New, high intensity, narrow-spot positron beams open new possibilities for solid state spectroscopy. Positron annihilation techniques, being non-destructive, allowing depth profiling down to a few micrometers and detecting open-volume defects (vacancies, dislocations etc.) at single ppm concentrations constitute a valuable and complementary method, compared to other solid-state-physics studies.
We give examples of investigation in the field of semiconductors, performed with experimental techniques available at Trento University: 1) Doppler broadening of the 511 keV annihilation line method studied with a slow-positron beam facility, 2) positron life-time techniques with about 160 ps resolution, 3) Doppler-coincidence method in bulk.
Examples of applications are shown with the aim:
i)to follow the interstitial oxygen atoms dynamics and oxygen precipitates in Czochralski-type silicon at different stages of annealing using Doppler coincidence;
ii)to monitor the reduced layers and metal nanocrystals in semiconducting glasses
iii)to follow ageing processes in high-porosity, novel dielectric materials.
i)Lowering of the coincidence parameter in Si samples annealed at 450C indicates migration of oxygen atoms from defect sites to interstitial positions. This type of annealing is a standard recipe of semiconductor industries, used for "as-grown" samples, in order to remove the electrical activity of oxygen atoms. The recipe was purely phenomenological and positron annihilation studies allow to explain it.
ii)Tracing the "valence" annihilation S-parameter allows to determine optimum thermal treatments for the desired reduced depth. Glasses are used for electron-multipliers and image-intensifiers.
iii)Ageing of high-porosity, low- ε materials, due to filling of pores by atmospheric gases. The lowering of 3- γ annihilation events indicated reductions of free-volumes inside the material. This processes is reversible.