Browsing by Subject "TFT"
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Item Open Access Air-stable n-channel diketopyrrolopyrrole-diketopyrrolopyrrole oligomers for high performance ambipolar organic transistors(American Chemical Society, 2016) Mukhopadhyay, T.; Puttaraju, B.; Senanayak, S. P.; Sadhanala, A.; Friend, R.; Faber, H. A.; Anthopoulos, T. D.; Salzner, U.; Meyer A.; Patil, S.n-channel organic semiconductors are prone to oxidation upon exposed to ambient conditions. Herein, we report design and synthesis of diketopyrrolopyrrole (DPP)-based oligomers for ambipolar organic thin-film transistors (OFETs) with excellent air and bias stability at ambient conditions. The cyclic voltammetry measurements reveal exceptional electrochemical stability during the redox cycle of oligomers. Structural properties including aggregation, crystallinity, and morphology in thin film were investigated by UV-visible spectroscopy, atomic force microscopy (AFM), thin-film X-ray diffraction (XRD), and grazing incidence small-angle X-ray scattering (GISAXS) measurements. AFM reveals morphological changes induced by different processing conditions whereas GISAXS measurements show an increase in the population of face-on oriented crystallites in films subjected to a combination of solvent and thermal treatments. These measurements also highlight the significance of chalcogen atom from sulfur to selenium on the photophysical, optical, electronic, and solid-state properties of DPP-DPP oligomers. Charge carrier mobilities of the oligomers were investigated by fabricating top-gate bottom-contact (TG-BC) thin-film transistors by annealing the thin films under various conditions. Combined solvent and thermal annealing of DPP-DPP oligomer thin films results in consistent electron mobilities as high as ∼0.2 cm2 V-1 s-1 with an on/off ratio exceeding 104. Field-effect behavior was retained for up to ∼4 weeks, which illustrates remarkable air and bias stability. This work paves the way toward the development of n-channel DPP-DPP-based oligomers exhibiting retention of field-effect behavior with superior stability at ambient conditions.Item Open Access Dynamic control of photoresponse in ZnO-based thin-film transistors in the visible spectrum(IEEE, 2013-04) Aygun, L. E.; Oruc, F. B.; Atar, F. B.; Okyay, Ali KemalWe present ZnO-channel thin-film transistors with actively tunable photocurrent in the visible spectrum, although ZnO band edge is in the ultraviolet. ZnO channel is deposited by atomic layer deposition technique at a low temperature (80), which is known to introduce deep level traps within the forbidden band of ZnO. The gate bias dynamically modifies the occupancy probability of these trap states by controlling the depletion region in the ZnO channel. Unoccupied trap states enable the absorption of the photons with lower energies than the bandgap of ZnO. Photoresponse to visible light is controlled by the applied voltage bias at the gate terminal. © 2009-2012 IEEE.Item Open Access ZnO based photo-thin-film-transistors with actively tunable photoresponse in the visible spectrum(Bilkent University, 2013) Aygün, Levent ErdalZinc oxide, ZnO, is an important material for wide range of optoelectronic device applications. Especially, ZnO is famous with its large exciton binding energy of 60 meV which makes it a good candidate for ultraviolet light emitting diodes and lasers. Moreover, its high carrier mobility and wide band gap of 3.37 eV (368 nm) makes it a promising material for transparent electronics and UV photodetectors. However, ZnO has crystallographic defect states (e.g. oxygen vacancies, zinc interstitials) which degrade the performance of ZnO based LEDs, lasers and UV photodiodes. In this thesis, ZnO based photo-thin-film-transistors (photo-TFTs) with visible light response by using their defect states to absorb subbandgap photons are investigated. The design, fabrication and characterization of ZnO based photoTFTs are presented. A photo-TFT is a three-terminal optoelectronic device that is a photoconductor structure with an additional gate terminal which actively tunes electrical and optical properties of photoconductive material. In a clean room environment, ZnO based photo-TFTs with various device sizes are fabricated at different ZnO channel layer deposition temperatures (ranging from 80 to 250 °C). Initially, TFT characteristics of fabricated devices are characterized to show that the gate terminal dynamically modulates ZnO’s channel conductivity. Moreover, the effects of the device size and the deposition temperature on device performance are investigated. Then, the optical characterization of ZnO film deposited at 250° C is conducted via absorption and photoluminescence measurements in order to investigate its visible light absorption characteristics and the energy levels of its defect states in the forbidden band gap of ZnO. After that, the responsivity measurements are reported from ZnO based photo-TFTs fabricated at 250 °C and the active tuning mechanism of visible light photoresponse is discussed. Finally, the effects of the deposition temperature and the device size on the visible light responsivity are presented.Item Open Access ZnO, TiO² and exotic materials for low temperature thin film electronic devices(Bilkent University, 2012) Oruç, Feyza BozkurtThe metal-oxide-semiconductor field-effect transistor (MOSFET) technology is the core of integrated circuit industry. Nearly all electronic devices around us contain transistors for various purposes like electronic switches, amplifiers or sensors. As the need for more complex and miniature circuits has arisen, scaling down transistor sizes become the top priority. As Moore’s law indicates, number of transistors on integrated circuits doubles every two years but in future fabrication challenges and limitations like quantum effects seen in small devices will block further miniaturization. New growth techniques are required for depositing conformal, high quality films -like high-k dielectrics instead of SiO2- with atomic thickness control to reduce possible problems. Atomic layer deposition techniques are developed to meet these requirements. The field of thin film transistors (TFT), which is a subset of MOSFET’s have first started to be used in flat panel displays but now they are used in various fields, since their functional properties make them powerful candidates for sensor applications. ALD technology is important also for TFT applications since its low temperature growth mechanism allows fabricating TFT’s on various substrates like flexible and/or transparent ones. With ALD technique, transistors can be built even on cloths which makes the dream of e-suits real. In this thesis, thin film transistors are designed and fabricated using atomic layer deposition technique both for channel and dielectric layer growth. Design and fabrication steps of the TFT devices are realized in a cleanroom environment. The fabricated TFT’s are mainly characterized by measuring their current-voltage relations. A parameter analyzer with a probe station is used for such measurements. ALD grown ZnO TFT’s and the effect of growth temperature on performance characteristics are examined. High performance devices having very high Ion/Ioff ratios are fabricated at a temperature low as 80°C. ALD grown TiO2 TFT’s are also fabricated and effects of annealing temperature on device performance are analyzed. This study is, to the best of our knowledge, the first demonstration of TiO2 TFT’s grown by a thermal-ALD system. GaN and pentacene TFT’s are also fabricated and showed promising results. Pentacene TFTs have a special importance since it is a p-type organic semiconductor which gives us the opportunity to work on hybrid organic-inorganic structures. In conclusion, TFT devices based on ALD grown channel and/or dielectric layers show very encouraging results in terms of low cost, low temperature fabrication opportunities and freedom of using any substrate that can handle ALD processing temperature.