First-principles study of surface charging inLaAlO3/SrTiO3heterostructures

Abstract

The two-dimensional electron gas (2DEG) observed at the interface between LaAlO3 (LAO) and SrTiO3 (STO) is known to be very sensitive to the proximity of the LaAlO3 surface and the conditions to which the surface is exposed. We use first-principles calculations to study surface reconstructions on LAO films, taking into account that the LAO surface can be charged. The results for the charged surfaces and for the coupling between the surface and the 2DEG enable us to account not only for the behavior of the 2DEG as a function of thickness of the LAO layer, but simultaneously determine the stable terminations and reconstructions on the LAO surface under a variety of conditions. Our studies of charged surfaces are based on an extension of the methodology of Lozovoi et al. [J. Chem. Phys. 115, 1661(2001]. From the calculated electronic structure of the unreconstructed (but relaxed) AlO2 and LaO surface terminations of LAO, we find surface states having excess holes (AlO2 termination) or excess electrons (LaO termination). This result is central to understanding the mechanism of 2DEG formation, and is consistent with a 2DEG of density 3.3×10 14 cm −2 being intrinsic to the LaO-TiO2 interface in the LAO/STO system. We explore the effects of the Al-adatom, O-vacancy and H-adatom surface reconstructions on the 2DEG density, and find that the stability of different reconstructions is tied to the thickness of the LAO layer as well as the surface exposure conditions. We find that including the effects of charging of the surface significantly stabilizes the AlO2 termination versus the LaO termination. Overall, our methodology has the advantage of decoupling first-principles calculations for the interface from those for the charged surface, and constitutes a general approach that can be applied to the commonly occurring problem of charge exchange between the surface and the interface of a thin film with a substrate, or between the surface and defects/impurities in the bulk of a material.

Polarization effects on the interfacial conductivity in LaAlO3/SrTiO3heterostructures: a first-principles study

“…Some research efforts are still being made to explore a comprehensive model that can account for all the experimental phenomena in the LAO/STO HS. 12,13 Several possible explanations for this discrepancy have been proposed from various viewpoints. [12][13][14][15][16][17] For example, from density functional theory (DFT) calculations, Popović et al suggested that the transferred electrons are not only confined to the interfacial TiO 2 layer but also extend over several deeper TiO 2 layers, which dilutes the sheet carrier density.…”

“…12,13 Several possible explanations for this discrepancy have been proposed from various viewpoints. [12][13][14][15][16][17] For example, from density functional theory (DFT) calculations, Popović et al suggested that the transferred electrons are not only confined to the interfacial TiO 2 layer but also extend over several deeper TiO 2 layers, which dilutes the sheet carrier density. 14 Using first-principles electronic structure calculations, Krishnaswamy et al proposed that the sheet carrier density of 3.3 Â 10 14 cm À2 is intrinsic to the LaO/TiO 2 interface in the LAO/ STO system, while the AlO 2 -terminated surface may transfer electrons from the 2DEG to the surface states, leading to a smaller sheet carrier density at the interface.…”

“…14 Using first-principles electronic structure calculations, Krishnaswamy et al proposed that the sheet carrier density of 3.3 Â 10 14 cm À2 is intrinsic to the LaO/TiO 2 interface in the LAO/ STO system, while the AlO 2 -terminated surface may transfer electrons from the 2DEG to the surface states, leading to a smaller sheet carrier density at the interface. 13 Pentcheva et al, for the first time, have proposed that the polar distortion in the LAO overlayers on the STO substrate can neutralize the polar catastrophe, leading to the insulating behavior up to five monolayers of LAO. 15 As a result, one may speculate that the polarization, i.e., the polar distortion, in the LAO film may be weak enough when the LAO film is more than five monolayers thick so that the polar distortion is not capable of fully neutralizing the polar catastrophe.…”

Predicting point defect equilibria across oxide hetero-interfaces: model system of ZrO2/Cr2O3

“…71,72 The localized field is important in understanding localized processes at the interface. Recent studies have both measured experimentally 73 and predicted theoretically 74,75 the existence of such large electric fields at a water/oxide interface, an oxide surface, and oxide heterostructures, and their impact on the field-dependent properties such as local bonding interaction, dielectric constant, and 2D electron gas density. When these properties are of interest, accurate prediction of local electric field accounting for both perfect crystal contribution and defect contribution would be necessary.…”

Doping dependence of the surface phase stability of polar O-terminated (0001̅) ZnO

“…Other approaches treat the charged surface defects by introducing an additional dopant charge into the calculation and by applying a posteriori correction schemes to subtract the contributions of the compensating homogeneous background in the supercell. These approaches, however, neglect the effect of the electric field in the space charge region [35,36]. Kempisty and Krukowski presented a method to account for the electric field of the space charge region by manipulating the location and charge of passivating atoms on the back side of the slab [37][38][39][40], but did not translate the added charge into the bulk doping concentration.…”