Thin film radiation

The photoinitiated polymerizations were followed by real-time infrared spectroscopy on thin films, radiation. The rates of polymerization were reported by them to increase with the light intensity according to a nearly square root law, up to an upper limit. The upper limit or the saturation effect was attributed by them to a fast consumption of flie photoinitiator under intense illumination. A strong correlation was found to exist between flie rate at which the temperature increases and fire rate of polymerization. The temperature shows the same light intensity dependence as the reaction rate, and levels off to a maximum value under intense illumination. Photopolymerization experiments carried out at a constant temperature of 25"C show that thermal runaway is not responsible for the increase of the polymerization rate observed at the beginning oftheUV exposure. 

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

Thin films (qv) of lithium metal are opaque to visible light but are transparent to uv radiation. Lithium is the hardest of all the alkaH metals and has a Mohs scale hardness of 0.6. Its ductiHty is about the same as that of lead. Lithium has a bcc crystalline stmcture which is stable from about —195 to — 180°C. Two allotropic transformations exist at low temperatures bcc to fee at — 133°C and bcc to hexagonal close-packed at — 199°C (36). Physical properties of lithium are Hsted ia Table 3. 

These provide thin films of a soHd, or a combination of soHds, interposed between two moving surfaces to reduce friction and wear. They are coming into more general use for high temperatures, vacuum, nuclear radiation, aerospace, and other environments that prohibit use of oils and greases. 

Copper Indium Diselenide. CuInSe2 (CIS) has proven to be one of the most promising thin-film photovoltaic materials. CIS ahoy materials have yielded smah-area (ca 1 cm ) laboratory devices with efficiencies in excess of 17% and large-area (ca 0.5 m ) monolithic integrated modules with efficiencies in excess of 11%, and have shown excehent radiation hardness. 

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... 

Track-etched membranes are made by exposing thin films (mica, polycarbonate, etc) to fission fragments from a radiation source. The high energy particles chemically alter material in their path. The material is then dissolved by suitable reagents, leaving nearly cylindrical holes (19). 

Numerous uses for PZT/PLZT thin films are under investigation. The device that, as of this writing, is closest to commercialization is a nonvolatile memory. This device, which utilizes a ferroelectric thin-film capacitor integrated onto siUcon circuitry, provides memory retention when the power is off because of the polarization retention of the ferroelectric capacitor. One and zero memory states arise from the two polarization states, — and +F, of the ferroelectric. Because PZT is radiation-hard, the devices are also of interest for military and space appHcations. 

RESEARCH OF DEPENDENCE OF INTENSITY OF X-RAY RADIATION FROM PHYSICAL AND CHEMICAL PROPERTIES OF THIN-FILM SPECIMENS... 

Chain reactions such as those described above, in which atomic species or radicals play a rate-determining part in a series of sequential reactions, are nearly always present in processes for the preparation of thin films by die decomposition of gaseous molecules. This may be achieved by thermal dissociation, by radiation decomposition (photochemical decomposition), or by electron bombardment, either by beams of elecuons or in plasmas. The molecules involved cover a wide range from simple diatomic molecules which dissociate to atoms, to organometallic species with complex dissociation patterns. The... 

There are therefore two ways in which lasers may be used to bring about photon-assisted film formation. If the laser emits radiation in the near-ultra-violet or above, photochemical decomposition occurs in the gas phase and some unabsorbed radiation arrives at the substrate, but this latter should be a minor effect in die thin film formation. This procedure is referred to as photolysis. Alternatively, if the laser emits radiation in the infra-red, and tire photons are only feebly absorbed to raise the rotational energy levels of the gaseous... 

The energy densities of laser beams which are conventionally used in the production of thin films is about 10 — 10 Jcm s and a typical subsU ate in the semiconductor industry is a material having a low drermal conductivity, and drerefore dre radiation which is absorbed by dre substrate is retained near to dre surface. Table 2.8 shows dre relevant physical properties of some typical substrate materials, which can be used in dre solution of Fourier s equation given above as a first approximation to dre real situation. 

In X-Ray Fluorescence (XRF), an X-ray beam is used to irradiate a specimen, and the emitted fluorescent X rays are analyzed with a crystal spectrometer and scintillation or proportional counter. The fluorescent radiation normally is diffracted by a crystal at different angles to separate the X-ray wavelengths and therefore to identify the elements concentrations are determined from the peak intensities. For thin films XRF intensity-composition-thickness equations derived from first principles are used for the precision determination of composition and thickness. This can be done also for each individual layer of multiple-layer films. 

Besides phase identification XRD is also widely used for strain and particle size determination in thin films. Both produce peak broadenings, but they are distinguishable. Compared to TEM, XRD has poor area resolution capability, although by using synchrotron radiation beam diameters of a few pm can be obtained. Defect topography in epitaxial films can be determined at this resolution. 

XRD offers unparalleled accuracy in the measurement of atomic spacings and is the technique of choice for determining strain states in thin films. XRD is noncontact and nondestructive, which makes it ideal for in situ studies. The intensities measured with XRD can provide quantitative, accurate information on the atomic arrangements at interfaces (e.g., in multilayers). Materials composed of any element can be successfully studied with XRD, but XRD is most sensitive to high-Z elements, since the diffracted intensity from these is much lar r than from low-Z elements. As a consequence, the sensitivity of XRD depends on the material of interest. With lab-based equipment, surface sensitivities down to a thickness of -50 A are achievable, but synchrotron radiation (because of its higher intensity)... 

XRD is an excellenr, nondestructive method for identifying phases and characterizing the structural properties of thin films and multilayers. It is inexpensive and easy to implement. The future will see more use of GIXD and depth dependent measurements, since these provide important information and can be carried out on lab-based equipment (rather than requiring synchrotron radiation). Position sensitive detectors will continue to replace counters and photographic film. 

When samples are heated, they emit infrared radiation with a characteristic spectrum. The IR emission of ceramics, coals, and other complicated solids and thin films can be studied. Also, if conditions make it difficult to use an infrared source... 

Yet another alternative is the thin-film solar cell. This cannot use silicon, because the transmission of solar radiation through silicon is high enough to require relatively thick silicon layers. One current favourite is the Cu(Ga, InjSci thin-film solar cell, with an efficiency up to 17% in small experimental cells. This material has a very high light absorption and the total thickness of the active layer (on a glass substrate) is only 2 pm. 

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