Flexible and Stretchable Electronic Devices using Inorganic Semiconductors

Abstract

The demand for flexible and stretchable electronic devices are groundbreaking, owing to the rapid advancement in the industrial revolution, expecting to achieve wearable property in devices, Internet of Things (IOT) era, in which technologies matures rapidly from lab-based workflows to industries. Recently, many researchers utilized organic or self-healing elastomeric semiconductors to achieve flexible and stretchable properties in the electronic devices but the stability is a key issue in organic electronic devices. Undoubtedly, inorganic materials are preferred in industries due to their excellent stability over time and the superior electrical properties owing to their excellent charge transport characteristics, but they are not flexible and stretchable. To solve the issues of inorganic semiconductors, in commercializing the stretchable and flexible electronic devices, a novel approach that relies on unusual device construction and patterning of semiconductor layer, which can offer stretchable and flexible property in inorganic semiconductor-based electronic devices, is proposed. This thesis begins with the overview of the electronic devices in the chapter 1, in which the basic working principle of different electronic devices, such as thin film transistors (TFTs), hydrogen gas sensors and photodetectors, their characterizations techniques, such as current-voltage or current-time measurement (graph), and the extraction of the electrical parameters from their respective graphs are described. As a preliminary work, we demonstrated the fabrication of In-Ga-ZnO (IGZO) based TFTs and gas sensors on a rigid substrate in chapter 2 and 3, respectively and optimized the channel thickness, annealing temperature and IGZO deposition conditions. The carrier concentration within the channel was controlled by adjusting those parameters, which in turn control the electrical properties. We found that the transistor had the optimized transistor performance with a mobility of 10.8 cm2/Vs at the channel carrier concentration of 3ⅹ1017 cm-3. In case of H2-sensors, the high operating temperature in semiconducting metal oxides (SMOs), is the considerable drawback, which should be overcome. To address these issues, we utilized Pd-decorated IGZO as a sensing channel on rigid insulator substrate, and the Pd decoration greatly enhanced the sensing performance of IGZO at room temperature with a high sensing response of 6.1 × 106 % at 5 % H2. We employed two different strategies to achieve ultra-flexible property in the IGZO-based electronic devices. In the first strategy, chapter 4, the non-elastic inorganic stacking has been changed into organic/organic or organic/inorganic stacking to accommodate the strain applied to the thin metal oxide layer. Accordingly, we demonstrated the fabrication of flexible bottom-gated IGZO TFTs on a 100 μm-thick polyimide (PI) substrate by replacing the conventional inorganic dielectric layer with thermally stable organic dielectric layer and the device was able to sustain an unprecedented bending strain of 3.33% (1.5 mm bending radius). The TFTs showed no degradation in the device performances after severe bending, even at a 1.5 mm bending radius, and maintained a superior reliability up to 1000 cycles. The typical electronic devices have an active device thickness of less than 1 μm; therefore, overall flexibility of the device is determined by the substrate thickness. Accordingly, in second strategy, chapter 5, a novel approach for fabricating ultraflexible IGZO-based electronic devices on ultrathin PMMA substrate is proposed and their conformal contact onto non-planar surfaces is also explored. The devices, such as TFTs and H2-sensors, exhibit unprecedented flexibility down to a bending radius of less than 1 mm without any performance degradation and the free-standing H2-sensors have a potential to be conformably attached to the fittings of containers or pipe lines to detect the leakage in time. These days, there has been an increased demand in the electronic devices, capable not only of bending but also of stretching, which can establish intimate, conformal contacts to complex curvilinear surfaces, applicable in vast areas such as wearable electronics, paper-like rollable displays, and medical diagnostic systems. In chapter 6, a novel patterning technique which offers a stretchable property in electronic devices using hexagonally patterned gold electrode is demonstrated. The patterned gold electrode on elastomeric substrate, endured 100 % strain without any performance degradation, has the potential to be utilized in future wearable electronics.