Structural phase transitions of Pb-adsorbed Si(111) surfaces at low temperatures

We found two types of commensurate-incommensurate transition on Pb-adsorbed Si(111) surfaces using reflection high-energy electron diffraction (RHEED) below room temperature (RT); a hexagonal incommensurate (HIC) phase changed to a √7×√3-Pb superstructure around 250 K, and a transient phase between the HIC phase and a striped incommensurate phase changed to a √43×√3-Pb superstructure at 130 K. We constructed a surface structure map in ranges of temperature from 90 K to 500 K and Pb coverage from 0.8 to 1.5 atomic layers including these new phases. We found another structure change where streaks with a 3×3 periodicity began to appear in RHEED just below RT when the √3×√3-Pb phase at 1/3 atomic layer of Pb was cooled.