Indium Oxide Thin-film Transistors Research Articles

Reliable Operation in High-Mobility Indium Oxide Thin Film Transistors.

Transparent oxide semiconductors (TOSs) based thin-film transistors (TFTs) that exhibit higher field effect mobility (µFE) are highly required toward the realization of next-generation displays. Among numerous types of TOS-TFTs, In2O3-based TFTs are the front-running candidate because they exhibit the highest µFE ≈100cm2V-1s-1. However, the device operation of In2O3 TFTs is unreliable; a large voltage shift occurs especially when negative gate bias is applied due to adsorption/desorption of gas molecules. Although passivation of the TFTs is used to overcome such instability, previously proposed passivation materials do not improve the reliability. Here, it is shown that the In2O3 TFTs passivated with Y2O3 and Er2O3 films are highly reliable and do not show threshold voltage shifts when applying gate bias. Positive and negative gate bias is applied to the In2O3 TFTs passivated with various insulating oxides and found that only the In2O3 TFTs passivated with Y2O3 and Er2O3 films do not exhibit threshold voltage shifts. It is observed that only the Y2O3 grew heteroepitaxially on the In2O3 crystal. This is the origin of the high reliability of the In2O3 TFTs passivated with Y2O3 and Er2O3 films. This finding accelerates the development of next-generation displays using high-mobility In2O3 TFTs.

Effects of iodine doping on structural and electrical characteristics of solution-processed indium oxide thin-film transistors and its potential application for iodine sensing

Effects of iodine doping on structural and electrical characteristics of solution-processed indium oxide thin-film transistors and its potential application for iodine sensing

Dependence of Positive Bias Stress Instability on Threshold Voltage and Its Origin in Solution-Processed Aluminum-Doped Indium Oxide Thin-Film Transistors.

The initial electrical characteristics and bias stabilities of thin-film transistors (TFTs) are vital factors regarding the practical use of electronic devices. In this study, the dependence of positive bias stress (PBS) instability on an initial threshold voltage (VTH) and its origin were analyzed by understanding the roles of slow and fast traps in solution-processed oxide TFTs. To control the initial VTH of oxide TFTs, the indium oxide (InOx) semiconductor was doped with aluminum (Al), which functioned as a carrier suppressor. The concentration of oxygen vacancies decreased as the Al doping concentration increased, causing a positive VTH shift in the InOx TFTs. The VTH shift (∆VTH) caused by PBS increased exponentially when VTH was increased, and a distinct tendency was observed as the gate bias stress increased due to a high vertical electric field in the oxide dielectric. In addition, the recovery behavior was analyzed to reveal the influence of fast and slow traps on ∆VTH by PBS. Results revealed that the effect of the slow trap increased as the VTH moved in the positive direction; this occured because the main electron trap location moved away from the interface as the Fermi level approached the conduction band minimum. Understanding the correlation between VTH and PBS instability can contribute to optimizing the fabrication of oxide TFT-based circuits for electronic applications.

High bias stability of Hf-doping-modulated indium oxide thin-film transistors

Device stability is one of the key parameters for transistor applications. To improve the stability of Indium oxide (In2O3) after a long-time gate bias, a synthetic solution of hafnium chloride (HfCl4) and indium nitrate (In(NO3)3∙xH2O) reagents were used to obtain 0% to 5-at.% Hf doped In2O3 thin-film transistors. With the increase of Hf doping concentration, oxygen vacancies and residual hydroxyl groups continue to decrease, suppressing the carrier concentration and influencing the trap state density of In2O3. The sub-threshold slope (SS) 0.78 V·dec−1 for the undoped In2O3 transistor in this work is a typical value. When the dopant dose is up to 5-at.%, SS decreases to 0.32 V·dec−1. According to the proportional relationship between SS and the density of trap states, it shows that the density of trap states in the dielectric layer and the semiconductor/dielectric interface SS is greatly reduced after 5-at.% Hf doping. The probability of the charge being trapped is dropped as well. At the same time, under the doping of Hf, the transistor exhibits a very small threshold voltage shift. Especially at the dopant dose of 5-at.%, the transfer characteristic curve hardly shifts. This work demonstrates an In2O3 transistor with high bias stability by doping method.

Low-temperature rapid preparation of high-performance indium oxide thin films and transistors based on solution technology

Indium oxide (In<sub>2</sub>O<sub>3</sub>) thin films and thin-film transistors (TFTs) based on the solution process are prepared by pulsed UV-assisted thermal annealing at a low temperature (200 ℃) for 5 min. The effects of pulsed UV-assisted thermal annealing on the surface morphology, chemical structure, and electrical properties of the In<sub>2</sub>O<sub>3</sub> thin films are investigated, and they are compared with those of conventional thermal annealing (300 ℃, 30 min). The experimental results show that the pulsed UV-assisted thermal annealing method can improve the quality of In<sub>2</sub>O<sub>3</sub> thin film and the performance of TFT in a short period. The results of atomic force microscopy and field emission scanning electron microscopy show that the surface of the In<sub>2</sub>O<sub>3</sub> film is denser and flatter than that of the conventional thermally annealed film, and X-ray photoelectron spectroscopy tests show that the pulsed UV-assisted thermal annealing process generates oxygen vacancies, which increases the carrier concentration and improves the electrical conductivity of the In<sub>2</sub>O<sub>3</sub> film. In addition, the effect of pulsed UV-assisted thermal annealing on the electrical characteristics of In<sub>2</sub>O<sub>3</sub> TFTs is investigated in a comparative way. The results show that the electrical characteristics of the device are significantly improved: the subthreshold swing decreases to 0.12 mV/dec, the threshold voltage is 7.4 V, the current switching ratio is as high as 1.29×10<sup>7</sup>, and the field effect mobility is enhanced to 1.27 cm<sup>2</sup>·V<sup>–1</sup>·s<sup>–1</sup>. Therefore, pulsed UV-assisted thermal annealing is a simple and fast annealing method, which can rapidly improve the performances of In<sub>2</sub>O<sub>3</sub> thin film and TFTs, even under low-temperature conditions.

Improvement of Electrical Characteristics and Stability in Low-Voltage Indium Oxide Thin-Film Transistors by Using Tungsten Doping

Improvement of Electrical Characteristics and Stability in Low-Voltage Indium Oxide Thin-Film Transistors by Using Tungsten Doping

Effect of the gas flow rate in the focused-oxygen plasma treatment of solution-processed indium oxide thin film transistors

In2O3 is a transparent semiconductor layer due to its transparency, high mobility, and solution processability. In this paper, we study the effect of focused oxygen plasma treatment on solution processed indium oxide films annealed at 250 °C and analyze the effect of the gas flow rate. Thin film transistor devices were prepared based on pristine In2O3 films and films treated with oxygen plasma using flow rates of 3, 6, and 9 sccm. The results show that controlling the plasma flow rate can be used to finetune the device characteristics, and the selecting the optimal flow rate is important to improve the electron mobility, threshold voltage, on/off current ratio, and the electrical stability of the devices. The In2O3 TFTs treated at 6 sccm showed the best performance with mobility of 2.19 cm2V−1s−1, threshold voltage of of 4.25 V, and on/off current ratio of 2.13 × 106. The successful fabrication of high-performance, low-temperature solution deposited indium oxide transistors using the focused oxygen plasma treatment for electrical device applications demonstrates the practical potential of this treatment method for future transparent electronics.

Low-temperature crystallization of indium oxide thin films with a photoactivable additive

In this study, we report a simple route to the low-temperature crystallization of solution-processed indium oxide thin films by introducing ammonium nitrate in the sol–gel metal oxide precursor solution as photoactivable additive and applying deep ultraviolet (DUV) irradiation onto the as-spun oxide films in an inert atmosphere. Thermal and structural analyses revealed that the initial temperatures for condensation and crystallization were reduced down to 130 and 200 °C, respectively, by the in situ generation of reactive chemical species enabled by DUV-assisted nitrate photolysis. Furthermore, transmission electron microscopy confirmed that the degree of indium oxide film crystallinity was gradually enhanced as the amount of nitrate in the precursor solution was increased. Finally, electrical characterizations showed that carrier mobility, threshold voltage, subthreshold swing, and threshold voltage shift under the positive bias stress of sol–gel indium oxide thin-film transistors were improved from 0.21 to 5.03 cm2/V s, from 4.18 to 1.64 V, from 1.33 to 0.72 V/dec, and from 6.44 to 4.04 V, respectively, by combining ammonium nitrate and DUV photoactivation.

Effect of Humidity on Properties of Aqueous-Processed Tb-Doped Indium Oxide Thin-Film Transistors

Aqueous route is readily for fabricating oxide thin-film transistors (TFTs), but the relative humidity (RH) impacts evolution of oxide thin films. Herein, the effect of RH on the electronic performance and bias stability of aqueous-processed Tb-doped indium oxide (Tb:In2O3) TFTs is investigated. The mobility is sensitive to RH. By tuning the RH for desirable thickness and density of the obtained Tb:In2O3 films, the TFTs present optimized mobility of 13.0 cm2/Vs, with high ON/OFF ratio of 108. Notably, low threshold voltage shift of −0.35 V and 0.42 V under negative and positive gate bias stress are simultaneously obtained with a fabricating RH of 40%. Moreover, dry ambience (RH <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\le23$ </tex-math></inline-formula> %) affords to abundant nanopores and defects, resulting in inferior performance and stability of the aqueous-processed oxide TFTs. And high humid ambience easily ruins the uniform of precursor films. This work indicates the strategy to fabricate highly stable oxide TFTs with high performance.

Overall enhancement of electrical properties of indium oxide thin-film transistors by surface doping

Doping is a powerful tool for modulating the charge carrier density in metal oxide thin-film transistors (TFTs). However, one crucial limitation is the disruption of microstructure after doping of crystalline semiconductors, which dramatically decreases the charge carrier mobility. Here, we report on a surface doping method that utilizes organic acceptor molecules to efficiently modulate the carrier concentration without disrupting the lattice of thin indium oxide channel layers. For the first time, we achieve electron transfer from indium oxide channel to the organic layer, instead conventional strategy of organic to metal oxide, to control the intrinsic carrier concentration and enhance the electrical performance of the indium oxide TFTs. Moreover, we use a complementary tool based on spectroscopic techniques to prove the fundamental charge transfer processes between metal oxide semiconductors and organic molecular dopants. The results indicate that surface doping is a simple and effective way to improve the performance of oxide TFTs and suitable for potential applications in large-area electronics. • Indium oxide TFTs were fabricated by solution process method. • Surface doping modulates the carrier concentration of the channel layer. • The electrical performances of the devices were enhanced by surface doping.

Fluorine-passivated In2O3 thin film transistors with improved electrical performance via low-temperature CF4/N2O plasma

In this Letter, we report the electrical performance improvement of indium oxide (In2O3) thin film transistors (TFTs) via a low-temperature CF4/N2O plasma treatment. It is found that the fluorination via CF4/N2O plasma can reduce the excessive electrons in the In2O3 channel more effectively compared to the oxidative annealing, providing the same low off-currents at a lower temperature of 200 °C, while the hydrogenation could not give rise to the off-current reduction. The fluorinated In2O3 TFTs with a channel thickness of 3.5 nm, a HfO2 dielectric thickness of 3.5 nm, and a channel length ranging from 80 nm to 1 μm demonstrate markedly improved electrical performances, including a high field effect mobility of 72.8 cm2/V s, a more positive threshold voltage, a higher on/off current ratio of ∼106, a smaller subthreshold swing below 200 mV/dec, and a higher stability to both negative and positive gate biases. X-ray photoelectron spectroscopy (XPS) confirms the fluorine incorporation in In2O3/HfO2 heterojunction upon CF4/N2O plasma, speculatively passivating the oxygen vacancies and explaining TFT performance enhancement. This study suggests that the anion doping such as fluorine incorporation could be an effective method to improve the performance of oxide semiconductor TFTs with ultrathin channel and dielectric.

High mobility silicon indium oxide thin-film transistor fabrication by sputtering process

Amorphous silicon indium oxide (a-ISO) thin-films were deposited by sputtering process at room temperature for thin-film transistor (TFT) active channel applications. The amorphous nature of the deposited ISO thin films was confirmed by structural analysis and the films had an average surface roughness of ∼0.11 nm. The oxygen partial pressure was varied from 0 to 20% to deposit ISO active channel layers which showed a transition from conducting to semiconducting behaviour. The a-ISO TFT deposited with 5% oxygen partial pressure and post annealed at 100 °C exhibits a saturation mobility of 31.7 cm2/V.s, on-off current ratio of 2.3 × 1010, turn-on voltage of −5 V and sub-threshold swing of 0.25 V/dec. Bias stress analysis further confirms the stable operation of the ISO TFT devices. Silicon dopant acts as a strong carrier suppressor and enhances performance of a-ISO TFT devices.

Vacuum-Free Liquid-Metal-Printed 2D Indium-Tin Oxide Thin-Film Transistor for Oxide Inverters.

A cost-effective, vacuum-free, liquid-metal-printed two-dimensional (2D) (∼1.9 nm-thick) tin-doped indium oxide (ITO) thin-film transistor (TFT) was developed at the maximum process temperature of 200 °C. A large-sized 2D-ITO channel layer with an electron density of ∼1.2 × 1019 cm-3 was prepared in an ambient atmosphere. The 2D-ITO-TFT operated in full depletion with a threshold voltage of -2.1 V and demonstrated good TFT device characteristics such as a high saturation mobility of ∼27 cm2 V-1 s-1, a small subthreshold slope of <382 mV decade-1, and a large on/off-current ratio of >109. The TFT device simulation analysis found that the 2D-ITO-TFT performances were controlled by the shallow acceptor-like in-gap defects spreading in the midgap region of over 1.0 eV below the conduction band minimum. Post-thermal annealing tuned the electron density of the 2D-ITO channel and enabled it to produce enhancement and depletion-mode 2D-ITO-TFTs. A full signal swing zero-VGS-load n-type metal-oxide semiconductor (NMOS) inverter composed of depletion-load/enhancement-driver 2D-ITO-TFTs and a complementary inverter with p-channel 2D-SnO-TFT were successfully demonstrated using all 2D-oxide-TFTs.

Flexible, solution-processed, indium oxide (In2O3) thin film transistors (TFT) and circuits for internet-of-things (IoT)

Over the last decade, novel approaches to explore low voltage flexible devices and low power flexible circuits are being widely researched by the scientific community. To realize the true potential of energy thrifty Internet-of-Things (IoT) objects, low power circuits and hence their low-voltage operating devices are a paramount prerequisite, especially when their power is constrained by autonomous energy scavenging. At present, through advanced manufacturing processes, silicon-based semiconductor devices are powering the modern electronics industry. However, processing temperatures are inhibiting them from flexible and printed electronics, as well as being too costly for scalability to the trillions of IoT objects anticipated. Therefore, development of solution-processed metal oxide semiconductors creates huge opportunities for IoT and wearables. Here, flexible solution-processed indium oxide (In2O3) thin film transistors (TFT) and inverter circuits with low operating voltage are reported. The operating voltage of the TFTs is ≤ 3 V with threshold voltage (Vth) 0.82 V, on/off ratio 105 and extracted mobility (μ) in saturation regime is 14.5 cm2/V·s. The gain of the inverter at VDD 1, 2 and 3 V was determined to be 10, 22 and 32 respectively. Furthermore, measured transconductance (gm) and subthreshold swing (S) are found to be 140 μS and 0.22 V/dec, respectively.

Elimination of Oxygen Defects in In-Si-O Film and Thin Film Transistor Performance

Effect of storage in air on silicon doped indium oxide (In-Si-O) thin-film transistors fabricated via solution processing has been investigated. The on/off ratio drastically increased from 101 just after fabrication to 105 on the 10-day storage, whereas the mobility slightly decreased from 1.4 cm2/Vs to 0.38 cm2/Vs. Time constant of aging effect was 3.6 days. The behavior suggests that oxygen defects in In-Si-O films, which may be produced during thermal evaporation of Al electrodes under high vacuum, are eliminated.

Zirconium-aluminum co-doping on solution-processed indium oxide thin film and deceives measured by a novel nondestructive method

Solution-processed co-doped indium oxide (In₂O₃) have broad application prospects in the display industry. In this paper, zirconium-aluminium co-doped indium oxide (InxZryAl1-x-yO) thin films and thin film transistors (TFTs) are prepared by the solution method. Doping ratio of Zr-Al is 1:0, 2:1, 1:1, 1:2 and 0:1, respectively, and the total atomic ratio of Zr-Al doping is 10 at.%. A novel nondestructive method, microwave photoconductivity decay (µ-PCD) is used to evaluate the quality of thin films by simply measuring their response under laser irradiation. The result shows that doping Zr-Al can reduce the defects in In₂O₃ thin films effectively. The addition of Zr can effectively reduce oxygen vacancies, and the addition of Al can inhibit the crystallization of In₂O₃ thin films. The optimal InxZryAl1-x-yO thin film with minimum defects is obtained with doping ratio of Zr:Al = 2:1. The roughness of thin films is below 0.37 nm, and possess exceptional transmittance (>95%) in the visible range. The optimized TFT exhibits mobility of 2.3 cm2µV−1µs−1, an on/off current ratio of 2.0 × 104, a threshold voltage of 2.29 V and a subthreshold swing of 1.61 Vµdec−1 with doping ratio of Zr:Al = 2:1.

The effect of charge transfer transition on the photostability of lanthanide-doped indium oxide thin-film transistors

Amorphous oxide semiconductors are promising for their use in thin-film transistor (TFT) devices due to their high carrier mobility and large-area uniformity. However, their commercialization is limited by the negative gate bias stress experienced under continuous light illumination. Here, we report an approach to improve the negative bias illumination stress (NBIS) stability of amorphous oxide semiconductors TFTs by using lanthanide-doped indium oxide semiconductors as the channel layer. The effect of different lanthanide dopants on performances of solution-processed Ln:In2O3 TFTs are investigated. All lanthanides exhibit strong suppression of oxygen vacancy, which shift the Von from −13.5 V of pure In2O3 TFT to −1~1 V of Ln:In2O3 TFTs (except Ce). However, only Pr:In2O3 and Tb:In2O3 TFTs exhibit much better NBIS stability with same ΔVon of −3.0 V, compared to much higher ΔVon of −7.9~−15.6 V for other Ln:In2O3 TFTs. Our comprehensive study reveals that praseodymium and terbium act as a blue light down-conversion medium with low charge transfer transition energy for lowing photosensitivity of oxide semiconductors.

Switching Properties Improvement of Tungsten-Doped Indium Oxide Phototransistor

High-performance and transparent tungsten-doped indium oxide (InWO) thin-film transistors (TFTs) with aluminum or indium oxide (In2O3) source and drain (S/D) electrodes were fabricated using the radiofrequency (RF) co-sputtering method. Due to the lower electron barrier height of the In2O3/InWO interface, the device with a homojunction structure exhibited better performance (μ eff = 20.1 cm2 V−1 ·s−1 and S.S. = 0.17 V dec−1) than the one with a heterojunction structure. Moreover, due to the high transparency of the S/D electrode, the light-sensing area was magnified. Thus, the responsivity of the phototransistor was significantly improved from 23.68 A W−1 to 89.47 A W−1. This result improved the properties of the interface between the S/D electrode and the channel layer and simplified the manufacturing process for InWO TFTs.

High-k erbium oxide film prepared by sol-gel method for low-voltage thin-film transistor

In this work, high-dielectric-constant (high-k) erbium oxide（Er2O3）film is fabricated using the spin coating method, and annealed at a series of temperatures (from 400 °C to 700 °C). The effect of annealing temperature on the microstructural and electrical properties of Er2O3 nanofilm is investigated. To demonstrate the applicability of the Er2O3 film, the indium oxide (In2O3) thin film transistor (TFT)-based amorphous Er2O3 dielectric film is fabricated at different temperatures. The TFT-based EO-600 shows a low-operating voltage and good electrical properties. The inverter demonstrates that the Er2O3 nanofilm synthesized by the sol-gel method could be a promising candidate as the dielectric layer in a low-voltage electronic device.

Influence of adsorbed oxygen concentration on characteristics of carbon-doped indium oxide thin-film transistors under bias stress

To clarify a factor on the reliability, we investigated the characteristics of carbon-doped indium oxide (InO1.16C0.04) thin-film transistors by varying the O2 concentration from 0.001% to 100% at atmospheric pressure under negative bias stress (NBS) and positive bias stress (PBS). A positive threshold voltage (V th) shift was observed when the bias stress was changed from NBS to PBS. The positive V th shift increased with increasing bias voltage irrespective of the O2 concentration. This behavior was attributed to the reaction between adsorbed O2 molecules on the back side of the InO1.16C0.04 channel and the electrons in the channel being strongly enhanced under PBS. We have found the magnitude and direction of the bias stress play an important role in the positive V th shift under environments included O2 at concentrations as low as 0.001%.