Origin of Metal-Insulator Transition at the LaAlO$_{3}$/SrTiO$_{3}$ interface induced by ion beam irradiation

A quasi-two-dimensional electron gas appears when 4 or more unit cells of the LaAlO$_{3}$ film are deposited on a top of the TiO$_{2}$-terminated STO substrate. We show that it is possible to make the interface insulating using low-energy Ar$+$ ion-beam irradiation. The low energy Ar$+$ ions do neither etch the film below critical thickness nor create oxygen vacancies if the etching is terminated in time. The conductivity can be completely recovered by annealing under low oxygen pressure conditions. The restored interface shows strikingly similar electrical properties to the non-irradiated one and is also resistant to annealing at high oxygen pressure. High resolution transmission electron microscopy revealed that the difference between conducting and non-conducting interfaces is related to a change in stoichiometry of the LAO film where the La/Al ratio is 1.2 in the irradiated non-conducting areas while it is 1.0 in the conducting areas.