Solution-processed Oxide Thin-film Transistors Research Articles

Bias-stress-stable sub-1.5V oxide thin-film transistors via synergistic composition of sol-gel quaternary high-k oxide dielectrics

A rising demand in reliable, energy-efficient, and large-area electronics, particularly in the realm of sol-gel oxide thin-film transistors (TFTs), has steered research focus away from semiconductor towards dielectrics. However, achieving both bias stability and low-voltage operation remains a significant hurdle. While typical oxide TFTs employ high-dielectric-constant (high-k) dielectrics with lowered film thickness to acquire low-voltage operation, they inevitably suffer from undesired defects at both bulk and interfacial trap sites in dielectric layer. In this study, bias-stress-stable all solution-processed oxide TFTs were demonstrated with operation voltage under 1.5 V via sol-gel quaternary high-k oxide dielectric (Al-Hf-Zr-O, AHZO). In-depth understanding of their individual contributions to dielectric performance leads to the acquisition of optimized composition ratios of AHZO with amorphous feature and outstanding dielectric performance, marked by dielectric constant (k) over 11, leakage current density (Jleak) below 10−5.5 A cm−2, and sturdy breakdown strength (EB) exceeding 5 MV cm−1. By integrating the AHZO with In-Ga-Zn-O (IGZO) layer, we achieved sub 1.5 V TFTs while maintaining excellent bias stability with threshold voltage (VTH) shift lower than 0.20 V for 3600 s. Our findings offer a detailed insight into the realm of multi-component oxide dielectrics, paving the way for miniaturization and reliability in functional devices and sensors.

High-performance ultrathin solution-processed SnO2 top-gate thin-film transistors by constructing high-quality dielectric/channel interface

In this work, we developed a low-thermal-budget approach to realize high-performance solution-processed oxide thin-film transistors (TFTs). To construct high-quality dielectric/channel interface, the corrosion-resistant SnO2 channel and further O2 plasma pre-treatment were adopted to minimize the problematic interface defects. With this strategy, the afforded solution-processed SnO2 TFTs exhibited improved electrical performance and stability, including a descent field-effect mobility (μFE) of 21.4 cm2/V·s, high on/off ratio (Ion/Ioff) of 109, positive threshold voltage (Vth) of 0.39 V, steep SS of 94.7 mV/dec, and small ΔVth of 0.08 and −0.03 V under PBS and NBS, respectively. This work demonstrates a promising fabrication approach for solution-processed oxide TFTs towards advanced applications.

Dependence of Positive Bias Stress Instability on Threshold Voltage and Its Origin in Solution-Processed Aluminum-Doped Indium Oxide Thin-Film Transistors.

The initial electrical characteristics and bias stabilities of thin-film transistors (TFTs) are vital factors regarding the practical use of electronic devices. In this study, the dependence of positive bias stress (PBS) instability on an initial threshold voltage (VTH) and its origin were analyzed by understanding the roles of slow and fast traps in solution-processed oxide TFTs. To control the initial VTH of oxide TFTs, the indium oxide (InOx) semiconductor was doped with aluminum (Al), which functioned as a carrier suppressor. The concentration of oxygen vacancies decreased as the Al doping concentration increased, causing a positive VTH shift in the InOx TFTs. The VTH shift (∆VTH) caused by PBS increased exponentially when VTH was increased, and a distinct tendency was observed as the gate bias stress increased due to a high vertical electric field in the oxide dielectric. In addition, the recovery behavior was analyzed to reveal the influence of fast and slow traps on ∆VTH by PBS. Results revealed that the effect of the slow trap increased as the VTH moved in the positive direction; this occured because the main electron trap location moved away from the interface as the Fermi level approached the conduction band minimum. Understanding the correlation between VTH and PBS instability can contribute to optimizing the fabrication of oxide TFT-based circuits for electronic applications.

Solution-Processed Metal Oxide Thin-Film Transistor at Low Temperature via A Combination Strategy of H2 O2 -Inducement Technique and Infrared Irradiation Annealing.

Solution processing has emerged as a promising technique for the fabrication of oxide thin-film transistors (TFTs), offering advantages such as low cost, high throughput, and exceptional compositional control. However, achieving reasonable electrical properties typically demands high annealing temperatures in the fabrication process. In addressing this challenge, a novel combination strategy is proposed that involves integrating the H2 O2 inducement technique with infrared (IR) irradiation annealing. The study investigates the effects of precursors and IR irradiation annealing temperatures on the electrical properties of In2 O3 TFTs. It is found that H2 O2 can help accelerate the decomposition of organic residues, while IR irradiation annealing could enhance the film densification. By employing the proposed strategy, metal oxide TFTs consisting of a Zr-Al-O dielectric fabricated at 230°C and an In2 O3 channel layer fabricated at 185°C demonstrated high performance with field-effect mobility = 31.7 cm2 V-1 ·s-1 , threshold voltage = 1.3 V, subthreshold swing = 0.13 V per decade, and on-to-off current ratio = 1.1 × 105 . This work demonstrates the proposed combinational strategy is a general method to fabricate not only metal oxide semiconductorsbut also dielectrics.

Continuous-Wave Green Laser Activation of Transparent InZnO Electrodes for Fully Solution-Processed Oxide Thin-Film Transistors

Continuous-Wave Green Laser Activation of Transparent InZnO Electrodes for Fully Solution-Processed Oxide Thin-Film Transistors

Effect of the gas flow rate in the focused-oxygen plasma treatment of solution-processed indium oxide thin film transistors

In2O3 is a transparent semiconductor layer due to its transparency, high mobility, and solution processability. In this paper, we study the effect of focused oxygen plasma treatment on solution processed indium oxide films annealed at 250 °C and analyze the effect of the gas flow rate. Thin film transistor devices were prepared based on pristine In2O3 films and films treated with oxygen plasma using flow rates of 3, 6, and 9 sccm. The results show that controlling the plasma flow rate can be used to finetune the device characteristics, and the selecting the optimal flow rate is important to improve the electron mobility, threshold voltage, on/off current ratio, and the electrical stability of the devices. The In2O3 TFTs treated at 6 sccm showed the best performance with mobility of 2.19 cm2V−1s−1, threshold voltage of of 4.25 V, and on/off current ratio of 2.13 × 106. The successful fabrication of high-performance, low-temperature solution deposited indium oxide transistors using the focused oxygen plasma treatment for electrical device applications demonstrates the practical potential of this treatment method for future transparent electronics.

Rapid and Highly Sensitive Detection of Leishmania by Combining Recombinase Polymerase Amplification and Solution-Processed Oxide Thin-Film Transistor Technology.

Nucleic acid detection is widely used to identify infectious diseases and ensure food safety. However, conventional PCR-based techniques are time consuming. Thus, this study aims to combine recombinase polymerase amplification (RPA), which enables the rapid amplification of even trace amounts of nucleic acid fragments within 10-40 min at 37-42 °C, and solution-processed oxide thin-film transistor (TFT) technology, which exhibits high detection sensitivity, to detect Leishmania. A single-stranded anti-probe was incorporated into the RPA primer to facilitate effective hybridization between the RPA product and the immobilized probe on the solution-processed oxide TFT. The RPA-amplified product carrying an anti-probe enabled specific binding to the chip surface. Changes in current were monitored before and after sample incubation to identify the target nucleic acids in the samples accurately. The proposed method achieved a remarkable limit of detection of 101 copies/μL of the Leishmania HSP70 fragment within 30 min. The design of the probes on the solution-processed oxide TFT surface and the anti-probe simplified the detection of other target nucleic acids, eliminating the need to denature DNA double-strands for specific binding during nucleic acid detection. Thus, the novel method offers the advantage of requiring minimal reagent resources and eliminates the need for complex procedures.

Heterojunction Channel Engineering in Performance Enhancement of Solution-Processed Oxide Thin-Film Transistors

With the continuous development of next-generation display techniques, more and more attention has been put on solution-processed oxide thin-film transistors (TFTs). Here, we report the solution-based growth of InZnO/AlInZnO (IZO/AIZO) heterojunction channel layers and their implementation in high-performance TFTs. It is found that the heterojunction transistors exhibit a band-like electron transport, with both current ON/OFF ratio and mobility values significantly higher than single-layer IZO and AIZO devices by more than one order of magnitude. The marked improvement here is demonstrated to originate from the presence of confined free electrons at the atomically sharp heterointerface induced by the large conduction band offset between IZO and AIZO. Further channel engineering including the modification of In:Zn ratios, IZO, and AIZO thicknesses allows us to obtain high-performance heterojunction oxide TFTs with a high current ON/OFF ratio of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$&gt;$</tex-math> </inline-formula> 5 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times$</tex-math> </inline-formula> 10 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\text{7}}$</tex-math> </inline-formula> , a mobility of 14.4 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pm$</tex-math> </inline-formula> 1 cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\text{2}}$</tex-math> </inline-formula> /Vs, a turn-on voltage close to zero-volt, and a superb stability against bias stresses and long-term storage. The high performance combined with the solution-processability enables the great potential of our reported TFTs in next-generation printable electronics.

Improvement in Switching Characteristics and Bias Stability of Solution-Processed Zinc-Tin Oxide Thin Film Transistors via Simple Low-Pressure Thermal Annealing Treatment.

In this study, we used a low-pressure thermal annealing (LPTA) treatment to improve the switching characteristics and bias stability of zinc-tin oxide (ZTO) thin film transistors (TFTs). For this, we first fabricated the TFT and then applied the LPTA treatment at temperatures of 80 °C and 140 °C. The LPTA treatment reduced the number of defects in the bulk and interface of the ZTO TFTs. In addition, the changes in the water contact angle on the ZTO TFT surface indicated that the LPTA treatment reduced the surface defects. Hydrophobicity suppressed the off-current and instability under negative bias stress because of the limited absorption of moisture on the oxide surface. Moreover, the ratio of metal-oxygen bonds increased, while the ratio of oxygen-hydrogen bonds decreased. The reduced action of hydrogen as a shallow donor induced improvements in the on/off ratio (from 5.5 × 103 to 1.1 × 107) and subthreshold swing (8.63 to V·dec-1 and 0.73 V·dec-1), producing ZTO TFTs with excellent switching characteristics. In addition, device-to-device uniformity was significantly improved because of the reduced defects in the LPTA-treated ZTO TFTs.

The impact of nickel doping on metal-oxide network in solution-processed indium zinc oxide transistors

Solution-processed nickel zinc oxide (IZO) thin-film transistors (TFTs) are of great interest as core elements for future display technology based on the low-cost manufacture. It still remains a challenge to ensure stability in the off-current state of IZO using a dopant that is more resource-friendly than Ga in solution processing. In this study, we suggest a solution-based doping process of the IZO semiconductors with a Ni carrier suppressor and investigated the effects of Ni on their transistor operation. Owing to similar size of Ni2+ to Zn2+, an appropriate concentration of Ni partially substituted Zn sites and the strong bonding force of Ni with O reduces hydroxide groups on the surface bonded to Zn, whereas excess Ni addition rather inhibited the formation of smooth thin film. The resulting Ni-doped IZO TFT devices from an optimized Ni concentration exhibited higher performances with significantly reduced off-state currents while maintaining high on-state currents, compared to IZO TFTs. This approach enriches the current family of solution-processed metal oxide semiconductors by providing another option for resource-friendly carrier suppressor, which can contribute to the inexpensive and simple solution process for fabricating TFTs.

Low-voltage, solution-processed InZnO thin-film transistors with enhanced performance achieved by bilayer gate dielectrics

Fully solution-processed oxide thin-film transistors (TFTs) have shown a great potential in future printable electronics. However, high defect densities at the dielectric/channel interface have limited the simultaneous achievement of low operating voltage and high device performance. In this Letter, we study the effects of bilayer gate dielectrics in potential performance enhancement of solution-processed indium zinc oxide (IZO) TFTs. Several single and bilayer gate dielectrics are studied in terms of their physical and electrical properties. Compared with IZO TFTs using single-layer ZrOx gate dielectrics, optimized TFTs with ZrOx/AlOx gate dielectrics show an increase in carrier mobility and current on/off ratio by a factor of 3.5 and 27, respectively. The inner mechanisms of the performance enhancement are systematically studied, showing that the significantly improved TFT performance originates from the passivation effects of AlOx, which reduce the trap/defect states at the dielectric/channel interface by approximately an order of magnitude. With a low operating voltage of 2 V, a high mobility of over 10 cm2/V s, a subthreshold swing as low as 89 mV/dec, and a high current on/off ratio of &amp;gt;105, the reported devices might have a great potential in future low-cost, low-power printable electronics.

Effects of Post-UV/Ozone Treatment on Electrical Characteristics of Solution-Processed Copper Oxide Thin-Film Transistors.

To realize oxide semiconductor-based complementary circuits and better transparent display applications, the electrical properties of p-type oxide semiconductors and the performance improvement of p-type oxide thin-film transistors (TFTs) are required. In this study, we report the effects of post-UV/ozone (O3) treatment on the structural and electrical characteristics of copper oxide (CuO) semiconductor films and the TFT performance. The CuO semiconductor films were fabricated using copper (II) acetate hydrate as a precursor material to solution processing and the UV/O3 treatment was performed as a post-treatment after the CuO film was fabricated. During the post-UV/O3 treatment for up to 13 min, the solution-processed CuO films exhibited no meaningful change in the surface morphology. On the other hand, analysis of the Raman and X-ray photoemission spectra of solution-processed CuO films revealed that the post-UV/O3 treatment induced compressive stress in the film and increased the composition concentration of Cu-O lattice bonding. In the post-UV/O3-treated CuO semiconductor layer, the Hall mobility increased significantly to approximately 280 cm2 V-1 s-1, and the conductivity increased to approximately 4.57 × 10-2 Ω-1 cm-1. Post-UV/O3-treated CuO TFTs also showed improved electrical properties compared to those of untreated CuO TFTs. The field-effect mobility of the post-UV/O3-treated CuO TFT increased to approximately 6.61 × 10-3 cm-2 V-1 s-1, and the on-off current ratio increased to approximately 3.51 × 103. These improvements in the electrical characteristics of CuO films and CuO TFTs can be understood through the suppression of weak bonding and structural defects between Cu and O bonds after post-UV/O3 treatment. The result demonstrates that the post-UV/O3 treatment can be a viable method to improve the performance of p-type oxide TFTs.

Solution processed low operating voltage SnO2 thin film transistor by using Li2SnO3/TiO2 stacked gate dielectric

A thin layer of sol–gel derived lithium stannate (Li2SnO3) was deposited by a spin coating method that shows high areal capacitance with low DC conductivity which originated due to the mobile lithium ion (Li+) and low carrier concentration. The DC conductivity of this Li2SnO3 thin film has been reduced further by adding a sol–gel derived titanium-oxide (TiO2) layer. Moreover, bilayer thin film effective areal capacitance also increases due to the higher relative permittivity of TiO2 layer. The measured areal capacitance of such Li2SnO3 /TiO2 bilayer thin films is 518 nF/cm2 with a low current density of 3.8 X 10-5 A/cm2 under 5 V external bias, can fulfil the requirement as gate dielectric of a low operating voltage (≤3V) thin film transistor (TFT). For identifying its applicability as gate insulator, a solution-processed tin oxide (SnO2) thin film transistor (TFT) was fabricated with Li2SnO3/TiO2 stacked gate dielectric. In this TFT structure, thin film of TiO2, SnO2 and Li2SnO3 are all polycrystalline in nature. The electrical characterization of TFT has been done under DC mode operation in ambient atmosphere measurement which remain almost unaltered under 11 cyclic measurements. The optimized TFT required only 3.0 V external bias that shows an electron mobility of 3.47 cm2 V−1 s−1 with on/off ratio of 50.

Effect of channel thickness on radiation hardness of solution-processed oxide thin film transistors

The electrical properties of a-ZITO TFTs with different channel thicknesses were characterized under proton irradiation with electrical stress. The device measurements showed stronger radiation-tolerance of oxide TFTs with smaller channel thickness.

Solution-processed amorphous zinc indium tin oxide thin-film transistors with high stability under AC stress

Amorphous zinc–indium–tin oxide thin-film transistors with various Zn : In : Sn ratios are investigated to improve AC stress stability by suppressing hot carrier effects in the channel layer.

Enhanced stability and mobility of solution-processed oxide thin-film transistors with bilayer terbium-incorporated indium oxide channel

The trade-off between mobility and stability in oxide thin-film transistors (TFTs) hinders further advances of an active-matrix flat panel display. Herein, a solution-processed bilayer active channel is designed to improve the stability and mobility simultaneously. The optical bandgap and work function of Tb:In2O3 films are modulated by tuning the film thickness and Tb concentration of Tb-doped indium oxide (Tb:In2O3) films. Large conduction band offset is achieved in a Tb:In2O3 bilayer channel, which induces accumulation of abundant electrons at the interface. The mobility is significantly improved to 38.2 cm2/V s, and the photoinduced stability of bilayer Tb:In2O3 TFTs is improved with low threshold voltage shift of 0.26 and −0.38 V under negative-bias illumination stress and negative-bias temperature illumination stress, respectively.

Highly conductive and low-work-function polymer electrodes for solution-processed n-type oxide thin-film transistors

We present an n-doped poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) polymer and its application in n-type oxide thin-film transistors (OxTFTs) as a source and drain electrode material. A reduced molecule of a cationic dye, methyl red (MR), was used as an effective solution-processed n-type dopant. The sequential de-doping and doping of the initially p-doped PEDOT:PSS polymer with the reduced MR (r-MR) effectively removed positive charges via cancellation by the added electrons. As a result, the electron conductivity of PEDOT:PSS increased from 3.4 S/cm to ∼51 S/cm, while its work function decreased from 4.8 eV to 3.5 eV. This is one of the lowest values of the work function reported for PEDOT:PSS. The n-doped PEDOT:PSS films were eventually applied as a suitable material to fabricate the contact electrodes of solution-processed bottom-gate top-contact amorphous indium-gallium-zinc oxide-based OxTFTs. The resultant devices exhibited electron mobility over ten times better compared to those with undoped PEDOT:PSS electrodes. Therefore, we suggest this method as a highly suitable and low-cost technique for improving electron transport in PEDOT:PSS and all solution-processed conductors. Further investigations with this method are expected to expand the application of PEDOT:PSS to other sectors of optoelectronics.

Importance of Solvent Evaporation Temperature in Pre-Annealing Stage for Solution-Processed Zinc Tin Oxide Thin-Film Transistors

We focused on the importance of solvent evaporation governed by the temperature of the pre-annealing stage (TS) in solution-processed zinc tin oxide (ZTO) thin-film transistors (TFTs). We controlled TS based on the boiling point (BP) of the solvent used. When TS reaches the BP, the field effect mobility is found to be about 1.03 cm2/V s, which is 10 times larger than the TS &lt; Bp case (0.13 cm2/V s). The reason is presumed to be that residual organic defects are effectively removed as TS increases. In addition, when Ts is beyond Bp, the mobility is rather decreased due to structural defects such as pores and pinholes. Based on our results, it is noted that TS plays a significant role in the enhancement of electrical performance and stability of solution-processed ZTO TFTs.

Significant performance and stability improvement of low-voltage InZnO thin-film transistors by slight La doping

Low-voltage, solution-processed oxide thin-film transistors (TFTs) have shown great potential in next-generation low-power, printable electronics. However, it is now still quite challenging to obtain low-voltage oxide TFTs with both high mobility and stability, especially for solution-processed ones. In this work, La-doped InZnO (IZO:La) channel for high performance and stable TFTs is developed using a simple solution process. The effects of La composition in IZO:La on the film and TFT performance are systematically investigated. It is confirmed that the incorporation of appropriate La could control the carrier concentration, improve the surface morphology, and passivate the oxygen-related defects, leading to a reduced trap density both at dielectric/channel interface and within the channel layer. As a result, the optimized TFTs with 1% La dopants exhibit the best overall performance, including a low operating voltage of 1 V, a high mobility of 14.5 cm2/V s, a low subthreshold swing of 109 mV/dec, a turn-on voltage close to 0 V, and negligible changes of performance under both positive and negative bias stresses. This work might support the development of all-solution-processed oxide TFT backplanes for battery-powered active-matrix displays.

Spray pyrolyzed fluorinated inorganic-organic passivation for solution-processed a-InZnO thin-film transistors

Reliability is largely considered an important device requirement for commercial electronic devices. In this work, the reliability of solution-processed amorphous indium zinc oxide (a-IZO) thin-film transistors (TFTs) were evaluated through incorporation of inorganic-organic fluorinated polysilsesquioxane (PSQ:F) passivation deposited through spray pyrolysis (SP) and spin coating (SC) techniques. The barrier property of SP and SC PSQ:F passivation layers were examined under positive and negative bias stress tests and both devices revealed superior stability with smaller threshold voltage shift of 0.9 V/−0.4 V and 1.0 V/−0.2 V respectively, after 10000 s. In addition, both PSQ:F films were confirmed to have an excellent hydrophobic property compared to a-IZO film hindering moisture adsorption. Futheremore, XRR data demonstrates that through SP deposition technique, higher density PSQ:F passivation layer was achieved which inhibits the interaction between the ambient atmosphere and the exposed back-channel region efficiently. Enhancement in the reliability and electrical performance of a-IZO TFTs were achieved through combination of additional fluorine additives from PSQ:F with spray pyrolysis deposition technique.