Unexpected Transport Behavior in (001) and (111) Crystal Oriented AlO_x / SrTiO₃ Devices

In the two-dimensional conducting gasses formed at the interfaces of SrTiO₃ (STO) based devices, an electric field applied via a back gate voltage (Vg) is known to significantly modulate the density of carriers, the strength of spin-orbit interactions, and disorder of the system. If Vg is applied through the STO substrate, however, gating effects are modulated by the unusual dielectric properties of the STO. Here we report temperature and gate voltage dependent transport measurements on AlO_x/STO interface devices. We find that initially increasing Vg at low temperatures results in an increase in the magnitude of the Hall coefficient RH, but subsequently decreasing Vg reduces RH. This is completely counter to what one would expect if the majority carriers in the interface gas were electrons, as is the case with this system. Additionally, there is a significant deviation of low temperature behavior upon reducing gate voltage after ramping. Measurements on (001) oriented STO devices show some agreement to the predictions of Raslan et al¹, but measurements on (111) oriented devices show completely unexpected behavior.

[1] Raslan et al., Phys. Rev. B 95, 054106 (2017)