CdS Compounds Research Articles

Determining the lattice parameter and the single-electron model potential of the compound CdS by means of the soft x-ray absorption spectra

The method of determining the structure parameters of multiatomic systems, using the spectra of soft x rays (XANES) proposed earlier, is extended to the case of nonmetallic crystal bodies. This method is used to determine the lattice parameter and the empirical muffin-tin potential from the positions of maxima of single-electron origin in the sulfur K spectrum in the compound CdS.

Dimensional reduction in II‐VI materials: A2Cd3Q4 (A = K, Q = S, Se, Te; A = Rb, Q = S, Se), novel ternary low‐dimensional cadmium chalcogenides produced by incorporation of A2Q in CdQ

AbstractThe synthesis of the isomorphous, layered chalcogenides K2Cd3S4 (I), Rb2Cd3S4 (II), K2Cd3Se4 (III), Rb2Cd3Se4 (IV), and K2Cd3Te4 (V) in molten A2Qx fluxes in reported (A = K, Rb; Q = S, Se, Te; x = 2 to 3). The compounds form as (Cd3Q4) layers interspersed with A + cations; the layers are composed of Cd3Q2−4 units shaped as truncated cubes. The compounds have room‐temperature band gaps of 2.75, 2.92, 2.36, 2.37, and 2.26 eV for I, II, III, IV, and V, respectively, and also display strong photoluminescence. The thermal analysis data for all compounds are reported. The properties of these compounds are compared with those of the three‐dimensional compounds CdS, CdSe, and CdTe, as well as those of the nanometer‐sized CdQ clusters. A conceptual context is presented to connect all these different types of compounds.

Crystalline phases of II-VI compound semiconductors grown by pulsed laser deposition

II-VI compound semiconductors, ZnS, ZnSe, CdS, CdSe, and CdTe, were grown epitaxially on (111) and (100) InP and GaAs substrates by excimer laser ablation. All of these films have good crystalline quality (fully in-plane aligned) and mirror-like surface morphology. It was found that, on (111)-oriented substrates, CdS and CdSe films were in the hexagonal phase with the c axis perpendicular to the surface, while ZnS and ZnSe films were in the cubic phase. The films grown on (100)-oriented substrates were all cubic. These high quality films should be useful in optoelectronics applications.

Dielectric function and critical points of cubic and hexagonal CdSe.

The dielectric function of cubic and hexagonal CdSe has been measured by spectroscopic ellipsometry in the 2--25-eV range. The results are compared with similar data for CdS and other II-VI compounds and also with band-structure calculations. Relevant critical points are listed. Transitions originating in the Cd 4d levels are identified and an estimate of the corresponding exciton binding energies is made.

Self-consistent electronic-structure calculations of the (101-bar0) surfaces of the wurtzite compounds ZnO and CdS.

We report ab initio calculations of the surface electronic structure of the hexagonal wurtzite semiconductors ZnO and CdS. The calculations are carried out self-consistently in the local-density approximation employing separable norm-conserving pseudopotentials. Localized Gaussian orbitals are used in the basis set for an efficient description of the strongly localized wave functions. The Zn 3d and Cd 4d electrons are explicitly taken into account as valence electrons since they are important for a quantitative description of the structural as well as the electronic properties, as our investigations of bulk ZnO and CdS have shown. Cationic d-d repulsion, p-d and s-d interactions, and orthogonalization effects are, therefore, included in our calculations of the ZnO and CdS (101\ifmmode\bar\else\textasciimacron\fi{}0) surfaces. The nonpolar cleavage face is described in a supercell geometry with eight atomic and four vacuum layers in each cell. The surface atomic structure is determined by elimination of the forces. For ZnO, we find a rotation relaxation, in which both the Zn and O surface atoms move inward towards the substrate. The calculated surface-perpendicular displacements of the Zn atoms relative to the O atoms in the top layer turn out to be slightly smaller than those determined experimentally by low-energy electron diffraction. The surface electronic structure exhibits an oxygen-derived dangling-bond band in the fundamental gap which is essentially unaffected by the calculated surface relaxation.This is in marked contrast to the results of previous studies using an empirical tight-binding approach which either find no surface states in the gap at all or one band of surface states that is shifted down into the projected bulk valence bands by the surface relaxation. For CdS(101\ifmmode\bar\else\textasciimacron\fi{}0) we find a surface structure that nicely agrees in general with the relaxation model experimentally determined for the related CdSe(101\ifmmode\bar\else\textasciimacron\fi{}0) surface. The surface electronic structure is compared with the results of polarization- and angle-resolved photoemission experiments. With the help of selection rules we are able to identify two occupied surface states at the top of the projected bulk valence bands in very good agreement with experiment.

Distortion in the CdS1−y Se y lattices: an example of relaxation in wurtzite compounds

Relaxation in pseudobinary wurtzite lattices is predicted using the valence force field (VFF) approach. In this hexagonal packing structure, the computer simulation of the quasi-tetrahedral alloy lattice at an arbitrary composition is carried out by searching atomic positions at which the elastic forces acting upon every atom vanish. The VFF model is applied to the II-VI compound CdS 1-y Se y : the bimodal distribution of the distance between first neighbouring atoms is split, taking into account the type of chalcogen atom bound to the Cd one and the distortion of tetrahedra in the non-ideal wurtzite structure

Jahn-Teller interaction at Cr2+(d 4) centres in tetrahedrally coordinated II?VI lattices studied by optical spectroscopy

Optical spectra of thed4 configuration have been studied with II–VI compounds. Failure of static crystal-field description led various authors to introduce static or dynamic Jahn-Teller models for the Cr2+ ground and excited states. With ZnS, the main optical transition5E(D)↔5T2(D) now appears as a doublet at 5218 and 5212 cm−1 coinciding for emission and absorption spectra. This no-phonon structure indicates a splitting of the gound-and excited-state multiplets under combined Jahn-Teller and spin-orbit interactions. In the cubic host ZnSe and in the hexagonal compounds CdS and CdSe, the optical properties exhibit the same general behaviour but with a decreasing shaping of the structural details. In addition to the NIR measurements, middle-infrared spectra have been recorded by Fourier spectroscopy in the transmission mode. For the first time a broad band could be detected for ZnS and interpreted as the\(^5 \hat E\left( {T_2 } \right) \leftarrow ^5 \hat B\left( {T_2 } \right)\) absorption between the sheets of the respective adiabatic potential surfaces inD2d symmetry. Its maximum near 1300 cm−1 indicates\(\bar v_{JT} \approx 430cm^{ - 1} \) cm−1 following the common interpretation of this transition.

Calculation of optical excitations in cubic semiconductors. II. Second-harmonic generation.

The second-harmonic generations in 15 noncentral symmetric cubic semiconductors are systematically studied by the first-principles full band-structure method. The crystals studied are the III-V compounds AlP, AlAs, AsSb, GaP, GaAs, GaSb, InP, InAs, InSb; and the II-VI compounds ZnS, ZnSe, ZnTe, CdS, CdSe, and CdTe. Calculations are focused on the frequency-dependent complex second-order nonlinear optical susceptibilities ${\mathrm{\ensuremath{\chi}}}^{(2)}$(\ensuremath{\omega}) up to 10 eV and their zero-frequency limits ${\mathrm{\ensuremath{\chi}}}^{(2)}$(0). A simple scissor operator is applied to adjust the band gaps from the local-density calculations to the experimental values. Large numbers of k points in the sum over Brillouin zone are used which are important in resolving structures in the dispersion curves. Comparison with available experimental data on ${\mathrm{\ensuremath{\chi}}}^{(2)}$(0) and ${\mathrm{\ensuremath{\chi}}}^{(2)}$(\ensuremath{\omega}) shows general good agreement. It is shown that for a well-converged result, sufficiently high conduction-band (CB) states at least 40 eV from the top of the valence band must be included because of the large CB-CB transition-matrix elements. Correlations between the calculated nonlinear optical parameters and other physical parameters such as band-gap and static dielectric constants are also investigated. It is shown that the validity of the Miller's rule with regard to the ratio between linear and nonlinear susceptibilities is limited to the low-frequency range.

Characterisation of ZnSe and other II–VI semiconductors by radioactive dopants

In the II–VI compounds ZnS, CdS, ZnSe, CdSe, ZnTe and CdTe doped with In, the formation of In M-V M pairs in shown to occur using the radioactive dopant 111In along with the perturbed γγ angular correlation technique. For CdS and ZnSe, the migration energy of the metallic vacancy defect V M and its binding energy to the donor In is determined. The creation of V M defects under different experimental conditions is investigated, such as stoichiometry, temperature, electron irradiation and doping with Li atoms.

Measurements of exciton diffusion by degenerate four-wave mixing in CdS1-xSex.

We performed transient-grating experiments to study the diffusion of excitons in ${\mathrm{CdS}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Se}}_{\mathit{x}}$ mixed crystals. The decay of the initially created exciton density grating is well described for t\ensuremath{\le}1 ns by a stretched-exponential function. For later times this decay changes over to a behavior that is well fitted by a simple exponential function. During resonant excitation of the localized states, we find the diffusion coefficient (D) to be considerably smaller than in the binary compounds CdSe and CdS. At 4.2 K, D is below our experimental resolution which is about 0.025 ${\mathrm{cm}}^{2}$/s. With increasing lattice temperature (${\mathit{T}}_{\mathrm{lattice}}$) the diffusion coefficient increases. It was therefore possible to prove, in a diffusion experiment, that at ${\mathit{T}}_{\mathrm{lattice}}$\ensuremath{\le}5 K the excitons are localized, while the exciton-phonon interaction leads to a delocalization and thus to the onset of diffusion. It was possible to deduce the diffusion coefficient of the extended excitons as well as the energetic position of the mobility edge.

CH4/H2: A universal reactive ion etch for II-VI semiconductors?

A high resolution reactive ion etching process, capable of producing nanostructures less than 50 nm wide in a variety of II-VI semiconductors, is described. Using a mixture of methane and hydrogen, binary II-VI compound, e.g., ZnTe, ZnSe, CdTe, ZnS, CdS, and ternary compounds, e.g., CdMnTe and ZnSSe, have been etched. It would appear that the CH4/H2 gas mixture will play the same role for the II-VI semiconductors as it does for the III-Vs, that of seemingly ubiquitous etching system.

Jahn-Teller effect for CdS:Fe 2+ and CdSe:Fe 2+

Theoretical calculations based on a linear Jahn-Teller effect are performed for the cases of the wurtzite-structure compounds CdS:Fe 2+ and CdSe:Fe 2+. Vibronic functions with vibrational quantum number N up to 14 are used in order to study the stability of the solutions. Predicted lines that would appear in absorption experiments, Jahn-Teller energies and other parameters are discussed and compared with available experimental and theoretical information.

Electronic structure of Cd1-xMnxS ternary semimagnetic alloys.

We present synchrotron-radiation photoemission studies of the electronic structure of ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$S alloys, together with results for the parent binary compound CdS. Analysis of the partial photoionization cross section at the Mn 3p-3d resonance was conducted to investigate the Mn 3d contribution to the valence bands. We use ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$S as a test case to examine different methods of data reduction. A comparative analysis of constant-initial-state (CIS) spectra, ternary-binary difference curves, as well as resonance-antiresonance difference curves identifies a major Mn 3d density of states feature 3.12\ifmmode\pm\else\textpm\fi{}0.05 eV below the CdS valence-band maximum, together with relevant Mn 3d contribution to the electron states near the valence-band maximum and a broad satellite in the 4--8-eV range. A quantitative analysis of the Fano parameters derived from a least-squares fit of the CIS spectra allowed us to characterize the varying amount of Te p--Mn 3d hybridization in the valence states.

Diffusive and thermal properties of the electron-hole plasma in CdS and CdSe

The electron-hole plasma has been studied in the II-VI compounds CdS and CdSe under inhomogenous excitation by spatially resolved pump-and- prove experiments for various temperatures. The investigation of the optical gain and the behaviour of excitonic reflection structures on the unexcited surface of the thin platelet-type samples as a function of sample thickness, show an increase of the density, and a decrease of the shift length of the electron-hole plasma above 70 K.

Investigation on the carcinogenic effects of various cadmium compounds after inhalation exposure in hamsters and mice

The inhalation of cadmium chloride aerosols induced lung carcinomas in rats (3). Subsequently, CdCl2 was classified as probably carcinogenic in humans (2). Thus the Cd compounds CdCl2, CdSO4, CdS and CdO were investigated for their carcinogenic potencies in the lungs of hamsters and mice. The same experiment was conducted by GLASER et al. with rats (1).

Electron-beam and optical strength of semiconductors subjected to pulsed excitation by a high-intensity electron beam

Brittle fracture, initiated by initial polishing defects and low-angle boundaries, occurred in semiconductor crystals under the action of a high-intensity pulsed electron beam. The damage threshold increased along the series of compounds CdS, ZnSe, ZnO, and GaAs. A reduction in the diameter of the irradiated region increased the electron-beam damage threshold. In contrast to the electron-beam interaction, the high-intensity radiation generated as a result of lasing of the semiconductor crystals caused local melting because of the strong absorption of light near the initial microdefects.

Refractive index of hexagonal II–VI compounds CdSe, CdS, and CdSeXS_1–X

The theoretical refractive index of the direct-gap II–VI compounds CdSe, CdS, and CdSexS1–x with the wurtzite lattice structure is calculated for frequencies below the fundamental absorption edge. Use is made of the band structure of the Kane theory, modified to include the dependence of the refractive index on the effective hole masses of the three valence bands, the lattice constants a(Å) and C(Å) of the hexagonal structure, and the direction of polarization of the incident wave. An extension of the previous result for the refractive index of a cubic zinc blende crystal, in which birefringence is absent, to the hexagonal wurtzite structure, where birefringence occurs, is given. Results are obtained for the refractive indices of the ordinary and extraordinary rays in terms of experimentally available quantities, which include the direct-band-gap energy G, the effective electron mass mn, the three effective hole masses mp1,mp2, and mp3 of the three valence bands at k = 0, and lattice constants a(Å) and C(Å). Numerical values of the refractive index for values of x in the range 0 ≤ x ≤ 1 are given and compared with experimental results.

Materials for Heterojunction Devices

Heterojunction devices are playing an increasingly important role in opto­ electronics. They are produced by combining semiconductors that have differing bandgap energies but closely matching lattice parameters. These devices are epitaxially formed on single-crystal substrates. Laser diodes ( 1) and photodetectors are the most important optoelectronic hetero­ structures, although other types of devices, such as transistors, can also be fabricated. The use of heterostructures has made possible dramatic improvements in 111-V compound laser diode performance. However, the addition of heterojunctions to optical detectors has had a lesser impact on their performance. Therefore, this review concentrates mostly on material considerations bearing on laser diode fabrication. The 111-V compound heterojunction lasers have been extensively developed in the past decade starting with the single-heterojunction AIGaAs/GaAs close confinement laser diode (2,3). Since then, structures have been developed with up to four heterojunctions and other III-V and IV-VI materials have been used for heterojunction laser fabrication. The addition of heterojunctions to lasers of the GaAs alloy family has led to a large reduction in the threshold current density at room tempera­ ture, thus making continuous wave operation possible. This development led to practical light sources for high data rate, optical fiber communication. Semiconductor lasers can emit in a spectral range extending from the far infrared (A � 33 11m) to the visible (A � 0.5 11m). However, because some interesting direct bandgap semiconductors cannot be doped both nand p-type, only part of the range is attainable with p-n junction injection devices. For example, the wide-bandgap II-VI compounds CdS and ZnO cannot be doped p-type, and therefore the production of the

Picosecond optoelectronic switching in CdS 0.5Se 0.5

So far only few materials have been investigated for their application in fast opto-electronic switching. These materials are Si, GaAs and InP. The difficulty in obtaining a high resistivity specimen of group IV elemental semiconductors puts constraints on their high power handling capability. Most of the group III–V compounds have the possibility of showing up of Gunn effect at high fields, which has undesirable effect in high voltage switching. We have, therefore, investigated a mixed II–VI compound CdS 0.5Se 0.5 for its potential use as an opto-electronic switch.

Problems of the OPW Method. II. Calculation of the Band Structure of ZnS and CdS

AbstractFor the first time the MOPW method is applied for calculating the band structure of ZnS and CdS of sphalerite modification. Computations are made from first principles in a non‐(m‐t) approximation. Good agreement is obtained with the experimental data of the valence band and conduction band structures. It is shown that d‐states of Zn (Cd) form a narrow subzone in the lower part of the valence band. The compound CdS is used to investigate the effect of d‐states of Cd upon the energy spectrum. The d‐symmetry states are shown to have a rather significant effect not only on the states in the valence band, but also on the states of the conduction band.