Hysteresis in the (1×1)-(7×7) first-order phase transition on the Si(1 1 1) surface

We used low-energy electron microscopy to study the (1×1)-(7×7) phase transition on step-free surfaces in two-dimensional (2D) bilayer vacancy islands and islands formed on the surface of Si(1 1 1). A small and yet distinct hysteresis effect is found in the phase transition on these step-free surfaces compared to the transition on surfaces with regularly spaced steps where no hysteresis is observed. The occurrence of hysteresis on step-free surfaces is explained thermodynamically by the consideration of the change in Gibbs free energy in the phase transition based on a 2D nucleation theory.