Comparative studies of tunnel injection and irradiation on metal oxide semiconductor structures

Abstract

Tunnel injection and irradiation experiments on metal oxide semiconductor (MOS) structures are performed in order to compare the results of both experiments and to check the feasibility of radiation hardness prediction of MOS devices. The comparison is based on the fact that in both hot electron and ionizing irradiation experiments electron-hole pairs are generated in the SiO2. Due to an applied electrical field, these pairs are separated. The fraction of holes, trapped by neutral centers and the number of subsequently captured electrons by these now positively charged traps depend on the amount of available carriers, the magnitude of the respective capture cross sections for electrons and holes, and the number of hole traps. In the case of the tunnel injection experiment the number of the available carriers is a strong function of the field dependent ionization coefficient α. Up to now, its magnitude has not been accurately known. For this reason, a new method is presented which yields additional and reliable figures of α. When comparing the charge accumulation in the SiO2 by means of the flatband voltage shift as a consequence of the tunnel injection and of the irradiation, we determine α for various fields, α=3.3×10−6 exp [78/E(MV/cm)] (cm−1). For this determination we show that it is mandatory to correct for the steady state compensation of positively charged centers by electrons. From the saturation of the flatband voltage shift, caused by these trapped electrons, the field dependent capture cross sections for electrons are deduced. Their values are 2.3×10−14 and 3.5×10−15 cm2 for electrical fields of 7.8 and 8.6 MV/cm, respectively. The feasibility and limitations to predict the radiation hardness of MOS devices are discussed.