Effect of oxygen vacancies in the ${\mathrm{SrTiO}}_3$ substrate on the electrical properties of the ${\mathrm{LaAlO}}_3∕{\mathrm{SrTiO}}_3$ interface

We experimentally investigated optical, electrical, and microstructural properties of heterointerfaces between two thin-film perovskite insulating materials, ${\mathrm{SrTiO}}_3$ (STO) and ${\mathrm{LaAlO}}_3$ (LAO), deposited at different oxygen pressure conditions. Cathode and photoluminescence experiments show that oxygen vacancies are formed in the bulk STO substrate during the growth of LAO films, resulting in high electrical conductivity and mobility values. In both high and low oxygen pressure interfaces, the electrical Hall mobilities follow a similar power-law dependence as observed in oxygen reduced STO bulk samples. The results are confirmed on a microscopic level by local strain fields at the interface reaching $10\mathrm{nm}$ into the STO substrate.

Figure 1

(Color) Cathode and photoluminescence from various STO substrates and LAO/STO systems: (a) STO substrate annealed at ${10}^{-6}\mathrm{mbar}{\mathrm{O}}_2$, $800°\mathrm{C}$, and (b) after deposition of LAO film in the same conditions. (c) Substrate annealed at ${10}^{-4}\mathrm{mbar}{\mathrm{O}}_2$, $800°\mathrm{C}$ and (d) after LAO deposition. (e) Ar-ion bombarded STO single-crystal substrate. (f) Cathode luminescence from an as-received LAO single-crystal substrate. (g) Photoluminescence measurements of LAO/STO films grown under low oxygen pressure conditions and of an Ar-ion bombarded STO substrate.

Figure 2

(Color) Cathode luminescence from LAO/STO heterointerfaces: (a) as deposited at ${10}^{-6}\mathrm{mbar}{\mathrm{O}}_2$, (b) after re-oxygenation, (c) after vacuum annealing $\left({10}^{-7}\mathrm{mbar}\right)$ at $600°\mathrm{C}$, and (d) after ${\mathrm{H}}_2∕\mathrm{Ar}$ annealing.

Figure 3

(Color online) Sheet resistivity $R_{XX}$, charge-carrier density $n_S$, and Hall mobility ${\mu }_H$ for STO substrates annealed in deposition conditions (${10}^{-6}\mathrm{mbar}{\mathrm{O}}_2$, $800°\mathrm{C}$) and Ar-ion bombarded ($U=300\mathrm{eV}$, $J=0.2\mathrm{mA}∕{\mathrm{cm}}^2$) (open and solid circles); LAO/STO heterointerfaces prepared at low oxygen pressure $\left({10}^{-6}\mathrm{mbar}\right)$ as well as annealed at $500\mathrm{mbar}$ during cooling (open and solid diamonds); LAO/STO heterointerfaces prepared at high oxygen pressure $\left({10}^{-4}\mathrm{mbar}\right)$ and annealed at $500\mathrm{mbar}$ during cooling (open and solid triangles).

Figure 4

TEM cross-section micrographs of the LAO/STO interface for a film deposited at an oxygen pressure of ${10}^{-6}\mathrm{mbar}$. (a) U-shaped dark contrast is marked with arrows. (b) High-resolution electron microscope image showing details of the LAO/STO interface between misfit dislocations. (c) A fast Fourier-transform filtered image of the coherent interface between the dislocations.