Epitaxy molecular beam , preparation

Epitaxial crystal growth methods such as molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) have advanced to the point that active regions of essentially arbitrary thicknesses can be prepared (see Thin films, film deposition techniques). Most semiconductors used for lasers are cubic crystals where the lattice constant, the dimension of the cube, is equal to two atomic plane distances. When the thickness of this layer is reduced to dimensions on the order of 0.01 )J.m, between 20 and 30 atomic plane distances, quantum mechanics is needed for an accurate description of the confined carrier energies (11). Such layers are called quantum wells and the lasers containing such layers in their active regions are known as quantum well lasers (12). 

Electrocatalytic activity of supported metal particles has been investigated on surfaces prepared in an ultrahigh vacuum (UHV) molecular beam epitaxy system (DCA Instruments) modified to allow high throughput (parallel) synthesis of thin-film materials [Guerin and Hayden, 2006]. The system is shown in Fig. 16.1, and consisted of two physical vapor deposition (PVD) chambers, a sputtering chamber, and a surface characterization chamber (CC), all interconnected by a transfer chamber (TC). The entire system was maintained at UHV, with a base pressure of 10 °mbar. Sample access was achieved through a load lock, and samples could be transferred... 

Quantum dots are the engineered counterparts to inorganic materials such as groups IV, III-V and II-VI semiconductors. These structures are prepared by complex techniques such as molecular beam epitaxy (MBE), lithography or self-assembly, much more complex than the conventional chemical synthesis. Quantum dots are usually termed artificial atoms (OD) with dimensions larger than 20-30 nm, limited by the preparation techniques. Quantum confinement, single electron transport. Coulomb blockade and related quantum effects are revealed with these OD structures (Smith, 1996). 2D arrays of such OD artificial atoms can be achieved leading to artificial periodic structures. 

Molecular beam epitaxy is an important technique for the preparation of semiconductors (III-V compounds). The finesse and sophistication of modern preparative solid... 

Preparation of Ill-V magnetic semiconductors by molecular beam epitaxy. 6... 

There are different criterion of how to classify solid-solid interfaces. One is the sharpness of the boundary. It could be abrupt on an atomic scale as, for example, in III-IV semiconductor heterostructures prepared by molecular beam epitaxy. In contrast, interdiffusion can create broad transitions. Surface reactions can lead to the formation of a thin layer of a new compound. The interfacial structure and composition will therefore depend on temperature, diffusion coefficient, miscibility, and reactivity of the components. Another criterion is the crystallinity of the interface. The interface may be crystalline-crystalline, crystalline-amorphous, or completely amorphous. Even when both solids are crystalline, the interface may be disturbed and exhibit a high density of defects. 

For the study of crystalline surfaces, ultrahigh vacuum (UHV) is required. The preparation of clean crystalline surfaces is usually carried out within the UHV system by cleavage, sputtering, evaporation, thermal treatment, or molecular beam epitaxy. 

Recently, many researchers have paid attention to the optical properties of lanthanide-doped III-V and II-VI semiconductor nanocrystals prepared by ion implantation, molecular-beam-epitaxy (MBE) or wet chemical syntheses. Although some controversies still exist, many important results have been achieved, which may be beneficial to the understanding of the basic physical or chemical properties of lanthanide-doped semiconductor nanocrystals. 

ZnO thin films can be prepared by a variety of techniques such as magnetron sputtering, chemical vapor deposition, pulsed-laser deposition, molecular beam epitaxy, spray-pyrolysis, and (electro-)chemical deposition [24,74]. In this book, sputtering (Chap. 5), chemical vapor deposition (Chap. 6), and pulsed-laser deposition (Chap. 7) are described in detail, since these methods lead to the best ZnO films concerning high conductivity and transparency. The first two methods allow also large area depositions making them the industrially most advanced deposition techniques for ZnO. ZnO films easily crystallize, which is different for instance compared with ITO films that can... 

High-vacuum dry-processes, such as chemical vapor deposition (CVD) and molecular beam epitaxy (MBE), have made it feasible to control precisely the thickness of metal oxide thin films. In these techniques, the preparative conditions like pressure and substrate temperature can be widely varied, and the elemental composition in individual atomic layers is controllable by sequential supply of precursor gases [1]. The dense, defect-less oxide films thus prepared are frequently used as underlayers of microelectronics devices. 

As the previous section showed, in a variety of examples severe enhancements of the ionic conductivity has been found and successfully attributed to space charge effects. Typical examples are silver halides or alkaline earth fluorides (see Section V.2.). How significantly these effects can be augmented by a particle size reduction, is demonstrated by the example of nano-crystalline CaF2.154 Epitaxial fluoride heterolayers prepared by molecular beam epitaxy not only show the thermodynamically demanded redistribution effect postulated above (see Section V.2.), they also highlight the mesoscopic situation in extremely thin films in which the electroneutral bulk has disappeared and an artificial ion conductor has been achieved (see Fig. 

In summary, MgO(OOl) surfaces can be prepared with a very high quality and are ideally suited to perform GIXS measurements. This offers the opportunity to investigate the atomic structure and morphology of metal/MgO interfaces by this technique, during in situ deposition in UHV by molecular beam epitaxy, from the very early stages of sub-monolayer deposition, up to fairly thick metallic layers [23]. 

Films of IVA-VIA compounds have been prepared by the aqueous reactions of group IV nitrates with thio- or selenourea, in basic solution. More recently, bulk crystals, especially of the alloys, have been made by direct reaction. Control of stoichiometry is always difficult. At present, molecular beam epitaxy (precise evaporation of the elements) has become preeminent, because alloys of PbTe with both SnTe and EuTe can be made. It is surprising that a rare earth atom can be substituted into such a lattice, and even more surprising that its electronic behavior appears to be that of a substituent with a valence of + 2. Sn02, while differing widely from the lead salts , is also a IV-VI compound that can be prepared as films by spray pyrolysis of the chloride, or by reactive evaporation or sputtering. 

Non-Silicon Semiconductor Materials. III-V compounds are finding wider uses in the semiconductor Industry. One area of primary Interest is the concern over the cleanliness of substrates used in Molecular Beam Epitaxy (MBE) studies. Substrate preparation is Important because the substrate may play a role in failure that occur after additional processing steps. The surface sensitivity of XPS makes it Ideally suited for such investigations. 

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