Observations of reversible and irreversible structural transitions of cobalt on Si(111) with LEEM

We present real time images of the evolution of the structure of the Si (11 1) surface during the deposition of cobalt at elevated temperatures, acquir ed using low-energy electron microscopy. The system follows a sequence of c oexisting ordered and disordered phases, consistent with two-dimensional eu tectic behavior. Reversible temperature driven transitions are observed bet ween the ordered Si (111)-(7 x 7) reconstructed phase and a disordered latt ice gas of Co-containing ring-clusters (RC), indicating a local equilibrium between these structures. Only irreversible temperature driven transitions from an ordered (root 7 x root 7)-RC phase to the disordered phase are obs erved. The nucleation and growth of stable islands, mostly CoSi2, depletes the Go-rich (root 7 x root 7) structure of Co, resulting in the formation a nd growth of adjacent regions of the Co-poor disordered "(1 x 1)"-RC phase. (C) 1999 Elsevier Science Ltd. All rights reserved.