High-dose helium-implanted single-crystal silicon: Annealing behavior

The modifications induced in single-crystal silicon by implanted helium have been investigated by ion beam techniques. The damage has been detected by 2 MeV 4He+ backscattering in channeling conditions and the helium in-depth distribution by 7 and 8 MeV 15N++ elastic recoil scattering. The samples prepared by implanting 2×1016 cm−2 helium ions at 20 keV in silicon wafers held either at 77 K (LNT sample) or at 300 K (RT sample) have been heat treated for 2 h in the 100–800 °C temperature range. In the as-implanted LNT sample the damage maximum is at 130±20 nm and shifts in-depth to 180±10 nm after annealing at 200 °C, in the as-implanted RT sample, the damage maximum is already located at 180±10 nm. In the 250–500 °C temperature range, the LNT and RT samples follow the same annealing path with only slight differences in the temperature values; in both cases, the dechanneling signal increases and reaches a maximum value of nonregistered silicon atoms of 2.2–2.5×1022 at/cm3. In the same temperature range, the helium signal becomes narrower, builds up in a region centered on 220±20 nm and no appreciable loss of helium can be detected. The growth of the damage is consistent with the creation of cracks and a etherogenous distribution of bubbles filled with high pressure helium which stress the lattice; for the channeling Rutherford backscattering technique, their action is similar to silicon interstitials. At temperatures above 500 °C, helium is released from the samples; this process is associated with a decrease of the damage and the formation and increase in size of voids. At 900 °C empty voids with a diameter around 20 nm are found.