Oxygen vacancies and hydrogen doping in LaAlO₃/SrTiO₃ heterostructures: electronic properties and impact on surface and interface reconstruction

Abstract

We investigate the effect of oxygen vacancies and hydrogen dopants at the surface and inside slabs of LaAlO3, SrTiO3, and LaAlO3/SrTiO3 heterostructures on the electronic properties by means of electronic structure calculations as based on density functional theory. Depending on the concentration, the presence of these defects in a LaAlO3 slab can suppress the surface conductivity. In contrast, in insulating SrTiO3 slabs already very small concentrations of oxygen vacancies or hydrogen dopant atoms induce a finite occupation of the conduction band. Surface defects in insulating LaAlO3/SrTiO3 heterostructure slabs with three LaAlO3 overlayers lead to the emergence of interface conductivity. Calculated defect formation energies reveal strong preference of hydrogen dopant atoms for surface sites for all structures and concentrations considered. Strong decrease of the defect formation energy of hydrogen adatoms with increasing thickness of the LaAlO3 overlayer and crossover from positive to negative values, taken together with the metallic conductivity induced by hydrogen adatoms, seamlessly explains the semiconductor-metal transition observed for these heterostructures as a function of the overlayer thickness. Moreover, we show that the potential drop and concomitant shift of (layer resolved) band edges is suppressed for the metallic configuration. Finally, magnetism with stable local moments, which form atomically thin magnetic layers at the interface, is generated by oxygen vacancies either at the surface or the interface, or by hydrogen atoms buried at the interface. In particular, oxygen vacancies in the TiO2 interface layer cause drastic downshift of the 3d eg states of the Ti atoms neighboring the vacancies, giving rise to strongly localized magnetic moments, which add to the two-dimensional background magnetization.

Possible flexoelectric origin of the Lifshitz transition in LaAlO3/SrTiO3 interfaces

“…1]. Several doping mechanisms, including electronic reconstruction [48], a nonstoichiometric LAO surface [49,50], and top-gating, contribute to the 2DEG in the STO substrate. For our purposes, these doping mechanisms can be modeled by a positive charge density en 2D on the LAO surface, and overall neutrality requires a 2D charge density of −en 2D in the STO substrate.…”

Magnetic Properties of LaAlO3/SrTiO3 Heterostructure Modelled on a Supercomputer

“…The contribution from a distant Ti atom and a Ti atom next to the vacancy differ significantly. In particular, we found the downshift of the e g states of the latter atom as compared to those of the former by about 2 eV [8], which leads to a finite occupation of these orbitals as well as a finite contribution to the local magnetic moment, which even exceeds that of the t 2g states (Fig. 2 b)).…”

“…To study the heterostructure with defects, we used 2 × 2 in-plane supercells of the bare heterostructures [7] with introduced oxygen vacancies and hydrogen dopants at the surface, or in an interfacial layer on both sides while preserving the inversion symmetry of the slabs. More details about computation parameters and slab geometry can be found in [5,7,8].…”

“…While investigating defects profile in the LAO/STO heterostructure we have found that for sufficiently large concentrations of oxygen vacancies the formation energy drops at the interface [8]. We started with a bare insulative and non-magnetic heterostructure containing three LAO overlayers.…”

“…2 a). Thus, oxygen vacancies at the interface induce atomically thin magnetic layers with a rather uniform background of magnetic moments generated by the Ti 3d t 2g states, which are complemented by well localized magnetic moments due to e g states of the Ti atoms neighboring the vacancy [8].…”