a-IGZO Thin-film Transistors Research Articles

Stress Release Effect of Micro-hole Arrays for Flexible Electrodes and Thin Film Transistors.

The effects of micro-hole arrays in the thin metal films were studied as a method to release bending stress in flexible electrodes and flexible thin film transistors (TFTs). Interest in flexible electronics is increasing, and many approaches have been suggested to solve the issue of the electrical failure of electrodes or electrical components such as TFTs after repeated bending. Here, we demonstrate a micro-hole array structure as a common solution to release bending stress. Although micro-size cracks were generated and propagated from the hole edges, the cracks stopped within a certain range when enough stress was released. Moreover, since the crack sites were predictable and controllable, a fatal electrical breakdown in a conductive layer such as a metal electrode or the semiconducting junction of a TFT can be prevented by specifically arranging the hole arrays. Thin film layers fabricated without holes suffered an electrical breakdown due to random crack propagation during bending tests. Aluminum thin film electrodes prepared with arrays of 3 μm diameter holes and 25% hole area showed excellent durability after 300,000 bending cycles. The change in resistance was below 3%. The electrical characteristics of an a-IGZO TFT with the micro-hole structure were almost equivalent to a standard a-IGZO TFT. After 10,000 bending cycles, ION and the ratio of ION/IOFF remained >107 A and ∼107, respectively. Since the effective hole diameter is micrometer in size, fabrication does not require additional process steps or expensive process equipment. Therefore, the approach can be an important way to enhance the reliability of various electrical devices in flexible and wearable applications.

Top-Gate Amorphous Indium-Gallium-Zinc-OxideThin-Film Transistors With Magnesium Metallized Source/Drain Regions

Magnesium (Mg)-induced metallization of amorphous indium-gallium-zinc-oxide (a-IGZO) films is investigated to develop a self-aligned (SA) top-gate (SATG) a-IGZO thin-film transistor (TFT) technology. The high-conductive and SA a-IGZO source/drain (S/D) regions are well realized by a short time of sputtered deposition of Mg onto the a-IGZO film on a heated substrate, followed by the removal of the spare Mg on the surface in hot water. It is shown that the resistivity of the a-IGZO films is lowered to about $4 \times 10^{-{3}}\,\,\Omega $ cm from over $10^{{4}}~\Omega $ cm with a 36-s Mg deposition at $300~^{\circ }\text{C}$ . The metallization effect is believed to be the consequence of a large number of donor-like defects (oxygen vacancies) generated by oxidation–reduction reaction at the interface between the Mg and a-IGZO films. The SATG a-IGZO TFTs fabricated by the proposed technology show excellent electrical characteristics, such as a field-effect mobility of 19.5 cm $^{{2}}\text{V}^{-{1}}\text{s}^{-{1}}$ , a subthreshold swing of 0.19 V/dec, an ON-/ OFF-current ratio of over $10^{{9}}$ , a low S/D series resistance of $2.1~\Omega $ cm, a small channel length shrinking of around $0.1~\mu \text{m}$ , and a high stability against electrical stresses.

Effect of Sputtering Working Pressure on the Optical and Electrical Properties of InZnO Thin-Film Transistors

Amorphous In-Ga-Zn-O (a-IGZO) thin film transistors, because of their relatively low mobility, have limits in attempts to fulfill high-end specifications for display backplanes. In-Zn-O (IZO) is a promising semiconductor material for high mobility device applications with excellent transparency to visible light region and low temperature process capability. In this paper, the effects of working pressure on the physical and electrical properties of IZO films and thin film transistors are investigated. The working pressure is modulated from 2 mTorr to 5 mTorr, whereas the other process conditions are fixed. As the working pressure increases, the extracted optical band gap of IZO films gradually decreases. Absorption coefficient spectra indicate that subgap states increase at high working pressure. Furthermore, IZO film fabricated at low working pressure shows smoother surface morphology. As a result, IZO thin film transistors with optimum conditions exhibit excellent switching characteristics with high mobility (≥ 30cm2/Vs) and large on/off ratio.

Study of Positive-Gate-Bias-Induced Hump Phenomenon in Amorphous Indium–Gallium–Zinc Oxide Thin-Film Transistors

A positive-gate-bias stress (PBS)-induced hump phenomenon in staggered bottom-gate amorphous indium–gallium–zinc oxide (a-IGZO) thin-film transistors (TFTs) is observed, and the possible reason is discussed. It is found that the hump phenomenon is closely related to device layouts. In conventional metal-over-active (MOA) structure, the hump current is independent of the channel width. However, for the active-over-metal (AOM) structure, the hump current disappears when the width of active layer exceeds a critical value. These results indicate that the hump current caused by PBS comes from parasitic edge transistors rather than back-channel transistors. Furthermore, the possible origin of channel edge conduction mechanism is checked by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) techniques. From AFM image, we notice that there is a long gentle slope at edge of active layer, which will enhance the electrical filed at local area and accelerate the device performance degradation under PBS condition. XPS measurements before and after PBS on devices at channel edge confirm our assumption, that is, the oxygen vacancy ( $\text{O}_{\mathrm {V}}$ ) content increases from 29.33% to 36.89% after PBS. This implies that the carrier concentration in channel edge is enhanced resulting in lower turn-on voltage of parasitic edge transistors, which, in turn, causes hump phenomenon.

Mechanochemical and Thermal Treatment for Surface Functionalization to Reduce the Activation Temperature of In-Ga-Zn-O Thin-film Transistors.

Amorphous indium-gallium-zinc oxide (a-IGZO) films, which are widely regarded as a promising material for the channel layer in thin-film transistors (TFTs), require a relatively high thermal annealing temperature to achieve switching characteristics through the formation of metal-oxygen (M-O) bonding (i.e., the activation process). The activation process is usually carried out at a temperature above 300 °C; however, achieving activation at lower temperatures is essential for realizing flexible display technologies. Here, a facile, low-cost, and novel technique using cellophane tape for the activation of a-IGZO films at a low annealing temperature is reported. In terms of mechanochemistry, mechanical pulling of the cellophane tape induces reactive radicals on the a-IGZO film surface, which can give rise to improvements in the properties of the a-IGZO films, leading to an increase in the number of M-O bonds and the carrier concentration via radical reactions, even at 200 °C. As a result, the a-IGZO TFTs, compared to conventionally annealed a-IGZO TFTs, exhibited improved electrical performances, such as mobility, on/off current ratio, and threshold voltage shift (under positive bias temperature and negative bias temperature stress for 10,000 s at 50 °C) from 8.25 to 12.81 cm2/(V·s), 2.85 × 107 to 1.21 × 108, 6.81 to 3.24 V, and -6.68 to -4.93 V, respectively.

Effects of hydrogen plasma treatment on the electrical performances and reliability of InGaZnO thin-film transistors

In this work, we have investigated the effects of hydrogen (H) plasma treatment on the electrical performances and reliability of amorphous InGaZnO (a-IGZO) thin film transistors (TFTs). By optimizing the H plasma treatment time, the a-IGZO TFT with H plasma treatment time of 1 min exhibits excellent electrical performances and reliability, such as high field-effect mobility (μFE) of 26.5 cm2/V s and small threshold voltage shifts (ΔVth) value of 2.5 V and −2.3 V under positive gate bias stress (PBS) and negative gate bias stress (NBS) measurements without any passivation layers. The increased performance relies that the H plasma treatment not only could increase the carrier concentration (Ne) but also reduce the surface defects of channel layer and interface trap density of a-IGZO TFTs. The X-ray photo-electron spectroscopy measurement reveals that the H treatment passivated the oxygen vacancy (VO) defects and formed center-bonded complex states (HO or VO-H) in the a-IGZO films, which act as a shallow donor in a-IGZO films. Therefore, the high performances and excellent reliability of a-IGZO TFTs is suggesting that H is a very promising treatment for TFTs to be used for flexible thin film electronic applications.

An Active Multielectrode Array for Collecting Surface Electromyogram Signals Using a-IGZO TFT Technology on Polyimide Substrate

The surface electromyogram (sEMG) signals produced by living bodies play a significant role in biological applications. However, an active multielectrode array (MEA) having a high spatiotemporal resolution, flexibility, and high gain is still an enormous challenge for accurate sEMG detection. Here, a flexible active MEA based on the diode-connected load inverter is proposed and fabricated for collecting sEMG signals using amorphous indium–gallium–zinc oxide (a-IGZO) thin-film transistors (TFTs) technology on the polyimide (PI) substrate. To obtain the good gain effect and low-noise active MEA, the width–length ratio of the transistors and the suitable impedances are systemically designed. Moreover, the optimized circuit exhibits good bias stability, excellent uniformity in electrical performance, and outstanding mechanical durability, which can ensure reliable sEMG detection. With the combination of the a-IGZO TFTs, the active MEA has an approximate 15-dB signal gain under a low driving voltage of 5 V. Measuring the sEMG of the muscle groups below the thumb with our developed flexible active MEA, the amplitude value of sEMG signals is successfully amplified and the noise fluctuation is obviously reduced. Thus, the active MEA has certain advantages in large-area preparation, stability, flexibility, and high gain, which opens a new avenue for the accurate sEMG signal measurements.

Influence of active layer thickness on the cut-off frequency of a-IGZO thin film transistors

Design optimizations like channel length and gate/contact overlap length scaling are generally used for cut-off frequency (fT ) enhancement of thin film transistors (TFT). But channel length scaling leads to mobility degradation due to contact resistance (RC ). RC in TFTs is directly proportional to the transfer length (LT ), and is inversely proportional to the overlap length (LOV ). LT depends upon the metal/semiconductor interfacial resistivity, the vertical bulk resistance and the semiconductor sheet resistance. For the same interfacial contact properties, active layer thickness (Ta ) scaling can be effectively utilized for lowering the vertical bulk resistance thereby lowering LT and enhancing fT . To study the effect of thickness scaling on the bandwidth of TFT based amplifiers, a-IGZO thin film transistor of effective channel thickness of 70nm are first fabricated and characterized. Density of states model of this TFT is extracted and numerical simulations are used to study the influence of active layer thickness scaling. By varying the channel lengths from 10μm to 100μm, transfer line method of contact resistance estimation, is used to extract LT and RC of devices. As the active layer thickness is varied from 70μm to 30μm it is found that LT decreases from 1μm to 60 nm, due to lower vertical bulk resistance. This results in a lower RC and higher μeff, facilitating further scaling of LOV , leading to enhanced fT of the device.

Bias stress and humidity exposure of amorphous InGaZnO thin-film transistors with atomic layer deposited Al2O3 passivation using dimethylaluminum hydride at 200 °C

Highly reliable amorphous InGaZnO (a-IGZO) (In:Ga:Zn:O = 2:2:1:7) thin-film transistors (TFTs) were fabricated with 25 nm-thick Al2O3 passivation deposited by atomic layer deposition (ALD) using dimethylaluminum hydride (DMAH). Al2O3 passivation deposited at various temperatures was studied to determine its effect on TFT behavior following gate bias stress and humidity exposure. The Al2O3-passivated a-IGZO TFTs demonstrated an on-off current ratio (Ion/Ioff) of ~108 and linear mobility (µ) of 9 to 13 cm2 V−1 s−1. An optimum ALD temperature of 200 °C was demonstrated to result in Al2O3-passivated a-IGZO TFTs with very small on-voltage shifts of 0.3 V and −2.7 V against positive bias stress and negative bias illumination stress (NBIS) after 10 000 s of stress time. Furthermore, negligible degradation was observed after humidity exposure. The results of x-ray photoelectron spectroscopic analysis of the O 1s spectra showed the highest peak area ratio for metal–oxygen (M–O) bonding and lowest metal–oxygen vacancy (M–Vo) bonding, relating to the least amount of trap sites both in the bulk region of a-IGZO and near the interface of Al2O3/a-IGZO, for the device passivated at 200 °C. It was also found that at a low deposition temperature (⩽100 °C), low-density Al2O3 was formed with high carbon contamination and hydrogen while at a high deposition temperature (⩾300 °C), high hydrogen concentration was also present in high-density Al2O3. The hydrogen incorporation from Al2O3 passivation to a-IGZO channel creates bulk defects and trap sites as well as in the interface of a-IGZO and SiO2 gate insulator. At 200 °C, high quality Al2O3 passivation was achieved with high density and reduced impurities leading to improved device reliability. These results can be applied to long term sustained device operations which are reliable against different stresses.

Verification of microwave annealing ambient effect on sol-gel-processed amorphous In–Ga–Zn–O thin-film transistors and application to inverter circuit

In this study, we conducted microwave annealing (MWA) on amorphous In–Ga–Zn–O (a-IGZO) thin films deposited by the sol-gel method, and then investigated the effects of ambient conditions on the electrical properties and reliability of a-IGZO thin-film transistors (TFTs) and on the optical and chemical properties of a-IGZO thin films. On the other hand, pure N2 ambient-treated TFTs exhibited the poorest transfer and output characteristics, but improved with increasing O2 mixing ratio. In terms of stability, pure O2 ambient-treated a-IGZO TFTs were excellent for both positive and negative gate bias stresses, but deteriorated with increasing N2 mixing ratio. Also, static and dynamic tests of resistor-loaded inverters were carried out. As a result, the pure O2 ambient-treated inverter showed excellent characteristics, while the pure N2 ambient-treated inverter had poor characteristics. Meanwhile, pure O2 ambient MWA has excellent carbon removal ability in sol-gel-processed a-IGZO films, lowering oxygen vacancies and impurities. Besides, it has an effect of increasing energy band gap, improving transmittance, increasing Fermi level and electron concentration, and contributing to improvement of mobility. Therefore, it was verified that MWA in pure O2 ambient is very beneficial to improve the optical and chemical properties of the sol-gel-processed a-IGZO thin film and the electrical properties of TFTs.

High-performance extended-gate ion-sensitive field-effect transistors with multi-gate structure for transparent, flexible, and wearable biosensors

ABSTRACT In this study, we developed a high-performance extended-gate ion-sensitive field-effect transistor (EG-ISFET) sensor on a flexible polyethylene naphthalate (PEN) substrate. The EG-ISFET sensor comprises a tin dioxide (SnO2) extended gate, which acts as a detector, and an amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistor (TFT) for a transducer. In order to self-amplify the sensitivity of the pH sensors, we designed a uniquely-structured a-IGZO TFT transducer with a high-k engineered top gate insulator consisting of a silicon dioxide/tantalum pentoxide (SiO2/Ta2O5) stack, a floating layer under the channel, and a control gate coplanar with the channel. The SiO2/Ta2O5 stacked top gate insulator and in-plane control gate significantly contribute to capacitive coupling, enabling the amplification of sensitivity to be enlarged compared to conventional dual-gate transducers. For a pH sensing method suitable for EG-ISFET sensors, we propose an in-plane control gate (IG) sensing mode, instead of conventional single-gate (SG) or dual-gate (DG) sensing modes. As a result, a pH sensitivity of 2364 mV/pH was achieved at room temperature – this is significantly superior to the Nernstian limit (59.15 mV/pH at room temperature). In addition, we found that non-ideal behavior was improved in hysteresis and drift measurements. Therefore, the proposed transparent EGISFFET sensor with an IG sensing mode is expected to become a promising platform for flexible and wearable biosensing applications.

A High-Gain CMOS Operational Amplifier Using Low-Temperature Poly-Si Oxide TFTs

A high-performance CMOS operational amplifier (op-amp) has been demonstrated with the low-temperature poly-Si oxide (LTPO) thin-film transistors (TFTs). The p-type TFT exhibits the maximum field-effect mobility of 80 cm2/V.s and subthreshold swing (SS) of 0.7 V/decade. The n-type amorphous indium–gallium–zinc oxide (a-IGZO) TFT exhibits the SS of 0.3 V/decade and the mobility of 11.5 cm2/V.s. The op-amp consisting of 13 TFTs functions in two stages. It exhibits a high voltage gain of 50.7 dB, a cutoff frequency of 200 kHz, a unit gain frequency of 7 MHz, and a very high gain-bandwidth product of 68.5 MHz. The first LTPO CMOS op-amp with low-power consumption of 0.6 mW demonstrates the potentiality of this emerging technology.

Defect gradient control in amorphous InGaZnO for high-performance thin-film transistors

The accuracy control of oxygen-correlated defect densities in the lattice of metal oxide semiconductors is one of the remaining issues restricting the performance of metal oxide-based devices and their commercialization process. This study has successfully realized the gradient control of defects in amorphous InGaZnO through various strategies. X-ray photoelectron spectroscopy results reveal that the 350 °C thermal annealing accompanied with 300 °C pre-treatment is more conducive to improve the quality of a-IGZO and suppress the density of oxygen vacancy defects. Correspondingly, the initial electrical properties of a-IGZO thin-film transistor treated by gradient annealing procedures is superior to those of the devices with one-step 350 °C routine annealing or combined with an intermediate N2O plasma treatment. It is demonstrated that the gradient annealing procedures effectively improve the quality of a-IGZO bulk and its adjacent interfaces. More importantly, the notorious instability induced by the negative bias and illumination stress is significantly suppressed.

Numerical Analysis on Effective Mass and Traps Density Dependence of Electrical Characteristics of a-IGZO Thin-Film Transistors

We have investigated the effect of electron effective mass (me*) and tail acceptor-like edge traps density (NTA) on the electrical characteristics of amorphous-InGaZnO (a-IGZO) thin-film transistors (TFTs) through numerical simulation. To examine the credibility of our simulation, we found that by adjusting me* to 0.34 of the free electron mass (mo), we can preferentially derive the experimentally obtained electrical properties of conventional a-IGZO TFTs through our simulation. Our initial simulation considered the effect of me* on the electrical characteristics independent of NTA. We varied the me* value while not changing the other variables related to traps density not dependent on it. As me* was incremented to 0.44 mo, the field-effect mobility (µfe) and the on-state current (Ion) decreased due to the higher sub-gap scattering based on electron capture behavior. However, the threshold voltage (Vth) was not significantly changed due to fixed effective acceptor-like traps (NTA). In reality, since the magnitude of NTA was affected by the magnitude of me*, we controlled me* together with NTA value as a secondary simulation. As the magnitude of both me* and NTA increased, µfe and Ion deceased showing the same phenomena as the first simulation. The magnitude of Vth was higher when compared to the first simulation due to the lower conductivity in the channel. In this regard, our simulation methods showed that controlling me* and NTA simultaneously would be expected to predict and optimize the electrical characteristics of a-IGZO TFTs more precisely.

Effects of X-ray Irradiation on the Noise Behavior of Amorphous Indium-Gallium-Zinc-Oxide TFTs

In this paper, the response of low-frequency noise under and after the X-ray irradiation for amorphous indium–gallium–zinc-oxide (a-IGZO) thin film transistors (TFTs) is analyzed. With fixed irradiation dose, the noise stays the same regardless of the TFT under and after X-ray irradiation. The noise mechanism is analyzed to be mainly the carrier mobility fluctuation. The coincidence of the noise of TFTs under and after X-ray irradiation further confirms that there is no direct relation between X-ray irradiation and the noise behavior. The mechanism of the positive charge trapping in the oxide layer under X-ray for the threshold voltage (Vth) shift of the TFT does not affect its noise behavior. With more stable and larger SNR by 20 dB than low-temperature polycrystalline-silicon (LTPS) TFT, a-IGZO TFT is believed to be a more suitable device for applying to X-ray active pixel sensing circuits.

High Performance Amorphous In–Ga–Zn–O Thin-Film Transistors with Low Temperature High-k Solution Processed Hybrid Gate Insulator

For flexible display applications, high performance amorphous In–Ga–Zn–O (a-IGZO) thin-film transistors (TFTs) fabricated by low temperature process are required. Moreover, high dielectric constant (high-k) material which enables higher mobility and lower threshold voltage while minimizing the device size are excellent candidates to replace SiO2 gate insulators in conventional oxide semiconductor TFTs. Therefore, nanocomposite material containing high-k BaTiO3 nanoparticles and transparent poly-siloxane (PSX) hybrid polymer were deposited as gate insulator in a-IGZO TFTs to facilitate the reduction of process temperature due to the high temperature requirement (>650 ºC) of BaTiO3. This paper presents the performance and analysis of a-IGZO TFTs with high-hybrid gate insulators at different ratios of BaTiO3/PSX deposited by solution process at a lower temperature (300 ºC). The TFT shows a high mobility of 35.59 cm2 V−1 s−1, low threshold voltage of 0.4 V, and gate leakage current of ∼10−10 A.

Hydrogen behavior under X-ray irradiation for a-IGZO thin film transistors

We studied hydrogen (H) behavior in amorphous In-Ga-Zn-O (a-IGZO) films under X-ray irradiation by evaluating the threshold voltage (VTH) shift in a-IGZO thin film transistors (TFTs) with different H concentrations in the active layers. We fabricated three types of a-IGZO TFTs: (i) one without a buffer layer and postannealed in N2, (ii) one with a H-resolved buffer layer and postannealed in N2, and (iii) one with a H-resolved buffer layer and postannealed in a mixture of N2 and H2. All three TFTs showed a negative VTH shift after 100 Gy of X-ray exposure. The degree of VTH shift correlated with an increase in conductivity, which, in turn, corresponds to the H concentration in the active layer of the as-fabricated TFTs. Based on spectroscopic ellipsometry analysis, we confirmed a large increase in the donorlike H related D1 state after X-ray irradiation in high-H concentration a-IGZO films. In addition, an increase in the number of H2 molecules in a-IGZO films after X-ray irradiation was observed via thermal desorption spectroscopy analysis. Therefore, we conclude that the increase in conductivity and/or the resulting negative VTH shift in a-IGZO TFTs during X-ray irradiation can be attributed not only to the state transition from acceptorlike to donorlike H in the as-prepared a-IGZO but also to the incorporation of additional H radicals generated by X-ray irradiation.

Rail-to-Rail Timing Signals Generation Using InGaZnO TFTs For Flexible X-Ray Detector

This paper reports on-chip rail-to-rail timing signals generation thin-film circuits for the first time. These circuits, based on a-IGZO thin-film transistors (TFTs) with a simple staggered bottom gate structure, allow row and column selection of a sensor matrix embedded in a flexible radiation sensing system. They include on-chip clock generator (ring oscillator), column selector (shift register) and row-selector (a frequency divider and a shift register). They are realised with rail-to-rail logic gates with level-shifting ability that can perform inversion and NAND logic operations. These logic gates are capable of providing full output swing between supply rails, V DD and V SS , by introducing a single additional switch for each input in bootstrapping logic gates. These circuits were characterised under normal ambient atmosphere and show an improved performance compared to the conventional logic gates with diode connected load and pseudo CMOS counterparts. By using these high-performance logic gates, a complete rail-to-rail frequency divider is presented from measurements using D-Flip Flop. In order to realize a complete compact system, an on-chip ring oscillator (output clock frequency around 1 kHz) and a shift register are also presented from simulations, where these circuits show a power consumption of 1.5 mW and 0.82 mW at a supply voltage of 8 V, respectively. While the circuit concepts described here were designed for an X-ray sensing system, they can be readily expanded to other domains where flexible on-chip timing signal generation is required, such as, smart packaging, biomedical wearable devices and RFIDs.

Parameter Extraction and Compact Modeling of 1/f Noise for Amorphous ESL IGZO TFTs

In this paper, the flicker noise properties of bottom-gate ESL structured amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) from two different technologies have been studied and modeled. Model development is carried out by adapting the Unified Model parameter Extraction Method (UMEM), developed for parameter extraction and compact modeling of TFT devices, and the unified 1/f noise model. Furthermore, comparative study of device figures of merits is performed to point out the device performance enhancements that the different fabrication technologies have brought about. It is found out that both devices have low deep state density, whereas the second device demonstrated higher performance in terms of carrier mobility and subthreshold slope. Both the DC and noise models are validated by matching them against experimental data obtained in different regimes of device operation.

3.93-MHz/328-μW Dynamic Frequency Divider in Flexible a-IGZO TFT Technology

The implementation of a dynamic frequency divider in a fully flexible amorphous indium–gallium–zinc–oxide (a-IGZO) thin-film transistor (TFT) technology on a sub-15 $\mu \text{m}$ polyimide substrate is presented. This frequency divider is regenerative and is also known as a Miller divider. In this letter, it is implemented using only a Gilbert cell with minimum-size LO transistors. Including the bias network, it has only eight transistors. Using a 6-V supply voltage, it operates up to 3.93 MHz, consumes 328 $\mu \text{W}$ , and has a speed over power Figure-of-Merit (FOM) of 12.0 MHz/mW. To the best of our knowledge, this FOM is the highest reported for circuits in this class of flexible TFT technologies.