Computer Simulation of Channeling Profile Analysis of Implantation Damage
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residual damage remaining after annealing. Compared to channeling-RBS this method is sensitive to lower defect concentrations. The present work reports on systematic binary collision (BC) computer simulations of experimental SIMS data obtained by CPA of silicon targets predamaged by Si+ ions of different energies [3, 4]. The damage accumulation during the preimplant is described by a phenomenological model which has been already employed in order to simulate the dose dependence of the shape of range profiles in ion implantation into virgin targets [5]. The procedure yields an estimation of the distribution of such as-implanted defects which are relevant for the dechanneling process during the CPA. A modified model is used to obtain informations about the profile of residual defects if samples were analyzed after annealing. SIMULATION MODEL CPA is simulated by the program Crystal-TRIM. This code is based on the binary collision (BC) approximation which assumes that the motion of a projectile in the target crystal may be described by a sequence of binary collisions of the projectile with target atoms. The formalism and basic physical inputs of Crystal-TRIM have been elucidated elsewhere [6, 7, 5]. In the following only the modeling of damage accumulation is briefly described. 133
Mat. Res. Soc. Symp. Proc. Vol. 532 01998 Materials Research Society
The introduction of defect accumulation into the simulation of ion implantation means the treatment of the ion impact into a dynamically changing target structure. BC codes which contain such a procedure are therefore called dynamic. Due to the lack of detailed microscopic information, the damage buildup must be described by a phenomenological model. A detailed review on the different models hitherto used in dynamic BC codes has been given recently [5]. The first problem to be considered is the identification of the relevant defects. In the framework of BC simulations this question is connected with the description of collisions of the projectile with target atoms in a damaged region. Some previous investigations were limited to the consideration of vacancies and silicon self-interstitials [5]. However, Molecular Dynamics (MD) simulations have recently shown that during single ion impact not only point defects are formed, but also a considerable amount of submicroscopic disordered regions consisting of clusters of displaced atoms or amorphous pockets [8]. Due to their greater extension, submicroscopic disordered regions are more important for enhanced dechanneling of incident particles than point defects. Therefore, they are considered as the relevant defects in the phenomenological damage buildup model. Unfortunately, there exists no unique description of binary collisions with target atoms in the region of such defects [5]. In the program Crystal-TRIM a statistically independent accumulation of amorphous pockets is assumed. The motion of projectiles in amorphous material is described like in the standard TRIM code [9]. The first advantage of such a model is, that it is
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