The Effect of Fluorine Implantation on Boron Diffusion in Metastable Si 0.86 Ge 0.14
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The Effect of Fluorine Implantation on Boron Diffusion in Metastable Si0.86 Ge0.14 Huda A. W. A. El Mubarek, Yun Wang1, Janet M. Bonar, Peter Hemment1, Peter Ashburn School of Electronics & Computer Science, University of Southampton, Southampton, UK 1 School of Electrical & Electronic Engineering, University of Surrey, Guildford, UK
ABSTRACT This paper investigates the effect of varying F+ implantation energy on boron thermal diffusion and boron transient enhanced diffusion (TED) in metastable Si0.86Ge0.14 by characterising the diffusion of a boron marker layer in samples with and without P+ and F+ implants. The effect of two F+ implantation energies (185keV and 42keV) was studied at two anneal temperatures 950°C and 1025°C. In samples implanted with P+ & 185keV F+, the fluorine suppresses boron transient enhanced diffusion completely at 950°C and suppresses thermal diffusion by 25% at 1025°C. In samples implanted with P+ & 42keV F+, the fluorine does not reduce boron transient enhanced diffusion at 950°C. This result is explained by the location of the boron marker layer in the vacancy-rich region of the fluorine damage profile for the 185keV implant but in the interstitial-rich region for the 42keV implant. Isolated dislocation loops are seen in the SiGe layer for the 185keV implant. We postulate that these loops are due to the partial relaxation of the metastable Si0.86Ge0.14 layer. INTRODUCTION
The minimisation of boron diffusion is vitally important in all types of Si and SiGe devices [1,2]. For example, in SiGe HBTs diffusion of boron from the SiGe base creates parasitic energy barriers [1] that degrade the current gain and limit the value of cut-off frequency that can be achieved. Similarly, in MOSFETs diffusion of boron in the pocket [2] has detrimental consequences on short channel effects. Recently, fluorine implantation has been investigated as an alternative method of suppressing boron diffusion in silicon, and shown to be extremely effective [3-8]. However to the authors’ knowledge, there have been no reports of the effects of fluorine on boron diffusion in SiGe. In this paper, a study is therefore made of the effect of fluorine on both boron thermal diffusion and boron transient enhanced diffusion in metastable Si0.86Ge0.14. EXPERIMENTAL PROCEDURE
Low pressure chemical vapour deposition (LPCVD) at 700°C was used to grow a 400nm Si starter layer, a 67nm Si0.86Ge0.14 layer and a 67 nm Si cap layer on a (100) silicon wafer. A boron doped marker layer was incorporated within the Si0.86Ge0.14 layer with a peak concentration of
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4.1×1019cm-3. Three types of samples were then produced from the same wafer; the first had no implants (unimplanted), the second a phosphorus and a low energy fluorine implant (P+ & low F+) and the third a phosphorus and a high energy fluorine implant (P+ & high F+). A 288keV, 6x1013cm-2 phosphorus implant was used to cause transient enhanced diffusion [7]. The low energy F+ was implanted at 42KeV, 9x1014cm-2, with the energy chosen to give a fluorine peak (at
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