The effect of an adsorbate upon secondary emission properties of low -energy ion bombarded metallic and semiconductor substrates
Doctor of Philosophy (Ph.D.)
Roy L Champion
The absolute probabilities for low energy ion bombardment induced secondary emission of electrons and anions have been measured as a function of adsorbate coverage of the surface. The primary ion beams were incident at less than 500 eV on metallic, semiconducting and insulating surfaces. The adsorbate used was chiefly oxygen, and the coverage range studied was zero to about one monolayer. The presence of an adsorbate was observed to significantly enhance secondary emission of electrons and anions in the case of O - and Na+ impacting metallic (W, Al) and semiconducting (Si) substrates; the effect of the adsorbate was little to minimal in the case of N2+, Ar+, Ne+ and He+ impacting these substrates, however. No appreciable adsorbate-induced changes in the secondary emission probability were measured for any of the probe beams incident on the insulating (MgO) substrate.;Secondary electron and anion kinetic distributions were also measured, as functions of projectile impact energy and of adsorbate exposure. The most probable energy of the secondary products was in the 1--3 eV region; the form of the distributions had little to no dependence on the impact energy or adsorbate exposure, but varied with different projectile and substrate species. The identities of the secondary anions were determined through mass spectroscopic techniques; atomic ion forms of the adsorbate and simple adsorbate-substrate molecular ions are the predominantly emitted species.;The data are discussed in terms of a model in which a molecular anion residing on the surface is collisionally excited, its subsequent decay giving rise to both electron and negative ion emission into the vacuum. The results of N2+, Ar+, Ne+ and He+ bombardment, in which secondary emission does not appear to be adsorbate-mediated, suggest that there exists a condition of excitation energy resonance which projectiles having high ionization potentials do not satisfy; experimental evidence shows that incident O- and Na+ satisfy this condition to a greater degree than do the above projectiles. The concepts of this excitation model can be represented mathematically and made to fit the observations with careful parameter choice; the parameters can be shown to reflect properties of the interaction.
© The Author
Vogan, Wendy Sara, "The effect of an adsorbate upon secondary emission properties of low -energy ion bombarded metallic and semiconductor substrates" (2003). Dissertations, Theses, and Masters Projects. William & Mary. Paper 1539623426.