Defect Induced Amorphization in Silicon: A Tight Binding Molecular Dynamics Simulation
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D. MARIC Swiss Scientific Computing Center, CH-6928 Manno (Switzerland) L. COLOMBO Dipartimento di Fisica, Universita' di Milano, via Celoria 16, 1-20133 Milano (Italy)
ABSTRACT We present an investigation on the amorphization process of crystalline silicon induced by ion beam bombardment by simulating the insertion of self-interstitials at different temperatures. The simulation is carried out by tight-binding molecular dynamics which allows for a detailed characterization of the chemical bonding and electronic properties of the irradiated samplesThe irradiation process consists of two steps: (i) insertion of defects at a constant rate; (ii) annealing of the sample and observation of its structural properties. Thanks to the large size of the simulation cell (up to 276 atoms) we can characterize the amorphous network both on the shortrange and medium-range length scale. Electronic properties are investigated as well and their evolution is monitored during the insertion process. Finally, we present a thorough comparison of the structural properties of the irradiated sample with amorphous silicon as obtained by rapid quench from the melt.
INTRODUCTION Electron-beam and ion-beam bombardment of solids is well know to induce a crystal-toamorphous transition in irradiated samples.", 2 Such a phenomenon has attracted a huge number of experimental and theoretical investigations because of its fundamental interest from the point of view of the formation, nature and stability of the amorphous state of matter. The key role in such a solid-state amophization process is played by point defects (interstitials or vacancies) and Frenkel pairs that are introduced in the host lattice. In this paper we present a molecular dynamics (MD) simulation of the response of a crystalline silicon lattice to random insertion of self-interstitials. Our main goals are to find the treshold concentration of defect above which the amorphization is observed and to study the dependence of the crystal-toamorphous transition upon the temperature of the substrate and the rate of insertion of selfinterstitials. Moreover we present a detailed characterization of both structural and electronic properties of the irradiated samples and show a comparison to amorphous silicon (a-Si) as 34 obtained by direct quenching from the melt. , MD simulations are particularly suitable to describe at the atomic level defect-induced structural relaxations and defect-defect interactions. 3 ,5,6 In this paper we make use of the tightbinding molecular dynamics (TBMD) scheme that we have successfully applied to study a-Si obtained by overcooling liquid silicon (I-Si). 7 The reliability of our method has been extensively discussed elsewhere. 3 In particular, we remark that the present TB Hamiltonian has been successfully applied to study intrinsic point defects in crystalline Si, like vacancies, interstitials, 5 divancancies and Frenkel pairs. 423 Mat. Res. Soc. Symp. Proc. Vol. 321. ©1994 Materials Research Society
In the TBMD simulations the atomic trajectories are calculat
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