Surface roughness of ion-bombarded Si(1 0 0) surfaces: Roughening and smoothing with the same roughness exponent

We have carried out scanning tunneling microscopy experiments under ultrahigh vacuum condition to study the roughness of pristine as well as ion-bombarded Si(1 0 0) surfaces and of ultrathin Ge films deposited on them. One half of a Si(1 0 0) sample (with native oxide layer) was irradiated at room temperature using 45 keV Si⁻ ions at a fluence of 4 × 10¹⁵ ions/cm² while the other half was masked. STM measurements were then carried out on the unirradiated as well as the irradiated half of the sample. Root-mean-square (rms) roughness of both the halves of the sample has been measured as a function of STM scan size. Below a length scale of ∼30 nm we observe surface smoothing and surface roughening is observed for length scales above this value. However, the surface is self-affine up to length scales of ∼200 nm and the observed roughness exponent of 0.46 ± 0.04 is comparable to earlier cases of ion sputtering studies where only roughening [J. Krim, I. Heyvart, D.V. Haesendonck, Y. Bruynseraede, Phys. Rev. Lett. 70 (1993) 57] or only smoothing [D.K. Goswami, B.N. Dev, Phys. Rev. B 68 (2003) 033401] was observed. Preliminary results involving morphology for Ge deposition on clean ion-irradiated and pristine Si(1 0 0) surfaces are presented.