GROWTH OF C-60 THIN-FILMS ON GES(001) STUDIED BY SCANNING FORCE MICROSCOPY

The epitaxial growth of C-60 thin films sublimed onto GeS(001) substra tes under ultrahigh-vacuum conditions was investigated in the range of 0.7-11 monolayers coverage by scanning force microscopy in air. The g rowth process follows basically a layer-by-layer mechanism if evaporat ed in the narrow temperature range from 180 to 200 degrees C, but was found to be very sensitive to even small changes in temperature. At su bstrate temperatures above 200 degrees C, the second and higher layers are not stable. For temperatures close to 200 degrees C, our studies indicate pure layer-by-layer-type growth. Somewhat fewer substrate tem peratures lead to the formation of two different types of triangular-s haped islands which originate from a different stacking of the molecul es, beginning with the second layer. Triangular islands belonging to t he same type tend to coalesce, whereas islands of different types are separated by grooves. At substrate temperatures close to 180 degrees C during evaporation, dendritic islands are formed due to the limited m obility of the C-60 molecules at this temperature. Furthermore, the gr owth mode changes to a nonideal layer-by-layer growth where the second and even higher layers start to grow before the first layer has been completed. This behavior can well be explained by a two-dimensional di ffusion-limited growth mechanism. Finally, substrate temperatures sign ificantly lower than 180 degrees C result in films without long-range order.