Interfacial lattice mismatch

Similarly, the (111) GaAs substrate could be used to achieve epitaxial growth of zinc blende CdSe by electrodeposition from the standard acidic aqueous solution [7]. It was shown that the large lattice mismatch between CdSe and GaAs (7.4%) is accommodated mainly by interfacial dislocations and results in the formation of a high density of twins or stacking faults in the CdSe structure. Epitaxy declined rapidly on increasing the layer thickness or when the experimental parameters were not optimal. 

Stress builds up at a coherent interface between two phases, a and / , which have a slight lattice mismatch. For a sufficiently large misfit (or a large enough interfacial area), misfit dislocations (= localized stresses) become energetically more favorable than the coherency stress whereby a semicoherent interface will form. The lattice plane matching will be almost perfect except in the immediate neighborhood of the misfit dislocation. Usually, misfits exist in more than one dimension. Sets (/) of nonparallel misfit dislocations occur at distances... 

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. 

The most commonly used methods for the preparation of ultrathin oxide films are (1) direct oxidation of the parent metal surface, (2) preferential oxidation of one metal of choice from a suitable binary alloy, and (3) simultaneous deposition and oxidation of a metal on a refractory metal substrate. The detailed procedures for (1) and (2) are discussed elsewhere [7,56,57] procedure (3) is discussed here in detail. Preparation of a model thin-film oxide on a refractory metal substrate (such as Mo, Re, or Ta) is usually carried out by vapor-depositing the parent metal in an oxygen environment. These substrate refractory metals are typically cleaned by repeated cycles of Ar sputtering followed by high-temperature annealing and oxygen treatment. The choice of substrate is critical because film stoichiometry and crystallinity depend on lattice mismatch and other interfacial properties. Thin films of several oxides have been prepared in our laboratories and are discussed below. 

In principle, epitaxial structures involve the deposition and growth of complete monolayers of the adatom which occupy the continuation sites of the substrate. Because of lattice mismatch, the epitaxial structure is subject to lateral stress which is relieved by defect formation. In reality, the distinction between alloy formation and epitaxy is often a function of the surface growth conditions and preparation. If grown at elevated temperatures, seemingly epitaxial structures can exhibit significant amounts of interdiffusion across the interfacial region. 

Another possible route that the h3djrid system could adopt to release the lattice interfacial strain consists in a progressive change of the chemical composition whereby atoms interdiffuse across the interface thus quietening the lattice mismatch, this is what happens in the case of alloy formation [17,19],... 

In nanocrystalline thin films, stresses within the fdm that result from lattice mismatch or thermal expansion mismatch between the substrate and the thin film or from the deposition process can affect the electrical conduction properties. Also, interfacial crystallographic faults may develop as a means of accommodating the film material on the substrate, which can lead to altered transport... 

From the experimental HRTEM images discussed in the previous sections, some conclusions may he drawn regarding the possible atomic arrangement of the r-plane sapphire with respect to the a-plane GaN. Figure 11.25a and b shows possible interfacial arrangements in the [0001 ]caN and [llOOJcaN directions for which the Al-N and Ga-O bonds have been adjusted in such a way that they have minimum lengths. The low mismatch in [llOOjcaN direction leads to an almost complete match between the O atoms of the sapphire and the Ga atoms of the nitride film. Contrary to this, the larger lattice mismatch in [0001 ]caN direction results in a rather poor match between the A1 and O columns of the sapphire with respect to the N and Ga columns of the film. This poor match results in shear forces parallel to the c planes, which are likely to increase the probability of BSE formation in the presence of stress fields, for example, those associated with the introduction of MFDs upon relaxation. 

The primary difficulty inherent in this issue is the small niunber of materials with suitable crystal structures and lattice constants. Some transition metals and ceramics, such as Ni, Cu, Fe, and cBN (Table 5, Ch. 3), are the few isostructural materials with sufficiently similar lattice constants (mismatch <5%). In addition, the extremely high surface energies of diamond (ranging from 5.3 to 9.2 J m for the principle low index planes) and the existence of interfacial misfit and strain energies between diamond films and non-diamond substrates constitute the primary obstacles in forming oriented two-dimensional diamond nuclei. Earlier attempts to grow heteroepitaxial diamond on the transition metals were not successful. The reasons may be related to the high solubility/ mobility of C in/on the metals (for example, Fe, Co, or the... 

Figure 8.2 shows schematically an example of a perfect lattice matched system in which atomic diameters are identical, and an example of a mismatch in which atomic diameters are drawn differing by 15% and for which a dislocation has formed. In a poorly matched system, atoms across the interface are in poor registry. This leads to high interfacial stress and high interfacial energy. Interfacial stress is relieved by periodic formation of dislocations [49]. In general, a lattice match of better than approximately 15% is required for a coherent interface [49]. 

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