Suppression of boron transient enhanced and thermal diffusion in silicon and silicon germanium by fluorine implantation

El Mubarek, Huda Abdel Wahab Abdel Rahim (2004) Suppression of boron transient enhanced and thermal diffusion in silicon and silicon germanium by fluorine implantation. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

In this thesis a study is made of the growth of buried boron marker layers with sharp and narrow boron profiles and of the effect of fluorine implantation on the diffusion of boron in buried marker layers in silicon and silicon germanium.

Initial experiments investigate the effect of varying F⁺ implantation energy on boron thermal diffusion and boron transient enhanced diffusion (TED) in Si_1-xGe_x. In samples implanted with 185keV F⁺, the fluorine suppresses boron transient enhanced diffusion completely and suppresses thermal diffusion, whereas in samples implanted with 42keV F⁺, the fluorine does not reduce boron transient enhanced diffusion. These results indicate that a high energy F⁺ implant is much more effective than a low energy implant for suppressing boron diffusion.

The effect of F⁺implantation dose on the diffusion of boron in silicon and silicon germanium is then studied. In silicon samples implanted with P⁺ and 2.3x10¹⁵cm^-2 F⁺, the fluorine completely suppresses boron transient enhanced diffusion. Reduction of boron thermal diffusion is observed for F⁺ doses at and above a dose of 1.4x10¹⁵cm^-2. In Si_1-xGe_x a reduction of boron thermal diffusion is observed for F⁺ doses at and above a dose of 9x10¹⁴cm^-2, whereas a suppression of boron transient enhanced diffusion is observed for all F⁺ doses. For F⁺ doses of 1.4x10¹⁵cm^-2 and 2.3x10¹⁵cm^-2 the fluorine reduces the boron thermal diffusion coefficient by factors of 1.9 and 3.7 in silicon and factors of 2.5 and 3.5 in Si_1-xGe_x respectively. The reduction of boron thermal diffusion correlates with the appearance of shallow fluorine peaks in the silicon layers and Si_1-xGe_x layers at and above the critical doses of 1.4x10¹⁵cm^-2 and 9x10¹⁴cm^-2 respectively. These shallow fluorine peaks are present in samples with and without boron marker layers in both silicon and Si_1-xGe_x and hence are not due to a chemical interaction between the fluorine and boron.

Transmission electron microscopy (TEM) micrographs show that there are no extended defects in both the silicon and Si_1-xGe_x layers, and hence it is proposed that the shallow fluorine peaks are due to vacancy-fluorine clusters. The reduction in boron thermal diffusion above the critical F⁺ dose is then explained by the presence of the vacancy-fluorine clusters, which suppress the interstitial concentration in the silicon and Si_1-xGe_x layers. The suppression of boron transient enhanced diffusion correlates with a deep fluorine peak around the range of the fluorine implant and TEM micrographs show that this peak is due to a band of dislocation loops. The suppression of TED by fluorine is then explained by the influence of the loops in suppressing the backflow of interstitials to the surface. Analysis of the SIMS profiles shows that fluorine is transported from the adjacent silicon into the Si_1-xGe_x layer during anneal, and reaches concentrations that are much higher than observed after implant. This mechanism would give benefits in devices like Si_1-xGe_x heterojunction bipolar transistors (HBTs), since a high fluorine concentration is automatically obtained in the vicinity of the boron profile, which maximises the effect of fluorine in suppressing boron diffusion.