Controlling the Carrier Density of SrTiO₃-Based Heterostructures with Annealing

The conducting interface between the insulating oxides LaAlO₃ (LAO) and SrTiO₃ (STO) displays numerous physical phenomena that can be tuned by varying the carrier density, which is generally achieved by electrostatic gating or adjustment of growth parameters. Here, it is reported how annealing in oxygen at low temperatures (T < 300 °C) can be used as a simple route to control the carrier density by several orders of magnitude. The pathway to control the carrier density relies on donor oxidation and is thus applicable to material systems where oxygen vacancies are the dominant source of conductivity. Using STO capped with epitaxial γ-Al₂O₃ (GAO) or amorphous LAO (a-LAO), the pathways for changing the carrier density in the two STO-based cases are identified where oxygen blocking (GAO) and oxygen permeable (a-LAO) films create interface conductivity from oxygen vacancies located in STO near the interface. For a-LAO/STO, the rate limiting step (E_a = 0.25 eV) for oxidizing oxygen vacancies is the transportation of oxygen from the atmosphere through the a-LAO film, whereas GAO/STO is limited by oxygen migration inside STO (E_a = 0.5 eV). Finally, it is showed how the control of the carrier density enables writing of conducting nanostructures in γ-Al₂O₃/STO by conducting atomic force microscopy.