First Principles Study of Boron in Amorphous Silicon
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1070-E05-07
First Principles Study of Boron in Amorphous Silicon Iván Santos1,2, Wolfgang Windl1, Lourdes Pelaz2, and Luis Alberto Marqués2 1 Dept. Materials Science and Engineering, The Ohio State University, Columbus, OH, 43201 2 Dpto. Electricidad y Electrónica, Universidad de Valladolid, Valladolid, Spain ABSTRACT We have carried out an ab initio simulation study of boron in amorphous silicon. In order to understand the possible structural environments of B atoms, we have studied substitutionallike (replacing one Si atom in the amorphous cell by a B atom) and interstitial-like (adding a B atom into an interstitial space) initial configurations. We have evaluated the Fermi-level dependent formation energy of the neutral and charged (±1) configurations and the chemical potential for the neutral ones. For the interstitial-like boron atom, we have find an averaged formation energy of 1.5 eV. For the substitutional case, we have found a dependence of the chemical potential on the distance to Si neighbors, which does not appear for the interstitial ones. From MD simulations, we could observe a diffusion event for an interstitial-like boron atom with a migration barrier of 0.6 eV. INTRODUCTION Future CMOS technology requires ultrashallow junctions with high dopant activation levels and very sharp doping profiles [1]. According to the ITRS [1], solid-phase epitaxial regrowth (SPER) of doped amorphous silicon (a-Si) is a promising technique to minimize channeling [2], transient-enhanced diffusion [3,4] and dopant deactivation [5,6] and it allows in the case of B to obtain high active concentrations of 2-4×1020 cm-3 [7,8], compared to 2-9×1019 cm-3 in cSi [9,10]. Thus, for the fabrication of state-of-the-art electronic devices, preamorphization implants and high dopant implantation fluences are being used. Because of these reasons, there is an increasing interest in studying and modeling dopant processes in a-Si. B has been extensively studied in c-Si and diffusion mechanism [11-14], interaction with Si atoms and segregation processes [15-18] are well known. However, its behavior in a-Si has been studied to a much lesser degree, especially at the atomic level, apart from one early study [19]. The aim of this work is to understand the possible structural environments of B in a-Si and eventually its diffusion. Evidently, there will be a continuous range of configurations, unlike the discrete set obtained in a crystalline environment. The key dynamical features of B in a-Si are naturally formulated as hopping between various metastable configurations in the energy landscape, where of course the a-Si network has a high probability of rearranging and thus changing the energy landscape during annealing. Thus, we believe that this paper is an important step to the larger problem of unraveling the dynamics of B in a-Si. SIMULATION DETAILS We have carried out an ab initio study of B in a 64-atom a-Si cell. The most realistic computer models of a-Si are made using the algorithm of Wooten, Weaire and Winer (WWW) [20]. The WWW model
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