Effect of stoichiometry on the LaAlO$_{3}$/SrTiO$_{3}$ 2-D electron gas grown by MBE

We find that through careful control of the stoichiometry in molecular-beam epitaxy grown LaAlO$_{3}$/SrTiO$_{3}$ samples, a 2-dimensional electron gas occurs at the interface between the two insulating oxides as extensively reported in samples grown by pulsed-laser deposition. Our results eliminate many extrinsic effects suggested as possible mechanisms of conductivity and are consistent with the polar catastrophe mechanism being responsible for the conductivity in our MBE-grown samples. We further show that the cation stoichiometry of the LaAlO$_{3}$ layer is key to the existence of the 2-dimensional electron gas and that a La/Al ratio less than or equal to 0.97 $\pm $ 0.03 is a necessary condition to obtain a conducting interface in this system.