  In the quest for high temperature superconductors, the interface between a
metal and a dielectric was proposed to possibly achieve very high
superconducting transition temperature ($T_c$) through interface-assisted
pairing. Recently, in single layer FeSe (SLF) films grown on SrTiO$_3$
substrates, signs for $T_c$ up to 65~K have been reported. However, besides
doping electrons and imposing strain, whether and how the substrate facilitates
the superconductivity are still unclear. Here we report the growth of various
SLF films on thick BaTiO$_3$ films atop KTaO$_3$ substrates, with signs for
$T_c$ up to $75$~K, close to the liquid nitrogen boiling temperature. SLF of
similar doping and lattice is found to exhibit high $T_c$ only if it is on the
substrate, and its band structure strongly depends on the substrate. Our
results highlight the profound role of substrate on the high-$T_c$ in SLF, and
provide new clues for understanding its mechanism.
