  We show that reflection high-energy electron diffraction (RHEED) can be used
as a highly sensitive tool to track surface and resulting film stoichiometry in
adsorption-limited molecular beam epitaxy of (001) SrTiO3 thin films. Even
under growth conditions that yield films with a lattice parameter that is
identical to that of stoichiometric bulk crystals within the detection limit of
high-resolution x-ray diffraction (XRD), changes in surface reconstruction
occur from (1x1) to (2x1) to c(4x4) as the equivalent beam pressure of the Ti
metalorganic source is increased. These surface reconstructions are correlated
with a shift from mixed SrO/TiO2 termination to pure TiO2 termination. The
crossover to TiO2 surface termination is also apparent in a phase shift in
RHEED oscillations observed at the beginning of growth. Comparison with prior
results for carrier mobilities of doped films shows that the best films are
grown under conditions of a TiO2-saturated surface [c(4x4) reconstruction]
within the XRD growth window.
