a-IGZO Films Research Articles

Electrical Performance and Bias-Stress Stability of Amorphous InGaZnO Thin-Film Transistors with Buried-Channel Layers

To improve the electrical performance and bias-stress stability of amorphous InGaZnO thin-film transistors (a-IGZO TFTs), we fabricated and characterized buried-channel devices with multiple-stacked channel layers, i.e., a nitrogen-doped a-IGZO film (front-channel layer), a conventional a-IGZO film (buried-channel layer), and a nitrogen-doped a-IGZO film (back-channel layer). The larger field-effect mobility (5.8 cm2V−1s−1), the smaller subthreshold swing value (0.8 V/dec, and the better stability (smaller threshold voltage shifts during bias-stress and light illumination tests) were obtained for the buried-channel device relative to the conventional a-IGZO TFT. The specially designed channel-layer structure resulted in multiple conduction channels and hence large field-effect mobility. The in situ nitrogen-doping caused reductions in both the front-channel interface trap density and the density of deep states in the bulk channel layers, leading to a small subthreshold swing value. The better stability properties may be related to both the reduced trap states by nitrogen-doping and the passivation effect of the nitrogen-doped a-IGZO films at the device back channels.

Control of the Boundary between the Gradual and Abrupt Modulation of Resistance in the Schottky Barrier Tunneling-Modulated Amorphous Indium-Gallium-Zinc-Oxide Memristors for Neuromorphic Computing

The transport and synaptic characteristics of the two-terminal Au/Ti/ amorphous Indium-Gallium-Zinc-Oxide (a-IGZO)/thin SiO2/p+-Si memristors based on the modulation of the Schottky barrier (SB) between the resistive switching (RS) oxide layer and the metal electrodes are investigated by modulating the oxygen content in the a-IGZO film with the emphasis on the mechanism that determines the boundary of the abrupt/gradual RS. It is found that a bimodal distribution of the effective SB height (ΦB) results from further reducing the top electrode voltage (VTE)-dependent Fermi-level (EF) followed by the generation of ionized oxygen vacancies (VO2+s). Based on the proposed model, the influences of the readout voltage, the oxygen content, the number of consecutive VTE sweeps on ΦB, and the memristor current are explained. In particular, the process of VO2+ generation followed by the ΦB lowering is gradual because increasing the VTE-dependent EF lowering followed by the VO2+ generation is self-limited by increasing the electron concentration-dependent EF heightening. Furthermore, we propose three operation regimes: the readout, the potentiation in gradual RS, and the abrupt RS. Our results prove that the Au/Ti/a-IGZO/SiO2/p+-Si memristors are promising for the monolithic integration of neuromorphic computing systems because the boundary between the gradual and abrupt RS can be controlled by modulating the SiO2 thickness and IGZO work function.

Electronic structure of interstitial hydrogen in In-Ga-Zn-O semiconductor simulated by muon

We report on the local electronic structure of an interstitial muon (Mu) as pseudohydrogen in In-Ga-Zn oxide (IGZO) semiconductors studied by muon spin rotation/relaxation (μSR) experiment. In polycrystalline (c-) IGZO, it is inferred that Mu is in a diamagnetic state, where the μSR time spectra under zero external field are perfectly described by the Gaussian Kubo-Toyabe relaxation function with the linewidth Δ serving as a sensitive measure for the random local fields from In/Ga nuclear magnetic moments. The magnitude of Δ combined with the density functional theory calculations for H (to mimic Mu) suggests that Mu occupies Zn-O bond-center site (MuBC) similar to the case in crystalline ZnO. This implies that the diamagnetic state in c-IGZO corresponds to MuBC+, thus serving as an electron donor. In amorphous (a-) IGZO, the local Mu structure in the as-deposited films is nearly identical to that in c-IGZO, suggesting MuBC+ for the electronic state. In contrast, the diamagnetic signal in heavily hydrogenated a-IGZO films exhibits the Lorentzian Kubo-Toyabe relaxation, implying that Mu accompanies more inhomogeneous distribution of the neighboring nuclear spins that may involve a Mu− H−-complex state in an oxygen vacancy.

Electron effective mass and electronic structure in nonstoichiometric amorphous Indium Gallium Zinc Oxide films

The transport properties and optical transmittance and absorption spectra for the nostoichiometric amorphous indium gallium zinc oxide (a-IGZO) films with gallium and zinc deficiencies are investigated. The resistivity and carrier concentration variation with temperature both reveal that the films possess degenerate semiconductor (or metal) characteristics. The thermopower is negative and decreases linearly with decreasing temperature, indicating the electron diffusion thermopower governs the thermal transport process in each film. Using free-electron-like model, we extracted the electron effective mass, which is about three times as large as that of the stoichiometric one and increases with increasing carrier (electron) concentration. Neglecting the variation in the energy with the wavevector near the valence band maximum and using the free-electron-like model, we also obtained the electron effective mass via the optical absorption spectra measurement. The magnitude of the effective mass obtained via optical spectra measurement is comparable to that obtained via thermopower measurement for each film. Our results strongly suggest that the nostoichiometric a-IGZO films possess free-electron-like pseudo-energy-bandstructure.

Room Temperature-Processed a-IGZO Schottky Diode for Rectifying Circuit and Bipolar 1D1R Crossbar Applications

In this paper, we demonstrate a tunable Schottky diode based on amorphous indium-gallium-zinc-oxide (a-IGZO) film with high performance for rectifying circuit and bipolar one diode-one resistor (1D1R) crossbar applications. By modulating the oxygen content in the a-IGZO film, the device shows adjustable behavior. Under the high oxygen content condition, the device displays outstanding diode features with a large rectification ratio up to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> at ±1 V and a high forward current density of 100 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at ±1 V when used Cu as the top electrode. The thermionic emission model is proposed to explain the Schottky diode behavior. And by utilizing the superior rectifying performance, we successfully demonstrate the dynamic rectifying application. In addition, we combine the optimized Schottky diode device with the bipolar-type resistive random access memory (RRAM) device to investigate the Schottky diode device in 1D1R crossbar application. As a result of its tunable nature and low processing temperature, the a-IGZO Schottky diode is a promising element for rectifying circuit and bipolar 1D1R crossbar applications.

Optimizing the Properties of InGaZnOx Thin Film Transistors by Adjusting the Adsorbed Degree of Cs+ Ions.

To improve the performance of amorphous InGaZnOx (a-IGZO) thin film transistors (TFTs), in this thesis, Cs+ ions adsorbed IGZO (Cs-IGZO) films were prepared through a solution immersion method at low temperature. Under the modification of surface structure and oxygen vacancies concentrations of a-IGZO film, with the effective introduction of Cs+ ions into the surface of a-IGZO films, the transfer properties and stability of a-IGZO TFTs are greatly improved. Different parameters of Cs+ ion concentrations were investigated in our work. When the Cs+ ions concentration reached 2% mol/L, the optimized performance Cs-IGZO TFT was obtained, showing the carrier mobility of 18.7 cm2 V−1 s−1, the OFF current of 0.8 × 10−10 A, and the threshold voltage of 0.2 V, accompanied by the threshold voltage shifts of 1.3 V under positive bias stress for 5000 s.

Low-temperature formation of high-mobility a-InGaZnOx films using plasma-enhanced reactive processes

This work demonstrated the low-temperature fabrication of high-mobility a-InGaZnOx (a-IGZO) films using plasma-enhanced reactive processing. The effect of the post-processing temperature on the performance of IGZO thin-film transistors (TFTs) using these a-IGZO films as a channel layer was also investigated. IGZO TFTs incorporating a-IGZO films plasma-treated at temperatures as low as 200 °C exhibited the expected TFT transfer characteristics, with field effect mobility values as high as 40 cm2 V−1 s−1. The fabrication of IGZO TFTs that exhibited homogeneous characteristics over large substrate areas was attempted, by assessing the effects of the plasma treatment on a-IGZO films deposited at various oxygen flow rate ratios. The electrical properties of IGZO TFTs using plasma-treated films were found to be dramatically improved, irrespective of the film deposition conditions prior to the plasma treatment. These results confirm the feasibility of producing IGZO TFTs with uniform characteristics over large substrates areas.

High-Speed and Low-Temperature Atmospheric Photo-Annealing of Large-Area Solution-Processed IGZO Thin-Film Transistors by Using Programmable Pulsed Operation of Xenon Flash Lamp

An efficient photo-annealing approach for high-performance solution-processed metal-oxide thin film transistors (TFTs) was demonstrated by using programmable pulse operation of xenon flash lamp. The flash lamp annealing (FLA) process could offer not only low-temperature (≈100° C) processing but also ultra-fast annealing speed of the order of seconds under air ambient conditions. Solution-processed amorphous indium-gallium-zinc-oxide (α-IGZO) TFTs implemented by the FLA process typically exhibited much improved electrical performance such as saturation mobility of >10.8 cm2V−11s−1, ION/IOPP of >108, and subthreshold slope of as steep as 0.24 V/dec. X-ray photoelectron spectroscopy analysis of a-IGZO films indicates that the FLA can provide sufficient activation energy for rapid formation of solid a-IGZO bonds within 30 s. The high-quality metal-oxide films achieved by an atmospheric and low temperature FLA method may represent a significant advance for scalable fabrication of flexible and printed metal-oxide electronics.

Hydrogen barrier performance of sputtered La2O3 films for InGaZnO thin-film transistor

We report the hydrogen barrier performance of sputtered La2O3 thin films for the device stability of amorphous indium–gallium–zinc–oxide (a-IGZO) thin-film transistors (TFTs). Hydrogen acts as a shallow donor in a-IGZO films, which makes TFTs conductive, resulting in degradation of their on/off properties. Since hydrogen can be easily incorporated by external environments or post-processing, an appropriate hydrogen barrier is essential for enhancing device stability. La2O3, with its extreme electronegativity, can provide excellent hygroscopic characteristics. Because hydrogen exists in the form of –OH groups inside a-IGZO films, La2O3 is expected to be a promising barrier material for preventing hydrogen incorporation. Therefore, we investigate the growth characteristics of sputtered La2O3 thin films as hydrogen barrier layers, focusing on variations in growth rate, refractive index, and film stress, which depend on various process parameters, such as radio-frequency (RF) power, O2 partial pressure, and substrate temperature during reactive magnetron sputtering. The effects of these parameters on hydrogen barrier properties are systematically investigated and correlated with the microstructures of La2O3 films. The results demonstrate that La2O3 films grown with low RF power and low O2 partial pressure have an amorphous phase and provide excellent hydrogen barrier performance. We anticipate that these experimental results will help improve the environmental stability of a-IGZO TFTs.

Ultraviolet laser damage mechanisms of amorphous InGaZnO4 thin films

Laser annealing is a usual step in the process to improve the performance of amorphous InGaZnO4 (a-IGZO) thin film transistors (TFTs). However, a high energy laser will induce damage to a-IGZO thin films during annealing. Knowing laser induced damage thresholds (LIDT) and the mechanisms of a-IGZO thin films helps to use appropriate laser energy density during annealing to avoid damage to the thin films and to achieve the best TFTs’ properties. In this article, the ultraviolet laser with a wavelength of 355 nm and a pulse width of 7.7 ns LIDT and damage mechanisms of a-IGZO thin films are reported. The damage morphologies are characterized with optical microscopy and scanning electron microscope and Raman spectra. The electrical and optical properties of a-IGZO thin films are studied. The a-IGZO thin films have LIDT increased from 0.12 J/cm2, 0.16 J/cm2, and 0.23 J/cm2 to 0.24 J/cm2 with absorbance decreased from 22.4%, 18.1%, and 17.3% to 12.3%. The concentrations of oxygen and free carrier (Ne) and thermal conductivity and optical band gap (Eg) and electrical effective mass (m*) are important factors affecting the LIDT of thin films. The thermal conductivity influences the surface temperature and LIDT of thin films. The increased Eg and m* with the decreased Ne in thin films are other important reasons for the increased LIDT. The laser mainly induces thermal damage of thin films with intrinsic processes including avalanche ionization and multi-photon absorption. No apparent phase transformation and lattice expansion exists during laser irradiation for the stable amorphous structures of a-IGZO thin films.

Comparative Study on Performance of IGZO Transistors With Sputtered and Atomic Layer Deposited Channel Layer

The structural, chemical, and electrical properties of amorphous indium gallium zinc oxide (a-IGZO) films by magnetron sputtering and atomic layer deposition (ALD) were investigated where both a-IGZO films had a comparable cation composition. The ALD-derived a-IGZO film exhibited the higher atomic packing density, the effective suppression of trap-like oxygen vacancy defect (VO), and the enhancement in the hybridization of the sp orbital of In, Ga, and Zn cations compared to those of the sputtered a-IGZO film. Hence, a significant improvement in terms of the field-effect mobility was observed for the thin-film transistors with an In0.50Ga0.34Zn0.16O channel by ALD (36.6 cm $^{\textsf {2}}/\text{V}\cdot \text{s}$ ) compared to that of the sputtered In0.48Ga0.38Zn0.14O transistor (20.1 cm $^{\textsf {2}}/\text{V}\cdot \text{s}$ ); the ${I}_{\text {ON/OFF}}$ ratios for both were ~107. Simultaneously, the gate bias stress stability and photobias stress stability were also improved for the IGZO transistors with an ALD-derived channel, which can be explained by its reduced trap-like VO density.

The Effect of Microwave Annealing of Reliability Characteristics on Amorphous IGZO Thin Film Transistors.

Amorphous oxide semiconductors (AOSs) are attracted much attention due to high mobility, low temperature deposition, flexible, transmission, and uniformity. The thin film transistors (TFTs) with a-IGZO thin film as active layer perform higher field-effect mobility (>10 cm²/V·S), larger Ion/Ioff ratio (106), smaller subthreshold swing and better stability against electrical stress. LaAlO₃/ZrO₂ is employed as gate electrode and gate dielectric layer for a-IGZO TFTs, under the premise that performance of a-IGZO TFTs without decreasing. Due to the good selectivity of energy transformation and rapid heating rate, microwave annealing is applied to improve the device reliability in the investigation. With adjusting the parameter of microwave annealing, the effect on reliability characteristics of a-IGZO TFTs is studied.

Investigation of Electrical Characteristics on AP-PECVD Fabricated Amorphous IGZO TFTs with Hydrogen Plasma Treatment.

TFT panel production process can be divided into three kinds of technology. There have amorphous silicon (a-Si), low-temperature polysilicon (LTPS) and amorphous IGZO (a-IGZO) oxide. Traditional amorphous silicon (a-Si) silicon has a lot of advantages such as good productivity, short process and low-cost. It also has a lot of shortcomings on these applications on TFTs such as photosensitivity, light degradation, and opacity, etc. The dispute of the material based on a-Si:H as an active layer in TFT is low field effect mobility (~1 cm²/V·S) (M. Shur and M. Hack, J. Appl. Phys. 55, 3831 (1984)), photo sensitivity (low band gap about 1.7 V) and high deposition temperature (~400 °C) (M. Shur, et al., J. Appl. Phys. 66, 3371 (1989); K. khakzar and E. H. Lueder, IEEE Trans. Electron Devices 39, 1438 (1992)). Amorphous In-Ga-Zn-O (IGZO) had attracted attention that compared with the conventional a-Si:H, in the past three years, a-IGZO thin film transistors is more popular which compared with the other oxide semiconductors, because of its larger Ion/Ioff ratio (>106, smaller subthreshold swing (SS), better field-effect mobility and better stability against electrical stress. Hydrogen plasma treatment is applied in improving a-IGZO TFTs active layer, which is fabricated by atmospheric pressure-plasma enhanced chemical vapor deposition (AP-PECVD), the electrical characteristics of a-IGZO TFTs is investigated.

Collaborative optimization of thermal budget annealing and active layer defect content enhancing electrical characteristics and bias stress stability in InGaZnO thin-film transistors

The impact of thermal treatment temperature on the density of oxygen-related defects in the lattice of amorphous InGaZnO is investigated. X-ray photoelectron spectroscopy results state that the oxygen-deficient defects are remarkably reduced and the quality of a-IGZO film is effectively improved after annealing under the appropriate temperature of 350 °C. With the help of synergistic effect of proper treatment temperature and high quality IGZO layer, the IGZO thin-film transistor exhibits outstanding electrical properties, including a field effect mobility of 8.70 cm2 V−1 s−1, a threshold voltage of 1.81 V, a subthreshold swing of 307 mV/dec., and a small hysteresis of 0.32 V. Moreover, the transmission line model clarifies that the suitable heating temperature is beneficial to alleviate the source/drain contact resistance as well as the effective channel length loss. More notably, the positive bias stress stability of a-IGZO TFTs is simultaneously improved.

Revisit of amorphous semiconductor InGaZnO4: A new electron transport material for perovskite solar cells

In this paper, we revisit the state-of-the-art amorphous semiconductor InGaZnO4 (a-IGZO), and demonstrate the application of a-IGZO thin film as an electron transporting layer (ETL) for efficient perovskite solar cells (PSCs) by a solution process. The champion device using such a-IGZO ETL has achieved an efficiency of 17.4%. The outstanding performance is attributed to the excellent properties of a-IGZO oxide thin films, including high carrier concentration, high electron mobility and high transparency. Our results definitely revel that amorphous semiconductors (e.g. a-IGZO) offer some unique features and have a great potential for large-area perovskite solar cells.

Design and Mechanism of Embedding Specific Carbon Nanotubes in Sputtered Sandwiched InGaZnO Thin-Film Transistors

Amorphous indium–gallium–zinc oxide (a-IGZO) thin-film transistors (TFTs) with high mobility are highly desirable to achieve high-speed operation in active-matrix displays and large-area sensors. Only a few works provide potential solutions. In this letter, we have embedded carbon nanotubes (CNTs) into sputtered a-IGZO film to form a-IGZO/CNT/a-IGZO (ACA) sandwiched channel. In the ACA channel, the CNT percolation networks connected by the a-IGZO film work as high-speed carrier paths to enable faster transport of carriers during the ON state, while it does not degrade the leakage performance during the OFF state. The type and the embedding location of the CNT percolation network are critical to determine the ACA device performance, which is analyzed and verified by experiment and simulation. The optimum ACA design has approximately doubled the mobility and the ON-current density of the TFT. The design owns relatively better uniformity and provides a high-speed TFT solution for the advanced electronics.

Ferroelectric Field Effect Transistors Based on PZT and IGZO

Ferroelectric field effect transistors (FeFETs) based on lead zirconate titanate (PZT) ferroelectric material and amorphous-indium-gallium-zinc oxide (a-IGZO) were developed and characterized. The PZT material was processed by a sol-gel method and then used as ferroelectric gate. The a-IGZO thin films, having the role of channel semiconductor, were deposited by radio-frequency magnetron sputtering, at a temperature of ~50°C. Characteristics of a typical field effect transistor with SiO2 gate insulator, grown on highly doped silicon, and of the PZT-based FeFET were compared. It was proven that the FeFETs had promising performances in terms of Ion/Ioff ratio (i.e., 106) and IDS retention behavior.

High Performance Amorphous IGZO Thin-Film Transistor Based on Alumina Ceramic

Ceramic materials are high-temperature resistant materials with promising prospects. In some applications, semiconductor devices need to be integrated with a ceramic substrate. Herein, we report on the stable operation of an Al 2 O 3 ceramic-based amorphous-Indium gallium zinc oxide (a-IGZO) thinfilm transistor (TFT) at room temperature up to 523 K. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were used to characterize the a-IGZO film. A mixed solution was printed on the surface of an insulating layer of alumina. After the combustion reaction, the metal electrode was printed on the surface of a-IGZO to obtain a TFT. The ION/IOFF ratio was 6.04 × 10 6 at 293 K, and it was maintained at 1.44 × 10 5 at 523 K. It was demonstrated that the parameters of a-IGZO TFTs such as the subthreshold swing (SS), g m and μ sat changed at different temperatures. As such, they can be used as building blocks for integrated circuits that can operate at high temperatures. The fabrication of TFT-based inverters, NAND and NOR gate circuits facilitate the exploration of the possibility of more complex digital circuits that operate at high temperatures, based on hybrid circuit design.

Flexible TEG Using Amorphous InGaZnO Thin Film

We have demonstrated flexible thermoelectric generators (TEGs) using transverse type structure with amorphous InGaZnO/Mo and amorphous InGaZnO/ITO materials. The amorphous-InGaZnO (a-IGZO) film has advantages such as its low thermal conductivity and relatively high carrier mobility. Additionally, the a-IGZO film can form on a flexible substrate using a low temperature fabrication process. The a-IGZO thin films were deposited at room temperature by RF magnetron sputtering on PEN substrate. The thin film TEG can create a temperature difference in the in-plane lateral direction by using a heat guide. For the heat guide, polymer photoresist material (KMPR 1035) was spin-coated and patterned using the standard photolithography process. Metal masks were used for all the deposition steps. A total of 676 a-IGZO/metal paired cells were connected in series. Temperature difference was applied between the heat guide and the back side of the substrate, and it was confirmed that both temperature difference and Seebeck voltage were generated in the IGZO cell. We have shown that the transparent and flexible TEG on PEN substrate can be fabricated using a-IGZO with the standard fabrication process.

Silicon germanium photo-blocking layers for a-IGZO based industrial display

Amorphous indium- gallium-zinc oxide (a-IGZO) has been intensively studied for the application to active matrix flat-panel display because of its superior electrical and optical properties. However, the characteristics of a-IGZO were found to be very sensitive to external circumstance such as light illumination, which dramatically degrades the device performance and stability practically required for display applications. Here, we suggest the use for silicon-germanium (Si-Ge) films grown plasma-enhanced chemical vapour deposition (PECVD) as photo-blocking layers in the a-IGZO thin film transistors (TFTs). The charge mobility and threshold voltage (Vth) of the TFTs depend on the thickness of the Si-Ge films and dielectric buffer layers (SiNX), which were carefully optimized to be ~200 nm and ~300 nm, respectively. As a result, even after 1,000 s illumination time, the Vth and electron mobility of the TFTs remain unchanged, which was enabled by the photo-blocking effect of the Si-Ge layers for a-IGZO films. Considering the simple fabrication process by PECVD with outstanding scalability, we expect that this method can be widely applied to TFT devices that are sensitive to light illumination.