Effect of impurities on pentacene island nucleation

Abstract

Pentacenequinone (PnQ) impurities have been introduced into a pentacene source material in a controlled manner to quantify the relative effects of the impurity content on grain boundary structure and thin film nucleation. Atomic force microscopy (AFM) has been employed to directly characterize films grown using 0.0-7.5% PnQ by weight in the source material. Analysis of the distribution of capture zones areas of submonolayer islands as a function of impurity content shows that for large PnQ content the critical nucleus size for forming a Pn island is smaller than for low PnQ content. This result indicates a favorable energy for formation of Pn-PnQ complexes, which in turn suggests that the primary effect of PnQ on Pn mobility may arise from homogeneous distribution of PnQ defects.Pac Numbers: 68.65.-k, 05.10.Gg, 73.63.-b, 68.35.bm, 68.55.-a Published as PRB 77, 205328 (2008) The study of organic materials, particularly the various roles of morphology and impurity doping, remains an active subject for device physics, materials design, and applied statistical mechanics.1-3 Studies of the most promising organic electronic semiconductor, pentacene (Pn), have shown that its transport properties are sensitively dependent on crystalline quality 4, 5 and thin film preparation: for the work here, observations that low concentrations of impurities significantly affect film nucleation and growth, electronic transport, and electronic signal noise are of particular interest [6][7][8][9] . Extensive studies of the initial stages of pentacene film growth 3, 10-15 have shown that it follows the classical picture of nucleation, island growth, aggregation and coalescence that was developed for the growth of inorganic films. [16][17][18][19][20] In later stages of growth, the two-dimensional domains formed from island coalescence serve as the basis for three-dimensional growth due to an Ehrlich-Schwoebel energy barrier that slows diffusion from higher to lower layers of the film 5,7,21 . Scaling analysis has proven powerful for evaluating island nucleation and grain boundary formation in such growth systems. 17,19,20,22 In particular, recent investigations using the Wigner surmise, which relates growth processes to To prepare materials with lower impurity content, source material was heated to a temperature slightly lower than its sublimation temperature for at least one hour prior to the thin film deposition. Previous measurements have shown that this treatment reduces the absolute source PnQ number fraction to less than 0.001 7 , and yields sample mobilities as high as those obtained withPn purified using gradient-sublimed material. The source concentration values used to quantify our results are the added number fraction plus the natural impurity level. This represents a readily reproducible quantity, but will not represent the absolute concentration in the thin film, due to the larger sublimation rate of PnQ than Pn at any given source temperature. Two film thicknesses were grown, submonolayer and 50 nm thick, with two diffe...

Effect of Impurities on Pentacene Thin Film Growth for Field-Effect Transistors

“…Together with the small grains, a low density of high-aspect-ratio protruding structures (appearing as white contrast in the AFM images in PnQ number fraction of 0.041. These structures are similar in density and shape to those that occur when equivalent amounts of pure PnQ are deposited on clean SiO 2 26 , suggesting phase separation of higher concentrations of PnQ from Pn 27,28 .…”

Following Chemical Charge Trapping in Pentacene Thin Films by Selective Impurity Doping and Wavelength‐Resolved Electric Force Microscopy

“…Considerable attention has also been paid to the impurity 6,13‐pentacenequinone (C 22 H 12 O 2 , PQ, Figure 2a) identified in commercial pentacene 29. The presence of PQ in pentacene films has been shown to have a significant effect on mobility,29, 30 island nucleation,31 and thin film morphology 30, 32, 33. However, the precise role of PQ in these effects remains unclear.…”

Determination of critical island size inpara-sexiphenyl islands on SiO2using capture-zone scaling

“…This method relies on the calculation of Wigner-Seitz cells surrounding each nucleation center, utilizing Voronoi tessellation, which was proposed earlier by Mulheran and Blackman as a way to characterize the capture zone of a growing island [14,15]. Experimental work [15][16][17][18][19], especially from the field of organic semiconductor thin-film growth, demonstrates the applicability of this method. The virtue of it -comparing with Venables' approach -is based on the fact that only one growth experiment is necessary to obtain i * for a given temperature.…”