Surface defects and conduction in polar oxide heterostructures

TL;DR

This paper investigates the origin of the 2DEG at LaAlO3/SrTiO3 interfaces, proposing surface oxygen vacancies as a key factor, supported by first principles calculations and aligning with recent experimental observations.

Contribution

It introduces a model linking surface redox reactions, especially oxygen vacancies, to 2DEG formation, explaining experimental results and suggesting ways to improve interface mobility.

Findings

Surface oxygen vacancies stabilize beyond a critical film thickness.

Surface defects influence interface electron mobility.

The model explains spectroscopic and transport measurement results.

Abstract

The polar interface between LaAlO$_{3}$ and SrTiO$_{3}$ has shown promise as a field effect transistor, with reduced (nanoscale) feature sizes and potentially added functionality over conventional semiconductor systems. However, the mobility of the interfacial two-dimensional electron gas (2DEG) is lower than desirable. Therefore to progress, the highly debated origin of the 2DEG must be understood. Here we present a case for surface redox reactions as the origin of the 2DEG, in particular surface O vacancies, using a model supported by first principles calculations that describes the redox formation. In agreement with recent spectroscopic and transport measurements, we predict a stabilization of such redox processes (and hence Ti 3$d$ occupation) with film thickness beyond a critical value, which can be smaller than the critical thickness for 2D electronic conduction, since the surface defects generate trapping potentials that will affect the interface electron mobility. Several other recent experimental results, such as lack of core level broadening and shifts, find natural explanation. Pristine systems will likely require changed growth conditions or modified materials with a higher vacancy free energy.