Boron diffusion in amorphous silicon and the role of fluorine

Abstract

We demonstrate that boron diffuses at high concentrations during low-temperature thermal annealing in amorphous silicon pre-amorphized by germanium ion implantation. For a typical boron ultrashallow junction doping profile, concentrations as high as 2ϫ10 20 cm Ϫ3 appear to be highly mobile at 500 and 600°C in the amorphous silicon region before recrystallization. In crystalline silicon at the same temperatures the mobile boron concentration is at least two orders of magnitude lower. We also show that boron diffusivity in the amorphous region is similar with and without fluorine. The role of fluorine is not to enhance boron diffusivity, but to dramatically slow down the recrystallization rate, allowing the boron profile to be mobile up to the concentration of 2 ϫ10 20 cm Ϫ3 for a longer time.

Boron activation and redistribution during thermal treatments after solid phase epitaxial regrowth

“…Experiments [5][6][7][8][9][10] and theoretical calculations [11,12] seem to evidence B clustering within the regrown layer for high B concentrations. Sheet resistance measurements [5,[7][8][9] and the hump observed in secondary ion mass spectrometry (SIMS) profiles (associated to B diffusion in amorphous Si) [5,7,8,10] are consistent with active B concentrations up to a few times 10 20 cm −3 for regrowth temperatures around 600 • C, and higher levels can be reached if the regrowth takes place at higher temperatures [27]. The study of the formation of B clusters in amorphous Si requires a complex modeling.…”

“…The implantation cascades are simulated with a binary collision approximation code, MARLOWE [15], which generates the coordinates of the displaced atoms in the lattice along with those of the implanted atom. In our simulations of SPER samples, we directly read the B profile experimentally obtained just after the regrowth, which includes the experimentally observed B diffusion in amorphous Si [10].…”

“…Thus, low-temperature solid phase epitaxial regrowth (SPER) of preamorphized samples appears to be one of the most promising techniques for achieving acceptable sheet resistance and junction depth values to meet the performance specifications of the international technology roadmap of semiconductors (ITRS) [3]. Experiments show that preamorphizing implants (PAI) enhances dopant activation up to concentrations levels ∼10 20 cm −3 during low-temperature SPER of the amorphous layer, with a minimal amount of dopant diffusion [4][5][6][7][8][9][10]. Higher B concentrations are electrically inactive after the recrystallization process [5][6][7][8], which has been associated to the formation of immobile boron clusters [11,12].…”

Mechanisms of boron diffusion in silicon and germanium

“…30,31 Conversely, theoretical calculations recently showed that the Si self-diffusion in a-Si is enhanced in presence of a high percentage of coordination defects, pointing out that selfdiffusion can be assisted by db or fb. 32 Similarly, B diffusion in a-Si was measured to be larger in the as implanted state than in the relaxed one, 10,11,33 showing peculiar features, such as a transient behavior and a dopant concentration dependence, which will be described in the next paragraph.…”

“…2). 33 The immobile portion of the B profile (above 2 Â 10 20 B/cm 3 ) was attributed to B clustering in a-Si. Actually, the formation of B-B pairs during the very early stages of annealing at 550 C, while B is still in the amorphous phase of Si, has been evidenced by x-ray absorption near-edge spectroscopy measurements of p þ /n ultrashallow junctions realized by solid-phase epitaxy.…”