I. TIME REVERSAL SYMMETRY AND LINEAR WAVE-VECTOR DEPENDENCE OF OPTICAL PHONONS: RAMAN SPECTRUM OF BISMUTH GERMANIUM-OXIDE. II. RESONANCE RAMAN EFFECT AND PHOTOLUMINESCENCE OF CINNABAR

WAYNE ISAMI IMAINO, Purdue University

Abstract

Triply degenerate F modes in the Raman spectrum of Bi(,12)GeO(,20), which has the cubic point group symmetry T, are studied as a function of the magnitude of the phonon wave-vector q. The small splittings, due to the long range polarization field (LO-TO splitting) and linear q-effects near the zone center are resolved using the high resolution of a piezoelectrically scanned Fabry-Perot interferometer, with samples cooled to liquid helium temperatures. Uniaxial stress is used as an external perturbation to characterize these effects further. Combining group theoretical arguments with time reversal symmetry, expressions are derived for the frequencies and the polarizability tensors of the F modes including the effects of linear q dependence, LO-TO splitting, and uniaxial stress. The experimental results are in excellent agreement with theory. In (alpha)-HgS, cinnabar, grown by chemical vapor transport, (CVT), a sharp photoluminescence feature is found at an energy E(X(,1)) = 1.873 eV; absorption measurements show that this peak is due to resonance fluorescence associated with an electronic transition. When the Raman spectrum of such samples is excited with laser energy, (H/2PI)(omega)(,L), in the range 1.865 to 1.885 eV provided by a dye laser, new Raman lines appear at 21 cm('-1), 33.5 cm('-1), 67 cm('-1) and 101 cm('-1); these lines exhibit a pronounced resonance in the spectral dependence of their scattering intensity. The 33.5 cm('-1) and the 67 cm('-1) lines, for which we have a complete spectral dependence, show a resonance peak at (H/2PI)(omega)(,L) = E(X(,1)) as well as for (H/2PI)(omega)(,L) = E(X(,1)) + the phonon energy. The shape of their resonance curves agrees well with that predicted by the theory of resonance Raman effect. These resonance curves also exhibit a peak at (H/2PI)(omega)(,L) = E(Z(,1)) + the phonon energy, where E(Z(,1)) = 1.870 eV. The 33.5 cm('-1), 67 cm('-1), and 101 cm('-1) Raman lines are ascribed to the first, second, and third harmonics of a Raman inactive A(,2)(TO) mode, activated under resonance conditions as a result of a breakdown of the usual selection rules. The 21 cm('-1) line can be interpreted as an 'in-band resonant mode'. The zone center Raman active phonons at 43 cm('-1) and 48 cm('-1), of A(,1) and E symmetry, respectively, also show a resonance enchancement for (H/2PI)(omega)(,L) = E(X(,1)). New photoluminescence (PL), optical absorption and PL excitation measurements on cinnabar are also reported. The details observed in these measurements, together with transport results enable us to present an energy scheme and Fermi level assignment for CVT-grown cinnabar which accounts for the photoluminescence features, which we label X(,1), X(,2), B(,1) and B(,2). In addition to the valence and conduction band continua, we postulate five energy states, three levels nearer to the conduction band and two levels nearer to the valence band. The binding energies of the former are 0.005, 0.402 and 0.420 eV and those of the latter are 0.25 and 0.05 eV. Based upon PL excitation spectra and optical absorption, we place (epsilon)(,F), the equilibrium Fermi energy, in the range 1,855 < (epsilon)(,F) < 1.873 eV. In order to explain the shape of the optical absorption and PL excitation spectra, we examined matrix element effects for a very simple band-to-impurity absorption process.

Degree

Ph.D.

Subject Area

Condensation

COinS