Crucial Role of Metal Surface Morphology in Photon Emission from a Tunnel Junction at Ambient Conditions

In this paper, we
study light emission from a tunnel contact between
the Au film on a glass substrate and the Au-coated tungsten probe
of the scanning tunneling microscope at ambient conditions. We investigate
the dependence of the intensity of the collected optical signal on
the film surface morphology, namely, on geometrical parameters of
nanometer-scale gold grains (islands) constituting the film. We reveal
that the magnitude of inelastic tunnel current and thus photon emission
intensity increase both with the decrease of island height and the
increase of island width. We show that the difference in the optical
emission intensity could reach 4 orders of magnitude for poly- and
monocrystalline (atomically flat) samples. The observed phenomena
are explained with dependence of the effective area of a tunnel contact
on the grain aspect ratio. The obtained results demonstrate the crucial
role of the sample morphology in influencing the efficiency of photon
emission from the tunnel junction.