Deposition epitaxial

Germane is used primarily to produce high purity germanium metal or epitaxial deposits of germanium on substrates for electronics by thermal decomposition at about 350°C (see Germaniumand germanium compounds). 

Fig. 12.6 (a) Co-ordination across a substrate S-electrodeposit D interface on the atomic scale produces epitaxy, (b) a non-epitaxial deposit has no co-ordination and (c) epitaxy would be expected to produce grain boundary continuation at the interface, though in fact grain boundaries often continue to thicknesses far greater than those at which epitaxy disappears... 

As the density of devices placed on the silicon wafer increases, the problems of autodoping and interdiffusion become more acute and the high temperature limitation of the above reactions has prompted much experimental effort to develop epitaxial deposition at lower temperature. This has been accomplished in the following experimental developments ... 

Gallium arsenide is epitaxially deposited on a silicon substrate and the resulting composite combines the mechanical and thermal properties of silicon with the photonic capabilities and fast electronics of gallium arsenide. 

Fig. 3.12 SEM micrographs of an epitaxial deposit of SnS nanodisks on Au(lOO), in two different magnifications. (Reprinted with permission from [198], Copyright 2009, American Chemical...

Zinc sulfide, ZnS, has been epitaxially deposited by the dual bath approach on Au(lll) surface and studied by STM and XPS [48]. The first complete ECALE cycle resulted in the formation of nanocrystallites of ZnS randomly distributed across Au(l 11) terraces, on account of lattice mismatch induced strain between ZnS and Au(lll) - although the mismatch is only 0.13% for ZnS/Au(lll). Atomically resolved STM images showed the ZnS/Au(lll) monolayer to be sixfold symmetric. The average diameter of the crystallites was 10 5 nm and the apparent coverage 0.38. 

Colletti LP, Teklay D, Stickney JL (1994) Thin-layer electrochemical studies of the oxidative underpotential deposition of sulfur and its application to the electrochemical atomic layer epitaxy deposition of CdS. J Electroanal Chem 369 145-152... 

Foresti ML, Pezzatini G, CavaUini M, Alois G, Innocent M, GuideUi R (1998) Electrochemical atomic layer epitaxy deposition of CdS on Ag(lll) An electrochemical and STM investigation. J Phys Chem B 102 7413-7420... 

Zhang X, Shi X, Wang C (2009) Optimization of electrochemical aspects for epitaxial depositing nanoscale ZnSe thin films. J SoUd State Electrochem 13 469-475... 

We and others have been involved in the study of such systems including Cu/Au(lll),85 86 Ag/Au(lll),87 Pb/Ag(lll),88 and Cu/Pt(lll).89 The first three systems involved the use of epitaxially deposited metal films on mica as electrodes.90 92 Such deposition gives rise to electrodes with well-defined single-crystalline structures. In the last case a bulk platinum single crystal was employed. Because of the single-crystalline nature of the electrodes, polarization dependence studies could be used to ascertain surface structure. 

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]. 

The primary difference in the operating conditions for growth of crystalline as compared with amorphous material is the deposition temperature. In the current design, 500 K is assumed for amorphous film deposition, while higher temperatures in the range 700-950 K are required for epitaxial growth. The low-temperature amorphous film deposition first is used to optimize the process, while the high-temperature epitaxial deposition subsequently is used as the basis for a detailed economic analysis. 

In those instances of very high C values, the fraction of surface uncovered by adsorbate again increases, as a result of epitaxial deposition on specific surface sites, which when widely spaced, would lead to high apparent cross-sectional areas. 

Apart from adhesion, the crystallographic properties of the CD film are sometimes dependent on the nature of the substrate (although more often there does not seem to be any dependence of this type). One example is epitaxial deposition on a crystallographically ordered substrate [epitaxial here means a struc-... 

As for CdS, CdSe has been epitaxially deposited on single crystal InP. As expected, epitaxy occurred only for the ion-by-ion mechanism, where individual species could either adsorb on or migrate to the ideal lattice position. 

When reading the literature, in many (probably most) cases it is not clear whether the deposition proceeds by an ion-by-ion process. The reason is that, unless another mechanism is specifically discussed, it is often assumed that the deposition proceeds via the ion-by-ion mechanism. If the exact deposition parameters are known, which mechanism is operative can, in most cases, be calculated. Two criteria have often been cited in the literature as proof of deposition via the ion-by-ion mechanism. One is epitaxial deposition of the CD film. (Epitaxy refers to growth of one material on another in such a way as to result in coherence between the lattice of the substrate and the deposit. Often—although not necessarily—the lattice of the deposit is aligned in the same direction as that of the substrate.) This is based on the expectation that a cluster mechanism will not result in an epitaxial film for this to occur, clusters of maybe thousands of atoms would need to be able to rearrange themselves on the substrate. Some examples of epitaxial growth are given in Sections 3.4.2 and 4.I.5.2. 

Growth of various semiconductors onto certain single-crystal substrates has resulted in epitaxial growth in a number of cases. This epitaxy has been well studied for CdS deposition by Lincot et al. [59-63]. Although the epitaxy requires a certain degree of lattice matching between semiconductor and substrate, chemical interactions between the constituents of the deposition solution and the substrate are important as well (discussed in more detail in Chap. 4). It is a reasonable assumption that epitaxial deposition occurs via an ion-by-ion process. Indeed, it has... 

There are a number of studies that report the effects of the substrate on the CdS films. With the exception of epitaxial deposition, which will constimte the main part of this section, it is usually difficult to explain any specific substrate effect. Also, it should be borne in mind that each specific study is confined to one deposition bath and that a substrate effect obtained for one bath need not necessarily be obtained for a different one. 

Various investigations into the epitaxial deposition of CdS onto different singlecrystal substrates have been carried out by Lincot et al. On InP, which is closely lattice matched to CdS (<0.1% difference), epitaxial deposition (c-axis of hexagonal CdS perpendicular to the substrate) occurs on the (111) P polar face of the InP but polycrystalline deposition on the (111) In face [49,56]. This difference was clearly due to differing chemical or electrostatic interaction between the InP faces... 

There have been a few reports on epitaxial deposition of PbS on various singlecrystal substrates. PbS (n-type) was epitaxially deposited on (111) Ge (5.4% mismatch) from a Pb(N03)2/KOH/thiourea solution at room temperature with (111) orientation [34] (although another study using apparently the same conditions found the deposit to be p-type and polycrystalline with some (100) preferred orientation [35]). From a similar solution (with addition of some ethanol), PbS was deposited on single-crystal CdS (ca. 6.6% mismatch) with varying degrees of epitaxy [36]. On the (0001) faces of CdS, the growth was (111) [(111) cubic corre-... 

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