Defects transition energies

An applied stress lowers the symmetry of the crystal and can make defects with different orientations inequivalent. A review of stress techniques has been written by Davies (1988). The degeneracy of the ground state and also of the spectroscopic transition energies can be lifted. In this section we suppose that the defects cannot reorient and consider only the splitting of the transition energies. The stress-induced reorientation of defects is discussed in the next section. 

The most fundamental transition that can take place is the transfer of an electron from the valence band to the conduction band. This creates a mobile electron and a mobile hole, both of which can often be treated as defects. Transitions of this type, and the reverse, when an electron in the conduction band drops to the valence band, eliminating a hole in the process and liberating energy, are called interband transitions. Apart from the electrons and holes themselves, interband transitions do not involve defects. All other transitions do. 

In measurements of the defect energies, it is essential to distinguish between the thermal emission and optical transition energies, to account properly for lattice relaxation effects (see Section... 

Defect level spectroscopy - optical transition energies... 

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... 

Indeed, for the activation energy of to be zero, the lattice relaxation energy should be equal to the trap depth, which is about 0.7-0.9 eV. Such a large lattice relaxation should give the defect transition a large Stokes shift, but none is observed. The optical and thermal transition energies have been measured for n-type a Si H and are shown in Fig. 

A fully microscopic interpretation of the temperature dependence of the absorption maximum, even well above any order-disorder transition temperature, is a formidable task because of the potential importance of many complicated physical factors (27-30). As a first attack on this problem, we have adopted a simple mean-field (or effective-medium) approach (28-30) with the assumption that the absorption peak (to) is linearly perturbed from its limiting dl -trans value ((Orod) by the presence of bond rotational defects (free energy of formation, e)... 

Fig. 23 Summary of band edge defect energies derived from spectroscopic data (a) energies of occupied and unoccupied d-state features in SXPS, and VUV SE and XAS, respectively, (b) occupied d-state features, and empty states determined by renormalizing VUV SE data for d-d transition energies...

The computed transition energies for the trapped electron, about 2eV, are close to values reported for optical excitations at the surface of polycrystalline MgO where Fg(H)+ defect centers have been created according to reactions (2.6) and (2.7). 

The proposed model assumes that the electronic states of these molecules are delocalized along an undefected portion of the polymer chain and that chain defects effectively isolate the absorbing segments. The transition energies for each segment are given by the expression... 

The ensemble of defect sites can be regarded as a solid with discrete disorder and this ensemble indeed exhibits many of the characteristics observed for dye molecules in truly disordered solids such as a glass [204]. The spread of transition energies of the defect sites over about 200 cm-1 is... 

The first difficulty is more fundamental, and suggests that, in complex cases, more information is needed to determine quantum defect parameters completely than is available in a set of experimental transition energies. 

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