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.     

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.     

Sol-gel processed high-k aluminum oxide dielectric films for fully solution-processed low-voltage thin-film transistors

High-k oxide dielectric films have attracted intense interest for thin-film transistors (TFTs). However, high-quality oxide dielectrics were traditionally prepared by vacuum routes. Here, amorphous high-k alumina (Al₂O₃) thin films were prepared by the simple sol-gel spin-coating and post-annealing process. The microstructure and dielectric properties of Al₂O₃ dielectric films were systematically investigated. All the Al₂O₃ thin films annealed at 300–600?°C are in amorphous state with ultrasmooth surface (RMS ~ 0.2?nm) and high transparency (above 95%) in the visible range. The leakage current of Al₂O₃ films gradually decreases with the increase of annealing temperature. Al₂O₃ thin films annealed at 600?°C showed the low leakage current density down to 3.9?×?10^?7 A/cm² at 3?MV/cm. With the increase of annealing temperature, the capacitance first decreases then increases to 101.1?nF/cm² (at 600?°C). The obtained k values of Al₂O₃ films are up to 8.2. The achieved dielectric properties of Al₂O₃ thin films are highly comparable with that by vapor and solution methods. Moreover, the fully solution-processed InZnO TFTs with Al₂O₃ dielectric layer exhibit high mobility of 7.23?cm² V^?1 s^?1 at the low operating voltage of 3?V, which is much superior to that on SiO₂ dielectrics with mobility of 1.22?cm²/V^?1 s^?1 at the operating voltage of 40?V. These results demonstrate that solution-processed Al₂O₃ thin films are promising for low-power and high-performance oxide 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.     

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.     

Aqueous Solution‐Processable,Flexible Thin‐Film Transistors Based on Crosslinked Chitosan Dielectric Thin Films

The hydrophilic character of chitosan (CS) limits its use as a gate dielectric material in thin‐film transistors (TFTs) based on aqueous solution‐processable semiconductor materials. In this study, this drawback is overcome through controlled crosslinking of CS and report, for the first time, its application to aqueous solution‐processable TFTs. In comparison to natural CS thin films, crosslinked chitosan (Cr‐CS) thin films are hydrophobic. The dielectric properties of Cr‐CS thin films are explored through fabrication of metal–insulator–metal devices on a flexible substrate. Compared to natural CS, the Cr‐CS dielectric thin films show enhanced environmental and water stabilities, with a high breakdown voltage (10 V) and low leakage current (0.02 nA). The compatibility of Cr‐CS dielectric thin films with aqueous solution‐processable semiconductors is demonstrated by growing ZnO nanorods via a hydrothermal method to fabricate flexible TFT devices. The ZnO nanorod‐based TFTs show a high field‐effect mobility (linear regime) of 10.48 cm² V^?1 s^?1. Low temperature processing conditions (below 100 °C) and water as the solvent are utilized to ensure the process is environmental friendly to address the e‐waste problem.     

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.     

Lightwave irradiation-assisted low-temperature solution synthesis of indium-tin-oxide transparent conductive films

Transparent conductive oxide (TCO) films have important applications in many areas. Unfortunately, TCOs are usually fabricated using vacuum and high-temperature methods, preventing them from applications in low-cost flexible devices. In this paper, facile low-temperature sol-gel method is described that can be used to fabricate high-quality TCO films. This study uses lightwave (LW) irradiation (at ～280 °C) with indium-tin-oxide (ITO) as a typical example. Both structure and key properties of ITO films are investigated for different LW irradiation conditions. ITO can be formed via LW irradiation after a period as short as 5 min. Furthermore, it is found that LW irradiation can promote the formation of M ? O framework, effectively remove Cl impurities, and facilitate the elimination of hydroxyl oxygen defects - even at temperatures as low as ～280 °C. The optimal ITO films show excellent electronic properties, including low sheet-resistance (14.5 Ω·sq^?1) and high conductivity (1.7 × 10³ S cm^?1). Moreover, ITO films also show high transmittance (above 87%). Overall, our ITO films have a figure of merit (FOM) of 1.72 × 10^?2 Ω^?1, which is comparable to (or higher than) those of previous ITO films that were produced using conventional vacuum and high-temperature methods. Our LW irradiation method provides facile and effective approach to produce high-performance TCO films at remarkably low cost. This means these films could be used in affordable flexible large-area devices.     

Preparation and effects of post-annealing temperature on the electrical characteristics of Li–N co-doped ZnSnO thin film transistors

In this study, transparent Li–N co-doped ZnSnO (ZTO: (Li, N)) thin film transistors (TFTs) with a staggered bottom-gate structure were fabricated by radio frequency magnetron sputtering at room temperature. Emphasis was placed on investigating the effects of post-annealing temperature on their physical and electrical properties. An appropriate post-annealing temperature contributes not only to achieving good quality thin films, but also to improving the electrical performance of the ZTO: (Li, N) TFTs. The ZTO: (Li, N) TFTs annealed at 675 °C showed the best electrical characteristics with a high saturation mobility of 26.8 cm²V⁻¹s⁻¹, a threshold voltage of 6.0 V and a large on/off current ratio of 4.5×10⁷.     

Diamond based field-effect transistors with SiNx and ZrO2 double dielectric layers

A diamond-based field-effect transistor (FET) with SiN_x and ZrO₂ double dielectric layer has been demonstrated. The SiN_x and ZrO₂ gate dielectric are deposited by plasma-enhanced chemical vapor deposition (PECVD) and radio frequency (RF) sputter methods, respectively. SiN_x layer is found to have the ability to preserve the conduction channel at the surface of hydrogen-terminated diamond film. The leakage current density (J) of SiN_x/ZrO₂ diamond metal-insulator-semiconductor FET (MISFET) keeps lower than 3.88 × 10^− 5 A·cm^− 2 when the gate bias was changed from 2 V to − 8 V. The double dielectric layer FET operates in a p-type depletion mode, whose maximum drain-source current, threshold voltage, maximum transconductance, effective mobility and sheet hole density are determined to be − 28.5 mA·mm^− 1, 2.2 V, 4.53 mS·mm^− 1, 38.9 cm²·V^− 1·s^− 1, and 2.14 × 10¹³ cm^− 2, respectively.     

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.     

Preparation and characterization of transparent indium- doped TiO2 films deposited by MOCVD

Titanium dioxide (TiO₂) films doped with different indium (In) concentrations have been prepared on SrTiO₃ (STO) substrates by high vacuum metalorganic chemical vapor deposition (MOCVD). X-ray diffraction (XRD) analyses revealed the TiO₂ films doped with low In concentrations to be [001] oriented anatase phase and the films with high In concentrations to present polycrystalline structures. The 1.8% In-doped TiO₂ film exhibited the best electrical conductivity properties with the lowest resistivity of 8.68×10⁻² Ω cm, a Hall mobility of 10.9 cm² V⁻¹ s⁻¹ and a carrier concentration of 6.5×10¹⁸ cm⁻³. The films showed excellent transparency with average transmittances of over 85% in the visible range.     

Surface morphology and electrical properties of boron-doped diamond films synthesized by microwave-assisted chemical vapor deposition using trimethylboron on diamond (100) substrate

Prime novelty: The smoothness of the synthesized boron-doped diamond was improved by the pre-treatment of a hydrogen plasma. Moreover, the Hall mobility also increased with this pre-treatment.Surface morphology and electrical properties, such as electrical conductivity, hole concentration and Hall mobility, were investigated for boron-doped diamond films, which were synthesized by microwave-assisted chemical vapor deposition (MPCVD) on a (100) diamond substrate. Trimethylboron (TMB) was used as a dopant source and methane (CH₄) was used as a carbon source. The morphology of the synthesized diamond surface depended on the MPCVD conditions such as TMB and CH₄ concentrations in the gas phase, and lower concentrations of TMB and CH₄ lead to a smoother surface. When the substrate was treated in a hydrogen plasma, the electrical properties of the boron-doped diamond films, as well as the smoothness of the surface, were improved. After optimizing the synthesis conditions, Hall mobility reached to 2020 cm² V⁻¹ s⁻¹ at 243 K for a diamond film with a hole concentration of 5×10¹² cm⁻³.     

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.     

Effect of total reaction pressure on electrical properties of boron doped homoepitaxial (100) diamond films formed by microwave plasma-assisted chemical vapor deposition using trimethylboron

Boron was doped into diamond films which were synthesized homoepitaxially on polished (100) diamond substrates by means of microwave plasma-assisted chemical vapor deposition (MPCVD) using trimethylboron as the dopant at a constant substrate temperature of 1073 K. The morphologies and electrical properties of the synthesized diamond films were dependent on the total reaction pressure. A maximum Hall mobility, 760 cm² V⁻¹ s⁻¹, was obtained for the film synthesized at 10.7 kPa. The values of Hall mobility were comparable with those obtained for B₂H₆-doped films at corresponding hole concentrations.     

Electrical properties of thick boron and nitrogen contained CVD diamond films

The acceptor and donor defects of thick (approx. 0.4 mm) free-standing boron and nitrogen containing microwave plasma CVD polycrystalline diamond films were investigated. Charge-based deep level transient spectroscopy (Q-DLTS) was applied to study impurity-induced defects, their density and energy distribution in the energy range of 0.01 eV≤E−E_v≤1.1 eV above the valence band. It was shown, that differential capacitance–voltage, and Hall effect measurements combined with DLTS data can be used to determine the degree of compensation, and the concentration of compensating donors (mostly the positively charged single-substitutional nitrogen (N⁺)) in p-type CVD polycrystalline diamond films. It was found, that incorporated boron atoms induce three levels of electrically active defects. Two of them with concentration (2–3)×10¹⁶ cm⁻³ each have activation energies of 0.36 and 0.25 eV with capture cross-sections of 1.3×10⁻¹³ and 4.5×10⁻¹⁹ cm², respectively. The third type of defect has an activation energy of 0.02 eV, capture cross-section 3×10⁻²⁰ cm² and concentration 10¹⁵ cm⁻³, this shallow trap being a probable general caterer of holes in low-doped films. The total concentration of electrically active uncompensated acceptors in all p-type diamond samples was approximately 2×10¹⁷ cm⁻³ with hole concentration of approximately 1.5×10¹⁴ cm⁻³ and hole mobility in the range of 30–40 cm² V⁻¹ s⁻¹ at room temperature. If assumed that compensating donors are mostly nitrogen, the films contained no less than 3×10¹⁶ cm⁻³ of N⁺.     

Photocarrier mobility in poly(2‐methoxyaniline)

In the current study the mobility of photogenerated charge carriers in PMA [poly(2‐methoxyaniline)] and their transport were investigated using time‐of‐flight (TOF) techniques. Also studied was the effect on hole mobility of film thickness and of the method of polymer processing during device fabrication. The highest value of hole mobility found was 4.5 × 10⁻⁴ cm² V⁻¹ s⁻¹ at an applied field of 1.3 × 10⁶ V/cm and 293 K in solution‐cast film of PMA. The hole mobility of solution‐cast films was about 2 orders of magnitude higher compared to spin‐coated films, for which the ordering of the polymer chains may be the reason. To our knowledge, this is the first time the TOF mobility of this material has been presented. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1506–1512, 2001     

Effects of yttrium additions on the microstructure and charge transport properties of indium tin oxide semiconductors

The effects of yttrium (Y) additions (x=0, 0.05, 0.1, and 0.2) on the microstructure, chemical structure, and electrical properties of Y_xInSnO_y (YITO) thin films, prepared using a sol-gel process were examined. The transmission electron microscopy (TEM) observations showed that the undoped InSnO (ITO) film consisted of an amorphous structure with local crystalline domains on the film surface, whereas the Y additions (x=0.05, 0.1, and 0.2) to ITO suppressed the formation of the crystalline phase. X-ray photoelectron spectroscopy (XPS) analysis showed that the Y content decreased the concentration of oxygen vacancies owing to the strong incorporation of Y with oxygen. As a result of the Y incorporation, the carrier concentration of ITO films decreased. The saturation mobility (μ_sat), the on-off ratios (I_on/off), and the sub-threshold swing (S.S) of YITO films were 1.1 cm² V⁻¹ s⁻¹, ~10⁶, and ~0.5 V decade⁻¹, respectively, which are comparable with 1.7 cm² V⁻¹ s⁻¹, ~10⁵, and ~1.17 V decade⁻¹ of ITO film. Additionally, the initial threshold voltage (V_TH) was positive shift with increased of Y addition and V_TH shift (ΔV_TH) under the positive bias stress (PBS) results decreased by Y addition.     

Electrical and chemical stability engineering of solution-processed indium zinc oxide thin film transistors via a synergistic approach of annealing duration and self-combustion process

The electrical and chemical stability of solution-processed indium zinc oxide (IZO) channel thin-film transistors (TFTs) were engineered via a synergistic approach of annealing duration and self-combustion process. In particular, the amorphous IZO TFTs that were thermally treated at 400 °C for 3 h using the specific precursor combination to generate internal self-combustion energy showed the best electrical performance [high saturation mobility (μ_SAT)=2.7 cm²/V s] and stability [low threshold voltage shift (ΔV_TH) under positive bias stress of 10.5 V] owing to the formation of oxide films with excellent metal–oxide–metal (M–O–M) bonds, fewer impurities, and an amorphous phase compared to IZO TFTs using other precursor formulas and annealing times. Longer annealing times led to a saturated M–O bond ratio and crystallization via extreme thermal annealing, which induced electrical degradation (low μ_SAT and high ΔV_TH) of IZO TFTs. In the wet chemical patterning of electrodes, conventional acidic and basic wet etchants cause severe damage to the surfaces of the IZO channels; thus, insufficiently annealed IZO TFTs exhibited considerable degradation in terms of their on-current level and mobility. Alternatively, the TFTs subjected to an excessively long-term thermal annealing showed only a moderate decrease in mobility with the formation of small nanocrystals.     

Semiconducting amorphous carbon thin films for transparent conducting electrodes

Semiconducting amorphous carbon thin films were directly grown on SiO₂ substrate by using chemical vapor deposition. Raman spectra and transmission electron microscopy image showed that the a-C films have a short-range ordered amorphous structure. The electrical and optical properties of the a-C thin films were investigated. The films have sheet resistance of 3.7 kΩ/□ and high transmittance of 82%. They exhibit metal-oxide-semiconductor field effect transistor mobility of 10–12 cm² V⁻¹ s⁻¹ at room temperature, which is comparable to previous reported mobility of amorphous carbon. The optical band gap was calculated by Tauc’s relationship and photoluminescence spectra showed that the films are semiconductor with an optical band gap of 1.8 eV. These good physical properties make the a-C films a candidate for the application of transparent conducting electrodes.