Silicon , refractive index

Antireflection coatings are used over the silicon surface which, without the coating, reflects ca 35% of incident sunlight. A typical coating consists of a single layer of a transparent dielectric material with a refractive index of ca 2, which is between the index of siUcon and ait or cover material. Materials such as titanium dioxide, tantalum pentoxide, Ta20, or siUcon nitride, Si N, ca 0.08-p.m thick are common. The coating and a physically textured... 

The refractive index of amorphous silicon is. within certain limits, a good measure for the density of the material. If we may consider the material to consist of a tightly bonded structure containing voids, the density of the material follows from the void fraction. This fraction / can be computed from the relative dielectric constant e. Assuming that the voids have a spherical shape, / is given by Bruggeman [61] ... 

Interferometry on porous silicon The average refractive index of the porous silicon layer is affected by analyte adsorption, resulting in a shift of the Fabry Perot fringes 6,18... 

At the smallest scale, the silicon photonic wire waveguides that are the subject of this chapter have a silicon core only a few hundred nanometers across and refractive index of n = 3.47 at a wavelength of /. 1550 nm31. Figure 9.3 shows an example... 

Fig. 9.6 The calculated effective index change in the silicon PWEF waveguide of Fig. 9.3 induced by adsorbed films of constant optical thickness (a) Dopt 0.4 nm and (b) Dopt 0.1 nm for film thickness between 0.5 and 16 nm. As the film thickness changes, the refractive index is adjusted so that the optical thickness remains constant. For comparison, the graphs also show 8iVeff and the corresponding SPR angle shift A6 for an SPR experiment...

The factor f reduces the oscillation amplitude symmetrically about R - R0, facilitating straightforward calculation of polymer refractive index from quantities measured directly from the waveform (3,). When r12 is not small, as in the plasma etching of thin polymer films, the first order power series approximation is inadequate. For example, for a plasma/poly(methyl-methacrylate)/silicon system, r12 = -0.196 and r23 = -0.442. The waveform for a uniformly etching film is no longer purely sinusoidal in time but contains other harmonic components. In addition, amplitude reduction through the f factor does not preserve the vertical median R0 making the film refractive index calculation non-trivial. 

The full expression for the reflected intensity of a laser interferometer in a plasma/polymer/silicon system can be used to measure the polymer refractive index to within about 3 percent. 

The fiber optic refractive index sensor finds use in biomedical applications. It uses a silicon chip with optical waveguides forming ring resonators. When the laser wavelength is scanned, the resonators cause dips in the power transmitted through the device. The wavelength at which these dips occur is a measure of the refractive index of the substance in contact with the chip surface. 

A certain anisotropy of the refractive index along specific crystallographic axes indicates that the microstructures in the porous network are not spherical but somewhat elongated along the PS growth direction [Mi4], This birefringence is below 1% for micro PS, while it may reach values in the order of 10% for meso PS films formed on (110) oriented silicon wafers [Ko22]. 

The refractive index of micro PS, nPS, for wavelengths in the visible or near IR regime is found to decrease with porosity p [Lol] and can be roughly approximated if the refractive index of bulk silicon nsi for the wavelength in question is known, using... 

A single layer of a micro PS film on a silicon substrate always reduces its reflectivity, because of its lower refractive index compared to bulk Si. Hence micro and meso PS films of a thickness around 100 nm have been proposed as anti-reflec-tive coatings for solar cells [Pr8, Gr9, Pol, Bi4, Scl8, StlO]. 

Wasserman [186] has described the use of both low-angle X-ray reflectivity and ellipsometry for the determination of thickness of Cio-Cig SAMs prepared on surface silanol groups of silicon plates. Ellipsometry is based on the reflection of polarized light from a sample and depends on the sample s thickness and refractive index. X-ray reflectivity measures the intensity of X-rays reflected from a surface (or interference pattern) that is characteristic of the distance between interfaces. The thickness of the SAMs was consistent with fully extended alkyl chains with all-trans conformations and excellent agreement was observed between the two methods. 

Through the photoelastic effect, stress will modify the refractive index of silicon according to the relations... 

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