Modification of silicon surfaces by molecule grafting of organo-germanium gas using vacuum-ultraviolet-light irradiation

A novel technique to modify hydrophobic surfaces using a photo-assisted reaction was developed for nano-scale science and technologies. This is the first time to use a xenon (Xe) excimer light (λ = 172 nm) to successfully graft organo-germanium on a silicon (111) surface that was hydrogen-terminated and atomically flattened. In the experiment, the gas source of the organo-germanium was either tetra-ethyl-germanium (TEGe) or tri-methyl-germanium-chloride (TMGC) for an investigation of a site-selective reaction under the vacuum ultraviolet (VUV) light. The substrate was exposed to the gas and the light simultaneously at room temperature for various exposure times (1 min to 10 min) and pressure (1.3 × 10−3 Pa to 1.3 Pa). For the TEGe exposure, angle-resolved x-ray photoelectron spectroscopy (XPS) showed that (i) Ge adsorption occurred independently of the excimer light irradiation, (ii) the adsorbed layers contained Ge, C, and O, (iii) the peak positions of these elements were independent of the detection angle, and (iv) there was no significant difference in the XPS spectra between the samples with and those without the excimer light irradiation. On the other hand, for the TMGC exposure, the XPS showed that (i) Ge adsorbed only when the substrate was directly irradiated with the excimer light, (ii) the adsorbed layers contained Ge, C, O, and Cl, (iii) the peak positions of C, O, and Cl were independent of the detection angle, but that of Ge shifted to a higher binding energy as the detection angle was increased. This dependence on detection angle indicates that the chemical state of Ge changed from metallic near the interface to compound near the surface. The metallic Ge was assigned to a Ge that was bonded with the substrate, and the compound Ge to physically adsorbed TMGC. Comparing the results for the TMGC and the TEGe exposures reveals that the photo-selectively adsorption (i.e., molecule grafting) of TMGC was caused by the formation of Ge-Si bonds with the Xe excimer light irradiation.