Epitaxial crystallization characteristics

In addition to epitaxial relations, characteristic textures appear due to the intergrowth of crystals of two different species in a certain crystallographic relation. Various terms have been used in the mineralogical field to describe textures, as summarized in Table 7.2 [15], [16]. Observations of descriptive and taxonomy type have been accumulated, since they show the origin of rocks and ores, but understanding the mechanism of their formation still remains a future subject of research. 

Electron diffraction from these crystals indicated a high degree of crystallinity in both the trimer and polymer, and indexing of the patterns required the use of new unit cells different from the usual structures(Figure 4). X-ray diffrac-tometry was used to provide structural information about the third dimension in these epitaxially crystallized films. It should be noted that the characteristic spacing determined by X-ray diffrac-tometry is the b-axis, only if the unit cell is orthorombic (Table II). 

Synthesis.—In efforts to increase crystal perfection and hence electrical characteristics, SjNj vapour has been epitaxially crystallized onto a variety of alkali halide substrates, and the resulting crystals polymerized in the solid state to (SN) . While the normal a form of (SN) was obtained on KCl, a novel y form was discovered... 

Hydroxyapatite (with some carbonate inclusions) is the most stable of the possible calcium phosphate salts that can be formed under physiological conditions. However, it is not the most rapid one to form. Instead, octacalcium phosphate (OCP) will precipitate more readily than hydroxyapatite. This led Brown in 1987 to propose that, as the kinetically favoured compound, OCP precipitates first, and then undergoes irreversible hydrolysis to a transition product OCP hydrolyzate [68]. This hypothesis is consistent with the observation that enamel comprises hydroxyapatite crystals that have the long, plate-like morphology that is generally considered characteristic of OCP crystals [69]. Overall, it seems that enamel crystals, with their elongated form, result from early precipitation of OCP, which forms a template on which hydroxyapatite units grow epitaxially [70,71]. This leads to enamel mineralisation with the observed thin, ribbon-like structure of crystals. 

Twinning and epitaxial growth are also methods of controlled growth, and therefore crystals grown by these methods often exhibit a very different characteristic morphology from that of coexisting single crystals. 

EPITAXY. An oriented crystalline growth between two crystalline solid surfaces of different chemical composition, in which the surface of one crystal provides, through its lattice structure, preferred positions lor the deposition of the second crystal. This behavior is characteristic of some types of high polymers. 

Yanagi, H., Araki, Y., Ohara, T., Hotta, S., Ichikawa, M. andTaniguchi, Y. (2003) Comparative carrier transport characteristics on organic field-effect transistors with vapor deposited thin films and epitaxially grown crystals of biphenyl-capped thiophene oligomers. Advanced Functional Materials, 13, 767-73. 

The prodnction of epitaxial thin films was developed particularly by the microelectronics indnstry, based on semi-condnctor materials [FEW 01], The continuons decrease in the thickness of films, combined with the miniatiuization of the systems, has led to the development of epitaxial films which are sometimes only a few nanometers thick. Fnrthermore, the development of new devices with magnetic, optical or electronic properties has enconraged the introdnction of oxide crystals in active components. The stracture of these oxides and the microstractures of the films prodnced are often complex, and improving the properties requires adequate characterization capabilities. X-ray diffraction, probably along with electronic or scanning probe microscopes, is one of the key techniques in the study and constant improvement of the characteristics of epitaxial films... 

If two crystals are placed side by side, it is possible to define vector relations that express the characteristic crystallographic directions of one of the ctystals in a set of coordinates defined by the cell of the other crystal. These are referred to as epitaxy relations. By extrapolation, when the grains that comprise a polyctystalUne film all have virtually the same orientation, it is possible to define ctystallographic axes specific to this orientation and to find the relation between these axes and those, for example, of the single ctystal on which the film is deposited. If the film and the substrate share the same ctystal nature, we are dealing with homoepitaxy, otherwise, it is referred to as heteroepitaxy. Epitaxy relations are three-dimensional and therefore they are usually written as follows ... 

Study of epitaxial growth by the LEED method is often quite clear-cut and uncomplicated by formation of unexpected structures. One simply observes development of a characteristic crystal plane of the deposit bulk structure, and this is usually quite obvious from simple inspection of the pattern. Orientation of the film on the substrate is usually easily evident also. In some cases, the overlayer is coherent with the substrate as a coincidence lattice producing complicated LEED patterns. Yet it is usually relatively straightforward to decipher such patterns, because spacings in the overlayer structure are often easily assigned from known X-ray spacings of the substance being deposited [see, for example, Bauer (95)]. 

The depth of the recrystallised layer is very small, of the order of 0.01 -0.1 pm. Thus, a possible way to eliminate its effect on p-n junction properties is to perform a drive-in diffusion of aluminium from the epitaxial layer. Unfortunately, the diffusivity of aluminium from an epitaxial layer is extremely slow in SiC. The diffusivity is 3 - 5 orders of magnitude lower than that observed for diffusing aluminium from the vapour phase [70]. The authors of [69] had to employ very high diffusion temperatures, over 2500 °C. The anneal produced a shift of the p-n junction into the crystal bulk and the electrical properties were substantially improved. However, this could not provide the elimination of the weak points of the junctions. The characteristics of the p-n junctions were worse than those with the recrystallised layer removed by sublimation etching. In addition, the surface evaporation and graphitisation at temperatures above 2500 °C severely reduces the reproducibility of the results. 

Some difference in condensation behaviour apparently occurs above and below 470 K. Below this temperature, the growth follows a layer-by-layer two-dimensional mode, while above it exhibits so-called Stranski— Krastanov characteristics in which an epitaxial mono- or multi-layer is first formed on the surface and three-dimensional crystals of the deposit then grow on top of (or from within) this layer. Clearly, when the number density of these crystals becomes high, the difference between the two growth modes is semantic and some workers take the viewpoint that all growth in this system follows the Stranski—Krastanov mode. 

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