ИСТИНА

Primary radiation defects (vacancies and interstitials) in Si single crystals are very mobile, and just after proton radiation interstitial Si atoms disappear in sinks or outer boundaries, but vacancies become decorated by impurities or dopant atoms forming vacancy–impurity complexes including monovacancy-oxygen (A-centers), monovacancy-phosphorus (E-centers) and Si divacancies [1]. Previous experiments with low dose proton irradiation (~10^14 cm−2) [2] have shown that concentration of radiation-induced defects is proportional to the radiation dose. In present work we study Si single crystals with the moderate concentrations of impurities ([P] ≈ 1×10^15 cm−3, [O] ≈ 8×10^17 cm−3, [C] ≈ 2.5×10^16 cm−3) irradiated by protons (21.5 - 22 MeV) with the dose ~1016 cm−2. Protons pass through a set of eight Si wafers with the thickness 0.4 mm each. According to SRIM simulation, concentration of the Frenkel pairs in the first wafer is up to 10^19 cm−3 and increase more than twice from the first to the sixth plate. However, estimation of the concentration of the vacancy-type radiation defects based on the positron lifetime (LT) spectroscopy data gives a value, which is order of magnitude less (about 10^18 cm−3). This value is practically equal for the first six wafers, and notable changes (2 times as large) are detected only in the seventh wafer, where practically all protons are stopped. Probably this discrepancy between simulation and experiment is due to we measure only the concentration of vacancies bounded with impurities (and particular oxygen), while the major fraction of primary vacancies produced by protons recombine with interstitials or reach free surface. The migration of vacancies and interstitials leads to formation of specific periodic pattern on the surface which is visible by AFM. The LT spectra are fitted with the help of a 4 state trapping model (with A-centers, 225 ps, E-centers, 270 ps, and divacancies, 320 ps). Oxygen complexes are effectively dissociated at T > 100 C. So, after annealing at 200 oC the positron lifetime in defects dramatically increases (up to 500 ps) and the short-lived defect components completely disappear. Concentration of these defects (formed after annealing) is comparable with the concentration of phosphorus. This may indicate that initial A-centers dissociate and the released vacancies attach to E-centers forming complexes of 5 and more vacancies. The number density of these complexes is comparable with the concentration of phosphorus. [1] Reinhard Krause-Rehberg, ‎Hartmut S. Leipner, Positron Annihilation in Semiconductors: Defect Studies. Springer, 1999 [2] N. Yu. Arutyunov, M. Elsayed, R. Krause-Rehberg, V.V. Emtsev, G.A. Oganesyan, V.V. Kozlovski, Positron annihilation on defects in silicon irradiated with 15 MeV protons, J. Phys.: Condens. Matter 25 (2013) 035801–035828.