InGaZnO Research Articles

3-Terminal Igzo FET Based 2T0C DRAM Combined Bit-Line Structure

The IGZO (InGaZnO)-based two-transistor zero-capacitor(2T0C) DRAM has attracted much attention as an alternative memory to overcome the scale-down limit of current 1T1C DRAM due to its low power consumption and monolithic 3D stacking capability. In particular, its low power consumption and the feasibility of low temperature process make it highly implementable for 3D DRAM. For operating the 2T0C DRAM, four metal lines are necessary, i.e., write word line and write bit line (WBL) for operating write transistor (WTR), and read word line and read bit line (RBL) for operating read transistor (RTR).In this study, we propose a 3-terminal 2T0C DRAM to enhance its memory characteristics, where RBL was combined with WBL, enabling 2T0C memory operation with only three metal lines as shown in Fig. 1(b). In addition, we investigated the dependency of retenetion time of the proposed 2T0C DRAM on channel length. In particular we evaluated the dependency of the sneak current of 2T0C DRAM on off-current of transistor. For the fabrication of 3-terminal IGZO 2T0C DRAM, a top gate IGZO transistor was fabricated using RF sputtering and two transistors were connected by storage node and bit line bridge as shown in Fig. 1(d).The storage node was formed by connecting the gate of RTR to drain of WTR. And source electrodes of both transistors were coupled. To determine the voltage values for memory operation, the transfer curve of each transistor was characterized and the threshold voltages for both RTR and WTR were around 1 V. The write and read voltages were set to 3 and 2 V, respectively. The retention time was measured by performing periodic read operations after writing and measuring the RTR currents. In addition, the retention time of transistors for varying channel length and gate capacitance was characterized. The retention time was defined as the time to be taken for decreasing the currents by 90 %. Furthermore, the disturbance of 2T0C DRAM with 2x2 array structure was investigated. The RTR currents of selected cells were measured depending on the memory state of adjacent cells.It was found that the retention times of transistors with 10- and 100-um RTR channel lengths were approximately 15 and 30 seconds, respectively. It was observed that the memory state was not changed from “0“ to “1“ by disturbant currents, but, RTR currents of “0“ state cell increased slightly when an adjacent cell was turned on. However, as shown in Fig.1(i) and (j), when off-currents of transistor of 2T0C DRAM decreased from 20 nA to 200 pA, it was confirmed the sneak current of 2T0C decreased. Finally, several technical apporaches how to suppress the disturbant currents will be presented in detail. Acknowledgement This research was supported by BrainKorea21 Four. Figure 1

Impact of Al2O3 spacers on the improvement in short-channel effects for the mesa-shaped vertical-channel In-Ga-Zn-O thin-film transistors with a channel length below 100 nm

To achieve a channel length scaling and an improvement in short-channel effect (SCE) for the mesa-shaped vertical thin-film transistors (VTFTs) using In-Ga-Zn-O (IGZO) channels prepared by atomic layer deposition (ALD), the impacts of two strategic approaches were investigated: the introduction of an ALD Al2O3 spacer and the modulation of ALD oxidants. The Al2O3 spacers were patterned with sound profiles by means of a hybrid-etching technique combining a wet- and plasma dry-etching processes to form vertical sidewalls corresponding to a channel length. For the cases when the Al2O3 spacers were prepared with H2O oxidant, the IGZO VTFT with an In/Ga ratio of 1.7 showed the degradations in device characteristics including a turn-on voltage (VON) of −8.0 V and a subthreshold swing (SS) of 1.50 V/dec. When the In/Ga ratio was increased to 2.9, the device failed to be operated with conductive properties. The excess hydrogen contained within the Al2O3 spacer was suggested to be thermally diffused into the IGZO from the back-channel interface, leading to marked increase in the number of conduction carriers within the channel. To suppress the doping effect of hydrogen from the spacer, the ALD oxidant was strategically modulated to O3. As results, the VON and SS of the IGZO VTFT were markedly improved to be −2.0 V and 0.483 V/dec, respectively. Additionally, the drain induced barrier lowering coefficient was also significantly enhanced from 2.9 to 0.13 V/V, demonstrating the excellent immunity against the SCE for the IGZO VTFTs with nanoscale channel lengths. It was concluded from these findings that the spacer engineering can be one of the most important guides for the implementation of IGZO VTFTs with channel lengths below 100 nm.

Synergism of spray-pyrolyzed aliovalent Ga (iii) ions and ZnO nanostructures for selective sensing of hazardous CO gas towards low ppm levels

Ga-doped ZnO (GZO) nanostructures deposited via low-cost spray pyrolysis process are employed in this study to identify low quantities of carbon monoxide gas. XRD studies reveal the single-phase wurtzite structure of ZnO thin films, thereby confirming the structural stability post-Ga doping. Raman examination discloses the vibrational characteristics and provides some early insights into the existence of defects in the films. The nanograins having well-defined grain boundaries noticed from the inspection of SEM images participate in the gas sensing performance as the site for the active adsorption of CO molecules. The defects, mainly oxygen vacancies responsible for the enhancement in the CO sensing characteristics were confirmed from room temperature photoluminescence (RTPL) spectra and XPS analysis. The CO sensing measurements reveal that 15 wt% GZO exhibited the highest sensing performance of 0.08 towards 1 ppm and 0.41 towards 5 ppm. GZO thin films had response and recovery times that were both extraordinarily quick under 40 s

Influence of Channel Surface with Ozone Annealing and UV Treatment on the Electrical Characteristics of Top-Gate InGaZnO Thin-Film Transistors.

The effect of the channel interface of top-gate InGaZnO (IGZO) thin film transistors (TFTs) on the electrical properties caused by exposure to various wet chemicals such as deionized water, photoresist (PR), and strippers during the photolithography process was studied. Contrary to the good electrical characteristics of TFTs including a protective layer (PL) to avoid interface damage by wet chemical processes, TFTs without PL showed a conductive behavior with a negative threshold voltage shift, in which the ratio of Ga and Zn on the IGZO top surface reduced due to exposure to a stripper. In addition, the wet process in photolithography increased oxygen vacancy and oxygen impurity on the IGZO surface. The photo-patterning process increased donor-like defects in IGZO due to organic contamination on the IGZO surface by PR, making the TFT characteristics more conductive. The introduction of ozone (O3) annealing after photo-patterning and stripping of IGZO reduced the increased defect states on the surface of IGZO due to the wet process and effectively eliminated organic contamination by PR. In particular, by controlling surface oxygens on top of the IGZO surface excessively generated with O3 annealing using UV irradiation of 185 and 254 nm, IGZO TFTs with excellent current-voltage characteristics and reliability could be realized comparable to IGZO TFTs containing PL.

Light-stimulated long-term potentiation behavior enhanced in a HfO2/InGaZnO photonic synapse

Photonic-based neuromorphic systems offer numerous advantages, including low crosstalk, high bandwidth, and low power consumption, making them an ideal choice for ultrafast neuromorphic systems. Although these neuromorphic systems promote the utilization of amorphous oxide semiconductors (AOSs) for promising photonic synapses, precise control of their unique persistent photoconductivity to mimic synaptic plasticity using an external light stimulus has rarely been studied. This study investigates the significant enhancement of the long-term potentiation (LTP) behavior in an InGaZnO (IGZO) photonic synapse through facile HfO2 passivation. In HfO2/IGZO synapses, the HfO2 layer is crucial in enhancing the LTP characteristics. The improvement in the persistent photocurrent behavior of the HfO2/IGZO synapse was attributed to the higher activation energy required for the neutralization of ionized oxygen vacancies, facilitating the LTP behavior. The LTP was dynamically modulated depending on external light pulse conditions. A large number of multilevel states (1024 states) were enabled by consecutive light simulations. Furthermore, HfO2/IGZO synapses exhibit different learning capacities depending on the external stimuli configuration. These findings demonstrate our ability to manipulate synaptic functionality in AOS-based artificial photonic synapses, paving the way for multifunctional photonic synapse circuitry. This advancement holds the potential for realizing an artificial vision system in the future.

Study on effective dry etching of InGaZnO thin films under capacitively coupled plasma-reactive ion etching with a nonfluorine-based etching gas

Dry etching of InGaZnO (IGZO) thin films was performed in a capacitively coupled plasma-reactive ion etching (CCP-RIE) mode using hydrochloride (HCl) and argon (Ar) gases. Dry-etching characteristics and surface properties of IGZO films were then investigated according to gas mixture ratio, radio frequency power, and chamber working pressure. With an optimized ratio of HCl : Ar at 20 : 20 SCCM, the IGZO film showed good etch characteristics with a moderate etch rate of 6.0 nm/min and a low roughness of 0.19 nm. During the etching process of IGZO thin films, nonvolatile by-products such as GaClx, InClx, and ZnClx generated on the surface could be effectively removed by physical bombardment with Ar+ ions. As a result, we successfully conducted dry etching of IGZO thin films in the CCP-RIE system under HCl-Ar gas conditions, which demonstrated dry etching on IGZO without needing a fluorine-based etching gas.

Stable and reliable IGZO resistive switching device with HfAlO x interfacial layer

The performance stability of the resistive switching (RS) is vital for a resistive random-access memory device. Here, by inserting a thin HfAlO x layer between the InGaZnO (IGZO) layer and the bottom Pt electrode, the RS performance in amorphous IGZO memory device is significantly improved. Comparing with a typical metal-insulator-metal structure, the device with HfAlO x layer exhibits lower switching voltages, faster switching speeds, lower switching energy and lower power consumption. As well, the uniformity of switching voltage and resistance state is also improved. Furthermore, the device with HfAlO x layer exhibits long retention time (>104 s at 85 °C) , high on/off ratio and more than 103 cycles of endurance at atmospheric environment. Those substantial improvements in IGZO memory device are attributed to the interface effects with a HfAlO x insertion layer. With such layer, the formation and rupture locations of Ag conductive filaments are better regulated and confined, thus an improved performance stability.

In–Ga–Zn–O memristor with double layers of different oxygen vacancy densities and long-term memory towards neuromorphic applications

An In–Ga–Zn–O (IGZO) memristor with double layers of different oxygen vacancy (VO) densities has been developed, and long-term memory towards neuromorphic applications has been confirmed. The IGZO layer of the higher VO density functions as a pseudo electrode to avoid the Schottky behavior, whereas that of the lower VO density functions as a conductance change layer. The long-term potentiation and long-term depression are observed based on the memristor characteristic by applying pulse voltages, which demonstrates the future possibility towards neuromorphic applications.

P‐1.2: Realization of Visible Detection of InGaZnO Thin‐Film Transistors by Coating (PEA)2PbI4 Light Absorbing Layer

In this work, the visible detection of RF‐sputtered InGaZnO (IGZO) thin‐film transistor (TFT) is realized by spin‐coating a light absorption layer of (PEA) 2 PbI 4 on the top of IGZO TFTs. A large number of carriers flow into the IGZO channel after visible light illuminations, resulting in the negative shift of the transfer curves and thus realizing the visible detection. The IGZO/(PEA)2PbI4 bilayer‐based phototransistors exhibit a photoresponsibility of 63.2 A/W and detectivity of 1.13 × 1014 Jones under the illumination of green light (550 nm).

Propagation characteristics of single and multilayer Ga:ZnO in the epsilon near zero region.

We numerically investigated the propagation characteristics of Ga:ZnO (GZO) thin films embedded in a ZnWO4 background in the epsilon near zero (ENZ) region. We found that, for GZO layer thickness ranging between 2 - 100 nm (∼ 1/600 - 1/12 of ENZ wavelength), such structure supports a novel non-radiating mode with its real part of effective index lower than surrounding refractive index or even less than 1. Such a mode has its dispersion curve lying to the left of the light line in the background region. However, the calculated electromagnetic fields display non-radiating nature contrary to the Berreman mode, because the transverse component of the wave vector is complex, ensuring a decaying field. Furthermore, while the considered structure supports confined and highly lossy TM modes in the ENZ region, no TE mode is supported. Subsequently, we studied the propagation characteristics of a multilayer structure constituting an array of GZO layers in the ZnWO4 matrix considering the modal field's excitation using the end-fire coupling. Such a multilayer structure is analyzed using high-precision rigorous coupled-wave analysis and shows strong polarization selective and resonant absorption/emission, the spectral location and bandwidth of which can be tuned by judiciously selecting the thickness of the GZO layer and other geometrical parameters.

Flexible, Transparent, High Mobility Amorphous In–Ga–Zn-O Thin Film Transistors Fabricated on Textile

To implement wearable textile displays (WTDs), it is essential that unique properties such as physical and chemical properties, aesthetics, and flexibility of the textile be maintained. Therefore, thin film transistors (TFTs) for WTDs should exhibit excellent electrical properties to enable 4K and 8K high resolution and high frame-rate (>120 Hz) displays while maintaining the properties of textiles. All layers of TFTs were formed by vacuum deposition and oxide-based transparent materials; amorphous In–Ga–Zn-O (a-IGZO), indium tin oxide (ITO), and Al2O3 were used. A transparent and flexible electrode with an ITO/Ag/ITO (IAI) structure was optimized and applied to the source/drain and gate. All processes were performed below 150 °C, and the wet-step was eliminated to prevent physical and chemical deformation of the substrate. The flexible, transparent, high mobility a-IGZO TFTs (FTH TFTs) for WTDs were fabricated on glass, polyethylene terephthalate (PET) film, and textile substrates, respectively. Their electrical performance, transparency, and flexibility were then compared. The textile-based TFTs showed transmittance of 72.98%, high mobility of 11.5 cm2/(V s), and an on/off ratio of 9.28 × 107 even in a low temperature process (<150 °C). In addition, the electrical properties of the textile-based TFTs were maintained despite testing entailing 1000 cycles of bending with a tensile strain of 0.8%. In particular, the FTH TFTs for WTDs were able to stably drive a red organic light-emitting diode on textile.

Multiple effects of hydrogen on InGaZnO thin-film transistor and the hydrogenation-resistibility enhancement

As the most common external dopant of amorphous oxide semiconductors (AOSs), the hydrogen (H) exhibits great influences on the performance of AOS thin-film transistors (TFTs) and is almost inevitable during device fabrication. In this work, the multiple effects of H doping on the amorphous InGaZnO (IGZO) TFT were systematically investigated by experimental and theoretical calculation methods, and the hydrogenation-resistibility was further investigated. The electrical performances of the hydrogenated IGZO TFT were determined by the concentrations of H itself and oxygen vacancy (Vo). With increasing H content, it separately serves as defect suppressor, donor defect, donor/acceptor transition state and then acceptor defect, resulting in the improved subthreshold slope (SS), negative shift of threshold voltage (Vth), fluctuation of mobility (μFE), and limitation of on-state current (Ion), respectively. The Vo in IGZO is considered to provide the diffusion channel of H dopant, thus determining the H concentration and the electrical behaviors. Furthermore, the hydrogen-resistibility of IGZO was enhanced by using the fluorine (F) dopant to suppress the intrinsic Vo content and weaken the dissociation energy of H-related bonds. The F-IGZO TFT exhibits excellent stabilities even in the H-rich environment. The findings not only deepen the understanding and manipulation of H doping in AOS devices, but also broaden the application scopes.

Editors’ Choice—Thin Film Transistor Response in the THz Range

Novel metal oxide materials such as InGaZnO (IGZO), ZnO, SnO, and In2O3 and improved fabrication processes dramatically enhanced the achieved and projected thin film transistor (TFT) performance. The record values of the effective field-effect mobility of Metal Oxide TFT (MOTFT) materials have approached 150 cm2/Vs. We report on an improved compact TFT model based on three models: the RPI TFT model, the unified charge control model (UCCM), and the multi-segment TFT compact model. This improved model accounts for a non-exponential slope in the subthreshold regime by introducing a varying subthreshold slope and accounts for non-trivial capacitance dependence on the gate bias, and parasitic impedances. The analysis of the TFT response using this model and the analytical calculations showed that TFTs could have a significant response to impinging THz and sub-THz radiation. Using a complementary inverter and the phase-matched THz signal feeding significantly improves the detection sensitivity.

Impact of Variant Gate Insulator Fabrication Process on Reliability of Dual-Gate InGaZnO Thin-Film Transistors

The effect of hydrogen diffusion in the bilayer bottom gate insulator (BGI) in dual-gate InGaZnO (IGZO) thin-film transistors (TFT) is investigated. It is discovered that hydrogen diffuses from the first deposited GI, migrates toward the second one, and forms a weak chemical bond with a dangling bond at the interface between the GI and the active layer. The stress condition with a positive gate bias at various temperatures makes the weakly bonded hydrogen break, thereby forming defects that can induce electron trapping in the interface and bulk, causing the threshold voltage to increase. With the rising temperature of positive bias temperature stress (PBTS), more broken bonds appear at the interface, which results in more trapped electrons. Through a charge trapping model and data fitting, the parameter that indicates the quality of the film can be extracted and compared. The trend of degradation between threshold voltage, bond breaking, and defect generation is examined.

Highly Sensitive Hybrid Oxide Phototransistors with Photoresponsive Zeolitic‐Imidazolate‐Frameworks for Real‐Time Light Detection

AbstractConventional oxide‐based thin‐film phototransistors (Ph‐TRs) with a hybrid structure with additional photoabsorbers show limitations in real‐time on/off operations without additional input bias. Here, hybrid InGaZnO (IGZO) Ph‐TRs utilizing zeolitic‐imidazolate‐framework‐67 (ZIF‐67), an electrically insulating material with a unique metal‐to‐metal charge transfer (MMCT) pathway enabling charge migration, are presented. A wide photodetection range and fast response/recovery speed without additional gate bias are achieved. In particular, ZIF‐67 used as a photoabsorber exhibits absorption of visible light over a wide range of wavelengths. Photoexcited electrons migrate through the MMCT pathway between Co ions and migrate to the IGZO conduction band, causing a significant change in the photocurrent (Iph) of the hybrid ZIF‐67/IGZO Ph‐TR. In the dark, ZIF‐67 immediately returns to the insulating state, resulting in a fast rise/fall time under dynamic photo‐stimulus. In addition, it exhibits repeatable photo‐sensing performance without persistent photocurrent behavior under pulsed light‐on/off cycles. A high photosensitivity of 108 for red light (592 nm, 0.1 mW cm–2) is obtained as it suppresses the dark current and amplifies photocurrent generation in the subthreshold gate voltage region. This combination of fast response time, high photosensitivity, and high stability is promising for ideal Ph‐TRs, enabling real‐time detection even for high‐frequency operations.

Effects of gate roughness on low voltage InGaZnO thin-film transistors with ultra-thin anodized AlxOy dielectrics

Low-voltage oxide semiconductors thin-film transistors (TFTs) with ultra-thin dielectrics are gaining attention in wearable electronics. However, it is a challenge for oxide semiconductor TFTs to operate at a low-voltage while maintaining high performance. In this article, ultra-thin Al x O y films (∼3 nm) are grown on aluminum (Al) electrodes with different surface roughness by anodization. The morphology and electrical properties of the anodized Al x O y films are studied. Furthermore, InGaZnO (IGZO) TFTs with the anodized AlxOy dielectrics are fabricated. It is revealed that the rougher Al gate electrode deposition resulted in a higher interface trap density, which lead to the degradation of device performance. Through optimizing the surface roughness of the initial Al gate electrodes that are used for anodization, the IGZO TFTs can operate at 1 V and show desirable properties including a reasonable saturation mobility of 5.5 cm2 V−1s−1, a low threshold voltage of 0.37 V, a small subthreshold swing of 79 mV decade−1, and a high current on-off ratio of over 106. This work shows the potential of using anodization in the future for low-power wearable electronics.

Improvement in current drivability and stability in nanoscale vertical channel thin-film transistors via band-gap engineering in In–Ga–Zn–O bilayer channel configuration

Vertical channel thin film transistors (VTFTs) have been expected to be exploited as one of the promising three-dimensional devices demanding a higher integration density owing to their structural advantages such as small device footprints. However, the VTFTs have suffered from the back-channel effects induced by the pattering process of vertical sidewalls, which critically deteriorate the device reliability. Therefore, to reduce the detrimental back-channel effects has been one of the most urgent issues for enhancing the device performance of VTFTs. Here we show a novel strategy to introduce an In–Ga–Zn–O (IGZO) bilayer channel configuration, which was prepared by atomic-layer deposition (ALD), in terms of structural and electrical passivation against the back-channel effects. Two-dimensional electron gas was effectively employed for improving the operational reliability of the VTFTs by inducing strong confinement of conduction electrons at heterojunction interfaces. The IGZO bilayer channel structure was composed of 3 nm-thick In-rich prompt (In/Ga = 4.1) and 12 nm-thick prime (In/Ga = 0.7) layers. The VTFTs using bilayer IGZO channel showed high on/off ratio (4.8 × 109), low SS value (180 mV dec−1), and high current drivability (13.6 μA μm−1). Interestingly, the strategic employment of bilayer channel configurations has secured excellent device operational stability representing the immunity against the bias-dependent hysteretic drain current and the threshold voltage instability of the fabricated VTFTs. Moreover, the threshold voltage shifts of the VTFTs could be suppressed from +5.3 to +2.6 V under a gate bias stress of +3 MV cm−1 for 104 s at 60 °C, when the single layer channel was replaced with the bilayer channel. As a result, ALD IGZO bilayer configuration could be suggested as a useful strategy to improve the device characteristics and operational reliability of VTFTs.

Improved Electron Transport Properties of Zn-Rich In–Ga–Zn–O Thin-Film Transistors

Amorphous transparent oxide semiconductor InGaZnO4 (IGZO)-based thin-film transistors (TFTs) have been practically used as the backplane of flat panel displays. For future higher-definition displays, alternative active materials with a higher field effect mobility (μFE) are necessary. Although there are a few reports on InGaO3(ZnO)m with Zn-rich composition (IGZOm)-based TFTs, their electron transport properties have not been clarified. Here, we show that a Zn-rich composition enhances the electron transport properties of IGZOm-TFTs. The best TFT performance was obtained for m = 7 (μFE ∼12 cm2 V–1 s–1, subthreshold swing ∼0.1 V decade–1, and a negligibly small bias stress shift). The carrier effective mass (m*) of IGZOm films was found to be 0.16 m0, independent of the m-value. We found that μFE of IGZOm-TFT increased with the m-value for m ≤ 7, whereas it decreased for m > 7 due to the crystallization. The thermopower modulation analyses revealed that the effective channel thickness increased with m (m ≤ 7), which resulted in a longer carrier relaxation time. The present results provide an improving strategy toward new material design for next-generation TFTs with higher μFE values.

Short- and Long-Term Memory Based on a Floating-Gate IGZO Synaptic Transistor

Short- and long-term neuroplasticity behaviors are key mechanisms for various activities. In this paper, we propose a synaptic transistor with a floating-gate (FG) node and an amorphous InGaZnO (IGZO) channel layer. The proposed device emulates the neuroplasticity functions of both short-term memory (STM) and long-term memory (LTM) through the control of the amplitude and the number of input pulses. The STM operated by ion movement in the gate dielectrics occurs when the input amplitude is relatively small (< 9 V). The LTM operated through the storage of electrons in the FG occurs when the input amplitude is relatively large (>10 V). In addition, as the number of input pulses increases, information is stored for a longer time. Our FG IGZO synaptic transistor could be a promising device solution for brain-inspired computing systems.

High gain complementary inverters based on comparably-sized IGZO and DNTT source-gated transistors

Complementary inverters using IGZO n-channel and DNTT p-channel source-gated transistors are demonstrated for the first time. They exhibit gain of 368 V V−1, 94% noise margin and matching on-current for relatively similar widths.