Uphill diffusion of ultralow-energy boron implants in preamorphized silicon and silicon-on-insulator

Abstract

Redistribution during annealing of low-energy boron ͑B͒ implants in silicon on insulator ͑SOI͒ structures and in bulk Si has been investigated by comparing secondary ion mass spectrometry ͑SIMS͒ and simulated profiles. All the samples have been preamorphized with Ge at different implantation energies in order to investigate the effects of the position of the damage on B diffusion. Different B doses in the range between 2 ϫ 10 13 and 2 ϫ 10 15 cm −2 and annealing temperatures between 700 and 1100°C have been investigated. All SIMS profiles show a B pileup in the first few nanometers of the Si matrix in proximity of the Si surface. The results of our simulations, performed on samples implanted at different doses ͑below and above the solid solubility͒, indicate that the B redistribution upon annealing can be explained with a simple model which considers the presence of traps in the surface region, without considering any asymmetric behavior of the dopant diffusion. The sink region is a few monolayers ͑1-2 nm͒ for doses of 2 ϫ 10 13 and 2 ϫ 10 14 cm −2 , and it extends to about 7 nm for the highest dose of 2 ϫ 10 15 cm −3 , in the region of very high B concentration where precipitates and clusters shrink the incoming B atoms. For the two lowest B doses, the amount of B trapped at the surface is maximum at temperatures around 800°C, when more than 80% of the implanted dopant is made immobile and electrically inactive. In our experimental conditions, i.e., preamorphization performed with constant dose and different implantation energies, the amount of trapped B increases with reducing the depth of the amorphous layer and it is higher in the bulk Si than in SOI.

Modeling of the transient interstitial diffusion of implanted atoms during low-temperature annealing of silicon substrates

“…All these phenomena substantially complicate the problem of the formation of very shallow junctions with high electrophysical parameters. For suppressing the TED of ionimplanted boron, a method of boron implantation in a silicon layer preamorphized by heavier germanium ions is widely used 9,[13][14][15][21][22][23][24][25][26][27][28][29][30][31][32][33] . Due to the solid phase epitaxial regrowth (SPER) of the amorphous layer, the region doped with boron is characterized by a perfect crystal structure, containing defects that are invisible by electron microscopy.…”

“…for a small thermal budget). Really, an increase in the "tail" extension occurs during subsequent thermal treatments of preamorphized silicon layers that were implanted with boron ions 23,27,31,32 . In the investigations carried out by 15,22,28,33 clearly identified "tails" after boron implantation were not observed.…”

Non-Fickian current enabling the uphill diffusion of impurities diffusing by the mechanism of mobile impurity-defect pairs

“…The mechanism of diffusion via mobile impurity-defect (I-d) pairs was developed in Refs. [1][2][3] in order to explain the unusual exponential diffusion profiles of boron impurities in silicon. Its peculiarity in comparison with the conventional mechanisms of defect-mediated diffusion is that the impurity within the pair exists in a mobile state where it may perform not one but many consecutive migration steps due to the closeness of the diffusionassisting defect.…”

“…For concreteness some characteristic numbers from Refs. [1,3,11] were used in the simulations as the input parameters. The stationary defect profile was modeled departing from the fact that in Ref.…”

“…[11] strong gradients of the defect concentration were observed at depth below 10 nm. So c d was assumed to be constant in Si bulk with the supersaturation maintained during the experiment, e. g., by the defect clusters [3], at the level of 10 2 [1] but starting from the depth 10 nm c d linearly dropped to the equilibrium value at the surface [3]. The equilibrium diffusion constant was calculated according to Ref.…”