Yi-Yu Zhang

In this review, the primary focus is the recent advances in the development of freestanding inorganic crystalline semiconductors and their manipulation technology for flexible optoelectronic applications. We firstly cover the details of the growth and processing techniques of freestanding inorganic crystalline semiconductors in various dimensions and their material property under strain condition. Finally, fabrication processes and opto-electrical properties of flexible optoelectronics are introduced. Future research directions are also discussed, including further enhancement of device performance, building more types of optoelectronic devices on flexible substrates, and process integration for the advanced optoelectronic circuits and systems. GRAPHICAL ABSTRACT

Amorphous InGaZnO (a-IGZO) UV photodetectors with a metal-semiconductor-metal structure were fabricated on polyethylene terephthalate (PET) substrates, and the effects of surface fluorine plasma treatment on a-IGZO were investigated. With this treatment, the oxygen vacancies (O _vac 's) at the a-IGZO surface were effectively passivated, thus reducing the shallow-level donor states and electron traps. Accordingly, the direct and/or trap assisted tunnelings through the metal/a-IGZO Schottky barrier were suppressed, leading to a significant reduction in the dark current. On the other hand, the photo current was nearly unchanged if the treatment time was less than 20 min. It is due to the simultaneous reductions in hole trap and recombination center, both of which are related to the Ovac's at deep levels. As a result, a high detectivity of 4.36 × 10 ¹³ Jones was realized with an increase of nearly 10 times. Moreover, a highly uniform trap distribution was achieved, and it could be beneficial to the photoconductive reliability of the device.

In the past decade, amorphous InGaZnO thin film transistors (a‐IGZO TFTs) have become a very promising candidate for application in flat panel displays (FPDs). However, it is difficult to break through the mobility bottleneck of a‐IGZO TFTs to obtain mobilities higher than 100 cm² V⁻¹ s⁻¹, thus limiting their use in more advanced applications. Construction of a high‐electron mobility transistor (HEMT) based on a heterojunction structure could provide a solution for this problem. In this work, the band alignment of a‐IGZO and amorphous InGaMgO (a‐IGMO) heterojunction has been investigated using X‐ray photoelectron spectroscopy (XPS) and transmission spectra measurements. The valence band (ΔE_V) and conduction band offsets (ΔE_C) were determined as 0.09 and 0.83 eV, respectively. The ΔE_C was large enough to construct a potential well that could favor the appearance of a two‐dimensional electron gas (2DEG). Hence, the achievement of an HEMT based on a‐IGZO/a‐IGMO heterojunction can be expected. Moreover, band bending contributed greatly to such a large ΔE_C, and thus to the formation of electrical confinement structure. Our findings suggest that a‐IGZO/a‐IGMO heterojunction is a potential candidate for constructing a HEMT and thus breaking through the mobility bottleneck of a‐IGZO‐based TFTs for the applications in next‐generation electronic products.

Amorphous InGaZnO ₄ thin film transistors (a-IGZO TFTs) with HfLaO gate dielectrics have been widely demonstrated to possess extremely excellent electrical characteristics, and thus show great potential for applications in various next-generation electronic products. Nevertheless, the in-depth understanding of HfLaO/IGZO interfacial features is still lacking, which makes further device optimization lack clear guidance. In this work, the band alignment and interfacial chemical structure of a sputtering-prepared HfLaO/IGZO heterojunction was investigated through x-ray photoelectron spectroscopy. The valence and conduction band offsets (Δ E _v and Δ E _c ) at the interface were determined to be 0.57 eV and 1.48 eV, respectively. The relatively large Δ E _v is mainly attributed to the formation of the interfacial layer (IL) and thus the upward band bending from IGZO to the surface of HfLaO. Furthermore, it was found that the oxygen vacancies on the surface of IGZO were significantly suppressed upon the deposition of HfLaO, which not only explained the previously reported ultrahigh performance of a-IGZO/HfLaO TFTs to some extent, but also additionally validated the formation of the IL. Our findings have successfully revealed the importance of ILs in modifying the band alignment and interfacial trap states of HfLaO/IGZO heterojunctions, thus suggesting a potential route to further optimizing a-IGZO/HfLaO TFTs so as to satisfy the requirements of next-generation technologies.

Recently, amorphous InGaZnO ultraviolet photo thin-film transistors have exhibited great potential for application in future display technologies. Nevertheless, the transmittance of amorphous InGaZnO (~80%) is still not high enough, resulting in the relatively large sacrifice of aperture ratio for each sensor pixel. In this work, the ultraviolet photo thin-film transistor based on amorphous InGaMgO, which processes a larger bandgap and higher transmission compared to amorphous InGaZnO, was proposed and investigated. Furthermore, the effects of post-deposition annealing in oxygen on both the material and ultraviolet detection characteristics of amorphous InGaMgO were also comprehensively studied. It was found that oxygen post-deposition annealing can effectively reduce oxygen vacancies, leading to an optimized device performance, including lower dark current, higher sensitivity, and larger responsivity. We attributed it to the combined effect of the reduction in donor states and recombination centers, both of which are related to oxygen vacancies. As a result, the 240-min annealed device exhibited the lowest dark current of 1.7 × 10⁻¹⁰ A, the highest photosensitivity of 3.9 × 10⁶, and the largest responsivity of 1.5 × 10⁴ A/W. Therefore, our findings have revealed that amorphous InGaMgO photo thin-film transistors are a very promising alternative for UV detection, especially for application in touch-free interactive displays.

In this paper, the impact of O ₂ postdeposition annealing treatment on the material and UV detection characteristics of amorphous InGaMgO (IGMO) thin films was intensively studied. With the replacement of Zn by Mg, amorphous InGaMgO possesses an optical bandgap larger than that of the conventional amorphous InGaZnO by over 0.5 eV. Furthermore, it was found the post-deposition annealing in O ₂ effectively suppressed the oxygen vacancies in the amorphous IGMO thin film, resulting in a significant reduction in its dark current. Due to strong hole trapping, all the samples except the 480-min annealed one exhibited large photocurrent and accordingly high responsivity over 1 × 10 ³ A/W @ 10 V. Moreover, both photocurrent and responsivity improve with increasing annealing time up to 240 min. As a result, the Metal-Semiconductor-Metal photodetector based on the 240-min annealed amorphous InGaMgO thin film exhibited a very prominent performance, including a high responsivity of 1.6 × 10 ⁴ A/W and a large photo-to-dark current ratio of 1.3 × 10 ³ . We attribute such excellent properties to the improved carrier mobility as well as the reduction of recombination centers in the O ₂ -annealed film. However, some degradations in device performance were observed when the annealing time reached 480 min, which can be explained by the suppression of localized tail states, as demonstrated by the abruptly reduced Urbach energy, and accordingly the inhibition of related extrinsic excitation. This work has provided a promising candidate for the application in transparent contact-free interactive display.