Surface sol-gel synthesis of ultrathin semiconductor films

Ultrathin films of ZnS, Mn-doped ZnS, ZnO, and SiO2 were grown on silicon s ubstrates 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) spectrosco py. The, Si substrates were pretreated by chemical oxidation, or by derivat ization with 4-((dimethylmethoxy)silyl)butylamine. On the oxidized Si/SiOx surface, nanoparticulate films of ZnS and Mn-doped ZnS were grown by sequen tial immersion in aqueous metal acetate and sodium sulfide solutions. Durin g the first four adsorption cycles, there was little film growth, but there after the amount of material deposited was linear with the number of adsorp tion cycles. This behavior is consistent with the formation of ZnS nuclei a t low coverage, followed by particle growth in subsequent cycles. PL spectr a are consistent with incorporation of Mn2+ into the ZnS nanoparticles. In contrast, the growth of SiO2 films from nonaqueous SiCl4 an 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 fil ms grew as relatively smooth islands, suggesting that the interaction of Zn 2+ ions or primary ZnO clusters with the amine surface priming layer was su fficiently strong to prevent the formation of isotropic nanoparticles upon exposure to aqueous base.