Amorphous semiconductor materials

Adachi Sadao, Optical Properties of Crystalline Solids and Amorphous Semiconductors. Materials and Fundamental Principles, Kluver Academic Publishers, Boston, 1999 Amirtharaj P.M., Dhart N.K, Baars J. and Seelewind H., SemicotuL Sci. Technol. 5, S68 (1990) Baars J. and Sorgers F., Solid State Commun.lO, 875 (1972)... 

S. Adachi, Optical Properties of Crystalline and Amorphous Semiconductors Materials and Fundamental Principles (Kluwer Academic, Boston, 1999). 

Figure 24.9 (a) Sketch summarizing the extended and localized states in amorphous semiconductor materials, and the possible transitions between them (b) Sketch showing... 

StiU another method used to produce PV cells is provided by thin-fiLm technologies. Thin films ate made by depositing semiconductor materials on a sohd substrate such as glass or metal sheet. Among the wide variety of thin-fiLm materials under development ate amorphous siUcon, polycrystaUine sUicon, copper indium diselenide, and cadmium teUuride. Additionally, development of multijunction thin-film PV cells is being explored. These cells use multiple layers of thin-film sUicon alloys or other semiconductors tailored to respond to specific portions of the light spectmm. 

Another parameter of relevance to some device appHcations is the absorption characteristics of the films. Because the k quantum is no longer vaUd for amorphous semiconductors, i -Si H exhibits a direct band gap (- 1.70 eV) in contrast to the indirect band gap nature in crystalline Si. Therefore, i -Si H possesses a high absorption coefficient such that to fully absorb the visible portion of the sun s spectmm only 1 p.m is required in comparison with >100 fim for crystalline Si Further improvements in the material are expected to result from a better understanding of the relationship between the processing conditions and the specific chemical reactions taking place in the plasma and at the surfaces which promote film growth. 

The way in which a semiconductor material is made has significant implications for NMR spectroscopy in several ways it governs the amount of sample available for analysis, it determines whether the material will be single-crystal or polycrystalline (or even amorphous), and it controls the nature and amounts of dopants, intentional or otherwise, and defects. In categories most relevant to the NMR spectroscopist, the ways in which most semiconductors are made can be classified as follows ... 

A turning point in the study of amorphous semiconductors was reached with the discovery that the addition of hydrogen to amorphous silicon could dramatically improve the material s optical and electrical properties. Unlike pure amorphous silicon, which is not photoconductive and cannot be readily doped, hydrogenated amorphous silicon (a-Si H) displays a photoconductive gain of over six orders of magnitude and its dark conductivity can be changed by over ten orders of magnitude by n-type or p-type... 

Does the above model for hydrogen motion apply to other amorphous semiconductors, such as a-Si H, a-Ge H, a-SixGe x-. H, a-SixCi-x Experimental determinations of whether the conclusions for a-Si H apply to these other amorphous alloys would greatly advance our understanding of these materials and would likely improve their technological usefulness. 

Hosono, H. 2006. Ionic amorphous oxide semiconductors Material design, carrier transport, and device application. J. Non-Cryst. Solids 352 851-858. 

Apart from the endeavor to optimize the stmcture property relations of materials used in modem optoelectronic devices there is the desire to understand the conceptual premises of charge transport in random organic solids. The use of amorphous, instead of crystalline, organic semiconductor materials is favored... 

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