Inevitable high density of oxygen vacancies on the surface of LaAlO3/SrTiO3 heterostructures

TL;DR

This study uses advanced DFT calculations to determine the critical thickness for intrinsic doping in LaAlO3/SrTiO3 heterostructures and finds that high-density oxygen vacancies are inevitable on the surface once the LAO layer exceeds 3 unit cells, regardless of growth conditions.

Contribution

It provides a detailed theoretical analysis of oxygen vacancy formation and the critical thickness for doping, clarifying the mechanisms behind surface defects in LAO/STO heterostructures.

Findings

Critical thickness for intrinsic doping is 6 unit cells.

Oxygen vacancies (~10^14 cm^-2) are inevitable when LAO thickness >= 3 unit cells.

High oxygen pressure during growth does not prevent surface oxygen vacancies.

Abstract

Using density-functional-theory (DFT) calculations with the HSE06 hybrid functional, we accurately evaluate the critical thickness of LaAlO3 film for the intrinsic doping in LaAlO3/SrTiO3 (LAO/STO) heterstructures. The calculated critical thickness of 6 unit-cell (uc) layers suggests to rule out the intrinsic doping mechanism. We also calculate the density of oxygen vacancies on the LAO surface at varying LAO thicknesses, preparation oxygen pressures and temperatures by using the condition of chemical equilibrium and DFT calculations. We find that once LAO thickness >=3 uc high-density (~ 10^14 cm^-2 ) oxygen vacancies will inevitably exist on the LAO surface of the LAO/STO heterstructures even though the samples are grown under high oxygen pressure. The oxygen vacancies are stabilized by releasing the electrostatic energy in the LAO film.