| Research Abstract |
We discovered that the X-ray photoelectron spectra (XPS) observed for dimethylaluminum hydride (DMAH) adsorbed on SiO_2 changed with time. The XPS measurement was originally undertaken to study the initial stage of Al deposition by photoinduced chemical vapor deposition using DMAH.As the diagnostic means the XPS signals should not be time-dependent. However, we realized that, from the point of view of the hard-photon processes currently under intensive investigation using mainly synchrotron radiation (SR), the present finding gave us a good chance to investigate the surface reactions induced by hard photons. In the present case, the hard photons were produced by the X-ray source used for XPS,namely al Kalpha line with a wavelength of 0.83 nm. As the hard photon source the X-ray characteristic line is monochromatic and can be generated by a relatively simplemachine. Although the intensity of the XPS X-ray source was not so great as that of SR,it was good enough to induce the surface reaction for adsorbates.
Under the present project we investigated the time-dependent XPS spectra systematically for DMAH adsorbates formed on SiO_2. We changed the amount of gas exposure, thus controlling the layr thickness of the adsorbates. Then, we measured the dependence of the time dependency on substrate temperatures where the adsorbates were prepared. It turned out that the adsorbates prepared at different temperatures changed with time differently, although the final product was identical. The dissociative product was assumed to react with surfaces, in particular with oxygen contained in SiO_2 surfaces.
As for the dissociation mechanism, there were two possibilities, namely, direct dissociation by photons and indirect dissociation by secondary electrons generated by hard photons. Since the secondary electrons exhibited an energy distribution peaked at 20 eV,which was adequate for dissociation, it was assumed that the adsorbates were dissociated by the secondary electrons.
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