Luminescence of fluctuation tails of disordered solid solutions

The form of the stationary luminescence spectra of excitons, localized by composition fluctuations, in disordered solid solutions under weak excitation is calculated. The tail states for which there are no nonradiative transition channels are distinguished by means of continuum percolation theory. Such states are responsible for the “zero-phonon” luminescence band. The shape of the short-wavelength luminescence band edge is determined mainly by the number of isolated localizing clusters and their smallest complexes, which decreases rapidly near the mobility threshold. The real luminescence spectrum is due to the simultaneous emission of phonons. The phonon emission determines the form of the long-wavelength wing of the emission band. The computed shape of the emission spectrum is compared with the experimental luminescence spectra of the solid solution CdS_(1−c)Se_c. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 3, 274–279 (10 February 1997)     

Intrinsic luminescence of amorphous and disordered crystalline systems

A unified method is developed for describing the steady-state luminescence of exciton fluctuation states for weak excitation in different disordered systems. The phononless luminescence band is found to be formed by “radiative” states of the fluctuation tail in the density of states, i.e., by states for which nonradiative states are either nonexistent or have a low probability. The shape of the emission spectra calculated including the phonon interaction is in good agreement with experimental luminescence spectra of α Si:H and of solid solutions of ZnSe_(1−c)Te_c and CdS_(1−c)Se_c. Fiz. Tverd. Tela (St. Petersburg) 40, 890–891 (May 1998)     

Picosecond time-resolved luminescence of localized excitons in CdS1-xSex

We present time-resolved luminescence results on CdS_0.36Se_0.64 which give a new insight on the kinetics of excitons localized by alloy potential fluctuations. By exciting in the localized exciton band with detection close to the exciting wavelength we obtain the lifetime across the band. Below the exciting laser energy two processes contribute to luminescence: transfer of localized excitons by tunnel effect assisted by acoustical phonons, and luminescence (assisted by acoustical phonons) of all the states excited at time t = 0 either directly or through their acoustical absorption wing. The time behavior of luminescence with respect to the detuning from the exciting energy helps to discriminate between those two contributions. Furthermore it shows that intermediate long-living states are involved in the exciton relaxation process.     

Direct observation of localized exciton states in CdS<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript>x</Subscript> solid solutions

A system of isolated localized exciton states corresponding to particular energies from the tail of the density of states is observed in CdS_1?xSe_x crystals. From an analysis of the obtained photoluminescence excitation spectra, the boundary between the free and localized excitons and the length of the tail region of the density of localized states that make an essential contribution to radiative recombination are determined. It is shown that the localized and free excitons prove to be separated and represent two exciton subsystems. The energies of actual optical photons are accurately determined to be 23.6±0.2 and 35.6±0.3 meV for CdS_0.70Se_0.30 and 20.7±0.2 and 31.4±0.3 meV for CdS_0.50Se_0.50.     

Fluctuation states and optical spectra of solid solutions with a strong isoelectronic perturbation

We propose an approach to describing the density of fluctuation states in a disordered solid solution with a strong perturbation introduced by isoelectronic substitution in the range of attraction-center concentrations below the threshold of percolation along the sites of a disordered sublattice. To estimate the number of localized states we use the results of lattice percolation theory. We describe a method for distinguishing, within the continuum percolation theory, among the various “radiating” states of the fluctuation-induced tail, states that form the luminescence band at weak excitation. We also establish the position of the band of radiating states in relation to the absorption band of the excitonic ground state and the mobility edge of the system. The approach is used to describe the optical spectra of the solid solution ZnSe_1−c Te_c, which at low Te concentrations can be interpreted as a system with strong scattering. We take into account the exciton-phonon interaction and show that the calculated and observed luminescence spectra of localized excitons are in good agreement with each other. Zh. éksp. Teor. Fiz. 115, 1039–1062 (March 1999)     

Exciton mobility edge in CdS1−xSex solid solutions

Low temperature emission spectra of localized excitons in CdS_1?xSe_x solid solutions under the monochromatic excitation with tunable laser have been studied. It has been found that the luminescence of localized excitons has a high degree of linear polarization with respect to the polarization direction of exciting light. This polarization reflects the “hidden” anisotropy of macroscopically isotropic localized exciton system and strongly depends on the frequency of exciting light. Study of this dependence has permitted for the first time a determination of position of the “mobility edge” for exciton migration in disordered semiconductor solid solution.     

X-ray diffraction and optical studies of CdS1−x Sex solid solutions with stacking faults

The existence in CdS_1?x Se_x crystals (with x=0.10–0.50) of crystalline regions with stacking faults (SF) was first demonstrated by x-ray diffraction and optical methods. X-ray diffraction studies showed SF to be present in all the samples investigated, but in different concentrations. The effect of SF present in CdS_1?x Se_x solid solutions of different compositions on their exciton reflectance and photoluminescence (PL) spectra has been studied. Crystals with high SF concentrations were found to exhibit new exciton bands, which are manifested in reflectance and PL spectra. In the CdS_1?x Se_x phase with SF, resonant emission due to free excitons and the corresponding phonon replicas have been observed. The effect of reabsorption, which can bring about a change in the zero-phonon emission line shape (doublet formation), as well as affect the intensity ratios of the zero-phonon line to the phonon replicas, has been analyzed. It is pointed out that the variation with temperature of the shape of the SF-induced PL exciton line indicates its complex structure, with the constituents of this structure varying in intensity with increasing temperature.     

Formation and optical properties of CdSSe semiconductor nanocrystals in the silicate glass matrix

Technological conditions providing the formation of CdS _x Se_1−x semiconductor crystal grains with sizes ranging from 2 to 8 nm in a silicate glass matrix have been determined. As the temperature of forming annealing increases, the size of crystal grains increases without changes in their crystal structure and composition. The observed short-wavelength shift of the optical absorption edge indicates that the quantum confinement affects the energy band structure of the nanocrystals. Intense luminescence of the samples is due to radiative transitions involving defects at the semiconductor nanocrystal-silicate matrix interface or intrinsic defects of nanocrystals.     

Photoluminescence and multiphonon resonant Raman scattering in Ni-and Co-doped Zn1−x MnxTe crystals

Low-temperature photoluminescence, exciton reflection, and multiphonon resonant Raman scattering spectra of Ni-and Co-doped Zn_1−x Mn_xTe crystals were investigated. Intense emission occurs in a broad spectral region (1100–17 000 cm⁻¹) in the crystals containing Ni atoms. It is caused by intracenter transitions involving Mn²⁺ ions and transitions between the conduction band and a level of the doubly charged acceptor. The features of the exciton photoluminescence and multiphonon resonant Raman scattering involving longitudinal-optical (LO) phonons at various temperatures are investigated. The insignificant efficiency of the localization of excitons on potential fluctuations in the Zn_1−x Mn_xTe:Co crystals is established. A temperature-induced increase in the intensity of the 5LO multiphonon resonant Raman scattering line due to the approach of the conditions for resonance between this line and the ground exciton state is observed in these crystals. Fiz. Tverd. Tela (St. Petersburg) 40, 616–621 (April 1998)     

Optical spectra and a tail in the density of states of the disordered solid solution ZnSe1−c Tec

A calculational scheme is presented to determine the density of states in the fluctuation tail for the disordered solid solution ZnSe_1−c Te_c at concentrations below the threshold for percolation over sublattice sites. Zero-phonon absorption and luminescence bands in the region of the exciton ground state are found using an approach developed earlier. Phonon coupling is taken into account, and vibronic absorption and luminescence bands are obtained. Experimental data are shown to be in a good agreement with the calculations. Fiz. Tverd. Tela (St. Petersburg) 40, 1420–1425 (August 1998)     

Optical properties of zinc telluride with cadmium telluride submonolayers

Reflection, luminescence, and Raman spectra of epitaxial ZnTe layers nominally incorporating double CdTe submonolayers were studied. The band of an exciton localized at the potential produced by narrow-gap planar inclusions dominated the luminescence of these heterostructures. The emission parameters of localized excitons (specifically, the ratio of integral emission intensity to localization energy) were determined, and it was found that excitons interact with longitudinal optical phonons of the layer enriched with cadmium. Giant amplification of the Stokes component resonant with the localized exciton level was observed in Raman scattering.     

Resonant exciton trapping at impurity states in silver bromide

Excitation spectra near the indirect exciton edge of AgBr at 1.8K are reported for several luminescence lines from weakly localized excitons. Excitation below the exciton absorption threshold reveals several excited bound exciton states the energetic positions of which are determined. For excitation above the threshold, strong energy dependent structure is observed. It is interpreted in terms of resonant trapping of free excitons in both ground and excited bound exciton states associated with emission of LO(Γ), long wavelength acoustic and intervalley TA(X) and LA(X) phonons as well as combinations and overtones of these. From measurements in doped crystals two bound exciton systems are found to be correlated with Cd²⁺ and Ca²⁺, respectively.     

Optical and thermal orientation of localized excitons in solid solutions under resonant excitation in a longitudinal magnetic field

A theoretical and experimental study of the effect of a longitudinal magnetic field on optical orientation and magneto-circular polarization of the luminescence of localized excitons in semiconducting solid solutions is reported. It is shown that recombination takes place through two types of emitting states differing substantially in the degree of anisotropy, g factor, and spin relaxation time. Estimates are made of the g factors, anisotropic and exchange splittings, lifetime, and spin relaxation time of localized states in a CdS_0.96Se_0.04/GaAs solidsolution epitaxial layer. Fiz. Tverd. Tela (St. Petersburg) 40, 900–902 (May 1998)     

CdSSe quantum dots: effect of the hydrogen RF plasma treatment on exciton luminescence

This work deals with effective passivation of CdS_XSe_1−X quantum dot surface after treating it by low-temperature hydrogen RF plasma. An enhancement of the exciton luminescence was observed, which can be interpreted as consequence of a decreasing number of surface non-radiative traps.     

Time-resolved spectroscopy of soantaneous luminescence of CdSSe1−x quantum dots

Picosecond time-resolved spectroscopy of the edge luminescence band of CdS _x Se_1–x quantum dots with crystallite diameters as small as a few nanometers under band-to-band excitation reveals strong enhancement of the radiative recombination rate compared to bulk CdS owing to quantum confinement. The splitting of the luminescence band into two lines originates from near-band-gap absorption. Analysis of the temperature as well as the spectral dependence of the decay time (leading to a red shift of the luminescence with increasing time) and of the total-light-decay law result in a new model for the dominant radiative recombination channel: donor-acceptor pair recombination instead of an excitonic mechanism as claimed in previous publications.Dedicated to H.-J. Queisser on the occasion of his 60th birthday     

Electronic structure of semiconductor solutions of cadmium chalcogenides

The electronic structure of the semiconductor solid solutions CdS_xSe_1−x x=(0, 0.17, 0.33, 0.5, 0.67, 0.83, 1.0) is investigated. The experimental x-ray and x-ray-induced electronic spectra of sulfur in CdS are compared with computed spectra. The calculations are performed using the FEFF (Version 7) program and a package of programs that implements a cluster version of the local-coherent-potential approximation. The effect of an electron vacancy in the 1s level on the x-ray absorption spectrum is investigated. The band gap as a function of the anion concentration in the solution is estimated. Fiz. Tverd. Tela (St. Petersburg) 41, 41–43 (January 1999)     

Temperature Dependence of the Transmission Spectra of Silicate Glasses Containing CdS<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Semiconducting Nanocrystals

The transmission spectra of silicate glasses containing CdS_{1 − x} Se _x semiconducting nanocrystals of various sizes are investigated in the temperature range 20–300°C. It is shown that for explaining optical properties of these materials, not only the nanocrystal sizes, but also the structural changes occurring in nanocrystals during their formation must be taken into account.     

Optical waveguide behavior of Se-doped and undoped CdS one-dimensional nanostructures using near-field optical microscopy

The optical waveguide behaviors of CdS and CdS _x Se_1−x nanostructures are studied using near-field optical microscopy. Optical measurements demonstrate that light may be guided on sub-wavelength scales along CdS nanoribbons in straight or bent structures. The photoluminescence (PL) spectra from nanoribbon emission using scanning near-field optical microscopy are analyzed under different incident laser intensities. The PL spectra along Se-doped and undoped CdS nanoribbons at different propagation distances are investigated. Both the guided PL spectra of Se-doped and undoped CdS nanoribbons show red-shifts because of the band-edge absorption. Our results are useful for the development of new kinds of functional nano devices. Supported by the National Natural Science Foundation of China (Grant Nos. 10574002, 90406007, and 50602015) and the National Basic Research Program of China (Grant No. 2007CB936800)     

Photoluminesence of Iodine‐Doped Crystals of ZnSe

The luminescence spectra and the luminescenceexcitation spectra of ZnSe crystals grown by the Piper–Polish method have been investigated in the temperature interval 81–300 K. An intense edge emission assigned to the presence of iodine has been detected. The analysis of the longwave photoluminescence band by the Alentsev–Fok method has revealed a yellow luminescence band which is attributed to the associative emission centers involving I _Se.