Precipitation and Diffusivity of Arsenic in Silicon

Abstract

Small angle x-ray scattering measurements were performed on arsenic-implanted laser-annealed silicon slices to investigate the features of precipitation at 900~ for different dopant supersaturations. With decreasing As concentration from 3.2 to 1.1 • 10 ~l cm -3, the size distribution of the precipitates shifts toward larger values and their shape evolves from that of thin platelets, becoming three-dimensional and approaching at last that of a sphere. The high concentration diffusivity of As at 900~ deduced from the size of the largest precipitates in specimens doped 1 • 102' cm -3, turns out to be 2 • 10 -'4 cm2-s -~. Determinations of the shift of the carrier concentration profiles and of the junction depth, after heating at 1000~ show that the diffusivity of As is not affected by the precipitation and, hence, is controlled by the concentration of the dopant.The physical nature of the electrically inactive As, which is present in Si at high doping levels, is receiving increasing attention, primarily due to the demands of VLSI. Well-known models hypothesize the formation of complex point defects, or "clusters," in thermal equilibrium (1-6).Previous work (7) performed on ion-implanted and laser-annealed samples, doped with As in a wide composition range, put into evidence that the electrically active concentration depends only on temperature, a finding leading to the conclusion that a two-phase equilibrium takes place.The formation of a second phase was also indicated by the isochronal heating behavior, typical of a precipitation process. Precipitates were in fact detected in very heavily doped specimens by small angle x-ray scattering (SAXS), a technique which is suitable because of the noticeably different atomic numbers of Si and As. These determinations provided information on the size distribution and shape of these precipitates, which were found to be in the form of thin platelets. These particles should be coherent with the silicon matrix, as suggested by the absence of extra lines or spots in the diffraction patterns.As an additional result, the displacement of the carrier concentration profiles due to the heat-treatments showed that the diffusivity of the dopant strongly increased with the implanted dose. This effect, which was particularly evident at high temperature, could be attributed either to the increase of As concentration or to the increased amount of precipitates per unit surface, i.e., to lattice defects in excess generated by the precipitation process.The present work extends SAXS examinations to less heavily doped compositions (a difficult task owing to the small thickness of the implanted layer) with the aim of controlling the occurrence of precipitation at lower dopant concentrations and of getting further information on this process. In addition, it was intended to verify whether the precipitation process can influence the high temperature diffusivity of As. ExperimentalFor these experiments, we used (100)-oriented, CZpulled, p-type, boron-doped silicon slices of 1 t2-cm resistivity.The s...

Electron Microscopy of As Supersaturated Silicon

“…Precipitates were, in fact, detected by small angle x-ray scattering (SAXS), a technique that is suitable because of the noticeably different atomic numbers of Si and As. These determinations provided information on the size distribution and shape of these precipitates that was found to be in agreement with the classical theory of nucleation, which predicts a decrease in particle density and an increase in particle size as the supersaturation decreases (9). Moreover, most of these particles should be coherent with the silicon matrix, as expected from the RBS experiments performed in channeling conditions (7,10), showing that the off-axis concentration is markedly lower than the precipitated one.…”

“…The subsequent thermal treatment of these supersaturated layers gives rise to a strong lowering of the carrier concentration, attributed to As precipitation (8,9). The values of the sheet carrier density Ns before and after different thermal treatments (including the low temperature one, performed at 450~ for 4h) are shown in Table I, to-2560 ) unless CC License in place (see abstract).…”

“…Liquid-phase epitaxial regrowth induced by pulsed laser irradiation produces a nearly complete electrical activation of the dopant, which is forced into substitutional positions at a concentration much higher than the solid solubility (15). After laser annealing, the implanted layers were single crystalline, without the presence of extended defects.The subsequent thermal treatment of these supersaturated layers gives rise to a strong lowering of the carrier concentration, attributed to As precipitation (8,9). The values of the sheet carrier density Ns before and after different thermal treatments (including the low temperature one, performed at 450~ for 4h) are shown in Table I, to-2560 ) unless CC License in place (see abstract).…”

“…Recent work (8,9) performed on ion-implanted and laser-annealed silicon, doped with As in a wide range of compositions, provided evidence that the electrically active concentration depends only on temperature, a finding leading to the conclusion that a two-phase equilibrium takes place. The formation of a second phase was also indicated by the isochronal heating behavior, showing a reverse annealing typical of a precipitation process (8).…”

Simulation and experimental study of the dynamics of arsenic clustering and precipitation including ramp-up and ramp-down conditions

“…al. [8] show that the diffusivity of arsenic increases with concentration at high temperatures ( 1000°C) even at concentrations as high as 2x loz1 ~m -~, which is not consistent with any model which holds that the electrically inactive fraction of arsenic is immobile, hecause the mobile, electr~cally active fraction normally is assumed to saturate or even decrease with increasing concentration. Furthermore, our inability to fit an Arrhenius type expression with a single activation energy to the coefficients K c and Kn for different temperatures and ramp-rates also implies that the process of deactivation of arsenic is apparently not governed by a single mechanism (e.g.…”