Anisotropic two-dimensional electron gas at SrTiO3(110)

Category: Scientific Highlights

Published in PNAS

Although still in its infancy, electronics based on all-oxide materials is a rapidly developing field, and strontium titanate is its key player. For this area to thrive, an atomic-scale control and understanding of the materials’ surfaces and interfaces needs to be achieved. A SrTiO₃ crystal with (110) orientation automatically forms an overlayer that is more insulating than the bulk and chemically less reactive, akin to the native SiO₂ on conventional wafer. With appropriate doping a two-dimensional electron gas (2DEG) forms underneath the SrTiO₃(110) surface. This (110) 2DEG is very different from (001): The effective mass here depends on the quantum number, and a completely flat band can be realized. Such a flat band bears good prospects for, among others, magnetism and thermoelectricity.

ABSTRACT:

Two-dimensional electron gases (2DEGs) at oxide heterostructures are attracting considerable attention, as these might one day substitute conventional semiconductors at least for some functionalities. Here we present a minimal setup for such a 2DEG––the SrTiO₃(110)-(4 × 1) surface, natively terminated with one monolayer of tetrahedrally coordinated titania. Oxygen vacancies induced by synchrotron radiation migrate underneath this overlayer; this leads to a confining potential and electron doping such that a 2DEG develops. Our angle-resolved photoemission spectroscopy and theoretical results show that confinement along (110) is strikingly different from the (001) crystal orientation. In particular, the quantized subbands show a surprising “semiheavy” band, in contrast with the analog in the bulk, and a high electronic anisotropy. This anisotropy and even the effective mass of the (110) 2DEG is tunable by doping, offering a high flexibility to engineer the properties of this system.

 

		Figure: SrTiO3(110)-(4 × 1) surface. (A) Structural model; an oxygen vacancy (VO) formed at the reconstructed surface spontaneously migrates to the (SrTiO)4+ plane beneath the top titania layer. The excess electrons from the VO form a 2DEG confined within a region of about 2 nm thickness. The layer-dependent charge is represented by the yellow lobes and plotted in B. (C) MD simulation of the VO diffusion from the surface (S) to the subsurface (S-2) sites represented by the accompanying upward oxygen migration from S-2 to S-1 (blue line) and from S-1 to S (black line). (D) Angle-integrated photoemission spectroscopy of the clean surface and after creating VOs by synchrotron radiation. An in-gap state and a metallic peak near the Fermi level (EF) develop (Inset). (E) Schematic band structure at a surface with VOs; the bands bend downward by 0.3 eV as deduced from the spectra in D. The dots denote the surface potential obtained from the shift of Ti 3s states in DFT+U calculations.

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