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Abstract
An extensive surface characterization of hydrofluoric acid (HF) etched and annealed SrTiO${}_{3}$ single crystals, vacuum-annealed below 300 ${}^{\ensuremath{\circ}}$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 $1\ifmmode\times\else\texttimes\fi{}1$ reconstructed SrTiO${}_{3}$ surface hosting two-dimensional carriers. Meanwhile, a gap opens in the tunneling spectrum of $2\ifmmode\times\else\texttimes\fi{}1$ 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.
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