Phonon‐Enhanced Near‐Field Spectroscopy to Extract the Local Electronic Properties of Buried 2D Electron Systems in Oxide Heterostructures

Abstract

In the family of functional oxide materials, the interface between LaAlO 3 and SrTiO 3 (LAO/STO) is an interesting example, as both materials are largebandgap insulators in their bulk state but give rise to a confined 2D electron gas (2DEG) when combined through thin-film deposition. While this 2DEG exhibits remarkable properties, its experimental investigation is mostly limited to destructive or non-local (i.e. averaging over larger areas) methods until recently. Scanning near-field optical microscopy is shown to overcome this limitation, detecting buried 2DEGs by using highly confined optical nearfields. Here, a full spectroscopic approach with phonon-enhancement and simulations based on the finite dipole model is combined to extract quantitative electronic properties of the interfacial LAO/STO 2DEG. This threefold improvement compared to previous work will enable the quantitative nanoscale, non-destructive, sub-surface analysis of complex oxide thin films and interfaces, as well as similar heterostructures.

Hybrid Machine Learning for Scanning Near-Field Optical Spectroscopy

“…Finally, to compare with the experimental observable S n , the demodulation is carried out as Here the tip is assumed to follow a harmonic tapping motion given by z ( t ) = A (1-cos­(Ω t )), where A is the tapping amplitude. The efficacy of the FDM has been demonstrated numerous times in the literature. , However, there are several noteworthy limitations: the real AFM tips are not elongated spheroids and the length of the tip is often close to or even larger than the incident wavelength such that the electrodynamical retardation effect might be significant. Although these inconsistencies obviously yield differences between the FDM prediction and the experimental data, the FDM still serves as a solid starting point for the HNN.…”

“…Here the tip is assumed to follow a harmonic tapping motion given by 𝑧(𝑡) = 𝐴(1 − cos(Ω𝑡)), where 𝐴 is the tapping amplitude. The efficacy of the FDM has been demonstrated numerous times in the literature 57,58 . However, there are several noteworthy limitations: the real AFM tips are not elongated spheroids; the length of the tip is often close to or even larger than the incident wavelength such that the electrodynamical retardation effect might be significant.…”

Local inhomogeneities resolved by scanning probe techniques and their impact on local 2DEG formation in oxide heterostructures

“…Correlative s-SNOM measurements lead to the same conclusion, confirming the presented interpretation of the c-AFM results. In the future, s-SNOM spectroscopy of the 2DEG 41 could be used to quantify the local charge carrier density for inhomogeneities. In particular, simulations show that changes of the charge carrier density and mobility could yield a different spectroscopic response in this sample ( cf.…”

“…36,37 Furthermore, s-SNOM has recently been shown to be able to optically detect free charge carriers in STO 38 as well as in LAO/ STO heterostructures. [39][40][41] Both these established techniques (phase contrast AFM and s-SNOM) will be used to show how c-AFM can contribute to scientic advance in the eld. Further scanning probe examples are scanning superconducting quantum interference device measurements, which revealed magnetic patches in LAO/STO that depend on the LAO thickness 42 and at low temperatures were able to identify highly conducting pathways, 31 and strain-tunable magnetic ordering.…”

“…[82][83][84][85] In the past, s-SNOM was shown to detect the presence of the 2DEG in LAO/STO 40 and even allow for determination of electronic properties (carrier density and mobility) using spectroscopy at different wavelengths. 39,41 Here, a correlative s-SNOM measurement on a 5 uc LAO/STO sample grown on a case [C] STO substrate is presented at a laser wavelength of l ¼ 10.3 mm (n ¼ 974 cm À1 ). As s-SNOM analysis is not hindered by the LAO thickness, 5 uc were chosen to avoid working at the critical thickness of LAO/STO.…”