  Sn1-xInxTe has been synthesized and studied recently as a candidate
topological superconductor. Its superconducting critical temperature increases
with Indium concentration. However, the role of Indium in altering the normal
state band structure and generating superconductivity is not well-understood.
Here, we explore this question in Sn1-xInxTe (0<x<0.3) thin films,
characterized by magneto-transport, infrared transmission and photoemission
spectroscopy measurement. We show that Indium is forming an impurity band below
the valence band edge which pins the Fermi energy and effectively generates
electron doping. An enhanced density-of-states due to this impurity band leads
to the enhancement of superconducting transition temperature measured in
multiple previous studies. The existence of the In impurity band and the role
of In as a resonant impurity should be more carefully considered when
discussing the topological nature of Sn1-xInxTe.
