  Measurements of conductance spectra in a superconductor - topological
insulator - normal metal thin film junctions of NbN-$\rm Bi_2Se_3$-Au are
reported. Junctions with ex-situ and in-situ prepared $\rm NbN-Bi_2Se_3$
interfaces were studied. At low temperatures, all the ex-situ junctions showed
coherence peaks in their conductance spectra, but imbedded robust zero bias
conductance peaks were observed only in junctions with a metallic or a metal to
insulator transition below $\rm T_c$ of the NbN electrode. The in-situ
junctions which had about two orders of magnitude lower resistance at low
temperatures, generally showed flat conductance spectra at low bias, with no
coherence or broad Andreev peaks, since the critical current of the NbN
electrode was reached first, at voltage bias below the energy gap of the
superconductor. A weak zero bias conductance peak however, was observed in one
of these junctions. We conclude that significant tunneling barriers, as in the
ex-situ prepared junctions, are essential for the observation of coherence
peaks and the zero energy bound states. The later seem to originate in the $\rm
Bi_2Se_3$-NbN interface, as they are absent in reference Au-NbN junctions
without the topological layer sandwiched in between.
