Direct observation of the phase transition between the (7 × 7) and (1 × 1) structures of clean (111) silicon surfaces
ABSTRACT The phase transition process of clean (111) silicon surfaces between the (7 × 7) and (1 × 1) structures at about 830°C was directly observed by reflection electron microscopy, which had been briefly reported in a previous short communication (Osakabe et al., Japan. J. Appl. Phys, 19 (1980) L309). Smooth atomic steps, whose shapes change spontaneously and continually in a microscopic scale at high temperature of the (1 × 1) structure, transform into zig-zag steps at low temperature of the (7 × 7) structure, where the changes of the step shape stop. On cooling, domains of the (7 × 7) structure nucleate preferentially on upper terraces along the steps and expand on the terraces to the neighbouring steps. Out of phase boundaries with phase differences of 2πn/7 are seen to be formed. On heating the reversed process takes place. The out of phase boundaries are easy places to transform to the (1 × 1) structure. The observations clearly suggest the phase transition of the first order and the models of the (7 × 7) structure of ordered vacancies or adatoms rather than of static displacements of surface atoms.
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ABSTRACT: 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 7′7 ⟷ 1′1 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 (5×2-Au or ×-Au) ⟷ 1×1-Au. The spot profiles were found to be broadened during the transition of Si(111)-×-Au ⟷ 1×1-Au, which was a signature of a continuous transition, while the profiles remained unchanged during the transitions of the 7×7 ⟷ 1×1 and 5×2-Au ⟷ 1×1-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 7×7 surface did not show any structural conversion with In adsorption, while the ×-In surface successively converted to a 2×2 and a × phase with coverage increase. The structural transitions at elevated temperatures were sensitively dependent on the temperatures. Sequences of transitions among the 7×7, 4×1, ×, 1×, and ×4 were quantitatively revealed as changes in RHEED spot intensity.Phase Transitions - PHASE TRANSIT. 01/1995; 53:87-114.
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ABSTRACT: Contrary to previous results concluding that all 7×7 boundaries in the down step edge were complete, 5×5 faulted half units were found to be introduced in part when the transition region was narrow. In addition, the formation characteristic of the 5×5 faulted half units was strongly influenced by the type of step configuration (nF or nU) and by the structure of the upper step edge (complete or incomplete). Even when the step configuration was the same, the difference in the upper step edge structures strongly affected introduction of 5×5 units. © 2001 American Vacuum Society.Journal of Vacuum Science & Technology A Vacuum Surfaces and Films 01/2001; 19(4):1549-1552. · 1.43 Impact Factor
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ABSTRACT: Studies of structural phase transitions on surfaces by electron microscopic techniques are reviewed. Structural phase transitions on surfaces induced by changes of temperature, amount of adsorbate and misorientation from low index flat surfaces and by DC current are mentioned. The usefulness and importance of in situ real space observations are stressed.Phase Transitions - PHASE TRANSIT. 01/1995; 53:197-214.