Quasi 2D Ultrahigh Carrier Density in a Complex Oxide BrokenGap Heterojunction

DOI: 10.1002/admi.201500432 however, only a few oxides accommodating high electron density confined to low dimensions.[12,17–21] Filling this materials gap would provide qualitatively new opportunities in the field of nanoscale oxide electronic devices including novel plasmonic and high charge-gain devices. NdTiO3 (NTO) and SrTiO3 (STO) are classified as a Mott and a band insulator, respectively. Moreover, the interface formed between polar NTO and nonpolar STO is metallic.[22] NTO/STO is thus expected to exhibit a 2DEG equivalent to 0.5 electron (e−) per lateral unit cell per interface. Here, we report the discovery of a quasi-2D ultrahigh-density electron gas exceeding 3 × 1015 cm−2, which is tunable by means of the NTO film thickness in molecular beam epitaxy (MBE)-grown heterostructures. This novel quasi-2DEG results from a brokengap band alignment[23,24] at the NTO/STO interface, which we have experimentally detected and theoretically verified. We note that such band alignments, which also form the basis for a lowpower tunnel FETs, are rare.[1] They have been positively identified in traditional III–V semiconductor heterostructures[24] and 2D crystals,[25] and have only indirectly been associated with oxide heterostructures.[26–32]