Thermal roughening of a thin film: A new type of roughening transition

The equilibrium thermal roughening of thin Ge layers (one and two monolayer s) deposited on Si(001) has been investigated with low-energy electron micr oscopy. A Ge-coverage-dependent roughening is observed. For two monolayers, the temperature at which imaging contrast is lost due to surface roughness is 900 +/- 25 degrees C. between the roughening temperatures of Ge(001) an d Si(001). Lower Ge coverages move this temperature closer to that of Si(00 1). The roughening is confined to the Ge overlayers. It is believed that th is phenomenon represents a new type of surface roughening transition that s hould be generally applicable for heteroepitaxial films.