PHOTOLUMINESCENCE AND RESONANT RAMAN SCATTERING IN SYNTHETIC CINNABAR (ALPHA-MERCURIC-SULFIDE)

CEBER THOMAS SIMPSON, Purdue University

Abstract

Photoluminescence, optical absorption and photoluminescence excitation measurements have been employed to study electronic states in synthetic cinnabar ((alpha)-HgS) grown by chemical vapor transport (CVT). Four luminescence features, labeled X(,1), X(,2), B(,1) and B(,2), have been observed in this material with below band-gap photoexcitation. Two of these features, X(,1) at 1.873 eV and X(,2) at 1.855 eV, are sharp, with half-widths of 3.7 and 6.6 meV, respectively, and two of them, B(,1) at 2.19 eV and B(,2) at 1.78 eV, are broad, with half-widths of (TURN) 100 meV. The photoluminescence, optical absorption and photoluminescence excitation measurements were performed in order to characterize the electronic states participating in these luminescence features. The details observed in these measurements, together with transport results indicate an energy level scheme and Fermi level assignment for CVT-grown cinnabar. Five energy states, three levels nearer to the conduction band and two levels nearer to the valence band, are located within the forbidden energy gap. 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 photoluminescence excitation spectra and optical absorption, the equilibrium Fermi energy can be placed in the range 1.855 to 1.873 eV. In order to explain the shape of the optical absorption and photoluminescence excitation spectra, matrix element effects for a very simple band-to-impurity absorption process were examined. The two photoluminescence features X(,1) and X(,2) can be interpreted as bound-to-free transitions which are weakly coupled to lattice vibrations, whereas the B(,1) and B(,2) features can be interpreted as bound-to-bound transitions strongly coupled to lattice vibrations. The temperature dependence of the X(,1) and B(,2) peak intensities are discussed in terms of a configuration coordinate model. The linear broadening of the X(,1) and X(,2) peaks with temperature above (TURN) 20 K is attributed to interactions with phonons. Optical absorption measurements show that the X(,1) photoluminescence feature at 1.873 eV is due to resonance fluorescence associated with an electronic transition. When the Raman spectrum of samples exhibiting this X(,1) luminescence 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 resonant Raman scattering. 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, show only a resonance enhancement for (H/2PI)(omega)(,L) = E(X(,1)).

Degree

Ph.D.

Subject Area

Condensation

COinS