A Method for the Determination of High-Field Conduction Laws in Insulating Films in the Presence of Charge Trapping

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Abstract
The investigation of high-field electronic conduction in metal-insulator-metal (MIM) or metal-insulator-semiconductor (MIS) structures utilizing dc methods is frequently complicated by the fact that the observed currents decay with time. In cases where this time dependence is caused by charge trapping in the insulating film, the most direct study of conduction involves a determination of the initial current, i.e., that which flows before charging becomes significant. Direct observation of this current is sometimes difficult because the onset of trapping can occur within either a very short time or a very short distance. In films characterized by trapping, the initial current can be determined by monitoring the time dependence of the charging process itself. This method requires the use of an MIS structure with a nondegenerate semiconductor, usually serving as a cathode. The state of charge of the insulator is determined by measuring the MIS flat-band voltage. The calculation, which assumes that the trapping of electrons is essentially permanent and that the current is determined by the magnitude of the electric field near the cathode, shows that over a significant interval of time the flat-band voltage has a logarithmic dependence, i.e., VFB∝ln(1+t/t0), where the quantity t0 is a characteristic time which is inversely proportional to the initial current. Hence, a measurement of t0 determines the conduction properties at a given field and temperature.