Vibrational energy transfer on hydrogen-terminated vicinal Si(111) surfaces: Interadsorbate energy flow

We report measurements of excited-state lifetimes for Si-H stretching vibrational modes of steps and terraces on chemically prepared, hydrogen-terminated vicinal Si(111) surfaces using picosecond pump-probe surface spectroscopy. The steps present on these vicinal surfaces are shown to play an important role in the vibrational energy relaxation pathways. Three types of vicinal Si(111) surfaces are studied, all having monohydride-terminated terraces but differing in step termination or in step density. Two surfaces are cut along the (112) direction, 9° and 5° from the (111) plane, respectively. Both of these surfaces have dihydride-terminated steps. A third surface is cut 9° from the (111) plane along the (112) direction and has monohydride-terminated steps. Two normal modes of the dihydride-terminated steps show vibrational energy relaxation times of ∼100 ps [≤80 ps and 130(20) ps, uncertainty in parentheses], while the monohydride-terminated steps relax 10 times more slowly with an 1100(120) ps lifetime. On the dihydride-stepped 9° surface the lifetime of the terrace mode is shortened to 420(40) ps from the flat surface lifetime of 950(100) ps, while on the monohydride-stepped surface the terrace mode lifetime is 820(80) ps. The results are explained by energy transfer between the terrace and the step Si-H modes. The different dynamics on the monohydride-and dihydride-stepped surfaces arise because the short-lifetime dihydride steps act as energy drains, while the long-lifetime monohydride steps do not. Dipole-dipole coupling between the Si-H stretching modes can account for the interadsorbate vibrational energy transfer observed.