Performance Improvement of In-Ga-Zn Oxide Thin-Film Transistors by Excimer Laser Annealing

We applied excimer laser annealing (ELA) on indium-zinc oxide (IZO) and IZO/indium-gallium-zinc oxide (IGZO) heterojunction thin-film transistors (TFTs) to improve their electrical characteristics. The IZO and IZO/IGZO heterojunction thin films were prepared by the physical vapor deposition method without any other annealing process. The crystalline state and composition of the as-deposited film and the excimer-laser-annealed films were analyzed by X-ray diffraction and X-ray photoelectron spectroscopy. In order to further enhance the electrical performance of TFT, we constructed a dual-heterojunction TFT structure. The results showed that the field-effect mobility could be improved to 9.8 cm2/V·s. Surprisingly, the device also possessed good optical stability. The electron accumulation at the a-IZO/HfO, HfO/a-IGZO, and a-IGZO/gate insulator (GI) interfaces confirmed the a-IGZO-channel conduction. The dual-heterojunction TFT with IZO/HfO/a-IGZO-assisted ELA provides a guideline for overcoming the trade-off between high mobility (μ) and positive VTh control for stable enhancement mode operation with increased ID.

c-Axis Aligned 3 nm Thick In2O3 Crystal Using New Liquid DBADMIn Precursor for Highly Scaled FET Beyond the Mobility-Stability Trade-off

Oxide semiconductors (OS) are attractive materials for memory and logic device applications owing to their low off-current, high field effect mobility, and superior large-area uniformity. Recently, successful research has reported the high field-effect mobility (μFE) of crystalline OS channel transistors (above 50 cm2 V-1 s-1). However, the memory and logic device application presents challenges in mobility and stability trade-offs. Here, we propose a method for achieving high-mobility and high-stability by lowering the grain boundary effect. A DBADMIn precursor was synthesized to deposit highly c-axis-aligned C(222) crystalline 3 nm thick In2O3 films. In this study, the 250 °C deposited 3 nm thick In2O3 channel transistor exhibited high μFE of 41.12 cm2 V-1 s-1, Vth of -0.50 V, and SS of 150 mV decade-1 with superior stability of 0.16 V positive shift during PBTS at 100 °C, 3 MV cm-1 stress conditions for 3 h.

Progress, Challenges, and Opportunities in Oxide Semiconductor Devices: A Key Building Block for Applications Ranging from Display Backplanes to 3D Integrated Semiconductor Chips

As Si has faced physical limits on further scaling down, novel semiconducting materials such as 2D transition metal dichalcogenides and oxide semiconductors (OSs) have gained tremendous attention to continue the ever-demanding downscaling represented by Moore's law. Among them, OS is considered to be the most promising alternative material because it has intriguing features such as modest mobility, extremely low off-current, great uniformity, and low-temperature processibility with conventional complementary-metal-oxide-semiconductor-compatible methods. In practice, OS has successfully replaced hydrogenated amorphous Si in high-end liquid crystal display devices and has now become a standard backplane electronic for organic light-emitting diode displays despite the short time since their invention in 2004. For OS to be implemented in next-generation electronics such as back-end-of-line transistor applications in monolithic 3D integration beyond the display applications, however, there is still much room for further study, such as high mobility, immune short-channel effects, low electrical contact properties, etc. This study reviews the brief history of OS and recent progress in device applications from a material science and device physics point of view. Simultaneously, remaining challenges and opportunities in OS for use in next-generation electronics are discussed.

Wide-range and area-selective threshold voltage tunability in ultrathin indium oxide transistors

The scaling of transistors with thinner channel thicknesses has led to a surge in research on two-dimensional (2D) and quasi-2D semiconductors. However, modulating the threshold voltage (VT) in ultrathin transistors is challenging, as traditional doping methods are not readily applicable. In this work, we introduce a optical-thermal method, combining ultraviolet (UV) illumination and oxygen annealing, to achieve broad-range VT tunability in ultrathin In2O3. This method can achieve both positive and negative VT tuning and is reversible. The modulation of sheet carrier density, which corresponds to VT shift, is comparable to that obtained using other doping and capacitive charging techniques in other ultrathin transistors, including 2D semiconductors. With the controllability of VT, we successfully demonstrate the realization of depletion-load inverter and multi-state logic devices, as well as wafer-scale VT modulation via an automated laser system, showcasing its potential for low-power circuit design and non-von Neumann computing applications.

Investigating an abnormal hump phenomenon in top gate a-InGaZnO thin-film transistors due to mobile sodium diffusion

Top gate a-InGaZnO (IGZO) thin-film transistors (TFTs) annealed at high temperature show excellent initial current-voltage (I-V) characteristics. However, when they are exposed to positive gate bias for a long time, hump can occur in the subthreshold region. This abnormal hump is accelerated at a higher positive gate voltage and mitigate by a negative gate voltage. While the strength of the hump is irrelevant to a change in channel width, it relies significantly on channel length. This phenomenon might be due to mobile Na ions diffused from a glass substrate migrating toward the back and edge side of the IGZO semiconductor by a vertical gate electric field. When a layer of Al2O3 is formed between the IGZO semiconductor and the glass substrate, the hump phenomenon could be successfully solved by serving as a barrier for Na ions moving into the IGZO.

Addressing the Conflict between Mobility and Stability in Oxide Thin-film Transistors

Amorphous oxide semiconductor thin-film transistors (AOS TFTs) are ever-increasingly utilized in displays. However, to bring high mobility and excellent stability together is a daunting challenge. Here, the carrier transport/relaxation bilayer stacked AOS TFTs are investigated to solve the mobility-stability conflict. The charge transport layer (CTL) is made of amorphous In-rich InSnZnO, which favors big average effective coordination number for all cations and more edge-shared structures for better charge transport. Praseodymium-doped InSnZnO is used as the charge relaxation layer (CRL), which substantially shortens the photoelectron lifetime as revealed by femtosecond transient absorption spectroscopy. The CTL and CRL with the thickness suitable for industrial production respectively afford minute potential barrier fluctuation for charge transport and fast relaxation for photo-generated carriers, resulting in transistors with an ultrahigh mobility (75.5 cm² V^-1 s^-1 ) and small negative-bias-illumination-stress/positive-bias-temperature-stress voltage shifts (-1.64/0.76 V). The design concept provides a promising route to address the mobility-stability conflict for high-end displays.

Charge Trapping and Emission Properties in CAAC-IGZO Transistor: A First-Principles Calculations

The c-axis aligned crystalline indium-gallium-zinc-oxide field-effect transistor (CAAC-IGZO FET), exhibiting an extremely low off-state leakage current (~10^-22 A/μm), has promised to be an ideal candidate for Dynamic Random Access Memory (DRAM) applications. However, the instabilities leaded by the drift of the threshold voltage in various stress seriously affect the device application. To better develop high performance CAAC-IGZO FET for DRAM applications, it's essential to uncover the deep physical process of charge transport mechanism in CAAC-IGZO FET. In this work, by combining the first-principles calculations and nonradiative multiphonon theory, the charge trapping and emission properties in CAAC-IGZO FET have been systematically investigated. It is found that under positive bias stress, hydrogen interstitial in Al₂O₃ gate dielectric is probable effective electron trap center, which has the transition level (ε (+1/-1) = 0.52 eV) above Fermi level. But it has a high capture barrier about 1.4 eV and low capture rate. Under negative bias stress, oxygen vacancy in Al₂O₃ gate dielectric and CAAC-IGZO active layer are probable effective electron emission centers whose transition level ε (+2/0) distributed at -0.73~-0.98 eV and 0.69 eV below Fermi level. They have a relatively low emission barrier of about 0.5 eV and 0.25 eV and high emission rate. To overcome the instability in CAAC-IGZO FET, some approaches can be taken to control the hydrogen concentration in Al₂O₃ dielectric layer and the concentration of the oxygen vacancy. This work can help to understand the mechanisms of instability of CAAC-IGZO transistor caused by the charge capture/emission process.

Heterogeneous Integration of Atomically-Thin Indium Tungsten Oxide Transistors for Low-Power 3D Monolithic Complementary Inverter

In this work, the authors demonstrate a novel vertically-stacked thin film transistor (TFT) architecture for heterogeneously complementary inverter applications, composed of p-channel polycrystalline silicon (poly-Si) and n-channel amorphous indium tungsten oxide (a-IWO), with a low footprint than planar structure. The a-IWO TFT with channel thickness of approximately 3-4 atomic layers exhibits high mobility of 24 cm² V^-1 s^-1 , near ideally subthreshold swing of 63 mV dec^-1 , low leakage current below 10^-13 A, high on/off current ratio of larger than 10⁹ , extremely small hysteresis of 0 mV, low contact resistance of 0.44 kΩ-µm, and high stability after encapsulating a passivation layer. The electrical characteristics of n-channel a-IWO TFT are well-matched with p-channel poly-Si TFT for superior complementary metal-oxide-semiconductor technology applications. The inverter can exhibit a high voltage gain of 152 V V^-1 at low supply voltage of 1.5 V. The noise margin can be up to 80% of supply voltage and perform the symmetrical window. The pico-watt static power consumption inverter is achieved by the wide energy bandgap of a-IWO channel and atomically-thin channel. The vertically-stacked complementary field-effect transistors (CFET) with high energy-efficiency can increase the circuit density in a chip to conform the development of next-generation semiconductor technology.

High-mobility and low subthreshold swing amorphous InGaZnO thin-film transistors byin situH2plasma 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 ZrO₂gate dielectric were fabricated using low-cost atmospheric pressure-plasma enhanced chemical vapor deposition (AP-PECVD) within situhydrogenation 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^-1and high field-effect mobility of 35 cm²V^-1s^-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 H₂plasma with NOBI-treated a-IGZO/ZrO₂TFT is a promising candidate for transparent electronic device applications.

Investigation on Atomic Bonding Structure of Solution-Processed Indium-Zinc-Oxide Semiconductors According to Doped Indium Content and Its Effects on the Transistor Performance

The atomic composition ratio of solution-processed oxide semiconductors is crucial in controlling the electrical performance of thin-film transistors (TFTs) because the crystallinity and defects of the random network structure of oxide semiconductors change critically with respect to the atomic composition ratio. Herein, the relationship between the film properties of nitrate precursor-based indium-zinc-oxide (IZO) semiconductors and electrical performance of solution-processed IZO TFTs with respect to the In molar ratio was investigated. The thickness, morphological characteristics, crystallinity, and depth profile of the IZO semiconductor film were measured to analyze the correlation between the structural properties of IZO film and electrical performances of the IZO TFT. In addition, the stoichiometric and electrical properties of the IZO semiconductor films were analyzed using film density, atomic composition profile, and Hall effect measurements. Based on the structural and stoichiometric results for the IZO semiconductor, the doping effect of the IZO film with respect to the In molar ratio was theoretically explained. The atomic bonding structure by the In doping in solution-processed IZO semiconductor and resulting increase in free carriers are discussed through a simple bonding model and band gap formation energy.

Transparent electronic and photoelectric synaptic transistors based on the combination of an InGaZnO channel and a TaOx gate dielectric

A transparent thin film transistor (TFT) based on the combination of an InGaZnO channel and a high-κ (the dielectric constant is about 42.6) TaO_x gate dielectric layer is fabricated. The TFT shows robust anticlockwise hysteresis under DC voltage sweep and synaptic behaviors (i.e., excitatory postsynaptic current, short-term memory plasticity, short-term memory to long-term memory transition, and potentiation and depression) under voltage pulse stimulus. In addition, the TFT shows high responsivity to illumination of light with various wavelengths (ultraviolet and visible light). Synaptic behaviors in response to light pulse stimuli, which could be employed in vision-based neuromorphic applications, are demonstrated. Large conductance change (G_max/G_min > 10) and ultra-low non-linearity (α < 0.5) of the potentiation and depression can be inspired by either gate bias pulses or photoelectric pulses with short pulse widths and small amplitudes.

Structural Engineering Effects on Hump Characteristics of ZnO/InSnO Heterojunction Thin-Film Transistors

Transparent conductive oxides (TCO) have been extensively investigated as channel materials for thin-film transistors (TFTs). In this study, highly transparent and conductive InSnO (ITO) and ZnO films were deposited, and their material properties were studied in detail. Meanwhile, we fabricated ZnO/ITO heterojunction TFTs, and explored the effects of channel structures on the hump characteristics of ZnO/ITO TFTs. We found that Vhump–VON was negatively correlated with the thickness of the bottom ZnO layer (10, 20, 30, and 40 nm), while it was positively correlated with the thickness of the top ITO layer (3, 5, 7, and 9 nm), where Vhump is the gate voltage corresponding to the occurrence of the hump and VON is the turn-on voltage. The results demonstrated that carrier transport forms dual current paths through both the ZnO and ITO layers, synthetically determining the hump characteristics of the ZnO/ITO TFTs. Notably, the hump was effectively eliminated by reducing the ITO thickness to no more than 5 nm. Furthermore, the hump characteristics of the ZnO/ITO TFTs under positive gate-bias stress (PBS) were examined. This work broadens the practical application of TCO and provides a promising method for solving the hump phenomenon of oxide TFTs.

Effects of Channel Thickness on Electrical Performance and Stability of High-Performance InSnO Thin-Film Transistors

InSnO (ITO) thin-film transistors (TFTs) attract much attention in fields of displays and low-cost integrated circuits (IC). In the present work, we demonstrate the high-performance, robust ITO TFTs that fabricated at process temperature no higher than 100 °C. The influences of channel thickness (t_ITO, respectively, 6, 9, 12, and 15 nm) on device performance and positive bias stress (PBS) stability of the ITO TFTs are examined. We found that content of oxygen defects positively correlates with t_ITO, leading to increases of both trap states as well as carrier concentration and synthetically determining electrical properties of the ITO TFTs. Interestingly, the ITO TFTs with a t_ITO of 9 nm exhibit the best performance and PBS stability, and typical electrical properties include a field-effect mobility (µ_FE) of 37.69 cm²/Vs, a V_on of -2.3 V, a SS of 167.49 mV/decade, and an on-off current ratio over 10⁷. This work paves the way for practical application of the ITO TFTs.

Continuous Tuning of the Fermi Level in Disorder-Engineered Amorphous Films of Li-Doped ZnO for Thermoelectric Applications

Amorphous metal-oxide semiconductors can be readily prepared by a solution process at low temperatures, and their energy band structures and carrier concentrations can be controlled based on the oxide composition or the addition of dopants in the design of thermoelectric (TE) materials. However, research on the correlation between the charge transport and TE performance of amorphous metal-oxide semiconductors is still in its infancy. Herein, we present the energy-dependent TE performance characteristics of Li-doped ZnO thin films with different doping levels and charge carrier concentrations. Thin films were prepared by the solution process, and the Li doping level was controlled by the Li precursor concentration added to a Zn precursor solution. Subsequently, a field-effect-modulated Seebeck coefficient measurement device was built to study the energy-dependent TE performance. Notably, the higher ratio of interstitial Li (Li_inter) and oxygen vacancies (O_va) in the Li-ZnO device indicates an improved n-type TE performance. To investigate more thoroughly the charge transport phenomena, the localized density of states (DOS) was derived from the temperature-dependent transfer curve; the higher ratio of interstitial Li (Li_inter) and oxygen vacancy (O_va) induces a reduction in the localized DOS and lowers the degree of disorder in their DOS. The determined energy-dependent TE characteristics can be used as guidance for the design of efficient TE devices with amorphous metal-oxide semiconductors.

New Simulation Method for Dependency of Device Degradation on Bending Direction and Channel Length

The dependency of device degradation on bending direction and channel length is analyzed in terms of bandgap states in amorphous indium-gallium-zinc-oxide (a-IGZO) films. The strain distribution in an a-IGZO film under perpendicular and parallel bending of a device with various channel lengths is investigated by conducting a three-dimensional mechanical simulation. Based on the obtained strain distribution, new device simulation structures are suggested in which the active layer is defined as consisting of multiple regions. The different arrangements of a highly strained region and density of states is proportional to the strain account for the measurement tendency. The analysis performed using the proposed structures reveals the causes underlying the effects of different bending directions and channel lengths, which cannot be explained using the existing simulation methods in which the active layer is defined as a single region.

Fully Printed High-Performance n-Type Metal Oxide Thin-Film Transistors Utilizing Coffee-Ring Effect

Metal oxide thin-films transistors (TFTs) produced from solution-based printing techniques can lead to large-area electronics with low cost. However, the performance of current printed devices is inferior to those from vacuum-based methods due to poor film uniformity induced by the "coffee-ring" effect. Here, we report a novel approach to print high-performance indium tin oxide (ITO)-based TFTs and logic inverters by taking advantage of such notorious effect. ITO has high electrical conductivity and is generally used as an electrode material. However, by reducing the film thickness down to nanometers scale, the carrier concentration of ITO can be effectively reduced to enable new applications as active channels in transistors. The ultrathin (~10-nm-thick) ITO film in the center of the coffee-ring worked as semiconducting channels, while the thick ITO ridges (>18-nm-thick) served as the contact electrodes. The fully inkjet-printed ITO TFTs exhibited a high saturation mobility of 34.9 cm² V^-1 s^-1 and a low subthreshold swing of 105 mV dec^-1. In addition, the devices exhibited excellent electrical stability under positive bias illumination stress (PBIS, ΔV_th = 0.31 V) and negative bias illuminaiton stress (NBIS, ΔV_th = -0.29 V) after 10,000 s voltage bias tests. More remarkably, fully printed n-type metal-oxide-semiconductor (NMOS) inverter based on ITO TFTs exhibited an extremely high gain of 181 at a low-supply voltage of 3 V, promising for advanced electronics applications.

Photonic Curing of Solution-Processed Oxide Semiconductors with Efficient Gate Absorbers and Minimal Substrate Heating for High-Performance Thin-Film Transistors

In this study, photonic curing is used to rapidly and effectively convert metal-oxide sol-gels to realize high-quality thin-film transistors (TFTs). Photonic curing offers advantages over conventional thermal processing methods such as ultrashort processing time and compatibility with low-temperature substrates. However, previous work on photonically cured TFTs often results in significant heating of the entire substrate rather than just the thin film at the surface. Here, sol-gel indium zinc oxide (IZO)-based TFTs are photonically cured with efficient gate absorbers requiring as few as five pulses using intense white light delivering radiant energy up to 6 J cm^-2. Simulations indicate that the IZO film reaches a peak temperature of ∼590 °C while the back of the substrate stays below 30 °C. The requirements and design guidelines for photonic curing metal-oxide semiconductors for high-performance TFT applications are discussed, focusing on the importance of effective gate absorbers and optimized pulse designs to efficiently and effectively cure sol-gel films. This process yields TFTs with a field-effect mobility of 21.8 cm² V^-1 s^-1 and an I _on/I _off ratio approaching 10⁸, which exceeds the performance of samples annealed at 500 °C for 1 h. This is the best performance and highest metal-oxide conversion for photonically cured oxide TFTs achieved to date that does not significantly heat the entire thickness of the substrate. Importantly, the conversion from sol-gel precursors to the semiconducting metal-oxide phase during photonic curing is on par with thermal annealing, which is a significant improvement over previous pulsed-light processing work. The use of efficient gate absorbers also allows for the reduction in the number of pulses and efficient sol-gel conversion.

Amorphous thin-film oxide power devices operating beyond bulk single-crystal silicon limit

Power devices (PD) are ubiquitous elements of the modern electronics industry that must satisfy the rigorous and diverse demands for robust power conversion systems that are essential for emerging technologies including Internet of Things (IoT), mobile electronics, and wearable devices. However, conventional PDs based on "bulk" and "single-crystal" semiconductors require high temperature (> 1000 °C) fabrication processing and a thick (typically a few tens to 100 μm) drift layer, thereby preventing their applications to compact devices, where PDs must be fabricated on a heat sensitive and flexible substrate. Here we report next-generation PDs based on "thin-films" of "amorphous" oxide semiconductors with the performance exceeding the silicon limit (a theoretical limit for a PD based on bulk single-crystal silicon). The breakthrough was achieved by the creation of an ideal Schottky interface without Fermi-level pinning at the interface, resulting in low specific on-resistance R_on,sp (< 1 × 10^-4 Ω cm²) and high breakdown voltage V_BD (~ 100 V). To demonstrate the unprecedented capability of the amorphous thin-film oxide power devices (ATOPs), we successfully fabricated a prototype on a flexible polyimide film, which is not compatible with the fabrication process of bulk single-crystal devices. The ATOP will play a central role in the development of next generation advanced technologies where devices require large area fabrication on flexible substrates and three-dimensional integration.

Quantum Confinement Effect in Amorphous In-Ga-Zn-O Heterojunction Channels for Thin-Film Transistors

Electrical and carrier transport properties in In–Ga–Zn–O thin-film transistors (IGZO TFTs) with a heterojunction channel were investigated. For the heterojunction IGZO channel, a high-In composition IGZO layer (IGZO-high-In) was deposited on a typical compositions IGZO layer (IGZO-111). From the optical properties and photoelectron yield spectroscopy measurements, the heterojunction channel was expected to have the type–II energy band diagram which possesses a conduction band offset (ΔE_c) of ~0.4 eV. A depth profile of background charge density indicated that a steep ΔE_c is formed even in the amorphous IGZO heterojunction interface deposited by sputtering. A field effect mobility (μ_FE) of bottom gate structured IGZO TFTs with the heterojunction channel (hetero-IGZO TFTs) improved to ~20 cm² V⁻¹ s⁻¹, although a channel/gate insulator interface was formed by an IGZO−111 (μ_FE = ~12 cm² V⁻¹ s⁻¹). Device simulation analysis revealed that the improvement of μ_FE in the hetero-IGZO TFTs was originated by a quantum confinement effect for electrons at the heterojunction interface owing to a formation of steep ΔE_c. Thus, we believe that heterojunction IGZO channel is an effective method to improve electrical properties of the TFTs.

Organic/Inorganic Hybrid Buffer in InGaZnO Transistors under Repetitive Bending Stress for High Electrical and Mechanical Stability

We investigated the influence of the multilayered hybrid buffer consisting of Al₂O₃/PA (polyacrylic) organic layer/Al₂O₃ on the electrical and mechanical properties of amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs). The multilayered organic/inorganic hybrid buffer has multiple beneficial effects on the flexible TFTs under repetitive bending stress. First, compared to the PA or Al₂O₃ single-layered buffer, the multilayered hybrid buffer showed an improved WVTR value of 1.1 × 10^-4 g/m² day. Even after 40,000 bending cycles, the WVTR value of the hybrid buffer increased only by 17%, while the WVTR value of the Al₂O₃ layer doubled after cyclical bending stress. We also confirmed that the hybrid buffer has advantages in mechanical durability of the TFT layers because of the change in the position of the neutral plane and the strain reduction effect by the PA organic layer. When we fabricate a top-gate a-IGZO TFT with the hybrid buffer layer (HB TFT), the device shows V_th = 0.74 V, μ_FE = 14.4 cm²/V·s, a subthreshold slope of 0.27 V/dec, and hysteresis of 0.21 V, which are superior to that of TFTs fabricated on an Al₂O₃ single-layer buffer (IB TFT). From the X-ray photoelectron spectroscopy and elastic recoil detection analysis, the difference in the electrical performance of TFTs could be explained by hydrogen-related molecules. After annealing at 270 °C, the amounts of hydrogen found in the a-IGZO layer for the IB, HB, and OB TFTs were 3.57 × 10²¹, 5.77 × 10²¹, and 7.34 × 10²¹ atoms/cm³, respectively. A top-gate bottom-contact structured a-IGZO TFT fabricated on the PA layer (OB TFT) showed a gate dielectric breakdown because of excessively high hydrogen content and high nonbonding oxygen content. On the other hand, HB TFTs showed better positive bias stability because of the higher hydrogen concentration, as hydrogen (when not excessive) is beneficial in passivating electron traps. Finally, we conducted 60,000 repetitive bending cycles on IB TFTs and HB TFTs with various bending radii down to 1.5 mm. The HB TFT shows improved mechanical durability and exhibits less electrical degradation during and after repetitive bending stress, compared to the IB TFT.

Investigation of the Capacitance-Voltage Electrical Characteristics of Thin-Film Transistors Caused by Hydrogen Diffusion under Negative Bias Stress in a Moist Environment

In this study, the impact of moisture on the electrical characteristics of an amorphous In-Ga-Zn-O thin-film transistor (a-IGZO TFT) was investigated. In commercial applications of such TFTs, high stability and quality performance in humid environments are essential. During TFT operation under ambient moisture, the electrolysis of water molecules occurs via the tip electric field effect. Hydrogen diffuses from the etch-stop layer or back-channel into the main channel under a negative electric field. The hydrogen atoms act as shallow donors (which causes the carrier concentration in the channel to rise), causing the threshold voltage (V_TH) to shift in the negative direction. Hydrogen diffusion from the overlap of the source/drain and gate electrodes to the channel center caused by the tip electric field induces a significant barrier lowering and V_TH shifts in a short-channel device. However, under negative bias stress (NBS) in ambient moisture, the negative V_TH shift is more obvious in short- than in long-channel devices, indicating suppressed hydrogen diffusion in long-channel devices. This is attributed to the electrolysis of water by the tip electric field at the source, drain, and gate electrodes, which causes hydrogen to diffuse to the center of the channel. Here, a novel physical model of the capacitance-voltage (C-V) electrical property changes under ambient moisture is proposed, based on the early appearance of abnormalities in the C-V measurements. The electrolysis of water caused by the tip electric field and electrical abnormalities caused by hydrogen diffusion into the a-IGZO active layer are explained by this model. A secondary-ion mass spectrometry analysis shows that hydrogen content in the channel generally increases under NBS in ambient moisture. The degradation behavior due to moisture in a-IGZO is clarified. Thus, inhibiting the tip electric field may benefit future flexible-display and gas-sensing applications.

Solution-Grown Homojunction Oxide Thin-Film Transistors

Growing attention has been given to low temperature, solution-processed metal oxide thin-film transistors because they can be applied in the emerging sector of flexible and large-scale electronics. However, major obstacles of solution-grown devices, such as their relatively low field-effect mobility and the difficulty of controlling carrier concentration, limit the further advancement of the electronics. Here, we overcome these constraints through a newly renovated structure, called a "homojunction", consisting of double-stacked semiconductors with same material. The homojunction oxide thin-film transistor has remarkable electrical performance with controllability, for example, tunable turn-on voltage (-80 V to -8 V) and high average field-effect mobility (∼50 cm²/V·s) are obtained via a low annealing temperature process (250 °C). Furthermore, notable achievements associated with stability, reliability, and uniformity are verified. These results are attributed to the unique phenomena of solution-grown thin films: the change of both chemical and physical properties of thin films. Our findings highlight that the thin films of high quality can be yielded through the solution process at low annealing temperatures, and thus solution-grown transistors hold great promise for widespread industrial applications.

Single crystal growth of bulk InGaZnO4 and analysis of its intrinsic transport properties

Large, high-quality InGaZnO₄ single crystals grown by high-pressure optical floating zone method and its electrical and optical properties.

Indium Tungsten Oxide Thin Films for Flexible High-Performance Transistors and Neuromorphic Electronics

Thin-film transistors (TFTs) with high electrical performances (mobility > 10 cm²/V s, V_th < 1 V, SS < 1 V/decade, on/off ratio ≈ 10⁶) obtained from the silicon- and oxide-based single-crystalline semiconductor materials require high processing temperature and hence are not suitable for flexible electronics. Amorphous oxide-based transparent electronic devices are attractive to meet emerging technological demands where crystalline oxide-/silicon-based architectures cannot provide a solution. Here, we tackle this problem by using a novel amorphous oxide semiconducting material-namely, indium tungsten oxide (IWO)-as the active channel in flexible TFTs (FTFTs). Post-annealing temperature as low as 270 °C for amorphous IWO thin films deposited by radio frequency sputtering at room temperature could result in smooth morphology ( R_rms ≈ 0.42 nm), good adhesion, and high carrier density ( n ≈ 7.19 × 10¹⁸ cm^-3). Excellent TFT characteristics of flexible devices could be achieved with linear field effect mobility μ_FE ≈ 25.86 cm²/V s, subthreshold swing SS ≈ 0.30 V/decade, threshold voltage V_th ≈ -1.5 V, and on/off ratio I_on/ I_off ≈ 5.6 × 10⁵ at 3 V and stable operation during bending of the FTFT. Additionally, IWO TFTs were implemented as synapses, the building block for neuromorphic computing. Paired-pulse facilitation up to 138% was observed and showed an exponential decay resembling chemical synapses. Utilizing this characteristic, a high-pass dynamic temporal filter was devised providing increased gain from 1.55 to 21 when frequency was raised from 22 to 62 Hz. The high performance and stability of flexible TFTs obtained with IWO films demonstrate their promise for low-voltage electronic applications.

Improving Thermal Stability of Solution-Processed Indium Zinc Oxide Thin-Film Transistors by Praseodymium Oxide Doping

Praseodymium-doped indium zinc oxide (PrIZO) channel materials have been fabricated by a solution process with conventional chemical precursor. The PrIZO-based thin-film transistors (TFTs) exhibited a field-effect mobility of 10.10 cm²/V s, a subthreshold swing value of 0.25 V/decade, and an I_on/ I_off ratio of 10⁸. The as-fabricated PrIZO-TFTs showed an improved device performance against positive bias temperature stress (PBTS shift of 1.97 V for 7200 s), which was evidently better than the undoped IZO-TFTs (PBTS shift of 9.52 V). This result indicates that the organic residual (-OCH₃ and -CH₂-) in metal-oxide semiconductor, which is confirmed to be a dominant effect on the performance of PBTS, can be passivated by the rare earth of praseodymium element. The residual is intended to be oxidized with a more stable ester group with the assistant of PrOx, weakening the electron-withdrawing characteristic during the thermal bias stress.

Large-scale transfer-free growth of thin graphite films at low temperature for solid diffusion barriers

Amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistors (TFTs) have been under intense investigation as one of the promising candidates for active matrix flat-panel displays. However, solid diffusion of a-IGZO to other layers during TFT device fabrication highly degrades their electrical and optical properties. It is expected that the diffusion-impenetrable properties of graphitic materials can be utilized as diffusion barriers. A conventional transfer method and direct growth on TFTs with high temperature are limited due to wet transfer conditions and low Tg (∼540 °C) of the glass substrates, respectively. Here we report the large-scale transfer-free growth of thin graphite films at low temperature (∼350 °C) for solid diffusion barriers in the a-IGZO TFTs using plasma enhanced chemical vapor deposition (PECVD), which can be widely used to protect solid-diffusion for sustainable and scalable future industrial technology.

Cyclical Annealing Technique To Enhance Reliability of Amorphous Metal Oxide Thin Film Transistors

This study introduces a cyclical annealing technique that enhances the reliability of amorphous indium-gallium-zinc-oxide (a-IGZO) via-type structure thin film transistors (TFTs). By utilizing this treatment, negative gate-bias illumination stress (NBIS)-induced instabilities can be effectively alleviated. The cyclical annealing provides several cooling steps, which are exothermic processes that can form stronger ionic bonds. An additional advantage is that the total annealing time is much shorter than when using conventional long-term annealing. With the use of cyclical annealing, the reliability of the a-IGZO can be effectively optimized, and the shorter process time can increase fabrication efficiency.

Instantaneous Semiconductor-to-Conductor Transformation of a Transparent Oxide Semiconductor a-InGaZnO at 45 °C

The emphasis on ubiquitous technology means that future technological applications will depend heavily on transparent conducting materials. To facilitate truly ubiquitous applications, transparent conductors should be fabricated at low temperatures (<50 °C). Here, we demonstrate an instantaneous (<100 ns) and low-temperature (<45 °C at the substrate) method, excimer laser irradiation, for the transformation of an a-InGaZnO semiconductor into a transparent highly conductive oxide with performance rivaling traditional and emerging transparent conductors. Our analysis shows that the instantaneous and substantial conductivity enhancement is due to the generation of a large amount of oxygen vacancies in a-InGaZnO after irradiation. The method's combination of low temperature, extremely rapid process, and applicability to other materials will create a new class of transparent conductors for the high-throughput roll-to-roll fabrication of future flexible devices.

Dimensional quantization effects in the thermodynamics of conductive filaments

We consider the physical effects of dimensional quantization in conductive filaments that underlie operations of some modern electronic devices. We show that, as a result of quantization, a sufficiently thin filament acquires a positive charge. Several applications of this finding include the host material polarization, the stability of filament constrictions, the equilibrium filament radius, polarity in device switching, and quantization of conductance.

Enhancement of a-IGZO TFT Device Performance Using a Clean Interface Process via Etch-Stopper Nano-layers

To overcome the technological and economic obstacles of amorphous indium-gallium-zinc-oxide (a-IGZO)-based display backplane for industrial production, a clean etch-stopper (CL-ES) process is developed to fabricate a-IGZO-based thin film transistor (TFT) with improved uniformity and reproducibility on 8.5th generation glass substrates (2200 mm × 2500 mm). Compared with a-IGZO-based TFT with back-channel-etched (BCE) structure, a newly formed ES nano-layer (~ 100 nm) and a simultaneous etching of a-IGZO nano-layer (30 nm) and source-drain electrode layer are firstly introduced to a-IGZO-based TFT device with CL-ES structure to improve the uniformity and stability of device for large-area display. The saturation electron mobility of 8.05 cm²/V s and the V_th uniformity of 0.72 V are realized on the a-IGZO-based TFT device with CL-ES structure. In the negative bias temperature illumination stress and positive bias thermal stress reliability testing under a ± 30 V bias for 3600 s, the measured V_th shift of CL-ES-structured device significantly decreased to - 0.51 and + 1.94 V, which are much lower than that of BCE-structured device (- 3.88 V, + 5.58 V). The electrical performance of the a-IGZO-based TFT device with CL-ES structure implies that the economic transfer from a silicon-based TFT process to the metal oxide semiconductor-based process for LCD fabrication is highly feasible.

Electrical Performance and Reliability Improvement of Amorphous-Indium-Gallium-Zinc-Oxide Thin-Film Transistors with HfO₂ Gate Dielectrics by CF₄ Plasma Treatment

In this work, amorphous indium-gallium-zinc oxide thin-film transistors (a-IGZO TFTs) with a HfO₂ gate insulator and CF₄ plasma treatment was demonstrated for the first time. Through the plasma treatment, both the electrical performance and reliability of the a-IGZO TFT with HfO₂ gate dielectric were improved. The carrier mobility significantly increased by 80.8%, from 30.2 cm²/V∙s (without treatment) to 54.6 cm²/V∙s (with CF₄ plasma treatment), which is due to the incorporated fluorine not only providing an extra electron to the IGZO, but also passivating the interface trap density. In addition, the reliability of the a-IGZO TFT with HfO₂ gate dielectric has also been improved by the CF₄ plasma treatment. By applying the CF₄ plasma treatment to the a-IGZO TFT, the hysteresis effect of the device has been improved and the device’s immunity against moisture from the ambient atmosphere has been enhanced. It is believed that the CF₄ plasma treatment not only significantly improves the electrical performance of a-IGZO TFT with HfO₂ gate dielectric, but also enhances the device’s reliability.

Effects of annealing temperature on properties of InSnZnO thin film transistors prepared by Co-sputtering

Indium-tin-zinc-oxide (ITZO) as the channel layer grown by co-sputtering of ZnO target and ITO target in the bottom gate thin-film transistors (TFTs) is proposed in this work. The microstructure and optical properties of ITZO thin films at different annealing temperatures were analyzed. The impact of various annealing temperatures on the ITZO TFT performance characteristics was systematically investigated as well. It was found that ITZO TFT with annealing temperature of 300 °C exhibits excellent electrical performance with a high saturation field-effect mobility (μ_sat) of 27.4 cm² V⁻¹ s⁻¹, a low threshold voltage (V_th) of −0.64 V, a small subthreshold swing (SS) value of 0.23 V per decade, and the high on-off current ratio (I_on/I_off) of 1.8 × 10⁷. In addition, it also shows good output curves including gate control capabilities and good electrode contact as well as extreme atmospheric stability. As shown by photoluminescence (PL) analysis and X-ray photoelectron spectroscopy (XPS) analysis, the beneficial effects of various annealing temperatures on device performance are attributed to the reorganization of the amorphous network and the control of defect chemistry in the films. The correlation between the post-deposition thermal treatment and the characteristics of a transistor was investigated and excellent performance of the transistor was demonstrated.

Indium-tin-zinc-oxide (ITZO) as the channel layer grown by co-sputtering of ZnO target and ITO target in the bottom gate thin-film transistors (TFTs) is proposed in this work.

Effect of hydrogen diffusion in an In–Ga–Zn–O thin film transistor with an aluminum oxide gate insulator on its electrical properties

We fabricated amorphous InGaZnO thin film transistors (a-IGZO TFTs) with aluminum oxide (Al₂O₃) as a gate insulator grown through atomic layer deposition (ALD) method at different deposition temperatures (T_dep). The Al₂O₃ gate insulator with a low T_dep exhibited a high amount of hydrogen in the film, and the relationship between the hydrogen content and the electrical properties of the TFTs was investigated. The device with the Al₂O₃ gate insulator having a high H content showed much better transfer parameters and reliabilities than the low H sample. This is attributed to the defect passivation effect of H in the active layer, which is diffused from the Al₂O₃ layer. In addition, according to the post-annealing temperature (T_post-ann), a-IGZO TFTs exhibited two unique changes of properties; the degradation in low T_post-ann and the enhancement in high T_post-ann, as explained in terms of H diffusion from the gate insulator to an active layer.

We fabricated amorphous InGaZnO thin film transistors (a-IGZO TFTs) with aluminum oxide (Al₂O₃) as a gate insulator grown through atomic layer deposition (ALD) method at different deposition temperatures (T_dep).

Activity Dependent Synaptic Plasticity Mimicked on Indium-Tin-Oxide Electric-Double-Layer Transistor

Ion coupling has provided an additional method to modulate electric properties for solid-state materials. Here, phosphorosilicate glass (PSG)-based electrolyte gated protonic/electronic coupled indium-tin-oxide electric-double-layer (EDL) transistors are fabricated. The oxide transistor exhibits good electrical performances due to an extremely strong proton gating behavior for the electrolyte. With interfacial electrochemical doping, channel conductances of the oxide EDL transistor can be regulated to different levels, corresponding to different initial synaptic weights. Thus, activity dependent synaptic responses such as excitatory postsynaptic current, paired-pulse facilitation, and high-pass filtering are discussed in detail. The proposed proton conductor gated oxide EDL synaptic transistors with activity dependent synaptic plasticities may act as fundamental building blocks for neuromorphic system applications.

Room-Temperature Activation of InGaZnO Thin-Film Transistors via He+ Irradiation

Amorphous indium gallium zinc oxide (a-IGZO) is a transparent semiconductor which has demonstrated excellent electrical performance as thin-film transistors (TFTs). However, a high-temperature activation process is generally required which is incompatible for next-generation flexible electronic applications. In this work, He⁺ irradiation is demonstrated as an athermal activation process for a-IGZO TFTs. Controlling the He⁺ dose enables the tuning of charge density, and a dose of 1 × 10¹⁴ He⁺/cm² induces a change in charge density of 2.3 × 10¹² cm^-2. Time-dependent transport measurements and time-of-flight secondary ion mass spectroscopy (ToF-SIMS) indicate that the He⁺-induced trapped charge is introduced because of preferential oxygen-vacancy generation. Scanning microwave impedance microscopy confirms that He⁺ irradiation improves the conductivity of the a-IGZO. For realization of a permanent activation, IGZO was exposed with a He⁺ dose of 5 × 10¹⁴ He⁺/cm² and then aged 24 h to allow decay of the trapped oxide charge originating for electron-hole pair generation. The resultant shift in the charge density is primarily attributed to oxygen vacancies generated by He⁺ sputtering in the near-surface region.

Origin of the apparent delocalization of the conduction band in a high-mobility amorphous semiconductor

In this paper, we show that the apparent delocalization of the conduction band reported from first-principles simulations for the high-mobility amorphous oxide semiconductor [Formula: see text] (a-IGZO) is an artifact induced by the periodic conditions imposed to the model. Given a sufficiently large unit-cell dimension (over 40 Å), the conduction band becomes localized. Such a model size is up to four times the size of commonly used models for the study of a-IGZO. This finding challenges the analyses done so far on the nature of the defects and on the interpretation of numerous electrical measurements. In particular, we re-interpret the meaning of the computed effective mass reported so far in literature. Our finding also applies to materials such as SiZnSnO, ZnSnO, InZnSnO, In₂O₃ or InAlZnO₄ whose models have been reported to display a fully delocalized conduction band in the amorphous phase.

Flexible and High-Performance Amorphous Indium Zinc Oxide Thin-Film Transistor Using Low-Temperature Atomic Layer Deposition

Amorphous indium zinc oxide (IZO) thin films were deposited at different temperatures, by atomic layer deposition (ALD) using [1,1,1-trimethyl-N-(trimethylsilyl)silanaminato]indium (INCA-1) as the indium precursor, diethlzinc (DEZ) as the zinc precursor, and hydrogen peroxide (H₂O₂) as the reactant. The ALD process of IZO deposition was carried by repeated supercycles, including one cycle of indium oxide (In₂O₃) and one cycle of zinc oxide (ZnO). The IZO growth rate deviates from the sum of the respective In₂O₃ and ZnO growth rates at ALD growth temperatures of 150, 175, and 200 °C. We propose growth temperature-dependent surface reactions during the In₂O₃ cycle that correspond with the growth-rate results. Thin-film transistors (TFTs) were fabricated with the ALD-grown IZO thin films as the active layer. The amorphous IZO TFTs exhibited high mobility of 42.1 cm² V^-1 s^-1 and good positive bias temperature stress stability. Finally, flexible IZO TFT was successfully fabricated on a polyimide substrate without performance degradation, showing the great potential of ALD-grown TFTs for flexible display applications.

Engineering of band gap states of amorphous SiZnSnO semiconductor as a function of Si doping concentration

We investigated the band gap of SiZnSnO (SZTO) with different Si contents. Band gap engineering of SZTO is explained by the evolution of the electronic structure, such as changes in the band edge states and band gap. Using ultraviolet photoelectron spectroscopy (UPS), it was verified that Si atoms can modify the band gap of SZTO thin films. Carrier generation originating from oxygen vacancies can modify the band-gap states of oxide films with the addition of Si. Since it is not easy to directly derive changes in the band gap states of amorphous oxide semiconductors, no reports of the relationship between the Fermi energy level of oxide semiconductor and the device stability of oxide thin film transistors (TFTs) have been presented. The addition of Si can reduce the total density of trap states and change the band-gap properties. When 0.5 wt% Si was used to fabricate SZTO TFTs, they showed superior stability under negative bias temperature stress. We derived the band gap and Fermi energy level directly using data from UPS, Kelvin probe, and high-resolution electron energy loss spectroscopy analyses.

Metal oxides for optoelectronic applications

Metal oxides (MOs) are the most abundant materials in the Earth's crust and are ingredients in traditional ceramics. MO semiconductors are strikingly different from conventional inorganic semiconductors such as silicon and III-V compounds with respect to materials design concepts, electronic structure, charge transport mechanisms, defect states, thin-film processing and optoelectronic properties, thereby enabling both conventional and completely new functions. Recently, remarkable advances in MO semiconductors for electronics have been achieved, including the discovery and characterization of new transparent conducting oxides, realization of p-type along with traditional n-type MO semiconductors for transistors, p-n junctions and complementary circuits, formulations for printing MO electronics and, most importantly, commercialization of amorphous oxide semiconductors for flat panel displays. This Review surveys the uniqueness and universality of MOs versus other unconventional electronic materials in terms of materials chemistry and physics, electronic characteristics, thin-film fabrication strategies and selected applications in thin-film transistors, solar cells, diodes and memories.

Rational Design of ZnO:H/ZnO Bilayer Structure for High-Performance Thin-Film Transistors

The intriguing properties of zinc oxide-based semiconductors are being extensively studied as they are attractive alternatives to current silicon-based semiconductors for applications in transparent and flexible electronics. Although they have promising properties, significant improvements on performance and electrical reliability of ZnO-based thin film transistors (TFTs) should be achieved before they can be applied widely in practical applications. This work demonstrates a rational and elegant design of TFT, composed of poly crystalline ZnO:H/ZnO bilayer structure without using other metal elements for doping. The field-effect mobility and gate bias stability of the bilayer structured devices have been improved. In this device structure, the hydrogenated ultrathin ZnO:H active layer (∼3 nm) could provide suitable carrier concentration and decrease the interface trap density, while thick pure-ZnO layer could control channel conductance. Based on this novel structure, a high field-effect mobility of 42.6 cm(2) V(-1) s(-1), a high on/off current ratio of 10(8) and a small subthreshold swing of 0.13 V dec(-1) have been achieved. Additionally, the bias stress stability of the bilayer structured devices is enhanced compared to the simple single channel layer ZnO device. These results suggest that the bilayer ZnO:H/ZnO TFTs have a great potential for low-cost thin-film electronics.

Carrier Transport at Metal/Amorphous Hafnium-Indium-Zinc Oxide Interfaces

In this paper, the carrier transport mechanism at the metal/amorphous hafnium-indium-zinc oxide (a-HIZO) interface was investigated. The contact properties were found to be predominantly affected by the degree of interfacial reaction between the metals and a-HIZO; that is, a higher tendency to form metal oxide phases leads to excellent Ohmic contact via tunneling, which is associated with the generated donor-like oxygen vacancies. In this case, the Schottky-Mott theory is not applicable. Meanwhile, metals that do not form interfacial metal oxide, such as Pd, follow the Schottky-Mott theory, which results in rectifying Schottky behavior. The Schottky characteristics of the Pd contact to a-HIZO can be explained in terms of the barrier inhomogeneity model, which yields a mean barrier height of 1.40 eV and a standard deviation of 0.14 eV. The work function of a-HIZO could therefore be estimated as 3.7 eV, which is in good agreement with the ultraviolet photoelectron spectroscopy (3.68 eV). Our findings will be useful for establishing a strategy to form Ohmic or Schottky contacts to a-HIZO films, which will be essential for fabricating reliable high-performance electronic devices.

Electronic Structure of Low-Temperature Solution-Processed Amorphous Metal Oxide Semiconductors for Thin-Film Transistor Applications

The electronic structure of low temperature, solution-processed indium-zinc oxide thin-film transistors is complex and remains insufficiently understood. As commonly observed, high device performance with mobility >1 cm² V^-1 s^-1 is achievable after annealing in air above typically 250 °C but performance decreases rapidly when annealing temperatures ≤200 °C are used. Here, the electronic structure of low temperature, solution-processed oxide thin films as a function of annealing temperature and environment using a combination of X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and photothermal deflection spectroscopy is investigated. The drop-off in performance at temperatures ≤200 °C to incomplete conversion of metal hydroxide species into the fully coordinated oxide is attributed. The effect of an additional vacuum annealing step, which is beneficial if performed for short times at low temperatures, but leads to catastrophic device failure if performed at too high temperatures or for too long is also investigated. Evidence is found that during vacuum annealing, the workfunction increases and a large concentration of sub-bandgap defect states (re)appears. These results demonstrate that good devices can only be achieved in low temperature, solution-processed oxides if a significant concentration of acceptor states below the conduction band minimum is compensated or passivated by shallow hydrogen and oxygen vacancy-induced donor levels.

Spatial atmospheric atomic layer deposition of InxGayZnzO for thin film transistors

We have investigated the nucleation and growth of InGaZnO thin films by spatial atmospheric atomic layer deposition. Diethyl zinc (DEZ), trimethyl indium (TMIn), triethyl gallium (TEGa), and water were used as Zn, In, Ga and oxygen precursors, respectively. The vaporized metal precursors have been coinjected in the reactor. The metal composition of InGaZnO has been controlled by varying the TMIn or TEGa flow to the reactor, for a given DEZ flow and exposure time. The morphology of the films changes from polycrystalline, for ZnO and In-doped ZnO, to amorphous for In-rich IZO and InGaZnO. The use of these films as the active channel in TFTs has been demonstrated and the influence of In and Ga cations on the electrical characteristics of the TFTs has been studied.

Low-temperature fabrication of high performance indium oxide thin film transistors

In this study, indium oxide (In₂O₃) thin-film transistors (TFTs) were fabricated by a fully-solution process at low temperature.