STM study of the surface morphology of gold on mica

The scanning tunneling microscope (STM) promises to be a useful tool for the study of thin film deposition processes. We have imaged the growth surface of vapor-deposited gold on mica with an STM. Monatomic steps, dislocations, grain boundaries and grain topographies have been mapped for various gold thicknesses and deposition temperatures. Complementary X-ray diffraction and transmission electron microscopy experiments have been used to determine the orientation, size, shape and defectiveness of the crystallites. Near room temperature, the nuclei do not fuse together into larger single crystallites as the film coalesces, and the mounded topography of the individual crystallites evolves only slightly as the film thickens. The mounds are about 500 Å across and 75 Å high. At higher temperatures (150°C to 300°C), the nuclei apparently fuse as the film coalesces, giving larger crystallites containing holes. The dislocations observed by STM are apparently created as these holes fill in, due to frequent misregistry of the gold lattice around the holes. In the latter stages of deposition, there is a temperature-dependent flattening of the irregular topography caused by the island growth mode, which we attribute to surface diffusion. Consequently, starting the deposition at lower temperatures and increasing the temperature during the deposition gives very flat surfaces. In contrast, depositing gold under the same conditions on a thin underlayer of silver results in large islands separated by grooves, presumably due to extensive fusion of the silver nuclei into larger crystallites before a complete film is formed.