  We report on the selective fabrication of high-quality Sr$_2$IrO$_4$ and
SrIrO$_3$ epitaxial thin films from a single polycrystalline Sr$_2$IrO$_4$
target by pulsed laser deposition. Using a combination of X-ray diffraction and
photoemission spectroscopy characterizations, we discover that within a
relatively narrow range of substrate temperature, the oxygen partial pressure
plays a critical role in the cation stoichiometric ratio of the films, and
triggers the stabilization of different Ruddlesden-Popper (RP) phases. Resonant
X-ray absorption spectroscopy measurements taken at the Ir $L$-edge and the O
$K$-edge demonstrate the presence of strong spin-orbit coupling, and reveal the
electronic and orbital structures of both compounds. These results suggest that
in addition to the conventional thermodynamics consideration, higher members of
the Sr$_{n+1}$Ir$_n$O$_{3n+1}$ series can possibly be achieved by kinetic
control away from the thermodynamic limit. These findings offer a new approach
to the synthesis of ultra-thin films of the RP series of iridates and can be
extended to other complex oxides with layered structure.
