  Aging, which manifests itself as an irreversible increase in electrical
resistance in island metal films is of considerable interest from both academic
as well as applications point of view. Aging is attributed to various causes,
oxidation of islands and mobility of islands followed by coalescence (mobility
coalescence) being the main contenders. The effect of parameters like substrate
temperature, substrate cleaning, residual gases in the vacuum chamber,
ultrasonic vibration of the substrate, suggest that the mobility coalescence is
responsible for the aging in island metal films. Electron microscopy studies
show evidence for mobility of islands at high substrate temperatures. The
comparison of aging data of island silver films deposited on glass substrates
in ultra high vacuum and high vacuum suggests that the oxidation of islands, as
being responsible for aging in these films, can be ruled out. Further, under
certain conditions of deposition, island silver films exhibit a dramatic and
drastic fall in electrical resistance, marking the insulator-metal transition.
This interesting transition observed in a conservative system - after the
stoppage of deposition of the film- is a clear evidence for mobility
coalescence of islands even at room temperature. The sudden fall in resistance
is preceded by fluctuations in resistance with time and fluctuations are
attributed to the making and breaking of the percolation path in the film.
