The Surface Chemistry of Triallylamine on Si(111) and its Coadsorption with Triethylgallium
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Triallylamine is a tertiary amine and the allyl radical which is formed after dissociation from the N atom is more stable than e.g. alkyl radicals with single C bonds. Each carbon in the allyl radical CH 2=CH-CH 2 is sp2 hybridized, giving rise to a-bonds with each neighbor atom; the remaining three p electrons contribute to a 'one and a half iT-bond among the C atoms (mesomery or delocalized C=C bond). TAA has been successfully employed in MOCVD for the doping of JI-VI semiconductors [1]. In a MOMBE process, due to the reduced gas pressures as compared with MOCVD, the reactions among the different precursors take place at the substrate surface rather than in the gas phase. Surface analysis methods are therefore particularly suited to follow the reaction/dissociation paths and the incorporation at the growing surface. After each deposition step, in-situ High Resolution Electron Energy Loss Spectroscopy (HREELS) yields the spectrum of vibration modes at the surface and X-ray Photoelectron Spectroscopy (XPS) the chemical bonding and the partial concentration of the different adsorbates in the near surface region. Due to its conductive properties and the absence of any spectral feature in the infra-red region, a silicon substrate has been chosen for the adsorption experiments. The insulating sapphire has been investigated at the end of this study to verify the potential of TAA as a precursor in the GaN growth, which is known to give the best results on this substrate so far. EXPERIMENTAL The experiments were carried out in a three-unit UHV system, equipped with a deposition and two analysis chambers [2]. The gases (TAA and TEG) were introduced into the deposition unit through separated Baratron controlled gas lines. The HREELS spectra are all measured under reflection geometry (dipole scattering regime) with a primary beam energy E=5 eV, an incidence angle 0=60" and a resolution (FWHM) of 3 meV for the electron beam in the spectrometer. For the XPS analysis monochromatized AI-KI radiation (hv=1486.6 eV) was used with different pass energies corresponding to overall resolutions of 1.6, 0.56 and 0.42 eV, chosen depending on the intensity of the different measured core levels. The Si(l 11) substrates were prepared in-situ by annealing at 820 "C for 15 minutes giving a (7x7) surface reconstruction as confirmed by Low Energy Electron Diffraction and HREELS. TAA ADSORPTION ON Si(1 11) Fig. 1 shows the HREEL spectrum after 1 minute exposure to triallylamine. Two main spectral regions are identified. The stretching bands of CH. complexes at 2700-3300 cm"1 and a broad band between 900 and 1500 cm 1 . On both sides of this latter region at 750 cm-l and at 1650 cm"' two small structures are also observed. For comparison the IR absorption spectrum of TAA is superimposed as a shaded area [3] and an overall good agreement is found, which gives evidence of molecular adsorption at RT. Assignements for each of the single bands are also given in the figure. A few points should be mentioned which are as follows. The presence of the bro
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