Band alignment, built-in potential and the absence of conductivity at the LaCrO3/SrTiO3(001) heterojunction

In order to advance microelectronic technologies, scientists and engineers are on the lookout for new semiconductor materials that can be used to control the flow of electrons in nanoscale devices. One of the promising directions is to exploit interfaces of so-called non-polar and polar oxides. The former are composed of neutral atomic planes, while in the latter the atomic planes have alternating charge.

It has been demonstrated previously that, remarkably, interfaces of polar (LaAlO3) and non-polar (SrTiO3) oxide insulators can conduct electricity. This phenomenon has been attributed to build up of the electrostatic potential, which alters the electronic structure of materials near the interface and makes it conductive by means of the electronic reconstruction.

Researchers from the Pacific Northwest National Laboratory and UCL have investigated properties of a polar/non-polar interface formed by LaCrO3 film grown on SrTiO3 substrate (shown in the figure) and found that there is, indeed, build up of electrostatic potential in the film but there is no interface conductivity.

So why is it that the electronic reconstruction model is not working in this case? The answer is that the interface is never ideal and the polarity mismatch at the interface is partially alleviated by inter-diffusion of metal ions. In turn, this cation intermixing allows for the electron redistribution between the partially occupied d-shells of Cr³⁺ ions and empty d-shells of Ti⁴⁺ ions diffused into the LaCrO3 film.

Such electron redistribution - a mechanism not available in the LaAlO3/SrTiO3 system - creates localized electronic states, which explains the absence of electric conductivity at the interface. Moreover, it reduces the polarity mismatch and, consequently, suppresses further cation intermixing. These phenomena, not previously considered, may be important for understanding and ultimately harnessing for technological purposes the electronic structure of polar/non-polar interfaces.

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