Papers by Keyword: II-VI Semiconductors

Abstract: This paper displays the fabrication of a thermoelectric (TE) generation module using n-ZnSb and p-Zn_0.25Cd_0.75Sb bulk TE materials. TE properties of the Zn_1-xCd_xSb bulks with x= 0, 0.5 and 0.75, in terms of the electrical conductivity () and Seebeck coefficient (S) were measured in the range of 300-500K. The higher power factor (S²σ) values for n-ZnSb and p-Zn_0.25Cd_0.75Sb bulks were obtained about 2.410^-4mW/mK² at 303K and 1.1810^-5 mW/mK² at 468K, respectively. By variation of the thermal conditions, the maximum output power (P_max) with two p-n couples generator module was 1.3810^-5 mW at hot side temperature of 355K and temperature difference () of 40K. The internal (R_in = 0.17 m) and contact resistances (R_c = 0.67 m) between legs and electrodes were discussed below.

Abstract: In this research, Zn_1-xCd_xSe alloys (x from 0 to 1) were synthesized by solid-state microwave (SSM) method of producing thermally evaporated thin films. The cubic structure and the elemental ratios of the films were studied using X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray spectroscopy. The optical characterizations of the as-deposited film in terms of the energy band gap (Eg), photoluminescence (PL), and Raman shift spectra were conducted at the room temperature. The Eg values for the thin films from ZnSe to CdSe were 3.4 to 1.7 eV, respectively. The PL orange emission for ZnSe thin film at 565 nm, whereas 590 nm in the yellow region for CdSe thin film. From Raman shift spectra, the two longitudinal-optical phonon modes (1LO and 2LO) at 240, and 490 cm^-1 are assigned for the ZnSe and CdSe thin films.

Abstract: In this work, the preparation of cadmium telluride (CdTe) nanoparticles by aqueous solution method is presented. The preparation process was performed in the presence of an acidic agent (L-cys) at certain pH value. The x-ray diffraction (XRD) patterns and scanning electron microscopy (SEM) of the prepared samples showed that poly crystalline structure with lowest particle size can be produced by controlling the period of heating process after the preparation step. Accordingly, this parameter can be used carefully to control the surface roughness of nanoscale CdTe samples to serve certain applications requiring some determined values of particle size and surface roughness.

Abstract: Currently there is a great interest in II–VI semiconductor nanoparticles, particularly organically capped soluble particles of cadmium or zinc sulphide and selenide, for their ready to use application in devices. For electroluminescence (EL) devices, it is expected to cover a broad spectrum and to tune various specific colours by preparing Cd_1-xZn_x Se instead of CdSe and ZnSe. Ternary alloys have composition dependent properties; therefore Cd_1-xZn_xSe has attracted much attention in the fields of luminescence and optoelectronic devices. It has wide optical band-gap and good stability with respect to environment. In this study, Cd_1-xZn_xSenanoparticles have been synthesized by using starch as a capping agent through a chemical synthesis route at room temperature. Samples have been prepared varying composition factor ‘x’ in ternary alloy Cd_1-xZn_xSe. Cubic structure of all has been confirmed by XRD. Crystallite size calculated from XRD was found in the range of 3-5 nm and it was observed that size reduces on increasing Zn content in ternary compound. Optical absorption spectra showed the blue shift in absorption edge with increasing Zn content. Band gap has been obtained by absorption studies and increase in band gap observed on increasing Zn content in the compound. Electroluminescence studies reveal that lower threshold voltage is required for samples with lower ‘x’ value. The Brightness increases on increasing the voltage above threshold voltage and the variation pattern is almost exponential for all samples. The voltage-current curve represents ohmic nature of the EL cell. Impedance was found to increase on increasing ‘x’ value. The increase in EL intensity is faster for higher frequency. EL spectra revealed that light emission is in violet-green region corresponding to band gap for both Cd_0.75 Zn _0.25Se and Cd_0.5 Zn _0.5Se, with a slight blue shift on increasing Zn content. A ternary system Cd_1–xZn_xSe, may be engineered better for application purpose by suitably choosing the composition parameter ‘x’.Contents of Paper

Abstract: Nanoparticle or an ultrafine particle is a small solid whose physical dimension lies between 1 to 100 nanometers. Nanotechnology is the coming revolution in molecular engineering, and therefore, it is curiosity-driven and promising area of technology. The field of nanoscience and nanotechnology is interdisciplinary in nature and being studied by physicists, chemists, material scientists, biologists, engineers, computer scientists, etc. Research in the field of nanoparticles has been triggered by the recent availability of revolutionary instruments and approaches that allow the investigation of material properties with a resolution close to the atomic level. Strongly connected to such technological advances are the pioneering studies that have revealed new physical properties of matter at a level intermediate between atomic/molecular and bulk. Quantum confinement effect modifies the electronic structure of nanoparticles when their sizes become comparable to that of their Bohr excitonic radius. When the particle radius falls below the excitonic Bohr radius, the band gap energy is widened, leading to a blue shift in the band gap emission spectra, etc. On the other hand, the surface states play a more important role in the nanoparticles, due to their large surface-to-volume ratio with a decrease in particle size (surface effects). From the last few years, nanoparticles have been a common material for the development of new cutting-edge applications in communications, energy storage, sensing, data storage, optics, transmission, environmental protection, cosmetics, biology, and medicine due to their important optical, electrical, and magnetic properties.

Abstract: Nanoparticles of wide band gap II-VI compounds doped with transition metal (TM) or rare earth (RE) ions are perspective phosphor materials and fluorescence labels for optoelectronic, biology and medical applications. The efficiency of 3d-3d and 4f-4f intra-shell transitions is shown to be enhanced in TM, RE doped nanoparticles. Two mechanisms of emission enhancement related to spin dependent interactions of free carriers with impurities are discussed. These interactions enhance the TM, RE intra-shell transitions by increasing the rate of host to impurity energy transfer. It is shown also that Al doping increases the intensity of light emission from ZnO nanoparticles.

Abstract: The lattice dynamics and thermodynamic properties of MgS and related II-VI compounds are studied by the first-principles linear-response function calculation in the framework of densityfunctional perturbation theory. The ab initio structural, mechanic and dielectric parameters of these phases are presented. From the theoretical phonon dispersion relations, the linear thermal expansion coefficient and its temperature dependence are calculated. The differences in structural and thermodynamic behaviors of these compounds are explained from their phonon dispersion characters.

Abstract: Ag and Cu diffused into CdTe exhibit unusual shapes of their concentration profiles, especially if the diffusion was performed under Cd vapor pressure. The shapes of the concentration profiles at low concentrations of Ag or Cu are well described by a model based on the interaction with intrinsic defects. In that model the characteristic features of the profiles can be independently reproduced by appropriate parameters. It turns out that the resulting profiles are determined by the diffusion of interstitial Cd atoms and reflect the actual distribution of the intrinsic defects in the crystal, i.e. the deviation from stoichiometry.

Abstract: A variety of electrochemical and electrical techniques is employed in order to determine useful parameters of the optical behaviour of thin semiconducting films. In particular, this work is intended to the characterization of cathodically electrodeposited binary and ternary cadmium and zinc selenides and tellurides by photoelectrochemical (PEC) tests. Typical solid-state techniques, such as reflection, laser assisted photoreflection, resistivity and Hall effect measurements are used as well. A plain relation between crystal structure/film morphology and PEC behavior is established so long as the electrochemical preparation method is capable to explicitly control the deposit structure. In certain cases, a particular charge transfer mechanism in the semiconductor, associated with the existence of a nanostructure, is shown to result in higher photoconversion efficiencies as compared to larger-grained films.