STM investigation of the alpha-Sn/Si(111) phase at 120 K

Low-temperature (120 K) scanning tunneling microscopy experiments on the one-third of a monolayer alpha-Sn/Si(111) surface have clearly shown that both the substrate and the defect density have an effect on the charge density wave transition from a root 3 x root 3 to a 3 x 3 structure, reported for the isoelectronic system Sn/Ge(111). Both systems show that defects generate exponentially (temperature-dependent) damped electronic disturbances. At room temperature this perturbation decays rapidly at the second-nearest neighbors of the defects while at 120 K its decay length increases to about 11 Angstrom [compared with 50 Angstrom for Sn/Ge(111)], clearly affecting the defect neighbors up to the third coordination shell. In contrast to the Sn/Ge(111) system, there is no sign of a phase transition down to 120 K. At 120 K the high defective surface clearly shows localized 3 x 3 symmetry domains, whilst the vast majority of the low defective surface retains its root 3 x root 3 symmetry. The role of defect-to-defect correlation in originating opposite-phase 3 x 3 symmetry patterns is discussed. An advantage of the Sn/Si(111) system is that the defect density can be controlled; in this study, from a low value of 1% to a high value of 20%. (C) 2000 Elsevier Science B.V. All rights reserved.