Two-dimensional electron gas in the Cu/Bi2O3 interface detected by mid-infrared ellipsometry

The formation of novel two-dimensional electron gas (2DEG) with high mobility in amorphous oxides/metal interfaces has motivated an ongoing interest regarding novel characteristics of these 2DEGs. Here we report an optical study, based on spectroscopic ellipsometry, of Cu/Bi$_{2}$O$_{3}$ interfaces that confirms the formation of a 2DEG with spin orbit coupling (SOC). In addition we detect Fano resonances which are attributed to the coupling of 2DEG quantum confined states with the continuum of the interface-bulk states. In particular, the line shape of the interfacial 2DEG dielectric function (DF) thus resolved in Drude and Lorentz components resulted very similar to theoretical predictions [M. Xie et al., Phys. Rev. B 89, 245417 (2014)] of a two dimensional electron gas confined in the normal direction of the system. Although the original constituent materials do not possess spin-orbit coupling (SOC), the resulting interfacial hybridization of such states induce electronic asymmetric wave functions. This work demonstrates the feasibility of ellipsometry as a non-invasive optical characterization of complex interfacial phenomena in challenging 2DEG formed by amorphous crystals, overcoming an experimental impediment which has hold back for decades important advancements for the understanding of 2DEGs in amorphous materials.