Nonlinear Insulator in Complex Oxides

For complex oxides, the very concept of an insulator must be re-examined as they behave differently from conventional insulators such as SiO$_{2}$ due to the presence of multiple defect levels within bandgap. As the semiconductor industry is moving to such oxides for high-$k$ materials, we need to truly understand the insulating properties of them under various electrical excitations. We report a class of material which we coin as nonlinear insulators that exhibit reversible electric-field-induced metal-insulator transitions (MIT). We show this behaviour for an insulating LaAlO$_{3 }$thin film with a large bandgap of $\sim $5.6 eV in a metal/LaAlO$_{3}$/Nb-SrTiO$_{3}$ heterostructure. The reversible MIT is attributed to the formation of a quasi-conduction band (QCB) in the defect states of LaAlO$_{3}$ that forms a continuum state with the conduction band of the Nb-SrTiO$_{3}$. An opposing voltage is required to deplete the charges from the QCB. The implications of these nonlinear insulators are far-reaching. For example, the use of multi-component oxides as insulators in devices (e.g., high-$k$ dielectrics in silicon CMOS devices) must be exercised with caution.