Experimental evidence about the states in the gap of chalcogenide glasses is discussed. The total concentration of states is estimated from the measurements of the magnetic susceptibility and their density distribution from the optical and photo-emission measurements. Possible models for the interpretation of the experimental facts are considered.

States in the gap

The sharp structure observed in the fundamental optical spectra of crystals, both vibrational and electronic, can be classified and interpreted by symmetry arguments based explicitly on the existence of long-range order. Indeed, this is one of the few properties of crystals which cannot be accounted for on the basis of short-range order alone: If the long-range order is destroyed, the sharp structural detail, which is typical for crystals, disappears. However, the broad features of the spectra are similar if the short-range order is similar.

Optical Properties of Amorphous Semiconductors

Given a spatially dependent mass distribution, we obtain potential functions for exactly solvable nonrelativistic problems. The energy spectrum of the bound states and their wave functions are written down explicitly. This is accomplished by mapping the wave equation for these systems into well-known exactly solvable Schrodinger equations with constant mass using point canonical transformation. The Oscillator, Coulomb, and Morse class of potentials are considered.

Solutions of the nonrelativistic wave equation with position-dependent effective mass

In an organic electroluminescent device including a substrate and formed thereon a multilayered structure successively having at least an anode layer, an organic electroluminescent layer and a cathode layer, a sealing layer having at least one compound selected from the group consisting of a metal oxide, a metal fluoride and a metal sulfide is further provided on the electrode layer formed later A hole injecting and transporting layer is preferably provided between the anode layer and the organic electroluminescent layer An electron injecting and transporting layer may also be provided between the organic electroluminescent layer and the cathode layer At least one layer of the hole injecting and transporting layer, organic electroluminescent layer and electron injecting and transporting layer may be formed of a polyphosphazene compound or a polyether compound or a polyphosphate compound having an aromatic tertiary amine group in its main chain

Organic electroluminescent device

Cr2+-doped II–VI materials for lasers and nonlinear optics

Semiconductors which are slowly graded in composition can be shown to have position-dependent band gaps and position-dependent effective masses, describable in terms of an effective Hamiltonian in an effective-mass equation. The effective Hamiltonian previously obtained is, in the present work, rendered Hermitian. Electronic minority-carrier transport for graded systems is described in terms of an effective field which includes the electrostatic field plus a term in the gradient of the band edge and another in the gradient of the effective mass. The local radiative-recombination lifetime and local density of states for inhomogeneous semiconductors are discussed. The equation for the excess minority-carrier concentration in an inhomogeneous semiconductor is deduced and is found to differ from that in an homogeneous system, by the effective field replacing the electric field, by the position dependences of lifetime and mobility, and by terms in the mobility gradient. Some phenomena specific to graded mixed semiconductors are considered on the basis of the theoretical analysis.

Theory of Electronic States and Transport in Graded Mixed Semiconductors

High-field electroluminescence☆

We have measured the optical absorption spectra of twinned cubic crystals of chromiumdoped zinc sulfide between 5000 and 29 000 ${\mathrm{cm}}^{\ensuremath{-}1}$ (2-0.34 \ensuremath{\mu}m) at 4.2-500\ifmmode^\circ\else\textdegree\fi{}K. Two broad absorption bands are specific to chromium; at 77\ifmmode^\circ\else\textdegree\fi{}K, one has a maximum at 24 400 ${\mathrm{cm}}^{\ensuremath{-}1}$, the other, at 5700 ${\mathrm{cm}}^{\ensuremath{-}1}$. Weak photoconductivity is observed in the former; but none in the latter. Twelve narrow bands between 5100 and 5250 ${\mathrm{cm}}^{\ensuremath{-}1}$ are identified as zero-phonon transitions in ${\mathrm{Cr}}^{2+}$ (3${\mathrm{d}}^{4}$) substitutional at cation sites, two for the cubic structure, ten for the twin boundaries. The cubic crystal field is determined. Thirteen phonon-assisted transitions are identified and the phonon wave numbers are evaluated. We propose that the 24 400-${\mathrm{cm}}^{\ensuremath{-}1}$ band involves transitions to the conduction band edge.

Optical Absorption Spectra of Chromium-Doped Zinc Sulfide Crystals

The effects of hydrostatic pressure on radiative electronic transitions of impurities in solids are analyzed in the adiabatic approximation. A procedure is established for including the effects of pressure on the adiabatic potential for each electronic state. From harmonic configuration coordinates the effects of pressure on peak energy and half-width are determined classically for broad-band, phonon-assisted, absorption and luminescent emission spectra. The conditions for linear dependence of the peaks on pressure are clarified. The impurity is then considered quantum mechanically, and the effects of pressure on vibronic transition energies and probabilities determined. The pressure at which the zero-phonon transition becomes most probable is predicted for some defects. Finally, the effect of anharmonicity on the pressure dependence of vibronic transition energies is evaluated. From this study it is concluded that impurities with vibronic structure can be understood in great detail from the application of the theory to experimental pressure-dependent spectra.

Theory of the effects of hydrostatic pressure on the radiative transitions of impurities in crystals

The dependence on hydrostatic pressure of the photoluminescent spectrum of alexandrite has been measured to 40 kbar, at 300 and 167 K. The ${R}_{1}$ and ${R}_{2}$ lines red shift linearly 0.27\ifmmode\pm\else\textpm\fi{}0.01 and 0.33\ifmmode\pm\else\textpm\fi{}0.01 \AA{}/kbar, the same for both temperatures. Narrowing of the $R$ lines with pressure is observed. The phonon sidebands are blue shifted with pressure. By analysis of these spectral shifts we calculate the anharmonicities of the $^{4}A_{2}$ and $^{2}E$ states and explain the line narrowing. Thus from these analyses we propose that the linear dependency of the $R$-line transition energies on hydrostatic pressure results from anharmonic effects on the zero-point levels of the $^{4}A_{2}$ and $^{2}E$ states, for ${\mathrm{Cr}}^{3+}$-doped materials in general.

Ferd Williams

Papers

Theory of Electronic States and Transport in Graded Mixed Semiconductors

High-field electroluminescence☆

Optical Absorption Spectra of Chromium-Doped Zinc Sulfide Crystals

Theory of the effects of hydrostatic pressure on the radiative transitions of impurities in crystals

Pressure dependence of the alexandrite emission spectrum