Posted: April 28, 2010

Electron-hole systems at oxide interfaces

(Nanowerk News) The interfaces of complex oxides can show unexpected characteristics distinct from the bulk materials. A targeted manipulation of these properties could yield electronics components with tailored functionalities. LMU materials scientist Dr. Rossitza Pentcheva and her team explored novel phenomena arising at the interface between the two insulators lanthanum aluminate (LaAlO3) and strontium titanate (SrTiO3). Recently, Mannhart and collaborators (U Augsburg) observed that beyond a critical thickness of the LaAlO3 layer the interface loses its insulating characteristics and becomes conductive.
“Within an international cooperation we were able to demonstrate that the effect can be manipulated by means of an additional SrTiO3 capping layer“, says Pentcheva. “Particularly fascinating is the formation of a bilayer consisting of mobile electrons at the interface and holes, the positively charged counter pieces of electrons, at the surface.“
Such excitonic or electron-hole systems only 1 to 2 nanometers apart could become key elements for novel electronics applications ("Parallel electron-hole bilayer conductivity from electronic interface reconstruction").
Transition metal oxides are extremely exciting materials, because their strongly correlated electrons lead to a manifold of properties. Some substances are high temperature superconductors that conduct electric current without any resistance. Others are magnetic or ferroelectric, the latter exhibiting a permanent electric dipole field. In some cases an external magnetic field can lead to an extreme change of the material’s electrical resistance, the so-called “colossal magnetoresistance”. Thus, transition metal oxides could lay the ground for spintronics devices, which utilize not only the electron’s charge, but also its spin to process information.
The growth of oxide based heterostructures with atomic precision has become possible only recently. The development of materials with tailored properties in the laboratory requires the variation of a large number of parameters. The interplay of different effects cannot always be disentangled based on the experimental data. Therefore, complex quantum mechanical simulations are vital to understanding the behaviour of a material on the atomic scale.
LMU scientist Dr. Rossitza Pentcheva and her team at the Department of Earth and Environmental Sciences explored the electronic phenomena in transition metal oxides by modelling the quantum mechanical interactions between the individual atoms at the interface between LaAlO3 und SrTiO3 layers. “We found a new and particularly effective parameter”, explains the materials scientist. “Along with colleagues at the University of California in Davis, USA, and the University of Twente in the Netherlands we demonstrated that an additional SrTiO3 capping layer triggers the formation of an electron gas at a significantly lower LaAlO3 film thickness.”
LaAlO3 is a polar material because it formally contains positively and negatively charged layers. The resulting charge difference between the polar LaAlO3 layers and the formally neutral SrTiO3 layers leads to the formation of a two-dimensional electron gas at the interface. Moreover, an additional layer of positively charged holes – the counterparts of the negatively charged electrons – forms at the system’s surface. Electron-hole-pairs are key elements in semiconductor electronics. They form, for instance, in solar cells, when light excites electrons to a higher energy level, leaving behind a positively charged hole.
Compared to semiconductors, however, in the current system the electron-hole sheets are only 1-2 nanometers apart. This opens new avenues to further miniaturizing electronics devices. “Within the research collaborative programme TRR80 ‘From electronic correlations to functionality’, funded by the German Science Foundation, we aim at a concerted search for material combinations with novel electronic properties”, says Pentcheva.
Source: Ludwig-Maximilians-Universität München