Carrier traps in ultra-high purity single crystals of anthracene

Abstract

Measurements of space-charge-limited currents, arising from electron-injection or hole-injection from suitable ohmic contacts, have been carried out at room temperature, on a variety of anthracene crystals differing only in the nature of the gas (He, Xe, N₂ or vacuum) exposed to the melt from which the crystals grew. The principal aim of the work is to characterize the energetics of the carrier traps present in samples not deliberately deformed. It is shown that a Gaussian distribution of trapping states, as well as the more generally used exponential one, can readily account for the observed current, voltage dependences. Correlations have been found between the calculated total trap density, H, and the characteristic energy of the trap distribution kT_c for crystals grown under different atmospheres. These parameters, as well as E_L, and h(E_L), which refer, respectively, to the depth and the density of the deepest traps, serve to define the properties of the traps, which are essentially the same (both with respect to their density and depth E_L∼0.7 eV) for electrons and for holes. This fact suggests that the origin of the traps may be ascribed to structural imperfections, which are, in turn, associated with crystallographic perturbations brought about by the presence of inert gas species.

It is also shown that explanations for the compensation law in electrical conductivity based on continuous distributions of traps need not have universal applicability, and, in particular, are invalid for anthracene.