Ion Channeling Techniques for Defect and Surface Studies
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and Tan,
eds.
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Defects in Semiconductors
ION CHANNELING TECHNIQUES FOR DEFECT AND SURFACE STUDIES* B. R. APPLETON Solid State Division, Oak Ridge National Laboratory,
Oak Ridge,
Tennessee 37830
ABSTRACT
The channeling and blocking phenomena are well enough understood that they can be combined with ion scattering or ion induced reactions to investigate defects and surface structures. A brief review is given of the origins of these phenomena, of associated concepts which are useful for defect and surface analysis, and of experimental constraints and requirements. Applications to selected problems are used to illustrate the advantages and limitations of these techniques.
INTRODUCTION This review is one of three companion papers designed to provide an introduction to the use of channeling for defect studies. The primary emphasis of this paper is on the origins of ion channeling and the associated concepts which make it useful for defect analysis and surface studies. Particular emphasis is given to surface and interface analysis. The companion reviews by Chu [1] and Picraux [2J concentrate more on specific applications. The interested researcher should be able to assess from these papers whether channeling techniques are suitable for his particular problems, and find ready reference to the publications necessary to effect their application. No attempt was made in this paper to provide a complete or historical accounting of the development of the ideas or the first applications of channeling. Consequently, the examples used and references given were chosen for emphasis and do not necessarily represent a balanced view. There are a number of excellent reviews where such information can be found [3-7].
ORIGINS OF THE CHANNELING EFFECT The circumstances responsible for the channeling of energetic ions in single crystals are represented schematically in Fig. 1 and are considered in numerous books and articles [1-5]. When ions impinge on a disordered solid or a misaligned single crystal, they encounter the atoms at random and sample a random distribution of impact parameters. This assumption underlies most descriptions of ion-solid interaction phenomena. However, if the bombarded sample is a single crystal aligned so that the ions are incident parallel to an axial or planar "channel," such as those shown in Fig. 1, then the ions experience successive, correlated collisions with the symmetrically arranged atoms (center schematic Fig. 1) and acquire oscillatory trajectories in the open regions, or channels, between the rows or planes of atoms. The minimum distance of approach of a channeled ion to atoms on normal lattice sites is Pmin - 10-9 cm. Since many ion-solid interactions such as Rutherford scattering require much closer impacts *Research sponsored by the Division of Materials Sciences, U.S. Department of Energy under contract W-7405-eng-26 with Union Carbide Corporation.
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CHANNELING EFFECT
GEOMETRIES ANALYSIS CHANNELING
RANDOM
AXIAL CHANNEL
•k•
CHANNEL
UNIAXIAL*1
BIAXIAL
I
CLOSE IMPACT COLLISbON
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