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Recently a metallic state was discovered at the interface between insulatingoxides, most notably LaAlO3 and SrTiO3. Properties of this two-dimensionalelectron gas (2DEG) have attracted significant interest due to its potentialapplications in nanoelectronics. Control over this carrier density and mobilityof the 2DEG is essential for applications of these novel systems, and may beachieved by epitaxial strain. However, despite the rich nature of straineffects on oxide materials properties, such as ferroelectricity, magnetism, andsuperconductivity, the relationship between the strain and electricalproperties of the 2DEG at the LaAlO3/SrTiO3 heterointerface remains largelyunexplored. Here, we use different lattice constant single crystal substratesto produce LaAlO3/SrTiO3 interfaces with controlled levels of biaxial epitaxialstrain. We have found that tensile strained SrTiO3 destroys the conducting2DEG, while compressively strained SrTiO3 retains the 2DEG, but with a carrierconcentration reduced in comparison to the unstrained LaAlO3/SrTiO3 interface.We have also found that the critical LaAlO3 overlayer thickness for 2DEGformation increases with SrTiO3 compressive strain. Our first-principlescalculations suggest that a strain-induced electric polarization in the SrTiO3layer is responsible for this behavior. It is directed away from the interfaceand hence creates a negative polarization charge opposing that of the polarLaAlO3 layer. This both increases the critical thickness of the LaAlO3 layer,and reduces carrier concentration above the critical thickness, in agreementwith our experimental results. Our findings suggest that epitaxial strain canbe used to tailor 2DEGs properties of the LaAlO3/SrTiO3 heterointerface.

Tailoring a two-dimensional electron gas at the LaAlO3/SrTiO3 (001) interface by epitaxial strain