(Invited) Oxide Electronics and Recent Progress in Bipolar Applications

In recent years, oxide electronics has emerged as one of the most promising new technologies for a variety of electrical and optoelectronic applications, including next-generation displays, solar cells, batteries, and photodetectors. Oxide electronics have a lot of potential because of their high carrier mobilities and ability to be manufactured at low temperatures. However, the preponderance of oxide semiconductors is n-type oxides, limiting present applications to unipolar devices and stifling the development of oxide-based bipolar devices like p-n diodes and complementary metal-oxide—semiconductors. We have contributed to oxide electronics, particularly on transition metal oxide semiconductors of which the cations include In, Zn, Sn and Ga. We have integrated these oxide semiconductors into thin film transistors (TFTs) as active channel layer in light of the unique combination of electronic and optical properties such as high carrier mobility (5-10 cm“/Vs), optical transparency in the visible regime (>~90%) and mild thermal budget processing (200-400°C). In this study, we achieved four different results. The first result is that unlike several previous reports on oxide p-n junctions fabricated exploiting a thin film epitaxial growth technique (known as molecular beam epitaxy, MBE) or a high-powered laser beam process (known as pulsed laser deposition, PLD) that requires ultra-high vacuum conditions, a large amount of power, and is limited for large-area processing, we demonstrate oxide-based heterojunction p-n diodes that consist of sputter-synthesized p-SnOx and n-IGZO of which the manufacturing routes are in-line with current manufacturing requirements. The second result is that the synthesized p-SnOx films are devoid of metallic Sn phases (i.e., sn? state) with carrier density tuneability and high carrier mobility (> 2 cm2/Vs). The third result is that the charge blocking performance of the metallurgical junction is significantly enhanced by the engineering of trap/defect density of n-IGZO, which is identified using photoelectron microscopy and valence band measurements. The last result is that the resulting oxide-based p-n heterojunction exhibits a high rectification ratio greater than 10° at +3 V (highest among the sputter-processed oxide junctions), a low saturation current of ~2x10 © A, and a small turnon voltage of ~0.5 V. The outcomes of the current study are expected to contribute to the development of p-type oxides and their industrial device applications such as p-n diodes of which the manufacturing routes are in-line with the current processing requirements. Reference: 1. Lee, S.; Paine, D. C., Metallization selection and the performance of amorphous In-Zn-O thin film transistors. Applied Physics Letters 2014, 104 (25), 252103. 2. Liu, M.; Kim, H.; Wang, X.; Song, H. W.; No, K.; Lee, S., Carrier Density-Tunable Work Function Buffer at the Channel/Metallization Interface for Amorphous Oxide Thin-Film Transistors. ACS Applied Electronic Materials 2021, 3 (6), 2703-2711. 3. Chu, X., Zhuang, S., Chi, C., Xu, H., Du, G., Dong, X., & Yin, J. (2017). Study on the electroluminescence properties of the P-nimgo:Li/mgo/n-zno nanowires/ito heterojunction. Journal of Luminescence, 187, 486-491. 4. Kawazoe, H.; Yanagi, H.; Ueda, K.; Hosono, H., Transparent p-Type Conducting Oxides: Design and Fabrication of p-n Heterojunctions. MRS Bull. 2000, 25 (08), 28-36. 5. Mo, X., Fang, G., Long, H., Li, S., Wang, H., Chen, Z., Huang, H., Zeng, W., Zhang, Y., & Pan, C. (2014). Unusual electroluminescence from n-ZnO@i-MgO core-shell nanowire color-tunable light-emitting diode at reverse bias. Phys. Chem. Chem. Phys., 16(20), 9302-9308. 6. Nomura, K.; Ohta, H.; Takagi, A.; Kamiya, T.; Hirano, M.; Hosono, H., Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors. Nature 2004, 432 (7016), 488-492.

Donor-acceptor conjugated polymers have been widely investigated in recent years. By optimization of donor and acceptor structures, the conjugated copolymers can exhibit broad absorption from visible region to near-infrared range for electronic and optoelectronic applications, such as light-emitting diodes, photovoltaic cells, and thin film transistor. However, the electronic and optoelectronic properties of thienopyrazine-based conjugated polymers have not been fully explored yet. The goal of this thesis is to investigate the effects of donor-acceptor or donor-acceptor-donor structures on the electronic properties of thienopyrazine based conjugated polymers. In chapter 2, the optical, electrochemical, and field effect charge transport properties of the new thienopyrazine-based alternating donor-acceptor conjugated copolymers were explored. The new copolymers, Poly[5-(2,5-bis(decyloxy)phenyl)-2,3- bis(4-(2-ethylhexyloxy)phenyl)thieno[3,4-b]pyrazine] (PDPTP), Poly[5-(9,9-dioctyl- 9H-fluoren-2-yl)-2,3-bis(4-(2-ethylhexyloxy)phenyl)thieno[3,4-b]pyrazine] (PFPTP), Poly[5-(9-(2-ethylhexyl)-9H-carbazol-3-yl)-2,3-bis(4-(2-ethylhexyloxy)phenyl)thieno[3,4-b]pyrazine] (PCPTP) and Poly[2,3-bis(4-(2-ethylhexyloxy)phenyl)-5-(thiophen-2-yl) thieno[3,4-b]pyrazine] (PTPTP) had excellent solubility and broad optical absorption bands with absorption maxima at 540-950nm in thin film. The PTPTP exhibits the smallest band gap (0.98eV), indicating a stronger intramolecular charge transfer. The four polymers show the low ionization potentials (4.57-4.99 eV) and high electron affinity (3.26-3.49eV). The hole mobilities of PDPTP, PCPTP, and PTPTP are 2.62×10-6, 2.74×10-5, and 2.00×10-4 cm2V-1s-1. PTPTP has the highest hole mobility due to low band gap and strong intramolecular charge transfer.The AFM topographic images of the copolymers show smooth and amorphous phases. From these results, intramolecular charge transfer might be main impact on hole mobility. In chapter 3, thienopyrazine-based donor-acceptor-donor alternating conjugated copolymers were synthesized. The copolymers including Poly[5-(5-(2,5-bis(decyloxy)-4-methylphenyl)thiophen-2-yl)-2,3-bis(4-(2-ethylhexyloxy)phenyl)-7-(5-methylthiophen-2-yl)thieno[3,4-b]pyrazine] (PDDTTP), Poly[5-(5-(9,9- dioctyl-9H-fluoren-2-yl)thiophen-2-yl)-2,3-bis(4-(2-ethylhexyloxy)phenyl)-7-(thiophen-2-yl)thieno[3,4-b]pyrazine] (PFDTTP), and Poly[2,3-bis(4-(2-ethylhexyloxy)phenyl)-5 ,7-di(thiophen-2-yl)thieno[3,4-b]pyrazine] (PDTTP) exhibit broad optical absorption bands (662-816nm) and small optical band gaps (1.15-1.57eV). All polymers exhibit reversible oxidation and reduction and low ionization potential (4.62-4.98 eV). The hole mobility of PDDTTP, PFDTTP, and PDTTP are 7.24×10-4, 1.61×10-3 and 1.16×10-3cm2V-1s-1. The hole mobility of PFDTTP is the highest probably due to the relatively smooth and amorphous phase in thin film. According to these results, thienopyrazine-based donor-acceptor-donor copolymers combine small band gap and high carrier mobility. Such polymers may have potential optoelectronic device applications, such as thin film transistor and photovoltaic cells.

This study aims to examine the studies regarding In doped ZnO published in the Web of Science database. A total of 777 articles were reached (31 March 2020). The articles were downloaded for the bibliometric analysis and collected in a file. The file was uploaded to VOSViewer programme in order to reveal the most used keywords, words in the abstracts, citation analyses, co-citation and co-authorship and countries analyses of the articles. The results showed that the most used keywords were “ZnO”, “photoluminescence”, “optical properties”, “thin films” and “doping”. These results indicate that the articles mostly focus on some characteristics of In doped ZnO thin films such as structural, optical and electrical features. When the distribution of the number of articles using the keywords by year was searched, it was found that recent articles focus mainly on synthesis of In doped ZnO film via chemical routes such as sol-gel and hydrothermal syntheses, and on ZnO-based device applications such as solar cells and gas sensors. The most used keywords were also found to be films, X-ray, glass substrate, X-ray Diffraction (XRD), spectra and layer. These results indicate that the studies mostly focus on In doped ZnO thin films as transparent conductive oxide (TCO) material used in device applications like solar cells. In this context, it was found that structural, topographical, optical, electrical and magnetic properties of In doped ZnO films were characterized in terms of defected structure or defect type, substrate temperature, film thickness and In doping content. When the distribution of these words is shown on a year-by-year basis, it is evident that more recent articles tend to focus both on efficiency and performance of In doped ZnO films as TCO in solar cells, diodes and photoluminescence applications both on nanostructures, such as nanoparticles, and nanorods for gas sensor applications. The results also indicated that Maldonado and Asomoza were the most cited authors in this field. In addition, Major, Minami and Ozgur were the most cited (co-citation) authors in this field. The most cited journals were found to be Thin Solid Films, Journal of Materials Science Materials in Electronics and Journal of Applied Physics and, more recently, Energy, Ceramics International, Applied Physics-A, Optik, Material Research Express, ACS Applied Materials and Interfaces and Optical Materials. The most co-cited journals were Applied Physics Letters, Thin Solid Films, Journal of Applied Physics, Physical Review B, and Applied Surface Science. Lastly, the countries with the highest number of documents were China, India, South Korea, USA and Japan. Consequently, it is suggested that future research needs to focus more on synthesis and characterization with different growth techniques which make In doped ZnO suitable for device applications, such as solar cells and diodes. In this context, this study may provide valuable information to researchers for future studies on the topic.

Despite the outstanding achievements in multiple areas such as displays and energy, oxide electronics has been limited to single-polar (n-type) applications due to the facile generation of oxygen vacancies as native donors. On the contrary, the processing of p-type oxides is restrained due to the high formation energy of native acceptors. Furthermore, the oxygen 2p orbitals of the majority of oxide semiconductors are anisotropic and localized to the valence band maximum (VBM), resulting in a large effective mass of holes and hence low carrier mobility.Hybrid orbital electronic configurations with cation d10 (closed shell structure) and cation s2 (pseudo-closed structure) have been suggested initially in complex oxides (e.g., CuMO2 where M= Al, Ga, and In; and SrCu2O2) to delocalize the oxygen 2p orbitals from the VBM. However, these complex oxides require high temperatures to process and are difficult to engineer the electrical properties of carrier density and carrier mobility due to the correlated nature of multi-cation species. Several single-cation p-type oxides such as PbO, Bi2O3, and SnO have emerged as well, where the energy level of a unique s-orbital of cations is similar to oxygen 2p orbitals, forming strong hybrid structures. In addition, a simpler single-cation structure leads to easier control of electrical properties required in practical device applications such as thin film transistors (TFT) and complementary logic inverters.We previously reported scalable processing of p-type SnOx through thermodynamic-based interfacial reactions as well as reactive sputtering.1 More recently, we also suggested multi-modal encapsulation to enhance TFT on- and off-state behaviors and identified a defect complex as an effective p-type doping agent.2 However, challenges remain since the TFT off-state current is large, and the defect/trap state density is high.In this presentation, we share our approaches to engineer the off-state current and passivate the defect/trap states. In addition to channel thickness optimization, intrinsic (Sn vacancy or oxygen interstitial) and extrinsic (H-related species) doping strategies to adjust channel carrier density will be compared. The performance of several surface treatments (oxygen plasma and UV) and TFT back channel encapsulations (SiO2 and Al2O3) will be systematically compared. Then, the device performance of optimized p-type SnO TFTs and complementary inverters with n-type InZnO TFTs will be discussed.References Lee et al., ACS Applied Materials & Interfaces, 13 (46), 55676–55686 (2021)Lee et al., ACS Applied Materials & Interfaces, 14 (48), 53999–54011 (2022) AcknowledgmentsThis work was partially supported by National Science Foundation, Award number ECCS-1931088 and CBET-2207302.

The rapid development of the semiconductor industry calls for the exploration of novel semiconductors to cater to modern technical and commercial needs. Recently, black phosphorus (BP) has emerged as a new class of 2D semiconducting material and has attracted intensive research attention. The high carrier mobility and tunable direct bandgap of BP deliver great promise in photonic and optoelectronic device applications. Furthermore, the unique intrinsic anisotropy arising from the puckered structure can be exploited in the design of new devices. This review briefly introduces the history of BP and puts emphasis on recent advances pertaining to its optical properties and applications in the photonic and optoelectronic fields. From the perspective of mass production and practical use of BP, some of the research challenges and opportunities are discussed.

The InGaZnO thin films with varied the O2 flow ratios were deposited by a magnetron sputtering system. The effect of O2 content on the structure, surface morphology, optical and electrical properties of InGaZnO thin films is studied. Then, we investigate the relationship between bias stress stability and density of states (DOS) of thin film transistors (TFTs) with various oxygen partial pressure InGaZnO (IGZO) as the channel layer. The temperature-dependent field-effect measurements are used to calculate DOS of IGZO TFTs. With the increase of the oxygen partial pressure, the value of DOS decreases from 2.2 × 1016 to 5.5 × 1015 cm-3. Moreover, the positive gate bias stress of IGZO TFTs are investigated. Under positive gate bias stress, the transfer curves of IGZO TFTs with various oxygen partial pressure show an obvious shift, the value of threshold voltage shift () is 5.1, 2.94, 2 and 0.8 V, respectively. The results suggest that the device with a smaller DOS is more stable than others. Therefore, this study offers a brief and accurate method to demonstrate instability mechanism of IGZO TFTs.

Semiconducting polymers have been paid great attention because their intensive application of light-emitting diodes, thin film transistors, and photovoltaic devices. The conjugated polymer is a class of semiconducting polymers under intensive research, because it owns tunable optoelectronic properties and good processibility, leading to great potential of device application. The main objective of this thesis is to synthesize a series of conjugated polymers composed of different electron donors and acceptor (mainly quinoxaline) for the fabrication and measurement of field effect transistors and photovoltaic cells. In the first part of this thesis (Chapter 2), five side-chain modified quinoxaline-containing conjugated copolymers with donor-acceptor or donor-acceptor -donor structures were synthesized and characterized, including poly[2,5-didecyloxy- 1,4-phenylene-alt-2,3–bis(4-(2-ethylhexyloxy)phenyl)-5,8–dithien-2–yl-quinoxaline] (POC10DTQ(EHP)),poly[2,5-didecyloxy-1,4-phenylene-alt-2,3-bis(4-(2-ethylhexyl oxy)phenyl)-5,8–dithien-2–yl-quinoxaline] (PFODTQ(EHP)), poly[thiophene-2,5-diyl-alt-2,3-bis(4-(2-ethylhexyloxy)phenyl)-5,8-quinoxaline] (PThQx(EHP)), poly[thiophene-2,5-diyl-alt-2,3-bis(4-(2-ethyl hexyloxy)phenyl)-5,8-dithien-2-yl-quinoxaline] (PThDTQ(EHP)), and poly[2,3bis(4-(2-ethylhexyloxy)phenyl)-5,8-dithien-2-yl- quinoxaline] (PDTQ(EHP)). The hybridization of the high-lying HOMO energy level of the donor and low-lying LUMO energy level of the acceptor could produce a relatively small band gap polymer. The synthesized polymers (except PThDTQ(EHP)) are soluble in common organic solvents. Their band gaps are in the range of 1.57 ~ 1.92 eV and 1.93 ~ 2.14 eV measured by optical spectra and cyclic voltammetry, respectively. PThQx(EHP) has the smallest band gap because it is composed of the donor (thiophene) with higher electron-donating ability with the balanced percentage of the acceptor among the analogous polymers. These copolymers were fabricated into thin film transistors by their chlorobenzene solutions and measured their hole mobility in the range of 4.34×10-3 ~ 4.70×10-5 cm2/(Vs) with on/off ratios in the order of 103 ~104. The smooth and homogeneous surface of PDTQ(EHP) film may be a main factor for the highest charge carrier mobility. Furthermore, the solar cell power conversion efficiencies based on polymers PFODTQ(EHP), POC10DTQ(EHP) and PThQx(EHP) achieved 1.76%, 0.64% and 0.92%, respectively. In the second part of this thesis (Chapter 3), five different carbazole-based copolymers were synthesized by palladium(0)-catalyzed Suzuki coupling reaction, including poly[3,6-(2-ethylhexyl)carbazole-alt-2,1,3-benzothiadiazole] (PCzBTD), poly [3,6-(2-ethylhexyl)carbazole-alt-2,3-dimethyl -5,8-quinoxaline] (PCzDQ), poly[3,6-(2-ethylhexyl)carbazole -alt-2,3-dimethyl-5,8–dithien-2–yl-quinoxaline] (PCzDTQ), poly[3,6-(2-ethylhexyl)carbazole-alt-2,3-bis(4-(2-ethylhexyloxy)phenyl)-5,8-quinoxaline] (PCzQx(EHP)) and poly[3,6-(2-ethylhexyl)carbazole-alt-2,3-bis (4-(2-ethylhexyloxy) phenyl)-5,8-dithien-2-yl-quinoxaline] (PCzDTQ(EHP)). The polymers containing quinoxaline with long alkoxy-phenylene side chains have better solubility, resulting larger molecular weights and meanwhile soluble in common organic solvents. Their optical and electrochemical band gaps were measured in the range of 1.91 ~ 2.37 eV and 1.63 ~ 2.34 eV, and the effective intramolecular charge transfer should account for the smallest band gap of PCzDTQ(EHP). Furthermore, by the red shifts of UV-visible spectra, we found that the alkoxy-phenylene side groups of quinoxaline both improve the solubility and enhance the effective conjugation length. In sum, the present study suggests the importance of donor/acceptor strength and backbone structure on the electronic and optoelectronic properties of conjugated polymers, and also demonstrated the potential on optoelectronic device application of these polymers.

Oxide semiconductors have been investigated as channel layers for thin film transistors (TFTs) which enable next-generation devices such as high-resolution liquid crystal displays (LCDs), organic light emitting diode (OLED) displays, flexible electronics, and innovative devices. Here, high-performance and stable Ga-Sn-O (GTO) TFTs were demonstrated for the first time without the use of rare metals such as In. The GTO thin films were deposited using radiofrequency (RF) magnetron sputtering. A high field effect mobility of 25.6 cm2/Vs was achieved, because the orbital structure of Sn was similar to that of In. The stability of the GTO TFTs was examined under bias, temperature, and light illumination conditions. The electrical behaviour of the GTO TFTs was more stable than that of In-Ga-Zn-O (IGZO) TFTs, which was attributed to the elimination of weak Zn-O bonds.

In bulk form the thermoelectric material bismuth telluride (Bi2Te3) has a high figure of merit (ZT) at room temperature. When bulk Bi2Te3 is exfoliated to smaller dimensions, such as its 2D form (nanosheets), its thermoelectric effects are improved by reduction in thermal conductivity due to phonon confinement and scattering, thus increasing the ZT. The difficulty involved in incorporating nanosheets into energy harvesting systems motivated the search for a form of Bi2Te3 that retains the high ZT apparent at the nanoscale with the capability of being deposited or painted onto device infrastructures.This work highlights a method whereby solvent exfoliation of Bi2Te3 into solution-dispersible 2D nanosheets can form a practical thin film that can be distributed across a surface. Optical transmission measurements quantify the relationship between efficient and stable exfoliation and the reduction in optical density. Optimized exfoliated suspension are also shown to form smooth, uniform blends when mixed with poly ethylene glycol and other polymers to produce a paintable Bi2Te3film that can be applied to surfaces using an innovative painting technique.Atomic force microscopy, transmission electron spectroscopy, Raman spectroscopy and scanning electron spectroscopy are used to examine the structure of the 2D nanosheets and the highly reproducible Bi2Te3 thin films. Electrical transport studies show that the films have conductive pathways over a range of surfaces and various structural formations, linking the conductivity to the percolating conduction through the nanosheet ensemble. The combination of the facile preparation method and the scope for diverse surface coating as a cohesive and conductive thin film offers a methods for integration with heat producing devices for energy harvesting applications.ReferencesK. M. F. Shahil, M. Z. Hossain, D. Teweldebrhan and A. A. Balandin, Applied Physics Letters, 96 (2010).B. Poudel, Q. Hao, Y. Ma, Y. Lan, A. Minnich, B. Yu, X. Yan, D. Wang, A. Muto, D. Vashaee, X. Chen, J. Liu, M. S. Dresselhaus, G. Chen and Z. Ren, Science, 320, 634 (2008).M. Saleemi, M. S. Toprak, S. Li, M. Johnsson and M. Muhammed, Journal of Materials Chemistry, 22, 725 (2012).L. E. Bell, Science, 321, 1457 (2008).S. Li, M. S. Toprak, H. M. A. Soliman, J. Zhou, M. Muhammed, D. Platzek and E. Müller, Chemistry of Materials, 18, 3627 (2006).Y. Hernandez, N. Valeria, L. Mustafa, B. F. M., S. Zhenyu, S. De, M. T., B. Holland, M. Byrne, Y. K. Gun'Ko, J. J. Boland, P. Niraj, G. Duesberg, S. Krishnamurthy, R. Goodhue, J. Hutchison, V. Scardaci, A. C. Ferrari and J. N. Coleman, Nature Nanotechnology, 3, 1748 (2008).V. Nicolosi, M. Chhowalla, M. G. Kanatzidis, M. S. Strano and J. N. Coleman, Science, 340 (2013).L. D. Hicks and M. S. Dresselhaus, Physical Review B, 47, 12727 (1993).

Owning to their superior electronic and optical properties, silicon nanomembranes (NMs) have attracted considerable attention to be exploited as fundamental building blocks for the applications in electronics, photonics, and optoelectronics. Nevertheless, small photon traveling distance in such ultra‐thin silicon (UT‐Si) nanomembranes with nanoscale thicknesses (50 nm) induces low total light absorption, which is crucial for optoelectronic applications. Here, a convenient and controllable strategy, involving self‐assembly of dielectric polystyrene (PS) microsphere array on UT‐Si, was proposed to enhance optoelectronic responses of UT‐Si‐based photodetectors in broadband. Scattering effect of PS microspheres facilitates to couple incident light into UT‐Si layer, thus enhancing the light absorption. UT‐Si‐based metal‐semiconductor‐metal (MSM) photodetectors with a PS microspehere array demonstrate significant enhancement in optoelectronic response compared with the photodetector without PS microspeheres. Furthermore, the response spectrum can be controllably tuned by adjusting the size of paved microspeheres. This research may provide a practical and cost‐efficient approach for enhancing the optoelectronic responses of nanomaterials with nanoscale thicknesses, thus expediting their potential applications in optoelectronic devices.

Gallium oxide is an exceptional ultra-wide bandgap material. Its most stable phase, β-Ga2O3, has been extensively studied and recently attracted much attention for various applications in electronics and optoelectronics. Recent research interest has also arisen in the study of two-dimensional β-Ga2O3, which might promise superior electrical, optical, thermal, and mechanical properties due to quantum confinement, high surface to volume ratio, and ease of integration with other materials. This work reviews and discusses facile, versatile, and economic methods such as exfoliation, spray pyrolysis, and oxide printing to produce few-nanometer thin β-Ga2O3 films. Optical properties and applications in optoelectronic devices are particularly highlighted, along with possible directions for future research.

This paper investigates the effects of different passivation layers (PVLs) on the electrical performance and reliability of amorphous indium gallium zinc oxide (a-In-Ga-ZnO) thin film transistors (TFTs). By rational design, the fabricated a-InGaZnO TFT with hafnium oxide and aluminum oxide (HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) dual PVLs exhibits a field-effect mobility of 13.5 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /Vs, low sub threshold swing of 0.32 V/decade, and especially, small threshold voltage shifts of 0.5 (-0.6) V and 1.1 (-1.2) V under positive (negative) gate bias, and light illumination stress at the relative humidity of 40%. Furthermore, the a-In-Ga-ZnO TFTs with HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> dual PVLs maintain reasonable mobility and electrical performance even exposure to ambient condition for up to four months. This enhanced stability is attributed to the presence of high-quality HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> dual PVLs, which not only could suppress the photodesorption, reduce the total trap density and subgap photoexcitation behavior, but also protect the channel from environmental effects. Thus, the rational-designed HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> dual PVLs passivated a-In-Ga-ZnO TFTs with superior reliability represent a great step toward the achievement of long-term reliable zinc oxide-based oxide TFTs.

In this study, we report a novel vertical diffusion method for fabricating solution-processed quaternary In-Ga-Zn-O (IGZO) (defined as vd-IGZO) thin-film transistors (TFTs). Compared with conventional IGZO (defined as con-IGZO) TFT annealed at 380°C, vd-IGZO thin film is to spin coat Ga2O3, ZnO and In2O3 binary oxide precursors consecutively and then anneal at 250°C. Finally, the IGZO film with a vertically gradient Ga, Zn, In diffusion is obtained and used as the channel layer of TFTs. The results showed that the novel method lead to an improvement in charge carrier mobility and an improvement in the on-off current ratio of IGZO TFTs along with an improved stability. Compared with those for con-IGZO TFTs, the mobility for vd-IGZO TFT reaches 2.81cm2 V−1 s−1 from 0.88cm2 V−1 s−1, the threshold voltage(Vth) changes from 3.7V to 0.3V, the sub-threshold swing (S.S) decreases from 0.51V/dec to 0.35V/dec, and the on-current/off-current (Ion/Ioff) increases from 2.1 × 106 to 2.3 × 107. Besides, the positive bias stress (PBS), negative bias stress (NBS) and environmental stability are all improved to some extent. Hence this novel vertical diffusion method shows great potential to be applicable in the fabrication of flexible TFT.

p-type conductivity and the modulation of bandgap of ZnO are crucial aspects for realization of optoelectronic devices’ applications. The Li and Li-Cu could be suitable doping agents for achieving the p-type conductivity and the modulation of bandgap of ZnO. To this point of view, the Zn[Formula: see text]Li[Formula: see text]O (x = 0 to 40 at.%) and Zn[Formula: see text]Li[Formula: see text]Cu[Formula: see text]O (fixed, x = 5 at.%, and y = 0.0 to 10 at.%) thin films were prepared on the microscopic glass substrates at a temperature of 350[Formula: see text]C using cost effective chemical spray pyrolysis (CSP) technique. Field emission scanning electron microscope images show the coexistence of interconnected fibrous and flat grains on the films surface. The grain size changes as function of Li- and Li-Cu concentrations, and at a higher doping granular grains are observed. The successful incorporation of Li and Cu-Li into ZnO crystal is confirmed by X-ray photoelectron spectroscopy (XPS) measurements. The X-ray diffraction (XRD) patterns exhibit hexagonal polycrystalline structure of doped ZnO. However, the crystallinity is deteriorated at higher Li- and Li-Cu doping concentrations. The optical bandgap study exhibits direct transition type and it is red shifted from 3.21 to 2.61 eV and 2.84 to 3.56 eV for Li and Li-Cu doping in ZnO thin films, respectively. The optical conductivity enhances as a result of Li- and Li-Cu doping in ZnO. Therefore, Li- and Li-Cu can effectively be doped to tune bandgap and enhance optical properties of ZnO for electronic and optoelectronic device applications.

Two dimensional (2D) semiconductors combine the advantages of both oxide and organic semiconductor world, namely, high carrier mobility, environmental stability, as well as room temperature processability, flexibility and the availability of both high carrier mobility n- and p-type semiconductor variants. However, for their realization in flexible, wearable electronics, high throughput solution processing techniques, such as printing is essential. However, when solution processed, the performance of the devices deteriorates substantially due to huge inter-flake resistance. To overcome this challenge, here we propose and demonstrate an unconventional thin film transistor (TFT) device architecture which can circumvent the shortcoming of large inter-flake resistance by transforming the TFTs into predominantly intra-flake transport edge-FETs. Using this edge-FET device architecture, here we present TFTs printed from chemically exfoliated WS2 and MoSe2 inks, with 106 µA/µm and 25 µA/µm width-normalized, On-state current density, respectively. On the other hand, the maximum On-Off ratio observed in these printed TFTs have also been large, as high as 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> has been recorded, which is surely a rarity in solution-processed 2D electronics. In addition, a tunable channel capacitance mediated subthermionic transport with minimum subthreshold slope of 36 mV/dec has also been observed.