Extracting the electronic properties of an oxide two-dimensional electron gas by scanning near-field optical microscopy and phonon modeling

In the family of functional oxide materials one interesting system is the interface between SrTiO3 and LaAlO3 (LAO/STO), which are large-bandgap insulators in their bulk state: combining the two insulating materials gives rise to a confined and highly conductive two-dimensional electron gas (2DEG) close to the interface [1], if the number of LAO unit cells is larger than three. This 2DEG exhibits remarkable properties such as superconductivity and gate tunability, and displays the possibility for future transistor applications. However, its investigation was thus far limited to non-local methods, that average over larger areas.
Here, scanning near-field optical microscopy (SNOM) in a nano-FTIR setup is used in conjunction with synchrotron radiation to obtain nanoscale IR spectra of LAO/STO interfaces in the range of their near-field phonon resonances, as the local electronic properties of complex oxides can be quantified by exploiting plasmon-phonon coupling [2].
Interestingly, both LAO and STO exhibit near-field phonon resonances in the spectral region of interest, leading to a non-trivial scattering spectrum that strongly depends on the number of LAO unit cells deposited on top. Additionally, growth strain and confinement effects of these ultra-thin films need to be taken into account, as their dielectric function deviates significantly from the bulk case [3].
The SNOM spectral response of different samples was predicted by using a combination of the transfer matrix method and the finite dipole model [4] and systematically compared to synchrotron nano-FTIR measurements. As a result, local electronic properties of the interfacial 2DEG could be extracted quantitatively, enabling the possibility of its nanoscale lateral mapping in the future.