Charge trapping phenomena in anthracene crystals and the influence of crystal deformation

Abstract

Carrier generation and transport in single crystals of anthracene either undeformed or deformed mechanically have been investigated using a drift technique in which carriers generated by a < 100 ns pulse of 10 keV electrons are studied as they are swept through the crystal in an applied electric field. In undeformed crystals the mobilities of electrons and holes are µ_e= 0.43 × 10^–4 and µ_h= 1.1 × 10^–4 m² V^–1 s^–1 respectively. The corresponding free lifetimes before trapping are τ^e_f > 2 ms and τ^h_t 100 µs. The temperature dependences of these parameters are also reported. The total carrier yield is also obtained as a function of field but the value of the product µτ deduced from the yield, assuming a Hecht-type formula, is much lower than is expected from the direct evaluation of µ and τ as above. This is attributed to field-sensitive space-charge which alters the sample capacitance. Space-charge may also be responsible for the delayed generation of holes.

Basal (001)[010] dislocations deliberately introduced by deforming give rise to deep electron traps (∼ 1 eV) with long lifetimes which reduce τ^e_t to ∼ 30 s. On the other hand τ^h_t is not reduced unless electrons are already trapped, whereupon they act as recombination centres.

Point indentation (h01)[010] dislocations in surface layers appear to produce either recombination centres or independent electron and hole traps. Shallow trapping processes and symmetrical trap behaviour as envisaged by Sworakowski do not appear to occur as a result of dislocation.