Indium in silicon: interactions with native defects and with C impurities
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Indium in silicon: interactions with native defects and with C impurities P. Alippi, A. La Magna, S. Scalese, V. Privitera CNR-IMM, Sezione Catania, Stradale Primosole 50, 95121 Catania (Italy) ABSTRACT
Equilibrium geometries and formation energies of neutral and charged In complexes with silicon native defects (vacancy (V) and self-interstitials (I)) and with C impurities are investigated within density functional theory, using the Vienna Ab-initio Simulation Package. We determine formation energies and ionization levels of different complexes and discuss the contribution of I and V to indium diffusion. We also identify the In-C defect responsible for the increased electrical activation in In+C-doped silicon samples. The ab initio energetics is then implemented in a continuum diffusion code in order to simulate the diffusion of as-implanted In profiles under different thermal treatments. INTRODUCTION Indium doping of silicon allows the realization of steep as-implanted profiles, due to indium heavy mass. Thus, in spite of the low electrical activation measured in indium-doped silicon samples, its implementation as possible alternative to boron as p-type silicon dopant is a matter of investigation. To this respect, the interactions between implanted indium and the defects (interstitial atoms, I's, and vacancies, V's) created by implantation or the impurities present in the crystal do play an important role. Transient enhanced diffusion is found in conditions of supersaturation of interstitials, while no oxidation enhanced diffusion is detected [1]. These results point out to a diffusion mechanism dominated by the interactions with self-interstitials rather than with vacancies, although a clear-cut experiment aimed at determining the relative weight of the two mechanisms to the total indium diffusivity has not been performed. Experimental evidence of increased activation in samples co-implanted with carbon is given in recent experimental works [2]: there, the electrical properties of In and In+C co-implanted silicon in different process conditions are investigated in an estensive and systematic way. On the theoretical side, a careful atomistic investigations of structures, energetics, and electronic properties of In-related defects in silicon is missing. As a consequence, the modeling of In diffusion in silicon has been often based on a priori hypothesis (as, for instance, similarity with B behaviour in silicon) or ad hoc fitted parameters. Although capable in many cases to simulate the experimental profiles, these models lack however of transferability and predictive power. The present work is aimed at filling this lack of atomistic investigations: we have in fact calculated first principle formation energies and ionization leves of different In-related defects in silicon (In-I, In-V and In-C), in order to clarify possible binding mechanisms, migration behaviour and electronic characteristics. A diffusion models is then set up, based on ab initio energetics. RESULTS The ab initio calculations are performed
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