Dissociation energies of alkali antimonides as thin layers

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Abstract
Optical transmittance and Auger Electron Spectroscopy (AES) analyses are used to derive some thermodynamic data from alkali antimonide compounds deposited on glass substrates as thin layers. These measurements are performed in the course of the deposition of the layers as well as during their thermal dissociation (linear temperature-time dependence) under UHV conditions. The optical and AES measurements recorded during the dissociation process are analysed in terms of dissociation energies and equilibrium alkali vapour pressures (PCs, PK, PNa) over the compounds. The dissociation phenomenon is analysed and considered to be neither alkali-diffusion limited nor layer morphology limited. Dissociation behaviours of alkali antimonides with respect to temperature and time are deduced and discussed from the above results. It is shown that PK=5*10-10 Torr and PNa=1*10-11 Torr at 200 degrees C over the Na2KSb. Without alkali pressures, at 200 degrees C, 6% of a Na2KSb layer is dissociated within 1 h whereas 50% of a NaK2Sb one is dissociated. The dissociation energy, Phi , of a Cs monolayer deposited on the top of the Na2KSb compound is also considered and is about Phi =35+or-1 kcal mol-1. At room temperature, the corresponding value of PCs is 2*10-17 Torr. Finally optical transmittance measurements associated with the AES analysis are shown to be powerful tools for the measurement of dissociation energies and the assessment of related phenomena.