Nb-doped Films Research Articles

Effect of dopants on ferroelectric and piezoelectric properties of lead zirconate titanate thin films on Si substrates

Lead zirconate titanate Pb(Zr0.52Ti0.48)O3 (undoped PZT) and doped PZT thin films with thickness of about 500 nm were grown on Pt/Ti/SiO2/Si substrates by pulsed laser deposition (PLD). In this study, 1.0 mol% Nb-doping (at Zr/Ti site) as donor, 1.0 mol% Fe-doping (at Zr/Ti) as acceptor and 10 mol% Ba-doping (at Pb site) as isovalence were used. The effects of the introduction of these dopants on the ferroelectric and piezoelectric properties were investigated and compared to the undoped PZT film. A noticeable improvement of the dielectric constant (ε) and effective piezoelectric coefficient (d33,f) are obtained. The maximum values of these parameters vary from 1280 and 126 pm/V for undoped PZT film to 1520 and 164 pm/V for Nb-doped PZT film. The largest remnant polarization (Pr) and coercive field (Ec) are obtained for Fe-doped PZT film, equal to 22.6 µC/cm2 and 35.3 kV/cm, as compared to 17.3 µC/cm2 and 31.2 kV/cm respectively for undoped PZT film. The Ba-doping decreases the dielectric constant but enhances the breakdown field of the film, reaching 800 kV/cm while it was only 660 kV/cm for undoped PZT film. The crystalline structure of Ba-doped film has a dominating (110) orientation instead of (100) in case of undoped, Nb-, and Fe-doped PZT films. The obtained results are important for the applications of doped PZT films in piezoMEMS devices.

Effect of thickness on optoelectrical properties of Nb-doped indium tin oxide thin films deposited by RF magnetron sputtering

Niobium-doped indium tin oxide (ITO:Nb) thin films are prepared on glass substrates with various film thicknesses by radio frequency (RF) magnetron sputtering from one piece of ceramic target material. The effects of thickness (60–360 nm) on the structural, electrical and optical properties of ITO: Nb films are investigated by means of X-ray diffraction (XRD), ultraviolet (UV)-visible spectroscopy, and electrical measurements. XRD patterns show the highly oriented (400) direction. The lowest resistivity of the films without any heat treatment is 3.1×10−4Ω·cm−1, and the resistivity decreases with the increase of substrate temperature. The highest Hall mobility and carrier concentration are 17.6 N·S and 1.36×1021 cm−3, respectively. Band gap energy of the films depends on substrate temperature, which varies from 3.48 eV to 3.62 eV.

The influence of experimental conditions on the morphology and phase composition of Nb-doped ZrO2 films prepared by spark anodization

This paper reports the morphological and microstructural characterization of Nb-doped ZrO2 films obtained by anodic doping in conditions of spark electrolytic breakdown. The oxide films were prepared by galvanostatic anodization of metallic Zr in oxalic acid solution containing ammonium oxaloniobate, NH4H2[NbO(C2O4)3]⋅3H2O using Nb(V) ions as the precursor dopant. The characterization was performed by SEM and XRD. A factorial design was used to optimize the experiments and study the influence of experimental variables on morphology and phase composition. The results demonstrated that the dopant concentration is the most important factor that affected the analyzed responses. A significant influence of this variable was observed on the pore average diameter and on the percentage of the stabilized tetragonal phase. The temperature also affected the phase composition, however no effect was observed on the morphological response.

Effect of niobium doping on opto-electronic properties of sol–gel based nanostructured indium tin oxide thin films

Abstract Optically transparent conducting niobium-doped indium tin oxide (ITNO) thin films were deposited on soda lime glass via sol–gel spin coating technique under various Nb-doping contents. The effect of different Nb-dopant contents on the opto-electronic as well as microstructural properties of the films were characterized by means of UV–vis spectroscopy, four point probe, X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and atomic force microscopy (AFM). For this purpose, addition of different Nb contents with tin at fixed 6 at% is considered as the first approach. For the second and third approaches, different Nb-doping contents were added at fixed atomic ratios of In:Sn=94:6 and In:Sn=95:5, respectively. Optimum opto-electronic properties, including minimum resistivity of 37.2×10 −3 Ω cm with average transmittance of about 85% in the visible region and band gap of 3.48 eV was achieved for ITNO thin films prepared by the first approach at Nb and Sn doping content of 1 and 5 at%, respectively. XRD analysis revealed patterns of cubic bixbyite structure of In 2 O 3 and rhombohedral structure of ITO for Nb-doped films with small shift in major peak position to lower diffraction angles with the addition of Nb. FESEM micrographs show that addition of Nb causes clustering of the grains at higher doping content. However, AFM studies show that addition of Nb dopant leads to the formation of films with compact surface and less average roughness compare to the undoped ITO films.

Preparation and characteristics of Nb-doped indium tin oxide thin films by RF magnetron sputtering

Niobium-doped indium tin oxide (ITO:Nb) thin films are fabricated on glass substrates by radio frequency (RF) magnetron sputtering at different temperatures. Structural, electrical and optical properties of the films are investigated using X-ray diffraction (XRD), atomic force microscopy (AFM), ultraviolet-visible (UV-VIS) spectroscopy and electrical measurements. XRD patterns show that the preferential orientation of polycrystalline structure changes from (400) to (222) crystal plane, and the crystallite size increases with the increase of substrate temperature. AFM analyses reveal that the film is very smooth at low temperature. The root mean square (RMS) roughness and the average roughness are 2.16 nm and 1.64 nm, respectively. The obtained lowest resistivity of the films is 1.2×10−4Ω·cm, and the resistivity decreases with the increase of substrate temperature. The highest Hall mobility and carrier concentration are 16.5 cm2/V·s and 1.88×1021cm−3, respectively. Band gap energy of the films depends on substrate temperature, which is varied from 3.49 eV to 3.63 eV.

Niobium aggregation and vacancylike defect evolution in nanostructured Nb-doped Mg: Their role in the kinetics of the hydride-to-metal phase transformation

The structural evolution of nanostructured Nb-doped magnesium film samples and its correlation with the change of the H${}_{2}$ desorption kinetics after successive H${}_{2}$ sorption cycles at 623 K was investigated by different techniques. The variation of the dispersed Nb fraction and the Nb clusterization was followed by extended x-ray absorption fine structure (EXAFS), while the progressive Mg nanostructuring was monitored by x-ray diffraction. The presence of vacancylike defects and their evolution was studied using positron annihilation lifetime spectroscopy and Doppler broadening spectroscopies. It was found that, with successive H${}_{2}$ sorption cycles: (i) the H${}_{2}$ desorption kinetics progressively becomes slower until stationary conditions are reached and (ii) the Nb dopant atoms, dispersed in the nanocrystalline Mg layers, aggregate, forming nanoclusters. Our results show that the progressive Nb aggregation drives the H${}_{2}$ desorption kinetics. EXAFS analysis show that fast desorption kinetics is due to the presence of small (\ensuremath{\sim}1 nm) Nb aggregates rather than Nb atoms dispersed into the Mg matrix. With cycling, the Nb aggregates progressively grow, forming larger bcc Nb nanoclusters and the H${}_{2}$ desorption kinetics becomes slower. In the as-deposited Nb-doped Mg samples, analysis of the positron data reveals the presence of intragranular vacancylike defects and of vacancy clusters which are inferred to be mainly located at the grain boundaries of the nanocrystalline Mg layers. With H${}_{2}$ cycling: (i) a decrease of the atomic fraction of the intragranular vacancylike defects after the first two sorption cycles was observed, and (ii) an increase of the atomic fraction of vacancy clusters at grain boundaries and the appearance of vacancylike defects located at the interface between the Nb aggregates and the Mg matrix was probed. It was also found that the kinetics follows a nucleation and growth mechanism and, under stationary conditions, the Mg nucleation is controlled by vacancy-decorated bcc Nb nanoclusters rather than by vacancy clusters, as in undoped Mg samples.

Electrical and optical properties of ZnO thin films doped with Nb deposited by rf magnetron sputtering

The optical and electrical properties are investigated of Nb doped ZnO (ZnO:Nb) thin films deposited by r.f. magnetron co-sputtering on glass substrates at different temperatures, Ts, between 50 °C and 500 °C. The optical transmittance and reflectance spectra are measured and used to determine the optical band gap. The values of the optical band gap are in the range 3.29- 3. 44 eV and are typical for ZnO thin films. The films have about 90 % transparency, while their resistivity increases in the range of 2.2×10 −3 Ω cm - 6.4×10 2 Ω cm as Ts is raised. The results demonstrate the potential of using Nb-doped ZnO films as a transparent conductive oxide in thin films solar cells and various optoelectronic devices.

Transparent Conductive Properties of TiOxand Nb-Doped TiOxFilms Produced by Reactive Co-Sputtering from Ti and Nb2O5Targets

We investigated the structural and transparent conductive properties of oxygen-deficient TiOx films that were deposited by metal-mode reactive electron cyclotron resonance plasma sputtering from a Ti target at 400 °C. Crystallites in a strongly reduced state (x≈1) had face centered cubic (fcc) structures with the resistivities ranging from 10-4 to 10-3 Ω cm, and the optical transmittance in the visible wavelength was between 25 and 55%. In a sufficiently oxidized state (x≈2), rutiles nucleated with resistivites higher than 10-2 Ω cm, and the optical transmittance was between 60 and 80%. The intermediate composition (1< x < 2) corresponded to fcc structures although the crystallinity approached an amorphous state with increasing x. Crystallization into magneli phases (TinO2n-1) was observed only for thick films at deposition temperatures higher than 500 °C. Carriers were n-type for rutile, but p-type for the fcc and magneli phases. Nb-doped TiOx films were produced by metal-mode sputtering of TiOx with co-sputtering Nb and O from an Nb2O5 target. The donor role of Nb5+ could be identified only in the oxidized rutile state, but the resistivity increased at higher Nb2O5 sputtering powers due to oxidation of Nb atoms that substituted Ti sites.

Transparent Conductive Properties of TiOx and Nb-Doped TiOx Films Produced by Reactive Co-Sputtering from Ti and Nb2O5 Targets

We investigated the structural and transparent conductive properties of oxygen-deficient TiOx films that were deposited by metal-mode reactive electron cyclotron resonance plasma sputtering from a Ti target at 400 °C. Crystallites in a strongly reduced state (x≈1) had face centered cubic (fcc) structures with the resistivities ranging from 10-4 to 10-3 Ω cm, and the optical transmittance in the visible wavelength was between 25 and 55%. In a sufficiently oxidized state (x≈2), rutiles nucleated with resistivites higher than 10-2 Ω cm, and the optical transmittance was between 60 and 80%. The intermediate composition (1< x < 2) corresponded to fcc structures although the crystallinity approached an amorphous state with increasing x. Crystallization into magneli phases (TinO2n-1) was observed only for thick films at deposition temperatures higher than 500 °C. Carriers were n-type for rutile, but p-type for the fcc and magneli phases. Nb-doped TiOx films were produced by metal-mode sputtering of TiOx with co-sputtering Nb and O from an Nb2O5 target. The donor role of Nb5+ could be identified only in the oxidized rutile state, but the resistivity increased at higher Nb2O5 sputtering powers due to oxidation of Nb atoms that substituted Ti sites.

Electrochromic Nb-doped WO3 films: Effects of post annealing

The Nb-doped WO3 films were deposited by e-beam co-evaporation method using ceramic WO3 targets and metal Nb slugs. The films were analyzed by glancing incident angle X-ray diffraction (GIAXRD), UV/visible spectrophotometer, electrochemical cyclic voltammetry, X-ray photoelectron spectroscopy (XPS). The as-prepared film is brown and amorphous in structure. The film has low transmission in optical visible region. The XPS results indicate that the as-deposited film is non-stoichiometric. By applying a negative potential, the as-deposited film does not show obvious electrochromic effect. However, the electrochromic properties of Nb-doped WO3 films are improved by post annealing treatment at 350, 400, and 450°C in oxygen atmosphere. The Nb-doped WO3 films transform into crystalline structure and become transparent after post annealing treatment. The energy band gap, optical modulation, and color efficiency increase with annealing temperature.

Structural, Optical, and Electrical Properties of Nb-Doped ZnO Thin Films Prepared by Spray Pyrolysis Method

Niobium (Nb) doping (0 at.% to 3 at.%) in ZnO thin films prepared by the chemical spray pyrolysis method at a substrate temperature of 400°C enhances the optical and electrical properties but deteriorates the structural quality of the films. The films are polycrystalline with hexagonal structure having a preferential orientation along the (002) crystallographic direction. The film doped with 3 at.% Nb demonstrates a maximum average transmittance of ~83% in the visible region. A strong blue emission is recorded for both pure and doped films, and the intensity is substantially enhanced with Nb doping due to interface and valence-band transitions. Vacuum annealing at 400°C for 60 min improves the electrical characteristics of the films, and the highest mobility of 71 cm2/V s is achieved for the 1 at.% Nb-doped ZnO films.

Investigations on the structural, optical and electrical properties of Nb-doped SnO2 thin films

Niobium-doped tin oxide thin films were deposited on glass substrates by the chemical spray pyrolysis method at a substrate temperature of 400 °C. Effects of Nb doping on the structural, electrical and optical properties have been investigated as a function of niobium concentration (0–2 at.%) in the spray solution. X-ray diffraction patterns showed that the films are polycrystalline in nature and the preferred growth direction of the undoped film shifts to (200) for Nb-doped films. Atomic force microscopy study shows that the surface morphology of these films vary when doping concentration varies. The negative sign of Hall coefficient confirmed the n-type conductivity. Resistivity of ~4.3 × 10−3 Ω cm, carrier concentration of ~5 × 1019 cm−3, mobility of ~25 cm2 V−1 s−1 and an average optical transmittance of ~70% in the visible region (500–800 nm) were obtained for the film doped with 0.5 at.% niobium.

Thermoelectric Properties of Nb-Doped Ordered Mesoporous TiO2

Mesoporous materials have pores with diameters between 2 nm and 50 nm, the presence of which generally decreases the thermal conductivity of the material. By incorporating mesoporous structures into thermoelectric materials, the thermoelectric properties of these materials can be improved. Although TiO2 is an ordinary insulator, reduced TiO2 shows better electrical conductivity and is therefore a potential thermoelectric material. Furthermore, the addition of a dopant to TiO2 can improve its electrical conductivity. We hypothesized that, by doping ordered mesoporous TiO2 films with niobium, we would be able to minimize the thermal conductivity and maximize the electrical conductivity. To investigate the effects of Nb doping and a mesoporous structure on the thermoelectric characteristics of TiO2 films, Nb-doped mesoporous films were investigated using x-ray diffraction, ellipsometry, four-point probe measurements, and thermal conductivity analysis. We found that Nb doping of ordered mesoporous TiO2 films improved their thermoelectric properties.

The effect of Nb substitution on synthesis and photo-response of TiO2 thin films prepared by direct current magnetron sputtering

TiO2 has a limited photoefficiency for solar energy harvesting and conversion due to its large bandgap. Nb substitution into the TiO2 lattice was investigated to red-shift its photoresponse by introducing defect energy levels in the host bandgap. Ti1−xNbyO2 thin films (0≥y≤0.52) were prepared by direct current reactive magnetron sputtering. The effect of Nb concentration on crystal phase formation and subsequent change in optical and electronic properties were determined. Films had an increasing rutile proportion as Nb concentration increased, and all films prepared for y>0.25 were pure rutile. Nb substituted as Nb(V) or a mixture of Nb(IV)/(V) while Ti remained Ti(IV). Therefore, in these materials cation vacancies formed to electronically compensate for group V doping. Nb-doped rutile films showed no red-shift in optical response while Nb-doped anatase/rutile composites were strongly red-shifted. Photocurrent measurements were made to determine if the enhanced visible light absorption generated mobile electron-hole pairs or was due to localized excitations at color centers. At low Nb concentrations (~2 at.%), visible light absorption generated mobile charge carriers, while at higher concentrations ( >9 at.%), no mobile charge carriers were created. At higher Nb concentrations, the formation of mobile charge carriers was hindered by charge recombination at cation vacancies.

Adjustment of thermal hysteresis in epitaxial VO2 films by doping metal ions

Thermal hysteresises of electrical resistance, accompanying with a structural phase transition, in epitaxial VO2 films have been successfully reduced to 1°C or less by doping Ti or Nb ions. We considered that owing to the metal-ion-substitutive structural defects induced by doping metal ions into VO2 films, the structural phase transition easily occurred without superheating or supercooling. In Nb-doped VO2 films, the hysteresis disappeared at a lower doping level than Ti-doped VO2 films. The maximum values of the temperature coefficient of the resistance of V0.91Ti0.09O2 and V0.982Nb0.018O2 films, which exhibited non-hysteretic MI transitions, were −24.8%/°C at 46°C and −21.6%/°C at 19°C, respectively.

Metal-semiconductor transition in Nb-doped ZnO thin films prepared by pulsed laser deposition

The structural, electrical and optical properties of Nb-doped ZnO films were investigated with different Nb contents (0, 0.15, 0.31, 0.46, 0.62, and 0.94 at.%) in this article. The film with 0.46 at.% Nb content showed the lowest resistivity of 8.95 × 10 − 4 Ω cm and high transmittance about 80% with high c-axis orientation. The undoped ZnO film showed a semiconducting behavior. And Nb-doped ZnO films showed a metal-semiconductor transition (MST), which was connected with localization of degenerate electrons. The films showed metallic conductivity at temperatures closer to the ambient temperature and semiconducting behavior at lower temperatures. It was noted that the NZO films with much lower Nb concentration of 0.15 at.% presented MST compared with other transparent conducting oxides films.

Surface segregation in Nb-doped BaTiO 3 films

We have used in situ photoemission spectroscopy to investigate Niobium doping in polycristalline BaTiO 3. The valence band maximum position progressively shifts from 2.5 eV for undoped to 2.84 eV for Nb-doped films. Ceramics and single crystal have been investigated for comparison with thin films. Nb-doped BaTiO 3 ceramics and Nb-doped SrTiO 3 single crystal show higher Fermi level position indicating that our doped films are less conducting regarding their bulk parents. This was confirmed by impedance spectroscopy under variable temperature. Large amount of niobium is clearly observable at surface but the amount of dopant is drastically reduced below the near-surface region, as evidenced by depth profile. Therefore, we provide evidence of surface segregation which would explain the contrasted resistivity values reported in literature for such donor-doped films.

Properties of Nb-doped ZnO transparent conductive thin films deposited by rf magnetron sputtering using a high quality ceramic target

Nb-doped ZnO films with (002) orientation have been grown on glass substrates by rf magnetron sputtering followed by vacuum annealing at 400°C for 3 h. The microstructures and surface figures of the Nbdoped ZnO films were investigated with X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. And its optical and electrical properties were measured at room temperature using a four-point probe technique and 756-type spectrophotometer, respectively. X-ray diffraction (XRD) revealed that the films are highly textured along the c axis and perpendicular to the surface of the substrate. After annealing at 400°C for 180 min under vacuum, transmittance of about 90% in visible region for Nb doped ZnO films was confirmed by the optical transmission spectra, and the low resistivity of 5·47 × 10−3 Ω·cm was obtained.

Enhanced ferro- and piezoelectric properties in (100)-textured Nb-doped Pb(Zr xTi 1 − x )O 3 films with compositions at morphotropic phase boundary

To develop high-performance piezoelectric films on conventional Pt(111)/Ti/SiO 2/Si(100) substrates, sol–gel-derived highly [100]-textured Nb-doped Pb(Zr xTi 1 − x )O 3 (PNZT) thin films with different Zr/Ti ratios ranging from 20/80 to 80/20 were prepared and characterized. The phase structure, ferroelectric and piezoelectric properties of the PZNT films were investigated as a function of Zr/Ti ratios, and it was confirmed that there was distinct phase transition of the PNZT system from tetragonal to rhombohedral when the Zr/Ti ratio varied across the morphotropic phase boundary (MPB). The Nb-doped PZT films showed enhanced remanent polarization but reduced coercive field, whose best values reached 75 μC/cm 2 and 82 kV/cm, respectively at the composition close to MPB. In addition, the [100]-textured PNZT film at MPB also shows a high piezoelectric coefficient up to 161 pm/V. All these properties are superior to those for undoped PZT films.

Preparation and Characterization of Transparent Conductive Nb-Doped ZnO Films by Radio-Frequency Sputtering

Niobium-doped ZnO (NZO) transparent conductive films are deposited on glass substrates by rf sputtering at 300° C. Effects of sputtering power on the structural, morphologic, electrical, and optical properties of NZO films are investigated by x-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), Hall measurement, and optical transmission spectroscopy. The obtained films are polycrystalline with a hexagonal wurtzite structure and preferentially oriented in the (002) crystallographic direction. The minimum resistivity of 4.0 × 10−4 Ω·cm is obtained from the film grown at the sputtering power of 170 W. The average optical transmittance of the films is over 90%.