Amorphous semiconductors structure

Oranges, citric acid in, 6 632t ORBIT PRINT SELECT software, 18 243 Orbitrap, 15 662-663 Orb web, structure of, 22 630 Ordered intermetallic alloys, 13 530 Order, in amorphous semiconductor structure, 22 128-129 Ordering, in ternary semiconductor alloy preparation, 22 158-159 Order of addition, in large-scale... 

D. Emin, in P. G. LeComber and J. Mort, eds.. Electronic and Structural Properties of Amorphous Semiconductors Academic Press, Inc., New York, 1973, Chapt. 7. 

Spectroscopic Studies of Gap States and Laser-Induced Structural Transformations in Se-Based As-Free Amorphous Semiconductors... 

N. Mott, Electronic and Structural Properties of Amorphous Semiconductors, Academic Press, New York, 1972. 

The disorder of the atomic structure is the main feature which distinguishes amorphous from crystalline materials. It is of particular significance in semiconductors, because the periodicity of the atomic structure is central to the theory of crystalline semiconductors. Bloch s theorem is a direct consequence of the periodicity and describes the electrons and holes by wavefunctions which are extended in space with quantum states defined by the momentum. The theory of lattice vibrations has a similar basis in the lattice symmetry. The absence of an ordered atomic structure in amorphous semiconductors necessitates a different theoretical approach. The description of these materials is developed instead from the chemical bonding between the atom, with emphasis on the short range bonding interactions rather than the long range order. 

The electronic structure is illustrated in Fig. 1.9. The energy of the mobiUty edge within the band depends on the degree of disorder and is typically 0.1-0.5 eV from the band edge in all amorphous semiconductors. The properties of states near the mobility edge are actually more complicated than in this simple model of an abrupt mobiUty edge and are described in more detail in Chapter 7. Nevertheless, the model of Fig. 1.9 provides a good description of amorphous semiconductors. 

The reduced RDF of a-Si H shown in Fig. 2.11 (Schulke 1981) has sharp structure at small interatomic distances, progressively less well-defined peaks at larger distances, and is featureless beyond about 10 A. This reflects the common property of all covalent amorphous semiconductors, that there is a high degree of short range order at the first and second neighbor distances, but then the spatial correlations decrease rapidly. 

The growth of a multilayer structure creates a new material with an imposed periodicity of the layer spacing. Such structures are familiar in crystalline semiconductors GaAs/GaAlAs multilayers have been particularly widely studied. Both the electronic and vibrational states of the material are inhuenced by the multilayer structure. The quantum conhnement of the electrons and holes in the narrow wells of the multilayer is of special interest because it is not obvious that such effects can occur in amorphous semiconductors. Quantum effects require that the coherence length is larger than the size of the confining well. The short mean free path of the carriers in a-Si H implies that quantmn effects are observed only in very narrow wells. 

This book describes the material properties and physical phenomena of hydrogenated amorphous silicon (a-Si H). It covers the growth of material, the atomic structure, the electronic and optical properties, as well as devices and device applications. Since it focusses on the specific properties of one amorphous material, there is a considerable emphasis on describing and interpreting the experimental information. Familiarity with semiconductor physics is assumed, and the reader is also referred to the excellent books by Mott and Davis, Elliott, and Zallen for further information about the general properties of amorphous semiconductors and glasses.f... 

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