On the Band-Gap Width of NiSi2 Nanocrystals Created in the Surface Region of Si Using Ion Implantation
Аннотация
Auger electron spectroscopy, high-speed electron diffraction, and light-absorption spectroscopy are used to study the composition, size, and electronic and crystal structures of NiSi2 nanoparticles and layers formed in the surface region of Si(111). Nanocrystalline structures are obtained by implanting Ni+ ions into Si(111) with an energy of E0 = 25 keV by varying the ion dose within the range from 1014 to 1017 cm—2. Prior to ion implantation, Si samples are degassed under ultrahigh vacuum conditions (10—6 Pa) first at Т ≈ 1200 K for 3–4 h and then at Т ≈ 1500 K for 10 min. It is shown that at doses of Ni+ ions of D ≤ 5 × 1015 cm–2, after heating, crystalline particles up to 10 nm in size are formed, and at D ≈ 6 × 1016 cm–2, a continuous nanolayer is formed. An analysis of the spectra obtained using Auger electron spectroscopy in combination with complete etching showed that, at doses less than D ≈ 1015 cm–2, the NiSi2 nanocrystalline phases are formed as spheres, at D > 1015 cm–2 they transform into a shape close to an ellipsoid, and at D = DS a continuous homogeneous NiSi2 layer is formed. An analysis of high-energy electron diffraction patterns shows that the Si films formed on the surface of the Si/NiSi2/Si(111) system are single crystal and have a structure with a cubic lattice. Due to the fact that the value of the lattice constant of NiSi2 (a ≈ 5.42 Å) barely differs from a of pure Si (5.41 Å), very narrow (d ≤ 3–5 nm) transition layers are formed at the Si/NiSi2 and NiSi2/Si(111) interfaces. A relationship is established between the size and band-gap width of the surface nanocrystalline phases of NiSi2. The sizes of nanophases, at which quantum-size effects begin to appear, are determined.