Role Transport and Charge Relaxation in Irradiated SiO2 MOS Capacitors

The transient response of SiO2 gate-insulator MOS capacitors to pulsed electron beam irradiation was studied. The radiation-induced flatband voltage shift (Δ VFB) in SiO2 MOS capacitors was measured with a fast C-V technique from 70 μsec to 1000 sec after a 60 krad radiation pulse for temperatures from 80 to 293 K. In complementary experiments, the post-irradiation charge displacement in the MOS capacitors was measured with an integrating picoammeter. By correlating the relaxation of the flatband voltage with integrated current measurements, it was established that the measured response is dominated by hole transport and trapping in the SiO2 film. The temporal and temperature dependences of the hole transport are well described by a stochastic hopping model based on a continuous time random walk (CTRW). The essential feature of the CTRW is that the transport occurs via a carrier hopping (phonon assisted) process between localized sites randomly distributed in the amorphous SiO2. Since the carriers do not require excitation to the band edges in order to be mobilized, the activation energy for conduction (tunneling) is independent of the optical excitation energy.