Breakdown Mechanisms of Thermally Grown

Mechanisms of high field electrical breakdown have been widely investigated in recent years in thermally grown silicon dioxide films at fields larger than 7 MV/cm. We find for oxide films thicker than 10 nm that constant voltage and constant current tests produce breakdown by different mechanisms. Specifically, the fast breakdowns of constant voltage tests can be explained by the IIR (Impact ionization-recombination) breakdown model by the growth of positive charge at the cathode at fields larger than a critical field, F_r. The slow breakdowns in constant current tests cannot be explained by the IIR mode, since growth of electron trapping during a test keeps the critical field increasing. It appears that breakdown in constant current tests may be explained as due to the effects of the generation of a very large density of trap states at the injection barrier. The density of trap states generated may grow to 1019/cm3. Such change in the oxide may produce current instability by diverse processes: barrier lowering, resonant tunneling, or transition of the oxide from an insulating to a conducting state. The mechanism of these breakdown processes has not yet been identified. Breakdown by effects of defect generation present novel models of insulator breakdown, and it is of interest to explore their range of validity for SiO₂ and also for other insulators.