Comparative experiments are made to study the nonequilibrium charge‐carrier recombination in silicon irradiated with γ‐rays of 50Co and of the bremsstrahlung spectrum of electrons with maximum energy 100 MeV. n‐ and p‐Si crystals with ϱ = 10 to 1000 Ωcm grown by Czochralski and vacuum float‐zone techniques are used. The temperature and injection dependences of nonequilibrium charge‐carrier lifetimes measured by a phase method are analyzed. It is found that when irradiating with γ‐rays of 60Co a great number of recombination centers are produced which are mainly complexes of irradiation‐generated vacancies and interstitials with dopants (phosphorus, boron) and technological (oxygen, carbon) impurities. As distinct from this, the dominant recombination centres in silicon irradiated with γ‐rays of the bremsstrahlung spectrum are divacancies involved in defect clusters, surrounded by the potential barrier Ψ for the majority charge carriers. The parameters of recombination centres (energy level spectrum, annealing temperatures, charge‐carrier capture coefficients) in crystals studied under γ‐irradiation are determined. The peculiarities in the charge‐carrier recombination processes due to the localization of recombination centres in defect clusters generated by photonuclear reaction products are discussed.
The processes of radiation defect formation and annealing in dislocation‐free n‐type Si (ϱ = 100 Ω cm) grown by the float‐zone technique in argon atmosphere or in vacuum are studied. In the latter case the ingot is grown at a varying rate and therefore contains A‐, A + B‐, and D‐type microdefects in different parts. Crystals obtained in argon atmosphere do not involve microdefects identified by selective etching. The temperature dependences of the charge carrier lifetime (τ) and the Hall coefficient (RH) are measured. The variation in τ and RH after 60Co γ‐ray irradiation at T = 330 K is found to be due to the accumulation of E‐centres with the level Ec −0.43 eV. However, the E‐centre introduction efficiency and annealing temperature in Si grown in argon atmosphere as well as in crystals with A + B‐type microdefects are lower than in control Si with small content of dislocations (ND = 1 × 104 cm−2) where microdefects are absent. It is concluded that under certain growing conditions (gaseous atmosphere, growth rate, axial temperature gradient) of dislocation‐free crystals, shallow (according to the estimates of 10 to 20 atoms) interstitial‐type inclusions unidentified by selective etching can be formed in their bulk which produce strain fields around themselves and thereby are surrounded by an impurity atmosphere. Affected by the strain fields induced by inclusions, primary radiation defects migrate directedly towards them whereby accumulation of secondary complexes occurs around inclusions within the impurity atmosphere. For the same reason the E‐centre formation efficiency in crystal matrix decreases and their annealing parameters change due to the effect to strain fields. Inclusions are assumed to be predecessors (fragments) of B‐ or D‐type microdefects.
The formation of donor‐type defects (T = 300 to 400 °c) is observed when annealing 60CO γ‐ray irradiated dislocation‐free float‐zone n‐Si (ϱ = 100 Ω cm). It is assumed that in the bulk of such crystals inclusions of interstitial type are present which produce anisotropic strain fields and as a result are surrounded by an atmosphere of back ground impurities within which irradiation‐induced accumulation of A‐centres and interstitial carbon–substitutional carbon complexes (level ≈ Ec — 0.18 eV) occurs. When annealed these defects interact with each other with the result that electrically inactive carbon–poxygen associates are formed which participate in the formation of donor centres whose properties resemble those of thermal donors I.
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