Resonant Raman, photoluminescence, and modulation spectroscopy of elemental and II-VI semiconductors

Stanislav Tsoi, Purdue University


Resonant enhancement enables the discovery and delineation of spin flip Raman scattering (SFRS) from free or donor-bound electrons in diluted magnetic semiconductors (DMSs) containing 3d transition metal ions (TMIs) at doping concentrations. In such studies, the intrinsic g-factor of the host, CdTe in the present case, has to be accounted for accurately. The SFRS in CdTe yields the conduction band electron g-factor of -1.676 ± 0.007 and displays two resonance peaks mediated by free and donor-bound excitons, respectively. Excitonic signature in modulated reflectivity signals the successful formation of Cd1-xVxTe as an alloy, whereas magnetization measurements show vanadium ions incorporated as V2+ with x ∼ 4 × 10-4 . SFRS of Cd1-xV xTe displays vanadium-related SFRS shifts of a sign opposite to that of the host. This indicates a ferromagnetic s-d exchange interaction between the s-like conduction electrons and the 3d-shell of V2+ in Cd1-xVxTe; from the linear dependence of the s-d exchange energy as a function of magnetization, α N0 the s-d exchange constant, is deduced to be (285 ± 8) meV. The Van Vleck paramagnetism of Cd1-xFe xTe, a diluted magnetic semiconductor, is explored with electronic Raman spectroscopy of an internal transition of Fe2+, on the one hand, and the spin flip Raman scattering (SFRS) from donor-bound electrons, on the other. Zeeman splitting of the Raman transition from the nonmagnetic ground state to the first excited state displays patterns consistent with energy levels responsible for the Van Vleck paramagnetism. SFRS, in turn, delineates characteristic features of the Van Vleck magnetization, as expected from s-d exchange interaction. The combination of SFRS and magnetization measurements yielded the s-d exchange constant in Cd1-xFe xTe, αN0 = (244 ± 10) meV. Photoluminescence and wavelength-modulated transmission spectra displaying phonon-assisted indirect excitonic transitions in isotopically enriched 28Si, 29Si, 30Si, as well as in natural Si, have yielded the isotopic mass (M) dependence of the indirect excitonic gap (Egx) and the relevant phonon frequencies. Interpreting these measurements on the basis of a phenomenological theory for (∂Egx/∂M), we deduce Egx(M = ∞) = (1213.8 ± 1.2) meV, the purely electronic value in the absence of electron-phonon interaction and volume changes associated with anharmonicity. Isotopic mass dependence of the [special characters omitted] and E1 direct excitonic gaps is investigated by photo-modulated reflectivity, yielding corresponding values of zero-point renormalizations (64 ± 4) meV and (129 ± 10) meV. The isotopic mass dependence of the A, B, and C excitonic band gaps of ZnO is investigated at liquid helium temperature exploiting wavelength-modulated reflectivity. The observed dependence is analyzed in terms of the zero-point renormalization of the band gap by electron-phonon interaction and the dependence of volume on isotopic mass. The analysis yields zero-point renormalizations by electron-phonon interaction 152 ± 20 meV (A), 146 ± 26 meV (B), and 158 ± 21 meV (C). The temperature dependence of the band gaps is studied in specimens having natural isotopic abundance with electro-, photo-, and wavelength-modulation reflectivity in the range 10-402 K. Zero-point renormalizations of the band gaps by electron-phonon interaction 184.6 meV (A), 163.6 meV (B), and 179.6 meV (C) are deduced from fits to the temperature data generated by the two-oscillator model.




Ramdas, Purdue University.

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