Dielectric relaxation behavior of CdS nanoparticles and nanowires

The dielectric relaxation and scaling behavior of CdS nanoparticles and nanowires were investigated in the frequency range 10²–10⁶ Hz and in the temperature range 373–573 K by complex impedance spectroscopy and electric modulus spectroscopy. Studies on the complex permittivity revealed that the dielectric relaxation in CdS nanostructures deviates from Debye like behavior. A detailed study on the grain and grain boundary charge transport was carried out. The charge carrier transport in CdS nanostructures was identified to be hopping of polarons. From the combined analysis of the variation of imaginary part of electric modulus and complex impedance with frequency, it was found that at high temperatures localized conduction is dominant in CdS nanoparticles where as the long range hopping process is dominant with nanowires. It was also found that the scaling behavior of CdS nanoparticles varied considerably from that reported earlier.     

The Poole-Frenkel conduction mechanismin Mo-Cu2O-Au thin film structures

Thin films of cuprous oxide (4.6 μm) were electrodeposited on molybdenum. Gold contacts were vacuum evaporated on the films to form devices. These films showed relatively low electrical resistivities at around 10⁶ Ω cm and a charge transport mechanism which is different from the space charge limited current conduction previously reported for the 10¹¹ Ω cm films. The charge transport mechanism in these films was determined by isothermal measurements of the devices current-voltage (I–V) characteristics at some selected temperatures in the range of 78–321 K. In this temperature range the dominant transport mechanism can be explained by the Poole-Frenkel effect through the relation I = VG₀exp(−φ_0L/kT)exp(B_LV^1/2)+I₀exp(−φ_0H/kT)exp(B_HV^1/2) where the numerical values of the parameters are measured. φ_0L = 0.12 eV is the zero-field ionization energy of a shallow acceptor-type level (measured from the edge of the valence band) which has the dominant effect in the range of 78–230 K. Similarly φ_0H = 0.70 eV corresponds to a deep level dominant in the high-temperature range 230–321 K. In the high-temperature region a 2.7 μm thick hole accumulation layer forms beneath the oxide-gold interface, assuming the ionized deep level is doubly charged. This revised version was published online in July 2006 with corrections to the Cover Date.     

Development of nanostructured CdS sensor for H2S recognition: structural and physical characterization

In the present work, we report on the performance of a nanostructured CdS gas sensor. The sensor was fabricated using spin coating technique on glass substrate. The CdS sensor was characterized for their, structural microstructural as well as optoelectronic and H₂S response was studied. The XRD analysis showed formation of nanocrystalline CdS. Morphological analysis using SEM revealed nanostructured morphology with average grain size in the range of 40–50nm. Optical investigations showed a high absorption coefficient (10⁴ cm⁻¹) with a direct band gap of 2.54 eV. Electrical transport studies revealed films shows n-type conduction mechanism with room temperature dc electrical conductivity 10⁻⁶ (Ω cm)⁻¹. The CdS sensors showed the maximum response of 13.2% upon exposure to 100 ppm H₂S at operating temperature 100 °C.     

Branched chain polymeric metal complexes containing Co(II) or Ni(II) complexes with a donor–π–acceptor architecture: synthesis,characterization, and photovoltaic applications

Four donor–π–acceptor type polymeric metal complexes (PCo–F, PCo–B, PNi–F, and PNi–B) with Co(II) or Ni(II) complexes in the branched chain were synthesized by the Heck coupling and utilized as dyes for dye-sensitized solar cells (DSSCs). The structures, photophysical, electrochemicals, and thermal properties of the four dyes were investigated in detail, and the results showed that dye containing Ni(II) complex and alkoxy benzene unit benefited the generation of photocurrent and the open-circuit voltages. The polymeric metal complexes possess good thermal stability and exhibit good solubility in common organic solvents such as chloroform, THF, and toluene. The maximal power conversion efficiency of 1.21% (J _sc = 2.49 mA/cm², V _oc = 0.695 V, FF = 0.59) was obtained with a DSSCs based on PNi–B dye under simulated air mass 1.5 G solar irradiation.     

Structural and electrical studies of Cd<Subscript>1−x</Subscript>Zn<Subscript>x</Subscript> Te thin film by vacuum evaporation technique

Cd_1−xZn_xTe (where x = 0.02, 0.04, 0.06, 0.08) thin film have been deposited on glass substrate at room temperature by thermal evaporation technique in a vacuum at 2 × 10⁻⁵ torr. The structural analysis of the films has been investigated using X-ray diffraction technique. The scanning electron microscopy has been employed to know the morphology behaviour of the thin films. The temperature dependence of DC electrical conductivity has been studied. In low temperature range the thermal activation energy corresponding to the grain boundary—limited conduction are found to be in the range of 38–48 μeV, but in the high temperature range the activation energy varies between 86 and 1.01 meV. The built in voltage, the width of the depletion region and the operating conduction mechanism have been determined from dark current voltage (I–V) and capacitor-voltage (C–V) characteristics of Cd_1−xZn_xTe thin films.     

Effect of deposition conditions on the InP thin films prepared by spray pyrolysis method

InP thin films were prepared by spray pyrolysis technique using aqueous solutions of InCl₃ and Na₂HPO₄, which were atomized with compressed air as carrier gas. The InP thin films were obtained on glass substrates. Thin layers of InP have been grown at various substrate temperatures in the range of 450–525°C. The structural properties have been determined by using X-ray diffraction (XRD). The changes observed in the structural phases during the film formation in dependence of growth temperatures are reported and discussed. Optical properties, such as transmission and the band gap have been analyzed. An analysis of the deduced spectral absorption of the deposited films revealed an optical direct band gap energy of 1.34–1.52 eV for InP thin films. The InP films produced at a substrate temperature 500°C showed a low electrical resistivity of 8.12 × 10³ Ω cm, a carrier concentration of 11.2 × 10²¹ cm⁻³, and a carrier mobility of 51.55 cm²/Vs at room temperature.     

Surface and grain-boundary energies as well as surface mass transport in polycrystalline yttrium oxide

The sessile drop technique has been used to measure the temperature dependence of the contact angle, θ, of the liquid metals Ag and Cu in contact with polycrystalline yttrium oxide (yttria, Y₂O₃) at the temperature range 1,333–1,773 K in Ar/4%H₂ atmosphere. Combination of the experimental results with literature data taken for nonwetted and nonreactive oxide/liquid metal systems permit the calculation of the surface energy of Y₂O₃ as γ_sv (J/m²) = 2.278–0.391 × 10⁻³ T. For the same atmospheric conditions, thermal etching experiments on the grain boundaries intersecting the surface of the polycrystalline ceramic allow to determine the groove angles, ψ, with respect to temperature and time as well as the grain-boundary energy of Y₂O₃ as γ_ss (J/m²) = 1.785–0.306 × 10⁻³ T. Grain-boundary grooving studies on polished surfaces of Y₂O₃ annealed in Ar/4%H₂ atmosphere between 1,553 K and 1,873 K have shown that surface diffusion is the dominant mechanism for the mass transport. The surface diffusion coefficient can be expressed according to the equation D _s (m²/s) = 1.22 × 10⁻³ exp(−343554/RT).     

Strain-induced artificial multiferroicity in Pb(Zr<Subscript>0.53</Subscript>Ti<Subscript>0.47</Subscript>)O<Subscript>3</Subscript>/Pb(Fe<Subscript>0.66</Subscript>W<Subscript>0.33</Subscript>)O<Subscript>3</Subscript> layered nanostructure at ambient temperature

Layered nanostructures (LNs) of the commercial ferroelectric Pb(Zr_0.53Ti_0.47)O₃ (PZT) and the natural ferroic relaxor Pb(Fe_0.66W_0.33)O₃ (PFW) were fabricated with a periodicity of PZT/PFW/PZT (~5/1/5 nm, thickness ~250 nm) on MgO substrates by pulsed laser deposition. The dielectric behavior of these LNs were investigated over a wide range of temperatures and frequencies, observing Debye-type relaxation with marked deviation at elevated temperatures (>400 K). High dielectric constant and very low dielectric loss were observed below 100 kHz and 400 K, whereas the dielectric constant decreases and loss increases with increase in frequency, similar to relaxor ferroelectrics. Asymmetric ferroelectric hysteresis loops across UP and DOWN electric field were observed with high remanent polarization (P_r) of about 33 μC/cm². High imprint (~5–7 V across 250 nm thin films) were seen in ferroelectric hysteresis that may be due to charge accumulation at the interface of layers or significant amount of strain (~3.21) across the layers. Room temperature ferromagnetic hysteresis was observed with remanent magnetization 5.32 emu/cc and a coercive field of ~550 Oe. Temperature and field dependent leakage current densities showed very low leakage ~10⁻⁷–10⁻⁵ A/cm² over 500 kV/cm. We observed imprint in hysteresis that may be due to charge accumulation at the interface of layers or active role of polar nano regions (PNRs) situated in the PFW regions.     

Conduction mechanism in doped polymethyl methacrylate (PMMA) films

The electrical transport behaviour of ferrocene mixed poly (methyl methacrylate) (PMMA) films (≈ 20 μm in thickness) deposited by the isothermal immersion technique has been studied in the temperature range of 333–373 K and field from (2·0–4·0)×10⁴ V/cm. It has been found that at higher fields and temperatures, the observed conduction behaviour could be consistently described by the Richardson-Schottky emission. The increase in current due to doping has been attributed to the formation of charge transfer complexes. The dopant molecules act as an additional trapping centre and provide a link between polymer molecules in amorphous region leading to the formation of charge transfer complex.     

Photoresponse of n -ZnO/ p -Si heterojunction towards ultraviolet/visible lights: thickness dependent behavior

A series of n-ZnO/p-Si thin film heterojunctions have been fabricated by a low cost sol–gel technique for different ZnO film thicknesses and the dark as well as photo current–voltage (I–V) characteristics have been investigated in details. The heterojunction with ZnO thickness of 0.46 μm shows the best diode characteristics in terms of rectification ratio, I _F/I _R = 5.7 × 10³ at 5 V and reverse leakage current density, J _R = 7.6 × 10⁻⁵ A cm⁻² at −5 V. From the photo I–V curves and wavelength dependent photocurrent of the heterojunctions, it is found that the junction with 0.46 μm ZnO thickness shows the highest sensitivity towards both UV and visible lights.     

Electrical studies on sputtered CuCl thin films

CuCl is a wide-direct band gap semiconductor, lattice matched to Si and it possesses excellent ultra violet (UV) emission properties. It is thus a promising candidate for the next generation Si based UV optoelectronics. CuCl films were deposited using RF magnetron sputtering technique. X-ray diffraction analysis reveals that the grains are strongly <111> oriented. Triangular crystallites of CuCl were observed in the AFM surface topograph. Au–CuCl–Si–Au structures were fabricated and field dependent electrical studies were carried out in the electric field range of 1.25 × 10⁶ to 2.5 × 10⁷ V/m. I–V characteristics show that ohmic conduction prevails in low electric fields up to 2.5 × 10⁶ V/m. In the higher field range, from 2.5 × 10⁶ to 2.5 × 10⁷ V/m, the conduction mechanism was Schottky emission controlled. There was no trap related charge transport observed at higher electric fields. Preliminary electrical studies are reported in this article.     

Current–voltage characteristics and impedance spectroscopy of LiNbO<Subscript>3</Subscript> films grown by RF magnetron sputtering

Polycrystalline LiNbO₃ films with random orientation of grains on (001)Si substrates have been grown by RF magnetron sputtering method. Electrical conductance of the formed (001)Si–LiNbO₃ heterostructures is defined through hopping mechanism by charge localization centers (CLC) in the band gap of LiNbO₃ with concentration N _t  = 2.3 × 10²⁴ m⁻³. Analysis of the impedance frequency spectrum has disclosed two relaxation processes of Maxwell–Wagner type with relaxation times τ₁ = 0.1 s and τ₂ = 1 × 10⁻⁴ s. Thermal annealing at T = 650 °C leads to an increase in the average grain size from 50 to 95 nm; it also leads to a decrease in the CLC concentration down to N _t  = 2.8 × 10²⁰ m⁻³. Electrical conductance of (001)Si–LiNbO₃ heterostructures after thermal annealing is determined by space charge limited conduction mechanism. There have been defined parameters of dielectric hysteresis loops. It has been demonstrated that thermal annealing leads to a decrease in values of remanent polarization and coercive field.     

MOS capacitors with metal gate/high-k dielectrics on GaAs bulk substrate

This paper investigates the electrical characteristics at low temperatures through C–V, I–V and conductance measurements to understand the interface behavior of HfO₂ and p-type GaAs bulk substrate. Room temperature interface state density, D _it , estimated for as-deposited Ti–Au/HfO₂/GaAs capacitors was found to be 3.68 × 10¹¹ cm⁻² eV⁻¹. Low temperature measurement suggests that only fast interface states contribute to the conduction process. When the characteristics of two different metal gates were compared, the accumulation capacitance density observed to be 1.4 and 8.98 fF/μm² for Be–Au/HfO₂/GaAs and Ti–Au/HfO₂/GaAs, respectively at 1 MHz.     

Interfacial interaction of solid cobalt with liquid Pb-free Sn–Bi–In–Zn–Sb soldering alloys

Dissolution kinetics of cobalt in liquid 87.5%Sn–7.5%Bi–3%In–1%Zn–1%Sb and 80%Sn–15%Bi–3%In–1%Zn–1%Sb soldering alloys and phase formation at the cobalt–solder interface have been investigated in the temperature range of 250–450 °C. The temperature dependence of the cobalt solubility in soldering alloys was found to obey a relation of the Arrhenius type c _s = 4.06 × 10² exp (−46300/RT) mass% for the former alloy and c _s = 5.46 × 10² exp (−49200/RT) mass% for the latter, where R is in J mol⁻¹ K⁻¹ and T in K. For tin, the appropriate equation is c _s = 4.08 × 10² exp (−45200/RT) mass%. The dissolution rate constants are rather close for these soldering alloys and vary in the range (1–9) × 10⁻⁵ m s⁻¹ at disc rotational speeds of 6.45–82.4 rad s⁻¹. For both alloys, the CoSn₃ intermetallic layer is formed at the interface of cobalt and the saturated or undersaturated solder melt at 250 °C and dipping times up to 1800 s, whereas the CoSn₂ intermetallic layer occurs at higher temperatures of 300–450 °C. Formation of an additional intermetallic layer (around 1.5 μm thick) of the CoSn compound was only observed at 450 °C and a dipping time of 1800 s. A simple mathematical equation is proposed to evaluate the intermetallic-layer thickness in the case of undersaturated melts. The tensile strength of the cobalt-to-solder joints is 95–107 MPa, with the relative elongation being 2.0–2.6%.     

Effect of CdS nanoparticles on photoluminescence spectra of Tb<Superscript>3 + </Superscript> in sol–gel-derived silica glasses

CdS nanoparticles doped with Tb^3 + were synthesized by sol–gel technique. The influence of CdS on the Tb^3 + glass was studied by UV-Visible and luminescence spectroscopy. The luminescence intensity of the glasses increased significantly in the presence of CdS nanoparticles. Terbium ions excited into the ⁵ D ₃ level have a rich emission spectrum in the 400–700 nm range decaying to different ⁷ F _J levels. The intensity of violet and blue luminescence from ⁵ D ₃ level is highly dependent on Tb^3 + concentration, on presence of CdS co-dopant and annealing conditions.     

Magnetic and electrical properties of ordered 112-type perovskite LnBaCoMnO<Subscript>5+δ</Subscript> (Ln = Nd,Eu)

Investigation of the oxygen-deficient 112-type ordered oxides of the type LnBaCoMnO_5+δ (Ln = Nd, Eu) evidences certain unusual magnetic behavior at low temperatures, compared to the LnBaCo₂O_5+δ cobaltites. The ordered NdBaCoMnO_5.9 depicts a clear paramagnetic to antiferromagnetic type transition around 220 K, whereas for EuBaCoMnO_5.7 one observes an unusual magnetic behavior below 177 K which consists of ferromagnetic regions embedded in an antiferromagnetic matrix. The existence of two sorts of crystallographic sites for Co/Mn and their mixed valence states favor the ferromagnetic interaction, whereas antiferromagnetism originates from the Co³⁺–O–Co³⁺ and Mn⁴⁺–O–Mn⁴⁺ interactions. Unlike the parent compounds, the present Mn-substituted phases do not exhibit prominent magnetoresistance effects in the temperature range 75–400 K.     

Dependence of efficiency of thin-film CdS/CdTe solar cell on parameters of absorber layer and barrier structure

Dependences of the open-circuit voltage, short-circuit current, fill factor, and efficiency of a CdS/CdTe solar cell on the resistivity and thickness of the p-CdTe absorber layer, the noncompensated acceptor concentration N_a-N_d, and carrier lifetime τ in CdTe, are investigated, and optimization of these parameters in order to improve the solar cell efficiency is performed. It has been shown that the observed low efficiency of CdS/CdTe solar cells is caused by the too short electron lifetime in the range of 10^− 10-10^− 9 s and too thin (3-5 µm) CdTe layer currently used for fabrication of CdTe/CdS solar cells. To achieve an efficiency of 28-30%, the resistivity and thickness of the CdTe absorber layer, the noncompensated acceptor concentration, and carrier lifetime should be ∼ 0.1 Ω·cm, ≥ 20-30 µm, ≥ 10¹⁶ cm^− 3, and ≥ 10^− 6 s, respectively.     

Comparison of charge distributions in CIGS thin-film solar cells with ZnS/(Zn,Mg)O and CdS/i-ZnO buffers

The commonly used CdS/i-ZnO buffer system in Cu(In,Ga)Se₂ (CIGS) thin-film solar cells was substituted by ZnS/(Zn,Mg)O. ZnS has a higher transmission in the short wavelength range due to the higher bandgap energy E_g = 3.7 eV compared to CdS with E_g = 2.4 eV. Unfortunately, in our experiments the resulting gain in short-circuit current density j_SC as the result of reduced absorption losses in the blue wavelength region is mostly accompanied by a decrease in open-circuit voltage V_OC of the devices with ZnS buffer. This contribution discusses possible explanations for the systematically lower open-circuit voltages of the devices with a ZnS buffer layer.The carrier collection properties of the devices with a ZnS buffer were investigated by electron beam induced current measurements in the junction configuration. The maximum of the collection probability for ZnS cells is located in the CIGS bulk and not near the buffer/CIGS interface like for solar cells with CdS buffer. Additionally, we observed a larger space charge width compared to devices with a CdS buffer. This finding concurs with the considerably lower capacitance values and also lower charge densities in ZnS-buffered devices, as determined by capacitance voltage measurements.Based on these findings, the main reason for the lower open-circuit voltages of our ZnS devices is that the charge densities are lower than for the CdS/i-ZnO cells.     

Novel direct MOCVD growth of In x Ga1−x As and InP metamorphic layers on GaAs substrates

A review of the technique of direct growing high-quality In_xGa_1−xAs or InP buffer layers on GaAs substrates by metal-organic chemical vapor deposition (MOCVD) is given. This low-temperature growth method benefits the improvement of metamorphic device performance. In this work, a simple and novel method of directly deposited thin In_xGa_1−xAs or InP buffer layers (< 1 μm) on GaAs substrates is presented, instead of strained-layer superlattice, two-step, graded or CS (compliant substrates) methods, while maintaining low dislocations, high crystal quality, and uniform and mirror like surfaces. For the direct growth technique of In_xGa_1−xAs on GaAs, we found an excellent quality In_0.54Ga_0.46As buffer of rms surface roughness of only 0.686 nm by AFM and FWHM of 925 arcsec by XRD can be obtained at a low growth temperature of 440^∘C with a constant Ga/In_gas partial pressure of 5. The superior results are mainly due to the use of low temperature growth technology. In addition, we also found this growth technology is available to directly grow InP buffer layers on GaAs. Experimental results conclude that the growth temperature of 480^∘C in harmony with the V/III ratios range of 130–210 is a suitable window to directly grow InP on GaAs substrates.     

Structure and piezoelectric properties of new (Bi0.5Na0.5)1?x?yBax(Yb0.5Na0.5)yTiO3 lead-free ceramics

New lead-free ceramics (Bi_0.5Na_0.5)_1−x−yBa_x(Yb_0.5Na_0.5)_yTiO₃ (x = 0.02–0.10 and y = 0–0.04) have been prepared by an ordinary sintering technique and their structure and piezoelectric properties have been studied. X-ray diffraction shows that Ba²⁺ and Yb³⁺ diffuse into the Bi_0.5Na_0.5TiO₃ lattices to form a solid solution with a pure perovskite structure and a morphotropic phase boundary (MPB) between rhombohedral and tetragonal phases is formed at 0.04 < x < 0.10. The partial substitutions of Ba²⁺ and Yb³⁺ for A-site ions of Bi_0.5Na_0.5TiO₃ decrease effectively the coercive field E _c and improve significantly the remanent polarization P _r. The ceramics with x = 0.06 and y = 0–0.02 situate within the MPB and possess the lower E _c and larger P _r, and thus exhibit optimum piezoelectric properties: d ₃₃ = 155–171 pC/N and k _p = 29.2–36.7%. The temperature dependences of the dielectric and ferroelectric properties suggest that the ceramics may contain both the polar and non-polar regions at temperatures near/above T _d.