Amorphous semiconductor alloys

From a consideration of the structure and bonding of amorphous materials, one can imagine how difficulties can arise. Consider the local bonding variations in crystalline 

Amorphous Si/a-Sii xGex H alloy heterojunctions have band edge offsets as do crystalline materials. Roughly 80% of the change in energy gap is accommodated in the conduction band for this alloy.[17] 

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Little band-gap bowing occurs in amorphous semiconductor alloys and most band offset in Si-Ge alloys occurs in the antibonding (conduction) band. 

There is another class of amorphous semiconductors based on chalcogens which predate the developments that have occurred in i -Si. Because their use has been limited, eg, to switching types of devices and optical memories, this discussion is restricted to the optoelectronic properties of i -Si-based alloys and their role in some appHcations. 

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. 

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... 

The charge transport in amorphous selenium (a-Se) and Se-based alloys has been the subject of much interest and research inasmuch as it produces charge-carrier drift mobility and the trapping time (or lifetime) usually termed as the range of the carriers, which determine the xerographic performance of a photoreceptor. The nature of charge transport in a-Se alloys has been extensively studied by the TOF transient photoconductivity technique (see, for example. Refs. [1-5] and references cited). This technique currently attracts considerable scientific interest when researchers try to perform such experiments on high-resistivity solids, particularly on commercially important amorphous semiconductors such as a-Si and on a variety of other materials... 

Both crystalline [168] and amorphous [169] alloys are considered as precursors in the preparation of HTSC films. Atomic-level uniformity of the component distribution in metallurgical alloys can be achieved. One more type of metal precursor, the oxidation of which gives good results under relatively mild conditions, are multilayer polymetallic coatings with nanometer-thick layers [170], Similar compositions are also the most frequently used type of precursors in the technology of semiconductors [171]. 

Another group of amorphous semiconductors are the A compounds and alloys. a-SiO, which is a much investigated material with large technical uses, a-SnTe, a-PbTe and the whole series of a-Ge Te. and a-Ge Se. alloys can be obtained by vacuum evaporation or sputtering. Bulk glassy Ge Tej j and Gej Sei j alloys can be obtained by splat-cooling the melt or even slower quenching if relatively small quantities of P, As, S, Si, or I are added to the melt. This is most easily achieved around the compositions close to an eutecticum (Hilton, Jones, and Brau, 1966 Feltz et al, 1971), e.g. for X — 0.15 in the Ge-Te system. 

The most important elemental constituents of amorphous semiconductors are Si and Ge in Group IV P, As, Sb and Bi in Group V and the chalcogenides (S, Se, and Te) in Group VI. Elements, compounds, or multi-component alloys varying widely in composition can be prepared in amorphous form variously by cooling a melt or condensing a vapor. 

In the next section of this chapter the amorphous semiconductors are classified according to their chemical bonds and composition. Chemical bonding arguments are used to explain the origin of the valence and conduction bands and the effects of alloying additives in producing donor and acceptor states in the gap. The different models which have been proposed for describing the electronic states and their conduction properties in amorphous semiconductors are then presented. 

The few surface studies reported suggest the presence of a considerable density of surface and gap states in the amorphous semiconductors studied. Kastner and Fritzsche (1970) found that one monolayer of H2O adsorbed on a 1000 A thick film of chalcogenide alloy increases its conductance by less than one percent. Amorphous Ge evaporated at room temperature is porous to H2 0 so that a large area of internal surfaces can be covered with water. A density of 5 X 10 H2 0/cm absorbed in a 1000 A thick Ge film produced an increase in conductance by only 10 percent. These small changes contrast strongly with the behavior of crystalline semiconductors. They suggest a large density of surface and gap states. 

The microscopic mechanisms for the MNM transition described in the previous section are quite general. They can be related to a wide variety of physical systems. These include not only expanded electronically conducting fluids, but liquid solutions such as the molten metal-salt solutions, metal-ammonia solutions, semiconducting liquid alloys, etc. The mechanisms are also relevant to the MNM transitions in various solids, including amorphous semiconductors, heavily doped crystalline semiconductors, and metal oxides. Our concern is with fluids and so we turn now to summarize briefly some of the theoretical investigations specifically focused on the MNM transition and its relation to the phase transition behavior of fluid metals. 

We must not lose sight of the fact that nature, as presently understood, often poses the same question with an unknown, or only partially specified structure. For example, an impurity in a semiconductor may involve a large local distortion rather than an ideal substitution of the foreign atom. In the absence of an exact specification of this distortion, it must be determined as part of the solution to the problem. A similar situation is confronted in the study of amorphous semiconductors, and indeed the whole field of alloys, so we are not speaking of some abstruse or unusual systems but rather a wide range of solids. 

All of the examples of the preceding sections involved perfectly periodic solids. While a great deal remains to be done in the application of the existing methods to more complicated (periodic) solids, it would seem that there are no very serious obstacles to progress in that direction, given accurate structure determinations. The same can hardly be said for disordered solids, be they alloys, amorphous semiconductors, or whatever. The formulation of a theory of electronic structure without Bloch s theorem has proved to be a difficult task. 

The choices of compounds from which to construct semiconductor alloys are limited. For example, the common elemental semiconductors are Si, and Ge. Other more rare examples include diamond, cubic Sn, and amorphous Se. The simple binary alloys therefore consist only of mixtures of these elements (excluding Se, which is not isovalent with the others) either in crystalline or amorphous form. All except Si-Ge alloys are extremely limited in usable compositions. Amorphous semiconductors have their own chapter (Chapter 8) and so will be ignored here. Carbon forms SiC rather than a Si-C alloy when mixed with Si. Sn has little or no solid solubihty with the other materials. By contrast. Si and Ge are completely miscible. Si-Ge alloys are of sufficient importance that they are discussed in detail in Section 6.4. Until then, we will leave the elemental alloys in favor of the compound semiconductor alloys. These provide much more flexibility in the resulting properties but are also much more complex and difficult to work with. 

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