A Novel Method for True Work Function Determination of Metal Surfaces by Combined Kelvin Probe and Photoelectric Effect
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4)is
an extremely sensitive indicator of changes in surface and interface
chemical composition, adsorbate induced surface dipole and surface roughness. Further, among
the various techniques for thin film characterisation, work function measurements offer the advantages of relatively simple and fast measurements and in-situ observations during film growth. The Kelvin Probe [ 1, 2] can be used to monitor these work function changes in a non-contact, non-invasive way with high accuracy (< 1meV) [3, 4] up to temperatures of approx. 900K. It has been utilised e.g. for thin film studies [5, 6, 7], characterisation of oxides and thin films [8], semiconductor surface processing [9] and surface charge imaging [10], investigation of the adsorption kinetics of oxygen on Si(l 11) surfaces [11 ] as well as for biological applications [12]. However, the Kelvin Probe is an inherently relative technique as it measures the average work function or contact potential difference (CPD) between a vibrating reference electrode (the tip) and the surface under investigation. Thus, in order to obtain the absolute value of the work function of the specimen, it is necessary to know the work function of the reference electrode, tý,p. This could be determined e.g. by a CPD measurement on a clean reference surface. The accuracy of this method however depends on tabulated work function values, which are often valid only for the given experimental setup and the assumption of a clean reference surface. We have developed a relatively simple extension of the Kelvin Probe technique as an approach for a more reliable method to determine 4t)p. This is achieved via measurement of the current voltage (I-V) characteristic of the photoelectric emission from a low work function surface, such as Gd, illuminated by monochromatic UV light of a fixed wavelength as discussed e.g. in [ 13].The 73 Mat. Res. Soc. Symp. Proc. Vol. 619 © 2000 Materials Research Society
work function of any conductor or semiconductor can then be obtained by subsequent Kelvin Probe CPD measurements. We have utilised this technique for oxidation studies of polycrystalline rhenium. The objective of this study was to determine the optimum conditions for the generation of high work function surfaces as part of an ongoing project searching for efficient target materials for use in Hyperthermal Surface Ionisation (HSI), a new mass spectroscopy ionisation technique [14]. HSI relies on high work function surfaces for the production of positive ions. Rhenium is particularly interesting in this respect as oxidation substantially increases 4)to approximately 7eV. EXPERIMENTAL METHOD The Kelvin Method The Kelvin Probe consists of a flat circular electrode (reference electrode, tip) suspended above and parallel to a stationary electrode (the specimen, S), thus creating a simple capacitor. If an external electrical contact is made between the two electrodes their Fermi levels equalise and the resulting flow of electrons from the metal with the lower work function produces a contact potential diff
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