Observation of smoothing and self-affine fractal roughness on MeV ion-irradiated Si surfaces

Scanning tunneling microscopy (STM) was used to quantitatively investigate the fractal nature of MeV ion irradiated silicon surfaces. Si(1 0 0) surfaces (with native oxide) were irradiated at room temperature using 2 MeV Si+ ions with a fluence of 4 x 1015 ions/cm2. One half of the sample was masked during irradiation. Root-mean-square roughness of both the irradiated and the pristine halves of the sample has been measured as a function of STM scan size. Ion beam induced smoothing has been observed at length scales below ∼50 nm. The roughness exponent of the smoothed surface is α=0.53±0.03. When the oxide is removed from the ion-bombarded surface by thermal flushing, the roughness exponent increases to 0.81 ± 0.04 with further smoothing below the length scale ∼50 nm and roughening at higher length scales upto ∼300 nm. The values of the roughness exponents indicate the self-affine nature of these surfaces