High-mobility low-temperature ZnO transistors with low-voltage operation

We report the fluorine (F) doping effect on zinc oxide (ZnO) thin-film transistor (TFT) fabricated by spray pyrolysis on spray-coated zirconia alumina oxide (ZAO) gate insulator. F doping was performed by NF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> plasma treatment on ZnO thin film. The ZnO film shows <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</i> -axis-aligned hexagonal structure, which remains unchanged by F doping. The photoluminescence (PL) spectra and X-ray photoelectron spectroscopy (XPS) show a reduction in oxygen-related defects by F doping. Valance band edge XPS spectra show the shift of Fermi energy level from 2.46 to 3.12 eV which reveals that the more carriers are generated in the conduction band and defect states are reduced. The F-doped ZnO (F:ZnO) TFT exhibits the saturation mobility of 31.59 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> V <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , the subthreshold swing of 238mV/dec, ON/ OFF current ratio of ~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> , and zero hysteresis voltage. The F:ZnO TFT also shows significant improvement of threshold voltage shift (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TH</sub> ) under negative/positive gate bias stress compared to pristine ZnO. The enhancement of mobility and stability is attributed to the substitution of oxygen (O) and passivation of oxygen vacancies (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> ) by F in ZnO. Therefore, it is expected that F doping is an effective method to improve the performance and stability of solution-processed oxide TFTs.

Photo-patterned oxide films produced using polymeric metal acrylate for low-voltage thin-film transistors

Improvement of on/off ratio in ZnO thin-film transistor by using growth interruptions during metalorganic chemical vapor deposition

Zinc oxide (ZnO) thin films have attracted significant attention for application in thin-film transistors (TFTs) due to their specific characteristics, such as high mobility and transparency. The atomic layer deposition (ALD) thin film is deposited with alternating exposures of a source gas and oxidant. The ALD method keeps fabrication temperature of ZnO TFTs low. In this study, we investigated the effects of gate insulator properties on the performance of TFTs with a ZnO channel layer deposited by plasma-assisted ALD (PAALD). The TFTs with <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\hbox{Al}}_{2}{\hbox{O}}_{3}$</tex> </formula> gate insulator indicated high performance (5.1 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">${\hbox{cm}}^{2}/{\hbox{V}}\cdot{\hbox{s}}$</tex></formula> field effect mobility) without thermal annealing. This result indicated a high-performance ZnO TFT with films deposited by PAALD can be obtained at temperatures below 100 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^{\circ}{\hbox{C}}$</tex> </formula> .

We investigated zinc oxide (ZnO) thin films prepared by plasma assisted atomic layer deposition (PA-ALD), and thin-film transistors (TFTs) with the ALD ZnO channel layer for application to next-generation displays. We deposited the ZnO channel layer by PA-ALD at 100 or 300 °C, and fabricated TFTs. The transfer characteristic of the 300 °C-deposited ZnO TFT exhibited high mobility (5.7 cm2 V-1 s-1), although the threshold voltage largely shifted toward the negative (-16 V). Furthermore, we deposited Al2O3 thin film as a gate insulator by PA-ALD at 100 °C for the low-temperature TFT fabrication process. In the case of ZnO TFTs with the Al2O3 gate insulator, the shift of the threshold voltage improved (-0.1 V). This improvement of the negative shift seems to be due to the negative charges of the Al2O3 film deposited by PA-ALD. On the basis of the experimental results, we confirmed that the threshold voltage of ZnO TFTs is controlled by PA-ALD for the deposition of the gate insulator.

We report on the morphological influence of solution-processed zinc oxide (ZnO) semiconductor films on the electrical characteristics of ZnO thin-film transistors (TFTs). Different film morphologies were produced by controlling the spin-coating condition of a precursor solution, and the ZnO films were analyzed using atomic force microscopy, X-ray diffraction, X-ray photoemission spectroscopy, and Hall measurement. It is shown that ZnO TFTs have a superior performance in terms of the threshold voltage and field-effect mobility, when ZnO crystallites are more densely packed in the film. This is attributed to lower electrical resistivity and higher Hall mobility in a densely packed ZnO film. In the results of consecutive TFT operations, a positive shift in the threshold voltage occurred irrespective of the film morphology, but the morphological influence on the variation in the field-effect mobility was evident. The field-effect mobility in TFTs having a densely packed ZnO film increased continuously during consecutive TFT operations, which is in contrast to the mobility decrease observed in the less packed case. An analysis of the field-effect conductivities ascribes these results to the difference in energetic traps, which originate from structural defects in the ZnO films. Consequently, the morphological influence of solution-processed ZnO films on the TFT performance can be understood through the packing property of ZnO crystallites.

The simultaneous doping effect of Gadolinium (Gd) and Lithium (Li) on zinc oxide (ZnO) thin‐film transistor (TFT) by spray pyrolysis using a ZrOx gate insulator is reported. Li doping in ZnO increases mobility significantly, whereas the presence of Gd improves the stability of the device. The Gd ratio in ZnO is varied from 0% to 20% and the Li ratio from 0% to 10%. The optimized ZnO TFT with codoping of 5% Li and 10% Gd exhibits the linear mobility of 25.87 cm2 V−1 s−1, the subthreshold swing of 204 mV dec−1, on/off current ratio of ≈108, and zero hysteresis voltage. The enhancement of both mobility and stability is due to an increase in grain size by Li incorporation and decrease of defect states by Gd doping. The negligible threshold voltage shift (∆VTH) under gate bias and zero hysteresis are due to the reduced defects in an oxide semiconductor and decreased traps at the LiGdZnO/ZrOx interface. Li doping can balance the reduction of the carrier concentration by Gd doping, which improves the mobility and stability of the ZnO TFT. Therefore, LiGdZnO TFT shows excellent electrical performance with high stability.

Tailored β-diketones as effective surface passivation for solution processed zinc oxide thin film transistors

Zinc oxide (ZnO) is a benign and earth-abundant semiconductormaterialthat has been applied in thin-film transistors (TFTs) for decadesand can be used in biodegradable, transient, and biocompatible devices.Printing as an alternative fabrication method to conventional TFTmanufacturing methods can deliver some benefits, such as simultaneousfilm deposition and patterning, good scalability, low cost, and material-savingfeatures. However, the high annealing temperature needed for ink-to-metaloxide conversion and film densification, compounded by the poor patterningresolution of conventional printing methods, still limits the useof printing in the fabrication of flexible metal oxide TFTs. Atomiclayer deposition (ALD) has recently emerged as a promising fabricationmethod for high-performance metal oxide TFTs that can offer more conformalfilm growth, precise film thickness, and higher film quality at lowtemperatures compared to sputtering, spin coating, or printing. AlthoughALD-based ZnO TFTs patterned with photolithography exhibit good electricalproperties, they cannot be readily scaled to a high-throughput fabrication.Very little attention has been paid so far to the combination of low-temperatureALD growth with printing to obtain more scalable manufacturing ofhigh-performance thin-film electronics. To overcome this challenge,we propose high-resolution reverse-offset printing (ROP) of a simplepolymer resist to pattern an ALD-grown ZnO film at few μm resolutionto fabricate TFTs. In this work, we report high-performance ZnO TFTsthat are ALD-grown at a low temperature of 150 °C and ROP-patternedwith promising stability and uniformity, a high field-effect mobility(μFE) of ∼16.6 cm2 (Vs)−1, an almost zero turn-on voltage (Von) of ∼−0.49 V, a high current on–off ratio (Ion/Ioff) of >108, a low operation voltage (Vop) of ≤5 V, and a negligible hysteresis (Vhyst) of ∼0.13 V. The combination of ALD and theROP-patterning process could be developed further to fabricate fullyflexible high-resolution metal oxide TFT-based circuits in the future.

This study explores the influence of nitrogen gas flow rate on the electrical characteristics of indium-gallium-zinc-oxide (IGZO) thin-film transistors (TFTs) annealed under heat-assisted UV illumination. The aim is to understand how nitrogen flow rates impact the performance of solution-processed IGZO TFTs annealed at low temperatures, which is crucial for developing high-performance devices for next-generation electronics and temperature-sensitive applications. The IGZO TFTs were fabricated on glass substrates using a bottom-gate top-contact configuration, with the IGZO thin film deposited by inkjet printing and annealed in a chamber with varying nitrogen gas flow rates (0.5, 1, 2, and 5 L/min) at 250°C for 2 hours under UV illumination. The electrical characteristics were extracted from transfer characteristics measurements. The results show that a nitrogen flow rate of 1 L/min enhances the electrical properties of IGZO TFTs, likely due to a suitable concentration of oxygen vacancies. Excessive N2 flow rates (&gt;1 L/min) negatively impact on the TFT characteristics, while lower flow rates (&lt;1 L/min) result in more negative threshold voltages and lower on/off current ratios. The study concludes that optimizing the nitrogen gas flow rate is critical for achieving desired TFT properties, offering a valuable tool for fine-tuning IGZO TFTs to meet specific application requirements.

The organic material based thin film transistors (TFTs) are attractive for flexible optoelectronics applications due to the ability of lager area fabrication by solution and low temperature process on plastic substrate. Recently, the research of organic TFT focus on low operation voltage and high output current to achieve a low power organic logic circuit for optoelectronic device,such as e-paper or OLED displayer. To obtain low voltage and high output current, high gate capacitance and high channel mobility are key factors. The well-arranged polymer chain by a high temperature postannealing, leading enhancement conductivity of polymer film was a general method. However, the thermal annealing applying heat for all device on the substrate and may not applicable to plastic substrate. Therefore, in this work, the low operation voltage and high output current of polymer TFTs was demonstrated by locally electrical bias annealing. The poly(styrene-comethyl methacrylate) (PS-r-PMMA) with ultra-thin thickness is used as gate dielectric that the thickness is controlled by thermal treatment after spin coated on organic electrode. In electrical bias-annealing process, the PS-r- PMMA is acted a heating layer. After electrical bias-annealing, the polymer TFTs obtain high channel mobility at low voltage that lead high output current by a locally annealing of P3HT film. In the future, the locally electrical biasannealing method could be applied on plastic substrate for flexible optoelectronic application.

High-performance zinc oxide (ZnO) bottom-gate (BG) thin-film transistors (TFTs) with a single vertical grain boundary in the channel have been successfully fabricated by a novel low-temperature (i.e., 85°C) hydrothermal method. The ZnO active channel was laterally grown with an aluminum-doped ZnO seed layer underneath the Ti/Pt film. Consequently, such BG-TFTs (W/L = 250 μm/10 μm) demonstrated the high field-effect mobility of 9.07 cm2/V - s, low threshold voltage of 2.25 V, high on/off-current ratio above 106, superior current drivability, indistinct hysteresis phenomenon, and small standard deviations among devices, attributed to the high-quality ZnO channel with the single grain boundary.

Thin film transistors (TFTs) based on active layers of zinc oxide prepared from a solution process were fabricated under different annealing conditions. The influence of the annealing gas as well as the influence of a subsequent exposure to synthetic air to the device properties is considered. Annealing under N2 or H2 atmosphere leads to a strong negative threshold voltage shift. With respect to known defect states in ZnO, two different donor states are suggested to be responsible for the negative threshold voltage. A subsequent synthetic air treatment causes in general a positive threshold voltage shift. However, transistors annealed under H2 degrade very fast under synthetic air in contrast to transistors annealed under N2. In order to obtain more information about the density of states (DOS) distribution, a transistor model for thin film transistors in the hopping transport regime (Vissenberg model) was utilized. For positive threshold voltages, the DOS distribution is independent from the gas treatment and the threshold voltage within the experimental accuracy. This indicates a shift of the Fermi-level within an exponentially decaying DOS. The change in the charge carrier density is either due to shallow donors or due to a charge transfer with acceptors at the surface. In contrast, for negative threshold voltages, the DOS distribution parameter rises, indicating a flatter DOS distribution. We suggest that the difference is due to the change from accumulation mode to the depletion mode of the device.

Zinc oxide (ZnO), as a wide band gap (3.37 eV) semiconductor, are useful materials for wide range of applications in thin-film transistor (TFT), photovoltaics, photocatalysts, sensors, and so on. Large exciton binding energy (60 meV) at room temperature (RT) makes ZnO a promising luminescent material especially in optoelectronic devices such as UV lasers and light emitting diodes (LED). Among them, ZnO nanocrystals (quantum dots) has attracted increasing attention because their optical properties can be tuned by varying the crystal size. However, its nanoscale crystallites tend to aggregate to undergo Ostwald ripening due to their high surface energy. To stabilize them, surface modification are usually necessary. At the same time, the whole processes for nanocrystal synthesis and film fabrication become complicated.Here, we report a simple novel photochemical gel conversion approach for one-step preparation of ZnO thin films with an average nanocrystals in about 2-3 nm size (inset in Fig. 1a), by employing vacuum ultraviolet (VUV) light irradiation in dry N2. Systematic characterization is performed to understand the surface chemistry and the optical properties of ZnO nanocrystals. Thus obtained ZnO films show unique absorption peak at 294 nm (4.2 eV) (Fig. 1a) and strong tunable photoluminescence in the range of 386-496 nm when expose to UV light, from violet, blue to bluish-green upon increasing time of VUV irradiation (Fig. 1b). Clearly blue-shifted absorption peaks of the VUV treated samples are indicative of quantum-confinement effect, as contrasted to the broad character of the annealed film, which originates from band structure of bulky ZnO. The extension of VUV time only results in increase of absorbance, with no change of the peak wavelength, indicating population increase but no crystal growth by Ostwald ripening during the VUV irradiation. Therefore, the color change of the emitted light should be associated with the change of defect state distribution, rather than the size effect. However, such photoluminescence from defects was fairly strong to be clearly recognized by naked eyes, so that the university logo could be patterned by photomasking VUV to be seen under black light was demonstrated (inset in Fig. 1b). Figure 1

Transparent thin film transistors (TFTs) have been the subject of extensive scientific research over the last couple of decades, for applications in displays and imaging, as their implementation in active-matrix liquid crystal displays backplanes is expected to improve their performance in terms of switching times and stability. To this end, several material systems have emerged as contenders to address this need for a high performance, low power, large-area electronics i.e. thin film silicon, organic semiconductors and metal oxides. The electronic limitations of thin film silicon are well documented, and although organic semiconductors have seen significant improvements in recent years, their persistent low mobility and instability means that they are unlikely to progress beyond niche applications. This thesis is focused on the investigation of the physical properties of metal oxides and their implementation in TFTs. Metal oxide based TFTs were fabricated by spray pyrolysis, a simple and large-area-compatible deposition technique. More precisely, the implementation of titanium-aluminate and niobium-aluminate both wide band gap and high-k gate dielectric metal oxides in solution processed ZnO-based TFTs was studied and high performance, low operational voltage devices were fabricated. ZnO-based TFTs employing stoichiometric Al2O3-TiO2 (k~13, Eg~4.5 eV) or Nb2O5-Al2O3 (k~13.5, Eg~5.1 eV) as gate dielectric exhibited low leakage currents, high on-off current modulation ratios, high field-effect mobilities and low subthreshold voltage swings. Furthermore, the implementation of solution-processed crystalline indium-zinc oxide (c-IZO) as active channel material in TFTs was equally investigated and high-performance c-IZO-based TFTs employing Al2O3 were fabricated. The effects of metal cation doping in c-IZO matrix were investigated in particular, and c-IZO:X (X:Ga,Y,Zr,Nb) based TFTs were fabricated and their properties were assessed for each dopant. Amongst them, Yttrium doped c-IZO (c-YIZO)-based TFTs exhibited the best performance in terms of low off-state currents, high field-effect mobilities and low subthreshold voltage swings.