Trapping effects on emission-limited current flow in narrow-band insulators

The effect of charge carrier traps on emission-limited injected currents in narrow-band insulators is studied. At low voltages the physical mechanism responsible for the current is the trapping of injected carriers in the crystal image force potential well between the emitter and the potential maximum; the Schottky emission is the dominant conduction mechanism at high voltages. A theoretical model is presented and the voltage dependence of currents is derived including the effects of trapping and barrier lowering. It is shown, in agreement with the model, that in injection experiments on anthracene and tetracene crystals the voltage (U) dependence of current (j) can be described by j ≈ j0 exp (aU1/2 — bU−1/2), where a and b are constants characterizing the crystals. The constant b dependent on the trap concentration, determines at what voltage the conductivity changes from trapping to Schottky barrier lowering mechanism. The hole diffusion length in five different anthracene and tetracene crystals is found to be within the range 19 to 310 Å and the trap concentration in the vicinity of the injection contact, within the range 6 × 1017 to 9 × 1020 cm−3.