Emission and capture of electrons at sulfur centers in silicon

Abstract

Some new techniques have been used to study the physical properties of sulfur centers in silicon. These techniques make use of the effects of the sulfur centers on the depletion region properties of a reverse-biased P-N junction. This study indicates that sulfur acts as a double donor in silicon. The ionization energy of electrons from neutral sulfur centers was found to be 0.275 eV, and the ionization energy of electrons from singly ionized sulfur centers was found to be 0.53 eV. The thermal emission rate of electrons from sulfur centers was found to be much greater than the thermal emission rate of holes. A least square fit of the measured electron emission rates to T^2 exp(-deltaE/kT) gave 1.64E10 (T/300)^2 exp(-0.276/kT) for the emission rate of electrons from neutral sulfur centers and 1.03E12 (T/300)^2 exp(-0.528/kT) for the emission rate of electrons from single ionized sulfur centers. The thermal electron emission rates were found to be quite field dependent. As the electric field was increased from 2E4 V/cm to 10^5 V/cm, the emission rate of electrons at 130 degrees K from neutral sulfur centers increased by a factor of approximately 1.5 while the emission rate of electrons at 200 degree K from singly ionized sulfur centers increased by a factor of 3.0. The photoionization cross-section of electrons from sulfur centers was found to be much greater than the photoionization cross-section of holes. In the photon energy range of 0.70 to 1.00 eV the photoionization cross-section of electrons from neutral sulfur centers is approximately 2E-16 cm^2, and the photoionization cross-section of electrons from singly ionized sulfur centers is approximately 10^16 cm^2. The spectral dependence of the photoionization cross-section of electrons from neutral sulfur centers fits a Lucovsky delta function potential model much better than a scaled hydrogenic model. For photon energies above threshold there was no observable field dependence or temperature dependence. As the field is increased from 2.14E4 V/cm to 1.11E5 V/cm, the threshold is lowered by approximately 0.01 eV. This reduction in the threshold is much lower than predicted by a Poole-Frenkel barrier lowering model. The photoionization cross-section of electrons from singly ionized sulfur centers was found to be very temperature dependent for photon energies near the absorption edge.

The capture rate of electrons by doubly ionized sulfur centers was measured as 5E-7 cm^3/sec at a field of 3E4 V/cm and decreased to approximately 10^-7 cm^3 /sec at a field of 10^5 V/cm. Little if any temperature dependence was observed. The capture rate of electrons by singly ionized sulfur centers was found to be approximately two orders of magnitude smaller than that of the doubly ionized sulfur center.