Electrical properties of electron-irradiated n-type silicon

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Abstract
Investigations of electron‐irradiated n‐type silicon show that the conductivity decreases with increasing dose, achieves intrinsic value, then converts into p type and increases again, reaching a saturation value. It is shown analytically that this behavior [for float‐zone silicon irradiated in the (111) direction] can be described by assuming that three main damage levels (two acceptor levels and one donor level) occur simultanously due to the irradiation. The deepest acceptor level determines the dose at which the minimum value of the conductivity occurs. The two acceptor levels (at Ec−0.44 eV and Ec−0.58 eV) are responsible for the decrease in conductivity at low doses. When the density of damage centers at Ec−0.58 eV exceeds the initial doping concentration, emission of holes from this level gives a significant contribution to the conductivity causing the conductivity to increase with dose. The donor level at Ev+0.27 eV causes the saturation at large doses. A zero‐current approximation is given for the conductivity versus dose dependence for an unlimited number of levels involved. These levels may be either independent levels or levels associated with different charge states of the same defect. By introducing the three levels above in this model a very good agreement with experiments on dose versus conductivity was obtained in the resistivity range 100 Ω cm to 50 kΩ cm. The results indicate that the two acceptor levels at Ec−0.44 eV and Ec−0.58 eV may be associated with the single‐ and double‐charge state, respectively, of the divacancy. The donor level does not seem to be correlated with the acceptor levels. The positive charge state of the divacancy may therefore have a larger energy or be masked by some other defect. Besides the zero‐current approximation, numerical calculations are performed, giving field distributions, charge distributions, and I‐V characteristics for one simple contract structure. These calculations show (assuming sufficient electron injection but negligible hole injection) that heavily irradiated n samples are p conducting at low currents but convert back to n conducting at large currents.