Vacuum Evaporation-Epitaxy

Kane take et al. (I987) have also obtained mono-oriented thin films by a 

Other techniques have been also used in oriented thin fim fabrication like shear technique (Meyler and Thakur (1985)) giving truly monocrystalline thin films. In this case, the film is grown from solution between two e.g. silica slides with a preorientation of monomers at low temperature by shearing and controlled solvent evaporation. It works for monomers characterized by a high ability for cristallization like diacetylenes TS and TCDU. 

Oriented thin films can be also obtained by stretching of poly acetylene, poly(alkyl)thiophene and some polydiacetylene isotropic thin films. In this case polymer chains align in the stretching direction. Some films like poly tiophene, polypyrrole can be obtained by electrochemical deposition. 

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The preparation method of the thin layers and the electroluminescent materials available are reviewed in Ref. [28J. Among the teehniques used are sputtering, vacuum evaporation, metal-organic chemical-vapor deposition (MOCVD) and atomic layer epitaxy (ALE). 

The most flexible methods, which can be applied in principle to nearly all molecules and which provide the most important techniques for the preparation of organic molecular films, are vacuum evaporation and molecular-beam epitaxy (cf [14]). In both methods, the substance required is evaporated or sublimed and captured on a substrate which is cooled as necessary. In molecular-beam epitaxy, the substrate must be an (inorganic) single crystal. The nature of the substrate s surface, its temperature and the velocity of evaporation or sublimation, adjustable... 

Recently, an evaporation deposition technique was employed for preparation of thin films, because it can maintain deposition conditions in a vacuum in the course of deposition. Therefore high-quality thin films can be obtained. Physical vapor deposition techniques include mainly vacuum evaporation, sputtering deposition ion-assisted deposition, molecular beam epitaxy (MBE), and ICB deposition. Table 1 shows characteristics of these deposition methods. 

These cells were not very reproducible, however, because CuInSe2 decomposes during such vacuum evaporation and, therefore, other ways of preparing thin films have been explored. These include a kind of molecular beam epitaxy by the Mickelson and Chen of Boeing [3], and rf-sputtering and chemical spray pyrolysis by the group at Brown University [7,8]. 

Molecular beam epitaxy is a non-CVD epitaxial process that deposits silicon through evaporation. MBE is becoming more common as commercial equipment becomes available. In essence, silicon is heated to moderate temperature by an electron beam in a high vacuum... 

Molecular beam epitaxy (MBE) is a radically different growth process which utilizes a very high vacuum growth chamber and sources which are evaporated from controlled ovens (15,16). This technique is well suited to growing thin multilayer stmctures as a result of very low growth rates and the abihty to abmpdy switch source materials in the reactor chamber. The former has impeded the use of MBE for the growth of high volume LEDs. 

Fig. 4. Schematic of an ultrahigh vacuum molecular beam epitaxy (MBE) growth chamber, showing the source ovens from which the Group 111—V elements are evaporated the shutters corresponding to the required elements, such as that ia front of Source 1, which control the composition of the grown layer an electron gun which produces a beam for reflection high energy electron diffraction (rheed) and monitors the crystal stmcture of the growing layer and the substrate holder which rotates to provide more uniformity ia the deposited film. After Ref. 14, see text.

One technique which has produced thin, but not epitaxial films of BaPbj.xBixOg, and which shows good promise is the use of laser evaporation methods (40). Since the compound is efficiently transported stoichiometrically from the target to the substrate at a high rate and does not require a vacuum, this method may be superior to sputtering techniques. 

PVD reactors may use a solid, liquid, or vapor raw material in a variety of source configurations. The energy required to evaporate liquid or solid sources can be supplied in various ways. Resistive heating is common, induction heating of the source bottle is sometimes used, and electron beams are also employed. Molecular-beam-epitaxy (MBE) systems are PVD-type reactors that operate at ultrahigh vacuum. Very low growth rates are used ( 1 xm/h), and considerable attention is devoted to in situ material characterization to obtain high-purity epitaxial layers (2). 

For the study of crystalline surfaces, ultrahigh vacuum (UHV) is required. The preparation of clean crystalline surfaces is usually carried out within the UHV system by cleavage, sputtering, evaporation, thermal treatment, or molecular beam epitaxy. 

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