Suppression of Boron Diffusion by Fluorine Implantation in Preamorphized Silicon
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Suppression of Boron Diffusion by Fluorine Implantation in Preamorphized Silicon
Giuliana Impellizzeri, José H. R. dos Santos, Salvatore Mirabella, Francesco Priolo MATIS-INFM and Dipartimento di Fisica ed Astronomia, Università di Catania, Via S. Sofia 64, 95123 Catania, Italy Enrico Napolitani, Alberto Carnera INFM and Dipartimento di Fisica, Università di Padova, Via Marzolo 8, 35131 Padova, Italy
ABSTRACT
We have investigated the role of fluorine in the reduction of transient enhanced diffusion (TED) and thermal diffusion (TD) of B in preamorphized Si layers implanted with F. For this purpose, we have employed B delta-doped layers, grown by molecular beam epitaxy (MBE), as markers for silicon self-interstitials (Is). We have shown that boron TED decreases with increasing amount of incorporated F up to the complete TED suppression. Furthermore, we have clearly demonstrated that the physical mechanism that suppresses the boron TED is not a B-F chemical bonding, but a strong interaction between F atoms and Is. In addition, we have seen that fluorine strongly reduces B diffusion also under Is thermal equilibrium concentration. Our results clearly show that the presence of F lowers the Is density very effectively, reducing the boron TED as well as the dopant diffusion under equilibrium conditions.
INTRODUCTION
In Si-based microelectronic technology, the exceptional shrinkage demanded for the fabrication of future complementary metal-oxide-semiconductor (CMOS) devices requires a reduction of electrical junction depths [1]. In this context, minimizing boron diffusion is essential, since B is the most used p-type Si dopant. However, achieving this goal is most difficult, mainly because of the well-known boron transient enhanced diffusion (TED) [2,3]. In fact, since B diffuses in Si exclusively by interacting with self-interstitials (Is) [3], the Is excess induced by B implantation dramatically increases B diffusivity. To solve this problem, different approaches have been investigated, such as ultra low-energy implantation, preamorphization by Si or Ge implantation, C doping engineering [4], laser annealing [5], BF2 implantation or B and F co-implantation [6]. As early as 1983 [6] it was already known that the addition of F to B implanted Si, either separately (B + F implants) or simultaneously with B (BF2 implant), was able to decrease boron TED. More recently, considerable interest has been devoted to study the F influence on B diffusion in silicon [7-11]. However, up to now, there is no clear understanding of the F role on boron TED and two conflicting hypotheses about the physical mechanism by which F reduces the boron TED have been put forward. On the one hand, it has been argued that F interacts with Is, reducing their ability to pair with B and diminishing, as a result, boron TED [7,8]. On the
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other hand, it has been asserted that the presence of F has no effect on the point-defect population, but there should be instead a strong B-F chemical interaction causing a drop of B diffusion [9]. Th
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