The Urbach rule for the PbO-SiO2 glasses

An analysis is made of the behavior of optical spectra of lead-silicate glasses, with variable lead content near the UV absorption edge, and within the 80–470-K range. A generalized formulation of the modified Urbach rule, applicable to glassy materials within a broad temperature range, is proposed for the interpretation of experimental spectral relations. Within this approach, the effective energies of the phonons responsible for the temperature-induced shift of the Urbach edge have been calculated. It is shown that the spectral and temperature parameters of the modified Urbach rule are structure-sensitive, and that their concentration behavior reflects the change of the type of short-range order in the glassy matrix.     

Low-voltage cathodoluminescence of ZnS single crystals

Bright blue and green cathodoluminescence from low resistivity ZnS crystals has been observed under the excitation of low-energy electron beams of several tens of volts; i.e., 40 fL at 50 V. Properties of the surface of the crystals are studied by the dependence of current and brightness on applied voltage and by the spectra of cathodoluminescence and photoluminescence.     

Polarized emission of doped ZnS crystals

Polarization of light emitted in various spectral ranges was studied in two single crystals ZnS:Cu, Cl and ZnS:Ag, Cu, Al. The G-Cu, B-Cu and a small amount of S-A centers have been identified in the ZnS:Cu, Cl crystal by the spectral and polarization methods. The B-Ag band was found in the spectrum of the ZnS:Ag, Cu, Al crystal and its polarization properties investigated. This emission appears to be always polarized perpendicular to the [111]_c axis of the stacking faults independently of the polarization of the exciting light. The symmetry of the B-Ag center is not lower than that of the host lattice. Analogy with G-Cu centers suggests a model for the B-Ag center in which the polarization comes from the symmetry properties of the Ag²⁺ orbitals in the trigonal field of stacking faults.     

Orientation of luminescence centers in self-activated ZnS crystals

Results concerning polarization of the self-activated (SA) luminescence in ZnS crystals are presented. It is assumed that the orientations of absorption centers — circular oscillators (rotators) — and emission centers — rotators and dipoles — are along the crystal 3-fold symmetry axes. The calculated polarization diagrams fit well to experimental results.     

Frequency dependence of electroluminescence processes in ZnS crystals

The frequency dependence of the integral electroluminescence brightness and the electrical conductivity of ZnS-Cu, Cl and ZnS-Mn single crystals was examined. It was established that the correlation between the integral electroluminescence brightness and the electrical conductivity in ZuS-Cu, Cl crystals improves slightly at high frequencies of the exciting field, whereas no such correlation exists for the frequency range 30 Hz-18 Hz in ZnS-Mn crystals. Conclusions regarding the various excitation mechanisms of the electroluminescence in ZnS-Cu, Cl and ZnS-Mn crystals are drawn.In conclusion, the authors express their sincere gratitude to P. E. Ramazanov for his valuable advice.     

The shape of the self-activated cathodoluminescence band in ZnS: Cl crystals

The shape of the self-activated (SA) cathodoluminescence (CL) band of ZnS: Cl crystals was measured at temperatures in the range from liquid nitrogen to room temperature. Around the SA CL band maximum the shape is Gaussian. For the low and high energy regions of the band the shape becomes exponential. These results are discussed in terms of the Dow and Redfield [1] internal electric microfield model for exponential edges. The shape parameters for the ZnS self-activated cathodoluminescence band are given in table 1. A spectral correction procedure was employed to obtain the spectra in absolute terms (photons emitted per second in a constant bandwidth resolution interval) from the raw count-rate spectra and the vital importance of this correction is emphasised. When the temperature is decreased, as usually reported, the peak of the uncorrected self-activated band shifts to lower energies. The peak of the corrected band however shifts in the opposite direction, i.e., to higher energies. This corrected change is in agreement with that of the edge emission band, that it is in agreement with the increase in the width of the forbidden energy gap with decreasing temperature in ZnS (table 2). The temperature dependence of the blue-silver (B-Ag_I) and the red-tin (R-Sn) impurity bands in cubic structure ZnS were also investigated. It was found that the uncorrected and corrected B-Ag_I peak energy decreases whilst that of the R-Sn band increases as the temperature is reduced.     

Red luminescence of ZnS: Fe crystals

In order to elucidate the nature of the red emission band at 660 nm in ZnS: Fe crystals, the equilibrium luminescence, the thermoluminescence and the photoinduced EPR signals were measured, using UV, or the simultaneous UV + IR excitations. In addition, the polarization properties and the light-pressure shift of the red luminescence were studied. On the basis of the obtained results and of the data reported in the literature and concerning the red and IR emissions in ZnS: Fe a donor-acceptor model is proposed, in which the transition Cl^2?→Fe³⁺ would give rise to the 660 nm band.     

The “Urbach” absorption edge in ALN

The shape of the intrinsic absorption edge of the AlN single crystals has been interpreted under assumption of the absorption of Wannier excitons in the electric field predominantly of charged impurities. The best fit of experimental data is obtained forE _G6·2–6·3 eV and exciton binding energyR70–80 meV.     

The basic growth parameters of ZnS single crystals grown by sublimation

The application of the crystal growth dislocation theory [11] to ZnS single-crystal growth is discussed. The experimental material, above all for determining the actual supersaturation in the growth region and the mechanism of ZnS transport (diffusion, transport gas, chemical transport) was found to be insufficient. An expression for the growth rate of the ZnS crystal plane (the dependence on the supersaturation is not linear) is derived and two different stages in growing ZnS crystals are described.     

On the Urbach rule in sulphur-implanted Cu6PS5I superionic conductors

Cu₆PS₅I superionic crystals, grown using chemical vapour transport, were implanted by sulphur ions. The ion implantation effect on the phase transitions is studied by temperature isoabsorption investigation of the optical absorption edge. For the implanted crystals the optical absorption edge shape is studied in the temperature range 77-320 K, the parameters of exciton-phonon interaction, resulting in the Urbach behaviour of the optical absorption edge, are determined, the temperature dependences of the optical pseudogap and Urbach energy are obtained. The implantation effect on the ordering-disordering processes in Cu₆PS₅I superionic conductors is studied.     

Brightness waves of different activators in ZnS single crystals

The brightness waves of ZnS single crystals with copper and manganese activators have been examined. The measurements were made on single emitting spots of the crystals with time resolution. It could be ascertained that the single spots act independently and that the copper and manganese centres show different electroluminescent properties. The observed phenomena have been investigated in connection with recent electroluminescence theories.     

Quenching of photoluminescence in ZnS:Cu single crystals

Experiments on the quenching of photoluminescence in ZnS:Cu single crystals by secondary radiation are reported. Quenching of emission at photon energies of 1.4, 1.7, 2.4 and 2.75 eV by photons at 0.93, 1.6, 2.0, 2.4 eV is found, with a possible fifth peak at 2.7 eV. The effect of each secondary band on each emission band is found to be equivalent. This is explained in terms of a model in which all quenching transitions effectively fill a common ground state for the green/blue emission.