Characterization of Vacuum Evaporated Polycrystalline Cd0.96Zn0.04Te Thin Films by XRD,Raman Scattering and Spectroscopic Ellipsometry

Cd_0.96Zn_0.04Te thin films are deposited onto well cleaned glass substrates (Corning 7059) kept at room temperature by vacuum evaporation and the films are annealed at 423 K. Rutherford Backscattering Spectrometry and X‐ray diffraction techniques are used to determine the thickness, composition and crystalline structure and grain size of the films respectively. The films exhibited zinc blende structure with predominant (111) orientation. The surface morphology of the films is studied by Atomic Force Microscopy. The rms roughness of the as‐deposited films is 3.7 nm and on annealing the films at 423K, the rms roughness value decreases to 3.4 nm. The Raman spectra of the Cd_0.96Zn_0.04Te films are recorded at room temperature to study lattice vibrations and their interactions with other excitations. The intensity of the peak increases and the FWHM decreases on annealing the films. The pseudodielectric‐function spectra, ε(E) = ε₁(E) + i ε₂(E), of polycrystalline Cd_0.96Zn_0.04Te thin films in the 1.3 ‐ 5.5eV photon energy range at room temperature are measured by spectroscopic ellipsometry. The measured dielectric function spectra reveal distinct structures at energies of the E₁, E₁+Δ₁ and E₂ critical points are due to interband transitions.     

Effect of substrate temperature on polycrystalline Cd0.9Zn0.1Te thin films studied by Raman scattering spectroscopy

Cd_0.9Zn_0.1Te thin films were prepared by vacuum evaporation onto well‐cleaned glass substrates maintained at 300, 373 and 473 K. X‐ray diffraction studies revealed that the films have zinc blende structure with preferential (111) orientation. Raman peak of the room temperature deposited film appeared at 140.30 cm^‐1 and 159.65 cm^‐1 were for the transverse optic (TO) and longitudinal optic (LO) phonons respectively. The XRD patterns of the higher substrate temperature deposited films exhibited an improvement in the crystallinity of the films. The Raman peak intensity increases and the FWHM decreases for the films deposited at higher substrate temperature. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Phase equilibria and crystal chemistry of silver-nickel-tellurium system

Ternary isotherm in the silver-nickel-tellurium system has been investigated using X-ray diffraction and metallographic techniques. At 873 K, only monoclinic Ag₂Te phase with a = 0.815 nm, b = 0.449 nm, c = 0.811 nm, and β = 112°60' is stable in the Ag-Te system. Existence of two phases viz., Ni₃Te₂ and NiTe_1+x has been established. NiTe_1+x phase has a large homogeneity range in the binary and ternary system. solid solubility of nickel in Ag₂Te as well of silver in Ni₃Te₂ is about 5 at.% at the temperature of investigation. The reasons for the large homogeneity range of NiTe_1+x in binary and ternary system has been discussed in detail.     

The effect of oxygen partial pressure on physical properties of nano‐crystalline silver oxide thin films deposited by RF magnetron sputtering

Nano‐crystalline silver oxide films were deposited on glass and silicon substrates held at room temperature by RF magnetron sputtering of silver target under different oxygen partial pressures. The influence of oxygen partial pressure on the structural, morphological, electrical and optical properties of deposited films was investigated. Varying oxygen partial pressure during the sputter deposition leads to changes of mixed phase of Ag₂O and Ag to a single phase of Ag₂O and to AgO. The X‐ray diffraction and X‐ray photoelectron spectroscopy results showed the formation of single phase Ag₂O with cubic structure at oxygen partial pressures of 2x10^‐2 Pa while the films deposited at higher oxygen partial pressure of 9x10^‐2 Pa showed the formation of single phase of AgO with monoclinic structure. Raman spectroscopic studies on the single phase Ag₂O showed the stretching vibration of Ag‐O bonds. Single‐phase Ag₂O films obtained at oxygen partial pressure of 2x10^‐2 Pa were nano‐crystalline with crystallite size of 20 nm and possessed an electrical resistivity of 5.2x10^‐3 Ωcm and optical band gap of 2.05 eV. The films deposited at higher oxygen partial pressure of 9x10^‐2 Pa were of AgO with electrical resistivity of 1.8x10^‐2 Ωcm and optical band gap of 2.13 eV. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical and opto‐electronic properties of polycrystalline Cd0.96Zn0.04Te thin films

Cd_0.96Zn_0.04Te thin films are deposited onto thoroughly cleaned glass substrates (Corning 7059) kept at room temperature by vacuum evaporation. The films are found to have good stoichiometry as analyzed by Rutherford Backscattering Spectrometry. The films exhibited zinc blende structure with predominant (111) orientation. The surface morphology of the films is studied by Atomic Force Microscopy. The rms roughness of the films evaluated by AFM is 3.7 nm. The pseudodielectric‐function spectra, ε(E) = ε₁(E) + i ε₂(E) at room temperature are measured by spectroscopic ellipsometry. The measured dielectric function spectra reveal distinct structures at energies of the E₁, E₁+ Δ₁ and E₂ critical points. The band gap energy of the films measured by optical transmittance measurement is 1.523 eV. The PL spectrum of the films shows intense emission due to free and bound exciton recombination and no emission associated with crystal imperfection and deeper impurity levels. The PL line shapes give indications of the high quality of the layers.     

Hydrothermal synthesis of Bi2Te3 nanowires through the solid-state interdiffusion of Bi and Te atoms on the surface of Te nanowires

Polycrystalline Bi₂Te₃ nanowires were prepared by a hydrothermal method that involved inducing the nucleation of Bi atoms reduced from BiCl₃ on the surface of Te nanowires, which served as sacrificial templates. A Bi–Te alloy is formed by the interdiffusion of Bi and Te atoms at the boundary between the two metals. The Bi₂Te₃ nanowires synthesized in this study had a length of 3–5 μm, which is the same length as that of the Te nanowires, and a diameter of 300–500 nm, which is greater than that of the Te nanowires. The experimental results indicated that volume expansion of the Bi₂Te₃ nanowires was a result of the interdiffusion of Bi and Te atoms when Bi was alloyed on the surface of the Te nanowires. The morphologies of Bi₂Te₃ are strongly dependent on the reaction conditions such as the temperature and the type and concentration of the reducing agent. The morphologies, crystalline structure and physical properties of the product were analyzed by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED) and X-ray photoelectron spectroscopy (XPS).     

Monocrystalline Cd0.2Zn0.8Te solid solution obtained by self‐selecting vapour growth

Cd_0.2Zn_0.8Te monocrystals with the sizes of about 15 mm have been produced by self‐selecting vapour growth (SSVG). High degree of structural perfection of monocrystalline Cd_0.2Zn_0.8Te was achieved. Excellent compositional uniformity was observed as well. To our knowledge, no comparable results are reported for this solid solution. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystal structures of X‐phase in the Sb–Te binary alloy system

It is well known that the Sb–Te binary system has a large number of incommensurately or commensurately modulated structures between Sb and Sb₂Te₃ compounds. These structures, which are long‐period trigonal stacking structures, possess their own modulation period γ, according to their composition in the thermal equilibrium. However, the structure of sputtered Sb–Te films with various compositions between the two compounds at both ends formed in a non‐thermal equilibrium showed smaller γ values, than those expected from their compositions without exception. A smaller γ value implies that its structure is closer to that of Sb with the shortest period in all Sb–Te modulated structures. With increase in temperature, all these transient structures with smaller γ, however, became stable, accompanying an increase of γ to acquire their original modulated structures.     

Optical Properties of Pb-Doped Bi2Te3 Single Crystals

The reflectance and the transmittance spectra in the IR region were measured on Pb-doped Bi₂Te₃ single crystal samples grown by a modified Bridgman technique. The plasma resonance frequency, the optical relaxation time, and the high-frequency permittivity were determined by fitting the Drude-Zener formulas to the reflectance spectra. It was found that Pb impurities in Bi₂Te₃ behave as acceptors. A part of incorporated Pb atoms behaves as electrically inactive. This effect is explained as due to the fact that electrically active Pb-atoms form substitutional defects, Pb′_Bi, and the others form inactive two-dimensional defects — seven-layer lamellae Te—Bi—Te—Pb—Te—Bi—Te. The transmittance spectra were used for the determination of the dependence of the absorption coefficient K on the energy of incident photons. The optical width of the energy gap increases with increasing Pb content. The values of the exponent α from the relation of K ∼ λ^α for the long-wavelength absorption edge range within the interval of 1.6 to 2.8, i.e. the dominant scattering mechanism of free current carriers in Pb-doped Bi₂Te₃ crystals is the scattering by acoustic phonons. By comparison of the effect of doping atoms of the IV.B group of the periodic table on the concentration of holes in Bi₂Te₃ crystal lattice was concluded that the tendency to form substitutional defects Me′_Bi (Me = Ge, Sn, Pb) in these crystals increases from Ge to Pb, whereas the tendency to the formation of seven-layer lamellae Te—Bi—Te—Me—Te—Bi—Te decreases.     

Bi2Te3 Crystals Heavily Doped with Germanium Atoms

Elemental Bi, Te, and Ge of 5N purity were used to prepare Bi₂Te₃(GeTe) single crystals with germanium content varying from 0 up to 2.3 × 10²¹ cm⁻³. The samples were characterized by reflectivity measurements in the plasma resonance frequency range and by measurements of the electrical conductivity. Germanium content in the samples was determined by means of energy dispersive analysis. The reflectivity spectra were interpreted on the basis of the Drude-Zener theory in the aim to obtain information on the concentration of the free carriers in the samples. It was found that Ge atoms in the Be₂Te₃ crystal lattice behave as acceptors. A comparison of the hole concentration with the amount of germanium built into the crystal lattice revealed that only about 1/100 of the total number of Ge atoms act as acceptors. This effect is explained by two different ways of incorporation of Ge atoms into the Bi₂Te₃ lattice, viz: the formation of substitutional Ge'_Bi defects acting as acceptors and the formation of seven-layer-lamellae of the Te Bi Te Ge Te Bi Te composition, which corresponds to the structure of GeBi₂Te₄.     

Charge transport in chlorine doped amorphous Se:Te xerographic photoreceptor films

Time-of-flight drift mobility experiments were carried out on a-Se_1?xTe_x (x = 0-0.1) with chlorine as an additive up to 70 atomic parts per million to investigate the charge transport mechanism in these xerographically important photoreceptor films. Hole drift mobility-temperature data indicate that hole transport in a-Se:Te alloys is controlled by a relatively discrete set of Te-introduced shallow traps (probably Te₁^? centres) at ～ 0.43 eV above E_v whose concentration increases nearly linearly with Te addition. Chlorine doping generates an additional set of traps (probably Cl₀^? centres) around the same energy as Te-induced traps in a similar fashion to the effect of Cl addition to a-Se. Electron drift mobility-temperature data for a-Se:Te alloys containing no Cl can also be interpreted by assuming Te-introduced electron traps at ～ 0.49 below E_c. There was no electron transport observable in Cl-doped a-Se:Te alloys.     

Impedance and Electric Modulus Analysis of Cd0.6Zn0.4Te Thin Films

Cd_0.6Zn_0.4Te thin films were deposited by single source vacuum evaporation technique. The impedance of polycrystalline Cd_0.6Zn_0.4Te thin films has been measured as a function of frequency. The experimental data were measured in the temperature range of 300 – 445K and have been analyzed in the complex plane formalism and suitable equivalent circuits have been proposed in different regions. The values of resistance and capacitance of bulk and grain boundary contributions have also independently calculated from the peak of spectroscopic plots. The role of bulk and grain boundary in the overall conduction process has been discussed with realistic justification. The frequency analysis of ac conduction properties showed distribution of relaxation times due to the presence of multiple grain boundaries in the films. The activation energy calculated from the complex impedance analyses was found to be 0.29eV. The values of activation energies decrease with increase in frequency and are in agreement with that calculated from the impedance plot.     

Te NMR chemical shifts and tellurium coordination environments in crystals and glasses in the Ge-As-Sb-Te system

The local coordination environments of Te atoms have been investigated in crystalline and glassy binary and ternary tellurides in the system Ge-As-Sb-Te using ¹²⁵Te solid-state wideline nuclear magnetic resonance (NMR) spectroscopy. The average ¹²⁵Te NMR chemical shifts in these materials range from 300 to 1050, 90 to 700 and − 2000 to − 4100 ppm for 2, 3 and 6-coordinated environments, respectively. Te atoms are predominantly 2-coordinated in binary Ge-Te, As-Te and ternary Ge-As-Te glasses. The ¹²⁵Te NMR spectrum of the cubic Ge₁Sb₂Te₄ phase with rock salt structure is consistent with a random distribution of Ge/Sb vacancies in the lattice. Besides the coordination number, the ¹²⁵Te chemical shifts in these materials are also found to be sensitive to the chemical identity of the nearest neighbors. ¹²⁵Te NMR spectroscopy shows significant future promise in its application as a technique complementary to diffraction and EXAFS in understanding the short-range structure of amorphous Ge-As-Sb tellurides.     

Laser-induced thermomigration of Te precipitates in CdZnTe crystals

The thermomigration of tellurium precipitates in a cadmium zinc telluride (CdZnTe) crystal was observed using an infrared (IR) CO₂ laser beam. Te precipitates present in CdZnTe have been shown to thermally migrate along a temperature gradient. Selective energy deposition from an IR laser beam in Te precipitates was investigated as a potentially advantageous approach to speed the annealing of Te precipitates in CdZnTe. Initial results indicate Te precipitates do thermally migrate under IR laser heating.     

Characteristics of Al/p‐Cu0.5Ag0.5InSe2 Polycrystalline Thin Film Schottky Barrier Diodes

Al/p‐Cu_0.5Ag_0.5InSe₂ polycrystalline thin film Schottky barrier diodes have been prepared. The current‐voltage, capacitance‐voltage and photores ponse have been investigated. Various important physical parameters of the sediodes were derived from these measurements.     

From Te nanotubes to CoTe2 nanotubes: A general strategy for the formation of 1D metal telluride nanostructures

The transformation of Te nanotubes to CoTe₂ nanotubes have been achieved through a solvothermal process in a mixed solvent of ethylene glycol (EG) and oleic acid (OA) at 200 °C for 24 h. Single crystalline Te nanotubes generated in situ could serve as the Te source and template, and then transformed as self-assembly CoTe₂ nanotubes. In the unique reaction system, CoTe₂ nanotubes were obtained when the volume ratio of EG and OA (V_EG/V_OA) was 5/3. The facile approach was extended to prepare other 1D telluride nanostructures, including CdTe, PbTe, Sb₂Te₃ and Bi₂Te₃.     

Growth,electrical and optical behaviour of nanocrystalline Ag2Cu2O3 films produced by RF magnetron sputtering

Thin films of Ag₂Cu₂O₃ were formed on glass substrates by RF magnetron sputtering technique under different oxygen partial pressures in the range 5 × 10^‐3 – 8 × 10^‐2 Pa using mosaic target of Ag₇₀Cu₃₀. The influence of oxygen partial pressure on the core level binding energies, crystallographic structure, and electrical and optical properties of the deposited films was studied. The atomic ratio of copper to silver in the films was 0.302. The oxygen content was in correlation with the oxygen partial pressure maintained during the growth of the films. The films formed at oxygen partial pressures < 2 × 10^‐2 Pa was mixed phase of Ag₂Cu₂O₃ and Ag. The films deposited at 2 × 10^‐2 Pa were single phase of Ag₂Cu₂O₃. The crystallite size of the films formed at 2 × 10^‐2 Pa was 12 nm, while those films annealed at 473 K was 16 nm. The nanocrystalline Ag₂Cu₂O₃ films formed at oxygen partial pressure of 2 × 10^‐2 Pa showed electrical resistivity of 8.2 Ωcm and optical band gap of 1.95 eV. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Infrared reflectivity of ZnIn2Te4

Infrared reflectivity spectra of ZnIn₂Te₄ single crystals are measured at room temperature in the wavenumber range from 170 to 4000 cm⁻¹. The spectra reveal a single vibrational mode the frequency of which compares well with the frequencies of the high-energy infrared active modes in HgIn₂Te₄, CuInTe₂ and AgInTe₂. It is concluded that these modes are sphalerite-like in nature and that they are essentially determined by the properties of the In—Te bond.     

Compositional dependence of the thermal,structural, and electrical properties of AsTeI glasses

Thermal, structural, and electrical properties of semiconducting AsTeI (and AsTe) glasses have been examined as a function of concentration. Analytical techniques have been developed for quantitative chemical analysis of all three components. Differential scanning calorimetry data indicate a broad endothermic reaction, T_min ∼ 145 °C, above the glass transition (T_g = 120 ± 8 °C) for iodine compositions of 0 to ∼ 20 at%. Above this reaction temperature the thermal data are composition dependent. For glasses with I ? 35 at %, T_g is much lower (65–70 °C) and the endothermic reaction is much sharper with a minimum at ∼ 133 °C. The wide variation in thermal properties with composition suggests that electrical effects associated with high temperatures (e.g. switching and memory phenomena) may also be composition dependent, as well as being dependent upon kinetics. Structural studies show that phase segregation above T_g is dependent upon kinetics as well as upon temperature. Thermal, structural, and electrical data give evidence that As₂Te₃ exists as a unit in the non-crystalline state. This structural unit, stable at high temperatures, is present in the molten material and is thought to be present as the supercooled liquid is quenched. The short-range order extends to long-range order upon devitrification and the first crystalline phase detected is monoclinic As₂Te₃. Apparently metastable under the conditions of formation, this phase converts to a previously unreported fcc phase upon further heat treatment. Similar crystalline structures are known to be associated with thermally- and electrically-induced memory phenomena in AsTeGe and AsTeI glasses. Density measurements at room temperature show that (1) there are no phase miscibility gaps in the glass substructure or different crystalline phases segregating as the I content is varied, and (2) there is a large change in molar volume with increasing I concentration, whereas the molar weight does not change significantly. Electrical conductivity, σ, data in the region around room temperature, show that the σ₀ values, determined from σ = σ₀e^−E/kt, range from 10² to 10³ (Ω-cm)⁻¹. A possible dependence of σ₀ on I content may be due to changes in molar volume. The more homogeneous glasses appear to show breakpoints in the σ versus 1/T data; the corresponding changes in E are only 0.02–0.06 eV, with E = 0.04 eV being most frequently observed. For As₅₀Te_50−xI_x glasses, σ at a given temperature increases as the iodine concentration is increased to about 5 at % and then decreases with further increase in I content. Correspondingly there is a minimum in the E versus I concentration data at I ∼ 5 at %. Results suggest that dependence of the σ breakpoints on glass homogeneity and the variation of E with I concentration may be due to trapping effects.     

Effect of tellurium deposition rate on the properties of Cu–In–Te based thin films and solar cells

To investigate the effects of tellurium (Te) deposition rate on the properties of Cu–In–Te based thin films (Cu/In=0.30–0.31), the films were grown on both bare and Mo-coated soda-lime glass substrates at 200 °C by co-evaporation using a molecular beam epitaxy system. The microstructural properties were examined by means of scanning electron microscopy and X-ray diffraction. The crystalline quality of the films was improved with increase in the deposition rate of Te, and exhibited a single CuIn₃Te₅ phase with a highly preferred (1 1 2) orientation. Te-deficient film (Te/(Cu+In)=1.07) grown with a low Te deposition rate showed a narrow bandgap of 0.99 eV at room temperature. The solar cell performance was affected by the deposition rate of Te. The best solar cell fabricated using CuIn₃Te₅ thin films grown with the highest deposition rate of Te (2.6 nm/s) yielded a total area (0.50 cm²) efficiency of 4.4% (V_oc=309 mV, J_sc=28.0 mA/cm², and FF=0.509) without light soaking.