Metal-to-insulator transition in LaAl1-xCrx O3/SrTiO3 oxide heterostructures guided by electronic reconstruction

Despite continuous efforts, a conceptual perception on the origin of quasi-two-dimensional electron gas (q-2DEG) in oxide heterostructures remains far from any generalization. Along this perspective, a systematic study identifying systems that exhibit heterointerface conductivity (LaAlO3/SrTiO3) and that do not (LaCrO3/SrTiO3) is undertaken. High quality LaAl1-xCrxO3 (0≤x≤1) films were fabricated on TiO2 terminated SrTiO3 (001) substrates using a pulsed laser deposition technique and its growth was monitored by reflection high energy electron diffraction (RHEED). The transport and photoemission spectroscopy experiments reveal that the the origin of heterointerface q-2DEG is associated with Ti3+ states and that it ceases to exist with increasing Cr ion concentration in the overlayer films. Following experimental evidences of the Cr concentration dependent metal-insulator electronic phase transition at the heterointerface, explanation is sought within the realms of a polar catastrophe model. Based on our transport and spectroscopy measurements, we propose that an electronic reconstruction drives the formation of q-2DEG for pristine LAO/STO and Al rich samples due to the formation of Ti3+/Ti4+ mixed valent state, while for heavily doped Cr samples, the required electron count necessary to solve the polar catastrophe instability are trapped in the LaAl1-xCrxO3 overlayers in the Cr 3d states. These trapped electrons in the overlayers are manifested in the form of Cr2+ ions.