Tomographic study of the three-dimensional distribution of a high-fluence implant in a polymer

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Li⁺ ions were implanted into PMMA at high flux up to fluences of 1×10¹⁵ cm^-2 under angles of 0° to 70° towards the surface normal. The Li depth distributions were determined by means of neutron depth profiling, and compared with theoretical simulations. The three-dimensional Li distribution was reconstructed from the one-dimensional depth profiles by means of a tomographic technique.

It turned out that the measured Li depth distributions can be described by a superposition of Gaussian and exponential functions. This points at considerable Li mobility during or after the ion implantation, with trapping in unsaturable traps in the ion-irradiated region which roughly follow the electronic energy transfer distribution. The Li redistribution is more pronounced along the track direction than transversely to it.

The normalized Li distributions in various implantation directions were fed into our tomographic program to reconstruct the three-dimensional distribution of the deposited lithium. As expected, the lithium preferentially distributes along the ion tracks. This work is another hint that mobility of implanted ions in solids does not proceed isotropically, but is strongly influenced by the radiation-damage distributions.