Trap generation and occupation dynamics in SiO2 under charge injection stress

Abstract

The effect of enduring charge injection on the physical properties of the SiO₂ layer of a metal‐oxide‐semiconductor structure is studied by means of a novel characterization method. It is based on the observation reported previously, that under charge injection conditions the density of occupied oxide traps reaches a value which is only a fraction of the total trap density. This trap occupation level is strongly dependent on the oxide electric field. The oxide trap density can be evaluated by measuring this field dependence, using a relatively small amount of charge injection. This method is used to distinguish between the process of trap generation and electron trapping in the generated traps, under conditions of continuous charge injection up to levels of more than 50 C/cm². The trap generation rate is found to be proportional to the flux of the injected charge, and to increase exponentially with the oxide electric field. At high oxide field only a small fraction of the newly generated traps are occupied; consequently, the measured oxide charge buildup does not reflect the actual increase in the density of generated traps. The density of the generated traps reaches high values of the order of 10²⁰ cm⁻³. It is suggested that these high values of oxide traps may be the cause of the SiO₂ ‘‘wear out’’ type breakdown, by forming a new path of conductance by electron tunneling between closely spaced generated traps.