Pentacene thin-film phase

Fig. 2.3. Five orders of reflection in a thin film results also show the presence of the two most X-ray diffraction scan, indicating the high level common polymorphs of pentacene, the bulk-of crystallinity often present in pentacene thin like phase and the thin-film phase [25]. films deposited on different dielectrics. These...

Figure 11.6 Evolution of pentacene films on Cu(l 10). After completion of the first monolayer revealing a predominant (6.5 X 2) phase and occasionally a coexisting c(13 X 2) phase (a) an intermediate phase A is formed whereas for thickness above 2 nm the molecules continue in an upright orientation a-dopting the well known thin film phase B (b).

Figure 11.10 Morphology and structure of thin pentacene films on a SAM pre-covered Au(l 11) substrate (a) SEM micrograph of a 2 nm film together with corresponding STM data (b), (c) showing a layered arrangement of upright standing molecules. XRD data recorded for a 30 nm film (d) clearly reveal the presence of (001) oriented films revealing the thin film phase while at larger thickness the bulk phase is adopted.

H. Yoshida, I. Katsuhiko, and N. Sato, X-ray diffraction reciprocal space mapping study of the thin film phase of pentacene, Appl. Phys. Lett. 90, 181930(2007). 

As a result of the above, and of the direct competition between molecule-substrate and intermolecular interactions, the presence of the metal or insulator can induce interface polymorphs which do not exist in the bulk. Examples for this are the specific thin film phases of pentacene on insulators [16, 74, 75] or metals (e.g., Cu(llO) [16]), the a- and 3-phases of tetraeene on Ag(l 11) [69], or the square phases of PTCDA on Ag(l 11) [30] and Au(l 11) [84]. It is evident that the charge carrier mobilities of organic semiconductors will depend on the crystal phase. 

The reason for pentacene being superior for the production of TFT devices [7, 8] when compared with other molecules [9] is still not obvious. In this chapter, we will discuss to what extent the peculiar growth properties [10] of pentacene on metallic contacts and gate dielectrics contribute to the device performance. For this purpose, first the early growth state of pentacene films and the molecular structure of the so called thin film phase is reviewed. Then, major sources of crystal defects in thin films as determined by advanced synchrotron diffiaction techniques are discussed. The relation of these defects to the frequently discussed electronic traps that strongly influence transport properties of TFTs [6, 11, 12] is indicated. Finally, the spatially resolved photo response of pentacene OTFTs will be discussed in the context of injection barriers and contact homogeneity. 

Surface energy calculations [28] reveal that the (001) cleaving plane is the surface with the lowest surface energy. In turn, the formation of (001) oriented films can be expected, if the interaction of the pentacene molecules with the surface is negligible to the pentacene-pentacene interaction. Experiments show that this condition apparently fulfilled for various inert substrates such as reduced and oxidised Si, as well as many polymeric films used as gate dielectric. Note that the surface energy of the thin film phase is rather isotropic... 

Figure 15.2 Thin film phase unit cell, (a) The side view illustrates the layered structure of the thin film phase, (b) The top view emphasises the herringbone ordering motive, which is a common feature of all pentacene polymorphs.

Another way to avoid dewetting is to passivate the metallic surface by a self assembled monolayer (SAM), e.g. an alkane thiol monolayer (C-18). After passivation, the growth structure resembles the growth mode on inert surfaces [33], the same holds for growth of pentacene on conducting polymers such as PEDOT PSS [poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate)] [39]. It is interesting to note that on bare Si, pentacene initially forms a flat lying monolayer, but on-top of this monolayer, the thin film phase readily forms [16] without need for passivation. 

Bouchoms, I.P.M., Schoonveld, W.A., Vrijmoeth J., and Klapwijk, T.M., Morphology identification of the thin film phases of vacuum evaporated pentacene on Si02, Synth. Met. 104, 175, 1999 Knipp, D., Street, R.A., Volkel, A., and Ho, J., Pentacene thin film transistors on inorganic dielectrics Morphology, stmctural properties, and electronic transport, J. Appl. Phys. 93, 347, 2003. 

Bouchoms, I.P.M. et ah. Morphology identification of the thin film phases of vacuum evaporated pentacene on SIOj substrates, Synth. Met. 104, 175-178, 1999. 

Fig. 4.11. A powder X-ray diffraction pattern taken of pentacene grown at room temperature. The polycrystalline structure is evident, as is the polymorphic structure of the film. Two sets of peaks form corresponding to the thin film phase, which is thermodynamically metastable, and the bulk phase, which is thermodynamically stable. Growth at different temperatures near room temperature can influence the ratio of these two phases through manipulation of the growth kinetics [56].

Max Shtein, Jonathan Mapel, Jay B. Benziger, and Stephen R. Forrest. Effects of film morphology and gate dielectric surface preparation on the electrical characteristics of organic-vapor-phase-deposited pentacene thin-film transistors. Applied Physics Letters, 81(2) 268-270, 2002. 

The thickness of the SIP is expected to be a function of the dielectric constant of the substrate. This fact is established by studies done on thin films of pentacene and polystyrene [15, 39, 40]. In our scenario, this theory does not apply, as the SIP is observed on Si/SiOx substrates (composed of a thin SiOx layer of 2 nm on top of silicon) having a dielectric constant of e = 11.9 as also on glass (Si02), PVP and AlOx with a dielectric constant of s = 3.9, 5.0 and 9.9, respectively. On the top, the substrate-induced phase exists in contact with air or a sacrificial layer, and the bulk phase. The rectangular lattice of the bulk DLC-phase is incommensurable with the tetragonal phase of the SIP. The substrate-induced phase thus forms regardless of the thickness and the structure of the bulk phase as a function of time due to nucleation events initiated by the solid substrate. 

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