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<SUP>+</SUP> ions with a fluence of 4 &#215; 10<SUP>15</SUP> ions/cm<SUP>2</SUP>. 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 &#945; =0.53&#177;0.03. When the oxide is removed from the ion-bombarded surface by thermal flushing, the roughness exponent increases to 0.81&#177;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