Defects optical transition energies

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 direct proof that H is present in certain centers in Ge came from the substitution of D for H, resulting in an isotopic energy shift in the optical transition lines. The main technique for unraveling the nature of these defects, which are so few in number, is high-resolution photothermal ionization spectroscopy, where IR photons from an FTIR spectrometer excite carriers from the ls-like ground state to bound excited states. Phonons are used to complete the transitions from the excited states to the nearest band edge. The transitions are then detected as a photocurrent. 

The situation is utterly different for semiconductors or insulators with a wide enough band gap 6.8,27,28). if the band gap exceeds the energy range of the fluorescence spectrum of the dye, energy transfer is either prevented or at least reduced to a very small rate which is controlled by the number of electronic energy levels within the band gap of the semiconductor or insulator. Such states are usually localized and can be caused by impurities or structural defects and represent optical transitions with a low oscillator strength. With a suitable position of the... 

Any defect with two states in the gap has four possible optical transitions which are denoted A-D in Fig. 4.3. The actual contribution to an optical absorption experiment depends largely on the position of the Fermi energy. Only transitions A and D in Fig. 4.3 contribute to the absorption when lies between the two levels and the defects are singly occupied, but when the defect states are either doubly occupied or empty, the only possible transitions are B and C respectively. (The... 

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. 

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). 

First-principles calculations including electron-electron interactions beyond the mean field theory have shown the excitonic character of optical transitions in SWNTs with large exciton binding energies (of up to 1 eV for the (8,0) SWNT) (Spataru et al. 2004). These predictions have been corroborated by experiments (Shaver et al. 2007 Wang et al. 2005), Unfortunately, these calculations are too demanding for routine calculations in large diameter tubes, and unlikely to be practical to study the effect of defects and functionalization. 

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