Localization and electronic properties in amorphous semiconductors

The review deals with the electronic properties and recent applications of amorphous silicon (a-Si), which can be regarded as the first member of a new generation of electronically viable thin-film materials. After a brief introduction to the structure and the distribution of electronic states in a-Si the preparation of the material by the decomposition of silane in a radio-frequency glow discharge is discussed. The presence of hydrogen in the deposition process is of crucial importance; saturation of defect states, particularly of dangling bonds in the growing structure, leads to a material with a remarkably low density of gap states. Effective substitutional doping from the gas phase now becomes possible with wide-ranging control of the electronic properties. A brief discussion of the doping mechanism in amorphous solids is followed by a summary of carrier transport mechanisms in a-Si, investigated by fast transient techniques. The possibility of doping in a-Si has removed a major limitation in the a-semiconductor field and has, during the past 10 years, led to an upsurge in applied interest in this electronically controllable thin film material. A summary of the present state of applied developments, many already in industrial production, is given. Two groups are discussed in some detail. The first, the photovoltaic development, is based on the a-Si p–i–n junction, and forms part of a wide range of consumer products, but larger area photovoltaic panels are now in production. In the second major development a-Si field effect transistors are used as the addressable elements in large area liquid crystal displays. Remarkable progress has been made with thin film colour displays for small portable television sets. The use of a-Si elements in addressable linear image sensing arrays for telefax applications, coupled with a-Si high-voltage transistor arrays in the associated printers, represents an important step towards an integrated a-Si technology in large-area applications.

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Amorphous Semiconductors Studied by First-principles Simulations: Structure and Electronic Properties

MRS Proceedings ◽

10.1557/proc-1153-a04-03 ◽

2009 ◽

Vol 1153 ◽

Cited By ~ 1

Author(s):

Karol Jarolimek ◽

Robert A. de Groot ◽

Gilles A. de Wijs ◽

Miro Zeman

Keyword(s):

Band Gap ◽

Electronic Properties ◽

First Principles ◽

Density Functional ◽

Large Cell ◽

Band Gap Energy ◽

Amorphous Semiconductors ◽

Structural Defects ◽

Defect States ◽

Wide Range

AbstractAtomistic models of amorphous solids enable us to study properties that are difficult to address with experimental methods. We present a study of two amorphous semiconductors with a great technological importance, namely a- Si:H and a-SiN:H. We use first-principles density functional theory (DFT), i.e. the interatomic forces are derived from basic quantum mechanics, as only that provides accurate interactions between the atoms for a wide range of chemical environments (e.g. brought about by composition changes). This type of precision is necessary for obtaining the correct short range order. Our amorphous samples are prepared by a cooling from liquid approach. As DFT calculations are very demanding, typically only short simulations can be carried out. Therefore most studies suffer from a substantial amount of defects, making them less useful for modeling purposes. We varied the cooling rate during the thermalization process and found it has a considerable impact on the quality of the resulting structure. A rate of 0.02 K/fs proves to be sufficient to prepare realistic samples with low defect concentrations. To our knowledge these are the first calculations that are entirely based on first-principles and at the same time are able to produce defect-free samples. Because of the high computational load also the size of the systems has to remain modest. The samples of a-Si:H and a-SiN:H contain 72 and 110 atoms, respectively. To examine the convergence with cells size, we utilize a large cell of a-Si:H with a total of 243 atoms. As we obtain essentially the same structure as with the smaller sample, we conclude that the use of smaller cells is justified. Although creating structures without any defects is important, on the other hand a small number of defects can give valuable information about the structure and electronic properties of defects in a-Si:H and a-SiN:H. In our samples we observe the presence of both the dangling bond (undercoordinated atom) and the floating bond (over-coordinated atom). We relate structural defects to electronic defect states within the band gap. In a-SiN:H the silicon-silicon bonds induce states at the valence and conduction band edges, thus decreasing the band gap energy. This finding is in agreement with measurements of the optical band gap, where increasing the nitrogen content increases the band gap.

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