Nitrogen sorption on titanium: Reconstruction of the subsurface composition profile using low- and high-energy Auger data
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abstract
Although Auger electron spectroscopy (AES) is the most widely used technique for surface analysis, there are many problems associated with its application to TiN, a technologically important material. The N and high-energy Ti peaks overlap completely and there is no elemental N standard available at ambient temperature. TiN standards can be used and the overlap problem can be overcome by computer simulation of the high-energy spectrum. Furthermore, the computer simulation technique can also be applied to the low-energy TiNx spectrum to give quantitative analysis with increased depth resolution. A procedure is proposed for deconvolution of the low- and high-energy AES data during nitrogen sorption on titanium to determine the composition profile based on a simple adsorption–diffusion model. Nitrogen sorption at various pressures has been followed at sample temperatures in the range 300–900 K. For a given nitrogen pressure, the experimentally observed escape depth averaged N/Ti ratios x̄ increase with sample temperature, pass through a maximum at Tm, and then decrease. At Tm the diffusion coefficient attains a value such that the diffusion rate and nitrogen gas arrival rates balance. For T<Tm the surface concentration x0 will be close to saturation, whereas for T>Tm, x0 will decrease rapidly. The concentration profiles obtained by computer deconvolution of the high- and low-energy Auger data confirm this model. As would be expected, Tm increases with increasing nitrogen pressure which increases the supply rate to the surface. The diffusion coefficients determined by computer deconvolution agree well with literature values for higher temperature studies. Prebombardment of the surface with Ar+ ions greatly increases the amount of nitrogen sorbed at room temperature and reduces Tm. The effect on the concentration profile is equivalent to an increase in diffusion coefficient caused by the excess defect concentration, viz., radiation enhanced diffusion.
