Existence of a Cu3Au-type ordered structure in CdxZn1−xTe epilayers grown on (100) GaAs substrates

Selected area electron diffraction pattern (SADP) and transmission electron microscopy (TEM) measurements were carried out to investigate the spontaneously ordered structure in Cd_xZn_1−xTe epilayers grown on GaAs (100) substrates. The SADP showed superstructure reflections with symmetrical intensity, and the high-resolution TEM (HRTEM) micrographs showed doublet periodicity in the contrast of the {100} lattice planes. The results of the SADP and the HRTEM measurements showed a Cu₃Au-type ordered structure was formed in the Cd_xZn_1−xTe epilayer. The present results can help improve understanding of the Cu₃Au-type ordered structures in Cd_xZn_1−xTe epilayers grown on GaAs substrates.     

Nanoscratch behavior of Zn1−xCdxSe heteroepitaxial layers

This paper describes the nanoscratch behavior of Zn_1−xCd_xSe epilayers grown using molecular beam epitaxy (MBE). Transmission electron microscopy (TEM), scanning electron microscopy (SEM), and Hysitron Triboscope nanoindenter techniques were employed to determine the microstructures, morphologies, friction coefficients (μ), and hardnesses (H) of these materials, and thereby propose an explanation for their properties in terms of nanotribological behavior. Nanoscratch analysis revealed that the coefficient of friction of the Zn_1−xCd_xSe epilayer system decreased from 0.172 to 0.139 upon increasing the Cd content (x) from 0.07 to 0.34. Furthermore, studies of the scratch wear depth under a ramping load indicated that a higher Cd content provided the Zn_1−xCd_xSe epilayers with a higher shear resistance, which enhanced the strength of the CdSe bonds. These findings suggest that the greater stiffness of the CdSe bond, relative to that of the ZnSe bond, enhances the hardness of the epilayers. Indeed, the effect of the Cd content on the growth of the Zn_1−xCd_xSe epilayers is manifested in the resulting nanotribological behavior.     

Effects of in situ thermal annealing on the structural, optical, and electrical properties in Hg0.7Cd0.3Te epilayers grown on CdTe buffer layers

Scanning electron microscopy (SEM), Fourier transform infrared (FTIR) transmission, and Hall effect measurements were performed to investigate the structural, optical, and electrical properties of as-grown and in situ-annealed Hg_0.7Cd_0.3Te epilayers grown on CdTe buffer layers by using molecular beam epitaxy. After the Hg_0.7Cd_0.3Te epilayers had been annealed in a Hg-cell flux atmosphere, the SEM images showed that the surface morphologies of the Hg_0.7Cd_0.3Te thin films were mirror-like with no indication of pinholes or defects, and the FTIR spectra showed that the transmission intensities had increased in comparison to that of the as-grown Hg_0.7Cd_0.3Te epilayer. Hall-effect measurements showed that n-Hg_0.7Cd_0.3Te epilayers were converted to p-Hg_0.7Cd_0.3Te epilayers. These results indicate that the surface, optical, and electrical properties of the Hg_1 − xCd_xTe epilayers are improved by annealing and that as-grown n-Hg_1 − xCd_xTe epilayers can be converted to p-Hg_1 − xCd_xTe epilayers by in situ annealing.     

NiP:Mn as a potential magnetic contacting material to Cd1−xMnxTe

Injection of spin-polarized current into spintronic devices is a challenge to the semiconductor physicists and technologists. II-VI compound semiconductors can act as the spin aligner on the top of GaAs light emitting diode. However, II-VI compound semiconductor like Cd_1−xMn_xTe is still suffering from contacting problem. Application of electroless deposited magnetic NiP:Mn contact would enhance efficient current injection into Cd_1−xMn_xTe than the standard gold contact. A technique for electroless deposition of NiP:Mn on Cd_1−xMn_xTe have been described here. The electronic and magnetic properties of the contact material NiP:Mn and the contact performance of NiP:Mn relative to evaporated gold have been evaluated. The contact fulfills the requirements of resistivity and ferromagnetism for application to Cd_1−xMn_xTe.     

The dependence of the structural and optical properties on the Te mole fraction in ZnS1−xTex/GaAs heterostructures

Lattice-mismatched ZnS_1−xTe_x epilayers with various Te mole fractions on GaAs (100) substrates were grown by double well temperature gradient vapor deposition. X-ray diffraction patterns showed that the grown ZnS_1−xTe_x layers were epitaxial films. The photoluminescence spectra showed that the peak position of the acceptor-bound exciton (A⁰, X) varied dramatically with changing the Te mole fraction and that the behavior of the (A⁰, X) peak position of the ZnS_1−xTe_x epilayers with a small amount of the Te mole fraction was attributed to a bowing effect. The reflectivity and ellipsometry spectra showed that the absorption energy peak was significantly affected due to the Stoke's effect. These results provide important information on the structural and optical properties of ZnS_1−xTe_x/GaAs heterostructures for improving optoelectronic device efficiencies operating in the spectral range between near ultraviolet and visible regions.     

Activation energies of the acceptor-bound excitons and the donor-acceptor pairs in nitrogen-doped p-type ZnSe, p-type ZnSySe1−y, and p-type Zn1−xMgxSySe1−y epitaxial films grown on GaAs (1 0 0) substrates

Nitrogen-doped p-type ZnSe, p-type ZnS_ySe_1−y, and p-type Zn_1−xMg_xS_ySe_1−y epilayers were grown on n-type GaAs (1 0 0) substrates by molecular beam epitaxy. Photoluminescence (PL) spectra for the p-type ZnSe and the lattice-matched p-type ZnS_0.06Se_0.94, and p-type Zn_0.92Mg_0.08S_0.12Se_0.88 epilayers showed a deep acceptor bound exciton emission and a donor-acceptor pair emission. Temperature-dependent PL measurements were carried out to determine the activation energies of these states. The activation energies of the acceptor-bound excitons and the donor-acceptor pairs were determined to be 40 and 65 meV in the p-type ZnSe epilayer, 20 and 45 meV in the p-type ZnS_0.06Se_0.94, and 45 and 43 meV in the p-type Zn_0.92Mg_0.08S_0.12Se_0.88 epilayers.     

Nanoscratch study of Zn1−xMnxO heteroepitaxial layers

We investigated the nanotribological properties of Zn_1−xMn_xO epilayers (0 ≤ x ≤ 0.16) grown by molecular beam epitaxy (MBE) on sapphire substrates. The surface roughness and friction coefficient (μ) were analyzed by means of atomic force microscopy (AFM) and hysitron triboscope nanoindenter techniques.The nanoscratch system gave the μ value of the films ranging from 0.17 to 0.07 and the penetration depth value ranging 294-200 nm when the Mn content was increased from x = 0 to 0.16. The results strongly indicate that the scratch wear depth under constant load shows that higher Mn content leads to Zn_1−xMn_xO epilayers with higher shear resistance, which enhances the Mn-O bond. These findings reveal that the role of Mn content on the growth of Zn_1−xMn_xO epilayers can be identified by their nanotribological behavior.     

The optical properties of CdxZn1−xS thin films on glass substrate prepared by spray pyrolysis method

A series of Cd_xZn_1−xS thin films have been deposited on glass substrates using spray pyrolysis technique. The crystallinity and microstructure of Cd_xZn_1−xS thin films have been investigated by X-ray diffraction (XRD). Based on the results of Hall measurements, the films obtained were an n-type semiconductor. The X-ray data analysis of Cd_xZn_1−xS thin films showed that the grain size of the Cd_xZn_1−xS increased with increase in Cd composition. It is observed that the band gap increases as the Cd composition decreases. The results also showed a blue shift of absorption edge of optical transmission spectra is increases as Zn ratio increases. The effects of Cd composition on the structural and optical properties of Cd_xZn_1−xS thin films were related to their grain size, stress and carrier concentration.     

Study on the doping stability and electronic structure of wurtzite Zn1−xCdxO alloys by first-principle calculations

An ab initio calculation based on density functional theory is applied to study the doping stability and electronic structure of wurtzite Zn_1−xCd_xO alloys. It is found that the different alloy configurations of Zn_1−xCd_xO with a given Cd content are possible thermodynamically, but having different band gaps. With increasing Cd content, the formation enthalpy of Zn_1−xCd_xO alloy increases sharply. The Cd-content dependence of the band-gap values can be fitted with a second-order polynomial. The reduction of band gap can be attributed to the contributions of the hybridization of Zn-4s and Cd-5s, the enhancement of p-d repulsion, and the tensile strain due to Cd-doping.     

Structural, optical and electrical properties of Zn1−xCdxO thin films prepared by PLD

Ternary polycrystalline Zn_1−xCd_xO semiconductor films with cadmium content x ranging from 0 to 0.23 were obtained on quartz substrate by pulse laser deposited (PLD) technique. X-ray diffraction measurement revealed that all the films were single phase of wurtzite structure grown on c-axis orientation with its c-axis lattice constant increasing as the Cd content x increasing. Atomic force microscopy observation revealed that the grain size of Zn_1−xCd_xO films decreases continuously as the Cd content x increases. Both photoluminescence and optical measurements showed that the band gap decreases from 3.27 to 2.78 eV with increasing the Cd content x. The increase in Cd content x also leads to the broadening of the emission peak. The resistivity of Zn_1−xCd_xO films decreases evidently for higher values of Cd content x. The shift of PL emission to visible light as well as the decrease of resistivity makes the Zn_1−xCd_xO films potential candidate for optoelectronic device.     

Structural,electronic and optical properties of CdxZn1 − xS alloys from first-principles calculations

Structural, electronic and optical properties as well as structural phase transitions of ternary alloy Cd_xZn_1 − xS have been investigated using the first-principles calculations based on the density functional theory. We found that the crystal structure of Cd_xZn_1 − xS alloys transforms from wurtzite to zinc blende as Cd content of x=0.83$x=0.83$. Effect of Cd content on electronic structures of Cd_xZn_1 − xS alloys has been studied. The bandgaps of Cd_xZn_1 − xS alloys with wurtzite and zinc blende structures decrease with the increase of Cd content. Furthermore, dielectric constant and absorption coefficient also have been discussed in detail.     

Growth and characterization of Cd1−xZnxTe thin films prepared from elemental multilayer deposition

Cd_1−xZn_xTe is a key material for fabrication of high-energy radiation detectors and optical devices. Conventionally it is fabricated using single crystal growth techniques. The method adopted here is the deposition of elemental multilayer followed by thermal annealing in vacuum. The multilayer structure was annealed at different temperatures using one to five repetitions of Cd-Zn-Te sequence. X-ray diffraction pattern for the multilayer with five repetitions revealed that annealing at 475 °C yielded single-phase material compared to other annealing conditions. EDX spectroscopy was carried out to study the corresponding compositions. Photoluminescence properties and change of resistance of the multilayer under illumination were also studied. The resistivity of the best sample was found to be a few hundreds of Ω cm.     

Ab-initio investigations of structural, electronic, magnetic and optical properties of ferromagnetic Cd1-xMnxTe

We report ab-initio calculations of the structural, electronic, magnetic and optical properties of the alloy Cd_1-xMn_xTe as a function of the Mn concentration ‘x’. Ab-initio calculations are based on the density functional theory (DFT) within the generalized gradient approximation (GGA). The calculated lattice constants of the Cd_1-xMn_xTe alloys exhibit Vegard's law downward bowing parameter. For the minority spin channel the Fermi level shifts toward higher energy with the value of ‘x’ in Cd_1-xMn_xTe. The spin-exchange splitting energy Δ_x(d) increases with increasing ‘x’ in Cd_1-xMn_xTe and the values of p-d exchange splitting energy Δ_x(pd) of Cd_1-xMn_xTe show that the effective potential for the minority spin is more attractive than that for the majority spin. The values of exchange constants N₀α and N₀β obtained for Cd_1-xMn_xTe are in agreement with the reported data. The magnetic moment per Mn atom reduces from its free space charge value of 5μ_B to around 4μ_B due to p-d hybridization and this results into an appearance of small local magnetic moments on the non-magnetic Cd and Te sites. The absorption threshold shifts toward higher energy and the static refractive index decreases with the increasing value of ‘x’ in Cd_1-xMn_xTe.     

Effect of microstructure on the nanomechanical properties of Zn1−xCdxSe alloys

We present a study of the nanoindentation behavior of Zn_1−xCd_xSe epilayers grown using molecular beam epitaxy; the surface roughness, microstructure, and crystallinity were analyzed using atomic force microscopy, cross-sectional transmission electron microscopy, and X-ray diffraction; the hardness H and elastic modulus E were studied using nanoindentation techniques. We found that these highly crystalline materials possessed no stacking faults or twins in their microstructures. We observed a very marked increase in the value of H and a significant decrease in the value of E upon increasing the concentration of Cd, presumably because of an increase in the stiffness of the CdSe bond relative to that of the ZnSe bond. We observed a corresponding shrinkage of the contact-induced damage area for those films having a small grain size and a higher value of H. It appears that resistance against contact-induced damage requires a higher Cd concentration.     

Structure and superconductivity of Mg1−xPbxB2

A series of polycrystalline samples of Mg_1−xPb_xB₂ (0≤x≤0.10) were prepared by a solid state reaction method and their structure, superconducting transition temperature and transport properties were investigated by means of X-ray diffraction (XRD) and resistivity measurements. Mg_1−xPb_xB₂ compounds were shown to adopt an isostructural AlB2-type hexagonal structure in a relatively small range of lead concentration, x≤0.01. The crystalline lattice constants were evaluated and were found to exhibit slight length compression as x increases. The superconducting transition temperature (T_c) steadily decreases with Pb doping. It is suggested that the mechanism of superconductivity reduction by lead doping can be attributed to the chemical pressure effect.     

Formation of ZnO and Zn1−xCdxO films on CdTe/CdZnTe single crystals

In this work we report on the properties of ZnO and Zn_1−xCd_xO films formed on top of CdTe and CdZnTe single crystals. The films have been obtained by thermal evaporation of Zn metal films and further oxidation in atmospheric conditions. The structural and compositional characteristics of the films have been analysed by means of scanning electron microscopy and energy-dispersive X-ray analysis. The chemical composition of the films as a function of growth parameters has been obtained. It has been possible to demonstrate by Raman spectroscopy the formation of both ZnO and Zn_1−xCd_xO films. The possible inter-diffusion effects between the films and the substrate, derived from the oxidation process, have been discussed. It has been possible to check by means of photoluminescence, the optical quality of the ZnO and Zn_1−xCd_xO films, also regarding to the presence of local changes. Differences between the optical spectra obtained from various ZnO films grown on top of CdTe and CdZnTe substrates enabled the determination of compositional differences introduced by the substrate when the deposition parameters are modified.     

On the magnetocaloric effect in Gd(Zn1−xCdx)

In this work, we calculate the magnetocaloric effect in the compounds Gd(Zn_1−xCd_x). We use a model Hamiltonian of interacting spins in which the indirect exchange interaction parameter between localized spins was calculated as a function of Cd concentration. The calculated isothermal entropy changes and the adiabatic temperature changes upon magnetic field variations are in good agreement with the available experimental data.     

EPR and magnetic properties of vapour phase grown Zn1−xCrxTe crystals

The EPR and magnetic properties were investigated on vapour phase grown Zn_1−xCr_xTe (0.001?x?0.005) crystal samples at room temperature. The EPR spectra were observed for samples with x=0.001, 0.002 only. The simulations of the spectra confirm Cr³⁺ charge state of the dopant ion at tetrahedral symmetry. The magnetic behaviour of the samples with x=0.001 and 0.002 is neither that of Brillouin paramagnets nor Van Vleck systems while the samples with compositions x=0.003, 0.004 and 0.005 exhibited hysteresis behaviour.     

Ab-initio calculations of Co-based diluted magnetic semiconductors Cd1−xCoxX (X=S, Se, Te)

Ab-initio calculations are performed to investigate the structural, electronic and magnetic properties of spin-polarized diluted magnetic semiconductors composed of II-VI compounds Cd_1−xCo_xX (X=S, Se, Te) at x=0.25. From the calculated results of band structure and density of states, the half-metallic character and stability of ferromagnetic state for Cd_1−xCo_xS, Cd_1−xCo_xSe and Cd_1−xCo_xTe alloys are determined. It is found that the tetrahedral crystal field gives rise to triple degeneracy t_2g and double degeneracy e_g. Furthermore, we predict the values of spin-exchange splitting energies Δ_x(d) and Δ_x(p−d) and exchange constants N₀α and N₀β produced by the Co 3d states. Calculated total magnetic moments and the robustness of half-metallicity of Cd_1−xCo_xX (X=S, Se, Te) with respect to the variation in lattice parameters are also discussed. We also extend our calculations to x=0.50, 0.75 for S compounds in order to observe the change due to increase in Co.     

Resonant photoemission study of Eu1−xGdxTe layers

Resonant photoemission study of electronic structure of molecular beam epitaxy grown Eu_1−xGd_xTe layers without and with cover protected layer of Te were performed using synchrotron radiation. The analysis of the valence band and shallow core levels spectra of the clean surface of Eu_1−xGd_xTe obtained in situ under UHV conditions showed the existence of Eu²⁺ and Eu³⁺ ions in the layers. The trivalent europium ions mostly are located at the surface and its amount strongly depends on sample surface preparation conditions. The prolonged annealing of Eu_1−xGd_xTe layers covered with protected layer of Te leads to formation of clean surface of the sample not changing the stoichiometry of it and without the accumulation of Eu³⁺ ions at the surface region.