Structural and morphological investigation of the development of electrical conductivity in ion-irradiated thin films of an organic material

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Abstract
Thin films of 3,4,9,10‐perylenetetracarboxylic dianhydride (PTCDA) develop low electrical resistivity (−3 Ω cm) after irradiation with 2‐MeV Ar+ ions. Electron microscopy and diffraction show that vacuum‐deposited films of this material consist, prior to ion‐irradiation, of discrete crystalline grains (∼20–50 nm diameter) in which the molecules are disposed closely parallel to the substrate (average inclination ∼10°–15°). Upon irradiation with up to ∼1014 Ar+/cm2, the grains become progressively more defect‐ridden and eventually amorphous. At that stage, the resistivity begins to decrease by ∼12 orders of magnitude at doses between ∼1014 and ∼5×1016 Ar+/cm2, while the intergranular boundaries become diffuse and the grains begin to merge. The temperature dependence of resistivity in this regime is as exp(const/T1/2), which is consistent with our morphological and structural results as it implies carrier hopping between conducting islands embedded in a nonconducting matrix. At the highest ion doses (>5×1016 Ar+/cm2) the grains become connected into a rather uniform and featureless network akin to amorphous carbon, and the resistivity reaches its lowest value and becomes independent of temperature. The amorphous‐carbon‐like character of the highly irradiated material is evidenced not only by its diffraction pattern but also by its crystallization with a graphitic‐type structure during annealing to 1200 °C.