Semiconductors impurity and defect

Common teniiinology used to characterize impurities and defects in semiconductors includes point and line defects, complexes, precipitates and extended defects. These teniis are somewhat loosely defined, and examples follow. 

However, most impurities and defects are Jalm-Teller unstable at high-symmetry sites or/and react covalently with the host crystal much more strongly than interstitial copper. The latter is obviously the case for substitutional impurities, but also for interstitials such as O (which sits at a relaxed, puckered bond-centred site in Si), H (which bridges a host atom-host atom bond in many semiconductors) or the self-interstitial (which often fonns more exotic stmctures such as the split-(l lO) configuration). Such point defects migrate by breaking and re-fonning bonds with their host, and phonons play an important role in such processes. 

Both anatase and mtile are broad band gap semiconductors iu which a fiUed valence band, derived from the O 2p orbitals, is separated from an empty conduction band, derived from the Ti >d orbitals, by a band gap of ca 3 eV. Consequendy the electrical conductivity depends critically on the presence of impurities and defects such as oxygen vacancies (7). For very pure thin films, prepared by vacuum evaporation of titanium metal and then oxidation, conductivities of 10 S/cm have been reported. For both siugle-crystal and ceramic samples, the electrical conductivity depends on both the state of reduction of the and on dopant levels. At 300 K, a maximum conductivity of 1 S/cm has been reported at an oxygen deficiency of... 

Many inorganic solids lend themselves to study by PL, to probe their intrinsic properties and to look at impurities and defects. Such materials include alkali-halides, semiconductors, crystalline ceramics, and glasses. In opaque materials PL is particularly surface sensitive, being restricted by the optical penetration depth and carrier diffusion length to a region of 0.05 to several pm beneath the surface. 

E J. Dean. Prog. Crystal Growth Charact. 5i 89, 1982. A review of PL as a diagnosdc probe of impurities and defects in semiconductors by an important progenitor of the technique. 

A. Mandelis, A. Budiman and M. Vargas, Photothermal Deep-Level Transient Spectroscopy of Impurities and Defects in Semiconductors... 

Reflection, emission and absorption in solids has long been studied. Intense photoluminescence is observed in many semiconductors at low temperatures. When spectrally analyzed, this photoluminescence provides an extensive source of experimental data which contributes to the ultimate identification of the electronic states of Impurities and defects in these semiconductors. Many sharp lines appear in such spectra, particularly from bound excitons, which provide a "finger print" of the impurities and defects which are present in the semiconductor lattice. 

In magnetic fields, bound excitons have unique Zeeman spectral characteristics, from which it Is possible to identify the types of centers to which the free excitons are bound. Bound exciton spectroscopy Is a very powerful analytical tool for the study and Identification of Impurities and defects in semiconductor materials. Magneto-Optical Spectroscopy Techniques... 

J. Chevallier, B. Pajot, Interaction of Hydrogen with Impurities and Defects in Semiconductors. Solid State Phenomena 85-86 203-284. (Scitec Publications, Switzerland, 2002)... 

The FEs produced at low temperature by illumination with photons in the vicinity or above Eg have finite lifetimes that depend on temperature (see [34] for silicon), their binding energies, and on the band structure of the semiconductor (the lifetime is larger in semiconductors with indirect gap than direct gap). During their lifetime, they can diffuse in the crystal and be trapped by impurities and defect to become bound excitons (BEs) with energies slightly different from that of the FE. 

Chapter 1 of the present volume provides the basic concepts related to the properties and characterization of the centres known as shallow dopants, the paradigm of the H-like centres. This is followed by a short history of semiconductors, which is intimately connected with these centres, and by a section outlining their electrical and spectroscopic activities. Because of the diversity in the notations, I have included in this chapter a short section on the different notations used to denote the centres and their optical transitions. An overview of the origin of the presence of H-related centres in crystals and guidelines on their structural properties is given in Chap. 2. To define the conditions under which the spectroscopic properties of impurities can be studied, Chap. 3 presents a summary of the bulk optical properties of semiconductors crystals. Chapter 4 describes the spectroscopic techniques and methods used to study the optical absorption of impurity and defect centres and the methods used to produce controlled perturbations of this absorption, which provide information on the structure of the impurity centres, and eventually on some properties of the host crystal. Chapter 5 is a presentation of the effective-mass theory of impurity centres, which is the basis for a quantitative interpretation... 

We normally define the energy level of electrons in a solid in terms of the Fermi level, eF, which is essentially equivalent to the electrochemical potential of electrons in the solid. In the case of metals, the Fermi level is equal to the highest occupied level of electrons in the partially filled frontier band. In the case of semiconductors of covalent and ionic solids, by contrast, the Fermi level is situated within the band gap where no electron levels are available except for localized ones. A semiconductor is either n-type or p-type, depending on its impurities and lattice defects. For n-type semiconductors, the Fermi level is located close to the conduction band edge, while it is located close to the valence band edge for p-type semiconductors. For examples, a zinc oxide containing indium as donor impurities is an n-type semiconductor, and a nickel oxide containing nickel ion vacancies, which accept electrons, makes a p-type semiconductor. In semiconductors, impurities and lattice defects that donate electrons introduce freely mobile electrons in the conduction band, and those that accept electrons leave mobile holes (electron vacancies) in the valence band. Both the conduction band electrons and the valence band holes contribute to electronic conduction in semiconductors. 

The motion of the carriers in a semiconductor is also affected by the presence of impurities and defects of the crystal. A small amount of impurities is always present, although impurities are usually introduced deliberately to make... 

The energy gap depends on temperature, as shown in Fig. 7.8, and on the number of impurities and defects of the crystal. With increasing temperatures, if Eg is small as in germanium, the electrical conduction is dominated by electron-hole pairs created by thermal excitation and not by the presence of the impurity atoms. Therefore, at high enough temperatures, any semiconductor can be considered as intrinsic. 

The limited number of pages of this Handbook has forced us, furthermore, to be even more restrictive with regard to the properties of semiconductors, which are considered as being of first-order importance for this data collection. So, semiconductor chemistry is beyond the scope of this Handbook. The very broad and inportant field of semiconductor technology could not be included at all. Also, the wide field of the influence of chemical doping, impurities, and defects on the properties of semiconductors had to be left out. The Handbook s emphasis is on the physical properties of a restricted number of most important pure semiconducting materials. 

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