Properties of fluorine doped ZnO thin films deposited by magnetron sputtering

Fluorine doped ZnO (FZO) films were deposited on Corning glass by radio frequency (rf) magnetron sputtering of pure ZnO target in CF₄ containing gas mixtures, and the compositional, electrical, optical, and structural properties of the as-grown films as well as the vacuum-annealed films were investigated. The fluorine content in FZO films increased with increasing CF₄ content in sputter gas. FZO films deposited at elevated temperature of 150 °C had considerably lower fluorine content and showed a poorer electrical properties than the films deposited at room temperature. Despite high fluorine contents in the films, for all the FZO films, the carrier concentration remained below 2×10²⁰ cm⁻³, leading to fairly low doping efficiency level. Vacuum-annealing of the FZO films deposited at room temperature resulted in substantial increase of Hall mobilities, reaching as high as 43 cm²/Vs. This was attributed partly to the removing of oxygen vacancies and/or the forming chemical bonds with interstitial zinc atoms by fluorine interstitials and partly to the passivation effect of excess fluorine atoms by filling in the dangling bonds at the grain boundaries. For all the films with thickness of around 300 nm, the optical transmissions in visible were higher than 80%, and increased with increasing fluorine content up to 85% for the film with highest fluorine content.     

Solvent-dependent growth of sprayed FTO thin films with mat-like morphology

Highly textured fluorine-doped tin oxide thin films have been deposited using cost-effective spray pyrolysis technique. Precursor solution for spraying is prepared with solvents viz. methanol, ethanol, propane-2-ol and distilled water. The optical, structural, morphological and electrical properties of thin films have been studied. X-ray diffraction studies revealed polycrystalline tin-oxide (SnO₂) phase with tetragonal crystal structure and predominantly (2 0 0) oriented films, irrespective of solvents. The novel mat-like morphology is observed by scanning electron microscope (SEM). Hall-effect measurements revealed that the films are heavily doped degenerate semiconductor with n-type conductivity. The variations in electrical resistivity, carrier concentration and mobility with respect to spray solvents have been discussed. The typical P_FTO sample has maximum value of figure of merit (φ=6.18×10⁻² Ω⁻¹) with lowest ever-reported sheet resistance of 3.71 Ω.     

Electrical conduction studies of hot wall deposited CdSexTe1−x thin films

CdSe_xTe_1−x thin films of different compositions have been deposited on cleaned glass substrates using the hot wall deposition technique under conditions very close to thermodynamical equilibrium with minimum loss of material. The electrical conductivity of the deposited films has been studied as a function of temperature. All the films showed a transition from phonon-assisted hopping conduction through the impurity band to grain-boundary-limited conduction in the conduction/valence band at temperature around 325 K. The conductivity has been found to vary with composition; it varied from 0.0027 to 0.0198 Ω⁻¹ cm⁻¹ when x changed from 0 to 1. The activation energies of the films of different compositions determined at 225 and 400 K have been observed to lie in the range 0.0031–0.0098 and 0.0285–0.0750 eV, respectively. The Hall-effect studies carried out on the deposited films revealed that the nature of conductivity (p or n-type) was dependent on film composition; films with composition x=0 and 0.15 have been found to be p-type and the ones with composition x=0.4, 0.6, 0.7, 0.85 and 1 have been observed to exhibit n-type conductivity. The carrier concentration has been determined and is of the order of 10¹⁷ cm⁻³. The majority of carrier mobilities of the films have been observed to vary from 0.032 to 0.183 cm² V⁻¹ s⁻¹ depending on film composition. The study of the mobility of the charge carriers with temperature in the range of 300–450 K showed that the mobility increased with power of temperature indicating that the type of scattering mechanism in the studied temperature range is the ionized impurity scattering mechanism.     

Evidence of ballistic thermal transport in lithium niobate at room temperature

In ballistic transport, heat carriers such as phonons travel through the solid without any scattering or interaction. Therefore, there is no temperature gradient in the solid, suggesting zero thermal conductivity by Fourier's law. Ballistic transport is typically seen in high purity crystals at either temperatures below ~ 10 K, or physical size below ~ 100 nm, where the mean free path of the carrier is larger than the solid itself. In this letter, we show evidence of ballistic transport at room temperature in lithium niobate wafers in the in-plane and cross-plane directions under both steady state and high frequency heating that are monitored using both infrared and resistance thermometry. We report phonon mean free path in lithium niobate around 425 μm, which is about 50 times higher than the largest phonon mean free path in the literature at room temperature.     

Correlation between micro-structural properties and ionic conductivity of Li1.5Al0.5Ge1.5(PO4)3 ceramics

We report on the structure and lithium ion transport properties of Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP). This material is commercially available and is prepared as amorphous powders via a flame spray technique called Flash Creation Method (FCM). We crystallize and sinter the amorphous powders at different temperatures in order to alter grain size and grain boundary properties. The structure is then characterized by means of powder X-ray diffraction, atomic force microscopy, scanning electron microscopy and transmission electron microscopy with energy dispersive X-ray spectroscopy. AC impedance spectroscopy is used to study lithium ion transport. A maximum total conductivity of 2 × 10⁻⁴ S cm⁻¹ at room temperature is found for a sample sintered at 750 °C for 2 h. In order to distinguish between grain and grain boundary contributions to the impedance spectra, equivalent circuit fits are carried out. The results are analysed in the framework of the classical brick layer model and of a finite-element approach taking into account non-ideal grain contacts. Our experimental results for the grain and grain boundary resistances are in good agreement with the predications of the finite-element approach.     

Ultra-thin nanocrystalline lanthanum strontium cobalt ferrite (La0.6Sr0.4Co0.8Fe0.2O3−δ) films synthesis by RF-sputtering and temperature-dependent conductivity studies

Nanocrystalline lanthanum strontium cobalt ferrite (LSCF) ultra-thin films with high in-plane electrical conductivity have been deposited by RF sputtering from composite targets. The films, with nominal thickness of 54 nm, are crystalline when annealed or deposited at temperatures above 450 °C. Effects of annealing temperature, annealing time, and substrate temperature on crystallization, microstructure, and room temperature lateral electrical conductivity have been systematically studied. No interfacial reaction products between the LSCF and single crystalline yttria-stabilized zirconia (YSZ) were observed from X-ray diffraction studies upon annealing until 750 °C. In-plane electrical conductivity as high as 580 S cm⁻¹ at 650 °C has been observed for LSCF thin films deposited on single crystalline YSZ substrates and sputtered nanocrystalline YSZ thin films; while activation energy for conductivity were determined to be 0.15 eV and 0.10 eV for the former and latter films, respectively, in 650–400 °C range. The high in-plane electrical conductivity for the nanocrystalline LSCF ultra-thin films is likely attributed to their low level of porosity. Micro-solid oxide fuels cells using 15 nm thick LSCF films as cathodes and sub-100 nm yttria-doped zirconia thin film electrolytes have been fabricated successfully and demonstrated to achieve peak power density of 60 mW cm⁻² at 500 °C. Our results demonstrate that RF sputtering provides a low-temperature synthesis route for realizing ultra-thin nanocrystalline LSCF films as cathodes for intermediate- or low-temperature solid oxide fuel cells.     

Electrochemical studies on epoxidised natural rubber-based gel polymer electrolytes for lithium–air cells

Gel polymer electrolyte films comprised of 50% epoxidised natural rubber polymer host, lithium triflate salt (LiCF₃SO₃), and ethylene carbonate (EC) or propylene carbonate (PC) plasticizer are prepared using the solution-casting technique. AC impedance studies show that the electrical conductivity of the electrolytes is dependent on both the salt and plasticizer concentrations. The highest room temperature conductivity of 4.92 × 10⁻⁴ S cm⁻¹ is achieved when 10 wt.% propylene carbonate is introduced into the system containing 1.0 g 50% epoxidised natural rubber polymer doped with 35 wt.% LiCF₃SO₃. Conductivity studies of these polymer electrolytes are carried out at various temperatures and are found to obey the Vogel–Tamman–Fulcher (VTF) rule. The highest conducting plasticized sample is used as a gelled electrolyte for lithium–air cells.     

Role of substrate temperature in controlling properties of sprayed CuInS2 absorbers

Optimization of substrate temperature of spray pyrolysed CuInS₂ absorber is discussed along with its effect on the photoactivity of junction fabricated. For CuInS₂ thin films, properties like crystallinity, thickness and composition showed progressive behavior with substrate temperature. X-ray photoelectron spectroscopic depth profile of all the samples showed that the concentration of copper on the surface of the films is significantly lesser than that in the bulk thus avoiding need for toxic cyanide etching. Interestingly, samples prepared at 623 K had higher conductivity compared to those prepared above and below this temperature. Also, the low energy transition, in addition to the direct band gap which was observed in other samples were absent in films prepared at 623 K. From thermally stimulated conductivity studies it was seen that shallow levels present in this sample contribute to its improved conductivity. Also, CuInS₂/In₂S₃ bilayer prepared at this substrate temperature showed higher photoactivity than those prepared at other temperatures.     

Effects of rapid process on the conductivity of multiple elements doped ceria-based electrolyte

The citric acid-based combustion technique (SV) for powder preparation and the rapid microwave sintering (MW) process are used to lower the synthesizing temperature and to shorten the processing time then to modify the grain boundary resistance and oxygen vacancies mobility in multiple elements doped ceria-based electrolyte (LSBC). Nanoparticles of less than 50 nm with a pure fluorite structure are prepared by SV method at a low temperature of 600 °C. Microwave sintering lowers the sintering temperature to 1400 °C from the conventional sintering (CS) temperature of 1500 °C needed for solid-state (SS) prepared LSBC, and requires only 15 min of sintering time. The SV sample conventionally sintered at 1400 °C-6 h reaches a conductivity of 0.006 S cm⁻¹. When the SV samples are microwave sintered at 1400 °C-15 min, they achieve a conductivity as high as 0.01 S cm⁻¹ measured at 600 °C. Microwave sintering reduces the grain boundary resistance of both SS and SV samples. The migration enthalpy (H_m) of 0.66 eV in the SS-MW and SV-MW samples is similar to that of the fully densified SS-CS sample. The Schottky barrier height can be adjusted by SV powder preparation and by the MW process using a slightly lower sintering temperature and with a shorter processing time for multiple elements doped solid electrolyte.     

Surface texturing of sputtered ZnO:Al/Ag back reflectors for flexible silicon thin-film solar cells

Al-doped zinc oxide/silver (ZnO:Al/Ag) back reflectors for silicon thin-film solar cells with an n-i-p configuration were prepared on flexible stainless steel substrates by dc magnetron sputtering. The surface morphologies of the back reflectors were modified by changing the deposition temperature of the Ag films to improve the light-scattering properties on the back reflectors, resulting in the enhancement of the light-trapping effect in the solar cells. By elevating the deposition temperature from room temperature to 500 °C, the surface roughness of the Ag films increased from 6.62 to 46.64 nm. The films at 500 °C had coarse surface features with irregular grain size distributions between 200 and 900 nm, whereas the films produced at low temperatures below 100 °C had smooth surfaces consisting of small grains between 100 and 200 nm. Even after the 100-nm thick ZnO:Al films were deposited on the modified Ag surfaces, the surface microstructure of the ZnO:Al/Ag bilayers was similar to that of the Ag films. The surface roughness of bilayers increased from 7.12 to 39.30 nm with coarsening the Ag surfaces. Haze factor (a ratio of diffuse reflectance to total reflectance) of Ag films was enhanced remarkably from 59% to 74% in a wide wavelength range from 350 to 1100 nm with increasing the surface roughness of the Ag films from 6.62 nm to 46.64 nm. Enhancement in the haze factor was due to the increase of diffuse reflectance on the Ag films, because the total reflectance did not change much with increasing surface roughness of the Ag films. This increasing roughness indicated that the light scattering from the rough surface of the back reflectors improved. The enhanced light scattering from the back reflectors influenced the performance of the solar cells mainly in terms of the short-circuit current density (J_sc). Compared to the back reflectors with smooth surface features, leading to a J_sc value of 9.94 mA/cm², the back reflectors with large surface roughness improved the J_sc value of the solar cells to 13.36 mA/cm² without detrimental changes in the fill factor (FF) and open circuit voltage (V_oc); they eventually increased the conversion efficiency of the solar cells from 5.59% to 7.60%.     

Synthesis and characterization of nanosized lithium manganate and its derivatives

Spinel lithium manganese oxide, LiMn₂O₄ and its derivatives are prepared by the sol–gel method. The lattice constant of the pure material is calculated as 8.23 Å. Different transition metal cations of chromium, iron, cobalt, nickel, copper and zinc (0.05 and 0.15 M) are doped in place of manganese in the LiMn₂O₄. X-ray powder diffraction data show that the spinel framework preserved its integrity upon doping. Formation of a single phase and the purity of the samples are confirmed by X-ray powder diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR). The crystallite size of the samples is calculated by use of the Scherrer formula and is found to be within a range of 43–66 nm. The electrical conductivity of the samples is determined over a temperature range of 200–300 K by means of four-point probe method. An increasing trend of conductivity with increase in temperature is noted for all the samples. The parent compound LiMn₂O₄ has a conductivity value of 3.47 × 10⁻⁴ ohm⁻¹ cm⁻¹ at room temperature. This value increases on doping with the above-mentioned transition metal cations.     

Experimental investigations of an ionic-liquid-based,magnesium ion conducting,polymer gel electrolyte

Studies on a novel magnesium ion conducting gel polymer electrolyte based on a room temperature ionic liquid (RTIL) is reported. It comprises a Mg-salt, Mg(CF₃SO₃)₂ [or magnesium triflate, Mg(Tf)₂] solution in an ionic liquid, 1-ethyl-3-methylimidazolium trifluoro-methanesulfonate (EMITf), immobilized with poly(vinylidene fluoride-hexafluoropropylene) (PVdF-HFP), which is a freestanding, semitransparent and flexible film with excellent mechanical strength. Physical and electrochemical analyses demonstrate promising characteristics of these films, suitable as electrolytes in rechargeable magnesium batteries. The material offers a maximum electrical conductivity of ∼4.8 × 10⁻³ S cm⁻¹ at room temperature (20 °C) with excellent thermal and electrochemical stabilities. Possible conformational changes in the polymer host PVdF-HFP due to ionic liquid solution entrapment and ion–polymer interaction are investigated by Fourier transform infra-red (FTIR), X-ray diffraction (XRD) and scanning electron microscopic (SEM) methods. The Mg²⁺ ion transport in the gel film is confirmed from cyclic voltammetry, impedance and transport number measurements. The Mg²⁺ ion transport number (t₊) is ∼0.26, which indicates a substantial contribution of triflate anion transport along with ionic conduction due to the component ions of the ionic liquid.     

Dynamics of hydrogen molecules in the channels of binary THF-H2 clathrate hydrate and its physicochemical significance on hydrogen storage

We have studied the dynamics of the H₂ loaded THF (tetrahydrofuran) clathrate hydrate by using an inelastic neutron scattering technique. At 2 K, a well defined single peak indicative of the ortho (J = 1) to para (J = 0) hydrogen transition is observed at 13.6 meV. The neutron spectrum undergoes two distinct changes as the temperature increases. First, additional peaks appear from splitting of the rotational transition above 10 K. Upon further heating, a quasi-elastic neutron scattering (QENS) signal due to hydrogen molecular diffusion through the THF hydrate crystalline lattice grows significantly above 65 K. These results indicate that H₂ molecules trapped inside the THF hydrate crystalline structure undergo two distinct transformations in their dynamical behavior.     

Carrier transport and structural properties of polysilicon films prepared by layer-by-layer technique

Polycrystalline silicon (poly-Si) films have been deposited on glass substrates by a layer-by-layer technique at very low temperature, 300°C, using fluorinated precursors. The electronic transport was characterized by Hall effect and conductivity measurements over a wide temperature range, 100 K to 400 K. The structure of the materials is a function of the film thickness. The measured Hall mobility increases as the thickness increases. The Hall mobility Arrhenius plot shows linear dependence with a negative slope over the temperature range examined, suggesting that carrier conduction is limited by grain boundary barriers.     

Structural, optical and luminescent characteristics of sprayed fluorine-doped In2O3 thin films for solar cells

Fluorine-doped indium oxide thin films, F-In₂O₃, prepared by the spray pyrolysis technique on glass substrates have been studied using cathodoluminescence spectroscopy, X-ray diffraction and spectrophotometry. These films, deposited at the optimal substrate temperature (T_s=450 °C), crystallize in a cubic structure with a preferential orientation along the (4 0 0) direction. For this temperature, the electrical resistivity is in the order of 6×10⁻³ Ω cm and the average optical transmission in the visible region is larger than 95%. At room temperature, the cathodoluminescence spectra of F-In₂O₃ present two emission peaks: blue indirect band gap peak at 410 nm and a red emission at 650 nm.     

Electroless Pd membrane deposition on alumina modified porous Hastelloy substrate with EDTA-free bath

This study demonstrates palladium membranes can be electrolessly plated on aluminum oxide-modified porous Hastelloy with hydrazine using an EDTA-free bath. The plating bath temperature affected the membrane surface morphology, with the palladium grain size increasing with increasing temperature. A 7.5 μm thick membrane plating was obtained at room temperature. Helium leak testing confirmed that the membrane was free of defects. Hydrogen permeation test showed that the membrane had a hydrogen permeation flux of 3.3 × 10⁻¹ mol m⁻² s⁻¹ at a temperature of 823 K and at a pressure difference of 100 kPa. There was no measurable interdiffusion between the membrane film and the porous Hastalloy substrate at 823 K. This room temperature membrane plating method provides several advantages such as very high selectivity, stability, favorable energy efficiency and simplicity.     

Influence of the growth parameters of p-CdTe thin films on the performance of Au-Cu/p-CdTe/n-CdO type solar cells

R.F. sputter deposition of Sb doped CdTe thin films was carried out with targets containing different amounts of antimony (C_T: 0, 2.5, 10 and 20 at.%). The substrates were kept at different temperatures (T_s) of 200, 275, 350 and 450 °C. Three different argon pressure values: 2.5, 5 and 15 mTorr were used. The lowest dark resistivity (ρ) at room temperature (RT) was 9.0 × 10⁵ Ω cm, which is one of the lowest values reported in the literature for Sb doped CdTe. Highly transparent (∼90%) and conductive (ρ = 3.7 × 10⁻⁴ Ω cm) F doped CdO (n-type) thin films, prepared at room temperature by the sol-gel method, were employed as window and top-contact. The configuration of the fabricated solar cell was (Au-Cu)/p-CdTe/n-CdO/glass. Open-circuit voltage (V_oc) and short-circuit current density (J_sc) at room temperature have the highest values for high T_s, low P_g and C_T = 10 at.%. Despite the fact that V_oc and J_sc are lower than those reported in the literature, we think this work is useful as a basis for the search of more competitive CdTe/CdO based PV devices.     

High quality ZnO and Ga:ZnO thin films grown onto crystalline Si (1 0 0) by RF magnetron sputtering

Undoped and 2% Ga-doped ZnO films have been deposited by RF magnetron sputtering onto single crystal Si (1 0 0) substrates equivalent to the commercial Si solar cells. The same films were also grown on amorphous silica substrates to complete their characterization. The films have been characterized by X-ray diffraction, electrical and optical measurements, X-ray photoelectron spectroscopy, Raman microspectroscopy and scanning and high-resolution transmission electron microscopy. Films present a very good quality crystalline wurtzite structure with the c-axis perpendicular to the substrate, with continuity of the (0 0 0 2) planes along the whole film, as shown by transmission electron microscopy. The doped sample shows an increase of two orders of magnitude of the electrical conductivity, an optical transmittance bigger than 85% along the visible spectrum, a diminution of the grain size in the direction parallel to the substrate and a lower surface roughness. The Ga-cations act only as substitutional impurities, they are homogeneously distributed in the whole film, maintaining the wurtzite structure and increasing the carrier density. The formation of any spurious phase or segregation of Ga₂O₃ clusters that can act as carrier traps can be discarded. The characterization results allow us to conclude that the doped film has improved electrical and optical properties with respect to the undoped one. Therefore, the Ga-doped films are very suitable candidates as transparent conducting electrodes for solar cells, displays and other photoelectronic devices.     

Phosphorus-doped glass proton exchange membranes for low temperature direct methanol fuel cells

Phosphorus-doped silicon dioxide thin films were used as ion exchange membranes in low temperature proton exchange membrane fuel cells. Phosphorus-doped silicon dioxide glass (PSG) was deposited via plasma-enhanced chemical vapor deposition (PECVD). The plasma deposition of PSG films allows for low temperature fabrication that is compatible with current microelectronic industrial processing. SiH₄, PH₃ and N₂O were used as the reactant gases. The effect of plasma deposition parameters, substrate temperature, RF power, and chamber pressure, on the ionic conductivity of the PSG films is elucidated. PSG conductivities as high as 2.54 × 10⁻⁴ S cm⁻¹ were realized, which is 250 times higher than the conductivity of pure SiO₂ films (1 × 10⁻⁶ S cm⁻¹) under identical deposition conditions. The higher conductivity films were deposited at low temperature, moderate pressure, limited reactant gas flow rate, and high RF power.     

Lifetime and recombination kinetics in a-Se thin films

Modulated photoconductivity measurements in amorphous selenium (a-Se) thin films were carried out. Especially, photocarrier lifetime as a function of applied electric field (d.c.) and temperature was determined by using the quadrature frequency-resolved spectroscopy (QFRS) method. At low temperature, two different carrier lifetime channels were observed. However, only one carrier lifetime channel was dominated at room temperature (297 K). The temperature dependence of the frequency-resolved photocurrent (FRPC) was investigated under different applied electric fields. At high temperatures, a small field independent activation energy value of 147 ± 35 meV was determined, in which hole transport is controlled by the valence band-tail states. The exponent ν in the power-law relationship (I_ph∝G^ν) between generating flux and photocurrent was obtained at different electric fields and excitation wavelengths. The value of ν increases very little with decreasing applied electric fields. However, ν shows a little stronger dependence on the excitation wavelengths. The applied electric field dependences of photocurrent at different excitation wavelengths were also directly measured. However, a little non-ohmic behaviour was observed at high applied electric fields and at low excitation wavelengths measured. We explained it in the frame of traditional models.