Helium in silicon: Thermal-desorption investigation of bubble precursors

Thermal desorption measurements are performed on (100)-oriented $p$-type Si wafers implanted with He ions at 20 keV. The doses have been selected in order to produce crystal damage avoiding the formation of detectable bubbles. The He effusion kinetics, studied both in isothermal and in constant heating rate conditions, exhibit effective activation energy heterogeneity indicating the presence of various kinds of traps, precursor of the bubbles. The energy distribution results peaked at about 1.1 eV with an exponential decay towards higher energies and a width of about 0.2 eV. A semiquantitative model, based on the present knowledge about the Si:He system, is proposed, that accounts for He filled nanoblisters formation through interstitial He clustering and precipitation. The observed energy heterogeneity is ascribed to variations of the He solution energy from these cavities due to He-He and He-wall interactions.