Introduction

Over the past several decades, there has been rapidly increasing interest in the preparation of nanometer-sized materials such as superlattices, quantum wires, and quantum dots. This enhanced interest is justified because these materials show a great variety of new properties with respect to three-dimensional ones. Nanostructured or nanophase materials can be classified into four categories according to the shape of their structural constituents: nanophase powders, nanostructured films (including single-layer, multilayer, composite film, compositionally graded film, etc.), mono-litic nanostructured materials, and nanostructured composite. The nanostructured materials can contain crystalline, quasicrystalline, and/or amorphous phases.

The search for ultra-thin materials (nanostructured films), in particular, semiconductors, can be traced quite far back (for a review of early work up to 1975, see [1-3]). However, the motivation for their production went up sharply when new types of devices were predicted [4, 5]. Studies of ultra-thin semiconductor layers and multilayer structures containing ultra-thin films have since then increased progressively. Single and multiple quantum wells, single- and double-barrier tunneling structures, incoherent multilayer tunneling structures, etc. have been prepared [6-10] as some of them have been used in production of various devices such as mobile phones, semiconductor photomultipliers, and lasers, etc. [11, 12]. It is important to notice that in crystalline superlattices (periodic multilayer structures built of two consecutively deposited materials) disorder and scattering must be low enough to allow building up of coherent superlattice bandstates and, thus, to prevent destruction of the phase coherence by interface fluctuations. This requirement means that for building up crystalline superlattices, pairs of materials have to be used with a difference in the lattice constants smaller that 1%. Thus, only a limited number of pairs can be applied in the superlattice preparation.

In the beginning of the 1980's, it was shown [13] that superlattices could also be fabricated from amorphous semiconductors (a-Si:H, a-Ge:H, SiOx, SiNx, etc.). The lack of long-range order in the structure of amorphous materials induces a great number of defects such as bond-length and bond-angle fluctuations, dangling and floating bonds, etc. This fact, along with the flexibility of amorphous materials, strongly reduces the requirements for matching lattice constants of the constituent materials and makes possible application of a great variety of materials in the fabrication of amorphous multilayers (a-MLs). Regular a-MLs, based on a-Si:H and a-Ge:H, have been prepared by a variety of rather simple and cheaper set-ups (glow-discharge [13], magnetron sputtering [14], plasma- and photochemically enhanced chemical vapor deposition [15-17] techniques), as compared to molecular beam epitaxy [18, 19] applied for crystalline MLs. It has been shown that applying bandgap engineering, new properties can be achieved, based both on classical effects, connected mainly with the influence of the interlayer barriers [20-25], and quantum-size effect (QSE) when the layers are sufficiently thin and the period (or the well layer width) is comparable with the de Broglie wavelengths of the charge carriers (electrons and holes) [20, 26-30]. Four-fold coordinated a-Si:H, a-Ge:H, and similar materials keep the rigid tetrahedrally bonded structure of the respective elemental crystalline semiconductors. In chalcogenide materials coordination number (that is, the number of the nearest neighbors) varies between 2 (for a-S, a-Se, a-Te, and their alloys) and <3 for GexS(Se)100-x compounds. This low-coordination number makes amorphous chalcogenides more flexible than a-Si:H, a-Ge:H, and similar four-fold coordinated materials. Thus, in chalcogenide a-MLs, the level of interface-induced network distortion (and, hence, the concentration of interface defects) is lower than in a-Si, a-Ge-based MLs [23].

The main peculiarity of nanoparticles in nanocompos-ite films and bulk nanostructured materials is the presence of a large fraction of atoms located at the surface of nanoparticles embedded in a matrix. The large fraction of

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Encyclopedia of Nanoscience and Nanotechnology Edited by H. S. Nalwa Volume 7: Pages (105-123)

low-coordinated atoms at the surfaces of semi-isolated nano-crystals or within the grain boundaries of nanometer-sized composite or polycrystalline materials determines many of their physical, chemical, and mechanical properties [32-35]. The electronic eigenstates and phonons are described by periodic wave functions localized within the nanocrystal. The quantum confinement results in an increase of the bandgap and formation of discrete states at the band edges of semiconductor nanocrystals. The selection rules for optical transitions are relaxed owing to the scattering of the electronic eigenstates and phonons at the grain boundaries, which leads to their mixing and increase of transition probabilities for light absorption and emission. The localization of phonons in the nanocrystals is seen as a shift of the bands in their Raman scattering spectra.

In this chapter, the focus is on fabrication of nano-crystalline materials by nonepitaxial vapor deposition techniques and post-treatment of regular (amorphous or nanocrystalline/amorphous) multilayer structures. Using this approach one can produce either continuous or discontinuous nanocrystalline layers from a great variety of materials. The main advantage of this approach is the possibility for a precise control of the layer thickness during the ML deposition procedure. This makes it possible for the fabrication of nanocrystals (NCs) with desired sizes. In Section 2, a brief description of vapor deposition techniques applied for fabrication of amorphous and crystalline MLs is given. The modes of film growth are also considered. In Section 3, attention is paid to preparation of high-quality periodic amorphous multilayer, thermally and laser-beam induced crystallization of their constituent layers. Special emphasis is put on changes in the crystallization temperature of two-dimensional amorphous materials. Crystallization-induced changes in MLs and material intermixing upon annealing is also considered in this section. In Section 4, preparation of discontinuous metal and semiconductor nanoparticle films is described by means of the multilayer approach and possible mechanisms of nanoparticle formation are discussed.

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