Vibrational Mode Spectroscopy and Uniaxial Stress Techniques

Local Vibrational Mode Spectroscopy and Uniaxial Stress Techniques 

An LVM is a vibration of a light impurity atom that does not propagate in the lattice. The atom motions are confined primarily to the impurity itself and its nearest neighbors, with rapidly decaying vibrational amplitude for more distant host atoms. Usually, the lighter the impurity, the higher the frequency of the vibration and the more localized the mode. 

While vibrational spectra do not always provide an unambiguous identification of the chemical species or the atomic arrangement of a defect, often sufficient clues are provided that a reasonable guess can be made. When combined with additional perturbations, such as uniaxial stress (Davies, 1988), or when used in conjunction with other techniques or theory the insights gained can be invaluable. 

The effect of isotopic or chemical substitutions on the vibrational frequency leads to the identification of the species that comprise a defect complex. For example, the large frequency shift that results upon the substitution of D for H leads to an unambiguous identification of the H motions. For a series of chemically similar complexes, such as the acceptor-H complexes, the frequency shifts that occur for group III substitutions show that the acceptor is indeed involved in the complex. 

In H-containing complexes another clue to the identity of the atom to which the H is attached comes from the ratio, r = o)(H)/(o(D), of the vibrational frequencies for the isotopic substitution, D for H. As an approximation, the reduced mass of an H vibration is taken to be mA (H) = (mH + mA), where mH is the FI mass and mA is the mass of the atom to which the H is attached. A similar expression can be written for the D vibration. In this approximation r is given by 

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II. Local Vibrational Mode Spectroscopy and Uniaxial Stress Techniques... 

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