Observations of reversible and irreversible structural transitions of cobalt on Si (1 1 1) with LEEM

We present real time images of the evolution of the structure of the Si (1 1 1) surface during the deposition of cobalt at elevated temperatures, acquired using low-energy electron microscopy. The system follows a sequence of coexisting ordered and disordered phases, consistent with two-dimensional eutectic behavior. Reversible temperature driven transitions are observed between the ordered Si (1 1 1)(7 X 7) reconstructed phase and a disordered lattice gas of Co-containing ring-clusters (RC), indicating a local equilibrium between these structures. Only irreversible temperature driven transitions from an ordered (√7 x √7)-RC phase to the disordered phase are observed. The nucleation and growth of stable islands, mostly CoSi₂, depletes the Co-rich (√7 x √7) structure of Co, resulting in the formation and growth of adjacent regions of the Co-poor disordered '(1 x 1)'-RC phase.