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Cryo-SNOM studies of LAO-STO interface

Oxide interfaces have attracted much attention after the discovery of a conducting 2D electron gas, 2DEG, (or 2D electron system, 2DES), with a high carrier density of ~5∙1013 cm-2 in between the two large-bandgap insulators SrTiO3 and LaAlO3 [1] (Fig. 1a). Interestingly, the 2DEG is formed only at and above a critical thickness of LAO of 4 unit cells [2]. Remarkable features of this system are a high electron mobility, magnetism, spin-orbit interaction and even superconductivity [3]. The 2D carrier density and the mobility are temperature dependent, the latter changing by two orders of magnitude between the ambient- and low-temperature. The residual resistivity at low temperature and the superconducting transition temperature can be controlled by the gate voltage [4], which makes this system interesting for oxide electronics.

Fig. 1.  a: Crystal structure of the LAO/STO interface, where the metallic 2DEG is formed. b: Schematics of the s-SNOM experiment on LAO/STO samples. Crystalline STO (c-STO) is covered by a few atomic layers of LAO. The left side contains the conducting 2DEG, while the right side is a reference, where no 2DEG is formed due to an additional layer of amorphous STO (a-STO). c-e: AFM topography, s-SNOM amplitude and s-SNOM phase in the area covering the main and the reference regions (T = 300 K).

Probing the local transport properties of two-dimensional electron gases confined at buried interfaces requires a non-invasive technique with a high spatial resolution operating in a broad temperature range. In Ref.[5], we used our cryo-SNOM to study the conducting SrTiO3/LaAlO3 interface from room temperature down to 6 K. We showed that the near-field optical signal, in particular the SNOM phase, is highly sensitive to the presence of the 2DEG (Fig.1b-e) and strongly dependent on its transport properties. Using modeling, we established that such sensitivity originates from the interaction of the AFM tip with coupled plasmon–phonon modes where the phonon-polaritons (PhPs) in STO are hybridized with plasmon-polaritons (PPs) in the 2DEG, with the dispersion shown in Fig.2.

Fig. 2.  Theoretical dispersion of plasmon–phonon polaritons in the LAO/STO interface. a: Optical dispersion for 2 nm of LAO on STO without a 2DES, b: LAO/STO containing a 2DES with the optical mobility of 10 cm2 V−1 s−1 and 2D carrier density of 8 × 1013 cm−2. The dashed white curves represent the momentum dependence of time-averaged near-field coupling weight function, which peaks at qopt/2π = 2.2 × 104 cm−1. The green dash-dotted lines show the experimental spectral range (CO2 laser).

The model explains the observed strong changes in the optical signal with the variation of the 2DES transport properties induced by cooling (Fig.3) and by electrostatic gating (Fig. 4).

Fig. 3. Temperature and frequency dependence of the near-field signal on the 2DES. a,b: Experimental temperature dependence of the near-field amplitude and phase at laser wavelengths of 9.3, 10.2, and 10.7 μm. The inset in the bottom panel shows the schematic view of the sample with 5 u.c. of LAO. c,d Calculated dependence of the near-field signal on the optical mobility for the same wavelengths.
Fig. 4. The effect of electrostatic gating on the electrical transport and near-field signal at 6 K. a,b: Experimental near-field amplitude and phase as a function of the gate voltage (the wavelength is 10.7 μm). c,d: Calculation of the gate dependence of the near-field signal performed using the experimentally measured gate dependence of the carrier density and mobility (the latter is scale by a factor of 20 to account for the decrease of the mobility at infrared frequencies).

To probe the spatial resolution of the technique, we imaged conducting nano-channels written in insulating heterostructures with a voltage-biased tip of an atomic force microscope (Fig.5). One can see that the written wires are clearly seen in the amplitude and especially in the phase. At the same time they are indistinguishable in topography.

Fig. 5. s-SNOM imaging of the AFM-written conducting wires in the LAO/STO interface with 3 u.c. of LAO (the wavelength is 10.7 μm). Images of the SNOM amplitude (a), SNOM phase (b) and AFM topography (c) with respect to the signal from the regions not affected by the writing procedure. The line profiles along the dashed lines are shown below.

The ability to visualize buried nanoscale conducting structures shows clearly the usefulness of this technique for the development of oxide interface based electronics. It complements other invasive and non-invasive techniques by offering nanoscale information about infrared optical response. We foresee that the use of this local optical probe, in combination with cryogenic performance and electrostatic gating, will provide important information on the possible phase separation and charge inhomogeneities due to ferroelectric domain walls, metal-insulator transitions and other emergent phenomena in a large family of 2D oxide interfaces.

References:

[1] A. Ohtomo and H.Y. Hwang, Nature 427 423 (2004).

[2] S. Thiel, G. Hammerl, A. Schmehl, C.W. Schneider, J. Mannhart, Science 313, 1942 (2006).

[3] N. Reyren, S. Thiel, A. Caviglia, L. F. Kourkoutis, G. Hammerl, C. Richter, C. Schneider, T. Kopp, A.-S. Ruetschi, D. Jaccard, M. Gabay, D. A. Muller, J.-M. Triscone, and J. Mannhart, Science 317, 1196 (2007).

[4] A. Caviglia, S. Gariglio, N. Reyren, D. Jaccard, T. Schneider, M. Gabay, S. Thiel, G. Hammerl, J. Mannhart, and J.-M. Triscone, Nature 456, 624 (2008).

[5] W.W. Luo, M. Boselli, J.M. Poumirol, I. Ardizzone, J. Teyssier, D. van Der Marel, S. Gariglio, J.M. Triscone, and A.B. Kuzmenko, Nature Communications, 10, 8 (2019).