Growth epitaxial

Spherulites are produced because the overall crystallization rate is the same for all directions in space. However, the growth rates of crystals in the spherulites may be directionally dependent. Conversely, if the overall 

Shish-kebab formation is a special case of epitaxial growth. The oriented growth of one crystalline substance on another is defined as epitaxy. 

Crystals possess three-dimensional long-range order, and amorphous polymers have no order. Therefore, cases with one- or two-dimensional order 

The molecules arrange themselves in parallel layers in smectic systems. In these cases, the molecular axis is perpendicular to the layer plane. Within the layers, the molecule may be arranged randomly or ordered with respect to other molecules. The molecules are also arranged parallel to each other but not in layers in nematic systems. The cholesteric state is midway between nematic and smectic the molecules are arranged in layers, but with the molecular axis being parallel to the plane of the layer. 

Smectic and nematic meso phases can be easily interconverted in the case of low-molar-mass compounds, and this gives melts of these compounds the characteristics of liquids. On the other hand, these melts are optically anisotropic because of their one-dimensional order, and so, have characteristic colors. Consequently, they are also known as liquid crystals. Solutions of rod-shaped macromolecules exhibit similar ordered behavior they are called tactoidal solutions. One-dimensional order can be induced in polymer melts by lowering the temperature below the melt or glass transition temperature, whereby the one-dimensional order is frozen in. The characteristic X-ray diagrams discussed above are then obtained. 

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The oriented overgrowth of a crystalline phase on the surface of a substrate that is also crystalline is called epitaxial growth [104]. Usually it is required that the lattices of the two crystalline phases match, and it can be a rather complicated process [105]. Some new applications enlist amorphous substrates or grow new phases on a surface with a rather poor lattice match. 

It has also been shown that sufiBcient surface self-diflfiision can occur so that entire step edges move in a concerted maimer. Although it does not achieve atomic resolution, the low-energy electron microscopy (LEEM) technique allows for the observation of the movement of step edges in real time [H]. LEEM has also been usefiil for studies of epitaxial growth and surface modifications due to chemical reactions. 

Another example of epitaxy is tin growdi on the (100) surfaces of InSb or CdTe a = 6.49 A) [14]. At room temperature, elemental tin is metallic and adopts a bet crystal structure ( white tin ) with a lattice constant of 5.83 A. However, upon deposition on either of the two above-mentioned surfaces, tin is transfonned into the diamond structure ( grey tin ) with a = 6.49 A and essentially no misfit at the interface. Furtliennore, since grey tin is a semiconductor, then a novel heterojunction material can be fabricated. It is evident that epitaxial growth can be exploited to synthesize materials with novel physical and chemical properties. 

The following two sections will focus on epitaxial growth from a surface science perspective with the aim of revealing the fundamentals of tliin-film growth. As will be discussed below, surface science studies of thin-film deposition have contributed greatly to an atomic-level understanding of nucleation and growth. 

Aspens D E and Dietz N 1998 Optical approaches for controlling epitaxial growth Appl. Surf. Sc/. 130-132 367-76... 

Sakurai M, Tada FI, Saiki K, Koma A, Funasaka FI and Kishimoto Y 1993 Epitaxial growth of Cgg and Cyq films... 

Koma A 1992 Van Der Waals epitaxy—a new epitaxial growth method for a highly lattice mismatched system Thin Soiid Fiims 216 72-6... 

Extended defects range from well characterized dislocations to grain boundaries, interfaces, stacking faults, etch pits, D-defects, misfit dislocations (common in epitaxial growth), blisters induced by H or He implantation etc. Microscopic studies of such defects are very difficult, and crystal growers use years of experience and trial-and-error teclmiques to avoid or control them. Some extended defects can change in unpredictable ways upon heat treatments. Others become gettering centres for transition metals, a phenomenon which can be desirable or not, but is always difficult to control. Extended defects are sometimes cleverly used. For example, the smart-cut process relies on the controlled implantation of H followed by heat treatments to create blisters. This allows a thin layer of clean material to be lifted from a bulk wafer [261. 

Peng Xet al 997 Epitaxial growth of highly luminesoent CdSe/CdS oore/shell nanoorystals with photostability and eleotronio aooessibility J. Am. Chem. Soc. 119 7019... 

During epitaxial growth, the semiconductor layers must be doped to form thep—n junction and conductive current spreading window layers. Eor III—V materials, zinc, Zn beryUium, Be carbon, C magnesium. Mg and siUcon, Si are commonly employed as -type dopants, whereas tellurium, Te ... 

Fig. 14. Phase diagrams of HgCdTe used to defiae the Hquid-phase epitaxial growth process where composition is ia mole fractioa, X, and the numbers represent temperatures ia °C (a) Te-rich corner where the dotted Haes A—F correspoad to values of of 0.1, 0.2, 0.3, 0.5, 0.8, and 0.9, respectively, and (b) Hg-rich corner where A—F correspond to values of X of 0.9, 0.8, 0.6, 0.4, 0.2, and 0.1, respectively.

The physical stmcture of mixed-layer minerals is open to question. In the traditional view, the MacEwan crystallite is a combination of 1.0 nm (10 E) non-expandable units (iUite) that forms as an epitaxial growth on 1.7 nm expandable units (smectite) that yield a coherent diffraction pattern (37). This view is challenged by the fundamental particle hypothesis which is based on the existence of fundamental particles of different thickness (160—162). 

Physics and chemistry researchers approach III—V synthesis and epitaxial growth, ie, growth in perfect registry with the atoms of an underlying crystal, differently. The physics approach, known as molecular beam epitaxy (MBE), is essentially the evaporation (14—16) of the elements, as illustrated in Figure 4. The chemistry approach, organometaUic chemical vapor deposition (OMCVD) (17) is exemplified by the typical chemical reaction ... 

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