Quasi-two-dimensional electron gas at the epitaxial alumina/SrTiO3 interface: Control of oxygen vacancies

Abstract

In this paper, we report on the highly conductive layer formed at the crystalline γ-alumina/SrTiO3 interface, which is attributed to oxygen vacancies. We describe the structure of thin γ-alumina layers deposited by molecular beam epitaxy on SrTiO3 (001) at growth temperatures in the range of 400–800 °C, as determined by reflection-high-energy electron diffraction, x-ray diffraction, and high-resolution electron microscopy. In situ x-ray photoelectron spectroscopy was used to confirm the presence of the oxygen-deficient layer. Electrical characterization indicates sheet carrier densities of ∼1013 cm−2 at room temperature for the sample deposited at 700 °C, with a maximum electron Hall mobility of 3100 cm2V−1s−1 at 3.2 K and room temperature mobility of 22 cm2V−1s−1. Annealing in oxygen is found to reduce the carrier density and turn a conductive sample into an insulator.

Anomalous orbital structure in a spinel–perovskite interface

“…The conductivity of the AlO/STO heterostructures used in this work is in good agreement with the previous reports 10,32,[37][38][39] , as shown in Sup- with a crystal field gap as large as ∼ 2 eV in the octahedral symmetry 9,[13][14][15][16][17][18] . Additionally, the strong spin-orbit interaction induces the splitting of the Ti 2p core level into 2p 1/2 and 2p 3/2 states.…”

“…As schematically shown in Supplementary Fig. 5, based on the interfacial atomic structure data 32,39 , for the case of γ-Al 2 O 3 spinel and in contrast to the previously reported perovskiteperovskite interfaces, the apical oxygen of Ti-O octahedra is not stable in a spinel-perovskite heterostructure. Thus, at the AlO/STO interface a unique Ti-O pyramid coordination is formed; in this distorted pyramid-like structure, d xz /d yz subband becomes the preferable state for the interfacial electrons (see Supplementary Fig.…”

“…Specifically, the orbital symmetry of conduction carriers is primarily linked to the symmetry of the superconducting gap 4 and the mobility of electrons [10][11][12] . To this end, the orbital configuration, responsible for the high mobility of electrons in a spinel-perovskite interface (for example γ-Al 2 O 3 /SrTiO 3 , AlO/STO) 10,11,32 is still an open question. From the experimental point of view, surface sensitive angle resolved photoemission spectroscopy (ARPES) with polarized photons is a suitable probe of symmetry of the surface electronic structure 19 , but it has limited applicability for the electronically active buried interfaces.…”

Suppressed carrier density for the patterned high mobility two-dimensional electron gas at γ-Al2O3/SrTiO3 heterointerfaces

“…The suppression of the carrier density in pattered samples is most likely due to the presence of the manganite buffer layer. This is because the GAO/STO heterostructure is one of the typical STO-based heterostructures, where the interface conductivity originates mainly from oxygen vacancies due to interfacial redox reactions 6,7,9,10 . At high deposition temperatures, the oxygen ions in STO can diffuse over many micrometers in minutes 25 .…”

“…Nevertheless, although extensive research has been carried out on this system, the typical mobility remains ~ 1,000 cm 2 V -1 s -1 or less (at low temperatures). Recently, a new 2DEG was discovered at the non-isostructural interface between perovskite STO and spinel -Al2O3 (GAO) with compatible oxygen sublattices [6][7][8][9][10] . Remarkably, the GAO/STO heterostructure shows much higher electron mobility (greater than 140, 000 cm 2 V -1 s -1 ) as well as extremely high carrier densities of more than 10 15 cm -2 .…”