Epitaxially textured films

Fig. 7.9. RHEED images of optimized ZnO thin film surfaces on r-plane, a-plane, and c-plane sapphire, in the two azimuthal orientations (top and bottom) separated by 45° (left) or 30° (middle and right), respectively. The RHEED patterns of the a-axis textured film on r-plane sapphire (left) indicate an epitaxial and three-dimensional, island-like growth. The ZnO films on a-plane (middle) and c-plane sapphire (right) exhibit a smoother surface structure, as indicated by the streaky RHEED patterns and the observation of additional weak reflections in the top images due to 3 x 3 surface reconstruction [51]...

Figure 13.3. (a) The carrier concentrations and (b) the mobilities of simultaneously deposited randomly oriented polycrystalline, highly oriented (lOO)-textured, and homo-epitaxial diamond films [294]. 

Diamond homoepitaxy and heteroepitaxy have been achieved on diamond and cBN, respectively. Epitaxially textured and fiber textured, highly oriented diamond films have been grown on Si(lOO). The feasibility of diamond heteroepitaxy on Ni, Ti, Co and Cu has also been confirmed by several experiments. Diamond can grow on Fe, but not yet by epitaxy. 

Secondary ion mass spectroscopy was used to study the chemical diffusivity of 1 0 in c-textured YBa2Cu307 5/SrTi03 epitaxial thin film. By fitting experimental data... 

As we have seen, the orientation of crystallites in a thin film can vary from epitaxial (or single crystalline), to complete fiber texture, to preferred orientation (incomplete fiber texture), to randomly distributed (or powder). The degree of orientation not only influences the thin-film properties but also has important consequences on the method of measurement and on the difficulty of identifying the phases present in films having multiple phases. 

The film thickness of epitaxial and highly textured thin films can be measured with XRD. Close to the usual or primary difftaction peaks there are secondary or subsidiary maxima in the difftacted intensity (see Figure 6), which are due to the finite film thickness. The film thickness is inversely proportional to the spacing between these maxima and is easily calculated. X-ray reflectivity is another accurate method for measuring a film s thickness. 

Another recently discovered form of epitaxy is graphoepitaxy (Geis et al. 1979). Here a non-crystalline substrate (often the heat-resistant polymer polyi-mide, with or without a very thin metallic coating) is scored with grooves or pyramidal depressions the crystalline film deposited on such a substrate can have a sharp texture induced by the geometrical patterns. More recently, this has been tried out as an inexpensive way (because there is no need for a monocrystalline substrate) of preparing oriented ZnS films for electroluminescent devices (Kanata et al. 1988). 

Epitaxial effects are not limited to single-crystalline substrates. The possibility for substrate-induced epitaxial development in the difficult case of ZnSe (cf. conventional electrodeposition) has been established also by using strongly textured, albeit polycrystalline, zinc blende (111) CdSe electrolytic films to sustain monolithic growth of ZnSe in typical acidic selenite baths [16]. Investigation of the structural relations in this all-electrodeposited ZnSe/CdSe bilayer revealed that more than 30-fold intensification of the (111) ZnSe XRD orientation can be obtained on the textured (111) CdSe films, compared to polycrystalline metal substrates (Fig. 4.2). The inherent problems of deposition from the Se(IV) bath, i.e., formation of... 

The fundamental issue of epitaxial growth on polycrystalline substrates has been addressed in a more refined manner in relation to the electrodeposition of CdSe on metals. Polycrystalline, lll -textured Au surfaces were shown [17] to promote the electrodeposition of coherent, epitaxial CdSe quantum dot films over areas micrometers in size, i.e., much larger than the polycrystalline Au grains, despite the numerous grain boundaries present in the substrate. The Au films (considered as... 

Cachet H, Cortes R, Froment M, Mamin G (1999) Epitaxial growth of electrodeposited cadmium selenide on (111) gallium arsenide. Philos Mag Lett 79/10 837-840 Muthuvel M, Stickney JL (2006) CdTe Electrodeposition on InP(lOO) via Electrochemical Atomic Layer Epitaxy (EC-ALE) Studies Using UHV-EC. Langmuir 22 5504-5508 Streltsov EA, Osipovich NP, Ivashkevich LS, Lyakhov AS (1999) Effect of Cd(ll) on electrodeposition of textured PbSe. Electrochim Acta 44 2645-2652 Beaunier L, Cachet H, Cortes R, Froment M (2000) Electrodeposition of PbSe epitaxial films on (111) InP. Electrochem Commun 2 508-510... 

Crystals growing on a substrate may be oriented every which way that is, the direction axes of individual crystallites can be randomly distributed. However, where one particular axis is oriented or fixed in nearly one direction, we speak about a single texture. When two axes are thus fixed or oriented, we speak about double texture. Monocrystalline orientation refers to a scenario in which there are three such nearly oriented axes, including epitaxial films. Orientation here is viewed with respect to any fixed (in space) frame of reference. Crystal planes and directions are illustrated in Figure 16.5. A brief discussion of the enumeration of these elements follows. 

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