Some problematic points in identification of bismuth impurity centers in floating-zone silicon irradiated with 15 MeV protons

N. Arutyunov
Martin Luther University Halle, IPLT-Institute of Electronics, Tashkent,

Keywords: bismuth, silicon, radiation defects, positron annihilation, nondestructive testing


The reliability of quantum computing on the basis of the donor centers of 209-Bi (J=9/2) is known to be affected by a shell of atoms containing Bi-related qubit forming centers in silicon having natural isotope composition. Meanwhile, a direct data on local electron/spin densities in the crystal open volume surrounding 209-Bi ion core is lacking. An admixture of positron to the spin/electron density of 209-Bi impurity center, -i.e. its positron probing, - allows one to obtain such information by detecting 2–gamma annihilation of the electron-positron pairs entangled with the polyelectron system of the Bi donor impurity center. Such testing of material is generally accepted to consider as a nondestructive one. Having applied positron annihilation lifetime spectroscopy, we have observed a decrease of 2–gamma annihilation rate with the increase of number of electrons contacting positron at substitutional Bi impurity center. This effect to be governed by spin-orbit field of Bi impurity atom in the crystal lattice of natural silicon is interpreted as an increase of number of electron-positron pairs in the polyelectron system with parallel spins whose decay on two gamma–quanta is prohibited by selection rules. Enormously large cross-section of localization of positron at Bi donor impurity center was found to be equal to ~ (1.3–1.7) ×10^(–13) cm^2 (ΔT~ 295 to ~ 25 K). The picture looks as if the admixture of the charge-even states of electron-positron pairs to the polyelectron in the spin-orbit field of Bi center decreases probability of long-lived 2–gamma decay, whereas the charge-odd states of the electron-positron pairs suppress the contribution of short-lived singlet electron-positron pairs to the resulting probability of 2–gamma annihilation. This effect is not observed for the substitutional phosphorus donor impurity center which is known to be characterized by a comparatively low spin-orbit interaction. Revealed for the substitutional Bi donor impurity center the suppression of 2–gamma annihilation rate is masked by its yet more pronounced decrease to be observed after irradiating the material with 15 MeV protons. Similarities and distinctions of 2–gamma annihilation processes in both Bi- and P-related centers formed as a result of the proton irradiation and subsequent isochronal annealing have been studied in parallel with low-temperature Hall effect measurements. On the whole, the accumulated database give an impression about the radiation resistivity of Bi- and P-doped moderately doped floating-zone n–type silicon which is subjected to irradiation by protons of MeV energies. It is argued that the microstructure of the Bi impurity center in the proton-irradiated material has an open volume, and this point defect should possess octahedral, probably distorted, symmetry. This suggestion is supported by the results of ab initio calculations (see, e.g. H. Höhler et al., Phys. Rev. B 2005, 71, 035212). The effects observed are discussed in the light of current reconsidering of a whole conception of formation of the positron-sensitive vacancy-Group-V-impurity centers in the moderately doped floating-zone n–type silicon subjected to irradiation by electrons and protons of MeV energies.