Using Atomic Steps to Control Pentacene Crystal Orientation Texture
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0965-S14-07
Using Atomic Steps to Control Pentacene Crystal Orientation Texture Valerian Ignatescu1, Jing-Chih M. Hsu1, Alex C. Mayer1,2, Jack M. Blakely1, and George G. Malliaras1 1 Materials Science and Engineering, Cornell University, 214 Bard Hall, Ithaca, NY, 14853 2 Materials Science and Engineering, Stanford University, Stanford, CA, 94305
ABSTRACT We have studied the effect of substrate atomic steps on the azimuthal alignment of vapordeposited pentacene crystals. Si(111) substrates with a low miscut angle were annealed at high temperature in ultra-high vacuum before the pentacene deposition; this produced surfaces with atomically flat terraces and arrays of parallel atomic steps. AFM analysis shows that pentacene deposited on these heated samples, at a low deposition rate, results in significant alignment of the pentacene crystals along the atomic steps.
INTRODUCTION Pentacene is an organic semiconductor being studied as candidate for cheap, large area organic based electronics [1-3]. While single-crystal field-effect transistors of pentacene show a charge carrier mobility of up to 2.3 cm2/(V·s) [4], the mobility measured on polycrystalline thin film transistors is almost an order of magnitude smaller [5,6]. One source of this reduced mobility in polycrystalline pentacene film is charge carrier scattering at the grain boundaries [7, 8]. In this paper, we report that using atomic steps as preferential sites for nucleation, under proper growth conditions, we have achieved good azimuthal alignment of the pentacene crystals so that only small angle boundaries are present. Texture control is necessary to increase the average carrier mobility thus improving device performance and may also be useful in probing interface effects on charge carriers’ transport. Furthermore, observations of growth on atomically flat silicon surfaces allow testing of theoretical predictions on the mode of growth of pentacene films. To our knowledge, the only other report of azimuthal texture orientation obtained in this way for organic semiconductor thin films was published by Ossó et al. [9] on hexadecafluorophthalocyanine deposited on annealed sapphire (112⎯0) samples. Itaka et al. [10] using sapphire (0001) for pentacene deposition only observed improved crystallinity on annealed sapphire substrates (larger grains and better c* axis alignment).
SUBSTRATE PREPARATION AND PENTACENE DEPOSITION The substrate used was Si(111) with a miscut angle of 0.1° – 0.5° from the (111) plane. The thermodynamically stable vicinal surface consists of atomically flat terraces separated by atomic steps (0.31 nm height for single bilayer atomic steps on Si(111)). One way to obtain such a smooth and clean surface is to perform a high temperature anneal in ultra-high vacuum (UHV). Therefore, before performing the pentacene deposition, we modified the substrate morphology by high temperature annealing in UHV following a procedure described in detail elsewhere [11, 12]. The samples, (18 x 6 mm) were heated by direct current. The method consists of slowl
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