Invited Paper MI+EM+MG-MoA6
Medard W. Welch Award Lecture - Complex Oxide Interfaces: Conquering the (Polar) Catastrophe

Monday, October 28, 2013, 3:40 pm, Room 202 A

The formation of a two-dimensional electron gas (2DEG) at the interface between two insulators, SrTiO₃ (STO) and LaAlO₃ (LAO), has sparked huge interest in oxide electronics. In spite of almost a decade of research, the mechanisms that determine the density of this 2DEG have remained controversial. The electronic behavior of these polar/nonpolar interfaces is often modeled using electrostatics based on an ionic representation of the solids. This leads to a “polar catastrophe” in which the potential in the overlayer diverges—similar to the case of (001) interfaces between heterovalent semiconductors (e.g., Ge/GaAs) that were studied 35 years ago [1].

In fact, if the electrons resulting from the polar discontinuity can be confined at the interface, the “catastrophe” can be entirely avoided, and a 2DEG with an electron density of 3.3x10¹⁴ cm^-2 (0.5 electrons per unit cell) can be generated. However, experimentally observed densities at the STO/LAO interface are more than an order of magnitude lower.

We have used a combination of first-principles calculations and Schrödinger-Poisson simulations to investigate this problem [2]. The termination of the wider-band-gap overlayer is key: surface states that act as a sink for electrons limit the 2DEG density. I will discuss the effects of LAO surface reconstructions, including hydrogenation. These results apply to oxide interfaces in general, and explain why the SrTiO₃/GdTiO₃(GTO) interface has been found to exhibit the full density of 0.5 electrons per unit cell [3].

An interesting question can be raised now: why is it that oxide interface can sustain this huge 2DEG density, while semiconductor interfaces are commonly accepted to undergo atomic reconstructions to eliminate the polar catastrophe? I will suggest that the insights gained from oxide interfaces may be used to design semiconductor interfaces that could sustain similar 2DEGs.

Work performed in collaboration with L. Bjaalie, L. Gordon, K. Krishnaswamy, and A. Janotti, and supported by the ARO, ONR, and NSF .

[1] W. A. Harrison, E. A. Kraut, J. R. Waldrop, and R. W. Grant, Phys. Rev. B 18, 4402 (1978).

[2] A. Janotti, L. Bjaalie, L. Gordon, and C. G. Van de Walle, Phys. Rev. B 86, 241108(R) (2012).

[3] P. Moetakef, T. A. Cain, D. G. Ouellette, J. Y. Zhang, D. O. Klenov, A. Janotti, C. G. Van de Walle, S. Rajan, S. J. Allen, and S. Stemmer, Appl. Phys. Lett. 99, 232116 (2011).