Figure - available from: Journal of Physics D: Applied Physics
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(a) Cross-sectional view of a flexible OLED display and (b) example of bending strain distribution at the bent state.
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Amorphous oxide semiconductor thin film transistors (AOS TFT) have recently attracted attention as next generation display backplane materials. In this article, the current research trends and status of AOS TFTs for flexible displays are discussed. First, the degradation mechanism of AOS TFTs via bending stress is examined. In this part, we investi...
Citations
Electronic devices based on flexible displays have been developed for use in various applications. Upon bending such devices, the applied mechanical force deteriorates the reliability of the devices. Therefore, a high bending reliability that is influenced by tensile and compressive forces that vary depending on the bending radius of curvature must be secured to ensure a reliable device operation. In this study, we investigated the electrical characteristics of flexible low-temperature polycrystalline silicon (LTPS) and amorphous indium–gallium–zinc–oxide (a-IGZO) thin-film transistors (TFTs) on polyimide (PI) substrate and subsequently conducted reliability evaluation of the devices under mechanical stress by applying negative bias temperature illumination stress (NBTIS). The degradations in the electrical properties, such as threshold voltage (
$\textit{V}_{\text{th}}$
) shift, OFF-state current (
$\textit{I}_{\text{off}}$
), and subthreshold slope (SS), and reliability, increased, and the reliability worsened, as the tensile force increased under decreasing radius of curvature, whereas relatively low degradation occurred under compressive force. A tensile stress-induced increase in grain boundary state density in polycrystalline silicon (poly-Si) and oxygen vacancy in a-IGZO were verified by density of state (DOS) extraction; the cracks occurring due to tensile stress were optically analyzed; the external moisture penetration along the cracks further degrades the device characteristics. This study provides an analysis of device design for ensuring reliability under the application of mechanical stress, thereby contributing to the development of reliable flexible technology.
Amorphous indium-gallium-zinc-oxide (α-IGZO) thin films were prepared by high-power impulse magnetron sputtering. The film composition and properties were investigated. The films were zinc-rich when compared to the target material due to the highest sputtering yield, the least collision scattering and the lowest self-sputtering yield of the zinc species among the metallic sputtered species. The carrier-density in the thin film was modulated by the process pressure. Increasing process pressure raised more oxygen radicals and enhanced the oxygen binding ability of the Ga additives. Therefore, the oxygen vacancies were passivated thereby the carrier density was inhibited. Specifically, the carrier density markedly decreased from 8.4 × 10¹⁸/cm³ to 9.9 × 10¹⁶/cm³ as the process pressure increased. Meanwhile, the electron mobility only slightly decreased from 11.9 cm²/V·s to 8.0 cm²/V·s. Inverted staggered IGZO-TFTs were fabricated. The switch characteristic of the TFTs can be effectively modulated by the pressure during the preparation of IGZO channel.
Flexible Ga2O3 photodetectors have attracted considerable interest owing to their potential use in the development of implantable, foldable, and wearable optoelectronics. In particular, β-phase Ga2O3 has been most widely investigated due to the highest thermodynamic stability. However, high-quality β-phase Ga2O3 relies on the ultrahigh crystallization temperature (usually ≥750 °C), beyond the thermal tolerance of most flexible substrates. In this work, we epitaxially grow a high-quality metastable κ-phase Ga2O3 (002) thin film on a flexible mica (001) substrate under 680 °C and develop a flexible κ-Ga2O3 thin film photodetector with ultrahigh performance. Epitaxial κ-Ga2O3 and the mica substrate are maintained to be thermally stable up to 750 °C, suggesting their potential for harsh environment applications. The responsivity, on/off ratio, detectivity, and external quantum efficiency of the fabricated photodetector are 703 A/W, 1.66 × 107, 4.08 × 1014 Jones, and 3.49 × 105 %, respectively, for 250 nm incident light and a 20 V bias voltage. These values are record-high values reported to date for flexible Ga2O3 photodetectors. Furthermore, the flexible photodetector shows robust flexibility for bending radii of 1, 2, and 3 cm. More importantly, it shows strong mechanical stability against 10,000 bending test cycles. These results reveal the significance of high-quality κ-phase Ga2O3 grown heteroepitaxially on a flexible mica substrate, especially its potential for use in future flexible solar-blind detection systems.
Aluminum-doped zinc oxide (AZO) thin films are popular transparent electrodes for optoelectronics and photovoltaics. However, next generation flexible devices demand not only high conductivities but also low thicknesses in AZO electrodes to stay robust against deformations. The two requirements contradict each other due to the scattering of carriers at grain boundaries in polycrystalline AZO electrodes. Herein, this work develops ultrathin, highly conductive, single-crystalline, and mechanically robust AZO electrodes on flexible Hastelloy substrates. The epitaxial growth of AZO on polycrystalline Hastelloy is empowered by an advanced buffer architecture topped with a biaxially-textured MgO layer. Thereby, a c-axis epitaxial strain is generated in the AZO electrodes and then regulated by varying the thickness of the electrodes. Abnormally high carrier concentrations are detected in the AZO electrodes likely caused by the piezopotential generated by the epitaxial strain. While the highest resistivity value of the epitaxial AZO electrodes is within the mainstream values of AZO electrodes on various substrates, a resistivity as low as ∼22 μΩ•cm is demonstrated in the ultrathin electrode with a thickness of ∼28 nm. The mechanical durability of the ultrathin AZO electrode is validated by bending tests of 1000 cycles.
The Hf‐doped indium zinc tin oxide (Hf:InZnSnO) channel for high performance and stable transparent thin film transistors (TFTs) is developed by using a simultaneous cosputtering of InZnSnO and HfO2 targets. The effects of In and Hf composition in Hf:InZnSnO channel on the performance and stability under bias stress for the Hf:InZnSnO channel‐based TFTs are investigated. Herein, the In cations enhance the electrical properties, while the Hf cations reduce the oxygen vacancies in the Hf:InZnSnO channel layer. Adjusting the atomic ratio of In of the InZnSnO target improves the performance of the Hf:InZnSnO‐based TFTs, while introducing an adequate amount of HfO2 improves the bias stabilities and hysteresis characteristics of the Hf:InZnSnO‐based TFTs. The transparent TFT with optimized Hf:InZnSnO channel, with a stoichiometry of Hf0.27In25.96Zn6.99Sn6.16O60.62, that is cosputtered at RF power of 100 W applied to InZnSnO target (In:Zn:Sn = 4:1:1 at%), and RF power of 50 W applied to HfO2 target exhibits a field effect mobility of 11.84 cm² V⁻¹ s⁻¹ and low shift of threshold voltage of less than 2.5 V under bias stress for 3000 s. The high performance and stability of the Hf:InZnSnO channel‐based TFTs demonstrate the feasibility of transparent TFTs‐related next‐generation display applications.
The dynamic inverter using amorphous indium-gallium-zinc oxide thin-film transistors (a-IGZO TFTs) is revealed to be more robust to tensile strain than the static inverter that is most widely used in TFT circuits. The results with the inverters can be reasonably extended to NAND or NOR gates, because all of them are commonly composed of pull-up and pull-down networks. The experimental results after tensile bending up to 20 000 times with a bending radius of 1.5 mm show that ΔVOH and ΔVOL in the dynamic inverter decrease to 85% and 0% of those in the static inverter, respectively. Also, while the power consumption of the static inverter increases by 36%, which is tens of μA, the dynamic inverter maintains low-power consumption, which is tens of nA. It is also worth noting that ratioed design and TFT operation in the saturation region make the circuit more sensitive to tensile strain than ratio-less design and TFT operation in the triode region.
Interfacial adhesive strength is an important characteristic considered in multilayered active-matrix organic light-emitting diode (AMOLED) architecture as it has an impact on the mechanical reliability issue generated from complicated organic/inorganic hybrid lamination and final debonding procedure. Thus, an appropriate released layer design is needed to manage the capability and reliability of the encapsulated AMOLED structure de-bonded from the temporary carrier. Accordingly, this research utilizes the plasma-assisted surface modification to improve the interfacial adhesive strength of the polyimide (PI)/debonding layer (DBL) during mechanical debonding. In addition, a fracture-based finite element model of peeling testing architecture is constructed to analyze the adhesive behavior of the PI/DBL interface via the utilization of the modified virtual crack closure technique. The analytic results indicate that the suitable plasma-assisted surface modification significantly increased the nitrogen concentration and generated the high dissociation energy N–O bonds among the bonded interface between the PI/DBL thin films. The opening mode-fractured energy also takes the dominance proportion in the entire peeling load-induced interfacial energy release rate. The application of plasma-assisted surface modification enhanced the delamination resistance of the concerned interface and maintained the structural stability during the lamination and related release of a whole flexible AMOLED architecture.
Wearable Gallium oxide solar-blind photodetector fabricated on muscovite mica is reported for room temperature as well as high temperature operations. The ultra-high photoresponsivity of 9.7 A/W is obtained for 5V applied bias at room temperature under 75 µW/cm2 weak illumination of 270 nm wavelength. The detector enables very low noise equivalent power (NEP) of 9×10⁻¹³ W/Hz1/2 and ultra-high detectivity of 2×10¹² jones which shows the magnificent detection sensitivity. Further, bending tests are performed for robust utilization of flexible detectors up to 500 bending cycles with each bending radius of 5 mm. After 500 bending cycles, device shows slight photocurrent decrease. The bending performances exhibit excellent potential for wearable applications. Moreover, photocurrent and dark current characteristics above room temperature demonstrate the outstanding functionalities till 523K temperature which is remarkable for flexible photodetectors. The obtained results show the potential of Gallium oxide solar-blind photodetectors at room temperature and high temperatures environments which pave the ways for futuristic smart and flexible sensors.
