INELASTIC LIGHT SCATTERING IN DILUTED MAGNETIC SEMICONDUCTORS: RAMAN SPECTRA OF CADMIUM(1-X)MANGANESE(X)TELLURIDE
Abstract
The vibrational Raman spectrum of the diluted magnetic semiconductor Cd(,1-x)Mn(,x)Te was investigated for the composition range 0.01 (LESSTHEQ) x (LESSTHEQ) 0.70. The spectra exhibit two pairs of sharp lines characteristic of the zone center "CdTe-like" and MnTe-like" LO-TO modes. These modes follow the so called "two-mode" behavior. The appearance of new quasi-continuous features in the vibrational Raman spectrum can be traced to first order processes which become allowed as a result of the absence of strict translational symmetry and reflect the one-phonon density of states. The behavior of the LO and TO phonons was studied close to band-gap resonance. Overtones, combination as well as difference modes of the LO modes, were observed close to resonance. Forbidden LO scattering was also observed and attributed to the presence of impurities in the crystals. A sharp Raman line due to the spin-flip of the 3d electrons of Mn('++), corresponding to the (DELTA)m(,s) = (+OR-)1 transition, occurs in the paramagnetic phase. Close to band-gap resonance, the (DELTA)m(,s) = (+OR-)1 transition in combination with the zone center LO phonons as well as the (DELTA)m(,s) = (+OR-)2 transition are present. In addition to the internal transitions of Mn('++), this resonance suggests a new Raman mechanism involving interband transitions in conjunction with the exchange interaction between band electrons and Mn('++). In the low temperature, magnetically ordered phases a new Raman feature, having antisymmetric polarization characteristics was observed for 0.40 (LESSTHEQ) x (LESSTHEQ) 0.70. This new feature was attributed to inelastic scattering involving the emission or absorption of one magnon. The polarization characteristics and the temperature dependence of its frequency are consistent with this assignment. The magnon splits into two components in the presence of an external magnetic field. As the temperature is lowered and the crystal becomes magnetically ordered, the Mn('++) spin-flip line of the paramagnetic phase evolves into the higher energy component of the magnon. Spin-flip Raman scattering was observed in Cd(,1-x)Mn(,x)Te in the paramagnetic phase for a wide range of compositions (0.01 (LESSTHEQ) x (LESSTHEQ) 0.18). The polarization characteristics establish that the line is associated with the spin-flip transition of an electron. The large effective g-factor as well as the temperature and magnetic field dependence of the Raman shift of this line can be traced to the especially important exchange interaction between Mn('++) and band electrons.
Degree
Ph.D.
Subject Area
Condensation
Off-Campus Purdue Users:
To access this dissertation, please log in to our
proxy server.