Energy-loss distributions for 2.5-MeVHe+ions incident on Si single crystals

Abstract

The energy distributions for 2.5-MeV He ions incident on thin Si single crystals are studied. Detailed angular scans are taken through the (110) and ( 100) axial directions along the [111]and [ 110] planar directions as well as perpendicular to the planar directions for Si(110) (0.74-and 1.4-pm-thick) and Si(100) (0.75-pm-thick) samples, respectively. Complex structures in the distributions are observed throughout the angular scans. The experimental distributions are reasonably well reproduced by a Monte Carlo simulation using the semiclassical approximation [N. M. Kabachnik, V. N. Kondratev, and O. V. Chumanova, Phys. Status Solidi 8 145, 103 (1988)] for energy loss to core electrons and the twocomponent free-electron-gas model for energy loss to valence electrons. Systematic deviations between theory and experiment are observed and discussed in terms of an increased penetration depth necessary for He+ ions to become fully ionized when channeled. PACS number{s): 61.80.Mk, 34.50.8w, 79.20.Rf, 61.80.Jh Investigations of energy-loss processes for ions interacting in single crystals have focused primarily on major axial and planar directions. This is due to a markedly reduced energy loss suffered by penetrating ions aligned with these high-symmetry directions over energy loss suffered when aligned with low symmetry ("random") directions [1]. The primary means of analysis of these early studies has been the peak or leading-edge value of the observed energy distributions [2]. This gives a narrow view of the impact-parameter dependence of the energy loss as essentially only impact parameters close to the channel radius are being explored. A much better method is to model individual trajectories in the crystal by the Monte Carlo technique, attempting to fit the full experimental distributions, not just peak or leading edge values, allowing a study over a broader range of impact parameters. Furthermore, by tilting the crystal away from the axial (planar) direction, smaller impact parameters play an increasingly important role and complex structures can be obtained in the energy distributions [3]. The energy-loss model thus proposed should be valid for impact parameters that span the channel radius rather than just near the channel center. Recently, we reported on the impact-parameter dependence of energy loss for 62S-keV H+ ions in Si single crystals [4]. Experimental energy distributions were obtained in the transmission geometry by scattering from a thin gold layer on the beam-exit side. Angular scans were taken through the ( 110) and ( 100) axial directions along the I111I and I 110I planar directions as well as perpendicular to the planar directions for Si(110) and Si(100), respectively. The energy distributions were very well reproduced by the corresponding Monte Carlo simulation when the semiclassical approximation (SCA) of Kabachnik, Kondratev, and Chumanova [5] was used to describe the energy loss to core electrons and the free-'On leave from the Soltan Institute for Nuclear Studies, Warsaw, Poland.electron-...

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