Epitaxial thinfilm ruby as an ionirradiation damage sensorThin films of 3,4,9, lO-perylenetetracarboxylic dianhydride (PTCDA) develop low electrical resistivity ( < 10-3 (J 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 parallelto the substrate (average inclination _10°_15°). Upon irradiation with up to _10 14 Ar+ I cm 2 , the grains become progressively more defect-rioden and eventually amorphous. At that stage, the resistivity begins to decrease by -12 orders of magnitude at doses between _10 14 and -5 X 10 16 Ar+ Icm 2 , while the intergranular boundaries become diffuse and the grains begin to merge. The temperature dependence of resistivity in this regime is as exp(constlT 1/2), 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 X 10 16 Ar+ Icm 2 ) 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. PACS numbers: 73.60. -n, 68.55. + b, 81.40.Rs, 61.65. + d Lovinger et a/. 478 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 130.88.90.110 On: Sun, 21 Dec 2014 05:04:25
We discuss the properties of organic-on-inorganic (OI) semiconductor contact barrier diodes. A model for charge transport is developed which suggests that thermionic emission over the organic/inorganic contact barrier dominates at low current densities, whereas space-charge effects dominate transport through the organic layer at high current densities. The effects of charge trapping in the organic layer are also considered. This model is applied to OI diodes using thin films of the prototypical aromatic compound; 3,4,9,10-perylenetetracarboxylic dianhydride, (PTCDA) vapor-deposited onto n- and p-Si substrates. Several electrical and optical properties of PTCDA are investigated to provide a basis for analyzing the OI diodes. Both ohmic and space-charge-limited transport are observed in the PTCDA. We discuss mobility, transient response, and photoresponse of the thin-film organic material. Also described are the general properties of organic-on-inorganic contact barrier diodes which employ PTCDA and related compounds on either p- or n-Si substrates. Comparisons between diode performance and the theory are made. The contact barrier diodes exhibit high breakdown voltages (≤230 V) and reverse dark currents limited by generation and recombination of carriers in the Si bulk. From the forward current-voltage characteristics, apparent OI contact barriers of φBp=(0.75±0.02) eV and φBn=(0.61±0.01) eV are formed with p- and n-Si substrates, respectively. The resulting diodes are superior, in many respects, to conventional Schottky diodes due to enhanced contact barriers and reduced edge effects.
The organic compound 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) has previously been observed to undergo a large increase in conductivity on irradiation with energetic particle beams. In this letter, we describe the electrical characteristics of novel rectifying junctions employing unirradiated PTCDA vapor deposited onto 10-Ω cm p-Si substrates. The PTCDA-Si contact barrier has a height of φB = 0.74 eV. The resulting diodes undergo avalanche breakdown at VB = 230 V, and exhibit current densities at 1/2 VB of ⩽50 μA/cm2. In addition, the forward current-voltage (IF−V) characteristics are strongly dependent on the contact metal used on the top PTCDA surface. The best results obtained were for diodes employing Ti contacts which gave nonhysteretic, stable IF-V characteristics with an ideality factor of n = 1.7. Several properties of the as-deposited PTCDA are also discussed. The rectifying characteristics reported here, coupled with the properties of irradiated PTCDA, suggest many unique device applications.
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