Control of a Two-Dimensional Electron Gas on ${\mathrm{SrTiO}}_3(111)$ by Atomic Oxygen

We report on the formation of a two-dimensional electron gas (2DEG) at the bare surface of (111) oriented ${\mathrm{SrTiO}}_3$. Angle resolved photoemission experiments reveal highly itinerant carriers with a sixfold symmetric Fermi surface and strongly anisotropic effective masses. The electronic structure of the 2DEG is in good agreement with self-consistent tight-binding supercell calculations that incorporate a confinement potential due to surface band bending. We further demonstrate that alternate exposure of the surface to ultraviolet light and atomic oxygen allows tuning of the carrier density and the complete suppression of the 2DEG.

Figure 1

(a) Schematic of a cubic perovskite unit cell of ${\mathrm{SrTiO}}_3$ with inequivalent (111) planes indicated in grey. (b) Top view of three consecutive ${\mathrm{Ti}}^{4+}$ (111) layers. Each of the three $t_{2g}$ orbitals are shown on the honeycomb lattice, formed by two consecutive layers (dark grey lines), to illustrate their rotational symmetry. The $xz$ orbital is shown in all three layers with large ($t_1$) and small ($t_2$) nearest neighbor hoppings indicated. (c) Sketch of the bulk FS of STO cut by the (001) plane. (d) The same bulk FS viewed down the [111] axis. A cut in the (111) plane through the $\mathrm{\Gamma }$ point is indicated by black lines to illustrate its different shape and size from the projection of the FS on the (111) surface plane.

Figure 2

(a) Constant energy surfaces of the STO(111) electronic structure measured at 108 eV with circularly polarized light. (b),(c) Energy—momentum dispersion along two high-symmetry directions (108 eV, $p$ polarization). A calculation (fit) of the bulk (2DEG) dispersion in the same direction is indicated by green (red) dashed lines, in (b). Inset of (c) is an energy distribution curve at $\mathrm{\Gamma }$, the red arrow indicates the peak due to the second subband.

Figure 3

Tight-binding supercell calculations of the electronic structure at the (111) surface of ${\mathrm{SrTiO}}_3$. (a) The calculated FS and orbital character superposed on the ARPES data. (b) The self-consistent band bending potential. One Ti layer corresponds to 2.25 Å. The square modulus of the wave functions of the lowest two subbands at $\mathrm{\Gamma }$ are plotted at their corresponding confinement energies. (c),(d) Calculated band dispersion along two high-symmetry directions, showing three confined 2DEG subbands and a “ladder” of states above ${\mathrm{E}}_F$ due to the finite size of the supercell.

Figure 4

Atomic oxygen treatment of the cleaved STO(111) surface. (a) Angle integrated photoemission spectra from a surface with a fully developed (suppressed) 2DEG after UV (atomic oxygen) [black (green)] exposure. (b) Occupied bandwidth of the 2DEG (blue) and valence band leading-edge midpoint (red) as a function of time exposed to UV radiation after atomic oxygen exposure which is indicated by arrows and marked by a break in the time axis. The dashed black lines are guides to the eye. Zero bandwidth indicates complete suppression of the 2DEG. (c) Dispersion plots in the $\overline{M^{\prime }}\text{−}\overline{\mathrm{\Gamma }}\text{−}\overline{M}$ direction from three stages of the 2DEG development as indicated in panel (b).