Dechannelling of MeV He ions by twinned regions in implanted Si crystals

Channelling-effect measurements with MeV 4He ions have been made in 28Si implanted and annealed (111)-oriented Si crystals. The bombarding dose and energies were chosen to produce a 4500 A amorphous layer. After annealing and recrystallization, there is a high concentration of twinned regions lying on the three brace111 planes inclined to the (111) surface. For particles channelled along a <111> axial direction in the host crystal, the inclined twins have a lattice with <115> orientation, and consequently the particles have a dechannelling rate which is an order of magnitude higher than along the original <111> directions. For disorder analysis the dechannelled fraction is evaluated using a linear combination of dechannelling in the host and twinned lattices, weighted by the volume concentration of each. The depth distribution of disorder obtained by this method agrees with that determined by layer-removal techniques. When the twinned regions are misoriented with respect to the host lattice by angles comparable to approximately one-half of the critical angle, the normalized aligned yield in the surface region shows a pronounced dependence on beam energy. (author)