Role of Interstitials in As TED and Clustering in Crystalline Silicon
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Role of Interstitials in As TED and Clustering in Crystalline Silicon Scott A. Harrison, Thomas F. Edgar and Gyeong S. Hwang Department of Chemical Engineering, University of Texas at Austin Austin, TX 78712 ABSTRACT In recent years, experimental studies [1-3] have suggested that Si interstitials may play a role in facilitating As transient enhanced diffusion during pn junction formation in silicon. These studies contradict conventional models that assume vacancy-mediated As diffusion. Using density functional theory calculations within the generalized gradient approximation, we have examined the structure, stability, and diffusion of the neutral As-Sii pair. We find the lowest energy structure is comprised of an As and Sii atom pair that is aligned in the [110] direction while sharing a lattice site. We have calculated the binding energy as well as diffusion pathways and barriers for the neutral As-Sii pair. Our results suggest that the neutral As-Sii pair has a binding energy relative to neutral Sii and neutral As of 0.63 eV. We also find an overall diffusion activation energy of 3.3 eV, which is similar to experimental observations for As diffusion and previous calculations for As-vacancy complex diffusion. These results clearly support that interstitials can contribute significantly to As transient enhanced diffusion, especially in regions where interstitials exist in excess. In addition, interstitial-mediated arsenic diffusion suggests that interstitials may also play a role in arsenic agglomeration. INTRODUCTION As silicon transistors dimensions are scaled down, the formation of ultrashallow junctions (< 20 nm in depth) is necessary to avoid short-channel effects. Currently, junctions are most often fabricated by using low energy ion implantation to inject dopant impurities into the silicon wafer surface. This step is followed by thermal annealing to re-crystallize the damaged silicon surface and electrically activate implanted dopants. During this annealing step, implanted dopants often show significant transient enhanced diffusion (TED). TED results in dopant profile elongation that makes meeting current junction depth requirements a great challenge. It is therefore necessary to determine the underlying mechanism of dopant TED. By understanding TED, process conditions that best minimize the TED while maximizing the electrical activity of dopants can be determined. At high temperatures (≥ 750 ºC) and highly doped conditions (> 3×1020 atoms/cm3), implanted arsenic (As) has been found to demonstrate significant TED during thermal treatment [1-5]. Earlier theoretical studies have proposed that Si vacancy (V) mediated diffusion in the form of mobile As-V and As2–V complexes may primarily be responsible for As TED [6-8]. However, recent experimental observations have suggested that Si interstitials (Sii) may also promote As TED [1-3], indicating further study of the TED mechanism is needed. Moreover, in a recent theoretical study, we showed that mono- and di-vacancy arsenic complexes (AsmV, AsnV2) are easi
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