Amorphous IGZO TFT with High Mobility of ∼70 cm2/(V s) via Vertical Dimension Control Using PEALD.

Amorphous InGaZnOx (a-IGZO) thin-film transistors (TFTs) are currently used in flat-panel displays due to their beneficial properties. However, the mobility of ∼10 cm2/(V s) for the a-IGZO TFTs used in commercial organic light-emitting diode TVs is not satisfactory for high-resolution display applications such as virtual and augmented reality applications. In general, the electrical properties of amorphous oxide semiconductors are strongly dependent on their chemical composition; the indium (In)-rich IGZO achieves a high mobility of 50 cm2/(V s). However, the In-rich IGZO TFTs possess another issue of negative threshold voltage owing to intrinsically high carrier density. Therefore, the development of an effective way of carrier density suppression in In-rich IGZO will be a key strategy to the realization of practical high-mobility a-IGZO TFTs. In this study, we report that In-rich IGZO TFTs with vertically stacked InOx, ZnOx, and GaOx atomic layers exhibit excellent performances such as saturation mobilities of ∼74 cm2/(V s), threshold voltage of -1.3 V, on/off ratio of 8.9 × 108, subthreshold swing of 0.26 V/decade, and hysteresis of 0.2 V, while keeping a reasonable carrier density of ∼1017 cm-3. We found that the vertical dimension control of IGZO active layers is critical to TFT performance parameters such as mobility and threshold voltage. This study illustrates the potential advantages of atomic layer deposition processes for fabricating ultrahigh-mobility oxide TFTs.

Design of InZnSnO Semiconductor Alloys Synthesized by Supercycle Atomic Layer Deposition and Their Rollable Applications.

Amorphous InGaZnO semiconductors have been rapidly developed as active charge-transport materials in thin film transistors (TFTs) because of their cost effectiveness, flexibility, and homogeneous characteristics for large-area applications. Recently, InZnSnO (IZTO) with superior mobility (higher than 20 cm2 V-1 s-1) has been suggested as a promising oxide semiconductor material for high-resolution, large-area displays. However, the electrical and physical characteristics of IZTO have not been fully characterized. In this study, thin IZTO films were grown using a novel atomic layer deposition (ALD) supercycle process consisting of alternating subcycles of single-oxide deposition. By varying the number of deposition subcycles, it was determined that the insertion of a Sn-O cycle improved the mobility and reliability of IZTO-based TFTs. Specifically, the IZTO TFT obtained using one In-O cycle, one Zn-O cycle, and one Sn-O exhibited the best performance (saturation mobility of 27.8 cm2 V-1 s-1 and threshold voltage shift of 1.8 V after applying positive-bias temperature stress conditions). Next, the production of rollable and flexible devices was demonstrated by fabricating ALD-processed IZTO TFTs on polymer substrates. The electrical characteristics of these TFTs were retained without drastic degradation for 240,000 bending cycles. These results indicate that the supercycle ALD technique is effective for synthesizing multicomponent oxide TFTs for electronic applications requiring high mobility and mechanical flexibility.

Performance and Stability Enhancement of In-Sn-Zn-O TFTs Using SiO2 Gate Dielectrics Grown by Low Temperature Atomic Layer Deposition.

Silicon dioxide (SiO2) films were synthesized by plasma-enhanced atomic layer deposition (PEALD) using BTBAS [bis(tertiarybutylamino) silane] as the precursor and O2 plasma as the reactant, at a temperature range from 50 to 200 °C. While dielectric constant values larger than 3.7 are obtained at all deposition temperatures, the leakage current levels are drastically reduced to below 10-12 A at temperatures above 150 °C, which are similar to those obtained in thermally oxidized and PECVD grown SiO2. Thin film transistors (TFTs) based on In-Sn-Zn-O (ITZO) semiconductors were fabricated using thermal SiO2, PECVD SiO2, and PEALD SiO2 grown at 150 °C as the gate dielectrics, and superior device performance and stability are observed in the last case. A linear field effect mobility of 68.5 cm2/(V s) and a net threshold voltage shift (ΔVth) of approximately 1.2 V under positive bias stress (PBS) are obtained using the PEALD SiO2 as the gate insulator. The relatively high concentration of hydrogen in the PEALD SiO2 is suggested to induce a high carrier density in the ITZO layer deposited onto it, which results in enhanced charge transport properties. Also, it is most likely that the hydrogen atoms have passivated the electron traps related to interstitial oxygen defects, thus resulting in improved stability under PBS. Although the PECVD SiO2 contains a hydrogen concentration similar to that of PEALD SiO2, its relatively large surface roughness appears to induce scattering effects and the generation of electron traps, which result in inferior device performance and stability.

Atomic Layer Deposition of an Indium Gallium Oxide Thin Film for Thin-Film Transistor Applications.

Indium gallium oxide (IGO) thin films were deposited via atomic layer deposition (ALD) using [1,1,1-trimethyl-N-(trimethylsilyl)silanaminato]indium (InCA-1) and trimethylgallium (TMGa) as indium and gallium precursors, respectively, and hydrogen peroxide as the reactant. To clearly understand the mechanism of multicomponent ALD growth of oxide semiconductor materials, several variations in the precursor-reactant deposition cycles were evaluated. Gallium could be doped into the oxide film at 200 °C when accompanied by an InCA-1 pulse, and no growth of gallium oxide was observed without the simultaneous deposition of indium oxide. Density functional theory calculations for the initial adsorption of the precursors revealed that chemisorption of TMGa was kinetically hindered on hydroxylated SiOx but was spontaneous on a hydroxylated InOx surface. Moreover, the atomic composition and electrical characteristics, such as carrier concentration and resistivity, of the ALD-IGO film were controllable by adjusting the deposition supercycles, composed of InO and GaO subcycles. Thus, ALD-IGO could be employed to fabricate active layers for thin-film transistors to realize an optimum mobility of 9.45 cm2/(V s), a threshold voltage of -1.57 V, and a subthreshold slope of 0.26 V/decade.

Flexible and High-Performance Amorphous Indium Zinc Oxide Thin-Film Transistor Using Low-Temperature Atomic Layer Deposition.

Amorphous indium zinc oxide (IZO) thin films were deposited at different temperatures, by atomic layer deposition (ALD) using [1,1,1-trimethyl-N-(trimethylsilyl)silanaminato]indium (INCA-1) as the indium precursor, diethlzinc (DEZ) as the zinc precursor, and hydrogen peroxide (H2O2) as the reactant. The ALD process of IZO deposition was carried by repeated supercycles, including one cycle of indium oxide (In2O3) and one cycle of zinc oxide (ZnO). The IZO growth rate deviates from the sum of the respective In2O3 and ZnO growth rates at ALD growth temperatures of 150, 175, and 200 °C. We propose growth temperature-dependent surface reactions during the In2O3 cycle that correspond with the growth-rate results. Thin-film transistors (TFTs) were fabricated with the ALD-grown IZO thin films as the active layer. The amorphous IZO TFTs exhibited high mobility of 42.1 cm2 V-1 s-1 and good positive bias temperature stress stability. Finally, flexible IZO TFT was successfully fabricated on a polyimide substrate without performance degradation, showing the great potential of ALD-grown TFTs for flexible display applications.

A Study on the Electrical Properties of Atomic Layer Deposition Grown InOx on Flexible Substrates with Respect to N2O Plasma Treatment and the Associated Thin-Film Transistor Behavior under Repetitive Mechanical Stress.

Indium oxide (InOx) films were deposited at low processing temperature (150 °C) by atomic layer deposition (ALD) using [1,1,1-trimethyl-N-(trimethylsilyl)silanaminato]indium (InCA-1) as the metal precursor and hydrogen peroxide (H2O2) as the oxidant. As-deposited InOx exhibits a metallic conductor-like behavior owing to a relatively high free-carrier concentration. In order to control the electron density in InOx layers, N2O plasma treatment was carried out on the film surface. The exposure time to N2O plasma was varied (600-2400 s) to evaluate its effect on the electrical properties of InOx. In this regard, thin-film transistors (TFTs) utilizing this material as the active layer were fabricated on polyimide substrates, and transfer curves were measured. As the plasma treatment time increases, the TFTs exhibit a transition from metal-like conductor to a high-performance switching device. This clearly demonstrates that the N2O plasma has an effect of diminishing the carrier concentration in InOx. The combination of low-temperature ALD and N2O plasma process offers the possibility to achieve high-performance devices on polymer substrates. The electrical properties of InOx TFTs were further examined with respect to various radii of curvature and repetitive bending of the substrate. Not only does prolonged cyclic mechanical stress affect the device properties, but the bending direction is also found to be influential. Understanding such behavior of flexible InOx TFTs is anticipated to provide effective ways to design and achieve reliable electronic applications with various form factors.