High resolution electron energy loss spectroscopy study of semiconductor surfaces

Abstract

In this thesis, High Resolution Electron Energy Loss Spectroscopy (HREELS)has been used as the main technique to study the phonons on boron nitride surfaces, as well as the surface chemistry of germanium and polycrystalline diamond towards hydrogen, oxygen and organic absorbates.HREELS has been used to characterize the phonon modes on chemical vapor deposited amorphous Hexagonal Boron Nitride (h-BN) films doped with different carbon content. The TOaSY, LO and TO|| modes of h-BN were observed on samples with C content < 10%, whilst only the TOaSY mode of h-BN can be observed for amorphous BCN samples with carbon content ~20%. Samples with C content ~30% do not exhibit TOaSY, LO and TO|| modes of h-BN. The acoustic phonon mode (Ka??0) shows ashift towards the higher wavenumbers with increasing matrix dilution by carbon.The surface vibration spectra of clean, hydrogenated, methanol-dosed and airexposedn-doped Ge(100) have been collected using HREELS. On clean Ge(100)2??1, we report the observation of a surface phonon peak between ~28-35 meV. The position and shape of this peak is sensitive to the presence of low surface coverage of hydrogen and oxygen. By adsorbing molecular hydrogen on the n-doped Ge, this peak shifts towards the elastic peak, and becomes attenuated. The HREELS spectrum of air-exposed Ge is similar to that created by dosing Ge with methanol. Methanolundergoes dissociation into methyl radicals and hydroxyl species on Ge surfaces at room temperature and oxidizes the Ge surface readily.The initial stage oxygenation of Ge(100) 2??1 was studied by dosing small amounts of molecular oxygen on dimerized germanium surface, and recording the HREELS spectrum at each stage. In the initial stage, molecular oxygen dissociatively chemisorbs on the Ge dimer at room temperature such that one O atom bridges between the Ge dimer atoms, whilst another O atom inserts into the Ge-O-Gebackbond. This configuration is stable at room temperature. At higher temperatures, anew configuration will be adopted with two O atoms inserted fully into the Ge backbonds. The experimentally observed vibrational peaks were compared to the theoretical vibrations from cluster models using density functional theory, and found to be in good agreement.Finally, the surface reaction mechanism of allyl alcohol-dosed polycrystalline diamond and single crystalline C(100) 2??1 surfaces was studied using HREELS, X-ray absorption spectroscopy (XAS) and valence band spectroscopy. Our results suggest that the bonding of the allyl alchohol proceeds via [2+2] type cycloaddition on the diamond dimer bonds, with retention of hydroxyl functionality. Multiple layer adsorption of allyl alchohol at room temperature can occur. Heating the adsorbed adlayer to 50 A?C results in the increased impact scattering from the C-H bonds on the surface as observed by HREELS, as well as strong enhancement in the NEXAFSsignal detected in the total yield mode. One possible reason is attributed to the thermally induced dissociation of C-H bonds, followed by radical initiated polymerization of the adlayer. Heating to temperatures above 200 A?C results in the desorption of the adlayer as judged by the vanishing of related signals in HREELS.The dissociation of C-H bonds in allyl alcohol during radicalization process transfers hydrogen to the diamond surface, and this chemisorbed hydrogen results in thepromotion of the negative electron affinity on the diamond surface.