Defect level spectroscopy - optical transition energies

4 Defect level spectroscopy - optical transition energies 

The optical absorption arising from the defect transitions is weak because of the low defect densities and in a thin film cannot be measured by optical transmission. The techniques of PDS, CPM and photoemission yield, described in Section 3.3, have sufficient sensitivity. Photocapacitance, which measures the light-induced change in the depletion layer capacitance, is similarly sensitive to weak absorption (Johnson and Biegelsen 1985). PDS measures the heat absorbed in the sample and detects all of the possible optical transitions. At room temperature virtually all the recombination is non-radiative and generates heat by phonon emission. CPM detects photocarriers and so is primarily sensitive to the optical transitions which excite electrons to 

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 shape of the defect absorption is given by the joint density of states and the matrix elements, and the position and width of the defect band can only be deduced by the appropriate deconvolution. The usual approach is to model the defect band, for example, by a gaussian, and to calculate the absorption from the known shape of the conduction band. 

2W enters because of the different energies for optical and thermal transitions due to lattice relaxation. is found to be 0.83 eV from 

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