Physical Review B 75,12 (2007) 125204;
Two types of localized vibrational modes of oxygen substituting for Te in CdTe, i.e., O-Te, are reported. In one, O-Te is associated with a nearest neighbor (NN) vacancy as a (O-Te-V-Cd) center and hence with C-3v symmetry, with its uniaxial axis along < 111 >, whereas in the other, O-Te is surrounded by all the four NN Cd's and thus possesses T-d site symmetry. By an appropriate control of stoichiometry it is possible to reproducibly generate the formation of either (O-Te-V-Cd) or O-Te centers. These configurations are deduced from their ultrahigh resolution infrared signatures. For the (O-Te-V-Cd) centers, consistent with their uniaxial symmetry, a pair of sharp local vibrational modes (LVM) are observed at nu(1)=1096.78 cm(-1) and nu(2)=1108.35 cm(-1), the latter nearly twice as intense as the former. In the LVM spectrum of O-Te centers with the full complement of NN Cd's, consistent with its T-d symmetry, only one LVM signature appears at nu(0)=349.79 cm(-1). With the increasing temperature, nu(1) and nu(2) approach each other and coalesce into a single triply degenerate line at nu(*)(0) for temperature T >= T-*similar to 300 K; the uniaxial (C-3v) symmetry of (O-Te-V-Cd) transforms to T-d symmetry at T-* and above, acquired by the (O-Te-V-Cd) centers due to the increasing rate of bond switching among the four possible O-Te-V-Cd < 111 > directions as T approaches T-*. The (O-Te-V-Cd) centers also display a fascinating pair of second harmonics including a coalescence at T-* and beyond.
Physics, Condensed Matter
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