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SALIENT FEATURES OF SHALLOW DONOR INTERACTIONS IN PROTON-IRRADIATED SILICON

SALIENT FEATURES OF SHALLOW DONOR INTERACTIONS IN PROTON-IRRADIATED SILICON. V.V. Emtsev and G.A. Oganesyan Ioffe Physicotechnical Institute Russian Academy of Sciences 194021 St.Petersburg, Russia V.V. Kozlovskii St. Petersburg State Polytechnical University

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SALIENT FEATURES OF SHALLOW DONOR INTERACTIONS IN PROTON-IRRADIATED SILICON

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  1. SALIENT FEATURES OF SHALLOW DONOR INTERACTIONS IN PROTON-IRRADIATED SILICON V.V. Emtsev and G.A. Oganesyan Ioffe Physicotechnical Institute Russian Academy of Sciences 194021 St.Petersburg, Russia V.V. Kozlovskii St. Petersburg State Polytechnical University 195251 St. Petersburg, Russia

  2. OUTLINE • Introductory remarks what we have learnt from literature • Aims of the present work defect formation kinetics, interactions with P, annealing • Experimental details irradiation and annealing conditions, electrical measurements etc • Results close attention to defect interactions with P • Conclusions

  3. Introduction • Defect production due to proton irradiation of Si at energies of a few MeV is generally considered to be very similar to electron irradiation at a few MeV but more effective. V.S. Vavilov, N.U. Isaev, B.N. Mukashev, A.V. Spitsyn. Sov. Phys. Semicond. (AIP), vol. 6 (6) (1972). L.C. Kimerling, P. Blood, W.M. Gibson. Defects and Radiation Effects in Semiconductors, 1978. Conf. Ser. No 46 The Institute of Physics, Bristol and London, 1979) pp. 273-280. • Experiments on proton-irradiated Si are usually carried out at low doses of 1011 to 1012 cm2 . Heavier doses at about 1014 cm 2 are mostly used in optical and positron annihilation measurements. • Very interesting data on CZ-Si irradiated with protons at 24 GeV/chave been obtained recently by Davies et al. The production rates of isolated vacancies and self-interstitials were reliably estimated to be close to 1 cm1, whereas the production rate of divacancies turned out to be one half of that value. Cf the production rates being about 200 cm1 for proton irradiation at 10 MeV and 0.1 cm1 for electron irradiation at 1 MeV. G. Davies, S. Hayama, L. Murin, R. Krause-Rehberg, V. Bondarenko, A. Sengupta, C. Davia, A. Karpenko. Phys. Rev. B 73,165202 (2006).

  4. AIMS • To investigate production and annealing processes of point defects in Si subjected to proton irradiation at 15 MeV. • Paying special attention to interactions of group-V impurities with intrinsic point defects. Thick samples of 0.5 to 0.9 mm to eliminate surface effects. Irradiation through the samples to suppress hydrogen-related defect formation. Samples cut from moderately doped FZ-Si crystals to suppress side reactions with oxygen and carbon.

  5. Experimental Starting material  Wafers of FZ-Si doped with P at (5-7)1015 cm3 .  The compensation ratio K= NA / ND is very low,  0.01. Irradiation and annealing conditions • Protons at 15 MeV, irradiation at room temperature. • Annealing steps of T=20C and t= 10 min. The annealing interval was from T=60C to 680C. • Reference temperature after each annealing step, T=300 K.

  6. Electrical measurements • The concentration and mobility of charge carriers in the FZ-Si vs temperature were measured with the aid of the Van der Pauw technique over a temperature range of T= 25 K to 300 K. • Analysis of n(1/T) curves was performed of the basis of the relevant equation of charge balance adopted for non-degenerate Si of n-type.

  7. Equation of charge balance in n-Si over a temperature range of ionisation of shallow donors • nis the electron concentration in the conduction band; ND is the total concentration of shallow donor states; NA is the total concentration of compensating acceptors;NC is the effective density-of -states in the conduction band; ED is the ionisation energy of the singlet ground states of shallow donors;  is the splitting of the ground states of shallow donor states; other symbols have their usual meanings.

  8. Electrical data on initial and proton-irradiated FZ-Si • Electron concentration against reciprocal temperature for the CZ-Si before irradiation (blue) and after the proton irradiation (red). Points, experimental; curves, calculated. • Ionisation energy of shallow donors of phosphorus impurity atoms is given.

  9. Results • We have first checked the defect formation in this material under fast electron irradiation at 1 MeV. As expected for a strongly dominant formation of E-centers, i e pairing of P atoms with vacancies, a 1:1 correspondence between  ND and  NA was observed, in accordance with the Watkins’ model.  In the case of the proton irradiation  ND was found to be much larger than  NA . In other words, the loss of shallow donor states of P due to interactions with intrinsic point defects exceeds substantially the concentration of radiation-produced acceptors.

  10. Results (continued) • As a result of the proton irradiation of the FZ-Si the loss of shallow donor states of P is equal to  ND= 3.31015cm-3 , whereas the concentration of radiation-produced acceptors was found to be  NA= 1.51015 cm-3 . • The removal rate of charge carriers in the FZ-Si irradiated with protons at 15 MeV was found to be  = (12010) cm-1. Interactions of P atoms with intrinsic point defects turned out to be the main way leading to the decreasing concentration of charge carriers under proton irradiation.

  11. Results (continued) • Taking into account a sizeable contribution of divacancies V2= / V2 to  NA= 1.51015 cm-3one can conclude that the E-centers play a subsidiary role in the defect formation. • A large fraction of the lost shallow donors, greater than  ND= 21015cm-3 , appears as radiation-produced deep donors being electrically neutral in n-FZ-Si. • This way of arguing is in good agreement with the results obtained in annealing experiments.

  12. Defect annealing • The annealing stages of E-centers and V2= / V2 are clearly seen. • Their contributions to  NA= 1.51015 cm-3 are evident. • Just above T=300C a strange stage of direct and reverse annealing is observed. • The complete annealing takes place at about T=680 C.

  13. Defect annealing (continued) • In the heavily irradiated n-FZ-Si this stage at T350C is practically lacking, though some weak signs of its presence are still observable. • The similarities of the two annealing curves at T400C suggest a saturation plateau in the formation kinetics of defects stable at high temperatures.

  14. Conclusions • Changes in the concentration of charge carriers in FZ-Si decreasing due to proton irradiation at 15 MeV are mainly caused by losses of shallow donor states of group-V impurity atoms. • At the same time the E-centers are present in relatively small concentrations. Therefore, there are other complex defects containing group-V impurity atoms.

  15. Conclusions • First, these complex defects appear to be deep donors. Second, they are thermally stable up to T=300C, i e much more stable than the E-centers. Together with this, their annealing was found to be very complicated. • The complete annealing of all radiation-induced defects in FZ-Si after proton irradiation at 15 MeV is observed around T=700C.

  16. Thank you for your attention !

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