Microstructure-properties of bismuth impurity centers in silicon irradiated by 15-MeV protons

N. Arutyunov
MLU Halle, IKZ Berlin, Germany,

Keywords: point radiation defects, group–V– impurity complexes, silicon, germanium, positron states, isochronal annealing


To gain deeper insight into properties of both the spin system and open volume related to the donor atoms to be used as qubit-forming elements, we have attempted to probe with positrons the bismuth Bi(J = 9/2) and phosphorus P(J = 1/2) impurity centers in silicon having natural isotope composition (J is the nuclear spin). Positron lifetime spectroscopy has been applied. In contrast to P – doped material, an inhibition and a delay of emission of two annihilation quanta has been revealed at the declining temperature when the electron density contacting positron at the Bi center increases. This observation looking paradoxically is suggested to be caused by non-local interaction between the nuclear and electron spins of atoms of magnetic isotope 29Si(J = 1/2) and Bi(J = 9/2) centers which generates triplet electron-positron states inhibiting partly two-quantum electron-positron decay. In the enriched 28Si(Bi) material ("a semiconductor vacuum") possessing very low concentration of 29Si(J = 1/2) isotope, on the contrary, under decreasing temperature a certain increase of two-quantum annihilation rate of the electron-positron pairs in Bi donor center is observed. Thus, the data obtained demonstrate for the first time the influence of, apparently, non-local interaction of the spin systems on the essentially local phenomenon of emission of the two-quantum electron-positron annihilation radiation out of the open volume of donor impurity atom in silicon. The hypotheses of forming emblematic D^0X (D = Bi) exciton involving positron is suggested for interpreting the data [1]. The features of radiation resistance of qubit-forming centers of Bi(J = 9/2) and P(J = 1/2) in silicon has been described including some challenging issues in identification of radiation defects in silicon and germanium [3, 5, 6]. The Bi center acquires an open volume Vop as a result of interaction with the point defects created by irradiation of material with 15 MeV protons. The thermally stable complex [Vop – Bi] having, as it is argued, D3d symmetry is decomposing during a fast stage of isochronal annealing over the temperature range ΔTann.= 320 to 470 °C [1]; the estimated activation energy is equal to Ea = 0.89 ± 0.08 eV. Thermally stable radiation complexes related to P impurity are annealed at a somewhat broader stage, ΔTann.= 430 to ~ 650 °C (Ea = 1.05±0.21 eV [2, 4]). The results available for the qubit-forming Bi and P centers are discussed in the light of reconsidering a whole conception of formation of point radiation defects in moderately doped n–type silicon subjected to irradiation with protons and electrons [3, 5]. [1] N. Arutyunov et al., J. Phys.: Condens. Matter, 33 (2021) 245702; [2] ibid. 25 (2013) 035801 [3] N. Arutyunov et al., Phys. Stat. Sol. (c) 13 (2016) 807; ibid. 14 (2017) 1700120 [4] N. Arutyunov et al., Journal of Physics: Conf. Ser. 618 (2015) 012013 [5] V. Emtsev et al., Semiconductors, 50 (2016) 1291 [6] N. Arutyunov et al., Solid State Phenomena, 156-158 (2010) 455