Low temperature near-field fingerprint spectroscopy of 2D electron systems in oxide heterostructures and beyond explores confined electron systems, such as 2D electron gases (2DEGs), 2D materials, and topological insulators. These systems hold great technological promise but are susceptible to defects resulting in nanoscale inhomogeneities. Scattering-type scanning near-field optical microscopy (s-SNOM) is a non-destructive method used to investigate buried confined electron systems with nanoscale resolution. However, a clear separation of carrier concentration and mobility has been challenging in s-SNOM. A characteristic “fingerprint” response of the LaAlO3/SrTiO3 2DEG was predicted and verified using a state-of-the-art tunable narrow-band laser in mid-infrared cryo-s-SNOM at 8 K. This allowed for the separation of carrier concentration and mobility to characterize 2DEG inhomogeneities on the nanoscale.
Layered heterostructures of complex oxides have become essential for novel electronic and spintronic devices. These structures often contain high local densities of electronic charge carriers, leading to correlation phenomena and emerging properties not found in the adjacent bulk materials. Characterizing high carrier concentrations in spatially confined and buried electronic systems poses new challenges, especially at cryogenic temperatures. Scattering-type near-field optical microscopy is ideal for investigating highly-confined electron systems, such as van-der-Waals materials or oxide heterostructures. The spectral information observed in s-SNOM is convoluted, posing challenges in understanding the acquired data, especially in layered structures.
The 2D electron gas at the LaAlO3/SrTiO3 interface has been extensively studied as a model system for high-concentration correlated electron systems. However, the local formation process and the influence of defects on the electronic properties are not fully understood. s-SNOM has shown sensitivity to the 2DEG in LAO/STO and allows for the extraction of local electronic properties with nanoscale lateral resolution. The 2DEG mobility increases at low temperatures, leading to a stronger sensitivity of s-SNOM to the 2DEG properties. Previous studies were limited to indirectly probing the 2DEG via secondary effects.
This work predicts a spectral fingerprint region where a characteristic scattering response of the 2DEG can be obtained directly, enabling the separation of carrier concentration and mobility. The predicted fingerprint spectra are a universal feature applicable to confined electron systems like topological insulators or stacked van-der-Waals materials. The study used a tunable narrow-band mid-infrared laser to investigate the 2DEG fingerprint region using cryo-s-SNOM, providing a detailed understanding of the local electronic properties.
Overall, near-field fingerprint spectroscopy offers a powerful tool for investigating nanoscale material properties, providing insights into the electronic properties of confined electron systems and offering potential applications in the design of nanoelectronic devices.
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