Observation of a two-dimensional electron gas at the surface of annealed SrTiO${}_3$ single crystals by scanning tunneling spectroscopy

An extensive surface characterization of hydrofluoric acid (HF) etched and annealed SrTiO${}_3$ single crystals, vacuum-annealed below 300 ${}^{\circ }$C, reveals the formation of a two-dimensional electron gas (2DEG). A joint scanning tunneling spectroscopy and low-energy electron diffraction analysis allows us to associate the surface metallic state (characterized by the presence of a nonzero density of states close to the Fermi level) with the low-temperature-annealed highly ordered $1\times 1$ reconstructed SrTiO${}_3$ surface hosting two-dimensional carriers. Meanwhile, a gap opens in the tunneling spectrum of $2\times 1$ reconstructed, high-temperature-annealed surfaces. X-ray photoemission spectroscopy shows that the metallic state is associated with the surface formation of Ti${}^{3+}$. Recently published photoemission data demonstrated the formation of a 2DEG on the surface of cleaved SrTiO${}_3$, while scanning tunneling spectroscopy on crystals heated at high temperature revealed gaplike features: Our results can help reconcile this seemingly contradicting phenomenology observed so far by scanning tunneling spectroscopy and photoemission spectroscopy.

Figure 1

(a) Noncontact AFM topography (1 $\mu$m $\times$ 1 $\mu$m) on a stoichiometric STO crystal showing the terrace structure of the surface. (b) STM topography (400 nm $\times$ 400 nm) on Sample A (annealed at 250 ${}^{\circ }$C), and (c) height profile along the shown dashed line.

Figure 2

(a) LEED image on Sample A (annealed at 250 ${}^{\circ }$C) exhibits a $1\times$1 pattern; the same result is obtained on Sample B (annealed at 350 ${}^{\circ }$C). (b) On Sample C (annealed at 900 ${}^{\circ }$C), the LEED pattern shows a $2\times 1$ reconstruction of the surface.

Figure 3

(a) Tunneling $I$-$V$ (inset: magnification around $V=0$) and (b) $dI$/$dV$ recorded on Samples A, B, and C ($T_A$ $=$ annealing temperature). Tunneling curves were recorded at several locations on the surfaces forming a grid of point for each measured crystal, and the displayed curves are an average of all such single measurements. The tunneling conditions were $V_b$ $=-1.5$ V, $I=0.7$ nA for Samples A and B; $V_b$ $=-1.5$ V, $I=0.1$ nA for Sample C. (c) Tunneling DOS estimation for each measured curve using the normalization to the tunneling matrix coefficient for Sample A, as described in Ref. 46; (d) magnification of some DOS curves (vertically displaced for clarity) to highlight the spectroscopic structures described in the text. (e) and (f) Same DOS estimation, performed on Sample B.

Figure 4

XPS spectra on Samples A and C around the binding energy range of the Ti $2p$ doublet states.