Silicon optical properties

As described in Volume 2IB, Hydrogenated Amorphous Silicon Optical Properties, the absorption coefficient of undoped a-Si H is strongly influenced by the deposition conditions. For example, the optical gap usually increases as the substrate temperature decreases, and this effect has been attributed to an increase in the hydrogen content (Zanzucchi et al.,... 

Fig. 8. Optical properties of silicon (a) transmissivity vs wavelength, (b) shortwavelength absorption coefficient where the transmissivity increases sharply...

The first type of polycarbosilane synthesized by using ADMET methodology was a poly[carbo(dimethyl)silane].14c Linear poly(carbosilanes) are an important class of silicon-containing polymers due to their thermal, electronic, and optical properties.41 They are also ceramic precursors to silicon carbide after pyrolysis. ADMET opens up a new route to synthesize poly(carbosilanes), one that avoids many of the limitations found in earlier synthetic methods.41... 

The III-V and II-VI compounds refer to combination of elements that have two, three, five, or six valence electrons. They have semiconductor properties and are all produced by CVD either experimentally or in production. The CVD of these materials is reviewed in Ch. 12. Many of their applications are found in optoelectronics where they are used instead of silicon, since they have excellent optical properties (see Ch. 15). Generally silicon is not a satisfactory optical material, since it emits and absorbs radiation mostly in the range of heat instead of light. 

Silicon is not as prominent a material in optoelectronics as it is in purely electronic applications, since its optical properties are limited. Yet it finds use as a photodetector with a response time in the nanosecond range and a spectral response band from 0.4 to 1.1 im, which matches the 0.905 im photoemission line of gallium arsenide. Silicon is transparent beyond 1.1 im and experiments have shown that a red light can be produced by shining an unfocused green laser beam on a specially prepared ultrathin crystal-silicon slice.CVD may prove useful in preparing such a material. 

On the other hand, the nonlinear optical properties of nanometer-sized materials are also known to be different from the bulk, and such properties are strongly dependent on size and shape [11]. In 1992, Wang and Herron reported that the third-order nonlinear susceptibility, of silicon nanocrystals increased with decreasing size [12]. In contrast to silicon nanocrystals, of CdS nanocrystals decreased with decreasing size [ 13 ]. These results stimulated the investigation of the nonlinear optical properties of other semiconductor QDs. For the CdTe QDs that we are concentrating on, there have been few studies of nonresonant third-order nonlinear parameters. 

Prakash, G. V., Cazzanell, M., Gaburro, Z., Pavesi, L., lacona, F., Franzo, G. and Priolo, F. (2002) Nonlinear optical properties of silicon nanocrystals grown by plasma-enhanced chemical vapor deposition. /. Appl. Phys., 91, 4607 610. 

The optical properties of a-Si H are of considerable importance, especially for solar-cell applications. Because of the absence of long-range order, the momentum k is not conserved in electronic transitions. Therefore, in contrast to crystalline silicon, a-Si H behaves as though it had a direct bandgap. Its absorption coefficient for visible light is about an order of magnitude higher than that of c-Si [74]. Consequently, the typical thickness (sub-micrometer) of an a-Si H solar cell is only a fraction of that of a c-Si cell. 

Non-oxide ceramics such as silicon carbide (SiC), silicon nitride (SijN ), and boron nitride (BN) offer a wide variety of unique physical properties such as high hardness and high structural stability under environmental extremes, as well as varied electronic and optical properties. These advantageous properties provide the driving force for intense research efforts directed toward developing new practical applications for these materials. These efforts occur despite the considerable expense often associated with their initial preparation and subsequent transformation into finished products. 

Snow, P. A. Squire, E. K. Russell, P. S. J. Canham, L. T., Vapor sensing using the optical properties of porous silicon bragg mirrors, J. Appl. Phys. 1999, 86, 1781 1784... 

Fig. 11.6 Optical properties of thermal oxide on silicon for three oxide thickness of 80, 100, and 120 nm. (a) The reflectance as a function of wavelength, (b) The relative change in reflectance upon immobilization of 1 nm protein...

The size of silica nanoparticles affects their physical, chemical, electronic, and optical properties. Proper size of silica nanoparticles is crucial for design of silica-based nanomaterials. In Stober methods, the size of silica nanoparticles is adjusted by changing the type of organic solvent, the amount of silicon alkoxide, and the... 

The smallest pores that can be formed electrochemically in silicon have radii of < 1 nm and are therefore truly microporous. However, confinement effects proposed to be responsible for micropore formation extend well into the lower mesoporous regime and in addition are largely determined by skeleton size, not by pore size. Therefore the IUPAC convention of pore size will not be applied strictly and all PS properties that are dominated by quantum size effects, for example the optical properties, will be discussed in Chapter 7, independently of actual pore size. Furthermore, it is useful in some cases to compare the properties of different pore size regimes. Meso PS, for example, has roughly the same internal surface area as micro PS but shows only negligible confinement effects. It is therefore perfectly standard to decide whether observations at micro PS samples are surface-related or QC-related. As a result, a few properties of microporous silicon will be discussed in the section about mesoporous materials, and vice versa. Properties of PS common to all size regimes, e.g. growth rate, porosity or dissolution valence, will be discussed in this chapter. 

The dependence of the optical properties of silicon on its microstructure is shown by the color figure in the lower left corner of this books cover. Despite the fact that the thickness of all PS samples shown has been chosen to correspond to a 20 pm thick film of bulk silicon, the difference in light transmission is dramatic [Le27]. 

The chemical properties of silicon are not particularly sensitive to small amounts of impurities and have mostly been determined using low purity material. However, many of the mechanical, electrical, and optical properties are substantially altered by the level of impurities. These have been reexamined in detail since high purity silicon first became available in the late 1940s. 

Polysilanes. Following the first reports of soluble and processable polysilanes in the late 1970s, these macromolecules have attracted substantial interest from both fundamental and applied perspectives." The backbone of silicon atoms gives rise to unique electronic and optical properties as a result of the delocalisation of a-electrons. Several polysilanes have also been found to function as useful thermal precursors to silicon carbide fibres and these materials have also attracted attention with respect to microlithographic applications and as polymerisation initiators." ... 

The surface of silicon has optimum optical properties and the adsorption of proteins... 

Octasilacubanes were used as a model in an attempt to understand the optical properties of porous silicon because both porous silicon and octasilacubane show a broad photoluminescence spectra and large Stokes shifts52. 16 for example, shows an absorption edge at ca 3.2 eV and a broad photoluminescence spectrum with a peak at 2.50 eV. 

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