Mechanisms for Interstitial-Mediated Transient Enhanced Diffusion of N-Type Dopants
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0912-C02-10
Mechanisms for Interstitial-Mediated Transient Enhanced Diffusion of N-Type Dopants Scott A. Harrison, Thomas F. Edgar, and Gyeong S. Hwang Chemical Engineering, University of Texas at Austin, 1 University Station, Austin, TX, 78712 ABSTRACT As silicon transistors scale down, transient enhanced diffusion of n-type dopants has become a barrier toward achieving required junction depths for transistors. In this paper, we use density functional calculations to identify a pathway by which silicon interstitials (Sii) mediate As and P diffusion. We show that As-Sii and P-Sii pairs in the neutral and negative charge states diffuse via a mechanism in which the dopant is bond-centered at energy minima and threefold coordinated at the high energy saddle point during dopant migration. For both As-Sii and P-Sii pairs, we conclude that neutral pairs will dominate under intrinsic conditions while the neutral and negatively charged pairs will both contribute under heavily doped extrinsic conditions. INTRODUCTION The 2005 International Technology Roadmap for Semiconductors predicts ultrashallow junctions less than 7 nm deep will be required for silicon transistors to be manufactured in 2010. To meet this depth requirement, it is necessary to have a better understanding of the dopant transient enhanced diffusion (TED) that undermines its achievement. As and P are the two most commonly used n-type dopants in junction formation. In this paper, first principles calculations based on gradient-corrected density functional theory are used to develop an understanding of ntype dopant TED mediated by silicon interstitials in crystalline silicon. It has long been known that dopant TED is mediated by defects present in silicon. Early experiments established that while P TED is exclusively mediated by silicon interstitials, As TED is mediated by both interstitials and vacancies [1]. A subsequent study determined that the fraction of As TED mediated by vacancies and interstitials is 0.6 and 0.4, respectively, while confirming the previous findings for P [2]. The challenge of scaling n-type junctions has led to recent experimental reports investigating how process conditions effect As [3-5] and P TED [68]. To complement these experimental observations, studies on DFT calculations have sought to clarify the microscopic mechanisms underlying As and P TED. In the 1990’s, reports focused on vacancy-mediated As TED and showed that AsV [9-11] and As2V [11] complexes can diffuse. However, recent experimental studies [3-5] have suggested a major role of interstitials in driving As TED at low junction depths. Recently, a DFT study found that As-interstitial (As-Sii) pairs in the neutral and negative charge state are the primary contributors to interstitial-mediated As TED [12]. With regard to interstitial-mediated P TED, a recent DFT study found that neutral and positively charged P-interstitial (P-Sii) pairs are responsible for P TED [13]. In this paper, we use gradient-corrected density functional calculations to determine a viable pathway b
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