Structural Phase Transitions at Clean and Metal-Covered Si(111) Surfaces Investigated by Rheed Spot Analysis

Structural phase transitions between various kinds of superlattice structures formed on a Si(111) surface have been investigated by spot analysis of reflection high-energy electron diffraction (RHEED). Reversible transitions induced by temperature changes and irreversible ones induced by metal depositions were observed. Detailed discussions on the dynamics of the phase transitions are made by quantitative analyses of integrated spot intensity and profile. For a phase transition of 7x7âŸ1x1 structures on a clean Si(111) surface, a hysteresis with temperature difference of 5° C between in heating and cooling processes was found in the spot intensity change, indicating a first-order transition. Hysteresis was hardly recognized, on the other hand, for transitions of Au-induced superstructures (5 x 2-Au or √3 x √-au). 1x1-Au. The spot profiles were found to be broadened during the transition of Si (111)-√3 x-√3-Au ⟠l x 1-Au, which was a signature of a continuous transition, while the profiles remained unchanged during the transitions of the 7x7âŸ.1x1 and 5x2-Au âŸ. 1x1-Au phases. Structural conversions induced by In adsorption on the Si(111) surface kept at constant temperatures were also analyzed. The conversions at room temperature were totally dependent on the initial substrate surface structures; the 7 x 7 surface did not show any structural conversion with In adsorption, while the √3x√3-In surface successively converted to a 2x2 and a √7x √3 phase with coverage increase. The structural transitions at elevated temperatures were sensitively dependent on the temperatures. Sequences of transitions among the 7x7, 4x1, √3x√3, √31 x √31, and √43x4 were quantitatively revealed as changes in RHEED spot intensity.