The Diffusion of Lead and Bismuth into Single Crystals of Undoped and Bismuth-Doped Lead Sulfide at 700°C

Abstract

The simultaneous diffusion of 210Pb and 210Bi into undoped single crystals of lead sulfide and single crystals doped with 4×1018 and 4×1019 bismuth atoms/cm3 has been studied as a function of deviations from stoichiometry at 700°C. The diffusion coefficients for lead and for bismuth were identical. The identical values of the lead and bismuth diffusion coefficients suggest that Coulombic attraction between VPb′ and BiPb is negligible. Although the addition of 4×1018 atoms of bismuth to lead sulfide does not change the diffusion coefficients of 210Pb and of 210Bi relative to undoped lead sulfide for −6.5<log sulfur pressure (logps2) <0.0, the addition of 4×1019 atoms of bismuth does. This result and other evidence indicates a higher concentration of native lattice defects than previously considered. For logps2>−3.9, the diffusion coefficients in undoped and lightly doped (4×1018 Bi) material are proportional to ¼ logps2. Because the concentration of singly ionized cation vacancies is also proportional to ¼ logps2, it is concluded that the singly ionized cation vacancy is the predominant defect through which 210Pb and 210Bi diffuse and may be the majority cation defect in lead sulfide at 700°C. For a logps2<−3.9, the diffusion coefficients are independent of composition presumably because the as-grown crystals contain microprecipitates of lead which dissolve extremely slowly under low sulfur pressures. Hence, these crystals contained essentially a constant concentration of defects.