Ta-doped SnO2 Research Articles

Growth of Ta-doped SnO2 on GaN as a UV-transparent conducting electrode and band alignment properties of the heterojunction

GaN-based ultraviolet light emitting diodes (UV LEDs) have attracted considerable attention in recent years and are required in various applications such as healthcare, light illumination, and optical communication. However, the limited UV transparency of the electrodes like indium-doped tin oxide has hindered the external quantum efficiency of current UV LEDs. In this work, we present the growth of UV-transparent Ta-doped SnO2 (TTO) thin films on GaN as a promising UV-transparent electrode for LEDs. TTO thin films with a thickness of 200 nm exhibit optical transmission exceeding 80% at the wavelength of 300 nm, with a low resistivity of 2.5 × 10−4 Ω·cm and a low contact resistance of 1.7 × 10−2 Ω cm2 to n-type GaN. High-resolution x-ray photoemission spectra were employed to reveal insight into the electronic structure of TTO and the interfacial band alignment of TTO/GaN heterojunction. The wide optical bandgap (∼4.6 eV) and high UV transparency of TTO films stem from a significant Burstein–Moss shift due to degenerate doping, giving rise to metal-like characteristics and a small barrier height at the interface of TTO/GaN. These findings imply the origin of low contact resistivity of TTO to n-type GaN and may be applicable to the development of UV-transparent electrodes of optoelectronic devices.

The origin of the n-type conductivity for Ta-doped SnO2: Density functional theory study

The electronic structure, intrinsic defects and extrinsic doping of SnO2 are studied by using the hybrid functional method. Research shows that SnO2 is a direct gap semiconductor. Along the Γ-M (Γ-Z) direction, the electron effective mass is 0.23 (0.19) m0 (m0: the mass of the free electron). When the Hartree-Fock mixing parameter (α) increases to 38%, the calculated band gap for SnO2 is 3.53 eV, which is close to the experimental results. For nonstoichiometric SnO2, both VO and Snint are major n-type defects, and the ionization energies ε(0/+) are above the conduction band minimum (CBM). About Ta-doped SnO2, obtained results show Ta substituting Sn (TaSn) is a predominate n-type defect. This work could promote the understanding of the n-type conductivity of SnO2 and provide significant guidance for their applications in transparent electronics.

Effects of Sb doping on the electrical characteristics of Ta-doped SnO2 varistors

This article describes the combined mixture of antimony and tantalum in SnO2 varistors. The results show that a moderate level of antimony can reduce the grain resistivity and maintain a high voltage gradient while providing good nonlinearity. The best electrical properties were obtained by doping 0.5 mol% antimony pentoxide and 0.003 mol% tantalum pentoxide. The nonlinear coefficient of 51, the leakage current of 5.7 μA/cm2, the voltage gradient of 480 V/mm and the grain resistance of 5.0 Ω.

Electron dephasing processes in the thin and thick Ta-doped SnO2 films

We have systematically investigated the electron dephasing processes of the thin and thick Ta-doped SnO2 films. All samples exhibit electron transport characteristics of degenerate semiconductors. At low temperature T (below ∼70 K), the weak-localization-induced negative magnetoresistance (MR) is observed. For the thick films with the thicknesses of ∼300 nm, the observed MR can be well described by three-dimensional weak-localization theory. The electron dephasing rate 1/τφ varies linearly with T3/2, indicating that the electron–electron scattering dominates the electron dephasing process. This is different from the common observation that the electron–phonon scattering is the main dephasing process for electrons in three-dimensional disordered conductors. For the thin films with the thicknesses of ∼30 nm, the MR can be well described by two-dimensional weak-localization theory. By analyzing the electron dephasing rate as a function of T, we find that both large- and small-energy-transfer electron–electron scattering processes play an important role in the electron dephasing.

Transparent conductive SnO2 thin films via resonant Ta doping

Transparent conductive oxide (TCO) thin films are highly desired as electrodes for modern flat-panel displays and solar cells. Alternative indium-free TCO materials are highly needed, because of the scarcity and the high price of indium. Based on the mechanism of resonant doping, Ta has been identified as an effective dopant for SnO2 to achieve highly conductive and transparent TCO. In this work, we fabricated a series of Ta-doped SnO2 thin films (Sn1−xTaxO2, x = 0.001, 0.01, 0.02, 0.03) with high conductivity and high optical transparency via a low-cost sol-gel spin coating method. The Sn0.98Ta0.02O2 film achieves the highest electrical conductivity of 855 S cm−1 with a carrier concentration of 2.3 × 1020 cm−3 and high mobility of 23 cm2 V−1 s−1. The films exhibit a very high optical transparency of 89.5% in the visible light region. High-resolution X-ray photoemission spectroscopy and optical spectroscopy were combined to gain insights into the electronic structure of the Sn1−xTaxO2 films. The optical bandgaps of the films are increased from 3.96 eV for the undoped SnO2 to 4.24 eV for the Sn0.98Ta0.02O2 film due to the occupation of the bottom of conduction band by free electrons, i.e., the Burstein-Moss effect. Interestingly, a bandgap shrinkage is also directly observed due to the bandgap renormalization arising from many-body interactions. The double guarantee of transparency and conductivity in Sn1−xTaxO2 films and the low-cost growth method provide a new platform for optoelectronic and solar cell applications.

Electrical transport properties of thick and thin Ta-doped SnO2 films

Ta-doped SnO2 films with high conductivity and high optical transparency have been successfully fabricated using the rf-sputtering method, and their electrical transport properties have been investigated. All films reveal degenerate semiconductor (metal) characteristics in electrical transport properties. For the thick films (t∼1μm with t being the thickness) deposited in pure argon, the electron–phonon scattering alone cannot explain the temperature-dependent behaviors of resistivity, the interference effect between electron–phonon and electron–impurity scattering should be considered. For t≲36 nm films, both the conductivity and the Hall coefficient show a linear relation with the logarithm of temperature (ln⁡T) from ∼100 K down to liquid helium temperature. The ln⁡T behaviors of conductivity and Hall coefficient cannot be explained by the Altshuler-Aronov type electron–electron interaction effect but can be quantitatively interpreted by the electron–electron interaction effects in the presence of granularity. Our results not only provide strong support for the theoretical results on the electron–phonon–impurity interference effect, but also confirm the validity of the theoretical predictions of charge transport in granular metals in a strong coupling regime.

Solid‐State Ligand‐Capped Metal Oxide Electron‐Transporting Layer for Efficient and Stable Fullerene‐Free Perovskite Solar Cells

Perovskite Solar Cells In article number 2100671, Yu-Ching Huang and co-workers developed a process for fabricating perovskite solar cells with all-oxide charge transporting layers under ambient condition. By using solid-state ligand exchange at room temperature, the more conductive Ta-doped SnO2 could be applied to manufacture perovskite solar cells with excellent operation stability.

Boltzmann conductivity approach for charge transport in spray-deposited transparent Ta-doped SnO2 thin films

Transparent conducting oxide (TCO) thin films should possess good resistance stability with temperature, apart from high figure of merit, to be suitable as electrodes for several potential optoelectronic device applications. In most of the metal oxide systems, an increase in the electrical conductivity with temperature indicates semiconducting behavior up to a certain critical temperature; and above this critical temperature, metal-like behavior is witnessed. Theoretical and experimental evidence for such change in charge transport behavior is crucial for further development and applicability of these TCO systems. In the present work, temperature-dependent electrical properties of spray-pyrolyzed Ta-doped SnO2 (TTO) thin films are explored for their resistance behavior in the high temperature regime. In order to justify the experimentally determined electrical conductivity of the TTO films, the values are correlated with theoretically estimated electrical parameters by adopting Boltzmann conductivity approach. The structural, chemical, morphological, optical and electrical properties of the TTO films indicate the optimal Ta dopant concentration to be 4 wt%. The film deposited with 4 wt% Ta dopant exhibits sheet resistance of 17.96 Ω/□, resistivity of 4.36 × 10−4 Ω cm, transmittance ≅ 85% (at λ = 550 nm) and band gap of 4.01 eV, indicating its potential to serve as wide band gap TCO electrode in several optoelectronic applications.

Enhanced oxygen reduction electrocatalysis on PtCoSn alloy nanocatalyst mediated by Ta-doped SnO2 support for polymer electrolyte fuel cells

For polymer electrolyte fuel cells (PEFCs), platinum-based catalysts for the air cathode supported on metal oxides such as SnO2 exhibit highly promising durability. Yet, the activity for the oxygen reduction reaction (ORR) has still not been sufficiently high toward widespread application. Here, we report a new type of catalyst with significantly boosted ORR activity, based on PtCoSn alloy nanostructures formed on Ta-doped SnO2 support (PtCoSn/Ta-SnO2). The ORR activity of the PtCoSn/Ta-SnO2 catalyst is evaluated in acidic media (0.1 M HClO4) from 20 to 80 °C. The apparent rate constants (kapp) for the ORR on PtCoSn/Ta-SnO2 are found to be about 4.5 and 1.5 times greater than those of commercial Pt and Pt3Co alloy catalysts supported on carbon black (c-Pt/C and c-Pt3Co/C), respectively, and increase with temperature, in accordance with an Arrhenius-type relation from 20 to 60 °C. The enhanced ORR rates of the PtCoSn/Ta-SnO2 catalyst can be correlated with an increase in the preexponential factor of the Arrhenius equation, together with a lowered activation energy. Experimental and density functional theory results reveal that the incorporation of Sn facilitates O2 adsorption and dissociation, which is beneficial to overcome the kinetic limitation of ORR. Moreover, the durability of PtCoSn/Ta-SnO2 examined by potential-step cycling between 0.6 and 1.0 V at 80 °C has been found to be much higher as compared with c-Pt/C and c-Pt3Co/C.

Optical, electrical, and structural properties of Ta-doped SnO2 films against substrate temperature using direct current magnetron sputtering

Ta-doped SnO2 (TTO) films were deposited at 3 × 10−3 Torr pressure from the 6 wt. % Ta2O5-doped SnO2 target at temperatures of 30–500 °C in Ar sputtering gas. The Ta dopants in the SnO2 host lattice were detected via X-ray photoemission, EDX mapping, and photoluminescence spectra. The Ta5+–Sn4+ substitution formed the OOx at VOxsites in the host lattice and the substitution occurred at deposition temperatures greater than 200 °C. The substitution leads to a decrease in the amount of VOx in the SnO2 lattice corresponding to the preferred rutile SnO2 (110) lattice reflection in the X-ray diffraction patterns. The ultraviolet-visible transmittance in visible light was approximately 80%. The lowest resistivity achieved was 2.0 × 10−3 Ω.cm, with a carrier concentration of 1.28 × 1020 cm−3 and carrier mobility of 24.5 cm2V−1s−1. The integration of the TTO-400 film with (n- and p-) Si exhibited an excellent photoelectric effect.

Short-Wave Infrared PbS Colloidal Quantum Dot ZnO Nanowire Solar Cells Aimed at Tandem Solar Cells

Solution-processed tandem solar cells, that stack two or more single junction subcells with different band gaps to harvest photons in the full solar spectrum more efficiently, have attracted increasing attention recently. Organic photovoltaics and perovskite solar cells are promising candidates for the top and/or middle subcells of tandem solar cells because the solar cells are able to capture visible and near-infrared photon energy. However, there are few materials to choose from for the infrared bottom subcell. While PbS and PbSe colloidal quantum dots (CQDs) have been gaining much attention for short-wave infrared solar cells owing to their wide-range bandgap tunability and solution process compatibility. Among various CQD-based solar cells, PbS / ZnO depleted heterojunction solar cells showed a power conversion efficiency (PCE) over 13% in 2020 (Nat. Commun. 2020, 11 , 103). But there is a limited number of literatures reporting PbS-QD-based solar cells working in the low photon energy region (< 1.0 eV) where optical absorption of PbS QDs is weak. Thus, we have focused on PbS QD/ZnO nanowire (NW) structures with the aim of achieving efficient carrier transport and light absorption simultaneously (J. Phys. Chem. Lett. 2013, 4, 2455). We then investigated the performance of PbS QD/ZnO NW solar cells using PbS CQDs with the first exciton absorption peak locating in the short-wave infrared region. (ACS Energy Lett. 2017, 2, 2110). Here, we develop high efficiency infrared PbS CQDs solar cells aimed at the bottom subcell of tandem solar cells, and discuss the potential for the bottom subcell.We constructed two different solar cells using infrared PbS QDs with the first exciton absorption peak at 1550 nm, which is nanowire-type solar cell (NW-cell) and planar-type solar cell(P-cell). The ZnO nanoparticles layer (P-cell) or ZnO nanowire (NW-cell) as an electron acceptor formed on highly infrared transparent 1% Ta-doped SnO2 (TTO) conductive glass substrates. PbS CQD were deposited on the ZnO layers from a PbS QD octane solution via a layer-by-layer dip coating method. The PbS-QD layer were treated with a tetrabutylammonium iodide (TBAI) methanol solution to replace the insulating oleic acid ligands with iodide anions (PbS-I). The external quantum efficiency (EQE) spectra and the structures of the P-cell and NW-cell are shown in the figure. In the almost entire wavelength region ranging from 380 to 1700 nm, the EQE gave higher in the NW-cell than in the P-cell. The best performing planar-type solar cell exhibited a PCE of 2.68% (Jsc= 18.9 mA/cm2; Voc=0.316 V; FF=0.451) under 100 mW/cm2 simulated AM 1.5G illumination. On the other hand, ZnO nanowire-type solar cells gave a PCE of 5.38% (Jsc=38.7 mA/cm2; Voc=0.335 V; FF=0.415), and showed almost twice as high Jsc value (38.7 mA/cm2) as that of the planar-type solar cells. The integrated Jsc of the nanowire-type solar cells obtained from the EQE spectrum 38.5 mA/cm2 is approximately the measured Jsc value. To investigate the potential for the bottom subcell of the tandem solar cell, infrared-performance of nanowire-type solar cells were studied using different short-cut filters. The solar cells exhibited 2.50 % PCE (Jsc=19.2 mA/cm2; Voc=0.307 V; FF=0.424) and 1.11% PCE (Jsc=8.6 mA/cm2; Voc=0.282 V; FF=0.460) under a long-pass filtered one-sun illumination (l >810 nm (perovskite solar cells equivalent), and l>1100 nm (Si solar cells equivalent)), respectively. All these results indicate that the ZnO nanowire PbS QD solar cells are promising candidates to construct tandem devices with perovskite and c-Si solar cells. Figure 1

Large-area spray deposited Ta-doped SnO2 thin film electrode for DSSC application

Large-area (10 × 10 cm2) Ta-doped SnO2 (TTO) thin film was deposited by a facile spray pyrolysis method. The crystallinity, phase purity, and texture properties of the film are studied using XRD analysis. The XPS study confirmed the charge state of Sn to be in 4+, O1s in 2−, and Ta to be in 5+ state. A homogeneous polyhedron morphology was witnessed owing to the Ta dopant effect. The TTO film was highly transparent in the visible to near infrared region (NIR) with an average transmittance value of 81% at 550 nm. The TTO film was having a free carrier concentration of 5.42 × 1021 cm−3 and mobility of 41.7 cm2/Vs. The 4 wt% Ta doped SnO2 electrode exhibits a low sheet resistance of 26 Ω/□ and a high figure of merit value of 6.15 × 10−3 Ω−1. The resistance stability of sheet resistance was fairly good up to 400 °C indicating suitability of the TTO electrode to serve as conducting photoanode/counter electrode in dye-sensitized solar cell (DSSC) device. The DSSC device fabricated with the TTO electrode exhibited appreciable efficiency of 3.26% which is attributed to the enhanced electrical and optical properties by way of doping with Ta element.

Enhancement of the Catalytic Activity and Load Cycle Durability of a PtCo Alloy Cathode Catalyst Supported on Ta-Doped SnO2 with a Unique Fused Aggregated Network Microstructure for Polymer Electrolyte Fuel Cells

We succeeded in the synthesis of two types of PtCo alloy catalysts supported on Ta-doped SnO2 without using carbon black additives. The PtCo disordered alloy supported on Ta-SnO2 (BET surface area,...

(Invited) Assessing Corrosion Resistance of Antimony-, Niobium- and Tantalum-Doped Tin Oxide Aerogels As Oxygen Evolution Reaction Catalyst Supports in Acidic Media

Despite the fact that iridium (Ir) is among the rarest elements on Earth [1], it remains the state-of-the-art anode electrocatalyst in Proton Exchange Membrane Water Electrolyzers (PEMWE). Indeed, Ir dioxide (IrO2) presents the best compromise between activity toward the Oxygen Evolution Reaction (OER) and stability under the harsh operating conditions of the anode: E > 1.5 V vs. the reversible hydrogen electrode (RHE), 60 < T < 80 °C, presence of oxygen and highly acidic environment [2].In this context, research efforts have been dedicated to minimize Ir loading without compromising OER activity and stability. One possible approach is to stabilize the metal nanoparticles (NPs) on a high surface area support. The classical carbon black supports, widely used in fuel cells, are easily oxidized in OER conditions: carbon corrosion leads to particle aggregation and eventually detachment from the support surface [3]. As an alternative to carbon, metal oxides such as tin dioxide (SnO2) [4] or titanium dioxide (TiO2) [5] feature a good corrosion resistance in PEMWE operating conditions and may strongly interact with the catalyst (strong metal–support interaction (SMSI) effect [6]).In this presentation, we shine a spotlight on iridium oxide (IrOx) NPs supported on SnO2 aerogels doped with, niobium (Nb), tantalum (Ta) or antimony (Sb). The aerogels, synthesized via a sol-gel process, feature a rutile structure after calcination, a specific surface area about 80 m2 g-1 and an electronic conductivity spanning from 5 mS cm-1 to 1 S cm-1 depending on the dopant [7-8]. A highly reproducible colloidal polyol route was used to deposit the IrOx NPs onto the aerogels, ensuring a straightforward comparison of their catalytic performance towards the OER and their resistance to corrosion. We used a home-made flow cell coupled to an inductively-coupled plasma mass spectrometer (ICP-MS) to assess metal dissolution under various potential conditions. As expected, carbon support proved to be unstable in OER conditions (Figure 1.a). In contrast, SnO2 aerogels clearly improved catalyst stability at high potential (Figures 1.b and 1.c). Especially, Ta-doped SnO2 (TaTO) aerogels showed an outstanding corrosion resistance with a negligible loss of dopant when compared to Sb-doped SnO2 (ATO). Based on our experimental observations, we demonstrate that a judicious doping of SnO2 aerogels could help developing highly active and robust OER electrocatalysts in acidic media.Figure 1. Metal concentration under staircase potential profiles from 0.9 V vs. RHE to 1.6 V vs. RHE in 100 mV steps with 300 seconds hold time at each potential for IrOx NPs deposed onto (a) Vulcan© XC-72, (b) ATO (10 at.% of Sb) and (c) TaTO (5 at.% of Ta) . The experiments were performed at room temperature by coupling a home-made flow cell to a PerkinElmer Nexion2000 ICP-MS instrument. A 0.05 M H2SO4 solution deaerated with argon was used as electrolyte. The catalyst ink was deposited on the surface of the glassy carbon electrode (4 mm in diameter) resulting in a loading of 20 μgIr cmgeo -2.

High mobility approaching the intrinsic limit in Ta-doped SnO2 films epitaxially grown on TiO2 (001) substrates

Achieving high mobility in SnO2, which is a typical wide gap oxide semiconductor, has been pursued extensively for device applications such as field effect transistors, gas sensors, and transparent electrodes. In this study, we investigated the transport properties of lightly Ta-doped SnO2 (Sn1−xTaxO2, TTO) thin films epitaxially grown on TiO2 (001) substrates by pulsed laser deposition. The carrier density (ne) of the TTO films was systematically controlled by x. Optimized TTO (x = 3 × 10−3) films with ne ~ 1 × 1020 cm−3 exhibited a very high Hall mobility (μH) of 130 cm2V−1s−1 at room temperature, which is the highest among SnO2 films thus far reported. The μH value coincided well with the intrinsic limit of μH calculated on the assumption that only phonon and ionized impurities contribute to the carrier scattering. The suppressed grain-boundary scattering might be explained by the reduced density of the {101} crystallographic shear planes.

Effect of tantalum doping on SnO2 electron transport layer via low temperature process for perovskite solar cells

The electron transport layer (ETL) plays an important role in determining the device performance of perovskite solar cells (PSCs). Recently, SnO2 has been used extensively as an ETL due to its many outstanding optoelectronic properties. Herein, we develop Ta doped SnO2 (Ta-SnO2) as an ETL grown by chemical bath deposition, allowing the fabrication of low-temperature PSCs. In contrast to pristine SnO2, the I-V curve and transmittance spectra show a significant conductivity improvement of Ta-SnO2 without declining the light transmittance property. Meanwhile, Ta-doping could accelerate the electron transfer and decrease the recombination probability at the SnO2/perovskite interface, as well as passivate the electron traps, leading to the improvement in the PSC performance. Through a series of optimization methods, the champion device shows a power conversion efficiency of 20.80%, with an open-circuit voltage of 1.161 V, a short-circuit current density of 22.79 mA/cm2, and a fill factor of 0.786. SnO2 with a suitable Ta content is a promising candidate as an ETL for fabricating high-efficiency PSCs via the low-temperature process.

On the Activity of Electrospun Tantalum Doped SnO2 for Durable ultra-Low PGM Fuel Cell Cathodes

Despite the advances in the development of Proton Exchange Membrane Fuel Cells (PEMFCs), some locks still need to be lifted before they are deployed on a large scale: i) Reduce the amount of platinum group metals (PGM) used in the electrodes by increasing the mass activity of the supported catalyst.ii) Improve durability of the electrodes by replace the conventional carbon support In particular, for the cathode side, carbon blacks (e.g. Vulcan XC-72) are conventionally used as a support materials for Pt and Pt-alloy nanoparticles. These materials exhibit high electric conductivity, controlled chemical properties, and high surface area that are of interest in PEMFCs application as electrocatalyst supports. However, carbon blacks are oxidized at the high voltage experienced at the cathode side (e.g. in stop/start conditions) leading to PEMFC major failure due to support corrosion leading to catalyst particles sintering or detachment [1,2]. Alternative supports based on tin oxide have attracted research interest due to their high corrosion resistance and promotion of activity for oxygen reduction reaction (ORR)[3]. The drawback is their low electrical conductivity, that in this work was tackled by doping with tantalum. Ta-doped SnO2 conducting fibers were synthetized by electrospinning followed by calcination [4] (Figure 1). This support was catalyzed with platinum nanoparticles prepared by a microwave-assisted polyol method and characterized for their physico-chemical and electrocatalytic properties towards ORR. In particular, stability to voltage cycling at high potential was evaluated by ex situ electrochemical analysis. A very significant mass activity of 470 A / gPt has been reached with 7.4wt.% of Pt homogenously deposited onto Ta-doped SnO2 nanofibers [5,6]. In addition, improved electrochemical stability was demonstrated with the retention of 70 % of the mass activity up to 10000 cycles from 1 to 1.6 V/RHE. Figure 1: SEM micrograph of 1%at. Ta-doped SnO2 nanofibers. References C. Meier, C. Galeano, I. Katsounaros, A. a. Topalov, A. Kostka, F. Schüth, K. J. J. Mayrhofer, ACS Catal. 2012, 2, 832–843. Yu, H. Li, H. Wang, X.-Z. Yuan, G. Wang, M. Pan, J. Power Sources 2012, 205, 10–23. Savych, S. Subianto, Y. Nabil, S. Cavaliere, D. Jones and J. Rozière, Chem. Chem. Phys. 2015, 17, 16970-16976 . Cavaliere, I. Jiménez-Morales, G. Ercolano, I. Savych, D. Jones and J. Rozière, ChemElectroChem 2015, 2, 1966-1973. Senoo, K. Taniguchi, K. Kakinuma, M. Uchida, H. Uchida, S. Deki, and M. Watanabe, ElectroChem. Commun. 2015, 51, 37–40. Jiménez-Morales, S. Cavaliere, D. Jones, and J. Rozière, Phys. Chem. Chem. Phys. 2018, 20, 8765–8772 . Figure 1

Atomic layer deposition of Ta-doped SnO2 films with enhanced dopant distribution for thermally stable capacitor electrode applications

Atomic layer deposition (ALD) on Ta-doped SnO2 thin films is proposed as a methodology for the fabrication of capacitor electrodes, for application in dynamic random-access memories (DRAMs). In ALD of doped materials, dopant concentration gradients inevitably occur due to the cyclic process-based characteristics of ALD. In this study, the dopant distribution improves drastically by the decrease in the growth per cycle of TaOx ALD. The modified ALD recipe exploits the low reactivity between Ta and Sn precursors to facilitate a reduction in the growth per cycle. The Ta-doped SnO2 films grown based on this modified ALD recipe exhibit improved crystallinity and conductivity. The ALD process yields excellent conformality of the Ta-doped SnO2 film over a hole structure with a high aspect ratio of ~10, both in terms of physical thickness and composition. Additionally, the Ta-doped SnO2 films serve as a template for the overgrowing dielectric TiO2 film, which induces the formation of a high-temperature phase with a high dielectric constant, rutile TiO2, and exhibit excellent thermal stability even after annealing at 400 °C in forming gas atmosphere. These findings demonstrate that the proposed methodology for the growth of Ta-doped SnO2 can facilitate the fabrication of capacitor electrodes for application in DRAMs.

Effect of high Al2O3 content on the microstructure and electrical properties of Co- and Ta-doped SnO2 varistors

Varistor behavior shows significant differences when the addition levels of different dopants like In2O3, Cr2O3, and Al2O3 are changed, thus stimulating current investigations on the SnO2–Co3O4–Ta2O5 ceramic system. In this contribution, the influence of high additions of Al2O3 on the microstructure, structure and the electrical properties of SnO2–Co3O4–Ta2O5 varistors ([98.95-X]%SnO2–1% Co3O4–X% Al2O3–0.05% Ta2O5, where X = 0, 0.05, 0.1, 1 or 2 mol%) is investigated. Characterization techniques such as thermal analysis, scanning electron microscopy and X-ray diffraction with Rietveld refinement were used for specimen analysis. The endothermic peaks in the ceramic system containing Al2O3 additions between 0.05 and 1% are ascribed to the formation of the Co2SnO4 and CoAl2O4 spinel type phases. Doping the ceramic system with 1 and 2 mol% Al2O3 leads to the formation of 1.163 and 3.449%, respectively, of the spinel phase Al2CoO4, which acts as a grain growth inhibitor because grain size decreases in about 16% for both addition levels. The apparent grains homogeneity and narrowest monomodal grain size distribution for the specimens with 2 mol% Al2O3 confirm the inhibitory role. With the lowest level of Al2O3 (0.05 mol%) the nonlinearity coefficient reaches a maximum, after which it decreases and fades at the highest alumina level. A remarkable decrease of about 50% in the leakage current from the reference specimen´s value to that of the one with 0.05 mol% Al2O3 concurrently with an increase in about 40% in the nonlinearity coefficient favors the potential use of alumina in the SnO2–Co3O4–Ta2O5 ceramic system.

UV–vis transparent conducting Ta-doped SnO2 epitaxial films grown by metal-organic chemical vapor deposition

Tantalum (Ta)-doped SnO2 films with different Ta concentrations have been deposited on α-Al2O3 (012) substrates by the metal organic chemical vapor deposition (MOCVD) method at 620 °C. Structural, electrical and optical properties of the obtained films have been investigated in detail. Structural analyses revealed that all the obtained films were rutile SnO2 growing along SnO2 [101] orientation. The 4% Ta-doped SnO2 film exhibited the best crystal quality and the in-plane epitaxial relationship between the film and substrate was SnO2 [010]//Al2O3 [110]. The highest Hall mobility of 58.1 cm2 V−1 s−1 was achieved for the 4 at.% Ta-doped SnO2 film and the minimum resistivity as low as 4.0 × 10-4 Ω cm was obtained at 6 at.% of Ta doping concentration. In the visible light region, all the obtained films showed average transmittances exceeding 88%. With Ta concentration increasing from 0 to 8 at.%, the transparent range of the films extended to the UV-B light region (280–320 nm), while the optical band gap of the films rose from 3.96 to 4.30 eV.