Distortion of na-concentration profiles in thin glassy surface layers by ion bombardment

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Abstract
The intensity/time plots of the photon emission of excited Na atoms recorded during the continuous removal of thin Na2O-containing glassy layers by an ion beam allow for the analysis of the Na2O concentration profiles in these layers. Different intensity/time plots were obtained when changing the angle of incidence β of the ions or applying different constant β during the ablation of thin Na2O-containing glassy layers by positive 5.6 keV Ne+, Ar+ and Xe+ ions. These differences indicated that the concentration of Na is changed by a drift of Na+ ions caused by a positive space charge below the ion-bombarded surface. The field strength of the space charge was greatest for Ne+ ions, smallest for Xe+ ions. Simultaneously, a repulsive or an attractive field strength acts on positive Na ions within the thin layer if the value of the dielectric constant e in the thin layer is greater or smaller than in the substrate respectively. The influence of the drift on the profiles could be separated from that of the disintegration of Na at the surface and the displacements of Na in the collision cascade reported earlier. For glasses containing the oxides of heavy elements, an additional influence of the back-scattering of Na atoms moved in the collision cascade from the interface between the glass substrates and layers on the intensity/time curves was also observed for Xe+ ions. The shapes of the recorded Na profiles are distinctly different for different stimulation depth distributions of the projectile ions. The different spatial extensions of the stimulation depths for Na moved in the collision cascade below the surface were estimated for 5.6 keV Xe+ ions at β = 20° and β = 70°. The consequences of the various findings for the quantitative analysis by ion-beam-induced radiation are considered.