Ge growth on ion-irradiated Si self-affine fractal surfaces

We have carried out scanning tunneling microscopy experiments under ultrahigh vacuum condition to study the morphology of ultrathin Ge films deposited on pristine Si(1 0 0) and ion-irradiated Si(1 0 0) self-affine fractal surfaces. The pristine and the ion-irradiated Si(1 0 0) surfaces have roughness exponents of α = 0.19 ± 0.05 and 0.82 ± 0.04, respectively. These measurements were carried out on two halves of the same sample where only one half was ion-irradiated. Following deposition of a thin film of Ge (∼6 Å) the roughness exponents change to 0.11 ± 0.04 and 0.99 ± 0.06, respectively. Upon Ge deposition, while the roughness increases by more than an order of magnitude on the pristine surface, a smoothing is observed for the ion-irradiated surface. For the ion-irradiated surface the correlation length ξ increases from 32 to 137 nm upon Ge deposition. Ge grows on Si surfaces in the Stranski–Krastanov or layer-plus-island mode where islands grow on a wetting layer of about three atomic layers. On the pristine surface the islands are predominantly of square or rectangular shape, while on the ion-irradiated surface the islands are nearly diamond shaped. Changes of adsorption behaviour of deposited atoms depending on the roughness exponent (or the fractal dimension) of the substrate surface are discussed.