  The structural and electronic properties of thermally reduced SrTiO3(100)
single crystals have been investigated using a probe with real- and
reciprocal-space sensitivity: a synchrotron radiation microsopic setup which
offers the possibility of Scanning Photoemission Microscopy and Angle Resolved
Photoelectron Spectroscopy (ARPES) down to the nanometric scale. We have
spectroscopically imaged the chemical composition of samples which present
reproducible and suitable low-energy electron diffraction patterns after
following well-established thermal reduction protocols. At the micrometric
scale, Ca-rich areas have been directly imaged using high-energy resolution
core level photoemission. Moreover, we have monitored the effect of Ca
segregation on different features of the SrTiO3(100) electronic band structure,
measuring ARPES inside, outside and at the interface of surface inhomogeneities
with the identified Ca-rich areas. In particular, the interaction of Ca with
the well-known intragap localized state, previously attributed to oxygen
vacancies, has been investigated. Moreover, the combination of direct imaging
and spectroscopic techniques with high spatial resolution has clarified the
long-standing dilemma related to the bulk or surface character of Ca
segregation in SrTiO3. Our results present solid evidence that the penetration
depth of Ca segregation is very small. In contrast to what has been previously
proposed, the origin of long-range surface reconstructions can unlikely be
associated to Ca due to strong local variations of its surface concentration.
