Amorphous Oxide Semiconductor Research Articles

Improving the photoswitching performance of a transistor with amorphous metal oxide semiconductor thin film by a gradient annealing approach

Metal oxides are attracting attention as electronic mate rials in research and industry. Thin films of amorphous indium gallium zinc oxide (a-IGZO) exhibit low absorbance in the visible spectrum, making them ideal components in transparent electronics. To widen the scope of use for thin film transistor (TFT) devices based on a-IGZO in on-chip sensing applications, photoresponsive behavior has been achieved by proper engineering of the active layers by either introducing a photosensitive top layer or using a method to generate localized states inside the band gap. In this paper, we propose a bilayer structured with the use of thermal annealing of a-IGZO film at different temperatures. Thermal annealing has been shown to improve the electrical performance of the TFT devices because of the improved film quality but negatively affects the photoresponsivity by removing tarp sites that play an important role in both charge generation and photomultiplication via the photogating effect. Therefore, here we propose an a-IGZO film with a high temperature-annealed bottom layer and pristine top layer. The bottom layer plays a vital role in the charge transport behavior, resulting in a low threshold voltage and subthreshold swing similar to the device with a fully annealed film, while the photoresponse of the device is driven by the higher density of gap states in the pristine top layer. This proposed method is advantageous to previously published procedures due to the simplicity of using no additional materials and complex steps to introduce trap sites into the photoactive layer, but only differential annealing temperature.

A study on the high mobility and improved reliability of pr-doped indium zinc oxide thin film transistors

Abstract It is generally accepted that there is a trade-off relationship between mobility and stability for oxide thin film transistor (TFT) devices. Different doping ratios of Ln praseodymium (Pr) into indium (In) zinc (Zn) oxide have been employed as the active layer to get 1# and 2# amorphous oxide semiconductor (AOS) TFTs in this work. The 1#-based TFTs exhibited a high mobility of 49.84 cm2 V−1 s−1 due to the increased concentration of In. By further elevating the Pr doping ratio and adjusting In/Zn ratio of the film, the 2#-based TFT obtained both a good mobility of 26.65 cm2 V−1 s−1, and a promising stability, showing a positive-bias temperature stress stability of ΔV TH = 1.56 V and a negative-bias temperature illumination stress stability of ΔV TH = −1.47 V. It was revealed that the low energy charge transfer state of Pr in 2# film absorbs the visible light, leading to suppressed photo-induced carriers and thus a good illumination reliability of the 2#-based TFTs. In practice, the LCD panel based 2# ACT TFT shows a well stable performance even under 10 000-nit illumination. The result indicates a promising strategy to accelerate the commercialization of AOS TFTs to large-panel display production.

Thermal atomic layer deposition-processed InHfZnO thin film transistors with excellent stability

In recent years, the downscaling of transistors has sparked considerable interest in the advancement of amorphous oxide semiconductors, particularly indium-hafnium-zinc oxide (IHZO) has remarkable potential in applications such as high-resolution displays and 3D NAND. For the first time, we propose the fabrication of IHZO thin film transistors (TFTs) via thermal atomic layer deposition (TH-ALD). The preparation, electrical properties, and stability of IHZO TFTs are investigated. The performance of TFT deposited at 250 °C and annealed in N2 atmosphere at 300 °C exhibits a high field-effect mobility (μ) of 22.53 cm2/Vs, a high Ion/Ioff of ∼107, a low threshold voltage (Vth) of 0.15 V, and a minimum subthreshold swing (SS) of 0.24 V/decade. Furthermore, the threshold voltage shifts (ΔVth) in TFTs annealed in N2 and O2 are 0.31 and 0.16 V, respectively. These enhancements are attributed to the defects suppression and remaining ionized oxygen vacancies result in increased electron concentration in N2-annealed films. In comparison, O2 annealing causes a reduction in field effect mobility but concurrently enhances stability due to the direct passivation of defects, which leads to fewer oxygen vacancies. Additionally, Hf content can also impact the performance of TFT devices, even a small addition of Hf also results in the effective reduction of the carrier concentration. These findings provide valuable insights into the device mechanism of IHZO TFTs, presenting a novel avenue for comprehending and augmenting their overall performance.

Recent Applications and Future Perspectives of Heterojunction Phototransistors Based on Amorphous Oxide Semiconductors

Recent Applications and Future Perspectives of Heterojunction Phototransistors Based on Amorphous Oxide Semiconductors

First demonstration of 2T0C-FeDRAM: a-ITZO FET and double gate a-ITZO/a-IGZO FeFET with a record-long multibit retention time of >4-bit and >2000 s.

Conventional DRAM, consisting of one transistor and one capacitor (1T1C), requires periodic data refresh processes due to its limited retention time and data-destructive read operation. Here, we propose and demonstrate a novel 3D-DRAM memory scheme available with a single transistor and a single ferroelectric field-effect transistor (FeFET) DRAM (2T0C-FeDRAM), which offers extended retention time and non-destructive read operation. This architecture uses a back-end-of-line (BEOL)-compatible amorphous oxide semiconductor (AOS) that is suitable for increasing DRAM cell density. Notably, the device structures of a double gate a-ITZO/a-IGZO FeFET, used for data storage and reading, are engineered to achieve an enlarged memory window (MW) of 1.5 V and a prolonged retention time of 104 s. This is accomplished by a double gate and an a-ITZO/a-IGZO heterostructure channel to enable efficient polarization control in hafnium-zirconium oxide (HZO) layers. We present successful program/erase operations of the double gate a-ITZO/a-IGZO FeFET through incremental step pulse programming (ISPP), demonstrating multi-level states with remarkable retention characteristics. Most importantly, we perform 2T0C-FeDRAM operations by electrically connecting the double gate a-ITZO/a-IGZO FeFET and the a-ITZO FET. Leveraging the impressive performance of the double gate a-ITZO/a-IGZO FeFET technology, we have effectively showcased an exceptionally record-long retention time exceeding 2000 s and 4-bit multi-level states, positioning it as a robust contender among emerging memory solutions in the era of artificial intelligence.

Tri‐Layer Heterostructure Channel of a‐IGZO/a‐ITZO/a‐IGZO Toward Enhancement of Transport and Reliability in Amorphous Oxide Semiconductor Thin Film Transistors

AbstractThin film transistors (TFTs) based on amorphous oxide semiconductors (AOS) are promising candidates for panel displays. However, the trade‐off between mobility and reliability in AOS‐TFTs hinders their further applications in next‐generation display techniques and newly developed logic and memory circuits. Here, a structural strategy is proposed for the mobility‐reliability trade‐off, via a triple‐layer channel containing a Ga‐free high‐mobility layer (amorphous InSnZnO, a‐ITZO) sandwiched by two Ga‐rich layers (amorphous InGaZnO, a‐IGZO) with higher reliability. Gate‐induced carrier accumulation is verified mainly being energetically confined within the high mobility a‐ITZO layer, at the newly defined a‐ITZO/a‐IGZO interface. Compared to single layer a‐ITZO‐TFTs, triple‐channel a‐IGZO/a‐ITZO/a‐IGZO TFTs (GTG‐TFTs) exhibit outstanding stability and electrical transport performances, with suppressed positive/negative‐bias‐stress voltage shifts from 1/0.3 to 0.1/0.004 V, enhanced field effect mobility from ≈40 to 56 cm2V−1s−1, and optimized sub‐threshold swing down to 80 mV dec−1. Further numerical simulations and charge transport characterizations, including magnetotransport and gate‐induced Hall effect, indicate that charge transport in tri‐layer structure is less affected by energetic disorders present at gate insulator interfaces.

Low Contact Resistance via Quantum Well Structure in Amorphous InMoO Thin Film Transistors

The amorphous oxide semiconductor (AOS) thin film transistor (TFT) shows promise for use in advanced integrated circuits, such as 2T0C dynamic random-access memory, due to its excellent electronic performance and ability to be fabricated at low temperatures. Nevertheless, the high contact resistance between the metal and AOS restricts the applicability of AOS-TFT. This study demonstrates the achievement of a reduced contact resistance in InMoO (IMO) transistors by using a MoOx interlayer during fabrication. Increasing the oxygen concentration alters the band structure of MoOx and creates a graded Mo-MoOx-IMO structure with a pronounced quantum well at the interlayer between the metal and channel. Consequently, the quantum well’s ability to attract electrons and shape the band edge suppresses the Fermi-level pinning effect, ultimately leading to the establishment of an ohmic contact. The optimized MoOx interlayer showed a significant improvement in contact resistance (∼400%) through the adjustment of oxygen content during annealing procedures. This finding suggests that it is an attractive approach to provide excellent source/drain contacts in future ultra-scaled amorphous oxide semiconductor thin-films.

The Electronic Impact of X-Ray on Atomically-Thin Oxide Semiconductor Devices

In the past few years, there has been a significant emphasis on atomically-thin metal oxide semiconductors (OS), driven by their adjustable electrical properties that open up a range of electronic applications [1.2]. However, the mechanisms of transport and electronic tunability in OS remain unclear [3]. X-ray characterization techniques, i.e. X-ray photoelectron spectroscopy (XPS) and X-ray diffractometer (XRD), are considered as non-destructive and are widely used to quantify the chemical and electronic state of OS. In this work, we discovered that X-ray has significant impact on the electronic properties of OS. We observed that low energy X-ray (<2 keV) induces a negative V T offset in OS devices [4]. The variation in V T primarily originates from the chemical interaction of oxygen molecules on the channel surface. OS devices absorb the low-energy X-ray, generating the holes, and they subsequently react with O2 - on the channel surface, resulting in the production of oxygen and its removal from the channel surface, as shown in Fig 1a. The V T shift caused by low energy X-ray will recover in air over time, as illustrated in Fig 1b. We incorporated the post low-energy X-ray devices into a vacuum environment with a pressure of 10-6 torr, and the V T of the device exhibited no recovery over time, providing additional support for oxygen absorption mechanism. Time-resolved electrical measurements conducted in different atmospheric conditions reveal that the variations in V T arise from the oxygen absorption mechanism. The research highlights the effect of X-ray on OS and analyzes the mechanism behind the increase in OS carrier concentration induced by the X-ray exposure.[1] Si, M. et al. Scaled indium oxide transistors fabricated using atomic layer deposition. Nat. Electron. 5, 164–170 (2022).[2] Charnas, Adam, et al. Extremely Thin Amorphous Indium Oxide Transistors. Adv. Mater. 2304044 (2023).[3] Conley, J. F. Instabilities in amorphous oxide semiconductor thin-film transistors. IEEE Trans. Device Mater. Rel. 10, 460–475 (2010).[4] Tseng, R., Wang, ST., Ahmed, T. et al. Wide-range and area-selective threshold voltage tunability in ultrathin indium oxide transistors. Nat. Commun. 14, 5243 (2023). Figure 1

Unlocking Neuromorphic Vision: Advancements in IGZO-Based Optoelectronic Memristors with Visible Range Sensitivity.

Optoelectronic memristors based on amorphous oxide semiconductors (AOSs) are promising devices for the development of spiking neural network (SNN) hardware in neuromorphic vision sensors. In such devices, the conductance state can be controlled by both optical and electrical stimuli, while the typical persistent photoconductivity (PPC) of AOS materials can be used to emulate synaptic functions. However, due to the large band gap of these materials, sensitivity to visible light (red/green/blue) is difficult to accomplish, which hinders applications requiring color discrimination. In this work, we report a 4 μm2 hydrogen-doped (H-doped) indium-gallium-zinc oxide (IGZO) optoelectronic memristor that emulates all of the important rules of SNNs such as short- to long-term memory transition (STM-LTM), paired-pulse facilitation (PPF), spike-time-dependent plasticity (STDP), and learning and forgetting capabilities. By the incorporation of hydrogen gas in the sputtering deposition of IGZO, visible sensitivity was achieved for green and blue wavelengths. Additionally, extremely high light/dark ratios of 179, 93, and 12 are demonstrated for wavelengths of 365, 405, and 505 nm, respectively, due to hydrogen-induced subgap states and device miniaturization. Therefore, the proposed device shows remarkable potential for integration with the pixel circuits of IGZO-based displays with extreme resolution for a true intelligent self-processing display.

Thermal Dehydrogenation Impact on Positive Bias Stability of Amorphous InSnZnO Thin-Film Transistors.

Recently, the growing demand for amorphous oxide semiconductor thin-film transistors (AOS TFTs) with high mobility and good stability to implement ultrahigh-resolution displays has made tracking the role of hydrogen in oxide semiconductor films increasingly important. Hydrogen is an essential element that contributes significantly to the field effect mobility and bias stability characteristics of AOS TFTs. However, because hydrogen is the lightest atom and has high reactivity to metal and oxide materials, elucidating its impact on AOS thin films has been challenging. Therefore, in this study, we propose controlling the hydrogen quantities in amorphous InSnZnO (a-ITZO) thin films through thermal dehydrogenation to precisely reveal the hydrogen influences on the electrical characteristics of a-ITZO TFTs. The as-deposited device containing 15.69 × 1015 atoms/cm2 of hydrogen exhibited a relatively low saturation mobility of 18.1 cm2/V·s and poor positive bias stress stability. However, depending on the extent of thermal dehydrogenation, not only did the hydrogen quantity and interface defect density (DIT) decrease but also the conductivity and surface energy increased due to the rise in oxygen vacancies and hydroxyl groups in a-ITZO thin films. As a result, the a-ITZO TFT with a hydrogen amount of 4.828 × 1015 atoms/cm2 showed that the saturation mobility improved up to 36.8 cm2/V·s, and positive bias stress stability was remarkably enhanced. Hence, we report the ability to manage the hydrogen quantity with thermal dehydrogenation and demonstrate that high-performance a-ITZO TFTs can be realized when an appropriate hydrogen concentration is achieved.

Tailoring the Ni-O Microenvironment in Amorphous-Dominated Highly Active and Stable Zn/NiO for Hydrogen Sulfide Detection.

Amorphous metal oxide semiconductor (MOS) materials are endowed with great promise to modulate electronic structures for gas-sensing performance improvement. However, the elevated-temperature requirement of gas sensors severely impedes the application of amorphous materials due to their low thermal stability. Here, a cationic-assisted strategy to tailor the Ni-O microenvironment in an amorphous-dominated Zn/NiO heterogeneous structure with high thermal stability was developed. It was found that 6 mol % Zn incorporation into amorphous NiO can effectively preserve the amorphous-dominated NiO phase even at high temperature. After calcination, the amorphous oxide can only be converted to crystals partly thus leading to the formation of amorphous/crystalline compounds, and the content of the amorphous phase can be adjusted by changing the calcination temperature. This amorphous/crystalline configuration can induce more electron transfer from Ni to Zn species, leading to the formation of active Niδ+ (δ>2) centers. Ex situ XPS and in situ Raman spectroscopy studies proved that the generated Niδ+ species pronouncedly promote the electron transfer during the H2S adsorption process. The amorphous/crystalline-6 mol % Zn/NiO sensor exhibits exceptional hydrogen sulfide response (2 ppm, 3.23), outstanding repeatability (as long as 5 weeks), and low limit of detection (as low as 50 ppb), surpassing most reported nickel-based gas sensors such as the crystal nickel oxide prepared in this work. The response and detection limit of the latter is only (2 ppm, 1.89) and (0.05 ppm) respectively. Our work thus opens up more opportunities for fundamental understanding and modulating of highly active amorphous sensing materials.

Dual-input optoelectronic synaptic transistor based on amorphous ZnAlSnO for multi-target neuromorphic simulation

Optoelectronic synapses can perceive both optical and electrical signals, which are critical for the realization of neuromorphic computing. We have rationally designed an optoelectronic synaptic transistor based on amorphous ZnAlSnO for multi-target neuromorphic simulation and recognition. The dual-input models are well operated by applying light pulses on the channel and electric pulses on the gate, and the transformation from short-term potentiation (STP) to long-turn potentiation (LTP) is identified for tunable synaptic plasticity. In the electrical operation mode, a single-layer artificial neural network was established to recognize handwritten digits by LTP/LTD (long-turn depression) modulation, with a recognition accuracy of 89.2 % for the actual device. In the optical operation mode, the processes of repetitive learning, image recognition, and biased/correlated random-walk learning are simulated on the basis of frequency, quantity, and power of light, with an energy consumption per event as low as 4.3 pJ. This work will facilitate the development of future artificial synapses and highlights the potential of amorphous oxide semiconductors for next-generation computer hardware applications.

Визначення величини енергетичної невпорядкованості в аморфних оксидних напівпровідниках

The amorphous material films are resistant to high-energy irradiation. Therefore, devices built using the properties of these materials can work in conditions of increased radiation much longer than devices using the properties of crystals. An important characteristic of these materials is their degree of disorder. To determine this characteristic, a model of random fluctuations of the local edge of the conduction band is considered for the theoretical study of magnetoconductivity in amorphous oxide semiconductors. The effective medium approximation is used. An approach to determining the amount of energy disorder based on experimental measurement of changes in longitudinal and transverse electrical conductivity in a magnetic field is proposed.

Electron conduction mechanism in indium oxide and its implications for amorphous transport

The electron conduction mechanism in indium oxide (In2O3) and its implications for amorphous transport have been investigated from an orbital overlap perspective. Combined density functional theory and empirical tight binding modeling reveal that the electron transport is facilitated by the neighboring metal atomic s orbital overlap “without” oxygen’s p-orbital involvement. In other words, the electron transport pathway in oxides is only due to the metal-metal medium range connection. This electron conduction mechanism is extended to amorphous In2O3 which unveils that the amorphous disorder influences the electron transport through impacting the metal-metal medium range order including metal-metal coordination number and metal-metal separation. Our results provide an insight into the current theoretical understanding of electron transport in amorphous oxide semiconductors.

Nanoscale-doped dual-channel for improved performance and reliability of Hf: IGTO / a-IGTO thin film transistor

Amorphous oxide semiconductors are widely used in the backplane thin film transistors (TFTs) of displays and are extensively researched as materials for next-generation memory devices. However, they face reliability degradation due to stress factors such as thermal, bias, and light exposure. To mitigate these issues, extensive research is focused on improvements, including doping, enhancing crystallinity, and applying pre-and post-treatments. This study investigates the significantly improved reliability and electrical properties of amorphous indium gallium tin oxide (a-IGTO) TFTs by adopting a dual-channel configuration through simultaneous co-sputtering of HfO2 and IGTO targets. The device fabrication involved co-sputtering HfO2 and IGTO at the gate oxide interface, followed by depositing an IGTO-only layer, rather than doping the entire channel material. The effect of varying Hf concentration was evaluated by applying different HfO2 sputtering powers. Optimal hafnium doping reduced the threshold voltage of the a-IGTO TFT from −0.85V to 0.15V compared to the pristine a-IGTO TFT, while enhancing the field-effect mobility (μFE). Furthermore, the Hf-doped device exhibited improved stability, with a reduced threshold voltage shift (ΔVth) under positive (+15 V) and negative(-15 V) bias stress at 3000s, decreasing from 11.5 V to 4.8 V, and 4.7 V–1.1 V, respectively. This study proposes a novel device fabrication method to enhance the reliability of amorphous oxide semiconductor, addressing their inherent stress-related issues.

Approach to Low Contact Resistance Formation on Buried Interface in Oxide Thin-Film Transistors: Utilization of Palladium-Mediated Hydrogen Pathway.

Amorphous oxide semiconductors (AOSs) with low off-currents and processing temperatures offer promising alternative materials for next-generation high-density memory devices. The complex vertical stacking process of memory devices significantly increases the probability of encountering internal contact issues. Conventional surface treatment methods developed for planar devices necessitate efficient approaches to eliminate contact issues at deep internal interfaces in the nanoscale complex structures of AOS devices. In this work, we report the pioneering use of palladium thin film as a high-efficiency active hydrogen transfer pathway from the outside to the internal contact interface via low-temperature postannealing in the H2 atmosphere, and the formation of highly conductive metallic interlayer effectively solves the contact issues at the deeply buried interfaces in devices. The application of this method reduced the contact resistance of Pd electrodes/amorphous indium-gallium-zinc oxide (a-IGZO) thin-film by 2 orders of magnitude, and thereby the mobility of thin-film transistor was increased from 3.2 cm2 V-1 s-1 to nearly 20 cm2 V-1 s-1, preserving an excellent bias stress stability. This technology has wide applicability for the solution of contact resistance issues in oxide semiconductor devices with complex architectures.

C-Axis Aligned Composite InZnO via Thermal Atomic Layer Deposition for 3D Nanostructured Semiconductor.

Amorphous oxide semiconductors have been widely studied for various applications, including thin-film transistors (TFTs) for display backplanes and semiconductor memories. However, the inherent instability, limited mobility, and complexity of multicomponent oxide semiconductors for achieving high aspect ratios and conformality of cation distribution remain challenging. Indium-zinc oxide (IZO), known for its high mobility, also faces obstacles in instability resulting from high carrier doping density and low ionization energy. To address these issues and attain a balance between mobility and stability, adopting a highly aligned structure such as a c-axis aligned crystalline IGZO could be advantageous. However, limited studies have reported enhanced electrical performance using crystalline IZO, likely attributed to the high thermal stability of the individual components (In2O3 and ZnO). Here, we first propose a c-axis aligned composite (CAAC) IZO with superior TFT properties, including a remarkable performance of field-effect mobility (μFE) of 55.8 cm2/(V s) and positive-bias-temperature-stress stability of +0.16 V (2 MV/cm, 60 °C, 1 h), as well as a low subthreshold swing of 0.18 V/decade and hysteresis as 0.01 V, which could be obtained through optimization of growth temperature and composition using thermal atomic layer deposition. These results surpass those of TFTs based on nanocrystalline/polycrystalline/amorphous-IZO. We conducted a thorough investigation of CAAC-IZO and revealed that the growth temperature and cation distribution profoundly influence the crystal structure and device properties. Finally, we observed excellent compositional conformality and 97% step coverage of IZO on a high-aspect-ratio (HAR) structure with an aspect ratio reaching 40:1, which is highly promising for future applications. Our results include a detailed investigation of the influence of the crystal structure of IZO on the film and TFT performance and suggest an approach for future applications.

Atomic layer deposition of multicomponent amorphous oxide semiconductors with sequential dosing of metal precursors

Atomic layer deposition (ALD) is a suitable method for depositing amorphous oxide semiconductors (AOSs) because it has the potential for achieving high performance in complex structures owing to its excellent step coverage and atomic-scale controllability. Multicomponent AOSs are typically deposited using the ALD with a super-cycle method. In this study, multicomponent AOSs, i.e., IZO, ITO, and ITZO, were deposited using ALD by employing a sequential dosing of metal precursors. Their compositions were adjusted by varying the dose time of the indium precursor. Subsequently, we applied the IZO, ITO, and ITZO films in thin-film transistors (TFTs), which demonstrated normal transfer characteristics. The TFT containing ITZO with 73.9 at% indium exhibited a turn-on voltage of −1.01 V, subthreshold swing of 0.09 V/dec, and field-effect mobility of 35.9 cm2/(V s).

Comparative study of indium tin oxide and indium gallium zinc oxide as potential n-type materials for the fabrication of all inorganic transparent solar cells

This study focuses on transparent solar cell device fabrication using amorphous and crystalline oxide semiconductors. Radio frequency magnetron sputtering was used to fabricate Fluorine doped Tin Oxide/n-Indium Gallium Zinc Oxide/p-Nickel Oxide/Silver nano wires and Fluorine doped Tin Oxide/n-Indium Tin Oxide/p-Nickel Oxide/Silver nano wires devices. Deposition conditions for n and p-type semiconductors were optimized and analyzed. Silver Nanowires were spin-coated as top contacts. The solar cells were exposed to 365 nm, and their current voltage characteristics were analyzed. This novel approach using amorphous oxide semiconductors provides insights for high-efficiency device fabrication.

Sn-doped n-type amorphous gallium oxide semiconductor with energy bandgap of 4.9 eV

Sn-doped n-type amorphous gallium oxide semiconductor with energy bandgap of 4.9 eV