Amorphous solids silicon

The same principles that are valid for the surface of crystalline substances hold for the surface of amorphous solids. Crystals can be of the purely ionic type, e.g., NaF, or of the purely covalent type, e.g., diamond. Most substances, however, are somewhere in between these extremes [even in lithium fluoride, a slight tendency towards bond formation between cations and anions has been shown by precise determinations of the electron density distribution (/)]. Mostly, amorphous solids are found with predominantly covalent bonds. As with liquids, there is usually some close-range ordering of the atoms similar to the ordering in the corresponding crystalline structures. Obviously, this is caused by the tendency of the atoms to retain their normal electron configuration, such as the sp hybridization of silicon in silica. Here, too, transitions from crystalline to amorphous do occur. The microcrystalline forms of carbon which are structurally descended from graphite are an example. 

This is a solid in which the atoms are not regularly arrayed as they are in a crystal, but are more jumbled. Glass is also amorphous, its silicon and oxygen atoms in mild disarray. Crystalline silicon was not made until 1854, by the French chemist Henri Deville. 

The acidic destruction of montmorillonite results in the release of silicon and aluminum. The initial fast exchange of surface cations by hydrogen ions is followed by the release of aluminum and silicon. The dissolution rate of Si is higher than that of A1 and is influenced by the relative ratios of basal siloxane and edge surfaces. The shift of pH to more basic values by the ion-exchange processes and the hydrolysis of dissolved species induce the formation of secondary amorphous solids, initiating the formation of amorphous aluminosilicates (Sondi et al. 2008). 

In one respect the defects of an amorphous solid are easier to deal with than those of a crystal. Any small deviation in the local structure of the defect in a crystal results in an identifiably different state, resulting in many possible defect structures. More than 50 point defects are known in crystalline silicon and there is probably an even larger diversity of extended defects. In the amorphous material, small differences in local structure which fall within the disorder of the ideal network cannot be resolved meaningfully. Thus one expects fewer separate classes of defects, but with their energy levels broadened out by the disorder, as illustrated schematically in Fig. 4.1. 

Silicon-backbone materials include silane oligomers, polysilanes, silicon clusters, and amorphous and crystalline silicons. These materials have been investigated independently in two different fields. Crystalline and amorphous silicon are studied in the field of solid-state physics (i), whereas polysilanes and related molecules are studied in the field of organosilicon chemistry (2). Crystalline silicon (c-Si) and amorphous hydrogenated silicon (a-Si H) are well known as two of the most useful semiconductors for electronic and optical devices. Polysilanes have been investigated for application as SiC ceramic binders (3) and photoresists (4). The methods of synthesizing... 

Even before alchemy became a subject of study, many chemical reactions were used and the products applied to daily life. For example, the first metals used were probably gold and copper, which can be found in the metallic state. Copper can also be readily formed by the reduction of malachite—basic copper carbonate, Cu2(C03)(0H)2—in charcoal fires. Silver, tin, antimony, and lead were also known as early as 3000 BC. Iron appeared in classical Greece and in other areas around the Mediterranean Sea by 1500 BC. At about the same time, colored glasses and ceramic glazes, largely composed of silicon dioxide (Si02, the major component of sand) and other metallic oxides, which had been melted and allowed to cool to amorphous solids, were introduced. 

Plasmas are also used for the low temperature deposition of thin solid films, for example amorphous hydrogenated silicon, diamond, and a host of other materials. Since the fundamentals of plasma physics and chemistry are the same for both plasma etching and plasma assisted chemical vapor deposition (PECVD), the latter will only be discussed briefly in Section 6.6. A review of PECVD can be foxmd in [14]. Sputtering is discussed by Chapman [15], and plasma polymerization is covered by Yasuda [16]. 

Since the first synthesis of silicon monoxide by Potter 1905, the chemical state and atomic structure of this amorphous solid have been discussed controversially. The known binary phase diagrams of silicon and oxygen that... 

Solid silicon monosulphide in an amorphous form has been prepared by the condensation of gaseous SiS at high temperatures. Vaporization and vapour pressure data suggest that SiS is metastable, disproportionating into SiS2 and silicon at an appreciable rate above 950 K. SiS appears to be thermodynamically unstable below 1452 K. Vaporization of solid SiS2 yields predominantly SiS and S2 species according to the dissociation process ... 

It is accepted that the features in the Raman spectra of amorphous solids resemble the vibrational densities of states (VDOS) of their crystalline counterparts [111]. The Raman spectrum of amorphous silicon is characterized by four broad bands around 160,300,390, and 470 cm [112,113]. They correspond to the features in the vibrational density of states of a-Si and are referred to as TA-, LA-, LO-, and TO-like bands, respectively [114]. In general, the TA/TO intensity ratio, their linewidths, and frequency positions depend on the method of preparation, deposition conditions, and the degree of structural disorder [115] (Fig. 16). 

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