Preparation of an ultraclean and atomically controlled hydrogen-terminated Si(111)-(1×1) surface revealed by high resolution electron energy loss spectroscopy, atomic force microscopy, and scanning tunneling microscopy: Aqueous NH4F etching process of Si(111)

We propose an improved wet chemical process for preparing a high-quality hydrogen-terminated Si(111)-(1×1) surface and show an atomically ordered and ultraclean surface without carbon and oxygen contamination. The vibrational properties and surface morphology are investigated by high-resolution electron energy loss spectroscopy (HREELS), atomic force microscopy (AFM), and scanning tunneling microscopy (STM). The HREELS spectra and images of AFM and STM reveal the precise aqueous NH₄F etching process of Si(111) and indicate the high controllability of steps and terraces at the atomic scale. The surface cleanliness and morphology strongly depend on the etching time. At the etching time of 10min, we obtain an ultraclean and atomically ordered surface with wide terraces of 36 ±7nm step distance. It is confirmed by AFM and STM that 1.0% ammonium sulfite is useful for removing dissolved oxygen in the 40% NH4F etching solution and for preparing a high-quality H:Si(111)-(1× 1) surface with a low density of etch pits. The onset of tunneling current and the gap of 1.39 eV are measured by scanning tunneling spectroscopy. There is no peak at -1.3 eV in comparison with the previous report [Phys. Rev. Lett. 65 (1990) 1917].