Recent related paper: Solid State Phenomena, (2011) 313 Positron Annihilation on Point Defects in n-FZ –Si:P Single Crystals Irradiated With 15 MeV Protons (or what has been observed beyond expectations) N. Yu. Arutyunov 1,2, M. Elsayed 1,R. Krause-Rehberg 1, 1 Department of Physics, Martin Luther University, Halle, Germany 2 Institute of Electronics, Tashkent , Uzbekistan V.V. Emtsev 3,G.A. Oganesyan 3,V.V. Kozlovski 4 3 Ioffe Physico-Technical Institute, St. Petersburg , Russia 4 St. Petersburg State Polytechnic University,St. Petersburg , Russia

Which Defects Are Produced In n-FZ-Si([P]) Under Irradiation with 15 MeV Protons? ● Oxygen [O] ≈ cm -3, carbon lean [C i ]≈ cm -3, n-FZ- Si([P]=7×10 15 cm -3 ) is of special interest because irradiation allows one to change position of Fermi level in wide range. Information about RD created in material is scarce. ●Most probable primary defects are separated Frenkel pairs, V and I; they are movable and they may create various complexes ● Established: Most probable secondary defects are divacancies (VV) and donor-vacancy pairs (or E-centers for D=P); ►Anticipated stages of annealing of defects are completed at: 180°C (E-centers), 280°C (VV) (EPR, DLTS, Hall’s measurements)

What we hoped to obtain creating defects in n- FZ-Si([P]) under irradiation with 15 MeV protons? E (P + )=15 MeV 0,8 mm Si E (P + )=8 MeV Projection range of protons > 1 mm: no stoppage effects ● Slowing down of protons + is due to ionization lost: roughly, average ionization energy I ≈ 172 eV, energy of recoil atom E R : 210 eV Generation of vacancies and interstitials dominates We hoped to optimize parameters of irradiation/annealing Goal: to control reliably the position of the Fermi level creating E-centers and divacancies We expected to reach the goal combining isochronal annealing and dose of irradiation Results of annealing surpass all expectations!

Characterization: Hall’s measurements and removal Of Carriers From Conduction Band of n-FZ-Si([P]) Irradiated with 15 MeV Protons ● Removal rate of electrons: ≈ 110–120 cm -1 (15 MeV H + ) and ≈ 0,11 cm -1 (1 MeV electrons)►tecnological advantage of H + beams ● Concentration of defects has been estimated using these data; Values have been applied for analysis of results obtained by PALS (Hall’s measurements)

Temperature Dependency of e + Lifetime In n- FZ-Si([P]) Irradiated with 15 MeV Protons ● We expected: E-centers, VV dominate, they are e + traps► isochronal annealing must be completed at 280 °C – 300 °C ● We obtained: T- dependency is preserved up to T ann. = 340 ° C ! ● τ av, - s trong T-dependency indicates effective positron- phonon interaction ● Value of e + lifetime suggests defects of a vacancy type (~ 254 ps) ● Shallow e + state (s): E st ≈ 0,2 – 0,6 meV (Krause- Rehberg, Leipner, 1999)

Positron Lifetime And Recovery Of Dopant Activity:Deep Donors Of Radiation Origin Hidden At Early Stages of Annealing Of n-FZ- Si([P]) Irradiated With 15 MeV Protons: ● e + lifetime ~251 ps is steady up to ~ 320°C, then its recovery begins ● E-centers, VV annealed, ~ 35-40% of atoms of P restored electrical activity ● At 500°C: e + lifetime vanishes, r ecovery of electrical activity of atoms of P continues up to 700 °C ● We identify thermally stable e + traps as deep donors hidden in early stages of annealing

Temperature Dependency Of Positron Trapping Rate For Deep Donors Of Radiation Origin In n-FZ-Si([P]) Irradiated With 15 MeV Protons ● For deep donors: reciprocal cubic temperature dependency of e + trapping rate► multi-phonon cascade trapping at attractive center (Abakumov et al., 1978) ● e + Trapping cross-section (averaged over temperature) ≈ 6,3× cm 2 for concentration of deep donors ≈ cm -3 (Hall effect and conductivity)

Activation Energy of Annealing Of Deep Donors Of Radiation Origin In n-FZ-Si([P]) Irradiated With 15 MeV Protons ● Annealing ► 1 st order of reaction γ=1 ►number of sinks is fixed, new e + traps are not formed ● Ea ≈ 0,74 eV corresponds to data of EPR: Ea ≥ 0,64 eV attributed to dissociation of VV in its neutral state (G.D. Watkins, 1964) ●k is e + trapping rate; λ 0 -1 =τ 0 ≈ 216 ps is e + lifetime in the bulk; λ av -1 = τ av is e + lifetime at 30K; τ d is maximal value of τ av Equation of kinetics of chemical reaction (Arutyunov et al., 1977)

Number of Vacancies In Deep Donors Of Radiation Origin In n-FZ-Si([P]) Irradiated With 15 MeV Protons ● Average e + lifetime τ av overlaps the range of numbers of vacancies in the cluster from 1 to 2 ● Long-lived τ av lifetime is in the range of numbers of vacancies from 2 to 3 ● Resume: deep donor contains more than 1 vacancy (most probably: 2 vacancies) ● Black squares: calculations, Hakala et. al, PRB,1998

Formal configuration of deep donor detected in n-FZ-Si ([P]) irradiated with protons 15 MeV ● Two vacancies are in close proximity to atom of P ● Atoms of silicon participate in the closing of bonds ● Decomposition of deep donor restores impurity atom of P as a shallow donor Relaxation is not shown

● Deep donors of radiation origin have been revealed in silicon of n-type conductivity ( n-FZ-Si([P]), irradiated with 15 MeV protons) ● Data of e + lifetime: Configuration of deep donor consists of 2 vacancies; it includes, at least, one atom of phosphorus ● Anealing of deep donors ranges 320°C to 700°C (i)Deep donors are hidden in stages of annealing of well known E-centers and divacancies (ii) Deep donor is effective trap of e + :σ ≈ 6,3× cm 2 ●Good electron wave functions are needed (to shed the light on details of configuration of deep donor) Conclusion

Defects In n-FZ-Si([P]) Single Crystal Irradiated With 15 MeV Protons: Why We Applied PALS? ● Sensitivity of the positron lifetime to vacancy defects and their complexes with atoms of dopant (P) Difficulties in EPR studying of E-center and VV: ►Absence of EPR signal for negatively charged phosphorus-vacancy complex (E-center) ● Insensitivity of EPR signal in the process of isochronal annealing to the charge state of divacancy (VV 1+ and VV 1– ) ● Necessity to assume the existence of the intermediate neutral state of VV which is undetectable by EPR; energy of dissociation of VV 0 ≈ 0,64 eV (G.D. Watkins, 1964)