Spatially localized electronic states at the LaAlO₃/TiO₂ interface: mapping of a 2D electron gas

Abstract text (incl. figure legends and references)

Perovskite heterostructures play an increasingly important role in optoelectronics applications, such as solar cells, LEDs and photo detectors [1]. The combination of two different semiconducting materials often leads to drastic changes of the electrical, magnetic or optical properties in the vicinity of the interface. Thus, full characterization of the localized electronic states is of utmost importance. We approach this goal with a combined ab-initio and experimental study of the electronic states localized at the LaAlO₃/TiO₂ interface.

The material of interest is a heterostructure consisting of the cubic perovskite LaAlO₃ and TiO₂ in anatase phase. Bulk anatase is typically a semiconductor with a few eV wide direct band gap. In combination with LaAlO₃, however, unoccupied states appear directly above the Fermi energy for 2-3 TiO₂ layers closest to the interface. When a small energy window is chosen for imaging, similar to elemental mapping albeit with a window of 0.5 eV or even smaller, the localized electronic states can be mapped in real space [2]. We compare the experimental results to inelastic channeling simulations of a Ti-La terminated interface based on [3].

The simulated spectrum image for an energy loss of 456.4 eV can be seen in Figure 1(c). Ti atoms at least 1 unit cell from the interface show negligible intensity or no intensity at all and can therefore be considered "bulk-like" in this context. Ti atoms close to the Ti-La terminated interface, however, show significant intensity at this energy loss, which is especially apparent in the second atomic column from the interface. The orbital there resembles the shape of a d_xy-orbital with x the horizontal axis and y the vertical axis in the figure. The experimental map of the Ti L₃-edge onset pre-peaks lies in close agreement with the simulations (see Figure 1(b)). Due to the difficult mapping conditions presented by the low signal-to-noise ratio and due to elastic channeling, the original shape of the orbitals is not exactly reproduced. More importantly, however, the spatial confinement of the relevant electronic orbitals and the resulting local increase in intensity is clearly visible.

Electrons occupying these states can, thus, be identified as a two-dimensional electron gas. The direct mapping of such an electron gas opens up new ways of investigating heterostructures in electronics with future application of understanding and designing new and improved optoelectronic materials. [4]

Figure 1. (a) HAADF image of a 12 nm thick heterostructure oriented in the [100] crystallographic directions of both anatase TiO₂ and LaTiO₃ recorded at a high tension of 300 kV. (b) STEM-EELS spectrum image for a collection semi-angle of 24 mrad and an energy window at the Ti L₃-edge onset. (c) Simulated spectrum image for an energy loss of 456.4 eV and a 1 unit cell thick sample with the same parameters as in (a) and (b).

[1] Cheng et al., Adv. Optical Mater. 10, 2102224 (2022)

[2] Löffler et al., Ultramicroscopy 177, 26 (2017)

[3] Wang et al., Journal of Applied Physics 108, 113701 (2010)

[4] The authors gratefully acknowledge funding from FWF under grant nr. I4309-N36.