Surface sol-gel synthesis of ultrathin semiconductor films

Ultrathin films of ZnS, Mn-doped ZnS, ZnO, and SiO₂ were grown on silicon substrates using surface sol-gel reactions, and the film growth process was characterized by ellipsometry, atomic force microscopy, X-ray photoelectron spectroscopy, UV-visible absorbance, and photoluminescence (PL) spectroscopy. The Si substrates were pretreated by chemical oxidation, or by derivatization with 4-((dimethylmethoxy)silyl)butylamine. On the oxidized Si/SiO(x) surface, nanoparticulate films of ZnS and Mn-doped ZnS were grown by sequential immersion in aqueous metal acetate and sodium sulfide solutions. During the first four adsorption cycles, there was little film growth, but thereafter the amount of material deposited was linear with the number of adsorption cycles. This behavior is consistent with the formation of ZnS nuclei at low coverage, followed by particle growth in subsequent cycles. PL spectra are consistent with incorporation of Mn²⁺ into the ZnS nanoparticles. In contrast, the growth of SiO₂ films from nonaqueous SiCl₄ on the same Si/SiO(x) substrates was regular from the first adsorption cycle, indicating a high density of nucleation sites. On amine-derivatized substrates, ZnO thin films grew as relatively smooth islands, suggesting that the interaction of Zn²⁺ ions or primary ZnO clusters with the amine surface priming layer was sufficiently strong to prevent the formation of isotropic nanoparticles upon exposure to aqueous base.