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

N. Arutyunov
MLU Halle, Germany / Inst. of Electronics, Tashkent, Uzbekistan,

Keywords: silicon, germanium, group V impurity centers, point radiation defects, positron annihilation, polyelectrons, excitons


In spin-dependent phenomena at the quantum computing on silicon platform the bismuth centers, Bi(J = 9/2), have been demonstrated to be active elements functioning as operators for information encoding (J is the nuclear spin). The cavity-like open volume ambient the remainder of Bi atom in the crystal lattice of silicon is being created for controlling spin relaxation. As the electron and spin exchange reactions are very fast and difficult for observing, we have used positron probing to study them in the electron systems of Bi centers. A radiation resistance of qubit-forming centers of Bi(J = 9/2) and the phosphorus impurity centers, P(J = 1/2), in silicon is outlined by these data [1–3]. It was established that the resulting electron-positron and spin exchange reaction manifests itself in inhibiting 2–gamma annihilation decay which is characterized by enormously large cross-section (1.2 – 1.5) × 10^(−13) cm^2. This inhibition looks paradoxically as it is accompanied by the increase of number of electrons contacting positron at Bi center [1]. Positron lifetime spectroscopy has been applied for studying the rate of inhibition of 2–gamma annihilation radiation emitted out of Bi impurity centers. This rate correlates with the growth of occupancy of the donor level E{Bi} = Ec − 0.069 eV to be observed by low-temperature Hall effect measurements. The hypotheses of forming both the polyelectron Wheeler’s compound (e−e+e−) and emblematic D0X (D = Bi) exciton involving positron are suggested for interpreting the data. Being formed at Bi center, the superpositioned long-lived triplet electron-positron states can not decay on two gamma-quanta, thus contributing to inhibition of the resulting 2–gamma annihilation rate. When the phonon effects are reduced with decreasing temperature, the interaction of positrons with D0X centers and excitations in them is accompanied by the positron/holes charge-exchange reaction controlling a correlation between the decay time of the excited state D0* and the positron lifetime. It is argued [1] that the parity of excited states D0* play a key role in forming electron-positron states with the prolonged positron lifetime observed. 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 and forms [Vop – Bi] complex having D3d symmetry. The isochronal annealing of the [Vop – Bi] complex occurs during a fast stage at temperatures ~430 – 470 °C, in contrast to P-related complexes which are annealed at a much broader stage, from ~ 430 to ~ 650 °C. The results available for the qubit-forming Bi and P centers are discussed in the light of current reconsidering of a whole conception of formation of point radiation defects in moderately doped n–type silicon subjected to irradiation with protons and electrons [2, 3]. [1] N. Arutyunov et al., J. Phys.: Condens. Matter, (2021), in press; [2] ibid. 25 (2013) 035801 [3] N. Arutyunov et al., Phys. Stat. Sol. (c) 13 (2016) 807; ibid. 14 (2017) 1700120