Comparative study of sputtered and electrodeposited CI(S,Se) and CIGSe thin films

Copper indium disulphide CuInS₂ (CIS) and diselenide CuInSe₂ (CISe) and their alloys with gallium CuIn_1 − xGa_xSe₂ (CIGSe) thin films have been prepared using both high- and non-vacuum processes. The well known two-stage process consisting in a sequential sputtering of Cu and In thin layers and a subsequent sulfurisation has led to the formation of good quality CuInS₂ ternary compound. The films exhibit the well known chalcopyrite structure with a preferential orientation in the (112) plane suitable for the production of the efficient solar cells. The absorption coefficient of the films is higher than 10⁴ cm^− 1 and the band gap value is about 1.43 eV. A non-vacuum technique was also used. It consists on a one step electrodeposition of Cu, In and Se and in a second time Cu, In, Se and Ga. From the morphological and structural point of view, the films obtained are similar to those prepared by the first technique. The band gap value increases up from 1 eV for the CIS films to 1.26 eV for the CuIn_1 − xGa_xSe₂ with 0 < x < 0.23. The resistivity at room temperature of the films was adjusted to 10 Ωcm after annealing. The films exhibit an absorption coefficient more than 10⁵ cm^− 1. The most important conclusion of this study is the interesting potential of electrodeposition as a promising option in low-cost CISe and CIGSe thin film based solar cells processing.     

Facing target sputtered iron-silicide thin film

The FeSi_x thin films with the flatness to an atomic scale for mechanical and optical applications can be grown by a facing target sputtering method. Optical properties indicate the films contain nano-crystalline FeSi₂. The nano-indentation hardness of 10 GPa was obtained. An interesting point is that Young's modulus is larger than that of the carbon-based materials with the same hardness. This means that the FeSi_x is more elastic, which could be characteristic of the chemical bonding of the FeSi₂ with the large contribution of d-electrons. Furthermore, results of the Hall measurement at room temperature shows that the films have p-type conductivity, a carrier concentration of p = 2.49 × 10²⁰ cm^− 3 and a Hall mobility of μ = 6.42 cm² V^− 1 s^− 1.     

Luminescence properties of Eu ions doped in Y2−xGdxO3 thin films grown by pulsed laser deposition method

Luminescence properties of Y_2−xGd_xO₃:Eu³⁺ (x = 0 to 2.0) thin films are investigated by site-selective laser excitation spectroscopy. The films were grown by pulsed laser deposition method on SiO₂ (100) substrates. Cubic phase Y₂O₃ and Gd₂O₃ and monoclinic phase Gd₂O₃ are identified in the excitation spectrum of the ⁷F₀ → ⁵D₀ transition of Eu³⁺. The emission spectra of the ⁵D₀ → ⁷F_J (J = 1 and 2) transition from individual Eu³⁺ centers were obtained by tuning the laser to resonance with each excitation line. The excitation line at around 580.60 nm corresponds to the line from Eu³⁺ with C₂ site symmetry of cubic phase. New lines at 578.65 and 582.02 nm for the C_S sites of Gd₂O₃ with monoclinic phase are observed by the incorporation of Gd in Y₂O₃ lattice. Energy transfer occurs between Eu³⁺ ions at the C_S sites and from Eu³⁺ ions at the C_S sites to those at the C₂ site in Y_2−xGd_xO₃.     

Structure and thermal conductivity of Gd2(TixZr1 − x)2O7 ceramics

The Gd₂(Ti_xZr_1 − x)₂O₇ (x = 0, 0.25, 0.50, 0.75, 1.00) ceramics were synthesized by solid state reaction at 1650 °C for 10 h in air. The relative density and structure of Gd₂(Ti_xZr_1 − x)₂O₇ were analyzed by the Archimedes method and X-ray diffraction. The thermal diffusivity of Gd₂(Ti_xZr_1 − x)₂O₇ from room temperature to 1400 °C was measured by a laser-flash method. The Gd₂Zr₂O₇ has a defect fluorite-type structure; however, Gd₂(Ti_xZr_1 − x)₂O₇ (0.25 ≤ x ≤ 1.00) compositions exhibit an ordered pyrochlore-type structure. Gd₂Zr₂O₇ and Gd₂Ti₂O₇ are infinitely soluable. The thermal conductivity of Gd₂(Ti_xZr_1 − x)₂O₇ increases with increasing Ti content under identical temperature conditions. The thermal conductivity of Gd₂(Ti_xZr_1 − x)₂O₇ first decreases gradually with the increase of temperature below 1000 °C and then increases slightly above 1000 °C. The thermal conductivity of Gd₂(Ti_xZr_1 − x)₂O₇ is within the range of 1.33 to 2.86 W m^− 1 K^− 1 from room temperature to 1400 °C.     

Epitaxial multi-component rare-earth oxide: A high-k material with ultralow mismatch to Si

New gate dielectric substitute for high-k application requires well matched lattice parameters and an atomically defined interface with Si for optimal performance. Using molecular beam epitaxy technique, we have grown on Si(111) crystalline rare-earth oxide ultrathin films, (Gd_xNd_1 − x)₂O₃ (GNO), a multi-component material that is superior to either of its binary host oxides. By carefully characterizing its crystal structure, we have found that the epitaxial GNO film exhibits a single bixbyite cubic structure with ultralow lattice mismatch to Si, which is indistinguishable even by the powerful synchrotron radiation. This structural perfection could make the GNO a promising high-k material in future devices.     

Influence of process steps on the performance of microcrystalline silicon thin film transistors

Bottom gate microcrystalline silicon thin film transistors (μc-Si TFT) have been realized with two types of films: μc-Si(1) and μc-Si(2) with crystalline fraction of 80% and close to 100% respectively. On these TFTs we applied two types of passivation (SiN_x and resist). μc-Si TFTs with resist as a passivation layer present a low leakage current of about 2.10^− 12 A for V_G = − 10 and V_D = 0.1V an ON to OFF current ratio of 10⁶, a threshold voltage of 7 V, a linear mobility of 0.1 cm²/V s, and a sub-threshold voltage of 0.9 V/dec. Microcrystalline silicon TFTs with SiN_x as a passivation present a new phenomenon: a parasitic current for negative gate voltage (− 15 V) causes a bump and changes the shape of the sub-threshold region. This excess current can be explained by and oxygen contamination at the back interface.     

Oxygen permeation and electrical transport of Gd1−xPrxBaCo2O5 + δ dense membranes

Double-perovskite Gd_1−xPr_xBaCo₂O_5 + δ membranes showed appreciable oxygen permeability at moderate temperatures. The overall oxygen permeation process of GdBaCo₂O_5 + δ was found to be controlled mainly by the bulk diffusion step with the membrane thickness larger than 0.8 mm, and the limitation by oxygen surface exchange came into play at reduced thickness of 0.8 mm. The electrical conductivity measurement showed Gd_1 - xPr_xBaCo₂O_5 + δ samples possessed a semiconducting behavior at a wide temperature range below 300 °C and a metallic behavior at 300-850 °C with a high conductivity of nearly 10³ S cm^− 1.     

R3Mn1.5CuV0.5O9 (R = Y, Ho, Er, Tm, Yb and Lu) and Lu3Mn3−3xCu2xVxO9: New noncentrosymmetric oxides related to YMnO3

We report formation of new noncentrosymmetric oxides of the formula, R₃Mn_1.5CuV_0.5O₉ for R = Y, Ho, Er, Tm, Yb and Lu, possessing the hexagonal RMnO₃ (space group P6₃cm) structure. These oxides could be regarded as the x = 0.5 members of a general series R₃Mn_3−3xCu_2xV_xO₉. Investigation of the Lu-Mn-Cu-V-O system reveals the existence of isostructural solid solution series, Lu₃Mn_3−3xCu_2xV_xO₉ for 0 < x ≤ 0.75. Magnetic and dielectric properties of the oxides are consistent with a random distribution of Mn³⁺, Cu²⁺ and V⁵⁺ atoms that preserve the noncentrosymmetric RMnO₃ structure.     

Ohmic contacts and n-type doping on TixCr2 − xO3 films and the temperature dependence of their transport properties

A procedure to dope n-type Cr_2 − xTi_xO₃ thin films is proposed. Besides doping the material, at the same time the method forms ohmic contacts on Ti_xCr_2 − xO₃ films. It consists on the deposition of 10 nm Ti and 50 nm Au, followed by thermal annealing at 1000 °C for 20 min in N₂ atmosphere. Ohmic contacts were formed on three samples with different composition: x = 0.17, 0.41 and 1.07 in a van der Pauw geometry for Hall effect measurements. These measurements are done between 35 K and 373 K. All samples showed n-type nature, with a charge carrier density (n) on the order of 10²⁰ cm^− 3, decreasing as x increased. As a function of temperature, n shows a minimum around 150 K, while the mobilities have an almost constant value of 11, 28 and 7 cm²V^− 1 s^− 1 for x = 0.17, 0.41 and 1.07, respectively.     

Synthesis of V-doped TiO2 films by chemical bath deposition and the effect of post-annealing on their properties

Amorphous composite films, composed of a Ti_1 − xV_xO₂ solid-solution phase and a V₂O₅ phase, were produced by chemical bath deposition and subsequently air-annealed at various temperatures up to 550 °C. The microstructure and chemical composition of the as-prepared and annealed films were investigated by a combinatorial experimental approach using Scanning electron microscopy, X-ray powder diffraction and X-ray photoelectron spectroscopy. Ultraviolet-Visible Spectrometry was applied to determine the optical band gap of the as-prepared and annealed films. It followed that the incorporation of vanadium in the as-deposited films reduces the optical band gap of TiO₂ from about 3.8 eV to 3.2 eV. Annealing of the films up to 350 °C leads to slight increase of band gap, as attributed to a reduction of the defect density in the initially amorphous oxide films due to the gradual development of long-range order and a concurrent reduction of the V⁴⁺-dopant concentration in the Ti_1 − xV_xO₂ solid-solution phase. The films crystallized upon annealing in air at 550 °C, which resulted in drastic changes of the phase constitution, optical absorbance and surface morphology. Due to the lower solubility of V⁴⁺ in crystalline TiO₂, V⁴⁺ segregates out of the crystallizing Ti_1 − xV_xO₂ solid-solution phase, forming crystalline V₂O₅ at the film surface.     

Structural and electrochromic properties of molybdenum doped vanadium pentoxide thin films by sol-gel and hydrothermal synthesis

Molybdenum-doped vanadium pentoxide (Mo-doped V₂O₅) thin films with doping levels of 3-10 mol% were prepared by dip-coating technique from a stable Mo-doped V₂O₅ sol synthesized by sol-gel and hydrothermal reaction. The Mo-doped V₂O₅ films had a layered V₂O₅ matrix structure along c-axis orientation with Mo⁶⁺ as substitutes. Values of the inserted and extracted charge density of 21.4 and 21.3 mC·cm^− 2 and the transmittance variation (ΔT at 640 nm) between anodic (+ 1.0 V) and cathodic (− 1.0 V) colored states of 41% were observed for the films with 5 mol% Mo⁶⁺ doping. Above this dopant concentration, the charge capacity and ΔT decreased. The enhancement of the electrochemical and electrochromic properties of the films is related to changes in the electronic properties of V₂O₅ films due to the creation of energy levels in the band gap of V₂O₅ by the Mo doping, accompanied by the reduction of the forbidden-band width and the increase of the conductivity.     

Antimony sulfide thin films in chemically deposited thin film photovoltaic cells

Antimony sulfide thin films of thickness ≈ 500 nm have been deposited on glass slides from chemical baths constituted with SbCl₃ and sodium thiosulfate. Smooth specularly reflective thin films are obtained at deposition temperatures from − 3 to 10 °C. The differences in the film thickness and improvement in the crystallinity and photoconductivity upon annealing the film in nitrogen are presented. These films can be partially converted into a solid solution of the type Sb₂S_xSe_3 − x, detected in X-ray diffraction, through heating them in contact with a chemically deposited selenium thin film. This would decrease the optical band gap of the film from ≈ 1.7 eV (Sb₂S₃) to ≈ 1.3 eV for the films heated at 300 °C. Similarly, heating at 300 °C of sequentially deposited thin film layers of Sb₂S₃-Ag₂Se, the latter also from a chemical bath at 10 °C results in the formation of AgSb(S/Se)₂ with an optical gap of ≈ 1.2 eV. All these thin films have been integrated into photovoltaic structures using a CdS window layer deposited on 3 mm glass sheets with a SnO₂:F coating (TEC-15, Pilkington). Characteristics obtained in these cells under an illumination of 850 W/m² (tungsten halogen) are as follows: SnO₂:F-CdS-Sb₂S₃-Ag(paint) with open circuit voltage (V_oc) 470 mV and short circuit current density (J_sc) 0.02 mA/cm²; SnO₂:F-CdS-Sb₂S₃-CuS-Ag(paint), V_oc ≈ 460 mV and J_sc ≈ 0.4 mA/cm²; SnO₂:F-CdS-Sb₂S_xSe_3 − x-Ag(paint), V_oc ≈ 670 mV and J_sc ≈ 0.05 mA/cm²; SnO₂:F-CdS-Sb₂S₃-AgSb(S/Se)₂-Ag(paint), V_oc ≈ 450 mV and J_sc ≈ 1.4 mA/cm². We consider that the materials and the deposition techniques reported here are promising toward developing ‘all-chemically deposited solar cell technologies.’     

RF diode reactive sputtering of n- and p-type zinc oxide thin films

We present the relationship between parameters of reactive RF diode sputtering from a zinc oxide (ZnO) target and the crystalline, electrical and optical properties of n-/p-type ZnO thin films. The properties of the ZnO thin films depended on RF power, substrate temperature and, particularly, on working gas mixtures of Ar/O₂ and of Ar/N₂. Sputtering in Ar+O₂ working gas (up to 75% of O₂) improved the structure of an n-type ZnO thin film, from fibrous ZnO grains to columnar crystallites, both preferentially oriented along the c-axis normally to the substrate (〈0 0 2〉 direction). These films had good piezoelectric properties but also high resistivity (ρ≈10³ Ω cm). ZnO:N p-type films exhibited nanograin structure with preferential 〈0 0 2〉 orientation at 25% N₂ and 〈1 0 0〉 orientation for higher N₂ content. The presence of nitrogen N_O at O-sites forming N_O-O acceptor complexes in ZnO was proven by SIMS and Raman spectroscopy. A minimum value of resistivity of 790 Ω cm, a p-type carrier concentration of 3.6×10¹⁴ cm⁻³ and a Hall mobility of 22 cm² V⁻¹ s⁻¹ were obtained at 75% N₂.     

Dielectric properties of sol-gel derived high-k titanium silicate thin films

High-k dielectric titanium silicate (Ti_xSi_1 − xO₂) thin films have been deposited by means of an optimized sol-gel process. At the optimal firing temperature of 600 °C, the Ti_0.5Si_0.5O₂ films are shown to exhibit not only a dielectric constant (k) as high as ∼ 23 but more importantly the lowest leakage current and dielectric losses. Fourier transform infrared spectroscopy shows an absorbance peak at 930 cm^− 1, which is a clear signature of the formation of Ti-O-Si bondings in all the silicate films. The developed sol-gel process offers the required latitude to grow Ti_xSi_1 − xO₂ with any composition (x) in the whole 0 ≤ x ≤ 1 range. Thus, the k value of the Ti_xSi_1 − xO₂ films can be tuned at any value between that of SiO₂ (3.8) to that of TiO₂ (k ∼ 60) by simply controlling the TiO₂ content of the films. The composition dependence of the dielectric constant of the Ti_xSi_1 − xO₂ films is analyzed in the light of existing models for dielectric composites.     

V-doping effects on ferroelectric properties of K0.5Bi4.5Ti4O15 thin films

V-doped K_0.5Bi_4.5Ti₄O₁₅ (K_0.5Bi_{4.5 − x/3}Ti_4 − xV_xO₁₅, KBTiV-x, x = 0.00, 0.01, 0.03, and 0.05) thin films were prepared on a Pt(111)/Ti/SiO₂/Si(100) substrate by a chemical solution deposition method. The thin films were annealed by using a rapid thermal annealing process at 750 °C for 3 min in an oxygen atmosphere. Among them, KBTiV-0.03 thin film exhibited the most outstanding electrical properties. The value of remnant polarization (2P_r) was 75 μC/cm² at an applied electric field of 366 kV/cm. The leakage current density of the thin film capacitor was 5.01 × 10⁻⁸ at 100 kV/cm, which is approximately one order of magnitude lower than that of pure K_0.5Bi_4.5Ti₄O₁₅ thin film capacitor. We found that V doping is an effective method for improving the ferroelectric properties of K_0.5Bi_4.5Ti₄O₁₅ thin film.     

Optical characterization of vanadium-titanium oxide films

Composite films of titanium and vanadium oxides (TVO) with various compositional ratios were prepared by sputtering deposition. The optical properties, crystalline structure and film morphology were investigated as a function of the composition. The results of thermochromism and X-ray diffraction suggest that the TVO films at any compositional ratios form substitutional solid solution of Ti and V, that is, Ti_xV_1−xO₂, where 0 ≤ x ≤ 1. Dielectric constants of the TVO films at any compositions were consistently determined at photon energies between 0.75 to 3 eV by employing the Lorentz-oscillator formula. With wide variation in x, the dielectric constants at visible and near-infrared wavelengths monotonically decrease down to the values of TiO₂, which suggests that dielectric constants of the TVO film can be precisely controlled by adjusting rf power in co-sputtering deposition.     

Capacitance-voltage and leakage-current characteristics of sol-gel-derived crystalline and amorphous SrTa2O6 thin films

SrTa₂O₆ (STA) is a promising high-dielectric-constant (ε) material. In this study, STA thin films were fabricated using the sol-gel method. The capacitance-voltage and leakage-current characteristics of crystalline and amorphous STA thin-film capacitors were investigated. STA thin films crystallized at an annealing temperature of 800 °C. Crystalline STA thin films exhibited a high ε of about 110, whereas amorphous STA thin films showed a much lower ε of about 26-41. However, amorphous STA thin films had a much more constant capacitance as a function of voltage. Of the amorphous thin films, the one annealed at 700 °C had the highest ε of about 41, the lowest leakage current of 10^− 8 A/cm², and a very constant capacitance as a function of voltage with a quadratic voltage-capacitance coefficient (α) of 27 ppm/V². The crystalline STA thin film had a negative α that was independent of frequency, which suggests that dipolar relaxation occurs and is responsible for the large change in the capacitance. The amorphous thin films had a positive α that decreased with increasing frequency, which implies that electrode polarization occurs.     

High performance Gd and Y co-doped ceria-based electrolytes for intermediate temperature solid oxide fuel cells

Co-doped ceria of Ce_1−xGd_x−yY_yO_2−0.5x, wherein x = 0.15 and 0.2, 0 ≤ y ≤ x, were prepared by glycine-nitrate method. Their structures and ionic conductivities were characterized by X-ray diffraction (XRD) and AC impedance spectroscopy (IS). All the electrolytes were found to be ceria-based solid solutions of fluorite type structures. Co-doping was found to effectively enhance the conductivity. In comparison to the singly doped ceria, the co-doped ceria showed much higher ionic conductivities at 673-973 K. At 773 K, the ionic conductivity of Ce_0.8Gd_0.05Y_0.15O_1.9 is 0.013 S cm⁻¹ which is three times as high as that of Ce_0.8Gd_0.2O_1.9. These Gd³⁺and Y³⁺ co-doped ceria are ideal electrolyte materials of intermediate temperature solid oxide fuel cells (SOFCs).     

Study of flash evaporated CuIn1 − xGaxTe2 (x = 0, 0.5 and 1) thin films

CuIn_1 − xGa_xTe₂ thin films with x = 0, 0.5 and 1, have been prepared by flash evaporation technique. These semiconducting layers present a chalcopyrite structure. The optical measurements have been carried out in the wavelength range 200-3000 nm. The linear dependence of the lattice parameters as a function of Ga content obeying Vegard's law was observed. The films have high absorption coefficients (4 · 10⁴ cm^− 1) and optical band gaps ranging from 1.06 eV for CuInTe₂ to 1.21 eV for CuGaTe₂. The fundamental transition energies of the CuIn_1 − xGa_xTe₂ thin films can be fitted by a parabolic equation namely E_g1(x) = 1.06 + 0.237x − 0.082x². The second transition energies of the CuInTe₂ and CuGaTe₂ films were estimated to be: E_g2 = 1.21 eV and E_g2 = 1.39 eV respectively. This variation of the energy gap with x has allowed the achievement of absorber layers with large gaps.     

Ce1−xNdxO2−δ/Si thin films obtained by pulsed laser deposition: Microstructure and conduction properties

Substituted Ce_1−xNd_xO_2−δ cerium dioxide thin films are obtained by pulsed laser deposition technique. The films are deposited for various deposition times and at.% Nd, on [100] Si substrates, covered by a thin native SiO₂ layer. The evolution of the cell parameters with Nd content shows that a solid solution is formed, up to x = 0.27. The thin films are homogenous in composition at a nanometer scale. The morphology of the grains does not change significantly with Nd content. The microstructure is columnar, with a preferential [100] growth direction. The width of the grains varies from 20 to 30 nm. The conductivities of the thin films are determined from impedance spectroscopy analyses, in the temperature range 200 °C to 600 °C. The experimental data are explained in the frame of the space charge layer model.