Giant Conductivity Switching of LaAlO3/SrTiO3 Heterointerfaces Governed by Surface Protonation
Of surfaces and interfaces: conductivity in LAO/STO
In their latest article, Jeremy Levy and his group put forward compelling experimental evidence of the role of surface chemistry on conductivity in complex oxides heterostructures.
A typical complex oxides heterostructure consists of lanthanum aluminate (LaAlO3, LAO) grown on strontium titanate (SrTiO3, STO). When 4 unit cells (u.c.) of LAO are grown on STO, a two-dimensional electron gas (2DEG)--a conductive layer--is formed at the interface between the two oxides, which are both insulating materials. The small distance between the interface and the surface of LAO raises the question of the influence of surface chemistry on the properties of the 2DEG, specifically on its conductivity.
In this study, PQI Professor Jeremy Levy and his group measure the change in the interfacial sheet resistance R of the heterostructure after immersing and drying a 4 u.c. sample of LAO/STO and after illumination with UV light.
Their results (Fig. 1) show that R increases upon immersion in water (as well as in other Lewis basic solvents) and decreases upon illumination, as the sample switches from a conductive state to an insulating state and back to a conductive state in a process called “conductive switching”.
The measurements then show that shining light of wavelength λ<450 nm restores the conductivity; the renewed flow of electrons arises from the optical transitions of oxygen vacancies in STO at 400 <λ<460 nm. The presence of ambient humidity accelerated the upward drift in R, suggesting that water is playing a role in the light-driven process.
The team therefore postulated a surface protonation-based mechanism for the role of solvent immersion and exposure to light in LAO/STO interfaces, where the conductive switching is due to the surface deprotonation of LAO with release of an e- into the environment, followed by the excitation of the STO Oxygen vacancies with splitting of the electron-hole pair. The hole can accept an electron from an adsorbed water molecule, thus creating another proton to complete the cycle (Fig. 4).
This work, published in Nature Communications on February 10, 2016 illustrates how the conductivity of the 2DEG layer in LAO/STO can be modulated using solvent immersion and illumination.