Analysis of Interface Phenomena for High-Performance Dual-Stacked Oxide Thin-Film Transistors via Equivalent Circuit Modeling.

Oxide thin-film transistors (TFTs) have attracted much attention because they can be applied to flexible and large-scaled switching devices. Especially, oxide semiconductors (OSs) have been developed as active layers of TFTs. Among them, indium-gallium-zinc oxide (IGZO) is actively used in the organic light-emitting diode display field. However, despite their superior off-state properties, IGZO TFTs are limited by low field-effect mobility, which critically affects display resolution and power consumption. Herein, we determine new working mechanisms in dual-stacked OS, and based on this, we develop a dual-stacked OS-based TFT with improved performance: high field-effect mobility (∼80 cm2/V·s), ideal threshold voltage near 0 V, high on-off current ratio (>109), and good stability at bias stress. Induced areas are formed at the interface by the band offset: band offset-induced area (BOIA) and BOIA-induced area (BIA). They connect the gate bias-induced area (GBIA) and electrode bias-induced area (EBIA), resulting in high current flow. Equivalent circuit modeling and the transmission line method are also introduced for more precise verification. By verifying current change with gate voltage in the single OS layer, the current flowing direction in the dual-stacked OS is calculated and estimated. This is powerful evidence to understand the conduction mechanism in a dual-stacked OS-based TFT, and it will provide new design rules for high-performance OS-based TFTs.

Highly transparent phototransistor based on quantum-dots and ZnO bilayers for optical logic gate operation in visible-light.

Highly transparent optical logic circuits operated with visible light signals are fabricated using phototransistors with a heterostructure comprised of an oxide semiconductor (ZnO) with a wide bandgap and quantum dots (CdSe/ZnS QDs) with a small bandgap. ZnO serves as a highly transparent active channel, while the QDs absorb visible light and generate photoexcited charge carriers. The induced charge carriers can then be injected into the ZnO conduction band from the QD conduction band, which enables current to flow to activate the phototransistor. The photoexcited charge transfer mechanism is investigated using time-resolved photoluminescence spectroscopy, scanning Kelvin probe microscopy, and ultraviolet photoelectron spectroscopy. Measurements show that carriers in the QD conduction band can transfer to the ZnO conduction band under visible light illumination due to a change in the Fermi energy level. Moreover, the barrier for electron injection into the ZnO conduction band from the QD conduction band is low enough to allow photocurrent generation in the QDs/ZnO phototransistor. Highly transparent NOT, NOR, and NAND optical logic circuits are fabricated using the QDs/ZnO heterostructure and transparent indium tin oxide electrodes. This work provides a means of developing highly transparent optical logic circuits that can operate under illumination with low-energy photons such as those found in visible light.

Atmospheric-pressure plasma treatment toward high-quality solution-processed aluminum oxide gate dielectric films in thin-film transistors.

We present an atmospheric-pressure plasma (APP) treatment technique to improve the electrical performance of solution-processed dielectric films. This technique can successfully reduce leakage current and frequency dependence of solution-processed dielectric films. The APP treatment contributes to the conversion of metal hydroxide to metal oxide, and in the case of a solution-treated AlO x dielectric thin film, it effectively ascribes to the formation of high-quality AlO x dielectric thin films. The capacitance of the untreated AlO x dielectric thin film varies up to 9.9% with frequency change, but the capacitance of the APP treated AlO x dielectric thin film varies within 1.5%. When the solution-processed InO x thin-film transistors (TFTs) were fabricated using these dielectric films, the field-effect mobility of TFTs with the APP-treated AlO x dielectric film was increased significantly from 9.77 to 26.79 cm2 V-1 s-1 in comparison to that of TFTs with the untreated AlO x dielectric film. We also have confirmed that these results are similar to the properties of the sample prepared at high annealing temperature including electrical performance, conduction mechanism and chemical structure. The APP treatment technique provides a new opportunity to effectively improve the electrical performance of solution-processed dielectrics in the atmosphere at low temperature.

Verification of Charge Transfer in Metal-Insulator-Oxide Semiconductor Diodes via Defect Engineering of Insulator.

In a MIS (Metal/Insulator/Semiconductor) structure consisting of two terminals, a systematic analysis of the electrical charge transport mechanism through an insulator is essential for advanced electronic application devices such as next-generation memories based on resistance differences. Herein, we have verified the charge transfer phenomenon in MIOS (Metal/Insulator/Oxide Semiconductor) diodes through a defect engineering of the insulator. By selectively generating the oxygen vacancies in the insulator (Al2O3), the MIOS diode rectification of the P++-Si anode/Al2O3/IGZO cathode reached 107 at 1.8 V and considerably suppressed the leakage current. Studying the current-voltage characteristics of MIOS diodes shows that the charge carrier transport mechanism can vary depending on the defect density as well as the difference between the CBM (conduction band minimum) of the semiconductor and the oxygen vacancy energy level of the insulator.

Vertical Transport Control of Electrical Charge Carriers in Insulator/Oxide Semiconductor Hetero-structure.

The technology for electrical current passing through an insulator thin-film between two electrodes is newly getting spotlights for substantial potentials toward advanced functional devices including a diode and a resistive switching device. However, depending on an electrode-limited conduction mechanisms of the conventional devices, a narrow processing window for a thickness of the insulator thin-film and an inability to control a magnitude and direction of the currents are challenges to overcome. Herein, we report a new approach to enable electrical charge carriers to pass stably through a relatively-thick insulator layer and to control a magnitude and polarity of the currents by applying an oxide semiconductor electrode in a metal/insulator/metal structure. We reveal that the electrical conduction in our devices follows a space charge-limited conduction mechanism which mainly depends on the charge carriers injected from contacts. Therefore, characteristics of the current including a current value and a rectification ratio of input signal are precisely controlled by electrical properties of the oxide semiconductor electrode. The unique current characteristics in metal/insulator/oxide semiconductor structures give extendable inspirations in electronic materials science, even a prominent solution for various technology areas of electronics.

Solution-processed amorphous ZrO2 gate dielectric films synthesized by a non-hydrolytic sol-gel route.

Solution-processed zirconium oxide (ZrO2) dielectrics were formed via a non-hydrolytic sol-gel route at low-temperature, and are suitable for flexible thin film transistor (TFT) devices. Precursor solutions with equimolar zirconium halide and zirconium alkoxide were prepared, and amorphous ZrO2 films were obtained by spin-coating and annealing at 300 °C through the direct condensation reaction between them. The ZrO2 films exhibited a high dielectric constant near 10, and a low leakage current density of 5 × 10-8 A cm-2 at a field of 1 MV cm-1. High mobility p-type pentacene TFTs were fabricated using the ZrO2 dielectrics, with a saturation field-effect mobility of 3.7 cm2 V-1 s-1, a threshold voltage of -2.7 V, an on/off ratio of 1.1 × 106 and a subthreshold swing of 0.65 V dec-1.

Strong Influence of Humidity on Low-Temperature Thin-Film Fabrication via Metal Aqua Complex for High Performance Oxide Semiconductor Thin-Film Transistors.

Oxide semiconductors thin film transistors (OS TFTs) with good transparency and electrical performance have great potential for future display technology. In particular, solution-processed OS TFTs have been attracted much attention due to many advantages such as continuous, large scale, and low cost processability. Recently, OS TFTs fabricated with a metal aqua complex have been focused because they have low temperature processability for deposition on flexible substrate as well as high field-effect mobility for application of advanced display. However, despite some remarkable results, important factors to optimize their electrical performance with reproducibility and uniformity have not yet been achieved. Here, we newly introduce the strong effects of humidity to enhance the electrical performance of OS TFTs fabricated with the metal aqua complex. Through humidity control during the spin-coating process and annealing process, we successfully demonstrate solution-processed InOx/SiO2 TFTs with a good electrical uniformity of ∼5% standard deviation, showing high average field-effect mobility of 2.76 cm2V-1s-1 and 15.28 cm2V-1s-1 fabricated at 200 and 250 °C, respectively. Also, on the basis of the systematic analyses, we demonstrate the mechanism for the change in electrical properties of InOx TFTs depending on the humidity control. Finally, on the basis of the mechanism, we extended the humidity control to the fabrication of the AlOx insulator. Subsequently, we successfully achieved humidity-controlled InOx/AlOx TFTs fabricated at 200 °C showing high average field-effect mobility of 9.5 cm2V-1s-1.