Structure Of CdTe Research Articles

Cadmium Telluride (CdTe) Thin Film for Photovoltaic Applications

Cadmium telluride (CdTe) thin films are prepared by the dip-coating deposition technique under atmospheric pressure at different temperature. The optical band gap obtained within the range 1.63-1.60 eV. Crystallite sizes are obtained from XRD that are dependent on composition (Cd/Te) and baking temperatures. Raman spectra confirms the presence of transverse (TO) and longitudinal (LO) optical phonons in the CdTe structure. Films are good photoconductive in nature and could be used in photovoltaic applications. Index Terms—CdTe, optical band-gap, photoconductivity. I. I NTRODUCTION The interest on the different properties of photonic CdTe material is of considerable, due to its practical importance in technological applications such as, integrated optics, optoelectronics, solar energy conversion, gamma ray detection, and x-ray imaging. CdTe is of low cost thin film photovoltaic cells because of its direct band gap and have high absorption coefficient. CdTe could be doped with both n- and p- type materials using a large number of of preparation methods / techniques such as vacuum deposition (1), electro-deposition (2), (3), molecular beam epitaxial (4), metal-organic chemical vapour deposition (5), (6), close-space sublimation (7), (8) and screen printing (9), (10). It is necessary to have a detailed understanding of the basic properties of the materials for the fabricating the photovoltaic applications. In this present work we focuses on the basic and fundamental properties like optical, structural and photoconductive response of CdTe thin films grown by the dip-coating for the photovoltaic applications.

Structural properties of Sb- and Te-based binary compounds: Spin-orbit effect

The band structure of AlSb, GaSb, ZnTe and CdTe is calculated using the empirical pseudopotential method (EPM) coupled with spin-orbit (SO) splitting. We applied our empirical model of bulk modulus with SO effect. It has been noticed that SO has a crucial effect on the band structure of these compounds but does not influence the structural phase transition. The calculated results are in good agreement with the experimental data.

Numerical Analysis of Novel Back Surface Field for High Efficiency Ultrathin CdTe Solar Cells

This paper numerically explores the possibility of high efficiency, ultrathin, and stable CdTe cells with different back surface field (BSF) using well accepted simulator AMPS-1D (analysis of microelectronics and photonic structures). A modified structure of CdTe based PV cell SnO2/Zn2SnO4/CdS/CdTe/BSF/BC has been proposed over reference structure SnO2/Zn2SnO4/CdS/CdTe/Cu. Both higher bandgap materials like ZnTe and Cu2Te and low bandgap materials like As2Te3 and Sb2Te3 have been used as BSF to reduce minority carrier recombination loss at the back contact in ultra-thin CdTe cells. In this analysis the highest conversion efficiency of CdTe based PV cell without BSF has been found to be around 17% using CdTe absorber thickness of 5 μm. However, the proposed structures with different BSF have shown acceptable efficiencies with an ultra-thin CdTe absorber of only 0.6 μm. The proposed structure with As2Te3 BSF showed the highest conversion efficiency of 20.8% ( V,  mA/cm2, and ). Moreover, the proposed structures have shown improved stability in most extents, as it was found that the cells have relatively lower negative temperature coefficient. However, the cell with ZnTe BSF has shown better overall stability than other proposed cells with temperature coefficient (TC) of −0.3%/°C.

Development of Cadmium Magnesium Telluride (Cd1−xMgxTe) for room temperature X- and gamma-ray detectors

Cadmium Magnesium Telluride (Cd1−xMgxTe/CMgT) offers several pronounced potential advantages over the well-studied CdZnTe and CdMnTe, possibly making it a good alternative for room-temperature X- and gamma-ray detectors. It possesses high crystallinity due to the near-similar lattice structure of CdTe (6.48Å) and MgTe (6.42Å). Its displays good homogeneity as the Mg segregation coefficient in CdTe is nearly 1. Furthermore, inhomogeneities in the crystal due to alloying effects can be minimized, because the optimal energy band-gap can be achieved using less Mg in CdMgTe compared to Zn and Mn needed in CdZnTe and CdMnTe. We recently grew an undoped- and a doped-ingot of CdMgTe, characterized its material properties, and tested its detection performance. We obtained some exciting results that demonstrated some of its potential advantages over the other materials. The band-gap was measured as 1.61eV at room temperature and 1.73eV at 4K. The yield was predominantly single crystals with about two orders-of-magnitude fewer Te inclusions and other growth defects compared to CdZnTe and CdMnTe crystals. The measured resistivity of the undoped annealed crystal was about ∼107Ω-cm; after doping in a second growth trial, it increased by 2–3 orders-of-magnitude (109–1010Ω-cm). We examined the doped as-grown crystal as a radiation detector and acquired reasonably good spectral response to an Am-241 source. The estimated mu-tau value was as high as 7×10−4cm2/V, which is an extraordinarily high value for such an early-phase investigation. We also analyzed point and extended defects using various techniques that will be discussed in this manuscript.

Structural and electronic properties of Pb1−xCdxTe and Pb1−xMnxTe ternary alloys

A systematic theoretical study of two PbTe-based ternary alloys, Pb${}_{1\ensuremath{-}x}$Cd${}_{x}$Te and Pb${}_{1\ensuremath{-}x}$Mn${}_{x}$Te, is reported. First, using ab initio methods, we study the stability of the crystal structure of CdTe-PbTe solid solutions, to predict the composition for which the rock-salt structure of PbTe changes into the zinc-blende structure of CdTe. The dependence of the lattice parameter on Cd (Mn) content $x$ in the mixed crystals is studied by the same methods. The obtained decrease of the lattice constant with $x$ agrees with what is observed in both alloys. The band structures of PbTe-based ternary compounds are calculated within a tight-binding approach. To describe correctly the constituent materials, tight-binding parametrizations for PbTe and MnTe bulk crystals as well as a tight-binding description of rock-salt CdTe are proposed. For both studied ternary alloys, the calculated band gap in the L point increases with $x$, in qualitative agreement with photoluminescence measurements in the infrared. The results show also that in p-type Pb${}_{1\ensuremath{-}x}$Cd${}_{x}$Te and Pb${}_{1\ensuremath{-}x}$Mn${}_{x}$Te mixed crystals an enhancement of thermoelectrical power can be expected.

Enhancing blue photoresponse in CdTe photovoltaics by luminescent down-shifting using semiconductor quantum dot/PMMA films

Summary Commercially available semiconductor quantum dots have been encapsulated within a poly(methyl, methacrylate) polymer matrix to form a luminescent down-shifting layer. The films were deposited, via doctor blading on thin film CdTe structures. This paper demonstrates an average increase of 16% in the conversion of photons with energies greater than the window layer band gap (>2.7 eV), which would normally not produce photocurrent due to the filtering effect of the window layer. The films have been characterised using surface mapping laser beam induced current measurements (LBIC) and external quantum efficiency (EQE). While these initial results show an overall decrease in the current output from the device, the possible loss mechanisms are explored and discussed.

Ab initio calculation of the CdSe/CdTe heterojunction band offset using the local-density approximation-1/2 technique with spin-orbit corrections

We performed ab initio calculations of the electronic structures of bulk CdSe and CdTe and of their interface. We employed the local-density approximation-1/2 self-energy correction scheme [L. G. Ferreira, M. Marques, and L. K. Teles, Phys. Rev. B 78, 125116 (2008)] to obtain improved band gaps and band offsets, as well as spin-orbit coupling to further correct the valence band edges. Our results are in good agreement with experimental values for bulk band gaps and reproduce the staggered band alignment characteristic of this system. We found that the spin-orbit effect is of considerable importance for the bulk band gaps, but has little impact on the band offset of this particular system. Moreover, the electronic structure calculated along the 61.4 Å transition region across the CdSe/CdTe interface shows a non-monotonic variation of the bandgap in the range 0.8-1.8 eV. This finding may have important implications to the absorption of light along the interface between these two materials in photovoltaic applications.

Ab Initio Studies of the Unreconstructed Polar CdTe (111) Surface

Ab initio electronic structure calculations were carried out for bulk cadmium telluride (CdTe) and the unreconstructed CdTe polar (111) Cd-terminated and ( $$ \bar{1}\bar{1}\bar{1} $$ ) Te-terminated surfaces. The hybrid functional for the exchange and correlation potential improves the overall description of the electronic structure of bulk CdTe, by lowering Cd 4d states and hence reducing the Cd 4d–Te 5p hybridization. The Cd–Te interlayer distance of the Cd-terminated surface exhibits a dramatic contraction in contrast with the expansion of the Te-terminated surface, and the surface relaxations decrease as the slab thickness increases. The underlying mechanism of the convergence of the electrostatic potential energy, work function, and electric dipole moment of the polar surfaces as a function of slab thickness is surface electron rearrangement leading to charge transfer from the Te- to the Cd-terminated surfaces. The surface electric polarization induces an internal electric field in the slab, which in turn tilts the bands of the slab double layers, thus rendering the surface layers metallic. The electric field decreases with increasing slab thickness due to convergence of the difference of electrostatic potentials between the Cd- and Te-terminated surfaces.

CdSe/CdTe interface band gaps and band offsets calculated using spin–orbit and self-energy corrections

We performed ab initio calculations of the electronic structures of bulk CdSe and CdTe, and their interface band alignments on the CdSe in-plane lattice parameters. For this, we employed the LDA-1/2 self-energy correction scheme [L.G. Ferreira, M. Marques, L.K. Teles, Phys. Rev. B 78 (2008) 125116] to obtain corrected band gaps and band offsets. Our calculations include the spin–orbit effects for the bulk cases, which have shown to be of importance for the equilibrium systems and are possibly degraded in these strained semiconductors. Therefore, the SO showed reduced importance for the band alignment of this particular system. Moreover, the electronic structure calculated along the transition region across the CdSe/CdTe interface shows an interesting non-monotonic variation of the band gap in the range 0.8–1.8 eV, which may enhance the absorption of light for corresponding frequencies at the interface between these two materials in photovoltaic applications.

Epitaxial Zinc-Blende CdTe Antidots in Rock-Salt PbTe Semiconductor Thermoelectric Matrix

The formation of zinc-blende CdTe antidots (bandgap of 1.5 eV at room temperature) embedded in a rock-salt PbTe semiconductor matrix with a narrow bandgap of 0.3 eV in properly annealed epitaxial CdTe/PbTe multilayers grown by molecular beam epitaxy on a GaAs(001) substrate is reported. Transmission microscopy and X-ray diffraction characterization revealed the monocrystalline zinc-blende crystal structure of the CdTe antidots. The CdTe antidots have a highly symmetric shape and size varying in a controlled way in the range from 5 to 30 nm, depending on the layer thicknesses in the initial multilayer CdTe/PbTe stack. The presented results indicate that the CdTe antidot growth mechanism is similar to that of PbTe dots embedded in a CdTe matrix and is driven by the nanoscale phase separation due to qualitative differences in the chemical bonding and crystal structure of PbTe and CdTe. The electrical characterization in terms of Hall effect, electrical conductivity, and Seebeck effect measurements showed that both n- and p-type conductivities can be obtained in these nanocomposite thermoelectric materials with carrier concentrations of 1017–1018 cm–3 and mobilities of about 200 cm2/(V s) at room temperature. About a 25% increase of the thermoelectric power as compared to that of the reference bulk thermoelectric PbTe crystals was found in heterostructures with the smallest CdTe antidots.

Accurate electronic properties for (Hg,Cd)Te systems using hybrid density functional theory

Hybrid screened density functional theory better describes the electronic structure of HgTe, CdTe, and HgCdTe systems in comparison with standard density functional theory. The unique hybrid functional reproduces the band inversion in the popular HgCdTe alloy, justifying it as a better method than standard density functional theory in the search for different topological insulators. In addition, the 0.53-eV valence-band offset obtained using the hybrid functional supports the recently observed higher band offset in the HgTe/CdTe heterostructure.

Single-Phase Cadmium Telluride Thin Films Deposited by Electroless Electrodeposition

By means of analysis of electrochemical processes taking place during electroless electrodeposition have elaborate method for obtaining of well adherent smooth and stoichiometric single-phase CdTe films with cubic structure. Illumination of the growing layers has increased their thickness and improved film crystallinity. Following improvement of CdTe structure was riched by “chloride treatment” of the films. So, the novel electroless electrodeposition technique gives possibilities for the manufacture of CdTe absorbers suitable for η-solar cells.

Investigation of ZnmCdnXy (y = m + n; X = Te, Se and S) Clusters with TDDFT Method

Based on zinc blende and wurtzite structures of experimental ZnTe and CdTe nanocrystals, ZnmCdnXy (X = Te, Se and S) clusters were investigated using DFT/B3LYP/LANL2DZ. From analyses of their characters of conformations, HOMO–LUMO gaps, Raman and absorption spectra, Mulliken charges and WBI (Wiberg Bond Index) values, we have discovered that ZnmCdnTey, ZnmCdnSey and ZnmCdnSy molecules had similar characters. In this paper, characters of ZnmCdnTey were investigated in detail. First, we have found that HOMO–LUMO gaps, Raman spectra, absorption spectra, bond lengths and Mulliken charges of doping Zn2CdTe3, ZnCd2Te3, Zn3CdTe4, Zn2Cd2Te4 and ZnCd3Te4 structures were in the scope of corresponding naked ZnTe and CdTe clusters. These characters of doping ZnmCdnTey molecules show that their stabilities are good. Second, comparing with ZnTe structures, the wavelengths of the absorption peaks of doping ZnmCdnTey clusters shift to red in water environment. Moreover, with increasing of the number of Cd atom, their wavelengths of the absorption peaks gradually shift to red. This conclusion is consistent with the experimental fact. Third, Raman spectra of pure ZnTe clusters have higher frequencies than corresponding naked CdTe structures. As for doping molecules, the frequencies of their Raman spectra gradually shift to low frequencies with increasing of Cd atoms’ number.

Properties of Cux(CdTe)yOz thin films: composition-dependent control of band gap and charge transport

Thin films of Cux(CdTe)yOz were grown by reactive rf co-sputtering using a simple method, in which all the growth parameters were kept constant except for the power applied to the copper target. This technique allowed controlling the relative concentrations of the chemical elements since the Cu addition to CdTe catalyzed, nearly in a one-to-one ratio, the incorporation of oxygen. As a result, it was possible to tailor the structural, optical and electrical properties of the CdTe host by adding Cu from 0 to 25 at.% for different runs. For films with 25 at.% of Cu and O, the crystalline structure changed from a hexagonal CdTe-like to a hexagonal Cu2Te-like. The optical band gap, obtained from UV-Vis spectroscopy, could be varied between 1.48 and 1.62 eV. All as-grown samples were p-type with a resistivity that could be controlled, depending upon the Cu/O concentration, over a wide range between 8 × 10−4 and 2 × 102 Ω cm without any post-growth activation treatment. These results are in agreement with a synergy effect of Cu and O when modifying the CdTe properties. The chemical properties were analyzed by energy dispersive and X-ray photoelectron spectroscopies. The X-ray diffraction and high-resolution transmission electron microscopy experiments showed that the films consist of grains with CdTe and/or Cu2Te structure coexisting with nanometric particles of binary or ternary metallic oxides such as TeO2 and CdTeO3, and for higher Cu concentrations, of Cu2−xTe. Raman scattering experiments showed that the long wavelength phonons of the CdTe structure are not significantly affected by the incorporation of Cu and O since the frequency of the CdTe-like longitudinal optic mode remains basically unchanged.

A complex study of structural imperfections of the CdTe and CdTe: Cl crystals grown by the sublimation method

The structure of CdTe and CdTe: Cl crystals grown by the modified physical vapor transport method has been investigated using selective chemical etching, electron diffraction, and X-ray diffractometry. It has been established that the structural perfection is improved in the growth direction and from the periphery of the radial cross section toward the center of the crystal. It has been shown that the thermomechanical stresses generated by the adhesion of the crystals to the walls of the growth ampoule are the main source of the formation of dislocations and the factor responsible for their nonuniform distribution over the volume of ingots.

Isothermal close space sublimation of CdTe/ZnTe heterostructures in vacuum conditions

AbstractThin films and structures of ZnTe and CdTe have been grown by isothermal close space sublimation using alternated exposure of single crystalline substrates to elemental Zn, Te and Cd sources. The results show that the use of vacuum conditions promotes the transport of vapors towards the surface and then efficient growth at lower temperatures. Films thickness ranged between 200 and 600 nm for 50 cycles samples. ZnTe and CdTe films were obtained even at temperatures as low as 310 °C. This is important because low temperatures worsen the inter‐ diffusion processes; as a consequence we have obtained relatively abrupt interfaces in the ZnTe/CdTe system. Its compositional, structural and optical properties are presented. These results offer new possibilities of this low cost technique for growing heterostructures. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Charge Separation in Type II Tunneling Multilayered Structures of CdTe and CdSe Nanocrystals Directly Proven by Surface Photovoltage Spectroscopy

Charge separation and diffusion in type II multilayered structures of CdTe and CdSe nanocrystals with a polymer spacer are unambiguously proven by surface photovoltage spectroscopy. Holes accumulate in CdTe nanocrystal layers, and the electrons in CdSe nanocrystal layers. An increase of thickness of the polymer spacer strongly decreases the charge separation efficiency. Surface photovoltage transients demonstrate diffusion of the separated charges over several layers of the same kind of nanocrystals.

Theoretical simulation of CdTe nanocrystals in aqueous synthesis

At the B3LYP/LANL2DZ theoretical level, Cd3Te3, (Cd3Te3)2, (Cd3Te3)3, Cd4Te4, and Te–Cd–ligand clusters were optimized. Firstly, hexagon Cd3Te3 and tetrahedron Cd4Te4 structures (with TD symmetry) may be the minimum units of CdTe nanocrystals. They have similar conformations with the experimental wurtzite and zinc blende structures, respectively. Secondly, the frequencies of calculated Raman peaks of four clusters appear in about 140 cm−1, which is close to the experimental data. Following, analysis of Te–Cd–ligand molecules elucidates that all our ligands have similar effect to CdTe structure, because the main influence of ligands comes from thiol, which is also the result of experiment. Finally, considering the influence of solvent and ligand, we believe that our wavelengths of absorption peaks which are calculated using the time-dependent density functional theory are perfectly identical with those of CdTe nanocrystals, according to quantum size effect. Moreover, we have testified that all these absorption peaks are the transition from d to p orbitals.

The adsorption of In on the surface of (001) CdTe

Abstract To understand CdTe doping with In, first-principle calculations are performed to obtain the various kinds of surface–structure for In on CdTe (0 0 1) surface. Of all the structures examined, the structure of CdTe (0 0 1) as caused by In adsorption atoms at the fourfold hollow sites with 0.25 monolayer coverage is the most energetically favorable. In atoms are adsorbed on the Cd-terminated surface, whereas below the Te-terminated surface. For the Cd-terminated surface, cadmium vacancy can form spontaneously and is energetically favorable. In atoms are likely to be adsorbed/incorporated at an interstitial site on Te-terminated CdTe (0 0 1) surfaces for most of the range of the chemical potential.

Structural growth behavior and polarizability of CdnTen (n=1–14) clusters

The lowest-energy structures of Cd(n)Te(n) (n=1-14) clusters have been studied by an unbiased simulated annealing search using first-principles molecular dynamics along with local optimization of "handmade" structures using density functional theory. After n>or=6, three-dimensional cage geometries are the lowest-energy configurations. Two families of low-lying structures, hollow cages, and endohedral or core-shell cages are found. The endohedral cages begin to appear from n=10, and they become more energetically preferred than the hollow cages for n>or=12. Cd(13)Te(13) with the core-shell cage structure is particularly stable. At the same size, the hollow cages possess smaller dipole moments and larger polarizabilities than the endohedral ones. The polarizabilities for the three-dimensional cage structures are insensitive to cluster size. The anisotropies in the polarizabilities mirror the anisotropies in cluster shapes.