SrTiO3(001)-(√5X√5)-R26.6° reconstruction:: A surface resulting from phase separation in a reducing environment

Scanning tunneling microscopy (STM) was used to produce atomic resolution images of the Nb-doped SrTiO3(001)-(root 5 x root 5)-R26.6 degrees reconstruction and its precursor surfaces. Low-energy electron diffraction was used to identify the surface structure in combination with the STM imaging. The (root 5x root 5)-R26.6 degrees reconstruction was observed growing as an adlayer on top of the underlying terraces. The formation of this structure could routinely be detected by an increase in the crystal's electrical conductivity during ultrahigh vacuum (UHV) annealing at 1300 degrees C. This rise in conductivity occurs due to significant reduction of the crystal. Scanning electron microscopy (SEM) on these samples showed small islands with a rectangular base separated by flat regions. Auger electron spectroscopy showed titanium enrichment, and strontium and oxygen depletion of the surface region. The islands seen in the SEM images are TiO crystals, as reported by S. B. Lee [Ultramicroscopy 104, 30 (2005)]. The flat regions between the islands are composed of a Sr adlayer on the surface that is consistent with the (root 5 x root 5)- R26.6 degrees adatom model of Kubo and Nazoye [Phys. Rev. Lett. 86, 1801 (2001)]. We propose that the TiO islands and the (root 5 x root 5)-R26.6 degrees Sr adlayer reconstruction form due to phase separation in the surface region of the crystal. This phase separation is driven by oxygen depletion which occurs during high-temperature UHV annealing of the sample.