Gallium Doping Effects for Improving Switching Performance of p-Type Copper(I) Oxide Thin-Film Transistors.

Copper(I) oxide (Cu2O), which is obtained from copper(II) oxide (CuO) through a reduction process, is a p-type oxide material with a band gap of 2.1-2.4 eV. However, the switching performance of typical Cu2O thin-film transistors (TFTs) is poor because the reduction process increases the concentration of oxygen vacancies (VO), which interfere with the conduction of hole carriers. Ga with high oxygen affinity was doped in Cu2O thin films to decrease VO during the reduction process. As a result, the VO concentration of 1.56 at % for Ga-doped Cu2O (Ga:Cu2O) thin films decreased from 20.2 to 7.5% compared to pristine Cu2O thin films. Accordingly, the subthreshold swing or S-factor, on/off-current ratio (Ion/off), saturation mobility (µsat), and threshold voltage (Vth) of Ga:Cu2O TFTs were improved compared to pristine Cu2O TFTs with values of 7.72 from 12.50 V/dec, 1.22 × 104 from 2.74 × 102, 0.74 from 0.46 cm2/Vs, and -4.56 from -8.06 V, respectively. These results indicate that Ga plays an important role in improving the switching performance of p-type Cu2O TFT.

Dual-Functional Superoxide Precursor To Improve the Electrical Characteristics of Oxide Thin Film Transistors.

We investigated a method to simultaneously improve the mobility and reliability of solution-processed zinc tin oxide thin film transistors (ZTO TFTs) using a dual-functional potassium superoxide precursor. Potassium cations in the potassium superoxide (KO2) precursor act as carrier suppliers in the ZTO thin film to improve the carrier (electron) concentration, which allows the potassium-doped ZTO TFT to exhibit high mobility. The anions in the precursor exist as superoxide radicals that reduce oxygen vacancies during the formation of thin oxide film. Consequently, the KO2-treated ZTO TFTs exhibited improved mobility and reliability compared with pristine ZTO TFTs, with an increase in field effect mobility from 5.57 to 8.74 cm2/V s and a decrease in the threshold voltage shift from 7.18 to 3.85 V, after a positive bias temperature stress test conducted over 5000 s.

Boosting Modulation of Oxide Semiconductors via Voltage-Based Ambi-Ionic Migration.

In recent years, high-performance amorphous oxide semiconductor thin-film transistor (AOS TFT) technology is required to meet the increasing demand for novel displays, such as rollable, transparent, or augmented reality head-up displays. It has been demonstrated that voltage-based modulation techniques for AOS-based active layers can achieve high-performance AOS TFTs. The voltage-based modulation technique allows specific ions to migrate into the active layer depending on the polarity of the applied voltage, thus easily modulating the active layer. Additionally, potassium superoxide (KO2) solution is employed in AOS TFTs as a source of potassium (K+) and highly reactive superoxide radical (O2•-) ions. The K+ and O2•- ions in the KO2 solution are controlled by an applied voltage bias and rapidly migrate into the active layer, directly changing its chemical composition and electrical properties. AOS TFTs that use this technique exhibit better electrical performance than conventional AOS TFTs: the field-effect mobility improved from 10.05 to 15.31 cm2/V·s; the subthreshold swing decreased from 0.44 to 0.33 V/dec; the Ion/off ratio increased from 1.24 × 107 to 3.17 × 108; and the threshold voltage shift decreased from 5.2 to 3.4 V under a positive bias stress test conducted over 10 000 s. Ultimately, this approach to modulating the internal ion distribution in oxide semiconductors could provide opportunities for various AOS devices to attain desirable electrical characteristics.

Enhancement of Switching Characteristic for p-Type Oxide Semiconductors Using Hypochlorous Acid.

We explored the effects of hypochlorous acid (HClO) oxidation on p-type oxide semiconductors. HClO generates oxygen radicals (O·) (strong reactive oxygen species) that affect the chemical state of p-type copper oxide (CuO x) thin films by reacting with CuO x. On robust oxidation by HClO, the numbers of Cu-O bonds increased and the numbers of copper vacancies serving as hole carriers decreased. In the modified CuO x thin-film transistors (TFTs), switching was evident. The subthreshold swing was 0.70 V/dec, the on-/off-current ratio was 4.86 × 104, and the field effect mobility was 2.83 × 10-3 cm2/V·s. Pristine CuO x TFTs did not exhibit switching.

Electric Field-aided Selective Activation for Indium-Gallium-Zinc-Oxide Thin Film Transistors.

A new technique is proposed for the activation of low temperature amorphous InGaZnO thin film transistor (a-IGZO TFT) backplanes through application of a bias voltage and annealing at 130 °C simultaneously. In this 'electrical activation', the effects of annealing under bias are selectively focused in the channel region. Therefore, electrical activation can be an effective method for lower backplane processing temperatures from 280 °C to 130 °C. Devices fabricated with this method exhibit equivalent electrical properties to those of conventionally-fabricated samples. These results are analyzed electrically and thermodynamically using infrared microthermography. Various bias voltages are applied to the gate, source, and drain electrodes while samples are annealed at 130 °C for 1 hour. Without conventional high temperature annealing or electrical activation, current-voltage curves do not show transfer characteristics. However, electrically activated a-IGZO TFTs show superior electrical characteristics, comparable to the reference TFTs annealed at 280 °C for 1 hour. This effect is a result of the lower activation energy, and efficient transfer of electrical and thermal energy to a-IGZO TFTs. With this approach, superior low-temperature a-IGZO TFTs are fabricated successfully.

High-pressure Gas Activation for Amorphous Indium-Gallium-Zinc-Oxide Thin-Film Transistors at 100 °C.

We investigated the use of high-pressure gases as an activation energy source for amorphous indium-gallium-zinc-oxide (a-IGZO) thin film transistors (TFTs). High-pressure annealing (HPA) in nitrogen (N2) and oxygen (O2) gases was applied to activate a-IGZO TFTs at 100 °C at pressures in the range from 0.5 to 4 MPa. Activation of the a-IGZO TFTs during HPA is attributed to the effect of the high-pressure environment, so that the activation energy is supplied from the kinetic energy of the gas molecules. We reduced the activation temperature from 300 °C to 100 °C via the use of HPA. The electrical characteristics of a-IGZO TFTs annealed in O2 at 2 MPa were superior to those annealed in N2 at 4 MPa, despite the lower pressure. For O2 HPA under 2 MPa at 100 °C, the field effect mobility and the threshold voltage shift under positive bias stress were improved by 9.00 to 10.58 cm(2)/V.s and 3.89 to 2.64 V, respectively. This is attributed to not only the effects of the pressurizing effect but also the metal-oxide construction effect which assists to facilitate the formation of channel layer and reduces oxygen vacancies, served as electron trap sites.

Low-cost label-free electrical detection of artificial DNA nanostructures using solution-processed oxide thin-film transistors.

A high-sensitivity, label-free method for detecting deoxyribonucleic acid (DNA) using solution-processed oxide thin-film transistors (TFTs) was developed. Double-crossover (DX) DNA nanostructures with different concentrations of divalent Cu ion (Cu(2+)) were immobilized on an In-Ga-Zn-O (IGZO) back-channel surface, which changed the electrical performance of the IGZO TFTs. The detection mechanism of the IGZO TFT-based DNA biosensor is attributed to electron trapping and electrostatic interactions caused by negatively charged phosphate groups on the DNA backbone. Furthermore, Cu(2+) in DX DNA nanostructures generates a current path when a gate bias is applied. The direct effect on the electrical response implies that solution-processed IGZO TFTs could be used to realize low-cost and high-sensitivity DNA biosensors.