The nucleation and growth behavior of two very different systems are s
tudied by low-energy electron microscopy (LEEM) and scanning tunneling
microscopy (STM). The first is a nonreactive system of Pb growth on t
he Si(100) surface at room temperature and the second is a reactive sy
stem in which nitride layers are formed on Si(111) during reaction wit
h NH3 at high temperatures. In both cases, the mobilities of the diffu
sing species have a strong influence on the resulting growth morpholog
ies.