SPATIALLY EXTENDED DEFECTS AS CHARGE TRAPS IN SPACE-CHARGE-LIMITED CURRENTS IN ORGANIC SOLIDS

Abstract

We propose a new interpretation of space-charge-limited current behaviour arising from spatial extension of defects present in real crystal lattices of insulators and semiconductors. The principal results of the standard (point defects) theory are derived from physical arguments, based on the trapping of injected charge carriers by spherical symmetry extended domains (macrotraps) consisted of local (point) traps (microtraps) with energy (E) distributed in space (r) such that E = (3kT/σ)ln(r o /r), where σ is a characteristic parameter of the exponential energy distribution function, and r o is the radius of the macrotrap. The potential of the neutral macrotraps can be effectively modified by an external electric field accessible in experiment leading to increasing ratio (Θ) of free to trapped carriers and thus governing the power n = 2 + 3/σ > 2 dependence of the measured current versus voltage applied to the crystal. These predictions of the model have been verified on high-quality anthracene crystals for which two distinct distributions of microtraps have been found with σ 1 = 1 and σ 2 = 0.3. Typical values of the concentration N o = 10 15 cm −3 and depth E t = 0.6eV of macrotraps are accompanied by a diversity of r o within the range 100A o 3 A. The origin of macrotraps is discussed in terms of dislocation properties of anthracene crystals.