Spatial Extension of Charge Traps: Consequences for Space-Charge-Limited Currents

Abstract

It is proposed that charge carriers are trapped by spatially extended domains (macrotraps) produced by physical perturbations of crystal lattice. These extended domains consist of local traps (microtraps) with energy (E) distributed in space (r) such that E = (3kT/[sgrave])ln(r_o/r), where [sgrave] is a characteristic parameter of the exponential energy distribution function, and r₀ is the radius of the macrotrap. The steady-state space-charge-limited currents (SCLC) are studied and interpreted by invoking the macrotrap concept. The results indicate that the potential of the macrotraps can be effectively modulated by an external electric field accessible in experiment and from the power law j ∼ Uⁿ (n > 2) of the measured current (j versus voltage (U) [sgrave] can be determined (n = 2 + 3/[sgrave]). In particular, we examined high-quality anthracene crystals for which two distinct distributions of microtraps have been found with [sgrave]₁ = 1 ± 0.2 and [sgrave]₂ = 0.3 ±; 0.05. Typical values of the concentration N_a = 10¹⁵cm⁻³ and depth E_t = 0.6 ± 0.05eV of macrotraps are accompanied by a diversity of r₀ within the range 100Å < r_o < 10³Å.

PACS numbers: 72.20.Jv; 72.80.Le; 72.80.Sk