Controlling the interfacial conductance in ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ in ${90}^{\circ }$ off-axis sputter deposition

We report on the fabrication of conducting interfaces between ${\mathrm{LaAlO}}_3$ and ${\mathrm{SrTiO}}_3$ by ${90}^{\circ }$ off-axis sputtering in an Ar atmosphere. At a growth pressure of 0.04 mbar the interface is metallic, with a carrier density of the order of $1\times {10}^{13}{\mathrm{cm}}^{-2}$ at 3 K. By increasing the growth pressure, we observe an increase of the out-of-plane lattice constants of the ${\mathrm{LaAlO}}_3$ films while the in-plane lattice constants do not change. Also, the low-temperature sheet resistance increases with increasing growth pressure, leading to an insulating interface when the growth pressure reaches 0.10 mbar. We attribute the structural variations to an increase of the La/Al ratio, which also explains the transition from metallic behavior to insulating behavior of the interfaces. Our research shows that the control which is furnished by the Ar pressure makes sputtering as versatile a process as pulsed laser deposition, and emphasizes the key role of the cation stoichiometry of ${\mathrm{LaAlO}}_3$ in the formation of the conducting interface.

Figure 1

AFM images of samples (a) 004 and (b) 010, using color code for the height. Insets are the height profiles of the surfaces.

Figure 2

High-angle annular dark field STEM (HAADF-STEM) images of (a) sample 004 taken along the [110] direction and (b) sample 010 taken along the [100] direction. (c) STEM electron energy loss spectroscopy (STEM-EELS) analysis of samples 004 and 010, the La-$M_{4,5}$ (solid circles) and Ti-$L_{2,3}$ (open circles) edges integrated unit cell by unit cell across the interface.

Figure 3

Reciprocal space maps (RSM) around the STO (103) diffraction peak of samples (a) 004 and (b) 010. (c) The $\theta \text{−}2\theta$ scans for samples grown at various Ar pressures. The dashed lines are the LAO (002) diffraction peaks.

Figure 4

Temperature dependence of (a) the sheet resistance ($R_{\text{s}}$) and (b) the carrier density ($n_{\text{s}}$) and the Hall mobility (${\mu }_{\text{H}}$) for samples grown at various Ar pressures.