Amorphous InGaZnO Thin Film Research Articles

Effects of the compositional ratios of sputtering target on the device performance and instability in amorphous InGaZnO thin film transistors

The effects of the compositional ratios of sputtering target on the device performance and instability in amorphous InGaZnO thin film transistors (a-IGZO TFTs) have been investigated. Four kinds of a-IGZO TFTs according to the contents of In, Ga, and Zn were fabricated by RF magnetron sputtering system. The electrical performances were enhanced with increasing In ratio in a-IGZO TFTs. The resistances of the source/drain electrodes and the effective channel length were decreased with increasing In ratio. Since the effective barrier energy is increased with increasing the sum of In and Zn ratios, the threshold voltage shifts under positive bias stress were decreased with increasing the sum of In and Zn ratios in a-IGZO TFTs. A-IGZO TFTs with increasing In ratio in a-IGZO TFTs exhibit a conductor behavior under negative bias illumination stress.

Degradation on the Current Saturation of Output Characteristics in Amorphous InGaZnO Thin-Film Transistors

Degradation on the current saturation of the output characteristics in amorphous indium–gallium–zinc–oxide (a-IGZO) thin-film transistors with the bottom-gate structure is investigated. As the drain-to-source voltage ( ${V}_{\textsf {DS}}$ ) increases at a fixed gate-to-source voltage ( ${V}_{\textsf {GS}}$ ), the current from drain to source ( ${I}_{\textsf {DS}}$ ) becomes nonsaturated and increases due to the mobility enhancement and ${I}_{\textsf {DS}}$ also decreases again due to the electron trapping into the gate insulator and/or an interface, which occurs mainly at the channel edge near a drain. Analysis is validated by using the pulsed ${I}_{\textsf {DS}}$ – ${V}_{\textsf {DS}}$ measurement. Nonsaturated current is attributed to the Joule heating-assisted mobility enhancement and the thermally activated electron trapping, which is the reason why the nonsaturation becomes more prominent as the IGZO active thin film becomes thinner. Furthermore, it is found that the rate of the increased ${I}_{\textsf {DS}}$ per increasing ${V}_{\textsf {DS}}$ gets higher as either the channel width ( ${W}$ ) increases or the channel length ( ${L}$ ) decreases; whereas, the rate of the decreased ${I}_{\textsf {DS}}$ per increasing ${V}_{\textsf {DS}}$ becomes higher as the device size, i.e., ${W}\times {L}$ , increases. The former is well correlated with Joule heating while the latter with the self-heating-assisted electron trapping due to a total heat accumulated in an active layer.

Effect of physical densification on sub-gap density of states in amorphous InGaZnO thin films

In this study, we investigate the relationship between the physical densification and sub-gap density-of-states (DOS) distribution of a-IGZO films prepared under various deposition pressures. For TFTs with a-IGZO channel films, the field effect mobility increases from 10.9 to 24.8 cm2/V·s, and the on-current increases from 1.5 to 20.8 μA as the deposition pressure for the channel films decreases from 7 to 1 mTorr. The sub-gap DOS distributions obtained from the electrical characteristics indicate the changes in the density of accepter-like tail states with deposition pressure. Further, the physical densification of the channel films increases as the deposition pressure decreases. Our study demonstrates that the lower density of accepter-like tail states and the higher physical densification contribute to the improved performance of a-IGZO TFTs.

Heterogeneous CFs induced unipolar and bipolar resistive switching behaviors in InGaZnO thin films

This paper investigates resistive switching behaviors of amorphous InGaZnO (a-IGZO) thin films for non-volatile resistive random access memory applications. Reversible transition process between bipolar and unipolar modes in Ag/InGaZnO/Pt memory cell takes place through changing the polarities of sweeping voltages, whereas the reset process cannot be operated by applying a positive voltage. The dominant conduction mechanisms in both modes are Ohmic conduction for the low resistance state, but different for the high resistance state, that is, Poole-Frenkel emission in the unipolar mode and trap-controlled space charge limited conduction in the bipolar mode. Temperature dependence of low resistance state and transmission electron microscopy analysis further indicate that the dominant conductive filaments (CFs) are composed of Ag atoms for the bipolar mode and oxygen vacancies for the unipolar mode, respectively. These two kinds of conductive filaments play a coordination role in resistive switching transition between bipolar and unipolar modes in Ag/InGaZnO/Pt memory devices. Based on the conductive filament model, the physical mechanisms for resistive switching are discussed.

Oxygen partial pressure ratio modulated electrical performance of amorphous InGaZnO thin film transistor and inverter

In this work, sputtering route has been pursued for the deposition of amorphous InGaZnO thin films under different oxygen partial pressure ratio (Oppr). The effect of oxygen partial pressure ratio on the microstructure, optical and electrical properties and chemical bonding states of the sputtering-derived amorphous indium-gallium-zinc oxide (α-IGZO) thin films have been investigated by X-ray diffraction, X-ray photoelectron spectroscopy, UV–vis spectroscopy, and a series of electrical measurements. Measurements based on α-IGZO/SiO2 thin film transistors (TFTs) have shown that oxygen partial pressure ratio can effectively regulate the carrier concentration, the defect trap density in the bulk channel and the interface between the channel and the gate insulator. The appropriate oxygen partial pressure ratio can reduce interface trap density (Nit) and decrease the off-state current (Ioff), which can obviously improve the on/off current ratio (Ion/Ioff) and the stability of the device. In addition, the appropriate oxygen partial pressure ratio increases the overlap of the ns-orbit of metal ions in the film, which is conducive to improve the electronic transport routes and increase the carrier mobility. As a result, the optimized TFTs performance with oxygen partial pressure ratio of 6.3% has been achieved, including a high saturation mobility (μsat) of 9.9 cm2V-1S−1, a large on/off current ratio of 3.5 × 109, a small subthreshold swing (SS) of 0.29 V/decade, a threshold voltage shift of 4 V under positive bias stress for 7200 s, respectively. A resistor loaded inverter was also constructed on α-IGZO/SiO2 TFTs and exhibited full swing characteristics with a high gain of 10.3, which is sufficient to drive the next stage component in a logic circuit. All the experimental results have indicated that the sputtering-derived α-IGZO thin films have potential application in large-area all-oxide flexible electronics.

P‐3: TCAD Modeling of Ion Transport for Simulation of Degradation in an Amorphous InGaZnO Thin Film Transistor

We have developed an ion transport model to simulate various degradation mechanisms in an amorphous InGaZnO (IGZO) thin film transistor (TFT). The simulation model reproduces the published data very well and demonstrates the evolution of charged ions in negative bias illumination stress, from which we can predict the ion contribution to the threshold voltage shift.

P‐10: A Study on Bending Tolerance of Hybrid IGZO TFTs Fabricated on PEN Films

Electrical behaviors of amorphous InGaZnO (a‐IGZO) thin film transistors (TFTs) with the flash lamp annealing process under mechanical stress were investigated in this study. While bending radius down to 5mm for tensile and compress stress, there is no obvious change for transfer characteristics. Hybrid IGZO TFTs show good bending tolerance, and it is useful for further applications of flexible electronics.

Hybrid PN Diode and CMOS Inverters Composed of MoTe2 Nanosheet-Amorphous in-Ga-Zn-O Thin Film

We report a hybrid material system which couples a dry-transfer-processed p-type MoTe2 nanosheet and a conventional photolithography-patterned n-type amorphous InGaZnO (IGZO) thin-film. The PN junction diode shows a good ideality factor of 1.57 and a high ON/OFF rectification ratio of ∼3 × 104. Under illumination, our PN diode shows somewhat stable only responding to high-energy photons of blue and ultraviolet. Our CMOS inverter composed of p-MoTe2 bottom-gate field-effect transistor and n-IGZO top-gate field-effect transistors. Our 2D nanosheet – oxide thin film CMOS inverter shows high voltage gains as high as ~40 at 5 V, low power consumption less than ~ a few nW at 1 V, and ~200 μs switching speed. Both hybrid PN junction diode and complementary (CMOS) inverters are realized by such novel hybrid material system that now opens any further possibilities toward practical applications of 2D nanosheets combined with existing technologies. Figure 1

Drain Current Stress-Induced Instability in Amorphous InGaZnO Thin-Film Transistors with Different Active Layer Thicknesses.

In this study, the initial electrical properties, positive gate bias stress (PBS), and drain current stress (DCS)-induced instabilities of amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs) with various active layer thicknesses (TIGZO) are investigated. As the TIGZO increased, the turn-on voltage (Von) decreased, while the subthreshold swing slightly increased. Furthermore, the mobility of over 13 cm2·V−1·s−1 and the negligible hysteresis of ~0.5 V are obtained in all of the a-IGZO TFTs, regardless of the TIGZO. The PBS results exhibit that the Von shift is aggravated as the TIGZO decreases. In addition, the DCS-induced instability in the a-IGZO TFTs with various TIGZO values is revealed using current–voltage and capacitance–voltage (C–V) measurements. An anomalous hump phenomenon is only observed in the off state of the gate-to-source (Cgs) curve for all of the a-IGZO TFTs. This is due to the impact ionization that occurs near the drain side of the channel and the generated holes that flow towards the source side along the back-channel interface under the lateral electric field, which cause a lowered potential barrier near the source side. As the TIGZO value increased, the hump in the off state of the Cgs curve was gradually weakened.

Doping Nitrogen in InGaZnO Thin Film Transistor with Double Layer Channel Structure.

This paper presents the electrical characteristics of doping nitrogen in an amorphous InGaZnO thin film transistor. The IGZO:N film, which acted as a channel layer, was deposited using RF sputtering with a nitrogen and argon gas mixture at room temperature. The optimized parameters of the IGZO:N/IGZO TFT are as follows: threshold voltage is 0.5 V, field effect mobility is 14.34 cm2V-1S-1. The on/off current ratio is 106 and subthreshold swing is 1.48 V/decade. The positive gate bias stress stability of InGaZnO doping with nitrogen shows improvement compared to doping with oxygen.

Effect of defect creation and migration on hump characteristics of a-InGaZnO thin film transistors under long-term drain bias stress with light illumination

We investigated the mechanism of formation of the hump that occurs in the current-voltage I-V characteristics of amorphous InGaZnO (a-IGZO) thin film transistors (TFTs) that are exposed to long-term drain bias stress under illumination. Transfer characteristics showed two-stage degradation under the stress. At the beginning of the stress, the I-V characteristics shifted in the negative direction with a degradation of subthreshold slope, but the hump phenomenon developed over time in the I-V characteristics. The development of the hump was related to creation of defects, especially ionized oxygen vacancies which act as shallow donor-like states near the conduction-band minimum in a-IGZO. To further investigate the hump phenomenon we measured a capacitance-voltage C-V curve and performed two-dimensional device simulation. Stretched-out C-V for the gate-to-drain capacitance and simulated electric field distribution which exhibited large electric field near the drain side of TFT indicated that VO2+ were generated near the drain side of TFT, but the hump was not induced when VO2+ only existed near the drain side. Therefore, the degradation behavior under DBITS occurred because VO2+ were created near the drain side, then were migrated to the source side of the TFT.

The Calculation of Negative Bias Illumination Stress-Induced Instability of Amorphous InGaZnO Thin-Film Transistors for Instability-Aware Design

We propose a systematic calculation method for negative bias illumination stress (NBIS)-induced instabilities in amorphous InGaZnO thin-film transistors (TFTs). The proposed method is based on activation energy window (AEW) in subgap energy range, and it can reproduce the NBIS time evolution of ${I}$ – ${V}$ characteristics without long-term stress test. Furthermore, it quantitatively explains the effect of oxygen content on the NBIS instability. The AEW, which is employed for emulating the oxygen vacancy ionization, peroxide formation, and hole trapping, has the order of magnitude of 1016–1017 cm−3 for the bias stress of −20 V and the commercial LED backlight of 300 cd/m2. The proposed method is expected to play such an important role in the instability awareness of amorphous oxide TFTs.

Combined Effects of Light Illumination and Various Bottom Gate Length on the Instability of Via-Contact-Type Amorphous InGaZnO Thin-Film Transistors

This paper utilizes electrical analyses and a study of physical mechanisms to investigate metal gate structure-dependent performance in amorphous InGaZnO (a-IGZO) thin-film transistors. The effects of different shielding areas between the IGZO layer and metal gate are investigated. In devices with shorter metal gate lengths, an abnormal rise in capacitance at the off-state in capacitance–voltage ( ${C}-{V}$ ) characteristic curves can be observed. This can be attributed to the stronger electric field induced by the edge of the metal gate under bias sweep when the metal gate length is shorter than the IGZO layer length. Light illumination measurements indicate a negative shift in threshold voltage and an increase in subthreshold-leakage current regardless of relative metal gate length. Moreover, negative threshold voltage shift becomes more severe with a more obvious hump in ${C}-{V}$ characteristic curves under back-light illumination of a shorter width device, a phenomenon which has been verified by simulation.

Lifetime prediction of InGaZnO thin film transistor for the application of display device and BEOL-transistors

In this work, the lifetime prediction models of amorphous InGaZnO thin film transistors (a-IGZO TFTs) were suggested for the application of display device and BEOL (Back End Of line) transistors with embedded a-IGZO TFTs. Four different types of test devices according to the active layer thickness, source/drain electrode materials and thermal treatments have been used to verify the suggested model. The device lifetimes under high gate bias stress and hot carrier stress were extracted through fittings of the stretched-exponential equation for threshold voltage shifts and the current estimation method for drain current degradations. Our suggested lifetime prediction models could be used in any kinds of structures of a-IGZO TFTs for the application of display device and BEOL transistors. The a-IGZO TFTs with embedded ITO local conducting layer under source/drain is better for BEOL transistor application and a-IGZO TFTs with InGaZnO thin film as source/drain electrodes may be better for the application of display devices.

Effect of Plasma Treatment on a SiO$_2$ Gate Insulator in Solution-Processed Amorphous InGaZnO Thin-Film Transistors

Effect of Plasma Treatment on a SiO$_2$ Gate Insulator in Solution-Processed Amorphous InGaZnO Thin-Film Transistors

Effects of SF6 plasma treatment on the properties of InGaZnO thin films

The effects of sulfur hexafluoride (SF6) plasma on the properties of amorphous InGaZnO (a-IGZO) thin films were examined. The properties of the a-IGZO thin films were characterized by Hall effect measurement, dynamic secondary ion mass spectroscopy (SIMS), and X-ray photoelectron spectroscopy (XPS). The IGZO thin films treated with SF6 plasma before annealing had a very high resistance mainly owing to the inclusion of S into the film surface, as evidenced by SIMS profiles. On the other hand, the samples treated with SF6 plasma after annealing showed better electrical properties with a Hall mobility of 10 cm2/(V·s) than the untreated samples or the samples SF6 plasma-treated before annealing. This was attributed to the increase in the number of oxygen vacancy defects in the a-IGZO thin films owing to the enhanced out-diffusion of O to the ambient and the increase in the number of F-related donor defects originating from the incorporation of a much larger amount of F than of S into the film surface, which were confirmed by XPS and SIMS.

非晶铟镓锌氧薄膜晶体管钼/铜源漏电极的研究

非晶铟镓锌氧薄膜晶体管钼/铜源漏电极的研究

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 (Tdep). The Al2O3 gate insulator with a low Tdep 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 (Tpost-ann), a-IGZO TFTs exhibited two unique changes of properties; the degradation in low Tpost-ann and the enhancement in high Tpost-ann, as explained in terms of H diffusion from the gate insulator to an active layer.

High-Performance Solution-Processed Amorphous InGaZnO Thin Film Transistors with a Metal–Organic Decomposition Method

A facile solution process was introduced for the preparation of IGZO thin films via a metal–organic decomposition (MOD) method. The IGZO ink was synthesized by mixing the solutions of gallium acetylacetonate [Ga(C5H7O2)3], zinc acetylacetonate hydrate [Zn(C5H7O2)2·xH2O] dissolved in ethanol, and indium acetylacetonate [In(C5H7O2)3] dissolved in tetrahydrofuran (THF). The deposited films by spin-coating were annealed at moderate process temperature (≤500°C). The relationship between device performance and postannealing temperature was studied. The result demonstrated that mobility of IGZO TFT increased as the annealing temperature increased. Based on the analysis of O 1s statement, the annealing temperature can influence the number of oxygen vacancy to further affect the carrier centration. In addition, the IGZO TFT devices with various Ga molar ratios were compared to demonstrate the influence of the Ga addition. The result demonstrated that the saturated mobilities (μe) decreased and VTH shifted to positive voltage as the Ga molar ratio was increased. It is likely that Ga can offer stronger chemical bonds between metal and oxygen that reduced the concentration of free carriers and thus help reducing VTH. As a result, the optimized performance of IGZO TFT with the mobility of 3.4 cm2V−1s−1 showed the MOD process was a promising approach.

Drain-Induced-Barrier-Lowing-Like Effect Induced by Oxygen-Vacancy in Scaling-Down via-Contact Type Amorphous InGaZnO Thin-Film Transistors

This investigation considers a method to ameliorate drain induced barrier lowing behavior in amorphous-indium-gallium-zinc-oxide thin-film transistors. The $V_{\mathrm{ th}}$ is found to shift negatively when increasing the $I _{D}$ – $V _{G}$ measurement condition $V _{D}$ from 0.1 to 15 V. The current–voltage curves show that this degradation is caused by the effective channel length ( $L _{\mathrm{ eff}}$ ) being shorter than the mask channel length ( $L$ ). Using the transmission line method to extract $L _{\mathrm{ eff}}$ , we discover that the degradation will be completely suppressed by an annealing treatment. As a result, the degradation mechanism of shorter channel length a-IGZO thin film transistors is due to oxygen-vacancies which are located between the channel and the source/drain junction.