Parallel Electron-Hole Bilayer Conductivity from Electronic Interface Reconstruction

The perovskite ${\mathrm{SrTiO}}_3\mathrm{\text{−}}{\mathrm{LaAlO}}_3$ structure has advanced to a model system to investigate the rich electronic phenomena arising at polar oxide interfaces. Using first principles calculations and transport measurements we demonstrate that an additional ${\mathrm{SrTiO}}_3$ capping layer prevents atomic reconstruction at the ${\mathrm{LaAlO}}_3$ surface and triggers the electronic reconstruction at a significantly lower ${\mathrm{LaAlO}}_3$ film thickness than for the uncapped systems. Combined theoretical and experimental evidence (from magnetotransport and ultraviolet photoelectron spectroscopy) suggests two spatially separated sheets with electron and hole carriers, that are as close as 1 nm.

Figure 1

(a) Layer-resolved density of states (DOS) of $\mathrm{STO}(001)/2\mathrm{LAO}$ (dotted line), and $\mathrm{STO}(001)/2\mathrm{LAO}/1\mathrm{STO}$ (black line, colored area) aligned at the bottom of the Ti $3d$ band at the interface. (b) Influence of $m=0–2$ STO capping layers on the band structure of $\mathrm{STO}(001)/2\mathrm{LAO}/m\mathrm{STO}$ The band gap closes due to overlap between surface O $2p$ states (black circles) and interface Ti $3d$ states (blue circles). (c) The electron density for the $m=2$ system, integrated between $-0.6$ and 0.0 eV, shows electrons in the Ti $3d_{xy}$ orbitals at the interface (top) and holes in the O $2p_{\pi }$ bands at the surface (bottom).

Figure 2

(a) Sheet resistance as a function of temperature for $\mathrm{STO}(001)/2\mathrm{LAO}/1\mathrm{STO}$ (red circles), $\mathrm{STO}(001)/2\mathrm{LAO}/2\mathrm{STO}$ (blue triangles), and $\mathrm{STO}(001)/2\mathrm{LAO}/10\mathrm{STO}$ (black squares). All samples are grown at $2\times {10}^{-3}\mathrm{mbar}$ of oxygen. Inset: $dR/dT$ as function of temperature with different linear fits below and above 100 K. (b) Sheet resistance at room temperature of $\mathrm{STO}(001)/n\mathrm{LAO}/10\mathrm{STO}$ samples for varying $n$ LAO thickness and a fixed number of 10 unit cells of STO capping. Red squares/blue circles correspond to oxygen pressure of $2\times {10}^{-3}\mathrm{mbar}$/$3\times {10}^{-5}\mathrm{mbar}$ during growth.

Figure 3

(a) UPS spectra of a $\mathrm{STO}(001)/2\mathrm{LAO}/1\mathrm{STO}$ sample taken at 80 K in situ after growth under various detector angles (the inset shows how the angle towards the detector is defined). A gradual filling of the valence band towards the Fermi energy is found in the surface-sensitive lower angle spectra. (b) STS conductance $(dI/dV)/(I/V)$ at 300 K for different tip-sample distances (current set-point, respectively, 1.5, 2.0, and 4.0 nA at a bias voltage of $-1.5\mathrm{V}$). Right inset: $I/V$ curve; left inset shows an STM image recorded at $-1.0\mathrm{V}$ and 300 K.

Figure 4

(a) Sheet resistance and (b) Hall coefficient ($R_H/H$) as a function of magnetic field at different temperatures for a $\mathrm{STO}(001)/2\mathrm{LAO}/1\mathrm{STO}$ sample. (c) Sheet carrier densities and (d) mobilities as obtained from a two-band fit to the magnetoresistance and Hall data at each temperature.