Università degli studi di Modena e Reggio Emilia

hermal desorption spectrometry (TDS) has been applied to investigate the thermal evolution of vacancy-like defects in helium-implanted (100) silicon samples with a dose of 2x1016 cm-2 at 20 keV. The measured spectra present features which can be interpreted as fingerprints of the modifications occurring in the sample. The defects that are recognized which affect the desorption are: thermally unstable helium-vacancy complexes, pressurized gas bubbles organized in planar structures (cracks) and thermally stable cavities. The attribution is supported by the results obtained by complementary techniques, such elastic recoil detection, channeling Rutherford backscattering spectrometry, cross sectional transmission electron microscopy and positron annihilation spectroscopy which have been employed on isothermally pre-annealed samples in the range 100-800oC

Large angle convergent beam electron diffraction (LACBED) technique has been applied to <110> TEM cross section of silicon samples implanted with 2x1016 cm-2 He ions at an energy of 20 KeV in order to evaluate the stress/strain field in the implanted layer. The stress/strain field depends on the mismatch between the lattice of the silicon substrate and the one of the defective layer which contains clusters of point defects and small He bubbles. Lattice mismatch causes changes in the spacing and inclination of diffraction planes producing a shift and a rotation of diffraction intensity lines (Bragg contours (BC)) visible in LACBED patterns. In particular, when the electron beam is parallel to the rotation axis, the BC simply rotate. This is the case of mismatches observed with (333) BC when the electron beam is incident on the sample along a direction close to <110> zone axis. Measurements show a peak of the stress/strain field (intensity 1.5 GPa) at a depth close to the helium projected range, where small bubbles occasionally form aggregate oriented along <011> direction.