  The interplay between substrates and superconducting thin films has attracted
increasing attention. Here, we report an in-depth investigation on
superconducting properties of the epitaxial TiN thin films grown on two
different substrates by dc reactive magnetron sputtering. The TiN films grown
on (0001) sapphire exhibit (111) crystal orientation, while that grown on (100)
Si substrates exhibit (100) orientation. Moreover, the samples grown on Si
reveal a relatively lower level of disorder, accompanied by the higher critical
transition temperature $T_c$ and smaller magnitude of upper critical field
slope near $T_c$. Remarkably, we uncovered a rather high value of
superconducting gap (with $\Delta_0/k_BT_c$ = 3.05) in TiN film on Si
indicating a very strong coupling superconductivity, in sharp contrast to the
case using sapphires as the substrate which reveals a weak-coupling feature.
Further analysis shows that the weakened electronic screening effect due to the
high level of disorder and the suppressed electronic density of states may be
the underlying reasons for the occurrence of weak coupling superconductivity in
the TiN films based on sapphire substrate.
