Electronic effects in LaTiO3/LaAlO3 superlattices

Abstract

This is a copy of the slides presented at the meeting but not formally written up for the volume. Description: Semiconductor physics contains a rich body of theory and working designs. However, their material properties seem to be reaching their limits. Perovskite oxides on the other hand have abundant physical properties, but are still under active investigation. The advent of RHEED-monitoring of pulsed laser deposition allows for the fabrication of structures with single unit cell (4 A) thick layers. In this way we may be able to fabricate quantum well structures for both applications and fundamental investigations. Superlattices of the Mott insulator LaTiO3 (LTO) and the band gap insulator SrTiO3 (STO) form such a structure. The superlattices are metallic, both as-grown and post-annealed [1]. This has been attributed to the existence of metallic states at the interfaces between LTO and STO [2]. At these interfaces the electron density is found to extend about 10 A into the STO. However, theoretically, the required length scale for quantum confinement is of the order of 4 A. A possible way to increase this confinement is to use a buffer material with a larger band gap than that of LTO (similar to semiconductor band gap engineering) and/or with a lower dielectric constant [2]. LaAlO3 (LAO) is such a material (ΔELAO = 5.6 eV vs. ΔESTO = 3.2 eV, eLAO = 24 vs. eSTO = 300). Here we report on the growth of LTO/LAO superlattices on STO substrates. As-grown superlattices of LTO/LAO are metallic, while post-annealing turns them insulating. This may be explained from a disorder-order transition in a 2D Mott-Hubbard model [3]. XPS and EELS measurements of the titanium valence show interesting differences for LTO layers close to and far away from the sample surface. The former, for thin LAO capping layers, show the presence of Ti4+ while the latter only have Ti3+. Hard XPS of samples with varying capping layer thickness shows an exponential dependence of the Ti3+ contents on a length scale of about 5 unit cells. [1] A. Ohtomo et al., Nature 419, 378-380 (2002). [2] S. Okamoto & A.J. Millis, Phys. Rev. B 70, 075101 (2004). [3] D. Heidarian & N. Trivedi, Phys. Rev. Lett. 93, 126401 (2004).