InGaZnO Thin Film Research Articles

Effects of SOG Passivation Layers Annealed in Various Ambient Conditions on the Stability of Amorphous InGaZnO Thin Film Transistors

Effects of SOG Passivation Layers Annealed in Various Ambient Conditions on the Stability of Amorphous InGaZnO Thin Film Transistors

Investigating degradation behaviors induced by hot carriers in the etch stop layer in amorphous InGaZnO thin film transistors with different electrode materials and structures

This work investigates the hot carrier effect in via-contact type amorphous indium gallium zinc oxide thin film transistors with various source/drain materials and structures. According to previous research, the redundant drain electrode plays an important role in hot carrier stress-induced degradation, which leads to carrier-trapping in the etch stop layer between the active layer and the redundant drain electrode. Hot carrier stress has different influences on device characteristics, depending on materials and structure. Hot carrier stress causes more electron trapping in the etch stop layer below the redundant drain electrode in the presence of smaller source/drain metal work function or a longer redundant drain electrode. To further verify the mechanisms of the degradation behavior, the barrier height for Fowler-Nordheim-tunneling is extracted by a fitting charge trapping model. It is found that the barrier height for Fowler-Nordheim-tunneling is different for different source/drain materials.

Investigating degradation behaviors induced by mobile Cu ions under high temperature negative bias stress in a-InGaZnO thin film transistors

This letter investigates the effect of negative bias temperature stress (NBTS) on amorphous InGaZnO4 thin film transistors with copper electrodes. After 2000 s of NBTS, an abnormal subthreshold swing and on-current (Ion) degradation is observed. The recovery of the Id-Vg curve after either annealing or positive bias temperature stress suggests that there are some native mobile copper ions in the active layer. Both the existence of copper and the degradation mechanism can be confirmed by AC stress with different frequencies and by transmission electron microscope energy-dispersive X-ray spectroscopy analysis.

Determination of the Band Alignment of a‐IGZO/a‐IGMO Heterojunction for High‐Electron Mobility Transistor Application

In the past decade, amorphous InGaZnO thin film transistors (a‐IGZO TFTs) have become a very promising candidate for application in flat panel displays (FPDs). However, it is difficult to break through the mobility bottleneck of a‐IGZO TFTs to obtain mobilities higher than 100 cm2 V−1 s−1, thus limiting their use in more advanced applications. Construction of a high‐electron mobility transistor (HEMT) based on a heterojunction structure could provide a solution for this problem. In this work, the band alignment of a‐IGZO and amorphous InGaMgO (a‐IGMO) heterojunction has been investigated using X‐ray photoelectron spectroscopy (XPS) and transmission spectra measurements. The valence band (ΔEV) and conduction band offsets (ΔEC) were determined as 0.09 and 0.83 eV, respectively. The ΔEC was large enough to construct a potential well that could favor the appearance of a two‐dimensional electron gas (2DEG). Hence, the achievement of an HEMT based on a‐IGZO/a‐IGMO heterojunction can be expected. Moreover, band bending contributed greatly to such a large ΔEC, and thus to the formation of electrical confinement structure. Our findings suggest that a‐IGZO/a‐IGMO heterojunction is a potential candidate for constructing a HEMT and thus breaking through the mobility bottleneck of a‐IGZO‐based TFTs for the applications in next‐generation electronic products.

Analytical Drain Current Model for Amorphous InGaZnO Thin-Film Transistors at Different Temperatures Considering Both Deep and Tail Trap States

Surface-potential-based drain current model is presented for amorphous InGaZnO thin-film transistors considering both exponential deep and tail trap states densities in the energy gap. The trap states densities are determined by the numerical calculation on the basis of the assumption that the trapped carrier concentration is much higher than the free carrier concentration. The analytical drain current model is developed consistent with the numerical calculation, and verified by the experimental data at different temperatures.

A threshold voltage and drain current model for symmetric dual-gate amorphous InGaZnO thin film transistors

Based on the drift-diffusion theory, a simple threshold voltage and drain current model for symmetric dual-gate (DG) amorphous InGaZnO (a-IGZO) thin film transistors (TFTs) is developed. In the subthreshold region, most of the free electrons are captured by trap states in the bandgap of a-IGZO, thus the ionized trap states are the main contributor to the diffusion component of device drain current. Whereas in the above-threshold region, most of the trap states are ionized, and free electrons increase dramatically with gate voltage, which in turn become the main source of the drift component of device drain current. Therefore, threshold voltage of DG a-IGZO TFTs is defined as the gate voltage where the diffusion component of drain current equals the drift one, which can be determined with physical parameters of a-IGZO. The developed threshold voltage model is proved to be consistent with trap-limited conduction mechanism prevailing in a-IGZO, with the effect of drain bias being also taken into account. The gate overdrive voltage-dependent mobility is well modeled by the derived threshold voltage, and comparisons of the obtained drain current with experiment data show good verification of our model.

Amorphous InGaZnO Thin Film Transistor Fabricated with Printed Silver Salt Ink Source/Drain Electrodes

Recently, amorphous indium-gallium-zinc-oxide thin film transistors (a-IGZO TFTs) with inkjet printing silver source/drain electrodes have attracted great attention, especially for large area and flexible electronics applications. The silver ink could be divided into two types: one is based on silver nanoparticles, and the other is silver salt ink. Organic materials are essential in the formulation of nanoparticle ink as a strong disperse stabilizer to prevent agglomeration of silver particles, but will introduce contact problems between the silver electrodes and the a-IGZO active layer after annealing, which is difficult to eliminate and leads to poor device properties. Our experiment is aimed to reduce this effect by using a silver salt ink without stabilizer component. With optimized inkjet printing conditions, the high performance of a-IGZO TFT was obtained with a mobility of 4.28 cm2/V·s and an on/off current ratio over 106. The results have demonstrated a significant improvement for a-IGZO TFTs with directly printed silver electrodes. This work presents a promising platform for future printed electronic applications.

Growth of InGaZnO nanowires via a Mo/Au catalyst from amorphous thin film

InGaZnO nanowires were grown from amorphous InGaZnO thin films using Mo/Au-catalyzed Vapor-Liquid-Solid (VLS) growth process. The Mo/Au/amorphous InGaZnO stacked sample was annealed at 700 °C in N2 atmosphere. The growth of InGaZnO nanowires was confirmed on the edge of the Mo thin film closest to the Mo/Au overlap. The Au or Mo-covered areas, on the other hand, were not found to be sites of nanowire growth. Nanowires have Mo contained Au nanoparticles at the tip, which suggests the VLS growth mechanism. TEM results also verified that the nanowires are single crystalline, and that their growth direction is along the c-axis.

Mechanical stress-induced degradation model of amorphous InGaZnO thin film transistors by strain-initiated defect generation

For the applications of next generation display, various types of thin-film transistors (TFTs) are actively researched. Among them, the indium‑gallium‑zinc-oxide (IGZO) TFTs are being focused because of its transparency and other great electrical properties. However, the effect of mechanical stress on amorphous IGZO (a-IGZO) TFTs has to be studied for the foldable display application. In this paper, we examine the reliability of mechanical stress on a-IGZO TFTs and report the degradation mechanism by analyzing sub-gap density of states (DOSs) extracted from the stress simulation model. The simulation model has been implemented through TCAD simulation tool and the electrical characteristics based on the extracted parameters are compared with the measured data.

InGaZnO transistor based on porous Ag nanowire-functionalized gate electrode for detection of bio-relevant molecules

We report on InGaZnO (IGZO) thin film transistors (TFTs)-based bio-chemical sensors which can detect the chemical/biological species. As novel sensing platform, the IGZO TFT with Ag nanowire (NW) mesh showed pronounced output voltage changes responding to all analytes of H2O2, b-d-glucose, d-glucono-1,5-Lactione, and lactic acid, which are reproducible and reversible. Herein, porous Ag NW-functionalized top gate electrode plays a major role in sensing platform for enhanced sensing capability in aqueous medium. Moreover, these top gate geometry serve as a stable backplane for electrical modulation. As a result, analytes solutions become acidic or basic and such pH alterations induce significant turn-on voltage shifts on our devices. For implementation of a resistive load inverter, the output sensing voltage signals can be directly extracted, and such signals are reproducible and reversible. The proposed IGZO TFTs with Ag NW mesh top gate electrode based sensing platform pave the way for development of portable and reusable real-time non-destructive label-free chemical/biological sensors.

Stability enhancement in InGaZnO thin-film transistor with a novel Al2O3/HfO2/Al2O3 as gate insulator

ABSTRACTThis work reports an efficient route for enhancing the stability of amorphous InGaZnO (a-IGZO) thin film transistors (TFTs). It is clearly observed that the off-current in IGZO-TFT decreases by using a novel Al2O3/HfO2/Al2O3 gate insulator. Temperature-stress measurement were carried out to investigate the stability and the parameters related to activation energy (EA) and density-of-states (DOS). The improved electrical properties are attributed to the suppression of leakage current and a lower trapping density at the channel-insulator interface. The results showed that temperature stability and electrical properties enhancements in a-IGZO thin film transistors can probably be ascribed to the smaller DOS.

Effects of Deposition Temperature on the Device Characteristics of Oxide Thin-Film Transistors Using In-Ga-Zn-O Active Channels Prepared by Atomic-Layer Deposition.

We demonstrated the physical and electrical properties of the In-Ga-Zn-O (IGZO) thin films prepared by atomic-layer deposition (ALD) method and investigated the effects of the ALD temperature. The film composition (atomic ratio of In:Ga:Zn) and film density were examined to be 1:1:3 and 5.9 g/cm3, respectively, for all the temperature conditions. The optical band gaps decreased from 3.81 to 3.21 eV when the ALD temperature increased from 130 to 170 °C. The amounts of oxygen-related defects such as oxygen vacancies increased with increasing the ALD temperature. It was found from the in situ temperature-dependent electrical conductivity measurements that the electronic natures including the defect structures and conduction mechanism of the IGZO thin films prepared at different temperatures showed marked variations. The carrier mobilities in the saturation regions (μsat's) for the fabricated thin film transistors (TFTs) using the IGZO channel layers were estimated to be 6.1 to 14.8 cm2 V-1 s-1 with increasing the ALD temperature from 130 to 170 °C. Among the devices, when the ALD temperature was controlled to be 150 °C, the IGZO TFTs showed the best performance, which resulted from the fact that the amounts of oxygen vacancies and interstitial defects could be appropriately modulated at this condition. Consequently, the μsat, subthreshold swing, and on/off ratio for the TFT using the IGZO channel prepared at 150 °C showed 10.4 cm2 V-1 s-1, 90 mV/dec, and 2 × 109, respectively. The threshold voltage shifts of this device could also be effectively reduced to be 0.6 and -3.2 V under the positive-bias and negative-bias-illumination stress conditions. These obtained characteristics can be comparable to those for the sputter-deposited IGZO TFTs.

Negative bias illumination stress instability in amorphous InGaZnO thin film transistors with ITO local conducting buried layer

The investigations on the device instabilities of amorphous InGaZnO thin film transistors (a-IGZO TFTs) with ITO local conducting buried layer (LCBL) under the source/drain region and in the middle of the active channel region have been performed under negative bias and illumination stress. From the increased drain current of a-IGZO with ITO LCBL, one can control the drive current by modulating the length of ITO LCBL without changing the ratio of channel width and length. The reason for the less degradation of a-IGZO TFTs with LCBL under negative bias stress than that of device without LCBL was explained by the fact that ITO LCBL could act to reduce the effective energy barrier and act as a hole damping layer. However, the device degradation of a-IGZO with ITO LCBL under negative bias and illumination stress was more significant than that of one without LCBL due to the electron hole pair generation in ITO layer under illumination.

Source-drain resistance characteristics of back-channel etched amorphous InGaZnO thin film transistors with TiO2:Nb protective layer

This work investigates the source-drain (S-D) parasitic resistance (RSD) characteristics of the back-channel-etched (BCE) a-IGZO TFTs with ultra-thin Nb doped TiO2 (TNO) protective layer. It is shown that RSD is strongly related to the thickness of the TNO protective layer although the electrical performances of the BCE a-IGZO TFTs with different TNO thickness are similar to each other. The BCE TFT with 3nm TNO shows an unusually large RSD value (300Ωcm). It is suggested that a ~3nm TNO depletion layer should be formed at the TNO/a-IGZO interface in the S-D region in this case. In addition, RSD of the BCE TFTs with 1 and 5nm TNO is 11 and 26Ωcm, respectively. The low RSD of these two devices is caused by much thinner TNO depletion layers in the S-D region. Besides, a moderate RSD of 53Ωcm for the S-D lift-off device can be ascribed to a lower a-IGZO band bending at the Mo/a-IGZO interface than that of the BCE devices at the TNO/a-IGZO interface.

Influence of Electron Beam Irradiation on the Electrical and Optical Properties of InGaZnO Thin Film Transistor

The effects of electron beam(EB) irradiation on the electrical and optical properties of InGaZnO(IGZO) thin films fabricated using a sol-gel process were investigated. As the EB dose increased, the electrical characteristic of the IGZO TFTs changed from semiconductor to conductor, and the threshold voltage values shifted to the negative direction. X-ray photoelectron spectroscopy analysis of the O 1s core level showed that the relative area of oxygen vacancies increased from 14.68 to 19.08 % as the EB dose increased from 0 to 1.5 × 1016 electrons/cm2. In addition, spectroscopic ellipsometer analysis showed that the optical band gap varied from 3.39 to 3.46 eV with increasing EB dose. From the result of band alignment, it was confirmed that the Fermi level(EF) of the sample irradiated with 1.5 × 1016 electrons/cm2 was located at the closest position to the conduction band minimum(CBM) due to the increase of electron carrier concentration

Device Characterization and Design Guideline of Amorphous InGaZnO Junctionless Thin-Film Transistor

Amorphous InGaZnO junctionless thin-film transistors (a-IGZO JL-TFTs) with different active layer thicknesses and thermal treatments were fabricated. The unique feature of a-IGZO JL-TFTs is that all of the active layer and source/drain (S/D) electrodes are realized by the deposition of the InGaZnO thin film. A quantitative analysis to completely deplete the active layer when the device is turned OFF has been performed according to the active layer thickness. The impact of active layer thickness and thermal treatment on the performance of a-IGZO JL-TFTs has been investigated. With the increase in active layer thickness and annealing temperature, the transfer curves shifted to the negative direction. From the effects of S/D series resistance on the performance of a-IGZO JL-TFTs, the series resistance cannot be a serious problem when the contact size is small enough and the active layer is thick enough. To completely deplete the active layer, the impacts of the key device design parameters such as gate oxide thickness, gate workfunction, and high-κ dielectrics on the device performance have been investigated using device simulation.

Electrical Properties of Sputter-Deposited InGaZnO Thin Film Transistors with an Etch Stopper

Electrical Properties of Sputter-Deposited InGaZnO Thin Film Transistors with an Etch Stopper

P‐18: Nitrogen‐Doped Amorphous InGaZnO Thin Film Transistors Capped with Molybdenum‐Doped ZnO Ultraviolet‐Shield Layers

Nitrogen‐doped amorphous InGaZnO (a‐IGZO:N) and Molybdenum‐doped ZnO (MZO) were combined to prepare more stable thin film transistors (TFTs). The superior positive bias stress, negative bias stress, and negative bias illumination stress stabilities were observed for the a‐IGZO:N TFTs capped with MZO films, which might be due to the channel‐layer improvement by the nitrogen‐doping, the shield of ultraviolet‐lights by the MZO, and the good interface between the a‐IGZO:N and MZO.

Highly hydrogen-sensitive thermal desorption spectroscopy system for quantitative analysis of low hydrogen concentration (∼1 × 1016 atoms/cm3) in thin-film samples.

We developed a highly hydrogen-sensitive thermal desorption spectroscopy (HHS-TDS) system to detect and quantitatively analyze low hydrogen concentrations in thin films. The system was connected to an in situ sample-transfer chamber system, manipulators, and an rf magnetron sputtering thin-film deposition chamber under an ultra-high-vacuum (UHV) atmosphere of ∼10-8 Pa. The following key requirements were proposed in developing the HHS-TDS: (i) a low hydrogen residual partial pressure, (ii) a low hydrogen exhaust velocity, and (iii) minimization of hydrogen thermal desorption except from the bulk region of the thin films. To satisfy these requirements, appropriate materials and components were selected, and the system was constructed to extract the maximum performance from each component. Consequently, ∼2000 times higher sensitivity to hydrogen than that of a commercially available UHV-TDS system was achieved using H+-implanted Si samples. Quantitative analysis of an amorphous oxide semiconductor InGaZnO4 thin film (1 cm × 1 cm × 1 μm thickness, hydrogen concentration of 4.5 × 1017 atoms/cm3) was demonstrated using the HHS-TDS system. This concentration level cannot be detected using UHV-TDS or secondary ion mass spectroscopy (SIMS) systems. The hydrogen detection limit of the HHS-TDS system was estimated to be ∼1 × 1016 atoms/cm3, which implies ∼2 orders of magnitude higher sensitivity than that of SIMS and resonance nuclear reaction systems (∼1018 atoms/cm3).

Systematic Decomposition of the Positive Bias Stress Instability in Self-Aligned Coplanar InGaZnO Thin-Film Transistors

We propose an experimental method to decompose the positive gate-bias stress (PBS)-induced threshold voltage shift ( $\Delta {V}_{\mathsf {th}})$ of amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) into the contributions of distinct degradation mechanisms. Top-gate self-aligned coplanar structure TFTs are used for this letter. Stress-time-divided measurements, which combine the subgap density-of-states (DOS) extraction and the analysis on recovery characteristics, are performed to separate the $\Delta {V}_{\mathsf {th}}$ components. Change in excess oxygen ( $\text{O}_{\mathsf {ex}})$ -related DOS is clearly observed, and $\Delta {V}_{\mathsf {th}}$ by PBS is quantitatively decomposed into the contributions of the active $\text{O}_{\mathsf {ex}}$ , and the deep and shallow gate insulator traps. The quantitative decomposition of PBS-induced $\Delta {V}_{\mathsf {th}}$ provides physical insight and key guidelines for PBS stability optimization of a-IGZO TFTs.