Antireflection structures

The use of AR structures in the case of IR detectors is even more important than in the visible spectrum, because materials used for the fabrication of both active regions of the detectors and optical concentrators usually have large real parts of 

Two main groups of antireflection stmctures are single- or multilayer AR layers composed of dielectric films (interference films) [152] and single or multiple periodical diffractive stmcture of either amplitude of phase type. Multiple AR structures may have quarterwave or quarterwave/half-wave periodicity or they may be aperiodic [153]. 

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Stoddart PR, Cadusch PJ, Boyce TM et al (2006) Optical properties of chitin surface-enhanced Raman scattering substrates based on antireflection structures on cicada wings. Nanotechnology 17(3) 680-686... 

Fig. 2.1 Methods of photon management use of optical concentrator, antireflection structure, stmctures for optical path increase (cavity enhancement) and light localization structures...

After arriving to the detector surface, the incident beam should enter the photodetector area with losses as low as possible. Semiconductor materials used for photodetectors have large values of refractive index and thus large values of reflection coefflcient at the device surface. This reflection is minimized using antireflection structures. Basically, these stmctures serve as impedance matching media between the detector environment (most often, but not always free space) and the active region. 

F. 2.23 Appl) ing effective medium approach to diffractive antireflection structures. Here fi denotes the filling factor at different heights, the largest value being 1 and the lowest 0 /i [Pg.80]

Since the strucmres in question are subwavelength and observed as an effective medium with a given filling factor at a given height from the detector surface, it is then irrelevant which profile exactly it follows, as far as the filling factor does not change. In other words, the profiles shown in Fig. 2.24 are identical from the point of view of their properties as antireflective structures. 

Fig. 2.27 Illustration of an artificial antireflection structure based on the moth-eye principle...

The simplest way to use metamateiials as antireflection structures is to make a ID multilayer in which isotropic layers of negative index metamaterial and lossless dielectric are alternating. Such structures were extensively researched [218-222]. Metamaterial-containing multilayers are characterized by many peculiar properties with both theoretical and practical interest. Using such structures it is possible to fabricate resonant cavities with subwavelength dimensions [223]. 

J.M. Dos Santos, L.M. Bernardo, Antireflection structures with use of multilevel subwavelength zero-order gratings. Appl. Opt. 36(34), 8935-8938 (1997)... 

The wafers are processed into solar cells, the majority of which have a diode structure, as sketched in Figure 11.4, characterized by a thin, diffused, doped emitter, screen-printed front and back contacts and a front-surface antireflective coating. Prior to the effective cell manufacturing step, a chemical treatment of the silicon wafers removes... 

The interfaces of importance in SECS are the solid/solid (S/S), solid/gas (S/G), and solid/ liquid (S/L) (4). The area-intensive nature of SECS components was established in the previous section. The major problem is collecting solar energy at a cost that is competitive with other energy forms. Thus, low initial cost is required for the materials, support structures, and production processes in the SECS of interest in Fig. 1 (6). This requires, for example, using thin films in mirrors, in photovoltaic systems, for antireflection coatings on windows, for passive collection, etc. in addition, these films must be made from inexpensive, durable, and easily processed materials (5). Inexpensive long-life materials in flat-plate collectors and durable, stable absorber coatings are also necessary. 

Another structure that has been used to make relatively efficient a-Si H solar cells is shown in Fig. 6b. In this case, the p layer is deposited on a steel substrate, and indium tin oxide (ITO) is electron-beam evaporated onto the flayer. The ITO serves as both a top contact layer and an antireflection coating. The steel substrate may be coated with Cr to improve the back surface reflection. Generally, in all p-i-n cells the top doped layer is thin (— 10 nm) in order to minimize losses due to absorption and recombination in that layer. 

Hamano et al. (1982) have fabricated an a-Si H photodiode array linear image sensor. The sensor structure is shown in Fig. 5. The sensor is constructed by first forming individual electrodes on a glass or a ceramic substrate. Then l-/mi-thick undoped a-Si H is produced at 230°C by glow-discharge decomposition of silane and finally 1500-A-thick ITO common electrode, which also acts as an antireflection coating, is deposited by dc sputtering. 

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