Diogo Castro Vaz

Edouard Lesne

Anke Sander

Hiroshi Naganuma

Eric Jacquet

Jacobo Santamaria

Agnès Barthélémy

Manuel Bibes

Diogo C Vaz
email: diogo.vaz@cnrs-thales.fr

Growth and electrostatic/chemical properties of hybrid metal/LaAlO3/SrTiO3 heterostructures

Keywords: 2D electron gas, oxide interfaces, lao, sto, magnetotransport, X-ray photoelectron spectroscopy, pulsed laser deposition, sputtering

We fabricate metal/LAO/STO 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.

LONG 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.

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 polarization 2 , extremely high electron mobilities 3 or ferroelectricity-coupled phenomena 4 . 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 play [5][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. To understand the effect of different surface conditions in the buried q2DES at the LAO/STO interface, an electronic and chemical study was then performed, using transport and electron spectroscopy experiments. This study shows 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. PROTOCOL: Note: All of the 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 2.4 to 4. 1. STO(001) Substrate Termination 1.1. Fill an ultrasonic cleaner with water and heat it to 60 °C. Ultrasound an out of the box mix-terminated single-side polished (001)-oriented STO single-crystal substrate (55 mm² in lateral size, 0.5 mm in thickness) in acetone for 3 min. Repeat the ultrasonication procedure in isopropanol, and then in deionized water. Dry substrates using a nitrogen blow gun. 1.2. Fill a large beaker with running deionized water. Wearing appropriate protection, fill a second beaker (typically made of Polytetrafluoroethylene) with a buffered hydrofluoric (HF) solution (HF:NH4F = 12.5:87.5%). Submerge the substrates in HF for exactly 30 seconds and immediately move them into the deionized running water to stop any subsequent chemical reactions. Dry substrates with a nitrogen blow gun. More details can be found within Kawasaki's recipe 8 . Caution: The HF solution used is highly corrosive and poisonous. Manipulation and disposal of used HF solutions should always be carried out 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. An alternative acidfree termination recipe might also be used 9 . 1.3. Bake the substrates in an oven for 3 hours at 1000 °C (20 °C/min ramp rate, for both warming and cooling) in an oxygen rich environment (about 1 atm.). Ramping is not included in the 3 hours procedure. 1.4. Repeat step 1.1. to remove subsequent surface contaminations promoted during annealing. 2. Growth of LAO Ultra-Thin Films 2.1. Prepare the single-crystal LAO target (1-inch diameter). 2.1.1. Mechanically polish the LAO target gently using sandpaper and isopropanol solution as a lubricant. Dry using a nitrogen blow gun. 2.1.2. Insert the target in the load-lock chamber. Let it degas in vacuum, while continuously pumping the chamber, for up to a few hours. Transfer the target in the PLD chamber and wait until it reaches a base pressure in the 10 -9 mbar range. Note: In the absence of a load-lock chamber and in vacuo transfer system, the typical waiting time and base pressure can be severely affected.

2.1.3. Inspect the laser energy using an excimer laser sensor. Use the same energy (or marginally higher) as the energy used during the growth. We 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). Note: Absolute values of the laser energy might vary depending on the geometry of the setup. However, for LAO target ablation, one should aim for a laser fluence of typically 1 J/cm 2 (fluence = energy/spot area).

2.1.4. Rotate the LAO target at about 10 rpm. Note: It is advised to 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.

2.1.5. Shoot at 3 or 4 Hz for 20000 pulses at an oxygen partial pressure of 2x10 -4 mbar.

The relatively long ablation of the LAO single-target we used was found to have a determinant role in LAO/STO sample-to-sample reproducibility. 2.2. Perform an AFM scan of the STO substrate surface to verify morphology and cleanness. 2.3. Glue the substrate to a sample holder using silver paste. 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. 2.4. Insert the sample holder in the vacuum chamber. At this step, the first XPS measurement can be performed to analyze the oxygen, carbon and titanium peaks (refer to step 4 for more details). 2.5. Insert the sample in the PLD chamber.

2.5.1. Raise the temperature of the sample holder to 730 °C (at 25 °C/min) in an oxygen partial pressure of 2x10 -4 mbar. 2.5.2. Using reflective high energy electron diffraction (RHEED) technique, align the electron beam at grazing angle with the substrate so that the diffraction spots are observed on the phosphor screen. Monitor in real-time the intensity of each spot by using a CCD camera. *We used a source voltage of 30 kV and current of 40 µA. 2.5.3. *Place the target 63 mm away from the substrate. 2.5.4. *Calibrate the laser energy. Use a slit (6x16 mm) and an external attenuator right after the laser exit to modulate the shape and energy of the beam. 2.5.5. Set the laser frequency to 1 Hz. 2.5.6. Ablate the target with the laser at constant energy (corresponding to about 1 J/cm 2 ) and constant oxygen partial pressure of 2x10 -4 mbar. Observe the RHEED oscillations. Close the laser shutter at the peak of one of the oscillation depending on the desired thickness. For the purpose of our experiment, we grew 1 and 2 uc for transport and spectroscopic experiments, respectively. 2.6. After LAO growth, shutdown the RHEED gun. Proceed to the post-annealing step.

2.6.1. Increase the oxygen partial pressure in the chamber to 1x10 -1 mbar. Decrease the temperature of the sample holder to 500 °C, and introduce a static oxygen partial pressure of about 300 mbar. Post-anneal the sample in these conditions for 30 min. 2.7. Cool down the sample at 25 °C/min while keeping it in the same oxygen partial pressure until it reaches room temperature. 2.8. The second XPS measurement can be performed at this stage to investigate possible valence changes in the titanium peak or the relative La/Al concentration. 2.9. To ensure that the LAO surface is kept pristine, transfer the sample in vacuo to the sputtering chamber. Performing these experiments ex situ will cause the accumulation of carbon and water on the surface which ultimately leads to altered results.

Magnetron Sputtering of Metallic Overlayers

3.1. *Use a pure Ar atmosphere of 4.5x10 -4 mbar. 3.2. *Position the substrate about 7 cm away from the target. 3.3. *Ramp the current up to about 100 mA (36 W) so that a stable plasma is created. 3.4. With the shutter closed, pre-sputter the metallic target for a few minutes to remove any oxidized layer that may have formed at its surface. 3.5. At room temperature, deposit the desired metal at the lowest possible rate, while keeping the plasma stable and uniform. For the purpose of our experiments we grew films of 2.5 and 0.3 nm (at a typical rate of 2 Å/s) for transport and spectroscopic experiments, respectively. For transport experiments, a subsequent capping layer of about 3 nm of Al (whose surface passivates, forming an AlOx protective layer upon exposure to air) is grown to prevent oxidation of the underlying metallic layer. 3.6. Ramp the current down to zero, close the Ar source and pump the chamber. 3.7. A last XPS experiment might be performed at this stage to inspect a valence change at the Ti 2p level as well as a possible oxidation at the metal/LAO interface. 4. X-ray Photoelectron Spectroscopy 4.1. Place the sample with the surface normal aligned parallel to the electron analyzer axis. 4.2. Turn on the X-ray gun and approach it as close as possible to the sample using a linear shift (avoid mechanical contact between the end of the gun and the sample holder to prevent damage). 4.2.1. We used 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. If your analyzer has different specifications, choose the slits in a way to avoid too high intensity in the electron counting unit (refer to the respective manual to know the specific limits). 4.3. In ultra-high vacuum conditions (10 -10 mbar range), collect the survey spectra (between 0 and 1200 eV binding energy) and locate the position of the relevant peaks. For improved statistics, measure each peak several times and average the spectra collected. 4.3.1. *Select a 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. 4.4. Analyze the spectra using an adequate XPS processing software.

4.4.1. In order to identify the electrons from a given transition, define an energy range that comprises the peak to analyze. 4.4.2. Create an appropriate background curve (normally a Shirley background) and subtract it from the original data. 4.4.3. Using bibliographic references 10 , locate the possible peaks that compose the measured peak. Pay special attention to tabulated distances and relative intensities for different peaks. 5. Magnetotransport Experiments 5.1. Wire-bond the metal/LAO/STO sample with Al or Au wires using an ultrasonic wedgebonding machine to contact the buried interface. Use an 8-wire geometry (4 in van der Pauw -channel 1-and 4 in Hall geometry -channel 2-). 5.2. Check if contacts are good by measuring the resistance. To ensure that the sample is uniform, verify that the resistance measured in different directions is the same. 5.3. Mount the sample in a transport setup. 5.3.1. Measure the resistance (channel 1) down to low temperature. 5.3.2. At low temperature, measure sequentially the magnetoresistance (channel 1) and Hall effect (channel 2) by sweeping an external and perpendicular magnetic field, sourcing a current of typically 10-100 µA for metal/LAO/STO samples. 5.3.3. Repeat 5.2.2 for several different temperatures.

REPRESENTATIVE RESULTS:

The full experimental system used for growth and characterization is shown in figure 1. 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 4) and XPS chamber (figure 5) are also described in detail. Additional information regarding the optical path in the PLD setup is displayed in figure 3c (adapted from Ref. 11 ). The conditions discussed in the protocol for each technique may vary depending on the exact geometry of each chamber, type of targets used or the type of equipment. For this reason, we have marked throughout the protocol (*) all the steps that highly rely on the features aforementioned and that ultimately will require some degree of optimization by the user. We confirmed atomically flat and clean STO surfaces, before starting the growth, through atomic force microscope (AFM), as seen in figure 2a. A "step and terrace" like structure with single unit cell step height is evidenced due to the miscut angle in respect to the (001) plane orientation. All samples were grown on substrates with a step size of about 250 nm (for a miscut angle between 0.01 and 0.02°). Both LAO and metal films reproduce the surface morphology of the layers beneath, as seen in figure 2b, respectively. During PLD growth, the plume generated through the process of ablating species from the target towards the substrate is shown in figure 3d. Note that the intensity (and color) of the plume depends on the oxygen pressure and fluence used. Altering the fluence was previously shown to modify the La/Al cationic ratio, which might ultimately lead to a modulation of the interfacial contacting properties 12,13 . RHEED monitoring is used to ensure layer-by-layer growth. Typical RHEED data is shown in figure 3e for a 2 uc growth of LAO on STO. A first XPS analysis of the substrate allows us to confirm the virtual absence of additional features in the oxygen peak, usually present due to attached water and hydrogen molecules, as well as a very small amount of adsorbed carbon. The heating process used during the PLD growth normally removes/reduces these features. A second analysis reveals the appearance of La and Al peaks and an additional intensity decrease of Ti and Sr peaks, due to the attenuation introduced by the LAO film. One last XPS analysis after metal deposition shows clear attenuation of all peaks coming from the LAO/STO. As we discuss ahead, analysis of the peaks associated with the deposited metal gives us information regarding its oxidation state. All survey spectra addressed are displayed in figure 5c. Transport experiments are performed at low temperature using an 8-wire geometry (4-wires for transverse measurements and 4-wire for Hall measurements), as shown in figure 6a. Measurements show strikingly different behavior depending on the metallic capping layer chosen. LAO(2 uc)/STO samples capped with noble metals such as Au, Pt or Pd showed a linear Hall trace with a change in resistance of a few tens of mΩ over 9 T (figure 6b). However, similar samples capped with reactive metals such as Ti, Ta, Co, Ni80Fe20 and Nb (2.5 nm) showed signatures of a 2DES, namely S-shaped Hall traces (figure 6c). We conclude that from a transport point of view, LAO/STO samples below the 4 uc critical thickness (that in regular conditions are insulating) form an interfacial 2DES if a reactive metal is added on top. Capping with noble metals leads to an insulating interface, where only the metal layer is detected (see figure 6b and inset of figure 6c). Indeed, these results are consistent with theoretical prediction performed on similar heterostructures 14 . Simultaneously fitting the extracted Hall and magnetoresistance for the 2DES enables us to extract values for carrier densities and mobilities. The results are summarized in table 1 for three LAO(2 uc)/STO samples capped with Au, Co or Ta. We observe that the carrier densities scale with the work function of the metal capping. Electrostatically, the Fermi level of lower work function metals will stay above the Fermi level of the STO, which ultimately promotes electron transfer towards it. Lower metal work function (higher Fermi level) leads therefore to stronger electron transfer, which explains the trend seen for these 3 samples. Although, an electrostatic approach seems to fully describe this system, chemical reactions must be considered [START_REF] Fu | Interaction of nanostructured metal overlayers with oxide surfaces[END_REF] . For this purpose, XPS allows us to investigate possible oxidation processes occurring at the metal/perovskite interface, as well as valence changes occurring at the interfacial Ti atoms in the STO (figure 7a). The results are summarized in table 2. The likeliness of the metal to oxidize, given by the enthalpy of oxide formation ΔfH 0 , scales with the Ti 3+ /Ti 4+ intensity ratio, translating in an increased number of electrons at the Ti interfacial atoms for highly reactive metallic capping layers, as seen in figure 7b and7d. As expected, the quantity of metal oxide formed at the metal/LAO interface scales similarly, although we suspect oxidation would continue for Ta capped sample if the metal layer was thicker (figure 7c). Additionally, from angle-dependent XPS experiments (inset of figure 7b) we observe that the thickness of the 2DEG formed extends beyond the XPS maximum probing depth. This result contradicts the transport experiments, since more carriers translate in a larger interfacial confining electric field that should shorten the 2DES thickness [START_REF] Copie | Towards two-dimensional metallic behavior at LaAlO3/SrTiO3 interfaces[END_REF] . The large spatial extension might be described through oxygen vacancies created by the highly reactive metal capping. Since the LAO layer is only 1 uc thick, if the concentration of vacancies formed is high enough then they might reach the 2DES region, expanding it while maintaining a high carrier concentration. These results clearly show the importance of backing transport experiments with electron spectroscopy and vice versa. For more details on the analysis refer to Ref. 7 .

FIGURE AND TABLE LEGENDS:

Figure 1 -Complete setup including the pulsed laser deposition chamber, magnetron sputtering and X-ray photoelectron spectroscopy. The three setups are connected through a cluster that allows for sample transfer without breaking vacuum (10 -9 mbar range). This geometry allows the collection of both Hall and magnetoresistance, sequentially sourcing between the two channels, yet in a single measurement run. Hall resistance Rxy as a function of applied perpendicular magnetic field µ0H of LAO(2 uc)/STO samples capped with (b) noble metals and (c) reactive metals. The inset shows a schematic circuit after wire bonding, where RM and Rq2DES represent the longitudinal resistances and VH,M and VH,q2DES the Hall voltages generated in each layer. More information in ref. 7 . Figure 7 -X-ray photoelectron spectroscopy of metal/LAO/STO heterostructures. (a) Schematic sketch of the measurement performed. Photoelectrons mitted by X-ray excitation carry information on both the LAO/STO and the metal/LAO interface. (b) Spectra collected on Ta/LAO/STO samples at the Ti 2p level show an additional (Ti 3+ ) shoulder at lower binding energy, associated with extra electrons hosted at the interfacial Ti atoms. The inset additionally shows the weak angle-dependence of the Ti 3+ shoulder, revealing that the extension of the q2DES formed at the LAO/STO interface is larger than the maximum electron probing depth. (c) The Ta 4f spectra informs on the oxidation level of the metallic capping used. The spectra can be fitted exclusively with different Ta oxide peaks, indicating that 100% of the metal layer has oxidized on top of the oxide heterostructure. (d) Fitting the Ti 2p peak with two components, 4+ and 3+, allows us to extract a Ti 3+ /Ti 4+ intensity ratio of 20%. More information in ref. 7 . Table 1 -Relation between metallic capping used, the work function of the metal and the carrier density found in the q2DES at the LAO/STO interface. Smaller work functions translate in a larger energetic difference between Fermi levels (metal and STO), which leads to stronger charge transfer, after band alignment, and ultimately an electro gas with higher carrier density. Table 2 -Relation between metallic capping used, the enthalpy of oxide formation ΔfH 0 , the intensity ratio between Ti 4+ and Ti 3+ peaks (extracted from XPS) and the relative concentration of metal oxide in the metallic capping. Higher (negative) enthalpies of formation mean that the oxide has a higher tendency to oxidize, which leads to a higher Ti 3+ shoulder in the Ti 2p XPS spectra (more electrons transferred towards the interface) and larger concentration of metal oxide formed.

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 5s 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 30s might lead to overetching, even though afterwards the annealing procedure might be sufficient to properly reconstruct the surface. Due to the risks of using HF based acids, we also advise the optimization of an acid-free termination technique such as the one described in Ref. 9 which should lead to similar results. Concerning the growth of LAO, we advise the use of single crystal targets to avoid the possible preferential ablation of specific species, which might occur for example in ceramic/sintered targets. In our case, samples grown with a ceramic target resulted in insulating samples, most likely due to off stoichiometry of the grown films 13 . The 20000 pulses pre-ablation step we describe might seem overly long, however we observed that the transport properties deteriorate extremely quickly by overlooking this step. Pre-ablating with less than 10000 pulses repeatedly showed insulating LAO/STO interfaces. As expected from PLD growth we additionally advise not to grow on substrates larger than 10x10 mm in lateral size. 4-point electrical probing on different regions of these 10x10 samples showed slight inconsistencies likely due to off-stoichiometry issues at the samples corners. We also note that all samples were grown with the center of the plume vertically aligned with the substrate. At low oxygen pressure (lower than 10 -2 mbar) and due to the low target-substrate distance, ablated species travel in a ballistic regime, which might also justify why the fluence utilized affects so drastically the properties of the interfacial conductivity 12 . Finally, since the laser energy measurement is done outside the chamber and the laser entrance window gets coated over time, which yield to a loss of transmission, we guide ourselves by targeting a constant growth rate value instead. The optimized growth rate for LAO was found to be approximately 25 pulses/unit cell for our specific PLD growth process. The RHEED oscillations measured during growth are extremely sensitive to the fine positioning of the substrate. The exact same growth process might result in substantial differences in terms of RHEED oscillations intensities. We advise using the same positioning parameters and probing the same diffraction spots for every growth. For consistency purposes, we have grown all of our samples monitoring only the (0-1) or (01) diffraction spots, off of the specular reflection. TEM imaging showed good uniformity and confirmed, together with standard X-ray diffraction, the correct corresponding number of LAO unit cells. Concerning the sputtering deposition, we emphasize that different metals have different wetting behaviors on top of oxides substrates [START_REF] Fu | Interaction of nanostructured metal overlayers with oxide surfaces[END_REF] . Consider increasing the thickness accordingly to achieve full coverage. Films under a few nm in thickness might result in non-percolated films. Perform SEM characterization after deposition to verify good uniformity. Using slower deposition rates and less energetic incoming species is also recommended, to finely control the film's thickness and avoid possible penetration of sputtered species into the sample. To do this, one might increase the target-substrate distance and/or reduce the plasma excitation current.

Regarding the XPS measurements, since the measurements are performed in situ, there is no electrical connection between the metallic overlayer (or the 2DEG) and the sample holder. This means that charging effects will be severe due to the insulating nature of the STO substrate. We therefore advise not to conclude on the position of the collected peaks, since they might be shifted several eV. It is also known that metals tend to screen electrons quite efficiently so that a 1 nm metallic capping might already impede probing other buried layers. An additional charge compensation by a flood gun might help to avoid peak shifting and deformation. In our case, peak deformation was negligible since we used a non-monochromated X-ray source providing enough secondary electrons. Angle-dependent studies can also provide valuable information on the depth profile. If the electron takeoff angle is 90° (surface normal parallel with the electron analyzer axis) then the maximum volume is probed. When the takeoff angle decreases (tilted sample) electrons from the same depth will travel larger distances, so that the total volume measured is reduced. One is then able to, for example, assess the thickness of a q2DEG through a valence state depthdependent analysis, as was previously shown for LAO/STO heterostructures [START_REF] Sing | Profiling the interface electron gas of LaAlO3/SrTiO3 heterostructures with hard x-ray photoelectron spectroscopy[END_REF] . We also bring the attention to additional peaks seen in the survey spectra. If the sample size is smaller than the detected area of the electron analyzer, traces of silver paste on the borders of the sample and of the species deposited on the substrate holder, such as La or Al, are collected. Note that these species will, to some extent, suffer different charging effects, which will lead for example to the appearance of double peak features. This can be easily fixed by reducing the entrance slit size for electron collection and increase the dwell time (for higher electron count). Overall, although the instructions provided in this paper serve as a guide to grow LAO ultra-thin films, they demonstrate a generic way of growing most ABO3 perovskites with PLD. Even though each material will require specific steps for optimized desired structural, electrical, or magnetic properties, one should be especially attentive to some essential features: type and composition of the target [START_REF] Preziosi | Reproducibility and off-stoichiometry issues in nickelate thin films grown by pulsed laser deposition[END_REF] , oxygen partial pressures during growth and annealing [START_REF] Hensling | Unraveling the enhanced Oxygen Vacancy Formation in Complex Oxides during Annealing and Growth[END_REF][START_REF] Xu | Disentanglement of growth dynamic and thermodynamic effects in LaAlO3/SrTiO3 heterostructures[END_REF] , substrate temperature, ablation frequency and laser fluence 12,[START_REF] Breckenfeld | Effect of growth induced (non)stoichiometry on interfacial conductance in LaAlO3/SrTiO3[END_REF] and type of substrates (and previous surface treatments). Through a combination of transport and spectroscopy experiments, we are also able to draw a more accurate and complete picture of a complex system where capturing the interplay between electrostatic and chemical phenomena is essential to understand the intrinsic versus extrinsic doping mechanisms. We reiterate that due to their strongly correlated character, these complex oxide systems are extremely sensitive to small stoichiometry and electronic changes such that their comprehensive study requires different complementary in situ and ex situ techniques.

ACKNOWLEDGMENTS:

This work received support from the ERC Consolidator Grant #615759 "MINT", the region Îlede-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

Figure 2 -

 2 Atomic force microscopy of (a) a STO substrate used for the growth of (b) a Co(2nm)/LAO(2 uc)/STO sample. Figure3-Pulsed laser deposition system. (a) Exterior of the PLD chamber. Although not visible, the phosphor screen and camera used to monitor the RHEED diffracted electrons are behind the chamber, aligned with the RHEED gun. (b) Interior of the PLD chamber. An infrared laser is used to locally heat the back-side of the sample holder. X and y positioning is controlled through knobs on the outside of the chamber. The target carousel allows us to have 5 different targets inside the chamber at the same time. (c) Detailed sketch of the optical apparatus. The beam exits the laser and is immediately reduced in size through a slit. The variable attenuator allows the control of the beam energy without varying the source laser parameters. Two converging lenses are then used to focus the beam on the target. Modified from Ref. 11 . (d) Characteristic plume-shaped plasma formed after ablating the LAO target with a single beam pulse, at 2x10 -4 mbar of oxygen. (e) Typical RHEED oscillations and diffraction spots. For consistency, monitoring is always performed on the (0-1) diffraction spot, located inside the yellow doted box.

Figure 4 -

 4 Magnetron sputtering system. (a) Exterior of the sputtering chamber. (b) Interior of the sputtering chamber. Ar atoms are accelerated towards the metal target, located inside the shown cylinder and mounted on a dc-magnetron source, hence generating a plasma.

Figure 5 -

 5 X-ray photoelectron spectroscopy system. (a) Exterior of the XPS chamber. Emitted electrons enter the transfer lens of the analyzer. There, they are retarded/accelerated to match the analyzer pass energy before entering the analyzer itself and being finally collected by the electron detector. A preamplified signal is then sent to the optical receiver before reaching the amplifier and computer. (b) Interior the XPS chamber. The X-ray gun can be retracted to allow positioning of the sample holder. (c) Typical survey scan performed on a bare STO substrate, an LAO(1 uc)/STO and a Co/LAO(1 uc)/STO. Note the evolution of the peaks. Ti and Sr peaks are gradually attenuated as the top layer thickness increases. La and barely visible Al peaks appear after LAO growth. The addition of an ultra-thin layer (3 Å) of Co rapidly attenuates all the other peaks. An additional zoomed O 1s peak (STO substrate) is displayed on the right. Figure 6 -Transport properties of metal/LAO/STO heterostructures. (a) Wire bonding of a sample in an 8-wire geometry (4-wire for Hall resistance and 4-wire for longitudinal resistance).

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.

DISCLOSURES:

The authors have nothing to disclose.

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