Controlling the Carrier Density of SrTiO₃‐Based Heterostructures with Annealing

Abstract

Abstract: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, we report how annealing in oxygen at low temperatures ( < 300 ℃) 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), we identify the pathways for changing the carrier density in the two STO-based cases 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 ( = 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 ( = 0.5 eV). Finally, we show how the control of the carrier density enables writing of conducting nanostructures in γ-Al2O3/STO by conducting atomic force microscopy (c-AFM).

Microscopic origin of the mobility enhancement at a spinel/perovskite oxide heterointerface revealed by photoemission spectroscopy

“…3(b)], hence suggesting a favored formation of type B oxygen vacancies and/or diffusion of vacancies from the bulk to the interface. A recent annealing study of γ-Al 2 O 3 /SrTiO 3 indeed identified such diffusion processes, in support of the mechanism proposed here [45]. Figure 4(a) schematically summarizes our experimental and theoretical findings.…”

Stimulating Oxide Heterostructures: A Review on Controlling SrTiO3‐Based Heterointerfaces with External Stimuli

“…The case of GAO/STO with a GAO film thicker than 1–2 nm is particularly interesting, as the conductivity here is stable at room temperature on the time scale of years at ambient conditions, but by elevating the temperature to 200–300 °C the conductivity could be tuned to a desired level . This is depicted in Figure where GAO was deposited by PLD at an oxygen partial pressure of 10 −5 mbar resulting in an initial sheet carrier density of around 2 × 10 14 cm −2 .…”

“…The transport properties and interface conductivity of STO‐based heterostructures can be readily tuned by varying the deposition conditions during the growth of the top oxide thin film . With the notable exception of interface conductivity originating from oxygen vacancies in STO, the conducting properties of STO‐based heterostructures are generally very stable. However, it is possible to electrostatically modulate the interfacial properties of STO‐based heterostructures by applying an external electric field using a gate.…”

“…For the heterostructures where the conductivity is formed by oxygen vacancies, the conductivity was found to weakly degrade over time at ambient conditions . The degradation ( δR s / δt , see Figure ) could be minimized by storing the samples in oxygen‐deficient environment or lowering the oxygen diffusion through the top film by increasing the layer thickness or utilizing a material with a high oxygen diffusion barrier such as GAO …”

“…Writing of nanocircuits was initially unsuccessful using an as‐deposited GAO/STO heterostructure with a low resistance. However, after raising the resistance of the same sample to an insulating state using annealing on a hot plate in ambient conditions, conducting AFM writing was possible …”