Oxide-Based Electric-Double-Layer Thin-Film Transistors on a Flexible Substrate

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.

Research interest in thin-film transistor (TFT)-based flexible electronics for next-generation foldable and rollable displays is increasing, centering on flexible plastic substrates such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and polyimide (PI). For flexible electronics using plastic substrates, it is important to select suitable semiconductor materials that can be deposited at low substrate temperature. Metal oxide (MO) semiconductors such as indium-gallium-zinc oxide (IGZO), zinc oxide (ZnO) are attracting attention as suitable candidates for plastic substrate electronics because of their higher mobility and optical transmittance than hydrogenated amorphous silicon (a-Si:H). In addition, since it can be processed at a lower temperature than low-temperature polysilicon (LTPS), it can be easily applied to flexible plastic substrates. One of the greatest advantages of MO is that it can be easily fabricated by conventional semiconductor processing methods such as sputtering, atomic laser deposition, and solution processes. Among them, solution-processed deposition method is particularly advantageous in terms of manufacturing efficiency because it does not require a separate vacuum system, and can efficiently utilize raw materials, so the fabrication cost is inexpensive. In particular, spray pyrolysis can guarantee environmental stability, high mobility, and large area production than other solution processes. In this work, we developed the high-performance, flexible a-IGZO/Al2O3 TFTs using spray pyrolysis fabricated on the flexible PI substrate. In a previous study, we developed a method to reduce curling and electrostatic discharge (ESD) damage to PI substrates by using a mixture of carbon nanotubes (CNTs) and graphene oxide (GO) between carrier glass and PI substrate. Therefore, it was possible to mechanically detach the TFTs on PI substrate and handle them easily even after detachment. The spray-pyrolyzed Al2O3 film is one of the high-k dielectric materials, which can lower the operating voltage, so the low-power consumption of the TFT backplane can be realized. The flexible spray-pyrolyzed a-IGZO/Al2O3 TFT exhibits the field-effect mobility (µFE) of 9.75 cm2 V−1 s−1, threshold voltage (VTH) of −0.8 V, subthreshold swing (SS) of 0.29 V dec−1, and on/off current ratio of 6.2 × 107. Even without TFT passivation layer, it shows environmental stability. The TFTs show robust electrical performance even under tensile strain with a bending radius of 1 mm. In addition, it showed stable electrical characteristics even in the repeated bending test of 30,000 cycles under compressive strain with a bending radius of 1 mm. This is due to fewer defects resulting from the smooth interface of the a-IGZO film and the Al2O3 film deposited through spray pyrolysis. Therefore, spray-pyrolyzed a-IGZO/Al2O3 TFT fabricated on PI substrate is a promising candidate for next-generation foldable display application. Figure 1

Impact of work function of the silicon bottom-gates on electrical instability in InGaZnO thin film transistors

Low temperature processed InGaZnO thin film transistor using the combination of hydrogen irradiation and annealing

Flat panel displays made on plastic substrates are envisioned for use in certain commercial and military systems because they are more rugged and lightweight than displays made on glass substrates. High information content can be attained for such displays using an active matrix array of thin film transistors (TFTs) for the pixels and high current TFTs for the drivers. In this work the fabrication of high performance polysilicon TFTs on flexible plastic substrates is presented along with corresponding electrical characteristics. Plastic substrates pose severe temperature constraints on the fabrication process. To overcome electrical characteristics. Plastic substrates pose sever temperature constraints on the fabrication process. To overcome these constraints, our group at LLNL has used low temperature silicon, oxide, and aluminum thin film deposition steps and pulsed excimer laser processing to perform the TFT channel crystallization and the source/drain doping. Sheet resistance values below 1k(Omega) /$DAL are obtained using our laser doping technique for 900 angstrom thick polysilicon films. Our n-channel polysilicon TFT electrical performance on plastic shows mobilities up to 50 cm<SUP>2</SUP>/V-sec and ON current to OFF current ratios of up to 1 X 10<SUP>6</SUP> for gate voltages from -1 to +35 V.

This paper investigated the degradation mechanism of InGaZnO (IGZO) thin-film transistors (TFTs) that depend on channel length under positive bias stress (PBS). Although the further degradation of short-channel IGZO TFTs has been previously reported, it has yet to be fully understood. Using technology computer-aided design (TCAD) simulations, we found that the primary mechanism of the channel length-dependent degradation is the additional electron trapping at channel edges where electrons are diffused from the n + region. We also observed that the degradation was minimized when the n + region had a relatively low carrier concentration. Based on these findings, we demonstrated a lightly doped drain-integrated oxide TFT and repositioned the source/drain electrodes. The TCAD simulation results verified that the proposed structure could prevent PBS-induced degradation in a short channel without compromising the drain’s current performance.

Flexible electronics, most prominently thin-film transistors (TFTs) on plastic substrates, are considered the prime building block for the realization of innovative wearable systems. Two of the currently most successful fabrication processes of transistors on free-standing polymer foils are large-area-compatible devices structured by conventional UV lithography and high-speed transistors realized by self-alignment. Here, both processes, based on InGaZnO (IGZO) technology, are combined for the first time. This not only demonstrates their compatibility, but also showcases the differences between the resulting devices. Concerning the geometry, TFTs with the same nominal designed channel length of 1.5μm exhibit a real channel length of 1.5μm (self-aligned) and 4.5μm (conventional). Furthermore, the integrated side-by-side fabrication enables the electrical comparison of both types of TFTs excluding external factors. While all TFTs exhibit similar threshold voltages around <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mathrm {0 V}$ </tex-math></inline-formula> and excellent on/off ratios of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\approx \mathrm {e10 }$ </tex-math></inline-formula> , conventional TFTs are easier to fabricate and have comparably higher mobilities up to 16cm2V−1s−1. At the same time, self-aligned TFTs demonstrate better ac performance, demonstrating a maximum oscillation frequency of 216 MHz. This integration shows new possibilities for the realization of complex systems made from building blocks optimized for reliability and speed.

Oxide thin film transistors (TFTs) have attracted much attention for next-generation electronics because of their superior performance compared with conventional a–Si TFTs such as high mobility and optical transmittance in the visible light.1 Some display manufacturing companies have already manufactured oxide TFTs based products, but there are some issues remained for oxide TFTs to be used for flexible displays. One of the issues is the high-temperature annealing process (> 300°C). Due to low thermal durability, flexible substrates cannot be processed in high temperatures.2-3 In this study, we propose various researches to lower the processing temperatures in oxide TFTs: photocatalytic reactions of titanium dioxide (PRT) and rotating magnetic field (RMF) treatment. In addition, we suggest ethyl cyanoacrylate (ECA) based instant glue (IG) as a new material for the passivation layers of oxide TFTs with low processing temperature. Photocatalytic Reaction of Titanium Dioxide (PRT) In this study, we fabricate indium–zinc oxide (IZO) TFTs using PRT. The hydroxyl radicals are generated by reaction between titanium dioxide (TiO2) and water molecules during UV irradiation. These hydroxyl radicals decompose the 2-methoxyethanol (2ME; one of the representative solvents) into smaller organic molecules. The decomposed 2ME has low boiling points and molar masses compared with non-PRT treated 2ME. Therefore, PRT treatment not only provides superior electronic devices caused by less defect sites in the oxide channel layers, but also allows low temperature process at 230°C. Additionally, since TiO2 particles can generate hydroxyl radicals permanently under UV irradiation, the PRT treated IZO TFTs show improved electrical characteristics even after four times recycling of TiO2. Rotating magnetic field (RMF) treatment In this work, we fabricate indium–gallium–zinc oxide (IGZO) TFTs by RMF treatment at a low temperature of 150°C. For RMF treatment, IGZO films are exposed to periodically rotating magnetic field and thermal energy of 150°C. The magnetic field and thermal energy simultaneously facilitate the interaction and attraction of metal and oxygen atoms leading to formation of M–O bonds in IGZO films. As a result, RMF treated IGZO TFTs exhibited improved electrical performances and positive bias stress (PBS) characteristics compared with those of IGZO TFTs treated by only thermal annealing process at 300°C. Instant Glue (IG) Passivation We introduce IG as a novel material for the passivation layer of IGZO TFTs. The ECA-based IG is a moisture-curing adhesive, which is liquid at room temperature and cures rapidly by reacting with moisture in atmosphere. As it is solidified rapidly at room temperature, it has shorter process time than that of conventional organic materials; the IG passivation layer is prepared successfully just for 1 min at 150°C. The IG passivation layer can modify the chemical stoichiometry of IGZO films and prevent any interaction between the back surface of the IGZO TFT and ambient gas. Therefore, the IGZO TFTs with an IG passivation layer show improved electrical characteristics and reliability compared to those without passivation layer. References Jae Won Na, You Seung Rim, Hee Jun Kim, Jin Hyeok Lee, Seonghwan Hong, and Hyun Jae Kim, ACS Appl. Mater. Interfaces 9, 35, 29849 (2017).Hua Xu, Dongxiang Luo, Min Li, Miao Xu, Jianhua Zou, Hong Tao, Linfeng Lan, Lei Wang, Junbiao Peng, and Yang Cao, J. Mater. Chem. C 2, 7, 1255 (2014).Seonghwan Hong, Sung Pyo Park, Yeong-gyu Kim, Byung Ha Kang, Jae Won Na, and Hyun Jae Kim, Scientific Reports 7, 1, 16265 (2017). Figure. Schematics of various techniques to lower the processing temperatures for oxide TFTs, (a) photocatalytic reaction of titanium dioxide, (b) rotating magnetic field treatment, (c) instant glue passivation. Figure 1

A process for the fabrication of organic thin-film transistors (TFTs) with channel lengths as short as 0.5 μm on flexible plastic substrates has been developed. The TFTs are fabricated in the bottom-gate, top-contact (inverted staggered) architecture and employ vacuum-deposited small-molecule organic semiconductors and a low-temperature-processed gate dielectric that is sufficiently thin to allow the TFTs to operate with voltages of about 2 to 3 V. For a channel length of 0.5 μm, the TFTs have effective field-effect mobilities of 0.08 cm2/Vs (p-channel TFTs) and 0.03 cm2/Vs (n-channel TFTs), on/off current ratios of at least 106, and subthreshold swings of about 140 mV/decade. For 11-stage complementary and unipolar ring oscillators based on TFTs with a channel length of 1 μm, signal propagation delays per stage as short as 6.6 μs and 420 ns have been measured at a supply voltage of 3 V.

Photon emission and the electrical characteristics of a InGaZnO (IGZO) thin-film transistor (TFT) were evaluated to clarify the existence of hot carriers and their effects. The photon emission was observed from the drain edge of the IGZO-TFT and the capacitance–voltage characteristics indicated that a potential barrier was formed at the drain region when the device is driven by high drain voltage of 30 V. Based on these results, the photon emission phenomenon from the IGZO-TFT was induced by hot carriers that should be considered as a significant degradation mode for a high-performance oxide semiconductor device.

This paper investigates degradation behavior induced by the self-heating effect for InGaZnO (IGZO) thin-film transistors (TFTs). Both the surrounding oxide and other thermal insulating materials, as well as the low thermal conductivity of the InGaZnO layer itself, cause the self-heating effect in InGaZnO TFTs. The heated channel layer enhances the threshold voltage shift, and the evolution of threshold voltage shift is found to be dominated by charge-trapping effects. Moreover, a nonuniform distribution of channel carrier concentration leads to an uneven temperature distribution throughout the IGZO active layer, which results in the asymmetrical degradation behavior after the self-heating operation. Further verifications indicate that the degree of the threshold voltage shift is only dependent on stress power, regardless of stress <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Vg</i> , <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Vd</i> , and channel length. Further, two-stage dependence of the threshold voltage shift on dynamic stress frequency is found.

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.

In recent years, environment-friendly and biocompatible electronics have received extensive attention. As a kind of natural biological material with rich sources, proteins have been widely used in electronic devices. In this work, electric-double-layer (EDL) thin-film transistors (TFTs) gated by natural chicken albumen are fabricated at room temperature. The indium-tin-oxide (ITO) conductive glass is employed as a substrate. The spin coated chicken albumen film is used as the gate dielectric. The indium-zinc-oxide (IZO) is sputtered on an albumen-coated ITO glass as the channel and the source/drain electrodes with only one shadow mask. The capacitance-frequency measurements demonstrate an ultra-large specific capacitance of the albumen film at low frequencies. For the physical understanding of the capacitive coupling within the albumen film, the phase angle is characterized as a function of frequency. The results indicate that such an ultra-large capacitive coupling can be attributed to the proton migration under the electric field, which results in the EDL effect at the interface of the albumen film. By DC sweep measurements, a low leakage current is observed (&lt;3.0 nA at &lt;i&gt;V&lt;/i&gt;&lt;sub&gt;gs&lt;/sub&gt;=1.5 V), which indicates a good isolation of the albumen-based dielectric. By transfer and output measurements, an ultralow operation voltage of 1.5 V, a high field-effect mobility of 38.01 cm&lt;sup&gt;2&lt;/sup&gt;/(V·s), a low subthreshold swing of 164 mV/decade, and a large on-off ratio of 2.4×10&lt;sup&gt;6&lt;/sup&gt; are obtained for such albumen-gated TFTs. The ultra-large EDL capacitive coupling is responsible for such good electrical characteristics. The dynamic bias stress stability of the albumen-gated TFTs is also investigated. The device exhibits a good reproducibility in response to the repeatedly pulsed gate voltage. A maintainable on-to-off ratio (&gt;10&lt;sup&gt;6&lt;/sup&gt;) and no obvious current loss are observed, which suggests that neither chemical doping nor chemical reaction occurs at the albumen-based dielectric/IZO channel interface when the gate potential is biased. After being aged one day in air ambient without surface passivation, the albumen-gated TFTs show a good stability of the electrical properties. Such ultralow-voltage EDL-TFTs gated by albumen electrolyte will be useful for the bioelectronic and low-energy portable electronic products. And our results will also have potential applications in biocompatible artificial neuron networks and brain-inspired neuromorphic systems.

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.

An amorphous InGaZnO film fabricated by radio frequency magnetron sputtering in only an Ar-reactive gas shows high conductivity, and a thin-film transistors (TFTs)-based IGZO active layer expresses a poor on/off current ratio with a high off current and high subthreshold swing (SS). This paper presents the post-annealing effects on IGZO thin films to compensate the oxygen deficiencies in films as well as on TFT devices to reduce the densities of the interface trap between the active layer and insulator. The ratio of oxygen vacancies over total of oxygen (O2/Otot) in IGZO estimated by the XPS measurement shows that they significantly diminish from 24.75 to 17.68% when increasing the temperature treatment to 350 °C, which is related to the enhancement in resistivity of IGZO. The TFT characteristics of IGZO treated in air at 350 °C show a high ION/IOFF ratio of ∼1.1 × 107, a high field-effect mobility of 7.48 cm2 V−1 s−1, and a low SS of 0.41 V dec−1. The objective of this paper is to achieve a successful reduction in the interface trap density, ΔDit, which has been reduced about 3.1 × 1012 cm−2 eV−1 and 2.0 × 1012 cm−2 eV−1 for the 350 and 200 °C treatment samples compared with the as-deposited one. The resistivity of the IGZO films can be adjusted to the appropriate value that can be used for TFT applications by controlling the treatment temperature.