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 (111) 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 (111)–(7×7) 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×√7)-RC phase to the disordered phase are observed.

The nucleation and growth of stable islands, mostly CoSi2, depletes the Co-rich (√7×√7) structure of Co, resulting in the formation and growth of adjacent regions of the Co-poor disordered “(1×1)”-RC phase.