Evidence for recombination at oxide defects and new SILC model

Abstract

This work presents experimental and computational investigations on the physical mechanisms of SILC. Carrier separation measurements are carried out on MOS samples with oxide thickness 6-8 nm, highlighting the electron and hole contributions to the SILC. We have investigated the relation between these components by means of time-relaxation. It is found that a linear relationship holds between electron SILC and hole SILC, measured at different times after the initial high-field stress. The same linearity has been observed for increasing fluences of injected electrons, at fixed stressing field. A correlation between electron and hole ILC is found also from a comparison between carrier separation data obtained in n/sup +/- and p/sup +/- polysilicon devices. These experimental data entails that hole SILC is due to a recombination current. As a result of these experimental findings, a new model for the SILC is developed. This model is based on trap-assisted tunneling, but also accounts for hole tunneling and includes Shockley-Hall-Read recombination process in the bulk oxide as a new leakage mechanism. Simulations in the oxide thickness range 5.9-8.2 nm show excellent agreement with I-V measurements and carrier-separation data. The resulting defect concentration scales with the oxide thickness, in agreement with published results. The energy distribution of defects responsible for the steady-state leakage is located 0.7-1.3 eV below the Si conduction-band minimum. Capture cross sections of 10/sup -13/ and 10/sup -16/ cm/sup 2/ have been assumed for electrons and holes respectively, compatible with a donor charge state of the SILC-related defect centers. Simulations are finally shown for oxide thickness t/sub 0x/=2.8 nm. The mechanism of recombination in the bulk oxide accounts very well for the observation of low-voltage SILC in ultrathin oxide, showing the effectiveness of the proposed SILC model.