Reflection thin film

Space-based solar ceUs are covered with a very thin layer of vitreous siHca to protect against the damaging environment of space such as atomic oxygen, micrometeorites, and radiation effects. Because the siHca is transparent to damaging uv radiation, it is normally coated with a uv-reflective thin film... 

Fig. 8.18 Total internal reflection thin-film optical switch (after K. Wasa el al. (1984) J. Lightwave Techn. IEEE., LT-2, 710-14).

We are now in a position to calculate the reflections from multiple mterfaces using the simple example of a thin film of material of thickness d with refractive index n.2 sandwiched between a material of refractive index (where this is generally air witii n = ) deposited onto a substrate of refractive index [35, 36], This is depicted in figure Bl.26.9. The resulting reflectivities for p- and s-polarized light respectively are given by ... 

Figure 3.18 Total reflection of radiation in a medium of refractive index 2 by a thin film of refractive index wj, where 2...

The real utility of d comes in the analysis of thin films. Consider a substrate of refractive index supporting a thin film of thickness d and refractive index in contact with an internal reflection element (the prism) of refractive index as shown in Figure 24. In this case, d depends on the polarization of the incident light beam and is given by... 

Atr—ftir can be readily performed on most commercial ftir spectrometers through the use of an attachment for atr spectroscopy. These devices provide ir-transparent internal reflection elements that are typically made of Ge, KRS-5, ZnSe, or ZnS. These internal reflection elements are made of materials that are of extremely high purity to avoid losses from absorption by impurities in these devices. Coupling of a thin film or surface sample to one of these reflection elements is accompHshed by pressing the sample against the element while acquiring the spectmm. 

Nonmilitary infrared apphcations for germanium include CO2 lasers (qv), intmsion alarms, and pohce and border patrol surveillance devices. Germanium is used as a thin-film coating for infrared materials to decrease reflection losses or to provide heavy filtering action below 2 p.m. 

Although the pure metal has a silvery-white color, in the cast condition it may have a yellowish tinge caused by a thin film of protective oxide on the surface. When highly poHshed, it has high light reflectivity. It retains its brightness well during exposure, both outdoors and indoors. 

Note that the 14 ppm of ammonia measured in the accumulated condensate may not accurately reflect the potentially higher level of ammonia dissolved in a thin film of moisture that could form on the metal wall in the region of initial condensation. This film of water could be saturated witb ammonia, increasing susceptibility of the metal to SCC. 

This chapter contains articles on six techniques that provide structural information on surfaces, interfeces, and thin films. They use X rays (X-ray diffraction, XRD, and Extended X-ray Absorption Fine-Structure, EXAFS), electrons (Low-Energy Electron Diffraction, LEED, and Reflection High-Energy Electron Diffraction, RHEED), or X rays in and electrons out (Surfece Extended X-ray Absorption Fine Structure, SEXAFS, and X-ray Photoelectron Diffraction, XPD). In their usual form, XRD and EXAFS are bulk methods, since X rays probe many microns deep, whereas the other techniques are surfece sensitive. There are, however, ways to make XRD and EXAFS much more surfece sensitive. For EXAFS this converts the technique into SEXAFS, which can have submonolayer sensitivity. 

Alternatives to XRD include transmission electron microscopy (TEM) and diffraction, Low-Energy and Reflection High-Energy Electron Diffraction (LEED and RHEED), extended X-ray Absorption Fine Structure (EXAFS), and neutron diffraction. LEED and RHEED are limited to surfaces and do not probe the bulk of thin films. The elemental sensitivity in neutron diffraction is quite different from XRD, but neutron sources are much weaker than X-ray sources. Neutrons are, however, sensitive to magnetic moments. If adequately large specimens are available, neutron diffraction is a good alternative for low-Z materials and for materials where the magnetic structure is of interest. 

The film thickness of epitaxial and highly textured thin films can be measured with XRD. Close to the usual or primary difftaction peaks there are secondary or subsidiary maxima in the difftacted intensity (see Figure 6), which are due to the finite film thickness. The film thickness is inversely proportional to the spacing between these maxima and is easily calculated. X-ray reflectivity is another accurate method for measuring a film s thickness. 

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