Modulation doping of a Mott quantum well by a proximate polar discontinuity

We present evidence for hole injection into ${\text{LaAlO}}_3/{\text{LaVO}}_3/{\text{LaAlO}}_3$ quantum wells near a polar surface of ${\text{LaAlO}}_3$ (001). As the surface is brought in proximity to the ${\text{LaVO}}_3$ layer, an exponential drop in resistance and a decreasing positive Seebeck coefficient are observed below a characteristic coupling length of 10–15 unit cells. We attribute this behavior to a crossover from an atomic reconstruction of the ${\text{AlO}}_2$-terminated ${\text{LaAlO}}_3$ surface to an electronic reconstruction of the vanadium valence. These results suggest a general approach to tunable hole doping in oxide thin-film heterostructures.

Figure 1

(Color online) (a) Schematic band and crystal structure of $\text{LAO}\left(n\right)/\text{LVO}\left(3\right)$ quantum wells grown on ${\text{AlO}}_2$-terminated ${\text{LaAlO}}_3$ (001) substrates. Filled and empty bands correspond to the valence and conduction bands, respectively, and $\text{O}2p$ bands are assumed to be aligned. (b) Typical reflection high-energy electron-diffraction (RHEED) oscillations during growth of LAO(8)/LVO(3). (c) Atomic force microscopy (AFM) image of LAO(50)/LVO(3) showing a clear step and terrace surface with step height of $\sim 0.4\text{nm}$.

Figure 2

(Color online) (a) Sheet resistance and (b) activation energy of $\text{LAO}\left(n\right)/\text{LVO}\left(3\right)$ as a function of ${\text{LaAlO}}_3$ cap thickness $n$. The curve is a guide for the eyes. Inset of (b) shows Arrhenius plots of the sheet resistance.

Figure 3

(Color online) Schematic diagrams of possible reconstruction processes. (a) Without reconstruction the structure is composed of negatively and positively charged $\left(\rho \right)$ layers from the surface, which induces a non-negative electric field $\left(E\right)$, leading to a divergence in potential $\left(-V\right)$. (b) Electronic reconstruction at the ${\text{LaVO}}_3$ quantum well layer with a net half hole per 2D unit cell induced. The potential divergence is canceled, but a finite shift $\Delta \left(n\right)$ in potential remains. (c) Atomic reconstruction of the ${\text{LaAlO}}_3$ polar surface.

Figure 4

(Color online) Schematic reconstructions of the top and bottom polar surfaces of (001)-oriented ${\text{LaAlO}}_3$ for two different terminations of the bottom surface.

Figure 5

(Color online) (a) Thermoelectric voltage measured as a function of applied temperature difference between two electrodes on $\text{LAO}\left(n\right)/\text{LVO}\left(3\right)$ quantum wells at room temperature. (b) Seebeck coefficient of $\text{LAO}\left(n\right)/\text{LVO}\left(3\right)$ as a function of $n$ (filled circles), plotted in comparison with bulk values for ${\text{La}}_{1-x}{\text{Sr}}_x{\text{VO}}_3$ (Refs. 24, 25, 26) and ${\text{La}}_{1-x}{\text{Ca}}_x{\text{VO}}_3$ (Ref. 27) at 300 K.

Figure 6

(Color online) (a) Sheet resistance and (b) activation energy of $\text{LAO}\left(3\right)/\text{LVO}\left(m\right)$ as functions of ${\text{LaVO}}_3$ layer thickness $m$. Inset of (a) shows Arrhenius plots of the sheet resistance. The curve is a guide for the eyes.

Figure 7

(Color online) Schematic reconstructions of the polar surface of (001)-oriented ${\text{LaAlO}}_3$ and the ${\text{LaAlO}}_3/{\text{SrTiO}}_3$ interface. (a) No reconstruction, (b) only the ${\text{LaAlO}}_3$ surface is reconstructed, (c) only the ${\text{LaAlO}}_3/{\text{SrTiO}}_3$ interface is reconstructed, and (d) both are reconstructed.