Studies of thermoelectric transport properties of atomic layer deposited gallium-doped ZnO

The thermoelectric transport properties of atomic layer deposited (ALD) gallium doped zinc oxide (GZO) thin films were investigated to identify their potential as a thermoelectric material. The overall thermoelectric properties, such as the Seebeck coefficient and electrical conductivity, were probed as a function of Ga concentration in ZnO. The doping concentration was tuned by varying the ALD cycle ratio of zinc oxide and gallium oxide. The GZO was deposited at 250 °C and the doping concentration was modified from 1% to 10%. Sufficient thermoelectric properties appeared at a doping concentration of 1%. The crystallinity and electronic state, such as the effective mass, were investigated to determine the enhancement of the thermoelectric properties. The efficient Ga doping of GZO showed a Seebeck coefficient of 60 μV/K and an electrical conductivity of 1808.32 S/cm, with a maximum power factor of 0.66 mW/mK².     

Effects of potassium incorporation on the structural,optical, vibrational and electrical properties of NiO sprayed thin films for p-type optical windows

Nickel oxide thin films were deposited on glass substrates by a simple mini spray technique at 460 °C. Alternatively, some of the obtained films were doped with potassium at the molar rates of: 1, 2 and 3% (K). In addition to the classical structural investigations including XRD, the opto-thermal studies, Raman spectroscopy and photoluminescence measurements were investigated.First, structural study by means of X-ray diffraction shows that all K-doped NiO thin films crystallized in cubic space group with some noticeable changes in terms of [K]/[Ni] ratio. In the same line, Raman spectroscopy reveals the principal NiO vibration’s mode with the shift related to K incorporation in NiO matrix. Second, some optical parameters, such as optical band gap, Urbach energy, refractive indices extinction coefficients and dielectric constant were studied in terms of K doping level. For all NiO:K prepared thin films, PL measurements show three large bands located at 405, 420, 485 and 529 nm.Furthermore, electrical properties were performed using impedance spectroscopy technique in the frequency range 5 Hz–13 MHz at various temperatures. DC conductivity is thermally activated showing a semiconductor behavior of NiO:K sprayed thin films. This study shows that the electrical conductivity is thermally activated. The calculated values of the activation energy show semiconductor behavior of such films.On the other hand AC conductivity is investigated through Jonscher law. The imaginary part of the complex impedance has a maximum whose relaxation frequency increases with temperature according to Arrhenius law.     

Formation and characteristics of undoped and doped tetrahedral amorphous carbon films

Synthesis of undoped and doped tetrahedral amorphous carbon (ta-C) films has been achieved using magnetic field filtered plasma stream system in an ambient gas of pure Ar and Ar with N₂, respectively. The optical and electrical properties of these films as a function of the substrate bias voltages (V_b) or nitrogen partial pressures (P_N) have been studied using UV-visible optical absorption spectroscopy, Fourier-transform infra-red spectroscopy (FTIR) and measurements of electrical conductivity. The results show that ta-C films with a high sp³ fraction were formed when the V_b was in the range of −10 to −50 V. The optical band gap of such ta-C films was found to be larger than 3 eV. The incorporation of nitrogen into the ta-C films deposited at low P_N (P_N<25%), results in a slight drop in activation energy, which indicates that there is evidently some doping effect of nitrogen. The configurations of N atoms in ta-C network are identified and discussed.     

Influence of Er doping on the structural,optical and luminescence properties of pulsed laser deposited Er: BaZrO3 thin films

Pure and erbium doped (1, 2, 3 and 5 at%) Barium zirconate (BZE) thin films have been deposited on Si (0 0 1) substrate via pulsed laser deposition using 100 mJ Nd: YAG laser operated at second harmonics (532 nm). Er doping significantly affects the surface morphology, microstructure and optical properties of grown thin films. All the films exhibit cubic BaZrO₃ structure and are polycrystalline in nature as extracted from XRD data. The optical band gap energies (3.75–3.63 eV) of doped (1, 2, 3, and 5 at%) BZE thin films are found to be less than that of pure BZO film (4.03 eV). PL spectra, excited at 328 nm, mainly consist of violet-blue (412 nm) and green (523–543 nm) emissions for all the doped films. The green emission increases with the increase in Er doping upto 3 at% and then concentration quenching effect appears at 5 at%. It is noted that the relative intensity of PL emission and the optical band gap can be tuned by varying Er concentration to alter the properties of the phosphor. The emission peaks in photoluminescence spectra makes the Er: BZO films potential candidates to be used in optoelectronic devices such as light emitting diodes (LEDs).     

Performance of spray deposited Gd-doped barium cerate thin films for proton conducting SOFCs

Doped barium cerate is a promising solid electrolyte for intermediate temperature fuel cells as a protonic conductor. In the present paper, the nanocrystalline Gd-doped barium cerate (BaCe_0.7Gd_0.1Y_0.2O_2.9) thin films have been successfully deposited on alumina substrate by spray pyrolysis technique. The films deposited from 0.1 M concentration and annealed at five different temperatures were characterized with different physio-chemical techniques. The BCGY is crystallized in orthorhombic perovskite structure with slight shift to the lower 2θ value compared with barium cerate (BC) and yttrium doped barium cerate (BCY). The grain growth and hence densification is also investigated by using SEM and AFM. The grain growth is almost complete at 1000 °C and the surface of the film appears to be smooth with typical roughness of 152 nm. Raman spectrum of BCGY film shows intense band at 463.8 cm⁻¹ compared to pure BC and BCY indicating the presence of more oxygen vacancies due to Gd doping. The proton conductivity of BCGY thin film in moist atmosphere is 1×10⁻³ Scm⁻¹.     

Samarium doped ceria (SDC) synthesized by a metal triethanolamine complex decomposition method: Characterization and an ionic conductivity study

Samarium doped ceria (SDC) powders as solid electrolyte ceramics were successfully prepared via thermal decomposition of metal organic complexes containing triethanolamine (TEA) as a ligand. The SDC powders synthesized using various samarium doping contents were characterized by X-ray diffractometry, scanning electron microscopy, X-ray absorption spectroscopy, energy dispersive X-ray spectroscopy and Brunauer-Emmett-Teller (BET) analysis. The influences of samarium doping and the calcination temperature on the characteristics of the SDC materials were thoroughly investigated. An appropriate temperature for SDC powder calcination was identified by thermogravimetric analysis to be 600 °C. After sintering the calcined SDC powders at 1500 °C to obtain highly dense ceramic pellets, the electrical conductivity of the materials was examined by impedance spectroscopy. The influence of percentage of Sm³⁺ dopants in SDC materials on the observed conductivity were explained by correlating with the detailed analysis of the local structure and environment of Sm³⁺ within the SDC materials by using X-ray absorption spectroscopy. The conductivities of the SDC products reported in this work indicate that they are promising candidates for solid electrolytes in solid oxide fuel cell applications.     

The role of substrate temperature on the properties of nanocrystalline Mo doped ZnO thin films by spray pyrolysis

Transparent conducting molybdenum (2 at.%) doped zinc oxide (MZO) films were prepared with various substrate temperatures by spray pyrolysis technique on glass substrates. The effect of substrate temperature on the structural, surface morphological, electrical, optical and photoluminescence properties of these films were studied. The X-ray diffraction analysis revealed that the films are polycrystalline in nature having a wurtzite structure with a preferred grain orientation in the (0 0 2) direction. The average crystallite size of the films increases from 17 nm to 28 nm with the increase of substrate temperature from 573 K to 623 K, thereafter it slightly decreases with further increase of substrate temperature to 723 K. Analysis of structural parameters indicates minimum strain and stress values for films deposited at a substrate temperature of 673 K. From atomic force microscopy (AFM) analysis, it is found that rms roughness of the films deposited at 623 K is a minimum, indicating better optical quality. The scanning electron microscopy (SEM) measurements showed that the surface morphology of the films changes with substrate temperature. Optical parameters such as optical transmittance, reflectance, refractive index, extinction coefficient, dielectric constant and optical band gap have been studied and discussed with respect to substrate temperature. Room temperature photoluminescence (PL) spectra show the deep-level emission in the MZO thin films. The films exhibit a low electrical resistivity of 6.22 × 10^?2 Ω cm with an optical transmittance of 75% in the visible region at a substrate temperature of 623 K.     

Characterization of spray pyrolytically deposited high mobility praseodymium doped CdO thin films

Praseodymium (Pr) doped CdO thin films with high transparency and high mobility were deposited, using a homemade spray pyrolysis setup, on micro-slide glass substrates preheated at 300 °C. Polycrystalline nature and Cd-O bond vibration of deposited films were confirmed by X-ray diffraction, micro-Raman and Fourier transform infrared spectroscopy analyses. The oxidation state of Cd²⁺, O²⁻, and Pr³⁺ was confirmed by X-ray photoelectron spectroscopy analysis. The highest average particle size (92 nm-FESEM) and high RMS (13.48 nm-AFM) values are obtained for 0.50 wt% Pr doped CdO thin film. The optical band gap is varied between 2.38 eV and 2.52 eV, depending on the Pr doping concentration. Photoluminescence spectra revealed that Pr doped CdO thin film exhibits strong green emission at 582 nm. High mobility (82 cm²/V s), high charge carrier concentration (2.19×10²⁰ cm⁻³) and high transmittance (83%) were observed for 0.50 wt% Pr doped CdO film. A high figure of merit (9.79×10⁻³ Ω⁻¹) was obtained for 0.50 wt% Pr doped CdO thin films. The mechanism behind the above results is discussed in detail in this paper.     

Electrical relaxation and conduction mechanisms in iron doped barium strontium titanate

Polycrystalline Ba_0.7Sr_0.3Ti_(1?x)Fe_xO₃ (x = 0.1) (BSTF) ceramics, synthesized via solid-state reaction route were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and complex impedance spectroscopy (CIS). The Rietveld refinement of the XRD data confirmed the presence of tetragonal and cubic phases in the prepared sample. The SEM image revealed that the sample has well distributed grains along with some degree of agglomerations. The electrical behaviour of the BSTF ceramic has been studied by complex impedance spectroscopy (CIS) as a function of frequency (1 Hz to 1 MHz) at different temperatures (RT to 700 K). Two semicircular arcs observed in the Cole-Cole plot confirm the contribution from the grain and grain boundary in overall impedance. Both the electrical as well as ac conduction phenomena take place via correlated barrier hopping (CBH) authenticated by detailed complex modulus analysis and ac fitted conductivity respectively. The values of activation energies calculated from electrical impedance, modulus, and conductivity data clearly reveal that the relaxation and conduction processes in BSTF ceramic are induced by doubly ionized oxygen vacancies.     

p-type conductive NiOx: Cu thin films with high carrier mobility deposited by ion beam assisted deposition

Transparent conductive NiO thin films with 18 at% Cu dopant were fabricated by ion beam assisted deposition (IBAD). Their structural and optoelectronic properties were compared with undoped NiO films and NiO films doped with 12 at% Cu, and also compared with NiO:Cu (18 at%) films deposited by RF sputtering as reported in our previous work. The results show that the crystallinity of NiO thin films deposited through IBAD technology is much better than that of the films deposited by RF sputtering. Thanks to this reason, the highest carrier mobility above 45 cm²V^?1s^?1 for NiO:Cu (18 at%) film can be realized here. Meanwhile, the films’ resistivity remains an acceptable value, varying from 2.05 to 0.064 Ω cm with oxygen ion beam current changing from 0.2 to 0.8 A. This feature is imperative for p-type transparent conductive oxides (TCOs) applied in various domains. In addition, with oxygen ion beam current increase, the increase of the Ni³⁺/Ni²⁺ ratio leads to more Ni²⁺ vacancies be introduced into NiO films, which is beneficial to generate holes and improve carrier concentration. In this work, the optimal carrier mobility of NiO film doped with 18 at% Cu is obtained when the oxygen ion beam current is 0.2 A. Its carrier concentration and electrical resistivity are 7.26 $\times$10¹⁶ cm^?3 and 2.05 Ω cm, respectively.     

Highly conducting and transparent antimony doped tin oxide thin films: the role of sputtering power density

Sb doped SnO₂ thin films were deposited on quartz substrates by magnetron sputtering at 600 °C and the effects of sputtering power density on the preferential orientation, structural, surface morphological, optical and electrical properties had been studied. The XRD analyses confirm the formation of cassiterite tetragonal structure and the presence of preferential orientation in (2 1 1) direction for tin oxygen thin films. The dislocation density analyses reveal that the generated defects can be suppressed by the appropriate sputtering power density in the SnO₂ lattice. The studies of surface morphologies show that grain sizes and surface roughness are remarkably affected by the sputtering power density. The resistivity of Sb doped SnO₂ thin films gradually decreases as increasing the sputtering power density, reaches a minimum value of 8.23×10⁻⁴ Ω cm at 7.65/cm² and starts increasing thereafter. The possible mechanisms for the change in resistivity are proposed. The average transmittances are more than 83% in the visible region (380–780 nm) for all the thin films, the optical band gaps are above 4.1 eV. And the mechanisms of the variation of optical properties at different sputtering power densities are addressed.     

Synthesis and characterization of ZnO:Ni thin films grown by spray-deposition

In the present study, nickel-doped zinc oxide thin films (ZnO:Ni) at different percentages (0–10%) were deposited on glass substrates by using a chemical spray technique. The effect of Ni concentration on the structural and optical properties of the ZnO:Ni thin films was investigated. The effect of Ni contents on the crystalline structure and optical properties of the films was systematically investigated by X-ray diffraction (XRD), scanning electronic microscopy (SEM), UV–vis, Photoluminescence spectra PL, and Raman spectrometry. The XRD analysis showed that both the undoped and Ni-doped ZnO films were crystallized in the hexagonal structure with a preferred orientation of the crystallites along the [002] direction perpendicular to the substrate. The XRD analysis also showed that the films were well crystallized in würtzite phase with the crystallites preferentially oriented towards (002) direction parallel to the c-axis. SEM study reveals the surface of NiZnO to be made of nanocrystalline particles. The SEM images showed a relatively dense surface structure composed of crystallites in the spherical form whose average size decreases when the [Ni]/[Zn] ratio increases. The optical study showed that all the films were highly transparent. The band gap decreased up to the 7 at% Ni doping level, but the band gap increased after 10 at% Ni doping level. All thin films exhibited approximately 80% and above transmittance in the visible region. PL spectra of undoped and Ni-doped ZnO thin films showed some marked peaks at 376, 389, 494, and 515 nm. The obtained results revealed that the structures and optical properties of the films were greatly affected by doping levels. These films are useful as conducting layers in electro chromic and photovoltaic devices. Finally, all results were discussed in terms of the nickel doping concentration.     

Physical properties of La-doped NiO sprayed thin films for optoelectronic and sensor applications

Lanthanum-doped nickel oxide NiO:La thin films were deposited onto glass substrates at 450 °C, by the spray pyrolysis technique using nickel and lanthanum chlorides as precursors. These films belonging to cubic structure, crystallize preferentially along (111) plane. First, Raman study shows the presence of bands corresponding to NiO structure. The same study confirms the presence of both Ni(OH)₂ and LaNiO₃ as secondary phases. Moreover, using SEM observations, all samples exhibit porous microstructures with rough surfaces and spherical nanoparticles of about 40 nm as size. Second, NiO:La films present a direct band gap energy value lying in the range of 3.63–3.84 eV. Also, the effect of the La incorporation in NiO matrix on the disorder is studied in terms of Urbach energy. Some optical constants (refractive index, extinction coefficient, dielectric constants, and dispersion parameters) are reached. On the other hand, the photoluminescence spectroscopy reveals the presence of peaks related to the electronic transition of the Ni²⁺ ions and others confirming the presence of some defects in NiO matrix in terms of La content. Finally, it has been found that La doping allows the improvement of the electrical conductivity as well as Haacke’s figure of merit of NiO sprayed thin films by at least, three orders of magnitude.     

B-site donor and acceptor doped Aurivillius phase Bi3NbTiO9 ceramics

The electrical properties of B-site donor and acceptor doped Aurivillius phase Bi₃NbTiO₉-based ceramics have been investigated. The effect of donor and acceptor doping on the dielectric constant, coercive field, dc conductivity and piezoelectric constant are presented. The band gap of Bi₃NbTiO₉ (BNTO) is about 3.4 ± 0.2 eV, determined from high-temperature dc conductivity measurements. All of the ceramics are ferroelectrics with high Curie points (∼900 °C). In acceptor doped ceramics, a low-temperature peak in the dielectric loss tangent is explained in terms of a Debye-type relaxation that results from an oxygen ion-jump mechanism. The activation energy for the relaxation is calculated as 0.93 ± 0.05 eV. The reduction of the piezoelectric constant below 500 °C is produced by depolarization, which is produced by the switching of thermally unstable non-180° domain walls.     

Effect of non-magnetic Al3+ doping on structural,optical, electrical,dielectric and magnetic properties of BiFeO3 ceramics

Pure BiFeO₃ (BFO) and Al doped BFO samples were synthesized via citrate precursor method and sintered at 500 °C for two hours. Effect of Al doping on the structural, optical, electrical, dielectric and magnetic properties were investigated. X-ray diffraction (XRD) confirmed the distorted rhombohedral structure without any merging of peaks which indicates no structural transformation. Average crystallite size was found to be in the range 28–39 nm. Field emission scanning electron microscopy (FESEM) images illustrated the dense, agglomerated, spherically shaped with reduced grain size nanoparticles. Increased value of dielectric constant with low dielectric tangent loss was observed for the Al doped BFO samples. The value of dielectric constant was found to be 51 at 100 kHz for x = 0.1 sample. Temperature dependent dielectric constant showed a dielectric anomaly, indicating the antiferromagnetic transition. The remanent polarization and the corresponding coercive field for x = 0.1 was found to be 0.0625 µC/cm² and 56.154 kV/cm at an operating voltage of 1000 V. The improved electrical properties with low leakage current density were ascribed to the stabilization of the pervoskite structure and reduced oxygen vacancies.     

Intermediate-temperature conductivity of B-site doped Na0.5Bi0.5TiO3-based lead-free ferroelectric ceramics

Na_0.5Bi_0.5TiO₃ (NBT) based oxide-ion conductor ceramics have great potential applications in intermediate-temperature solid oxide fuel cells (SOFCs) and oxygen sensors. Na_0.5Bi_0.49Ti_1−xMg_xO_3−δ ceramics with x=0, 0.01, 0.02, 0.03, 0.05 and 0.08 were prepared by conventional solid-state reaction. XRD measurement and SEM analysis revealed the formation of pure perovskite structures without secondary phase. MgO doping greatly decreased the sintering temperature and inhibited grain growth. AC impedance spectroscopy measurement was adopted to measure the total conductivity, which was found to increase with MgO doping content ranging from 0 to 3 mol% and subsequently to decrease. High oxygen ionic conductivity σ_t=0.00629 S/cm was achieved for sample doped with 3 mol% MgO at 600 °C in air atmosphere.     

X-ray photoemission spectroscopy of nitrogen-doped UNCD/a-C:H films prepared by pulsed laser deposition

Nitrogen-doped ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon (a-C:H) films were deposited by pulsed laser deposition (PLD). Nitrogen contents in the films were controlled by varying a ratio in the inflow amount between nitrogen and hydrogen gases. The film doped with a nitrogen content of 7.9 at.% possessed n-type conduction with an electrical conductivity of 18 Ω^? 1 cm^? 1 at 300 K. X-ray photoemission spectra, which were measured using synchrotron radiation, were decomposed into four component spectra due to sp², sp³ hybridized carbons, C=N and C–N. A full-width at half-maximum of the sp³ peak was 0.91 eV. This small value is specific to UNCD/a-C:H films. The sp²/(sp³ + sp²) value was enhanced from 32 to 40% with an increase in the nitrogen content from 0 to 7.9 at.%. This increment probably originates from the nitrogen incorporation into an a-C:H matrix and grain boundaries of UNCD crystallites. Since an electrical conductivity of a-C:H does not dramatically enhance for this doping amount according to previous reports, we believe that the electrical conductivity enhancement is predominantly due to the nitrogen incorporation into grain boundaries.     

Room temperature preparation of high performance AZO films by MF sputtering

Aluminum-doped zinc oxide (AZO) thin films have been deposited by MF magnetron sputtering from a ceramic oxide target without heating the substrates. This study has investigated effects of sputtering power on the structural, electrical and optical properties of the AZO films. The films delivered a hexagonal wurtzite structure with (002) preferential orientation and uniform surface morphology with 27–33 nm grain size. The results indicate that residual stress and grain size of the AZO films are dependent on sputtering power. The minimum resistivity of 7.56×10^?4 Ω cm combined with high transmittance of 83% were obtained at deposited power of 1600 W. The films delivered the advantages of a high deposition rate at low substrate temperature and should be suitable for the fabrication of low-cost transparent conductive oxide layer.     

Methacryloxypropyltrimethoxysilane films on aluminium: Electrochemical characteristics,adhesion and morphology

The electrochemical characteristics, adhesion and morphology of methacryloxypropyltrimethoxysilane (MAPT) films on aluminium were investigated during exposure to 3 wt.% NaCl. The MAPT films were deposited on aluminium surface from 2 to 5 vol.% methacryloxypropyltrimethoxysilane solutions, with the aim to investigate the influence of deposition parameters (silane solution concentration and curing time) on electrochemical characteristics, adhesion and morphology of MAPT films on aluminium.Using electrochemical impedance spectroscopy (EIS), potential–time measurements, adhesion measurements and optical microscopy coupled with image analysis, it was shown that films deposited from 5 vol.% solution exhibited better corrosion stability and adhesion, as well as lower porosity comparing to 2 vol.% solution and improved the corrosion protection of aluminium substrate, while the curing time had no influence on these characteristics.     

One-step synthesis of Li-doped NiO as high-performance anode material for lithium ion batteries

The poor electronic conductivity and huge volume expansion of NiO are the vital barriers when used as anode for lithium ion batteries. In order to solve above issues, Li-doped NiO are prepared by a facile one-step ultrasonic spray pyrolysis method. The effects of Li doping on the morphology, structure and chemical composition of the Li-doped NiO powders are extensively studied. When used as lithium ion batteries anode, it is demonstrated that the doping of Li has significant positive effect on improving the electrochemical performance. After 100 cycles at 400 mA g⁻¹, The Li-doped NiO samples deliver a discharge capacity of 907 mAh g⁻¹, much more than that of un-doped sample (736 mAh g⁻¹). The improved electrochemical performances can be ascribed to the improved p-type conductivity and lower impedance, which are confirmed by Rietveld refinement, X-ray photoelectron spectroscopy and electron impedance spectroscopy.