Amorphous to crystalline phase transition: onset of pattern formation during ion erosion of Si(001)

The morphological evolution of Si(001) is investigated for normal incidence 2 keV Kr+ ion irradiation under ultra high vacuum conditions as a function of temperature and ion fluence through scanning tunneling microscopy and low energy electron diffraction. Under the conditions chosen, the selvage of Si(001) amorphizes below the critical temperature T_c of 670 K, while above it remains crystalline. Below T_c the sample remains flat, irrespective of the ion fluence. Above T_c, the crystalline sample displays for fixed ion fluence and as a function of sample temperature a pronounced roughness maximum at 700 K. Around this temperature, with increasing ion fluence a strong increase of roughness as well as coarsening are observed. Pyramidal pits and mounds develop, with facets formed by Si steps and narrow reconstructed terraces. Most exciting, with increasing ion fluence the pattern reorients from pits and mounds with edges along the ⟨110⟩ directions to ridges and valleys rotated ≈45° to the ⟨110⟩ directions. Pattern formation and the reorientation transition are discussed in terms of a step edge barrier for vacancies, terrace and step edge diffusion.