Reaction of NH4F/HF solutions on Si (100) and Si (111) surfaces

The reaction of NH4F/HF solutions on Si (100) and Si (111) surfaces was investigated by means of x-ray photoelectron spectroscopy and high-resolution electron energy loss spectroscopy (HREELS). The pH of the HF treatment was varied in the range from pH 1 to pH 8. Exactly oriented Si (111) surfaces show an overall smoothing of the surface with increasing pH as can be seen by the vanishing of the SiH2 scissor vibration in HREELS spectra. In contrast, Si (100) surfaces tend to roughen by using HF solutions with higher pH. At intermediate pH (pH 5–6) the Si (100) surface regularity gets disturbed and new Si (111) like facets are formed. At higher pH the etching speed increases and the Si (111) facets again disappear.