Crystal Structure of and Defects in the Pentacene Thin Film Phase
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Crystal Structure of and Defects in the Pentacene Thin Film Phase Lawrence F. Drummy, Paul K. Miska and David C. Martin Department of Materials Science & Engineering University of Michigan 2022 HH Dow Building Ann Arbor, MI 48109-2136 [email protected], [email protected], [email protected]
INTRODUCTION The aromatic hydrocarbon pentacene is currently under investigation for use as the active layer in electronic devices such as thin film field effect transistors [1]. Understanding the relationship between processing, crystal structure, defects, morphology and resulting properties will be imperative for bringing pentacene and materials like it to commercial use. Pentacene has a triclinic structure with lattice parameters a=0.628 nm, b=0.771 nm, c=1.444 nm, α=76.75°, β=88.01°, γ=84.52° and ρ=1.37 g/cm3 [2]. Several polymorphs have been characterized in pentacene, which have an increased (001) plane spacing. Research groups have investigated thin films which show (001) plane spacings of 1.45 nm, 1.50 nm and 1.55 nm, however a full set of lattice parameters for any of these phases has been elusive due to the small crystal sizes in these thin films [3]. In a bottom gate field effect transistor, electron and hole transport through the active layer will take place in the region of the film nearest to the dielectric layer. It is likely that this region of a pentacene thin film would consist solely out of the 1.55 nm phase, which has been termed the “thin film phase” [4-6]. The crystal structure of this phase will be very important for understanding and controlling processing the parameters that influence the pentacene morphology and the final device performance. We have used X-Ray Diffraction (XRD), Electron Diffraction (ED), Low Voltage Electron Microscopy (LVEM), High Resolution Electron Microscopy (HREM) and molecular modeling to investigate the thin film phase of pentacene. We will report the orthorhombic symmetry and lattice parameters of the thin film phase measured experimentally from these techniques. The structure of extended defects such as dislocations and grain boundaries will influence the electrical and mechanical characteristics of the films [7-10]. The ability of the pentacene lattice to locally relax near defects will govern the size of electrical deep traps. We have seen evidence for both continuous bending and sharp kinks of the triclinic pentacene lattice near high angle grain boundaries in lattice images of nanocrystals [11]. It is possible that the types of defects that commonly form and locally relax will be different in triclinic bulk phase and orthorhombic thin film phase. Here we show a direct image of an edge dislocation in the thin film phase and discuss the way in which the lattice accommodates the defect. EXPERIMENTAL Thin films of pentacene were grown in a vacuum evaporator from a tungsten wire basket onto room temperature amorphous-carbon-coated mica substrates. The current across the wire basket was ~5 amperes and the vacuum was ~10-4 torr. XRD of the pentacene on amorphous carbon coated
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