Eco-friendly,low-temperature solution production of oxide thin films for high-performance transistors via infrared irradiation of chloride precursors

In this paper, we present the infrared (IR) irradiation of chloride precursors as a promising method for the eco-friendly, low-temperature solution fabrication of oxide film devices, which typically require thermal annealing at temperature over 450?°C. By the IR irradiation of AlCl₃ precursor, high-quality Al₂O₃ dielectric films were prepared at a low temperature of 230?°C. The obtained Al₂O₃ dielectric layers had high dielectric properties, such as a high capacitance of 158?nF/cm² and a small leakage current of 5.4?×?10^?8 A/cm². Various structure characterizations confirmed the high quality of Al₂O₃ films produced by IR irradiation. Moreover, full low-temperature solution-produced thin-film transistors (TFTs) were fabricated through the IR irradiation of chloride precursors. The In₂O₃ TFTs achieved a high mobility of 33.6 cm²V^?1s^?1?at a small operation voltage of 4?V. Compared with the common thermal annealing method, IR irradiation results in better precursor conversion, higher oxygen lattice, and fewer oxygen defects. These results suggest that IR irradiation can serve as a new approach for the eco-friendly, low-temperature solution production of various oxide thin films and devices. The method is also very promising for the low-energy production of functional materials and devices.     

Solution-processed ytterbium oxide dielectrics for low-voltage thin-film transistors and inverters

High permittivity (high k) metal-oxide thin films fabricated via solution processes have recently received much attention for the construction of low-operating voltage and high-performance thin-film transistors (TFTs). In this report, amorphous ytterbium oxide (Yb₂O₃) thin films were fabricated by spin coating and their applications in TFTs were explored. The physical properties of the solution-processed Yb₂O₃ thin films processed at different annealing temperatures were systematically investigated using various characterization techniques. To explore the feasibility of the Yb₂O₃ thin films as gate dielectrics for oxide TFTs, In₂O₃ TFTs based on Yb₂O₃ dielectrics were integrated. All the devices could be operated at 3 V, which is critical for the applications in portable, battery-driven, and low-power electronic devices. The optimized In₂O₃/Yb₂O₃ TFT exhibits high electrical performances, including field-effect mobility of 4.98 cm²/V s, on/off current ratio of ~ 10⁶, turn-on voltage around 0 V, and subthreshold swing of 70 mV/decade, respectively. To demonstrate the potential of In₂O₃/Yb₂O₃ TFT toward more complex logic application, the unipolar inverter was further constructed.     

Sol-gel derived low temperature HfO2-GPTMS hybrid gate dielectric for a-IGZO thin-film transistors (TFTs)

In this study, we prepared inorganic-organic HfO₂-GPTMS hybrid films by a simple sol-gel method at low temperature for high-k dielectric gate applications. The hybrid films were deposited by spin coating process, followed by annealing at 150?°C. The hybrid dielectric material was characterized by Spectroscopic ellipsometry (SE), AFM, FESEM, FTIR, TGA, and XPS techniques. The resulting hybrid films exhibit homogeneous and smooth surface with high optical transparency. Their dielectric properties were analysed by measuring leakage current and capacitance versus voltage of metal-insulator-metal (MIM) capacitor structures. From this analysis, the leakage current density at ??5?V, capacitance and dielectric constant at 1?MHz measured on the hybrid films were 10^?7 A/cm², 51.3?nF/cm² and of 11.4 respectively. Finally, to investigate the electrical performance of the hybrid thin films as a dielectric gate in thin film transistors (TFTs), bottom-gate TFTs were fabricated by depositing the HfO₂-GPTMS dielectric gate layer on ITO-coated glass substrate and subsequently a sputtered a-IGZO thin film as the channel layer. The electrical response of the resulting TFTs demonstrated good saturation mobility of 4.74?cm² V^?1 s^?1, very low threshold voltage of 0.3?V and I_on/I_off current ratio of 10⁴, with low operating voltage under 8?V.     

Rapid and facile low-temperature solution production of ZrO2 films as high-k dielectrics for flexible low-voltage thin-film transistors

A rapid lightwave (LW) irradiation method was presented for the low-temperature solution production of ZrO₂ films as high-k dielectrics for flexible high-performance thin-film transistors (TFTs). The LW irradiation process markedly decreased the required processing temperature and processing time. Microstructure characterizations confirmed the successful formation of ZrO₂ films with an ultrasmooth surface, large band gap (>5 eV) and low defect level. The ZrO₂ film produced via LW irradiation at ∼200 °C in only 8 min presented excellent dielectric properties, including a small leakage current of 3.3 × 10⁻⁸ A/cm² and a large capacitance of 296 nF/cm², significantly outperforming the films by the conventional high-temperature annealing process at 400 °C for 60 min. Furthermore, LW irradiation was extended to the channel layer. The rapid low-temperature solution-processed InZrO_x TFTs exhibited superior electrical characteristics, such as a high carrier mobility of 41.3 cm²V⁻¹s⁻¹ and a high on-off current ratio of 10⁵∼10⁶ at a low operation voltage of 3 V due to the employment of high-quality ZrO₂ dielectric films. Moreover, the flexible TFT on a polyimide (PI) plastic substrate achieved a high mobility of nearly 30 cm²V⁻¹s⁻¹, indicating that LW irradiation is highly promising for the rapid and low-temperature solution production of high-quality and flexible oxide electronic devices.     

Strontium doping effects on the characteristics of solution-processed aluminum oxide dielectric layer and its application to low-voltage-operated indium-gallium-zinc-oxide thin-film transistors

In this paper, we investigated the strontium doping effects on the electrical and physical characteristics of solution-processed aluminum oxide dielectric layer and its application to low-voltage-operated indium-gallium-zinc-oxide (IGZO) thin-film transistors (TFTs). With an optimized doping concentration of strontium (5 at%) in aluminum oxide (Al₂O₃), an oxide gate dielectric layer having a dielectric constant of ~7 and low leakage current characteristics (~4 × 10⁻⁷ A/cm² at 3 MV/cm) could be achieved by a solution process, which are comparably better than those of pristine Al₂O₃ film. The enhanced dielectric properties from strontium doping can be attributed to the change in the physical properties of Al₂O₃ film incorporated with strontium, providing charge relaxation of defect states in Al₂O₃ film. Also, since the strontium is highly reactive with oxygen, the strontium substitution through a doping leads to more strongly bound structure in an Al₂O₃ film without considerable lattice distortion. Using the strontium-doped aluminum oxide film as a gate dielectric layer, having a thickness less than 10 nm, solution-processed IGZO TFTs operating at ≤ 1 V were demonstrated showing a field-effect mobility of 1.74 ± 1.10 cm²/V s and an on-current level of ~10⁻⁵ A.     

The effect of solvent water content on the dielectric properties of Al2O3 films grown by atmospheric pressure mist-CVD

Aluminum oxide (Al₂O₃) dielectric layers were grown by a mist-chemical vapor deposition (mist-CVD) process at 300 °C, using solvent mixtures containing acetone and water. As the acetone to water ratio was varied from 9:1 to 7:3, the leakage current of Al₂O₃ at an electric field of 7 MV/cm² decreased from 9.0 × 10^?7 to 4.4 × 10^?10 A/cm², and the dielectric constant increased from 6.03 to 6.85 with improved hysteresis during capacitance-voltage measurements. Consequently, the most robust Al₂O₃ films were obtained at an acetone to water ratio of 7:3, with a dielectric constant (κ) close to the ideal value 7.0, and a breakdown field of approximately 9 MV/cm. Thin film transistors (TFTs) incorporating In-Sn-Zn-O (ITZO) as the semiconductor were fabricated with the Al₂O₃ (7:3) dielectric onto p⁺⁺-Si substrates. The devices exhibit high electrical performance, with a high field effect mobility of 42.7 cm²V^?1s^?1, and a small subthreshold swing (S.S.) value of 0.44 V/decade.     

High carrier mobility low-voltage ZnO thin film transistors fabricated at a low temperature via solution processing

Wide-bandgap ZnO TFTs have many potential applications in large-area, flexible electronics and transparent devices because of their low cost, high performance and excellent optical transmittance. High-performance ZnO TFTs fabricated via simple solution processing have been widely studied. However, the key issues of solution-processable ZnO TFTs are the relatively high processing temperature (> 300?°C) and the high operating voltage for achieving the desired electrical properties. Here, we successfully fabricated low-voltage ZnO TFTs at an annealing temperature of ≤?250?°C. The resulting ZnO transistors with high-k terpolymer P(VDF-TrFE-CFE) showed a mobility of up to 5.3?cm² V^?1 s^?1 and an on/off ratio of >?10⁵ at 3?V. Furthermore, the influence of the dielectric constant on the carrier mobility was investigated. A lower k-value dielectric resulted in a high carrier mobility under the same carrier density. Therefore, with a low-k CYTOP dielectric applied to modify the interface between the ZnO semiconductor and the P(VDF-TrFE-CFE) layer, ZnO transistors annealed at 250?°C showed an electron mobility of 13.6?cm² V^?1 s^?1 and an on/off ratio of >?10⁵ at 3?V. To the best of our knowledge, this mobility is the highest value reported to date among the low-voltage solution-processable undoped ZnO TFTs annealed at temperatures of ≤?300?°C.     

Significantly enhanced energy density of amorphous alumina thin films via silicon and magnesium co-doping

The different Si-Mg co-doping content was explored to improve the dielectric properties of amorphous Al₂O₃ thin film. According to the analysis of DSC, FT-IR, and XPS spectra, it can be confirmed that a novel structure of glass network is formed in the co-doped Al₂O₃ thin film. More importantly, compared to Al₂O₃ thin film, the leakage current of (Al_.97Si_.02Mg_.01)₂O_y thin film is reduced by 2 orders of magnitude and the breakdown strength is improved from 276?MV/m to 544?MV/m. The corresponding energy density of the modified sample is up to 9.2?J/cm³, which is an enhancement of 6.2?J/cm³ over that of the undoped Al₂O₃ thin film. Based on finite element analysis, the simulation results show that the applied electric field is mainly focused on the glass network, which could strengthen the stability of Al₂O₃ structure and decrease the breakdown probability of the films. From the viewpoint of defect chemistry, another reason for the enhancement of the dielectric properties is that Si-Mg co-doping results in the generation of cation vacancies and thus the formation of oxygen vacancies could be effectively prevented. This work could provide a new design strategy for high-performance dielectric capacitor devices.     

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.     

Thickness dependence of microstructure,dielectric and leakage properties of BaSn0.15Ti0.85O3 thin films

The BaSn_0.15Ti_0.85O₃ (BTS) thin films are prepared on Pt-Si substrates with thickness ranging from ~ 60?nm to ~ 380?nm by radio frequency magnetron sputtering. The effects of thickness on microstructure, surface morphologies and dielectric properties of thin films are investigated. The thickness dependence of dielectric constant is explained based on the series capacitor model that the BTS thin film is consisted by a BTS bulk layer and an interfacial layer (dead layer) between the BTS and bottom electrode. The thin films with thickness of 260?nm give the largest figure of merit of 76.9@100?kHz, while the tunability and leakage current density are 64.6% and 7.46?×?10^?7 A/cm² at 400?kV/cm, respectively.     

Effects of annealing temperatures on energy storage performance of sol-gel derived (Ba0.95, Sr0.05) (Zr0.2, Ti0.8) O3 thin films

Dielectric polarization and breakdown strength of dielectrics generally show directly and inversely dependent upon their crystallization, respectively. Therefore, achieving the maximum energy storage density should be expected by controlling the crystallization. A serial of ferroelectric (Ba_0.95, Sr_0.05)(Zr_0.2, Ti_0.8)O₃ (BSZT) thin films were prepared by the sol-gel method. Effects of annealing temperatures on the microstructure, dielectric and energy storage performance of the films were investigated. The results indicate that BSZT thin films annealed at 600 °C for 30 min demonstrate the highest recoverable energy density and efficiency (50.5 J/cm³ and 91.9%). Such superior energy storage performance is attributed to an ultrahigh electric breakdown strength (6.65 MV/cm) induced by the dense amorphous-nanocrystalline microstructure. This work creates a new way for optimizing the energy storage performance of dielectric thin films via balancing their dielectric polarization and breakdown strength at appropriate heating processing temperature.     

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.     

Extremely Thin Al2O3 Surface‐Passivated Nanocrystalline ZnO Thin‐Film Transistors Designed for Low Process Temperature

Nanocrystalline ZnO (nc‐ZnO) thin‐film transistors (TFTs) exhibit inherent instability under bias/photo stresses, which originates from the oxygen molecules adsorbed on the surface of the crystal grains. The space charge region at nanocrystal surfaces that is induced by adsorbed oxygen molecules produces a high electrical potential barrier and significantly interrupts charge transport between the source and drain in nc‐ZnO TFTs. In this article, we developed high‐performance TFTs via the continuous deposition of an extremely thin Al₂O₃ layer on a nc‐ZnO channel. These devices were fabricated by atomic layer deposition at an extremely low process temperature of 150°C, including both the deposition and postannealing temperatures. The nc‐ZnO TFT with an extremely thin Al₂O₃ layer (1.8 nm) showed a significantly higher mobility (25 cm²/Vs) compared to devices without an Al₂O₃ layer (3.6 cm²/Vs). This dramatic difference was ascribed to the suppression of the chemisorption of oxygen molecules at the nanocrystal surface during thermal annealing (reducing the potential barrier width/height between adjacent nanocrystals). Furthermore, ultrathin Al₂O₃‐covered nc‐ZnO TFTs exhibited considerably enhanced electrical/photo stability due to the reduction in adsorption/desorption events of oxygen molecules on the nanocrystal surfaces (with no change in the depletion width after illumination) under gate bias or illumination stress.     

Structural and electrical properties of sol–gel-derived Al-doped bismuth ferrite thin films

Al-doped BiFeO₃ (BiFe_(1?x)Al_xO₃) thin films with small doping content (x=0, 0.05, and 0.1) were grown on Pt(111)/TiO₂/SiO₂/Si substrates at the annealing temperature of 550 °C for 5 min in air by the sol–gel method. The crystalline structure, as well as surface and cross section morphology were studied by X-ray diffraction and scanning electron microscope, respectively. The dielectric constant of BiFeO₃ film was approximately 71 at 100 kHz, and it increased to 91 and 96 in the 5% and 10% Al-doped BiFeO₃ films, respectively. The substitution of Al atoms in BiFeO₃ thin films reduced the leakage current obviously. At an applied electric field of 260 kV/cm, the leakage current density of the undoped BiFeO₃ films was 3.97×10^?4 A/cm², while in the 10% Al-substitution BiFeO₃ thin films it was reduced to 8.4×10^?7 A/cm². The obtained values of 2P_r were 63 μC/cm² and 54 μC/cm² in the 5% and 10% Al-doped BiFeO₃ films at 2 kHz, respectively.     

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.     

Preparation of thin electrolyte film via dry pressing/heating /quenching/calcining for electrolyte-supported SOFCs

The development of technologies used to prepare thin electrolyte films will stimulate the application of electrolyte-supported SOFCs since thin electrolyte films typically have low ohmic resistances and good electrochemical performance. This paper presents a novel method for the preparation of thin electrolyte films for yttria-stabilized zirconia (YSZ)-supported solid oxide fuel cells (SOFCs) via dry pressing/heating/quenching/calcining. The thicknesses of the as-prepared YSZ films were as low as 78?μm, which is significantly thinner than those prepared using a traditional method (greater than 200?μm) via dry pressing/calcining/polishing. More importantly, the preparation process was quicker. Using this novel method, a YSZ-supported cell with a configuration of (La_0.6Sr_0.4)_0.9Co_0.8Fe_0.2O_3-δ (LSCF)–Ce_0.8Sm_0.2O_2-δ(SDC)/SDC/YSZ/SDC/Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ(BSCF)–SDC was fabricated and tested. The results showed promising electrochemical performance and a peak power density of 0.64?W?cm^?2 at 850?°C was obtained, which was much higher than the cell fabricated using the traditional method (0.29?W?cm^?2). The ohmic resistance (R_O) at 850?°C is 0.19?Ω?cm², which is much lower than that of the cell fabricated using the traditional method (0.33?Ω?cm²) at an identical temperature. The modified method described in this work is shown to be a promising technique to prepare thin electrolyte films for high-performance, electrolyte-supported SOFCs.     

Dielectric properties of composition spread SiO2–Al2O3 mixed phase thin films deposited at room temperature by off-axis RF magnetron sputtering

The dielectric properties of composition spread SiO₂–Al₂O₃ thin films deposited by off-axis radio-frequency magnetron sputtering at room temperature were explored to obtain optimized compositions, which have low dielectric constants and losses. The specific points (compositions) showing superior dielectric properties of low dielectric constants (8.13 and 9.12) and losses (tanδ ～0.02) at 1 MHz were found in area of the distance of 25.0 mm (Al₂Si₃O₈) and 42 mm (Al_2.4Si₃O₈) apart from SiO₂ target side in 75 mm × 25 mm sized Pt/Ti/SiO₂/Si(1 0 0) substrates, respectively. The specific thin films were amorphous phase and the compositions were Al₂Si₃O₈ (k ～8.13) and Al_2.4Si₃O₈ (k ～9.12).     

Effect of sputtering pressure on structural and dielectric tunable properties of BaSn0.15Ti0.85O3 thin films grown by magnetron sputtering

Lead?free ferroelectric BaSn_0.15Ti_0.85O₃ (BTS) thin films are grown on Pt-coated Si substrates by magnetron sputtering at 650?°C, the effect of sputtering pressure on the microstructural, surface morphological, dielectric properties and leakage characteristic is systematically investigated. XRD analysis shows the crystallinity of BTS thin films with perovskite structure can be improved by appropriate control of the sputtering pressure. The surface morphology analyses reveal that grain size and roughness can be affected by sputtering pressure. The BTS thin films prepared at sputtering pressure of 3.0?Pa exhibit a low dispersion parameter of 0.006, a medium dielectric constant of ~357, a high dielectric tunability of 65.7%@?400?kV/cm and a low loss tangent of 0.0084?@?400?kV/cm. Calculation of figure of merit (FOM) displays a high value of 84.1, and the measurement of leak current shows a very low value of 4.39?×?10^–7 A/cm² at 400?kV/cm. The results indicate that BTS thin film deposited sputtering pressure of 3.0?Pa is an excellent candidate for electrically steerable applications     

Preparation of dense 0.9Al2O3−0.1TiO2 ceramics with highly improved microwave dielectric properties by a noncontaminated DCC method

In this paper, dense 0.9Al₂O₃ ??0.1TiO₂ ceramics with highly improved microwave dielectric properties were prepared by a noncontaminated direct coagulation casting (DCC) method. The suspension was destabilized and coagulated by consumption of the dispersant without introducing impurity ions. The effect of dispersant content, pH value and solid loading on the rheological properties of 0.9Al₂O₃ ??0.1TiO₂ suspension was investigated. It was found that 0.9Al₂O₃ ??0.1TiO₂ suspension with a high solid loading of 50?vol% and low viscosity of 0.7?Pa?s could be prepared by adding 0.5?wt% TMAOH at the pH in the range of 10–12. The suspension was coagulated by adding 2?vol% GDA when it was treated at 60 ~ 80?°C for 40 ~ 60?min. Compared with dry pressing method, more homogeneous and denser microstructure could be obtained in 0.9A1₂O₃ ??0.1TiO₂ ceramics prepared by DCC via dispersant reaction which were sintered at 1550?°C for 3?h and annealed at 1100?°C for 5?h. The Al₂TiO₅ second phase in 0.9A1₂O₃ ??0.1TiO₂ ceramics prepared by DCC via dispersant reaction could be eliminated more easily by annealing treatment. After annealing treatment, only Al₂O₃ and TiO₂ phases could be detected. Therefore, higher density and much better microwave dielectric properties with ρ?=? 3.81?±?0.02?g/cm³, ε_r =?12.17?±?0.02, Q ×?f =?25,637?±?749?GHz, τ_f =?13.12?±?1.62?ppm/°C were obtained by DCC via dispersant reaction, and the Q ×?f value almost improved by 25%. Without introducing impurity ions, it provides a new insight into preparing complex shaped function ceramics with high properties.     

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.