Quasi 2D Ultrahigh Carrier Density in a Complex Oxide Broken-Gap Heterojunction

Two-dimensional (2D) ultra-high carrier densities are of considerable current research interest for novel plasmonic and high charge-gain devices. However, the highest 2D electron density obtained is thus far limited to 3 x 10(14) cm(-2) (1/2 electron/unit cell/interface) at GdTiO3/SrTiO3 interfaces, and is typically an order of magnitude lower at LaAlO3/SrTiO3 interfaces. We show from experiment and modeling that carrier densities much higher than expected based on resolution of the polar discontinuity at perovskite oxide heterojunctions can be achieved via band engineering. The SrTiO3 (8 u.c.)/NdTiO3 (t u.c) /SrTiO3 (8 u.c.) /LSAT(001) heterostructure shows the expected electronic reconstruction behavior starting at t = 2 u.c., but then exhibits a higher carrier density regime at t >= 6 u.c. due to additional charge transfer from band alignment.