  The evidence for proximity-induced superconductivity in heterostructures of
topological insulators and high-Tc cuprates has been intensely debated. We use
molecular beam epitaxy to grow thin films of topological insulator Bi2Te3 on a
cuprate Bi2Sr2CaCu2O8+x, and study the surface of Bi2Te3 using low-temperature
scanning tunneling microscopy and spectroscopy. In few unit-cell thick Bi2Te3
films, we find a V-shaped gap-like feature at the Fermi energy in dI/dV
spectra. By reducing the coverage of Bi2Te3 films to create nanoscale islands,
we discover that this spectral feature dramatically evolves into a much larger
hard gap, which can be understood as a Coulomb blockade gap. This conclusion is
supported by the evolution of dI/dV spectra with the lateral size of Bi2Te3
islands, as well as by topographic measurements that show an additional barrier
separating Bi2Te3 and Bi2Sr2CaCu2O8+x. We conclude that the prominent gap-like
feature in dI/dV spectra in Bi2Te3 films is not a proximity-induced
superconducting gap. Instead, it can be explained by Coulomb blockade effects,
which take into account additional resistive and capacitive coupling at the
interface. Our experiments provide a fresh insight into the tunneling
measurements of complex heterostructures with buried interfaces.
