We report on the In-Ga-Zn-O thin-film transistors (IGZO TFTs) with outstanding mechanical stretchability, which were fabricated on ultrathin polyimide (PI) film/prestrained elastomer with a wavy-dimensional structure. The device characteristics of the fabricated devices were evaluated under mechanically strained conditions with various strains. The operational reliabilities against the bias stress conditions and during the cyclic stretching tests were also carefully examined. The stretchable IGZO TFTs exhibited good device operations without any marked degradation under stretching/compressed conditions with a strain of 40%. Under positive bias stress with a prestrain of 50%, the turn-on voltage instabilities for the TFTs prepared on 0.9 and 2.0 μm-thick PI films were estimated to be 1.5 and 3.9 V, respectively. During the cyclic stretching tests with a strain of 50%, the device operations failed after 20,000 and 100,000 stretching cycles for the TFTs fabricated on 2.0 and 0.9 μm-thick PI films, respectively. As a result, the IGZO TFTs fabricated on a thinner PI film presented more reliable operations after the repeated stretching events. The robust mechanical stretchability dependent on the PI film thickness was suggested to be due to the difference in critical values of bending radii and the influence of the local strain induced by the spatial fluctuations of the wavy structures.

We investigate the effects of atomic layer deposition (ALD)-grown Al2O3 buffer layer on the device characteristics of flexible amorphous InGaZnO thin-film transistors (TFTs) fabricated on ultrathin polyimide (PI) films. The TFT with a buffer layer exhibited a saturation mobility of 8.6 cm2 V−1 s−1 and a subthreshold swing of 0.16 V dec−1 after annealing at 150 °C. Under negative bias temperature stress at 40 °C, the turn-on voltage instabilities of TFTs with and without the buffer layer were estimated to be −1.0 and −13.2 V, respectively. This marked difference is mainly due to the adsorption of water molecules on the PI film resulting in a positively charged surface.

Charge-trap memory thin-film transistors (CTM-TFTs) were fabricated with wavy-dimensional structures, and their device performances were demonstrated under mechanically stretching conditions. The fabricated CTM-TFTs obtained a wide memory window of 23.8 V, a steep subthreshold swing of 0.31 V/dec, and a large memory margin (106) with program pulses as short as 1 μs. Furthermore, the total variations in device parameters could be suppressed within a range of <12% even under stretching conditions with a prestrain as large as 60%. The mechanical durability during the cyclic stretching test with a strain of 50% was significantly improved from 2000 to 30000 cycles when the PI film thickness was reduced from 5.0 to 1.2 μm. The effects of PI thickness on the mechanical stability of the CTM-TFTs were quantitatively discussed from a viewpoint of surface strain. The difference in critical radius of curvature and the influence of the local strain induced by the spatial fluctuations of the wavy structure were suggested to be critical issues, and hence, the neutral mechanical plane was introduced for further improvement. As a result, the stretchable CTM-TFTs exhibited stable operations until 150000 stretching cycles by the contributions of an ultrathin substrate and a neutral mechanical plane.

Mechanically flexible In−Ga−Zn-O (IGZO) thin-film transistors (TFTs) were fabricated and characterized on poly(ethylene naphthalate) (PEN) with various film thicknesses. The physical origins of mechanical and electrical degradations in device characteristics were extensively and systematically investigated under various mechanical strain conditions to improve the device operational reliabilities. To investigate the mechanical durability of the IGZO TFTs, the effects of PEN thickness on the variations in critical radius of bending curvature were analyzed, which were estimated to be 3, 7, and 14 mm, when the PEN thickness was 25, 50, and 125 μm, respectively. During the cyclic bending tests for the TFTs fabricated on 25 μm thick PEN, the operation cycles were estimated to be markedly reduced from 10 000 to 150 when the applied strain increased from 0.3 to 0.45% (critical strain), respectively. It was clearly suggested that the main origin for the device degradation under bending deformation could be altered from electrical to mechanical ones in the vicinity of the critical strain determined by critical radius of curvature. In other words, the evolution of microcracks and the variations in the density of states within the IGZO channel were found to have critical impacts on the device characteristics of the flexible IGZO TFTs above and below the critical strain, respectively. Consequently, the systematic analysis and the obtained results provided design guidelines and insights for implementing more mechanically robust devices in flexible electronics.

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We introduce mechanically flexible nonvolatile memory thin-film transistors (MTFTs) for future highly functional flexible and stretchable electronic devices and systems. The proposed memory devices are uniquely composed of oxide-semiconductor thin films and operated with charge-trap and detrap mechanisms for nonvolatile memory operations. Charge-trap-assisted MTFTs (CT-MTFTs) using In-Ga-Zn-O active and ZnO charge-trap layers are designed and fabricated on flexible poly(ethylene naphthalate) and colorless polyimide films prepared on carrier glass substrates. We describe the fabrication processes and evaluation methodologies for realizing flexible CT-MTFTs in a detailed way and discuss related technical issues. The device characteristics and memory performance of the fabricated flexible CT-MTFTs, including when the devices are mechanically strained, are extensively discussed. We also propose further improvements for remaining issues on device performance from the viewpoints of memory operations and mechanical flexibility. © 2019 The Japan Society of Applied Physics PI (CPI) film substrates in Chap. 3. Furthermore, the memory operation mechanisms based on the device physics including the material properties and considerations on the designed device structures will be examined. Finally, in Chap. 4,

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