Fully solution-processed metal oxide thin-film transistors via a low-temperature aqueous route

We report a facile and low-temperature aqueous route for the fabrication of various oxide thin films (Al₂O₃, In₂O₃ and InZnO). A detail study is carried out to reveal the formation and properties of these sol-gel-derived thin films. The results show that the water-based oxide thin films undergo the decomposition of nitrate group as well as conversion of metal hydroxides to form metal oxide framework. High quality oxide thin film could be achieved at low temperature by this aqueous route. Furthermore, these oxide thin films are integrated to form thin-film transistors (TFTs) and the electrical performance is systematically studied. In particular, we successfully demonstrate In₂O₃/Al₂O₃ TFTs with high mobility of 30.88 cm² V⁻¹ s⁻¹ and low operation voltage of 4 V at a maximum processing temperature of 250 °C.     

Performance enhancement in InZnO thin-film transistors with compounded ZrO2–Al2O3 nanolaminate as gate insulators

We fabricated compounded ZrO₂–Al₂O₃ nanolaminate dielectrics by the atomic layer deposition (ALD) and used them to successfully integrate the high-performance InZnO (IZO) thin-film transistors (TFTs). It is found that nanolaminate dielectrics combine the advantages of constituent dielectrics and produce TFTs with improved performance and stability compared to single-layer gate insulators. The mobility in IZO-TFT was enhanced about 22% by using ZrO₂–Al₂O₃ gate insulators and the stability was also improved. The transfer characteristics of IZO-TFTs at different temperatures were also investigated and temperature stability enhancement was observed for the TFT with ZrO₂–Al₂O₃ nanolaminates as gate insulators. A larger falling rate (∼1.45 eV/V), a lower activation energy (Ea, ∼1.38 eV) and a smaller density-of-states (DOS) were obtained based on the temperature-dependent transfer curves. The results showed that temperature stability enhancement in InZnO thin-film transistors with ZrO₂–Al₂O₃ nanolaminate as gate insulators was attributed to the smaller DOS.     

Inkjet-printed thin film radio-frequency capacitors based on sol-gel derived alumina dielectric ink

There has been significant interest in printing radio frequency passives, however the dissipation factor of printed dielectric materials has limited the quality factor achievable. Al₂O₃ is one of the best and widely implemented dielectrics for RF passive electronics. The ability to spatially pattern high quality Al₂O₃ thin films using, for example, inkjet printing would tremendously simplify the incumbent fabrication processes – significantly reducing cost and allowing for the development of large area electronics. To-date, particle based Al₂O₃ inks have been explored as dielectrics, although several drawbacks including nozzle clogging and grain boundary formation in the films hinder progress. In this work, a particle free Al₂O₃ ink is developed and demonstrated in RF capacitors. Fluid and jetting properties are explored, along with control of ink spreading and coffee ring suppression. The liquid ink is heated to 400 °C decomposing to smooth Al₂O₃ films ~120 nm thick, with roughness of <2 nm. Metal-insulator-metal capacitors, show high capacitance density >450 pF/mm², and quality factors of ~200. The devices have high break down voltages, >25 V, with extremely low leakage currents, <2×10⁻⁹ A/cm² at 1 MV/cm. The capacitors compare well with similar Al₂O₃ devices fabricated by atomic layer deposition.     

Room-temperature UV-ozone assisted solution process for zirconium oxide films with high dielectric properties

A facile, low-cost, and room-temperature UV-ozone (UVO) assisted solution process was employed to prepare zirconium oxide (ZrO_x) films with high dielectric properties. ZrO_x films were deposited by a simple spin-coating of zirconium acetylacetonate (ZrAcAc) precursor in the environment-friendly solvent of ethanol. The smooth and amorphous ZrO_x films by UVO exhibit average visible transmittances over 90% and energy bandgap of 5.7 eV. Low leakage current of 6.0 × 10⁻⁸ A/cm² at 3 MV/cm and high dielectric constant of 13 (100 Hz) were achieved for ZrO_x dielectrics at the nearly room temperature. Moreover, a fully room-temperature solution-processed oxide thin films transistor (TFT) with UVO assisted ZrO_x dielectric films achieved acceptable performances, such as a low operating voltage of 3 V, high carrier mobility of 1.65 cm² V⁻¹ s⁻¹, and on/off current ratio about 10⁴–10⁵. Our work indicates that simple room-temperature UVO is highly potential for low-temperature, solution-processed and high-performance oxide films and devices.     

Preparation of IGZO sputtering target and its applications to thin-film transistor devices

Nano-scale In₂O₃, Ga₂O₃ and ZnO powder mixture prepared by a hybrid process of chemical dispersion and mechanical grinding was adopted for the In–Ga–Zn–O (IGZO) sputtering target fabrication. A pressure-less sintering at 1300 °C for 6 h yielded the target containing sole InGaZnO₄ phase with relative density as high as 93%. Consequently, the thin-film transistor (TFT) devices containing amorphous IGZO channels were prepared by using the self-prepared target and the electrical measurements indicated the TFT subjected to a post annealing at 300 °C exhibits the best device performance with the saturation mobility = 14.7 cm²/V s, threshold voltage = 0.57 V, subthreshold gate swing = 0.45 V/decade and on/off ratio = 10⁸. Capacitance–voltage measurement indicated that post annealing effectively suppresses the interfacial traps density at the IGZO/SiO₂ interface and thus enhances the electrical performance of TFT.     

Robust SiO2 gate dielectric thin films prepared through plasma-enhanced atomic layer deposition involving di-sopropylamino silane (DIPAS) and oxygen plasma: Application to amorphous oxide thin film transistors

DIPAS (di-isopropylamino silane, H₃Si[N(C₃H₇)₂]) and O₂ plasma were employed, using plasma-enhanced atomic layer deposition (PEALD), to deposit silicon oxide to function as the gate dielectric at low temperature, i.e., below 200 °C. The superior amorphous SiO₂ thin films were deposited through the self-limiting reactions of atomic layer deposition with a deposition rate of 0.135 nm/cycle between 125 and 200 °C. PEALD-based SiO₂ thin layer films were applied to amorphous oxide thin film transistors constructed from amorphous In-Ga-Zn-O (IGZO) oxide layers, which functioned as channel layers in the bottom-gated thin film transistor (TFT) structure, with the aim of fabricating transparent electronics. The SiO₂ gate dielectric exhibited the highest TFT performance through the fabrication of heavily doped n-type Si substrates, with a saturation mobility of 16.42 cm²/V·s, threshold voltage of 2.95 V and large on/off current ratio of 3.69 × 10⁸. Ultimately, the highly doped Si was combined with the ALD-based SiO₂ gate dielectric layers, leading to a saturation mobility of 16.42 cm²/V·s, threshold voltage of 2.95 V, S-slope of 0.1944, and on/off current ratio of 3.69 × 10⁸. Semi-transparent and transparent TFTs were fabricated and provided saturation mobilities of 22.18 and 24.29 cm²/V·s, threshold voltages of 4.18 and 2.17 V, S-slopes of 0.1944 and 0.1945, and on/off current ratios of 9.63 × 10⁸ and 1.03 × 10⁷, respectively.     

Sintering of Yb3+:Y2O3 transparent ceramics in hydrogen atmosphere

Highly transparent Yb³⁺:Y₂O₃ ceramics with doping concentration up to 40.0 at.% had been fabricated successfully via hydrogen atmosphere sintering, where the raw powders were synthesized by co-precipitation method. The sintering temperature is about 600 °C lower than its melting temperature. SEM investigation revealed the average grain size of Yb³⁺:Y₂O₃ ceramics sintered at 1850 °C for 9 h was about 7 μm. The highest transmittance of as-prepared 1 mm thickness samples around wavelength of 1050 nm reached 80%, which is close to the theoretical value of Y₂O₃. The optical spectroscopic properties of Yb³⁺:Y₂O₃ transparent ceramics have also been investigated, which shows that it is a very good laser material for diode laser pumping and short pulse mode-locked laser.     

Structural,electrical and optical properties of stoichiometric In2Te3 thin films

In₂Te₃ thin films were grown by thermal evaporation technique. The annealing of films played a major role to obtain stoichiometry, regardless of substrate temperature. Annealing at 300 ⁰C resulted in well oriented, mono-phased and nearly stoichiometric In₂Te₃ thin films. The variation in grain size of In₂Te₃ films associated with the substrate temperatures provides a significant control over the resistivity of the films, and the resistivity decreased with an increase in the grain size. The activation energy and optical band gap of stoichiometric In₂Te₃ films were found to be 0.01±0.005 eV and 0.99±0.02 eV, respectively. The absorption co-efficient of these films was found to be of the order of 10⁵ cm⁻¹.     

Anode-supported SOFCs based on Sm0.2Ce0.8O2−δ electrolyte thin-films fabricated by co-pressing using microwave combustion synthesized powders

Decreasing the electrolyte thickness is an effective approach to improve solid oxide fuel cells (SOFCs) performance for intermediate-temperature applications. Sm_0.2Ce_0.8O_2−δ (SDC) powders with low apparent density of 32±0.3 mg cm⁻³ are synthesized by microwave combustion method, and SDC electrolyte films as thin as ~10 μm are fabricated by co-pressing the powders onto a porous NiO–SDC anode substrate. Dense SDC electrolyte thin films with grain size of 300–800 nm are achieved at a low co-firing temperature of 1200 °C. Single cells based on SDC thin films show peak power densities of 0.86 W cm⁻² at 650 °C using 3 vol% humidified H₂ as fuel and ambient air as oxidant. Both the thin thickness of electrolyte films and ultra-fine grained anode structure make contributions to the improved cell performance.     

Dielectric properties and bright red emission of Y3+/Eu3+-codoped ZrO2 thin films prepared by chemical solution deposition

We report on an effective combination of good dielectric properties with bright red emission in Y³⁺/Eu³⁺-codoped ZrO₂ thin films. The thin films were deposited on fused silica and Pt/TiO₂/SiO₂/Si substrates using a chemical solution deposition method. The crystal structure, surface morphology, electrical and optical properties of the thin films were investigated in terms of annealing temperature, and Y³⁺/Eu³⁺ doping content. The 5%Eu₂O₃–3%Y₂O₃–92%ZrO₂ thin film with 400 nm thickness annealed at 700 °C exhibits optimal photoluminescent properties and excellent electrical properties. Under excitation by 396 nm light, the thin film on fused silica substrate shows bright red emission bands centered at 593 nm and 609 nm, which can be attributed to the transitions of Eu³⁺ ions. Dielectric constant and dissipation factor of the thin films at 1 kHz are 30 and 0.01, respectively, and the capacitance density is about 65.5 nf/cm² when the bias electric field is less than 500 kV/cm. The thin films also exhibit a low leakage current density and a high optical transmittance with a large band gap.     

Tb3+/Yb3+ co-doped Y2O3 upconversion transparent ceramics: Fabrication and characterization for IR excited green emission

Tb³⁺/Yb³⁺ co-doped Y₂O₃ transparent ceramics were fabricated by vacuum sintering of the pellets (prepared from nanopowders by uniaxial pressing) at 1750 °C for 5 h. Zr⁴⁺ and La³⁺ ions were incorporated in Tb³⁺/Yb³⁺ co-doped Y₂O₃ nanoparticle to reduce the formation of pores which limits the transparency of ceramic. An optical transmittance of ∼80% was achieved in ∼450 to 2000 nm range for 1 mm thick pellet which is very close to the theoretical value by taking account of Fresnel’s correction. High intensity luminescence peak at 543 nm (green) was observed in these transparent ceramics under 976 and 929 nm excitations due to Yb–Tb energy transfer upconversion.     

Preparation and electrical properties of Bi2Zn2/3Nb4/3O7 thin films deposited at room temperature for embedded capacitor applications

Bi₂Zn_2/3Nb_4/3O₇ thin films were deposited at room temperature on Pt/Ti/SiO₂/Si(1 0 0) and polymer-based copper clad laminate (CCL) substrates by pulsed laser deposition. Bi₂Zn_2/3Nb_4/3O₇ thin films were deposited in situ with no intentional heating under an oxygen pressure of 4 Pa and then post-annealed at 150 °C for 20 min. It was found that the films are still amorphous in nature, which was confirmed by the XRD analysis. It has been shown that the surface roughness of the substrates has a significant influence on the electrical properties of the dielectric films, especially on the leakage current. Bi₂Zn_2/3Nb_4/3O₇ thin films deposited on Pt/Ti/SiO₂/Si(1 0 0) substrates exhibit superior dielectric characteristics. The dielectric constant and loss tangent are 59.8 and 0.008 at 10 kHz, respectively. Leakage current density is 2.5 × 10^?7 A/cm² at an applied electric field of 400 kV/cm. Bi₂Zn_2/3Nb_4/3O₇ thin films deposited on CCL substrates exhibit the dielectric constant of 60 and loss tangent of 0.018, respectively. Leakage current density is less than 1 × 10^?6 A/cm² at 200 kV/cm.     

Structural-phase state and lasing of 5–15 at% Yb3+:Y3Al5O12 optical ceramics

Yttrium aluminum garnet (Yb³⁺:Y₃Al₅O₁₂) laser ceramics doped by 5, 10 and 15 at% of ytterbium ions were obtained by reactive sintering. Optimal sintering temperature range for the formation of highly-dense transparent Yb³⁺:Y₃Al₅O₁₂ ceramics under normal recrystallization conditions was found to be T = 1750–1800 °C. The influence of Yb³⁺ ions on structural-phase state, phase composition, microstructure, optical and luminescent properties of sintered samples was experimentally investigated. It was shown that lattice parameter a of Yb³⁺:Y₃Al₅O₁₂ ceramics decreases linearly with increasing of Yb³⁺ concentration in a good agreement with L. Vegard’s rule, that indicates to the formation of (Y_1−xYb_x)₃Al₅O₁₂ (х = 0.05–0.15) substitutional solid solutions. No concentration quenching of Yb³⁺ luminescence was observed in Yb³⁺:Y₃Al₅O₁₂ within the 5–15 at% doping range. Quasi-CW lasing of Yb³⁺:Y₃Al₅O₁₂ ceramics was studied under diode-pumping at 970 nm. A highest slope efficiency of about 50% was obtained for 15 at%-doped Yb³⁺:Y₃Al₅O₁₂ ceramics sintered at T = 1800 °C for 10 h.     

Graphene quantum dots directly generated from graphite via magnetron sputtering and the application in thin-film transistors

This work presents a novel method to prepare graphene quantum dots (GQDs) directly from graphite. A composite film of GQDs and ZnO was first prepared using the composite target of graphite and ZnO via magnetron sputtering, followed with hydrochloric acid treatment and dialysis. Morphology and optical properties of the GQDs were investigated using a number of techniques. The as-prepared GQDs are 4–12 nm in size and 1–2 nm in thickness. They also exhibited typical excitation-dependent properties as expected in carbon-based quantum dots. To demonstrate the potential applications of GQDs in electronic devices, pure ZnO and GQD–ZnO thin-film transistors (TFTs) using ZrO_x dielectric were fabricated and examined. The ZnO TFT incorporating the GQDs exhibited enhanced performance: an on/off current ratio of 1.7 × 10⁷, a field-effect mobility of 17.7 cm²/Vs, a subthreshold swing voltage of 90 mV/decade. This paper provides an efficient, reproducible and eco-friendly approach for the preparation of monodisperse GQDs directly from graphite. Our results suggest that GQDs fabricated using magnetron sputtering method may envision promising applications in electronic devices.     

Effects of element chemical states and grain orientation growth of ITO targets on photoelectric properties of the film

A study was carried out to compare element chemical states and grain orientation growth between two ITO targets, which were respectively sintered at 1560 °C (target A#) and 1600 °C (target B#). The lower sintering temperature can be beneficial to increase mass content ratio of In₂O₃ to SnO₂, reduce the production of Sn²⁺ ions and the component of O-In as well as increase oxygen holes, and can also promote grain orientation growth of In₂O₃ and In₄Sn₃O₁₂ phase. Three groups of ITO films were deposited at 230 °C using these targets to investigate the effects of sputtering parameters on the photoelectric properties of ITO films. Under different sputtering pressures, the sheet resistance for target A# has higher sensitivity to low O₂ flow, while target B# displays higher sensitivity to high O₂ flow. In the case of sputtering pressure of 0.5 Pa, ITO films for target A# displays the highest visible transmittance. In addition, annealing process could decrease the sheet resistance and improve the transmittance of ITO films because of its effect on the crystallinity and surface morphology of ITO films.     

Controlled Zr doping for inkjet-printed ZTO TFTs

This study examined the effects of controlled Zr doping on the electrical properties of inkjet-printed zinc-tin oxide (ZTO) thin-film transistors (TFTs). In contrast to previous reports, below a certain doping concentration, improved electrical properties were obtained due to the effectively suppressed oxygen vacancies, reduced trapped electrons, and controlled carrier concentrations. The 0.0025 M Zr-doped inkjet-printed ZTO TFTs showed higher mobility, higher on-to-off current ratio, lower threshold voltage, and lower subthreshold slope of 6.43 cm²/V s, 3.72×10⁸, 3.35 V, and 0.53 V/dec, respectively, compared to the un-doped TFTs. The bias stability of the Zr-doped inkjet-printed ZTO TFT was also improved.     

YBO3: Ce3+, Yb3+ based near-infrared quantum cutting phosphors: Synthesis and application to solar cells

Ce³⁺ and Yb³⁺ co-doped YBO₃ phosphors were facilely fabricated by a hydrothermal method. The investigations reveal that hexagonal YBO₃: Ce³⁺, Yb³⁺ nanoparticles aggregate to form cyclic structure after annealing at 900 °C. An efficient near-infrared (NIR) quantum cutting phenomenon involving the emission of two NIR photons (971 nm) for each ultraviolet (UV) photon (360 nm) absorbed is observed based on the cooperative energy transfer (CET) from Ce³⁺ to Yb³⁺ in YBO₃ with a CET efficiency of 41.9%. Moreover, YBO₃: Ce³⁺, Yb³⁺/SiO₂ films with anti-reflection and NIR quantum cutting abilities were prepared by dip-coating method. The as prepared composite films can convert UV photons into NIR photons between 950 nm and 1050 nm, which well matched with the spectral response of the silicon-based solar cell. The experimental results indicate that the photoelectric conversion efficiency of silicon solar cell can be effectively improved by assembling the YBO₃: Ce³⁺, Yb³⁺/SiO₂ bi-functional film, and the corresponding conversion efficiency is about 0.521% higher than the pure glass and 0.252% higher than the pure SiO₂ anti-reflection (AR) film. In a word, this work provides a simple strategy to develop optical films with AR and NIR quantum cutting abilities for solar energy conversion.     

Structural,optical and methanol sensing properties of sprayed In2O3 nanoparticle thin films

Indium oxide (In₂O₃) nanoparticle thin films were grown on cleaned glass substrates by the chemical spray pyrolysis technique using the precursor solution of indium nitrate (In (NO₃)₃). The XRD studies confirm that the films are polycrystalline In₂O₃, possessing cubic structure with lattice parameters, a = b = c = 10.17 Å. The optical studies show a direct optical band gap of 3.32 eV and an indirect band gap of 2.6 eV in the prepared films. The films exhibit high optical transparency >80% in the visible region, reaching a maximum of 85% at 684 nm wavelength. Further, the gas sensing properties of the films have been investigated for various concentrations of methanol in air at different operating temperatures. At 300 °C the film exhibits a very high response 99% to methanol vapor at a concentration of 40 ppm in air, which is ideal to be used as a methanol sensor. The film shows fast response and recovery to methanol vapor at higher operating temperatures. A possible methanol sensing mechanism has been proposed.     

Synthesis of In2O3 porous nanoplates for photocatalytic decomposition of perfluorooctanoic acid (PFOA)

Indium oxide (In₂O₃) porous nanoplates (PNPs) were synthesized by an ethylenediamine-assisted hydrothermal process followed by calcination at 270 °C. Compared with In₂O₃ non-porous nanoplates (NPs) obtained at higher temperature (500 °C), PNPs possessed a high value of specific surface area (156.9 m² g^− 1) and a meso-microporous structure. As-synthesized In₂O₃ PNPs showed a superior activity for photocatalytic decomposition of an emerging persistent organic pollutant-perfluorooctanoic acid (PFOA), the decomposition half-life of PFOA was shortened to 4.4 min.     

Solution-based fabrication and electrical properties of CaBi4Ti4O15 thin films

Ferroelectric CaBi₄Ti₄O₁₅ (CBT) thin films were prepared by spin coating technology using solution-based fabrication. The as-deposited CBT thin films were crystallized below 600 °C and the layered perovskite were crystallized at 700 °C using CFA processing in air. The enhancement of ferroelectric properties in CBT thin films for MFIS structures were investigated and discussed. Compared the Bi₄Ti₃O₁₂ (BIT), the CBT showed the better physical and electrical characteristics. The 700 °C annealed CBT thin films on SiO₂/Si substrate showed random orientation and exhibited large memory window curves. The maximum capacitance, memory window and leakage current density were about 250 pF, 2 V, and 10^?5 A/cm², respectively.