Methods for defect characterisation in thin film materials by depth-selective 2D-ACAR

The advent of intense positron beams makes it possible to perform depth-selective 2D-ACAR (two-dimensional angular correlation of annihilation radiation) studies. The Delft POSH–ACAR setup employs a strong permanent magnet for focusing of the POSH beam on the sample, which leads to a ∼15% spread in implantation energy. The effects of this spread on positron depth-profiling data are discussed, and are shown to be consistent with Doppler experiments on Si(1 0 0) with a subsurface layer of nanocavities. A method is presented to decompose depth-selective 2D-ACAR spectra reliably into their various (layer) components. This is used to reveal strong positron trapping in the nanocavities in Si(1 0 0).