DISSOLUTION AND SEGREGATION OF MONOLAYER CU, NI AND CO ATOMS ON THE SI(111)-ROOT-3X-ROOT-3-AG SURFACE-INDUCED BY THERMAL ANNEALING

Concentration changes of monolayer Cu, Ni, and Co atoms on the Si(111) -root 3x root 3-Ag surface by isochronal annealing at temperatures fro m 150 to 700 degrees C have been studied by means of LEED-AES-RBS tech niques. It is shown that Cu atoms on the Si(111)-root 3 x root 3 Ag su rface dissolve into the Si bulk at a temperature of 250 degrees C, and segregate back to the surface when Ag atoms decay from the surface on annealing at temperatures higher than 400 degrees C. It is also shown that Ni atoms, which have once dissolved into the bulk, segregate bac k to the surface on annealing above 400 degrees C, In the case of the Co/Si(111)-root 3x root 3-Ag surface, Co atoms preferentially dissolve into the bulk, a situation similar to that of Cu and Ni atoms on the Si(111)-root 3x root 3-Ag surface; however, no segregation of Co atoms has been observed upon annealing at higher temperatures. These result s are discussed in terms of the heats of mixing between the Ag atoms a nd the co-adsorbates and also between the co-adsorbates and the Si sub strate.