The formation of nanovoids in Si(100) and MgO(100) by 3He ion
implantation has been studied. Contrary to Si in which the voids are
generally almost spherical, in MgO nearly perfectly rectangular nanosize
voids are created. Recently, the 2D-ACAR setup at the Delft Positron
Research Center has been coupled to the intense reactor-based
variable-energy positron beam POSH. This allows a new method of monitoring
thin layers containing nanovoids or defects by depth-selective
high-resolution positron beam analysis. The 2D-ACAR spectra of Si with a
buried layer of nanocavities reveal the presence of two additional
components, the first related to para-positronium (p-Ps) formation in the
nanovoids, and a second one most likely related to unsaturated Si-bonds at
the internal surface of the voids. The positronium is present in excited
kinetic states with an average energy of 0.3 eV. Refilling of the cavities
by means of low dose 3He implantation (1×1014
cm−2) followed by annealing reduces the formation of Ps and
the width of the Ps peak in the ACAR spectrum. This width reduction is due
to collisions of Ps with He atoms in the voids. In MgO, p-Ps formed with an
initial energy of ~3 eV shows a final average energy of 1.6 eV at
annihilation due to collisions with the cavity walls. Possibilities of this
new, non-destructive method of monitoring the sizes of cavities and the
evolution of nanovoid layers will be discussed.