dynacool9T magnet.

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Metal LAO STO HeterostructuresPulsed Laser DepositionMagnetron SputteringX ray Photoelectron SpectroscopyMagnetotransport ExperimentsQuasi 2D Electron SystemCritical LAO ThicknessUltra high VacuumOxide Heterostructure GrowthInterfacial Conductivity

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