  The bound state of a He-3 atom at the interface between coexisting
crystalline and superfluid phases of He-4 is studied theoretically by means of
first principle Quantum Monte Carlo simulations. We consider both the case of a
solid/superfluid He-4 interface, as well as that of a few solid layers of He-4
forming on an attractive substrate. It is observed that the He-3 bound state is
sharply localized in a well-defined, quasi-2D layer of He-4 in the region
intermediate between the solid and the liquid. The layer in which the He-3 atom
resides displays an intriguing interplay of atomic localization and
quantum-mechanical exchanges; is not necessarily the first "non-solid" layer,
as suggested in previous studies, and it is affected by the attractive strength
of the substrate.
