  Using the superconducting proximity effect for engineering a topological
superconducting state in a topological insulator (TI) is a promising route to
realize Majorana fermions. However, epitaxial growth of a superconductor on the
TI surface to achieve a good proximity effect has been a challenge. We
discovered that simply depositing Pd on thin films of the TI material
(Bi$_{1-x}$Sb$_x$)$_2$Te$_3$ leads to an epitaxial self-formation of PdTe$_2$
superconductor having the superconducting transition temperature of ~1 K. This
self-formed superconductor proximitizes the TI, which is confirmed by the
appearance of a supercurrent in Josephson-junction devices made on
(Bi$_{1-x}$Sb$_x$)$_2$Te$_3$. This self-epitaxy phenomenon can be conveniently
used for fabricating TI-based superconducting nanodevices to address the
superconducting proximity effect in TIs.
