Ga Zn Research Articles

Implementation of sub-100 nm vertical channel-all-around (CAA) thin-film transistor using thermal atomic layer deposited IGZO channel

In–Ga–Zn–O (IGZO) channel based thin-film transistors (TFT), which exhibit high on–off current ratio and relatively high mobility, has been widely researched due to its back end of line (BEOL)-compatible potential for the next generation dynamic random access memory (DRAM) application. In this work, thermal atomic layer deposition (TALD) indium gallium zinc oxide (IGZO) technology was explored. It was found that the atomic composition and the physical properties of the IGZO films can be modulated by changing the sub-cycles number during atomic layer deposition (ALD) process. In addition, thin-film transistors (TFTs) with vertical channel-all-around (CAA) structure were realized to explore the influence of different IGZO films as channel layers on the performance of transistors. Our research demonstrates that TALD is crucial for high density integration technology, and the proposed vertical IGZO CAA-TFT provides a feasible path to break through the technical problems for the continuous scale of electronic equipment.

Improved high voltage operation of amorphous In–Ga–Zn–O thin film transistor by carrier density enhancement

We report on improved high voltage operation of amorphous-In–Ga–Zn–O (a-IGZO) thin film transistors (TFTs) by increasing carrier density and distributing the high bias field over the length of the device which utilizes an off-set drain structure. By decreasing the O2 partial pressure during sputter deposition of IGZO, the channel carrier density of the high voltage a-IGZO TFT (HiVIT) was increased to ∼1018 cm−3. Which reduced channel resistance and therefore the voltage drop in the ungated offset region during the on-state. To further decrease the electric field in the offset region, we applied Ta capping and subsequent oxidation to locally increase the oxygen vacancy levels in the offset region thereby increasing local carrier density. The reduction of the drain field in the offset region from 1.90 V μm−1 to 1.46 V μm−1 at 200 V drain voltage, significantly improved the operational stability of the device by reducing high field degradation. At an extreme drain voltage of 500 V, the device showed an off-state current of ∼10−11 A and on-state current of ∼1.59 mA demonstrating that with further enhancements the HiVIT may be applicable to thin-film form, low leakage, high voltage control applications.

Analysis on charge-retention characteristics of sub-threshold synaptic IGZO thin-film transistors with defective gate oxides

We provide a quantitative analysis on the charge-retention characteristics of sub-threshold operating In–Ga–Zn–O (IGZO) thin-film transistors (TFTs) with a defective gate-oxide for low-power synaptic applications. Here, a defective SiO2 is incorporated as the synaptic gate-oxide in the fabricated IGZO TFTs, where a defect is physically playing the role as an electron trap. With this synaptic TFT, positive programming pulses for the electron trapping are applied to the gate electrode, followed by monitoring the retention characteristics as a function of time. And this set of the programming and retention-monitoring experiments is repeated in several times for accumulating effects of pre-synaptic stimulations. Due to these accumulated stimulations, electrons are expected to be getting occupied within a deeper trap-state with a higher activation energy, which can lead to a longer retention. To verify these phenomena, a stretched exponential function and respective inverse Laplace transform are employed to precisely estimate a retention time and trap activation-energy for transient experimental results.

Reliable measurement of the density of states including occupied in-gap states of an amorphous In–Ga–Zn–O thin film via photoemission spectroscopies: Direct observation of light-induced in-gap states

Illumination stress (IS) and negative bias under illumination stress (NBIS) cause considerable device instability in thin-film transistors based on amorphous In–Ga–Zn–O (a-IGZO). Models using in-gap states are suggested to explain device instability. Therefore, to provide reliably their density of states (DOS), this study investigated the valence band, conduction band, and in-gap states of an a-IGZO thin film. The DOS of in-gap states was directly determined in a dynamic range of six orders of magnitude through constant final state yield spectroscopy (CFS-YS) using low-energy and low-flux photons. Furthermore, light irradiation irreversibly induced extra in-gap states near the Fermi level and shifted the Fermi level to the vacuum level side, which should be related to the device instability due to IS and NBIS. Hard x-ray photoemission spectroscopy and ultraviolet photoemission spectroscopy using synchrotron radiation observed the large DOS of in-gap states near the Fermi level as in previous works. Here, we reveal that they are not intrinsic electronic states of undamaged a-IGZO, but induced by the intense measurement light of synchrotron radiation. This study demonstrates that CFS-YS is useful for determining the reliable DOS of the in-gap states for samples that are sensitive to light irradiation. The absorption spectrum measured through photothermal deflection spectroscopy is interpreted based on DOS directly determined via photoemission spectroscopies. This indicates that the line shape in the energy region below the region assigned to the Urbach tail in previous works actually roughly reflects the DOS of occupied in-gap states.

Effective strategies for current boosting in a mesa-shaped In–Ga–Zn–O vertical-channel thin-film transistor with a short-channel length of 40 nm

The key strategies for enhancing the performance of an InGaZnO vertical channel transistor are to scale down the channel and gate insulator thicknesses and to control the incorporation of hydrogen from the Al2O3 spacer.

Soft nanocomposites of lead bromide perovskite and polyurethane prepared via coordination chemistry for highly flexible, stable, and quaternary metal alloy-printed light emitting diodes

Highly flexible and luminescent MAPbBr3/PU nanocomposites are synthesized via metal-amide coordination. In–Ga–Zn–Sn alloy cathode-printed PeLEDs incorporating the nanocomposites exhibit an impressive maximum luminance of 1458.6 Cd m−2 at r = 7 mm.

Rationally designed synthesis of bright Cu–Ga–Zn–Se-based nanocrystals for efficient green quantum-dot light-emitting diodes

Cd-free QLEDs with tunable emission from 603 to 524 nm were constructed by regulating the non-stoichiometric Cu : Ga molar ratio, and the green QLEDs showed high performance with an EQE of 5.8% and a high brightness of 7016 cd m−2.

Optimization of post-annealing temperature for high-performance synaptic transistors based on In–Ga–Zn–O channel and trap layers

Synaptic transistors using an indium–gallium–zinc oxide (IGZO) charge trap layer (CTL) have great potential for high-performance neuromorphic applications due to their excellent stability and high linearity in conductance modulation. Since the high conductivity of the IGZO CTL enhances charge de-trapping performance, the post-annealing process, which directly affects the conductivity of the film, should be carefully conducted. In this study, we investigated the effect of post-annealing temperature (TPA) on the charge de-trapping performance of synaptic transistors. The synaptic transistor showed superior charge de-trapping performance at a TPA of 250 °C, resulting from the increased conductivity of the CTL. Through various electrical and chemical analysis methods, it was revealed that the hydrogen decrease within the CTL and the oxidation–reduction reaction at the interface between the tunneling layer and the CTL contributed to the significant increase in the conductivity of the CTL. The synaptic transistor with the optimal TPA exhibited high linearity of potentiation (αp: 0.56) and depression (αd: −0.65) with 64 conductance levels, enabling the spiking neural network simulation to achieve a high accuracy of 88.75% using a Fashion-MNIST dataset. This work shows that TPA is one of the critical process strategies for the high charge de-trapping performance of the synaptic transistor by inducing the chemical reaction at the CTL interface and hydrogen content change within the CTL.

Sequential Design of PEALD In–Ga–Zn–O Active Layer for Enhancing TFT Stability

Sequential Design of PEALD In–Ga–Zn–O Active Layer for Enhancing TFT Stability

Composition-Engineered Heavy-Metal-Free Cu–Ga–Zn–S Nanorods for Efficient Photocatalytic Water Splitting

Composition-Engineered Heavy-Metal-Free Cu–Ga–Zn–S Nanorods for Efficient Photocatalytic Water Splitting

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.

High-mobility and low subthreshold swing amorphous InGaZnO thin-film transistors by in situ H2 plasma and neutral oxygen beam irradiation treatment

In this work, staggered bottom-gate structure amorphous In–Ga–Zn–O (a-IGZO) thin film transistors (TFTs) with high-k ZrO2 gate dielectric were fabricated using low-cost atmospheric pressure-plasma enhanced chemical vapor deposition (AP-PECVD) with in situ hydrogenation to modulate the carrier concentration and improve interface quality. Subsequently, a neutral oxygen beam irradiation (NOBI) technique is applied, demonstrating that a suitable NOBI treatment could successfully enhance electrical characteristics by reducing native defect states and minimize the trap density in the back channel. A reverse retrograde channel (RRGC) with ultra-high/low carrier concentration is also formed to prevent undesired off-state leakage current and achieve a very low subthreshold swing. The resulting a-IGZO TFTs exhibit excellent electrical characteristics, including a low subthreshold swing of 72 mV dec−1 and high field-effect mobility of 35 cm2 V−1 s−1, due to conduction path passivation and stronger carrier confinement in the RRGC. The UV–vis spectroscopy shows optical transmittance above 90% in the visible range of the electromagnetic spectrum. The study confirms the H2 plasma with NOBI-treated a-IGZO/ZrO2 TFT is a promising candidate for transparent electronic device applications.

Room-Temperature Preparation of Large-Area Transparent Two-Dimensional ZnO-Doped Ga2O3 Nanostructure-Based Layers: Implications for Optoelectronic Nanodevices

Herein, we demonstrated the controllable synthesis of a centimeter-scale two-dimensional (2D) ZnO-doped Ga2O3 nanostructure layer by a liquid Ga–Zn alloy printing strategy at near room temperature. Different from the liquid Ga–In and Ga–In–Sn alloys, the surface oxidation behavior of a liquid Ga–Zn alloy follows an obvious competition and cooxidation characteristics instead of the dominant oxidation characteristic of Ga, which could be effectively used to precisely tailor the Zn content of 2D Ga2O3 films. With an increase of the nominal Zn content in the Ga–Zn alloy from 0 to 8 atom %, the real Zn content of 2D ZnO-doped Ga2O3 films gradually increases and finally reaches a maximum saturated value of 16–18 atom % at the eutectic component of 3.87 atom %. Correspondingly, the transmittance and band gap of 2D ZnO-doped Ga2O3 films could also be tuned by changes of the Zn content and crystallinity. The method proposed in this work provides a general route toward the doping synthesis of a diverse nonlayered 2D structure, which will shed light on the applications of various displays and deep-ultraviolet optoelectronic devices.

Exploration of Chemical Composition of In–Ga–Zn–O System via PEALD Technique for Optimal Physical and Electrical Properties

AbstractIn–Ga–Zn–O (IGZO) material has been researched due to its favorable electrical characteristics for application in thin‐film transistor (TFT) applications such as low off current and relatively high mobility. However, most recently, as the developing and expanding application fields, conventional IGZO is a challenging aspect because higher mobility and excellent step‐coverage are required to be applied to high‐resolution displays and 3D NAND. In this regard, atomic layer deposition (ALD) is suggested as a novel deposition method for tackling issues. Here, this work systematically synthesizes IGZO films with various compositions by the supercycle technique of plasma‐enhanced ALD (PEALD) to determine the optimum metal cation composition range of the IGZO system for high‐mobility TFTs. The trends in the metal composition dependent electrical properties of ALD processed IGZO films are comparable to the previously reported results, while the structural properties are exclusive. Since both microstructure and carrier concentration affect to device characteristics complexly, the optimal In–Ga–Zn region is newly demonstrated via PEALD (XIn: 0.56–0.63, XGa: 0.13–0.17, XZn: 0.17–0.34). In this region, the device exhibits a remarkably high µFE of 41.4–43.7 cm2 V−1 s−1, a low subthreshold swing (SS) of 0.24–0.25 V decade−1, an initial threshold voltage (Vth) of −0.9 to −1.0 V, and a slight Vth shift (0.01 V) under the positive bias temperature stability (PBTS).

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.

Electron transport mechanisms in amorphous zinc oxysulfide thin films

Amorphous mixed-anion semiconductors (AMASs) such as amorphous zinc oxynitride and amorphous zinc oxysulfide (a-ZnOS) have attracted attention as rare-metal-free amorphous semiconductors that exhibit electron mobility comparable to or greater than the electron mobilities of typical amorphous oxide semiconductors (AOSs), including amorphous In–Ga–Zn–O (a-IGZO). A characteristic feature of AMASs is that their conduction-band minimum (CBM) mainly consists of s-orbitals of the single cation, in contrast to conventional AOSs, whose CBM is composed of s-orbitals of multiple cations. This unique band structure suggests that the potential of carrier electrons in AMASs exhibits less spatial fluctuation than that of carrier electrons in AOSs. In this study, we analyzed the temperature dependence of the electron transport properties of a-ZnOS thin films using the random barrier model to evaluate the potential barrier height and its spatial variation. The analyses revealed that the barrier height of a-ZnOS is comparable to that of a-IGZO. This result was attributed to the large covalent nature of Zn–S bonds strongly influencing the potential at the CBM through the antibonding interaction.

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.

Hydrogen-Doping-Enabled Boosting of the Carrier Mobility and Stability in Amorphous IGZTO Transistors.

This study investigated the effect of hydrogen (H) on the performance of amorphous In-Ga-Zn-Sn oxide (a-In0.29Ga0.35Zn0.11Sn0.25O) thin-film transistors (TFTs). Ample H in plasma-enhanced atomic layer deposition (PEALD)-derived SiO2 can diffuse into the underlying a-IGZTO film during the postdeposition annealing (PDA) process, which affects the electrical properties of the resulting TFTs due to its donor behavior in the a-IGZTO. The a-In0.29Ga0.35Zn0.11Sn0.25O TFTs at the PDA temperature of 400 °C exhibited a remarkably higher field-effect mobility (μFE) of 85.9 cm2/Vs, a subthreshold gate swing (SS) of 0.33 V/decade, a threshold voltage (VTH) of -0.49 V, and an ION/OFF ratio of ∼108; these values are superior compared to those of unpassivated a-In0.29Ga0.35Zn0.11Sn0.25O TFTs (μFE = 23.3 cm2/Vs, SS = 0.36 V/decade, and VTH = -3.33 V). In addition, the passivated a-In0.29Ga0.35Zn0.11Sn0.25O TFTs had good stability against the external gate bias duration. This performance change can be attributed to the substitutional H doping into oxygen sites (HO) leading to a boost in ne and μFE. In contrast, the beneficial HO effect was barely observed for amorphous indium gallium zinc oxide (a-IGZO) TFTs, suggesting that the hydrogen-doping-enabled boosting of a-IGZTO TFTs is strongly related to the existence of Sn cations. Electronic calculations of VO and HO using density functional theory (DFT) were performed to explain this disparity. The introduction of SnO2 in a-IGZO is predicted to cause a conversion from shallow VO to deep VO due to the lower formation energy of deep VO, which is effectively created around Sn cations. The formation of HO by H doping in the IGZTO facilitates the efficient connection of atomic states forming the conduction band more smoothly. This reduces the effective mass and enhances the carrier mobility.

Operating region-dependent characteristics of weight updates in synaptic In–Ga–Zn–O thin-film transistors

We present a study on characteristics of operating region-dependent weight updates in a synaptic thin-film transistor (Syn-TFT) with an amorphous In–Ga–Zn–O (IGZO) channel layer. For a synaptic behavior (e.g. a memory phenomenon) of the IGZO TFT, a defective oxide (e.g. SiO2) is intentionally used for a charge trapping due to programming pulses to the gate terminal. Based on this synaptic behavior, a conductance of the Syn-TFT is modulated depending on the programming pulses, thus weight updates. This weight update characteristics of the Syn-TFT is analyzed in terms of a dynamic ratio (drw) for two operating regions (i.e. the above-threshold and sub-threshold regimes). Here, the operating region is chosen depending on the level of the gate read-voltage relative to the threshold voltage of the Syn-TFT. To verify these, the static and pulsed characteristics of the fabricated Syn-TFT are monitored experimentally. As experimental results, it is found that the drw of the sub-threshold regime is larger compared to the above-threshold regime. In addition, the weight linearity in the sub-threshold regime is observed to be better compared to the above-threshold regime. Since it is expected that either the drw or weight linearity can affect performances (e.g. a classification accuracy) of an analog accelerator (AA) constructed with the Syn-TFTs, the AA simulation is performed to check this with a crossbar simulator.

Quantum confinement effects in amorphous In–Ga–Zn–O thin-film transistors with quantum well channel

We confirmed that quantum confinement effects are clearly observed even in a disordered material. Employing amorphous In–Ga–Zn–O (a-IGZO) thin-film transistors (TFTs) with quantum well channels, it was observed that field-effect mobility–gate voltage relations exhibited plateaus when the well thickness was 5 nm or less. Device simulation incorporating the quantum confinement effects reproduced the well-thickness dependence of the plateau onset voltage, verifying that the quantized electronic levels in the a-IGZO well dominate the TFT characteristics and, thus, the electron transport is free electron like with a coherent length of ∼5 nm. The field-effect mobility did not exceed the drift mobility of bulk a-IGZO, which is different from amorphous silicon superlattice TFT.