Thermal emission and capture of electrons at sulfur centers in silicon

Abstract

The electron thermal emission and capture rates and the electron impact ionization rate of trapped electrons at the sulfur donor centers in the depletion region of reverse biased silicon p- n junctions have been measured by the dark capacitance and current transient methods. Least square fits of the low field data give the following electron thermal emission rates: e n0 t = 1.64 × 10 10(T/300°K) 2exp [−276/kT] sec for the neutral center and e n−1 t = 1.03 × 10 12 (T/300°K) 2 exp [−528/kT] sec for the singly ionized center where kT is in meV. The thermal activation energies are then 276 and 528 meV respectively. The hole emission rates are much smaller and not determined. The electric field dependences of the thermal emission rates of electrons are considerably smaller than that predicted by the Poole-Frenkel theory applied to the ground state: e n0 t increased by 1.5 at 130°K from 0.2 to 1.0 × 10 5 V/cm and e n−1 t by 3 at 200°K. Electron capture coefficients are obtained from capacitance transient during steady state electron injection into the junction depletion layer either by transistor emitter or by optical generation at the surface next to the junction. The electron capture rate at the doubly charged center, c n−2 t , decreases from 5 × 10 −7 cm 3/sec at 3 × 10 4 V/cm to 10 −7 cm 3/sec at 1.0 × 10 5 V/cm with essentially no temperature dependence. The electron capture rate at the singly ionized centers, c n−1 t , obtained at 82°K was about two orders of magnitude smaller than c n−2 t but had essentially the same electric field dependence. The electron impact ionization rate of trapped electrons at the neutral centers and its electric field dependences were also determined at 82°K.