  The Fe-based superconductor Fe (Se,Te) combines non-trivial topology with
unconventional superconductivity and may be an ideal platform to realize exotic
states such as high-order topological corner modes and Majorana modes. Thin
films of Fe (Se,Te) are important for device fabrication, phase sensitive
transport measurements and for realizing proposals to engineer higher-order
modes. However, while bulk Fe (Se,Te) has been extensively studied with a
variety of techniques, the nature of the superconducting order parameter in the
monolayer limit has not yet been explored. In this work, we study monolayer
films of Fe (Se,Te) on Bi2Te3 with scanning tunneling spectroscopy and
Bogoliubov quasiparticle interference (BQPI). We discover that the monolayer Fe
(Se,Te)/Bi2Te3 heterostructures host a robust, multigap superconducting state
that strongly resembles the bulk. BQPI maps at the gap energies show a strong
spatial modulation, oriented 45 degrees to the Fe-Se bond direction. Analysis
of the phase-referenced quasiparticle interference signal reveals a
sign-changing s-wave order parameter similar to the bulk. Moreover, we observe
a unique pattern of sign changes in the BQPI signal which have not been
observed in the bulk. Our work establishes monolayer Fe (Se,Te)/Bi2Te3 as a
robust multi-band unconventional superconductor and sets the stage for
explorations of non-trivial topology in this highly-tunable system.
