Polycrystalline thin films morphology

Each organic layer should have uniform and pinhole-free thin films at a thickness of about 50 nm. Amorphous morphology was found to satisfy this requirement. Sometimes, polycrystalline thin films are also acceptable, when their grain sizes are less than 50 nm and they form dense pinhole-free films. The previous systematic studies proposed a molecular design for the formation of uniform thin films. It was reported that the introduction of bulky substituents and steric hindrance into molecular structures, as well as an appropriate three-dimensional steric configuration resulted in amorphous morphology.35,58,61... 

Thin films of metals, alloys and compounds of a few micrometres diickness, which play an important part in microelectronics, can be prepared by die condensation of atomic species on an inert substrate from a gaseous phase. The source of die atoms is, in die simplest circumstances, a sample of die collision-free evaporated beam originating from an elemental substance, or a number of elementary substances, which is formed in vacuum. The condensing surface is selected and held at a pre-determined temperature, so as to affect die crystallographic form of die condensate. If diis surface is at room teiiiperamre, a polycrystalline film is usually formed. As die temperature of die surface is increased die deposit crystal size increases, and can be made practically monocrystalline at elevated temperatures. The degree of crystallinity which has been achieved can be determined by electron diffraction, while odier properties such as surface morphology and dislocation sttiicmre can be established by electron microscopy. 

These model compounds can also be used in device fabrication, since thin films of appropriate thickness can be obtained by sublimation and subsequent deposition onto a substrate in vacuum. Electrical as well as optical properties of such devices have turned out to be strongly dependent on both the molecular packing within the crystallites and the polycrystalline morphology. Understanding and control of this aspect is one of the current scientific challenges. 

Figure 6.9. TMAFM image taken at ambient conditions of a TTF-TCNQ thin film (thickness um) grown on KCl(lOO), revealing its polycrystalline morphology. Reprinted from Surface Science, Vol. 482 85, C. Rojas, J. Caro, M. Gri-oni and J. Fraxedas, Surface characterization of metallic molecular organic thin films tetrathiafulvalene tetracyanoquinodimetane, 546-551, Copyright (2001), with permission from Elsevier.

When we consider silicon films, on the other hand, the nature of the solid deposit is crucial to the behavior of the film. Depending on deposition conditions, we can deposit amorphous, polycrystalline, or single crystal films. As was noted in Chapter 1, the morphology of polycrystalline films can be complex. In the present section, we will review some aspects of polysilicon (poly) thin films deposited by CVD. The final section of this chapter will be devoted to epitaxial silicon thin films. 

As an example of a chemisorption reaction where the control on the surface defects is crucial we mention here the case of CO on MgO. This process has been studied on polycrystalline samples, on MgO thin films supported on a metal, and on MgO single crystals, three forms of the material where the surface morphology and the defect concentration are different. This experimental... 

Knipp, D. et ah, Pentacene thin film transistors on inorganic dielectrics Morphology, structural properties, and electronic transport, J. Appl. Phys. 93, 347-355, 2003. Jentzsch, T. et ah, Efficiency of optical second harmonic generation from pentacene films of different morphology and structure. Thin Solid Films 315, 273-280, 1998. Knipp, D., Street, R.A., and Volkel, A.R., Morphology and electronic transport of polycrystalline pentacene thin-fihn transistors, AppZ. Phys. Lett. 82, 3907-3909, 2003. Mattheus, C.C. et ah, Identification of polymorphs of pentacene, Synth. Met. 138, 475-481, 2003. 

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