Lanthanum is a known dopant in SrTiO 3, so it has been suggested that La from the LaAlO 3 mixes into the SrTiO 3 and dopes it n-type. Multiple studies have shown that intermixing takes place at the interface; however, it is not clear whether there is enough intermixing to provide all of the free carriers. For example, a flipped interface between a SrTiO 3 film and a LaAlO 3 substrate is insulating.