Measurement of microscopic surface deformation due to low energy ion bombardment on Si(111)

A low energy electron diffraction spot profile analysis of the Si(111) surface, after argon ion bombardment at an elevated temperature, finds a continuous, low amplitude distribution of surface height, in addition to the atomic-step-and-terrace structure. With an amplitude of tenths of an Angstrom or less in height, correlated laterally over tens of Angstroms, this microscopic surface deformation is measured versus ion dose and for various sample temperatures during bombardment and annealing. For 230 eV argon ion doses increasing in the range of 1015 − 1016 cm− 2 with the sample held at 580 K and 800 K, the amplitude of surface deformation, measured as the standard deviation wz of the continuous height distribution, increases steadily in the range 0.06 − 0.10 Å. For higher ion doses in the range of 3 × 1016 − 1 × 1017 cm− 2, saturation-like behavior with wz ≈ 0.14 − 0.15 Å is measured at 800 K. After an ion dose of 1017 cm− 2, the surface deformation shows a lateral correlation length of 20 Å, implying an average lateral feature size of ≈ 40 Å, slightly less than the average terrace width of ≈ 60 Å. On the other hand, after the same ion dose at 300 K followed by annealing at 800 K, the surface deformation has a similar correlation length but a much smaller height amplitude of wz = 0.06 Å. It is notable that wz depends quite differently on sample preparation conditions overall, compared to the average terrace width and to the degree of (7 × 7) order indicated by diffraction intensities, both of which are found to evolve much more slowly with ion dose in conditions for which the height amplitude increased in the range of 0.06 − 0.14 Å. The surface height deformation due to strain around buried, subsurface defects is estimated as a function of defect concentration using a simple, order-of-magnitude theoretical model, in which a distribution of subsurface defects is modeled as small inclusions in a continuous elastic medium. This approximate model is consistent with the measured growth of the deformation amplitude wz, for a range of possible parameter choices for small defect clusters located in a shallow layer below the surface. The surface deformation is partially removed by annealing above 870 K.