Superlattices compositional

Esaki L (1985) Compositional Superlattices. In Parker EHC (ed) The Technology and Physics of Molecular beam epitaxy, pl85 Plenum Press, New York... 

Future work on these composite superlattices will focus on patterning applications (e.g., using microcontact printing) and the generation of magnetic and redox-tunable multilayers that take advantage of the unique features of polyferrocenylsilanes. 

Superlattices of metals, semiconductors, and ceramics - can be produced by single bath electrodeposition.il In addition to superlattices in which the composition is modulated, it is also possible to alternate layers of metal oxides with varying defect chemistry. The defect chemistry superlattices are produced by simply pulsing the applied potential during deposition. A cross-sectional SEM image of a compositional superlattice based on magnetite is shown in Figure 17.18. ... 

Figure 2. Band structures of doping and compositional superlattices.

Heterostructures and Superlattices. Although useful devices can be made from binary compound semiconductors, such as GaAs, InP, or InSb, the explosive interest in techniques such as MOCVD and MBE came about from their growth of ternary or quaternary alloy heterostmctures and supedattices. Eor the successful growth of alloys and heterostmctures the composition and interfaces must be accurately controlled. The composition of alloys can be predicted from thermodynamics if the flow in the reactor is optimised. Otherwise, composition and growth rate variations are observed... 

Colloidal crystals . At the end of Section 2.1.4, there is a brief account of regular, crystal-like structures formed spontaneously by two differently sized populations of hard (polymeric) spheres, typically near 0.5 nm in diameter, depositing out of a colloidal solution. Binary superlattices of composition AB2 and ABn are found. Experiment has allowed phase diagrams to be constructed, showing the crystal structures formed for a fixed radius ratio of the two populations but for variable volume fractions in solution of the two populations, and a computer simulation (Eldridge et al. 1995) has been used to examine how nearly theory and experiment match up. The agreement is not bad, but there are some unexpected differences from which lessons were learned. 

A dependence of w upon composition must also be adduced in the case of the Fe-Ni solid solutions. Over the range from 0 to 56 at. per cent Ni, these solid solutions exhibit essentially ideal behavior,39 so that w 0. Since the FeNi3 superlattice appears at lower temperatures, either w is markedly different at compositions about 75 at. per cent Ni than at lower Ni contents, or w 0 for the solid solutions about the superlattice. Either possibility represents a deviation from the requirements of the quasi-chemical theories. 

Figure 9. Streamlines (top) and relative gas phase composition of A1 species (bottom) In a vertical axlsymmetrlc reactor at five different times during growth of an AlAs/GaAs superlattice. Red corresponds to all A1 species, violet to no A1 species. The corner Insert portrays the variation In solid fraction of A1 across the Interface. Buoyancy dominated flow.

When the silver nanocrystals are organized in a 2D superlattice, the plasmon peak is shifted toward an energy lower than that obtained in solution (Fig. 6). The covered support is washed with hexane, and the nanoparticles are dispersed again in the solvent. The absorption spectrum of the latter solution is similar to that used to cover the support (free particles in hexane). This clearly indicates that the shift in the absorption spectrum of nanosized silver particles is due to their self-organization on the support. The bandwidth of the plasmon peak (1.3 eV) obtained after deposition is larger than that in solution (0.9 eV). This can be attributed to a change in the dielectric constant of the composite medium. Similar behavior is observed for various nanocrystal sizes (from 3 to 8 nm). 

A smaller class of type II alloys of II-VI binaries also exists, including the (CdS) ,(ZnSe)i (CdS) ,(ZnTe)i (CdSe) ,(ZnSe)i (CdS) ,(CdTe)i-. (CdSe)x(CdTe)i i , and (CdS) c(ZnS)i i systems, which transform at some critical composition from the W to the ZB structure. Importantly, the transition temperatures are usually well below those required to attain a thermodynamically stable wurtzite form for the binary constituents (e.g., 700-800 °C for pure CdS and > 1,020 "C for pure ZnS). The type 11 pseudobinary CdxZni jcSe is of considerable interest in thin film form for the development of tandem solar cells as well as for the fabrication of superlattices and phosphor materials for monitors. The CdSe Tei-x alloy is one of the most investigated semiconductors in photoelectrochemical applications. 

Switzer JA, PhUlips RJ, Golden TD (1995) Composition profiles in electrodeposited ceramic superlattices. Appl Phys Lett 66 819-821... 

A second application of current interest in which widely separated length scales come into play is fabrication of modulated foils or wires with layer thickness of a few nanometers or less [156]. In this application, the aspect ratio of layer thickness, which may be of nearly atomic dimensions, to workpiece size, is enormous, and the current distribution must be uniform on the entire range of scales between the two. Optimal conditions for these structures require control by local mechanisms to suppress instability and produce layer by layer growth. Epitaxially deposited single crystals with modulated composition on these scales can be described as superlattices. Moffat, in a report on Cu-Ni superlattices, briefly reviews the constraints operating on their fabrication by electrodeposition [157]. 

In addition, the measurements are rapid and simple, and are now even used in 100% inspection for quality control of multiple-layer semiconductors. An example is shown in Figure 1.6. This is a GaAs substrate with a ternary layer and a thin cap. The mismatch between the layer and the substrate is obtained immediately from the separation between the peaks, and more subtle details may be interpreted with the aid of computer simulation of the rocking curve. This curve can be obtained in a matter of minutes. Routine analysis of such curves gives the composition of ternary epilayers, periods of superlattices and thicknesses of layers, whilst more advanced analysis can give a complete strain and composition profile as a lunction of depth. 

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