Defect energy levels

Therefore, there could exist rich defects in Ba3BP30i2, BaBPOs and Ba3BP07 powders. From the point of energy-band theory, these defects will create defect energy levels in the band gap. It can be suggested that the electrons and holes introduced by X-ray excitation in the host might be mobile and lead to transitions within the conduction band, acceptor levels, donor levels and valence band. Consequently, some X-ray-excited luminescence bands may come into being. 

Processes involving defect energy levels are responsible for coloration of diamonds containing races of nitrogen or boron impurities. Diamond has a band gap of about 8.65 x 10-19 J (5.4 eV), which is too large to absorb visible light and... 

The defect energy levels are also obtained from optical emission transitions. Measurements of luminescence in a-Si H are described in more detail in Chapter 8. Transitions to defects are observed as weak luminescence bands. The transition energies are about 0.8 eV and... 

Perhaps the best way of summarizing the experimental information about the defect energy levels is to attempt to answer the following questions. 

Taking the different arguments together, it is the author s opinion that the dangling bond model remains the more plausible explanation of the 2.0055 defect. Perhaps within a short time, further studies of the hyperfine interaction or calculations of the defect energy levels, etc. will be able to provide definitive proof one way or the other. In the remainder of this book, for the sake of definiteness, we refer to the 2.0055 ESR spin and the associated deep trap as the dangling bond, recognizing that the interpretation of electrical data involves only the gap state levels and the electron occupancy, not the atomic structure. 

The random network of an amorphous material such as a-Si H implies that the formation energy varies from site to site. A correct evaluation of the equilibrium state must include this distribution and also the width of the defect energy levels (Smith and Wagner 1987). It is instructive, however, first to solve the simpler problem in which the distributions are replaced by single formation energies and discrete gap states (Muller, Kalbitzer, and Mannsperger 1986, Street et al. 1988a). 

Figure 5. The diagram of defect energy levels determinedfor YSZ and ScSZ [8],...

The Green-function method appeared to be very useful for displaying the chemical trends in defect energy levels [727,728]. However, the calculation of other defective-crystal properties (defect-formation energy, lattice relaxation, local-states localization) requires approaches based on molecular cluster or supercell models. Only recently have these models been used in the first-principles calculations to study point defects in SrTiOs. 

Table 4.2 Calculated nearest-neighbor bond lengths, the defect energy levels (E,) relative to the valence band maximum for negatively charged substitutional impurities, and the energy (AE) required to form the positively charged AX center from the substitutional acceptors.

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