Epitaxy perfect

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 ... 

Epitaxial growth of thin films usually involves the formation of strained material as a result of mismatch between the film and substrate and because of the large surface to volume ratio in the film. Surface stress can be a major factor, even when the lattice constants of film and substrate are perfectly matched. Although it appears to be difficult to eliminate the stress totally, it is important to be able to control it and even use it to produce desired qualities. 

Two diacetylenes have been epitaxially polymerized as thin films in contact with alkali halide substrates. These films in contact with alkali halide substrates. These films consisted of highly oriented single crystals aligned along both <110> directions of the substrate. The structures of both poly(TCDU) and poly(DMDA) were modified by this technique and, in all cases, highly crystalline near-perfect films were achieved. 

No epitaxy could be obtained by reaction of the metal films with reactive gas for short reactions times. This is understandable as the carburization and nitridation reactions progress from the surface of the metal films to the substrate and occur with a change in crystal structure of the film (for instance bcc to hex). So even if the starting metal film is epitaxial, the final carbide or nitride compound could be polycrystalline. For high temperatures and for long time treatments (>15 h), however, perfect epitaxial Y Mo2N films could be obtained on MgO (100).17 In this last case, the crystalline state of the precursor metal film had no effect on the final parallel orientation of the nitride. 

In general, unlike for the perfect epitaxial structures of fully strained materials, for nitride heteroepitaxial layers it is essential to perform not a single scan for a symmetrical reflection, but a set of two- or even three-dimensional maps of symmetrical and asymmetrical reflections. Additionally, for some applications, an intense beam is needed and therefore low-resolution X-ray diffractometry can be sometimes a preferable technique to the commonly used high-resolution XRD. For example, if we examine a heterostructural nitride superlattice, low resolution diffractometry will give us a broader zeroth-order peak (information on the whole layer) but more satellite peaks (information on the sublayers). Therefore, multipurpose diffractometers with variable configurations are the most desirable in nitride research. 

While it is important to control the stoichiometry of each layer to adjust their bandgaps, it is equally important to have as few interfacial mismatches as possible. That is, each layer must be epitaxially grown to ensure that the lattice constants are perfectly matched. It has been shown that a lattice mismatch of only ca. 0.01 % is enough to cause significant electron-hole recombinations, resulting in lower cell efficiency. CVD is the method of choice for the fabrication of these multilayer devices as you might expect, cells of this variety are relatively quite expensive. 

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