Nb-doped TiO2 Thin Films Research Articles

Nb-doped TiO2 thin films for photovoltaic applications

Abstract Polycrystalline titania and Nb:TiO 2 thin films were deposited by RF magnetron sputtering. The influence of post-deposition annealing in vacuum and hydrogen atmosphere on the structure, morphology, oxidation states and optical properties was studied by X-ray diffraction, atomic force microscopy, XPS and UV–VIS spectroscopy. The heat treatment of titanium dioxide thin films in vacuum and H 2 atmosphere induces structural and morphological changes. The band gap narrowing was observed for the transparent as-deposited Nb:TiO 2 films, while annealing at 420 °C in H 2 atmosphere resulted in an enhancement of the electrical conductivity. Further on, TiO 2 / p -CdTe photovoltaic devices with efficiency of 1.8% were fabricated and their characteristic ‘enhancement’ is discussed.

Synthesis and Characterization of Nb-Doped TiO<sub>2</sub> Thin Films Prepared by RF Magnetron Sputtering

Amorphous Nb-doped TiO2 thin films were deposited on (100) Si and glass substrates at room temperature by RF magnetron sputtering and a mosaic-type Nb2O5-TiO2 sputtering target. To adjust the amount of the niobium dopant in the film samples, appropriate numbers of Nb2O5 pellets were placed on the circular area of the magnetron target with intensive sputtering. By adjusting the discharge conditions and the number of niobium oxide pellets, films with dopant content varying between 0 and 16.2 at.% were prepared, as demonstrated by X-ray photoelectron spectroscopy data. The X-ray diffraction patterns of the as-deposited samples showed the lack of crystalline ordering in the samples. Surfaces roughness and energy band gap values increase with dopant concentration, as showed by atomic force microscopy and UV-Vis spectroscopy measurements.

Effects of Nb doping on microstructure and photocatalytic properties of TiO2 thin film

Pure TiO2 and Nb-doped TiO2 thin films deposited on quartz glass substrate were prepared by the sol-gel method. The morphologies, microstructure and spectral absorption properties of the as-prepared TiO2 thin films were investigated by scanning electron microscope (SEM), atomic force microscope (AFM) and UV–VIS absorption spectra (UV–VIS). The photocatalytic activity of the TiO2 thin films was evaluated by the degradation of methyl orange under ultraviolet illumination. The results showed that after Nb-doping the grain size of Nb-doped TiO2 particles was decreased to some extent and the pore size distribution was affected, but the morphology was changed negligibly. Moreover, the Nb-doping could produce a blue shift in the optical absorption edge compared to that of the pure TiO2 thin film. The Nb-doped TiO2 film showed lower photocatalytic activity for the degradation of methyl orange. We also investigated the effect of the number of layers on methyl orange degradation. The more the coating amount, the thicker the TiO2 thin film and the higher photocatalytic efficiency.

Structural and vibrational investigations of Nb-doped TiO2 thin films

Acid-catalyzed sol–gel and spin-coating methods were used to prepare Nb-doped TiO2 thin film. In this work, we studied the effect of niobium doping on the structure, surface, and absorption properties of TiO2 by energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), X-ray reflectometry (XRR), X-ray photoelectron spectroscopy (XPS), Raman, and UV–vis absorption spectroscopy at various annealing temperatures. EDX spectra show that the Nb:Ti atomic ratios of the niobium-doped titania films are in good agreement with the nominal values (5 and 10%). XPS results suggest that charge compensation is achieved by the formation of Ti vacancies. Specific niobium phases are not observed, thus confirming that niobium is well incorporated into the titania crystal lattice. Thin films are amorphous at room temperature and the formation of anatase phase appeared at an annealing temperature close to 400°C. The rutile phase was not observed even at 900°C (XRD and Raman spectroscopy). Grain sizes and electron densities increased when the temperature was raised. Nb-doped films have higher electron densities and lower grain sizes due to niobium doping. Grain size inhibition can be explained by lattice stress induced by the incorporation of larger Nb5+ ions into the lattice. The band gap energy of indirect transition of the TiO2 thin films was calculated to be about 3.03eV. After niobium doping, it decreased to 2.40eV.

Effect of annealing treatment on electrical and optical properties of Nb doped TiO2 thin films as a TCO prepared by sol–gel spin coating method

Anatase niobium doped TiO2 thin films (Ti0.94Nb0.06O2) were prepared on quartz substrates by sol–gel spin coating technique and various initial concentrations ([Ti+Nb] in the solution) of 0.1M and 0.4M. All the films dried at 100°C and then the annealing treatment performed by three different procedures. The influence of different annealing processes and total metal concentrations was studied on crystallinity, transparency, conductivity and morphology of thin films by XRD, EDX, FT-IR and UV–vis transmittance spectroscopy, four point probe, AFM and SEM images. It was found that two-step annealing procedure under vacuum is a more proper way to increase the conductivity of TNO thin films compared to one-step annealing process. Two-step method exhibited the minimum resistivity of 4.4Ωcm and the transparency of about 50% in the visible region with 36nm thickness.

Lateral Solid-Phase Epitaxy of Oxide Thin Films on Glass Substrate Seeded with Oxide Nanosheets

We developed a technique to fabricate oxide thin films with uniaxially controlled crystallographic orientation and lateral size of more than micrometers on amorphous substrates. This technique is lateral solid-phase epitaxy, where epitaxial crystallization of amorphous precursor is seeded with ultrathin oxide nanosheets sparsely (≈10% coverage) deposited on the substrate. Transparent conducting Nb-doped anatase TiO2 thin films were fabricated on glass substrates by this technique. Perfect (001) orientation and large grains with lateral sizes up to 10 μm were confirmed by X-ray diffraction, atomic force microscopy, and electron beam backscattering diffraction measurements. As a consequence of these features, the obtained film exhibited excellent electrical transport properties comparable to those of epitaxial thin films on single-crystalline substrates. This technique is a versatile method for fabricating high-quality oxide thin films other than anatase TiO2 and would increase the possible applications of oxide-based thin film devices.

Mechanism of insulator-to-metal transition in heavily Nb doped anatase TiO2

Heavily Nb-doped anatase TiO2 (TNO) thin films were prepared by pulsed dc magnetron sputtering using an Nb-doped TiO2 target. The as-grown films exhibit high resistivity whose resistance decreases by ∼2 × 104-fold upon vacuum annealing. The ∼40% Nb-doped anatase TiO2 film shows a low resistivity of 5.7 × 10−4 Ω cm and a high electron concentration of 3.07 × 1021 cm−3. Combined in situ x-ray photoelectron spectroscopy (XPS) measurement and density-functional theory (DFT) calculations show that oxygen interstitial (Oint) and Nb interstitial (Nbint) defect clusters introduce localized shallow p-type accepter states that trap the electrons and reduce the conductivity. These defect clusters can be eliminated by vacuum annealing which is companied by outward diffusion of Nb. As a result, the trapped electrons backfill the Ti sites which are delocalized to promote conductivity.

Effects of annealing in H2 atomsphere on optoelectronical properties of Nb-doped TiO2 thin films

Niobium-doped TiO2 thin films are deposited on strontium titanate substrates by E-beam evaporation deposition. Effects of post-annealing in hydrogen atmosphere on their optoelectrical properties are studied. The results show that the annealing in hydrogen atmosphere can enhance their conductivity values efficiently. The corresponding optium resistivity reaches 5.46×10-3 Ω·cm, and the transmittance values of the thin films are 60%–80%. The improvement in the conductive performance is attributed to the increase of the grain size of polycrystalline thin film, the formation of a lot of oxygen vacancies and H-doping caused by annealing in hydrogen atmosphere.

Solution processed thin films of Nb-doped TiO2 nanoparticles as hole blocking layer for organic solar cells

In this study, Nb-doped TiO2 nanoparticles were synthesized by sol–gel method with controlled doping ratios up to 6mol%. Catalyzed by p-toluene sulfonic acid, alkoxide precursors of Ti and Ni were stabilized by acetylacetone ligand, and converted to the organically modified nanoparticles which were well dispersible in alcohols. Transmission electron microscopy and X-ray diffraction measurements revealed that TiO2 nanoparticle possesses anatase morphologies, and the doping ratios were confirmed by X-ray photoelectron spectroscopy. Uniform thin films of Nb-doped TiO2 were obtained by spin coating dilute alcohol dispersions of Nb-doped nanoparticles. Ellipsometric characterizations were carried out for the as-coated film and the sintered film as well to characterize the indices of refraction, and atomic force microscopy showed subnanometer-scale roughness of Nb-doped films. Owing to the improved roughness via doping, Nb-doped TiO2 thin films exhibited enhanced solar cell efficiencies when used as hole-blocking layer of organic hetero-junction solar cell.

Conducting Nb-doped TiO2 thin films fabricated with an atomic layer deposition technique

Highly conducting Ti1−xNbxO2 thin films have been grown on glass substrates from TiCl4, Nb(OEt)5 and H2O with an atomic layer deposition (ALD) technique. The films become electrically conducting and crystallize with the anatase structure upon a reductive post-deposition annealing. A highly c-axis oriented Ti0.75Nb0.25O2 film exhibits room-temperature resistivity as low as 1.4×10−3Ωcm. The charge carrier density and electron mobility, as estimated from Seebeck and resistivity measurements, are 0.21–1.1×1021cm−3 and 4.2–22cm2/Vs, respectively. The electrical properties of the ALD-fabricated Nb-doped anatase films are comparable with those of sputter-deposited polycrystalline films on glass.

Oxygen sensing performance of Nb-doped TiO2 thin film with porous structure

In this paper, Nb-doped titanium dioxide (TiO2) porous thin films in application of oxygen sensors were studied. A facile sol–gel method was used to prepare the undoped and Nb-doped TiO2 thin films. The microstructure and oxygen-sensing behavior were investigated. The results indicate that Nb-doping can retard the phase transition from anatase to rutile and reduce the grain size. The oxygen responses of the thin film can be greatly improved by Nb-doping, which are 73.2 and 78.7 for sample with 6% Nb-doping measured at 700 °C in the atmosphere with 2% and 20.9% oxygen, while those for undoped TiO2 are only 6.5 and 6.6 in the same condition. The 6% Nb-doped TiO2 thin film also shows the fastest response time of only 1.1 s at 700 °C, which is only less than 1/10 of those for the undoped TiO2 thin film.

Wet Etching of Amorphous TiO2 Thin Films Using H3PO4–H2O2 Aqueous Solution

We report on the wet etching of amorphous undoped and Nb-doped TiO2 thin films using H3PO4–H2O2 etching solution. The etching rate (R) showed a maximum at a H3PO4 concentration of approximately 50 wt % at 80 °C, suggesting that H2PO4- and/or H3O+ is responsible for the etching reaction. The addition of H2O2 to H3PO4 solution significantly enhanced R, and an optimized solution exhibited an R of 13 nm/min at 80 °C, which is one order of magnitude higher than that using H2SO4. These results demonstrate that H3PO4–H2O2 aqueous solution is an effective etchant for TiO2-based amorphous thin films.

Comparison of optical and structural properties of nanostructure TiO2 thin film doped by Sn and Nb

The thin films of TiO2 doped by Sn or Nb were prepared by sol–gel method under process control. The effects of Sn and Nb doping on the structural, optical and photo-catalytic properties of applied thin films have been studied by X-ray diffraction (XRD) high resolution transmission electron microscopy and UV–Vis absorption spectroscopy. Surface chemical state of thin films was examined by atomic X-ray photoelectron spectroscopy. XRD results suggest that adding impurities has a great effect on the crystallinity and particle size of TiO2. Titania rutile phase formation in thin film was promoted by Sn4+ addition but was inhibited by Nb5+ doping. The activity of the photocatalyst was evaluated by photocatalytic degradation kinetics of aqueous methylene blue under UV and Visible radiation. The results show that the photocatalytic activity of the Sn-doped TiO2 thin film have a larger degradation efficiency than Nb-doped TiO2 under visible light, but under UV light photocatalytic activity of the Nb-doped TiO2 thin film is better.

Low temperature resistivity, thermoelectricity, and power factor of Nb doped anatase TiO2

The resistivity of a very high quality anatase TiO2 doped with 6% of Nb was measured from 300 K down to 40 mK. No sign of superconductivity was detected. Instead, a minute quantity of cation vacancies resulted in a Kondo scattering. Measurements of thermo-electric power and resistivity were extended up to 600 K. The calculated power factor has a peak value of 14 μW/(K2cm) at 350 K, which is comparable to that of Bi2Te3 [Venkatasubramanian et al., Nature 413, 597 (2001)], the archetype thermolectrics. Taking the literature value for the thermal conductivity of Nb doped TiO2 thin films, the calculated figure of merit (ZT) is in the range of 0.1 above 300 K. This value is encouraging for further engineering of the material in order to reach ZT of 1 suitable for high temperature thermoelectrics.

Optical properties of Nb-doped TiO2 thin films prepared by sol–gel method

Abstract In this work, we studied optical properties of pure and Nb-doped TiO 2 synthesized using a sol–gel method and deposited as thin films by spin-coating followed by annealing in air at 500 °C for 1 h. The surface elemental composition was derived from X-ray photoelectron spectra, while structure and surface morphology were investigated using X-ray diffraction and atomic force/scanning electron microscopy. Finally, the optical properties were investigated by means of UV–vis spectrophotometry and spectroscopic ellipsometry. The Nb content was determined from XPS measurements to vary between 1.8 and 4.3 at%. The XRD patterns of the deposited thin films, with a maximum thickness of about 56 nm, showed no diffraction peaks. As proven both by microscopy and spectroscopic ellipsometry studies doping TiO 2 with Nb modified the surface morphology of the samples; the grain size is increasing while the surface roughness decreases with the increase in Nb content. This is accompanied by a decrease in the refractive index and an increase of the extinction coefficient.

Fabrication of Nb-Doped TiO2Transparent Conducting Films by Postdeposition Annealing under Nitrogen Atmosphere

Here, we report that highly conductive polycrystalline anatase Nb-doped TiO2 (TNO) thin films can be prepared via crystallization of amorphous precursors under N2 atmosphere. An optimized TNO film on a glass substrate exhibited a low resistivity of 8.4 ×10-4 Ω cm and an absorbance of 6% at a wavelength of 460 nm. These transport and optical properties were comparable to those of TNO films fabricated by vacuum annealing. This demonstrates the potential of TNO as an electrode for GaN-based light-emitting diodes.

Fabrication of Nb-Doped TiO2 Transparent Conducting Films by Postdeposition Annealing under Nitrogen Atmosphere

Here, we report that highly conductive polycrystalline anatase Nb-doped TiO2 (TNO) thin films can be prepared via crystallization of amorphous precursors under N2 atmosphere. An optimized TNO film on a glass substrate exhibited a low resistivity of 8.4 ×10-4 Ω cm and an absorbance of 6% at a wavelength of 460 nm. These transport and optical properties were comparable to those of TNO films fabricated by vacuum annealing. This demonstrates the potential of TNO as an electrode for GaN-based light-emitting diodes.

Effect of hot-filament annealing in a hydrogen atmosphere on the electrical and structural properties of Nb-doped TiO2 sputtered thin films

In this work Nb-doped TiO2 thin films were deposited by d.c.-pulsed reactive magnetron sputtering at 500°C from a composite target with weight fractions of 96% Ti and 4% Nb, using oxygen as reactive gas. In order to enhance the conductive properties, the as-deposited samples were treated in vacuum with atomic hydrogen at a substrate temperature of 500°C. The atomic hydrogen flow was generated by a hot filament, inside a high-vacuum chemical vapour deposition reactor, at a temperature of 1750°C. In order to optimise the hydrogen hot-wire treatments, the H2 pressure was varied between 1.3 and 67Pa, the treatment time was monitored between 1 and 5min and the hot-filament current was changed between 12 and 17A. Dark conductivity was measured as a function of temperature and its value at room temperature was extrapolated and used to assess the effect of the hydrogen annealing on the charge transport properties. A two-order of magnitude increase in dark conductivity was typically observed for optimised hydrogen treatments (10Pa), when varying the hydrogen pressure, resulting in a minimum resistivity of ~3×10−3Ωcm at room temperature. The maximum amount of atomic H incorporation in oxygen vacancies was determined to be ~5.7at.%. Carrier mobility and resistivity were also investigated using Hall effect measurements. Correlations between structural and electrical properties and the hydrogen treatment conditions are discussed. The purpose of these films is to provide a transparent and conductive front contact layer for a-Si based photovoltaics, with a refractive index that better matches that of single and tandem solar cell structures. This can be achieved by an appropriate incorporation of a very small amount of cationic doping (Nb5+) into the titanium dioxide lattice.

Influence of annealing process on conductive properties of Nb-doped TiO2 polycrystalline films prepared by sol–gel method

Nb-doped TiO2 (TNO) thin films were prepared by sol–gel dip-coating method with Nb content in a wide range of 0–20at.%. The prepared films were preheated at 400°C and then undertaken by two different post-annealing processes: (a) three times vacuum annealing and (b) multi-round annealing. The designed multi-round annealing was shown to be an effective way to improve the conductive properties of the films, compared to the traditional vacuum annealing process. The minimum resistivity reached approximately 0.5Ωcm with Nb doping concentration around 12at.%, and the carrier density increased with Nb-doping concentration until the critical point of 12at.%, which might be the optimal doping content for our TNO films prepared by sol–gel method.

Optical Properties of Ultrathin Copper Thin Films Sandwiched between Nb-Doped TiO2 Films Studied with Spectroscopic Ellipsometry

Cu thin films sandwiched between Nb-doped TiO2 (NTO) thin films were grown on glass substrates using tilted-dual target DC magnetron sputtering deposition. The thicknesses of the top and bottom NTOs were nominally 30 nm, and the thicknesses of the Cu films (t) varied between 1.5 nm and 50 nm. We measured the ellipsometric angles (Ψ,Δ) of the NTO/Cu/NTO/glass by using spectroscopic ellipsometry and estimated the thicknesses and dielectric functions of the Cu films by using a multi-layer model analysis. Transmission electron microscopy measurements showed that the Cu layers evolved from aggregates of Cu nanoparticles to coalesced Cu thin films as the Cu film thickness increased. Sheet resistance data also showed that the t = 8 nm film was near the percolation threshold, suggesting that films thinner than 8 nm were aggregations of Cu nanoparticles that were not well-connected. The films thicker than 8 nm were above the percolation threshold. From the Drude model, we estimated the plasmon frequency (ωp) and the electron relaxation time (τ), which were found to increase with increasing film thickness. By applying standard critical point analysis to the second derivatives of the dielectric function spectra, we found several peaks near 1.5, 2.1, 2.5, 3.5, and 4.3 eV, and attributed to interband transitions. The peak energies (except 1.5 eV) matched to the band structure calculations of bulk Cu found in the literature.