Semiconductor polymer films

For studies of laser action in organic semiconductors polymer films with uniform thickness, d, ranging from 0.5 to 4 p.m were slowly spin-coated from fi-esh chloroform solutions onto quartz substrates. The variation in d was typically less than 5% over the film length of 1 mm [96]. For polymers in solutions we thoroughly mixed the polymer powder in good solvents such as THF or chlorophorm typically with concentration of few milligram per milliliter. Then the polymer solution was placed in a transparent cuvette with flat windows, in which the side windows were tiled to avoid optical feedback. 

The apphcation of a high electric field across a thin conjugated polymer film has shown the materials to be electroluminescent (216—218). Until recentiy the development of electroluminescent displays has been confined to the use of inorganic semiconductors and a limited number of small molecule dyes as the emitter materials. Expansion to the broad array of conjugated polymers available gives advantages in control of emission frequency (color) and facihty in device fabrication as a result of the ease of processibiUty of soluble polymers (see Chromogenic materials,electrochromic). 

Three common uses of RBS analysis exist quantitative depth profiling, areal concentration measurements (atoms/cm ), and crystal quality and impurity lattice site analysis. Its primary application is quantitative depth profiling of semiconductor thin films and multilayered structures. It is also used to measure contaminants and to study crystal structures, also primarily in semiconductor materials. Other applications include depth profilii of polymers, high-T superconductors, optical coatings, and catalyst particles. ... 

Polymer films that are sensitive to light, x-rays, or electrons— known as photoresists—are nsed extensively to transfer the pattern of an electronic circuit onto a semiconductor surface. Such films must adhere to the semiconductor surface, cross-link or decompose on exposure to radiation, and nndergo development in a solvent to achieve pattern definition. Virtually all aspects of photoresist processing involve surface and interfacial phenomena, and there are many outstanding problems where these phenomena mnst be controlled. For example, the fabrication of multilayer circuits requires that photoresist films of about 1-pm thickness be laid down over a semiconductor surface that has already been patterned in preceding steps. 

The above mechanistic aspect of electron transport in electroactive polymer films has been an active and chemically rich research topic (13-18) in polymer coated electrodes. We have called (19) the process "redox conduction", since it is a non-ohmic form of electrical conductivity that is intrinsically different from that in metals or semiconductors. Some of the special characteristics of redox conductivity are non-linear current-voltage relations and a narrow band of conductivity centered around electrode potentials that yield the necessary mixture of oxidized and reduced states of the redox sites in the polymer (mixed valent form). Electron hopping in redox conductivity is obviously also peculiar to polymers whose sites comprise spatially localized electronic states. 

Another role for polymer film and surfaces in the world to come is already firmly founded in the notion of modern thin film and integrated electronic circuitry. The era of solid state electronics determines nowadays our use of automata and other elements of highest productivity in international economy, as well being increasing factors in science, eduction, and national security. These capabilities are now primarily embodied in micro circuits, whose integrated form is made directly on single crystal surfaces of silicon or similar semiconductor. 

Photodiodes utilize principally the photophysical process of semiconductors. The most typical juctions to attain photoinduced charge separation are shown in Fig. 27 a c. If a photoexcited compound (P) is arranged with donor and/or acceptor on an electrode as shown in Fig. 25 (d), it must work as a kind of photodiode based on new principle of photochemical reaction. A polymer film must be most promising to construct such photoconversion element. 

Bureau et al. (1) developed a method for forming a polymer film that behaved as a conductor or semiconductor on its surface by layering using electrografting. 

Thin polymer films have many possible technical applications. Transistors and light-emitting diodes are the obvious ones. In ultra-thin films, one may even approach an electronics of molecular dimension. Molecular electronics will be a future challenge for basic and applied science. Nature applies it on a large scale in the reaction centers of the photosynthetic process, where photoinduced mobile charges are separated in some analogy to the separation of the photo-(p-n)-pair in the junction zone of a semiconductor (see Section 13.3.1). 

A number of different methods can be used to prepare polymer film-coated electrodes. The simplest is to dip the surface to be coated into a solution of the polymer, remove the electrode from the solution, and allow the solvent to evaporate. While this method is simple, it is difficult to control the amount of material that ends up on the electrode surface. Alternatively, a measured volume of solution can be applied to the surface to be coated. This allows for accurate control of the amount of polymer applied. The polymer film may also be spin-coated onto the electrode surface. Spin-coating is used extensively in the semiconductor industry and yields very uniform film thicknesses. 

Such bilayers can conveniently be built up by successive electropolymerization of complexes containing ligands with vinyl substituents, e.g. 4-vinylpyridine or 4-vinyl-4 -methyl-2,2 -bipyridyl. The films may be deposited on metallic or semiconductor electrodes (e.g. Pt, glassy carbon, Sn02, Ti02). More efficient metailation of the films is obtained by polymerization of coordinated ligand than by subsequent metailation of a preformed polymer film. An alternative to discrete films would be a copolymer with distinct redox sites, or a combination of a single polymer film with a copolymer film in a bilayer device. 

Electroanalytical sensors based on amperometric measurements at chemically modified electrodes are in the early stages of development. The modes of modification can take many forms, but the most common approach at the present time is the immobilization of ions and molecules in polymer films which are applied to bare metal, semiconductor, and carbon electrodes. Such surface-modified electrodes exhibit unique electrochemical behavior which has been exploited for a variety of applications. 

The materials (metals and conjugated polymers) that are used in LED applications were introduced in the previous chapter. The polymer is a semiconductor with a band gap of 2-3 eV. The most commonly used polymers in LEDs today are derivatives of poly(p-phenylene-vinylene) (PPV), poly(p-phenylene) (PPP), and polythiophene (PT). These polymers are soluble and therefore relatively easy to process. The most common LED device layout is a three layer component consisting of a metallic contact, typically indium tin oxide (ITO), on a glass substrate, a polymer film r- 1000 A thick), and an evaporated metal contact4. Electric contact to an external voltage supply is made with the two metallic layers on either side of the polymer. 

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