  Reliable chemical state analysis of Sn semiconductors by XPS is hindered by
the marginal observed shift in the Sn 3d region. For hybrid Sn-based
perovskites especially, errors associated with charge referencing can easily
exceed chemistry-related shifts. Studies based on the modified Auger parameter
${\alpha}'$ provide a suitable alternative and have been used previously to
resolve different chemical states in Sn alloys and oxides. However, the
meaningful interpretation of Auger parameter variations on Sn-based perovskite
semiconductors requires fundamental studies. In this work, we perform a
comprehensive Auger parameter study through systematic compositional variations
of Sn halide perovskites. We find that in addition to the oxidation state,
${\alpha}'$ is highly sensitivity to the composition of the halide-site,
inducing shifts of up to ${\Delta}{\alpha}' = 2 eV$ between ASnI$_3$ and
ASnBr$_3$ type perovskites. The reported dependencies of ${\alpha}'$ on the Sn
oxidation state, coordination and local chemistry provide a framework that
enables reliable tracking of degradation as well as X-site interaction for
Sn-based perovskites and related compounds. The higher robustness and
sensitivity of such studies not only enables more in-depth surface analysis of
Sn-based perovskites than previously performed, but also increases
reproducibility across laboratories.
