Internal Charge Transfer and Quasi-Two Dimensional Electron Gases at NdTiO$_{3}$/SrTiO$_{3}$ interfaces

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 3x10$^{14}$ cm$^{-2}$ (\textonehalf electron/unit cell/interface) at GdTiO$_{3}$/SrTiO$_{3}$ interfaces, and is typically an order of magnitude lower at LaAlO$_{3}$/SrTiO$_{3}$ interfaces. In this work, we will present detailed study that carrier densities much higher than expected based on resolution of the polar discontinuity at perovskite oxide heterojunctions can be achieved via band engineering and internal charge$^{\, [1]}$. The SrTiO$_{3}$(8 u.c.)/NdTiO$_{3}$(2 u.c)/SrTiO$_{3}$(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 $\ge $ 6 u.c. due to additional charge transfer from band alignment. Combining DFT modeling and experiments using x-ray photoelectron spectroscopy, scanning transmission electron microscopy, electron energy loss spectroscopy, energy dispersive x-ray spectroscopy and electronic transport measurements, we will discuss the origin of these carriers, dimensionality and transport mechanisms. [1] Peng Xu, et al. Advanced Material Interface (2015), in press.