Cd-rich Region Research Articles

Structural and morphological characterization of the Cd-rich region in Cd1-xZnxO thin films grown by atmospheric pressure metal organic chemical vapour deposition

We have analysed the growth, morphological and structural characterization of Cd1-xZnxO thin films grown on r-sapphire substrates by atmospheric pressure metal organic chemical vapour deposition, mainly focusing on the Cd-rich rock-salt phase for its promising optical and technological applications. The evolution of the surface morphology and crystalline properties as a function of Zn content has been studied by means of high resolution x-ray diffraction and electron microscopy techniques. Monocrystalline (002) single-phase cubic films were obtained with Zn contents up to 10.4%, and with a low density of dislocations as a consequence of the optimized crystal growth process. Particular attention has been paid to the alloy characterization for compositions when cubic and wurtzite phases coexist.

Nonuniform Excitonic Charge Distribution Enhances Exciton-Phonon Coupling in ZnSe/CdSe Alloyed Quantum Dots.

Zinc to cadmium cation exchange of ZnSe quantum dots has been used to produce a series of alloyed Zn1-xCdxSe quantum dots. As x increases and the lowest-energy exciton shifts to the red, the peak initially broadens and then sharpens as x approaches 1. Resonance Raman spectra obtained with excitation near the lowest excitonic absorption peak show a gradual shift of the longitudinal optical phonon peak from 251 cm-1 in pure ZnSe to 210 cm-1 in nearly pure CdSe with strong broadening at intermediate compositions. The LO overtone to fundamental intensity ratio, a rough gauge of exciton-phonon coupling strength, increases considerably for intermediate compositions compared with those of either pure ZnSe or pure CdSe. The results indicate that partial localization of the hole in locally Cd-rich regions of the alloyed particles increases the strengths of local internal electric fields, increasing the coupling between the exciton and polar optical phonons.

Thermodynamic properties of actinides and rare earth fission products in liquid cadmium

Liquid cadmium has been applied as a liquid solvent or a liquid cathode in pyroprocessing to separate actinides from used fuels or high level radioactive nuclear wastes. The thermodynamic properties, such as the element solubility, the stable metallic compounds, the activity coefficients, and the diffusion coefficients, of actinides and rare earth fission products in liquid Cd determine or strongly affect the separation performance of pyroprocessing. In the present article, the existing experimental data of these properties are critically reviewed and analyzed. Considering the liquid cadmium applications in pyroprocessing, the present article focuses on the properties in the Cd-rich region.

Photoluminescence in undoped (CdO)1−x–(InO3/2)x thin films at room temperature, 0≤x≤1

By means of the sol–gel growth technique undoped (CdO)1−x–(InO3/2)x thin films were prepared in the entire 0≤x≤1 range. The values of x studied were 0.0, 0.16, 0.33, 0.50, 0.67, 0.84 and 1. X-ray diffraction measurements showed that the films were mainly composed of CdO, In2O3, and CdIn2O4. CdO and In2O3 were obtained for x=0 and 1, respectively, and for x=0.67, which is the stoichiometric composition of the CdIn2O4 compound, only this oxide was formed. CdO and CdIn2O4 crystals were obtained in the Cd-rich region, whereas In2O3 and CdIn2O4 crystals were formed in the In-rich region. X-ray diffraction, atomic force microscopy, UV–vis spectroscopy and photoluminescence (PL) measurements were carried out at room temperature for the material characterization. At x=0, i.e. for CdO, PL showed the presence of three main emission bands centered at energies 2.0, and 2.3 and 2.9eV. Even with the fact that films do not form a continuous solid solution of CdO and In2O3 in all the 0≤x≤1 range studied, the material seems to have a graded evolution of these three bands observed in CdO as x moves from 0 to 1, that is from CdO to In2O3. The band at 2.0eV has a red shift when x increases; on the contrary, a blue-shift of the bands at 2.3 and 2.9eV takes place for increasing In concentration, which is related to the increase in the band-gap energy of the mixed system. Actually, this work is a continuation of a previous report of PL at low temperatures.

Low-temperature photoluminescence spectra of CdO–In2O3 thin films prepared by sol–gel

(CdO)1−x–(InO3/2)x thin films were deposited on glass substrates by the sol–gel method. The precursor solutions for the mixed oxide films were obtained from the mixture of the precursor solutions for CdO and In2O3 prepared separately. The investigated In atomic concentrations in the solution, x, were 0.0, 0.16, 0.33, 0.50, 0.67, 0.84, and 1. X-ray diffraction measurements showed that the films were mainly constituted of CdO, In2O3, and CdIn2O4. CdO and In2O3 were obtained for x=0 and 1, respectively. For x=0.67, which is the stoichiometric composition of the CdIn2O4 compound, only this oxide was formed. CdO and CdIn2O4 crystals were obtained in the Cd-rich region, whereas In2O3 and CdIn2O4 crystals were formed in the In-rich region. The PL spectra at 15 K for CdO showed the presence of two main emission bands at energies close to 2.2 and 3.0 eV. A blue-shift of these bands took place for increasing In concentration, which is related to the increase in the band gap energy of the mixed system in going from CdO, with a band gap energy of 2.46 eV, to CdIn2O4, with 3.2 eV, to In2O3, with 3.6 eV.

Thermodynamic properties and microscopic structure of liquid Cd–Na alloys by estimating complex concentration in a regular associated solution

The thermodynamic and microscopic structure of Cd – Na liquid alloy at 673 K has been studied by using regular associated solution model. This model has been utilized to determine the concentration of the complexes Cd 2 Na in Cd – Na alloys in molten state at 673 K. The concentration of the complexes is then used to find free energy of mixing, heat of mixing, entropy of mixing, activity, concentration fluctuations in long wavelength limit S cc (0) and the Warren Cowley short-range parameter α 1. The analysis suggests that this alloy is of a weakly interacting nature. The theoretical analysis reveals that the pairwise interaction energies between the species depend considerably on temperature and the alloy is more ordered in Cd-rich region. The alloy behaves like a segregating system in Na-rich region. The observed asymmetry in the properties of mixing of Cd – Na alloy in molten state is successfully explained by assuming Cd 2 Na complex on the basis of regular associated solution model.

Phase-separation in ZnxCd1-xSe/C Core/shell nanocrystals studied with cathodoluminescence spectroscopy

AbstractZnxCd1-xSe/C core/shellnanocrystals with 31-39 nm diameter semiconductor cores and 11-25 nm-thick carbon shells were synthesized from solid state precursors. Transmission electron microscopy showed striations on a scale of ˜1 – 5 nm in the nanocrystals that are indicative of a composition modulation, and reveal a chemical phase separation and possible spinodal decomposition within the nanocrystals. Such a composition modulation within the ternary nanocrystals represents a rarely reported phenomenon that could lead to additional applications. The optical properties and carrier relaxation kinetics of the nanocrystals were examined with variable excitation cathodoluminescence (CL). As the excitation level is increased, carrier filling in two-dimensional (2D) phase-separated Cd-rich regions leads to a partial saturation of states before the onset of carrier filling in the higher bandgap homogenous Zn0.47Cd0.53Se regions. In order to model the state filling using Fermi-Dirac statistics for non-interacting carriers, a random quantum well model was used to determine the electron and hole eigensates in the Cd-rich regions of the nanocrystals.

Highly Luminescent ZnxCd1−xSe/C Core/Shell Nanocrystals: Large Scale Synthesis, Structural and Cathodoluminescence Studies

Zn(x)Cd(1-x)Se/C core/shell nanocrystals with 31-39 nm semiconducting core and 11-25 nm carbon shell were synthesized from solid state precursors in large scale amounts. A mixture of spherical and tripod nanostructures were obtained only in the one-step reaction (ZC3), where the Zn- and Cd-precursors were reacted simultaneously, rather than in the two step reactions (ZC1 and ZC2), where largely spherical nanostructures were observed. Rietveld analysis of the X-ray diffraction patterns of the samples prepared in three different ways, all under their autogenic pressure, reveal varying compositions of the Zn(x)Cd(1-x)Se nanocrystal core, where the cubic phases with higher Zn content were dominant compared to the hexagonal phases. Carbon encapsulation offers excellent protection to the nanocrystal core and is an added advantage for biological applications. Cathodoluminescence (CL) measurements with spatially integrated and highly localized excitations show distinct peaks and sharp lines at various wavelengths, representing emissions from single nanostructures possessing different compositions, phases, and sizes. Transmission electron microscopy (TEM) showed striations in the nanocrystals that are indicative of a composition modulation, and possibly reveal a phase separation and spinodal decomposition within the nanocrystals. Thermal quenching of the luminescence for both the near band-edge and defect related emissions were observed in the range 60-300 K. The measured activation energies of ∼50-70 meV were related to the presence of shallow donors or acceptors, deep level emissions, and thermal activation and quenching of the luminescence due to the thermal release of electrons from shallow donors to the conduction band or a thermal release of holes from shallow acceptors to the valence band. Spatially integrated CL spectra revealed the existence of broadening and additional components that are consistent with the presence of a composition modulation in the nanocrystals. Spatial localization of the emission in isolated single nanocrystals was studied using monochromatic CL imaging and local CL spectroscopy. CL spectra acquired by a highly localized excitation of individual nanocrystals showed energy shifts in the excitonic luminescence that are consistent with a phase separation into Zn- and Cd-rich regions. The simultaneous appearance of both structural and compositional phase separation for the synthesis of Zn(x)Cd(1-x)Se nanocrystals reveals the complexity and uniqueness of these results.

Electron microscopy investigation of the defect configuration in CdSe/ZnSe quantum dot structures

Abstract In this study we report the dependence of the defect configuration observed in CdSe/ZnSe quantum dot structures on the ZnSe cap layer thickness by transmission electron microscopy (TEM) and reflection high-energy electron diffraction (RHEED). The samples were grown by molecular-beam epitaxy at 350°C. The nominal thickness of the CdSe layers was approximately 3 monolayers and the cap layer thickness was varied in the range from 3 to 60 nm. In all samples, RHEED showed a transition from the two-dimensional (2D) to the three-dimensional (3D) growth mode during the CdSe deposition. By TEM we found pairs of stacking faults (SFs) lying on one of the two pairs of lattice planes given by {(111)-(1 11)} and {(111)-(111)}. The SFs are bound by Shockley partial dislocations. Both of the SFs forming one pair originate from the same stair-rod dislocation with Burgers vector b = ⅙{110} lying at the CdSe-ZnSe interface inside a Cd-rich region (island). Measuring the atomic displacements in the vicinity of the S...

Electron microscopy investigation of the defect configuration in CdSe/ZnSe quantum dot structures

Abstract In this study we report the dependence of the defect configuration observed in CdSe/ZnSe quantum dot structures on the ZnSe cap layer thickness by transmission electron microscopy (TEM) and reflection high-energy electron diffraction (RHEED). The samples were grown by molecular-beam epitaxy at 350°C. The nominal thickness of the CdSe layers was approximately 3 monolayers and the cap layer thickness was varied in the range from 3 to 60 nm. In all samples, RHEED showed a transition from the two-dimensional (2D) to the three-dimensional (3D) growth mode during the CdSe deposition. By TEM we found pairs of stacking faults (SFs) lying on one of the two pairs of lattice planes given by {(111)-(1 11)} and {(111)-(111)}. The SFs are bound by Shockley partial dislocations. Both of the SFs forming one pair originate from the same stair-rod dislocation with Burgers vector b = ⅙{110} lying at the CdSe-ZnSe interface inside a Cd-rich region (island). Measuring the atomic displacements in the vicinity of the SFs, we obtained that all SFs have an intrinsic nature. At the line of contact of two SFs that belong to different pairs, we observed stair-rod dislocations with b = ⅓{100} which are inclined to the interface. The length of all dislocation lines and the sizes of the SFs increase with increasing ZnSe cap layer thickness. We find that the 2D-3D transition observed by RHEED is most probably caused by defect formation, which is discussed with respect to the relaxation of strain in the CdSe layer.

Transmission Electron Microscopy Investigation of CdSe/ZnSe Quantum Dot Structures

Different techniques of transmission electron microscopy were applied to study the Cd distribution and the formation of defects in CdSe/ZnSe structures with a nominal CdSe layer thickness t of three monolayers (ML) and different ZnSe cap layer thicknesses which were varied from 20 to 60 nm. The samples were grown by standard molecular-beam epitaxy at 350 °C. We report on the configuration of defects which appear in the CdSe/ZnSe structure from a critical thickness t > 2.5 ML. The defects originate in Cd-rich regions close to the CdZnSe layer and consist of stacking fault (SF) pairs on {111} crystal planes. The lengths of the dislocation lines and the sizes of the SFs increase with the cap layer thickness. The quantitative measurement of the Cd distribution indicates segregation with an efficiency of R = 0.74 ± 0.04.

Cd Distribution and Defects in Single and Multilayer CdSe/ZnSe Quantum Dot Structures

Conventional and high-resolution transmission electron microscopy (TEM) was applied to a study of the Cd distribution and structure of defects in single and multiple CdSe layers in a ZnSe matrix. The samples containing nominal CdSe layer thicknesses tn between 1.7 and 3.5 monolayers (ML) were grown by molecular beam and atomic layer epitaxy under different conditions. In all samples ternary CdZnSe wetting layers with Cd-rich regions with sizes of less than 10 nm (small islands: SIs) and a density of ∼1011 cm—2 are observed. The Cd concentration in the wetting layer and the SIs increase with tn. In addition, regions with sizes of 20–30 nm (large islands: LIs) and Cd concentrations >40% occur in specimens with tn > 2.5 ML. In the vicinity of the LIs stacking faults are preferably generated leading to a “coffee-bean” contrast in plan-view TEM images. The multilayer structures with tn ≈ 2.5 ML display a predominant vertical correlation of the SIs at a ZnSe spacer thickness of 12 ML.

Study of zinc inclusions/precipitates in CdZnTe crystals

Zn inclusions/precipitates in Bridgman-grown CdZnTe crystals, for the first time, have been characterized by scanning electron microscopy with an electron probe X-ray microanalyzer (SEM/EPMA). The Zn inclusions/precipitates in CdZnTe crystals are possibly due to Zn segregation effects resulting from the deviation from the congruent melting point. It is also shown that Te- and Cd-rich regions surrounding the Zn inclusions/precipitates edge, due to the non-stoichiometric solid compositions, lead to the formation of Te and/or Cd precipitates around the Zn inclusions/precipitates. The infrared transmittance of these stoichiometric-type CdZnTe is about 64% in the wavenumber range of 400–4000 cm −1.

Phase studies of the Cd-Fe-Se system in the Cd-rich region

The cadmium-rich region of the semi-magnetic semiconductor Cd-Fe-Se phase diagram has been partially determined, for the first time, by x-ray diffraction techniques and by metallurgical microscopy. The maximum solid solubility of iron in CdSe, employing annealing techniques, has been found in this work to be about 19 at. % at 925°C, while the solid solubility limit is generally discussed to be about 12 at %. The CdFeSe system appears to be a pseudo-binary one. Further considerations about the structure of this system are discussed.

Electrochemical control of the CuxS phase in CuxS-CdS photovoltaic cells

The phase of the CuxS layers formed on single-crystal substrates of CdS by dipping in a solution containing Cu+ ions has been controlled by applying a potential to the CdS during the plating process. The phase is extremely sensitive to the magnitude of the bias potential, and the most efficient cells with the preferred chalcocite, Cu2.0S, phase of CuxS were prepared with biases in the very narrow range between +0.01 and 0.02 V on the back contact to the CdS. In all, biases between -0.1 and +3.0 V have been investigated. The phases of the CuxS have been identified directly by RHEED and indirectly from spectral response measurements. The most efficient devices plated at +0.01 V have hard characteristics as-prepared, show no cross-over effect, and are impaired by subsequent heat treatment C-V measurements support the suggestion that the Cd2+-Cu+ ion-exchange process during plating is strongly affected by the applied bias, so that in the optimum cell a Cd-rich region is produced at the interface inhibiting the in-diffusion of Cu+ ions.

The entropy of mixing of liquid Na-Cd alloys

The observed entropy of mixing of liquid Na-Cd alloys at 673 K can be explained using the binary hard sphere model provided diameter shrinkages are assumed to accompany the measured total volume contraction. No chemical short range order need be postulated. An implication of this result is that the system remains nearly free electron-like at all concentrations, notwithstanding the fact that, on the evidence of the concentration fluctuations, there is a pronounced tendency to compound formation in the Cd-rich region.

Positron Annihilation Study on the Fermi Surface of Cd–Mg Alloys

Angular correlation measurements of annihilation photons have been made on single crystals of the Cd–Mg alloys over all compositions. It is found that the Fermi surfaces of the alloys in the Cd-rich region distort considerably from a free-electron sphere; the 3rd band “stars” and the 4th band horizontal “cigars” around the points L do not exist in Cd, and appear at 10 at% Mg accompanying an appreciable decrease of the energy gap at L . It is proposed that changes in the 4 d core states cause the sudden decrease of the energy gap.