Defect centres

Experimentally, local vibrational modes associated witli a defect or impurity may appear in infra-red absorjrtion or Raman spectra. The defect centre may also give rise to new photoluminescence bands and otlier experimentally observable signature. Some defect-related energy levels may be visible by deep-level transient spectroscopy (DLTS) [23]. 

A primary motivation behind most magnetic resonance experiments is the chemical identification of the resonance centres and on this count the work outlined above has met rally very limited success. However, a number of defect centres have been catalogued and interesting ideas put forth to understand the defect centres in GaN and its alloys. The results for centres in GaN and GaN-based devices are summarised in TABLE 1. We note that (where determined) the principal axis is always the c-axis of the crystal and no deviation from axial symmetry has been observed. 

In alkaline-earth oxides CaO and MgO, F-type defect centres (i.e. anion vacancies with trapped electrons) are responsible for many of the luminescence bands which have been observed [29], The defects produced by elastic collisions act as traps for free electrons [29]. The trapping may be described as the donation of one or more electrons to a Mg + site which is adjacent to the anion vacancy. The donation of one electron into the magnesium 3s AO, which overlaps with the doubly-occupied 2pO AO of an adjacent O2-, enables an (Mg 0) three-... 

Initially the calcium ions were placed on the faces of rings as well as at random locations in the cavity. Once many of the distinct minima had been found for specific aluminium configurations, calciums were placed in these positions to verify that no minima had been missed and optimised for all of the aluminium configurations The calcium ion positions were obtained in Cartesian co-ordinates relative to the centre of region 1, rather than fractional coordinates since ions in a defect model are not replicated periodically. In order to compare with X-ray diffraction data, approximate fractional co-ordinates for the calcium ion were found by assuming that the fractional co-ordinates of the defect centre did not change when the geometry of the defect was relaxed. Positions could then be related to the unit cell of the pure material. 

Site Idealised Mott-Littleton Defect centre ... 

The incorporation on a cation site of an impurity cation of higher charge than the host cation can stabilize a lower valence state of the host cation. The addition of titanium (as Ti02> to the spinel, zinc ferrite, leads to an B-type semi-conductor of this type of formula, Zn (Ti + Fe t28 Fe ) O. The defect centre may be described as an impurity cation of higher relative charge on a cation site plus a quasi-free electron bound on a neighbouring host cation. A necessary condition for the application of the prindple of controlled valency is that the impurity cation should be of much the same size as the host cation. 

In addition to surface redox (external) processes, there is formation of defects and recombination events occurring in the bulk lattice that can also be considered as redox processes, albeit internal processes. For instance, formation of electron colour centres (e.g. Ti " in Ti02, Zr in Zr02, and F -type centres) is formally a reduction of lattice defect sites, whereas hole trapping by a lattice defect centre is formally an oxidative event (see e.g. eq. 5.72). 

For technological purposes, it can be also desirable to obtain a distribution map of a defect centre in a semiconductor wafer. One then chooses an absorption or PL line of this centre and scans its intensity at different points of the wafer. 

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... 

Quantum characterization of single paramagnetic defect centres in connection with their applications in QITs... 

Precise characterization and understanding of quantum properties of single colour centers in diamond is still under progress. A great variety of the colour centers and the complex environment conditions together with the strict limitations posed on the candidates for QIT applications makes the characterization an important and hard problem. The mostly investigated potential candidate is the nitrogen vacancy [N-V]" defect centre in diamond [8]. 

For the alkaline earth oxides. Winter originally suggested that R2 defect centres, i.e., pairs of associated F centres, were of considerable importance.It was proposed that the activation of oxygen by such centres was rapid and non-activated with the reverse reaction (i.e., the elimination of exchanged oxygen) being responsible for the observed kinetics ... 

Crosslinking reduces the polyolefin crystallinity. Crosslinks play the role of defect centres which impede the folding of macromolecules and thus decrease the sizes of the lamellar crystals. Even a small number of crosslinks reduces the crystallinity of low density polyethylene provided that it is performed above the melting tonperature of the polymer crystallites. The linkage of polymer chains impedes their independoit displacement and the degree of crystallinity is distinctly Iowa even when only one macromolecule involves one crosslink [160]. 

The discontinuity in this measurement suggested a change in the defect centre induced by oxygen in the lattice this was subsequently confirmed using UV-induced photo-absorption measurements. This change occurs at oxygen concentrations of about 0.75 atomic% X-ray measurements confirmed a change in the unit cell volume at this point. 

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