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

Engineering a Subnanometer Interface Tailoring Layer for Precise Hydrogen Incorporation and Defect Passivation for High-End Oxide Thin-Film Transistors

Top-gate self-aligned structured oxide thin-film transistors (TFTs) are suitable for the backplanes of high-end displays because of their low parasitic capacitances. The gate insulator (GI) deposition process should be carefully designed to manufacture a highly stable, high-mobility oxide TFT, particularly for a top-gate structure. In this study, a nanometer-thick Al2O3 layer via plasma-enhanced atomic layer deposition (PE-ALD) is deposited on the top-gate bottom-contact structured oxide TFT as the interface tailoring layer, which can also act as the hydrogen barrier to modulate carrier generation from hydrogen incorporation into the active layer of the TFT during the following process such as postannealing. Al-doped InSnZnO (Al/ITZO) with an Al/In/Sn/Zn atomic ratio composition of 1.7:24.3:40:34 was used for high mobility oxide semiconductors, and an Al2O3/Si3N4 bilayer was used for the GI. The degradation issue due to the excellent barrier characteristics of Al2O3 and Si3N4 can be minimized. An oxide TFT fabricated without the interface tailoring layer exhibits conductor-like characteristics owing to the excessive carrier generation by hydrogen incorporation. However, TFTs with additional interface layers exhibit reasonable characteristics and distinct trends in electrical characteristics depending on the thicknesses of the interface layers. The optimized devices exhibit an average turn-on voltage (Von) of -0.31 V with 33.63 cm2/(V s) of high mobility and 0.09 V/dec of subthreshold swing value. The interfaces between the active layer and hydrogen barriers were investigated using a high-resolution transmission electron microscope, contact angle measurement, and secondary ion mass spectroscopy to reveal the origin of the trends in properties between the devices. The top-gate device with a hydrogen barrier using the four-cycle deposition exhibits optimum electrical characteristics of both high mobility and good stability with only a 0.04 V shift of Von under positive-bias temperature stress (PBTS). We realize a high-end, self-aligned TFT with high mobility [34.7 cm2/(V s)] and negligible Von shift of -0.06 V under PBTS by applying a subnanometer hydrogen barrier.

A Comprehensive Large Signal, Small Signal, and Noise Model for IGZO Thin Film Transistor Circuits

We report a new physics-based model for dual-gate amorphous-indium gallium zinc oxide (a-IGZO) thin film transistors (TFTs) which we developed and fine-tuned through experimental implementation and benchtop characterization. We fabricated and characterized a variety of test patterns, including a-IGZO TFTs with varying gate widths (100-1000 μm) and channel lengths (5-50 μm), transmission-line-measurement patterns and ground-signal-ground (GSG) radio frequency (RF) patterns. We modeled the contact resistance as a function of bias, channel area, and temperature, and captured all operating regimes, used physics-based modeling adjusted for empirical data to capture the TFT characteristics including ambipolar subthreshold currents, graded interbias-regime current changes, threshold and flat-band voltages, the interface trap density, the gate leakage currents, the noise, and the relevant small signal parameters. To design high-precision circuits for biosensing, we validated the dc, small signal, and noise characteristics of the model. We simulated and fabricated a two-stage common source amplifier circuit with a common drain output buffer and compared the measured and simulated gain and phase performance, finding an excellent fit over a frequency range spanning 10 kHz-10 MHz.

Quantitative analysis of defect states in InGaZnO within 2 eV below the conduction band via photo-induced current transient spectroscopy

This work investigates the function of the oxygen partial pressure in photo-induced current measurement of extended defect properties related to the distribution and quantity of defect states in electronic structures. The Fermi level was adjusted by applying a negative gate bias in the TFT structure, and the measurable range of activation energy was extended to < 2.0 eV. Calculations based on density functional theory are used to investigate the changes in defect characteristics and the role of defects at shallow and deep levels as a function of oxygen partial pressure. Device characteristics, such as mobility and threshold voltage shift under a negative gate bias, showed a linear correlation with the ratio of shallow level to deep level defect density. Shallow level and deep level defects are organically related, and both defects must be considered when understanding device characteristics.

Defect Passivation and Carrier Reduction Mechanisms in Hydrogen-Doped In-Ga-Zn-O (IGZO:H) Films upon Low-Temperature Annealing for Flexible Device Applications

Low-temperature activation of oxide semiconductor materials such as In-Ga-Zn-O (IGZO) is a key approach for their utilization in flexible devices. We previously reported that the activation temperature can be reduced to 150 °C by hydrogen-doped IGZO (IGZO:H), demonstrating a strong potential of this approach. In this paper, we investigated the mechanism for reducing the activation temperature of the IGZO:H films. In situ Hall measurements revealed that oxygen diffusion from annealing ambient into the conventional Ar/O2-sputtered IGZO film was observed at >240 °C. Moreover, the temperature at which the oxygen diffusion starts into the film significantly decreased to 100 °C for the IGZO:H film deposited at hydrogen gas flow ratio (R[H2]) of 8%. Hard X-ray photoelectron spectroscopy indicated that the near Fermi level (EF) defects in the IGZO:H film after the 150 °C annealing decreased in comparison to that in the conventional IGZO film after 300 °C annealing. The oxygen diffusion into the film during annealing plays an important role for reducing oxygen vacancies and subgap states especially for near EF. X-ray reflectometry analysis revealed that the film density of the IGZO:H decreased with an increase in R[H2] which would be the possible cause for facilitating the O diffusion at low temperature.

Highly-stable flexible pressure sensor using piezoelectric polymer film on metal oxide TFT

In this study, a flexible pressure sensor with highly stable performance is presented. The pressure sensor was fabricated to work under low voltage conditions by using a high mobility amorphous indium–gallium–zinc oxide (a-IGZO) thin-film transistor (TFT) and a stretched polyvinylidene fluoride (PVDF) film. To prepare a stable sensor suitable for practical use, we designed a device structure that shields ambient noise by grounding the control gate. The shielding structure significantly improves the stability of the device. Moreover, the sensor was fabricated on a flexible substrate and delaminated via a laser lift-off (LLO) technique to meet the urgent needs for flexibility. The pressure sensor showed good sensitivity and reliability over a pressure ranging from 0 to 75 kPa which covers the human touch pressure range. Especially, good linearity over a wide pressure range and high stability over 1000 repeated loadings were realized. Due to the simple structure, the pressure sensor demonstrates the advantage of being inexpensive to be manufactured and holds the potential to be integrated into the display backplane. Therefore, the proposed sensor has great potential in the production of flexible touch screens, human–machine interacting applications, and even electronic skins in the future.

Flexible piezoelectric pressure sensor using a-IGZO TFT was prepared and a shielding structure was proposed to stabilize the response current.

Atomic-Layer-Deposited SiOx/SnOx Nanolaminate Structure for Moisture and Hydrogen Gas Diffusion Barriers

High-density SnO_x and SiO_x thin films were deposited via atomic layer deposition (ALD) at low temperatures (100 °C) using tetrakis(dimethylamino)tin(IV) (TDMASn) and di-isopropylaminosilane (DIPAS) as precursors and hydrogen peroxide (H₂O₂) and O₂ plasma as reactants, respectively. The thin-film encapsulation (TFE) properties of SnO_x and SiO_x were demonstrated with thickness dependence measurements of the water vapor transmission rate (WVTR) evaluated at 50 °C and 90% relative humidity, and different TFE performance tendencies were observed between thermal and plasma ALD SnO_x. The film density, crystallinity, and pinholes formed in the SnO_x film appeared to be closely related to the diffusion barrier properties of the film. Based on the above results, a nanolaminate (NL) structure consisting of SiO_x and SnO_x deposited using plasma-enhanced ALD was measured using WVTR (H₂O molecule diffusion) at 2.43 × 10^-5 g/m² day with a 10/10 nm NL structure and time-lag gas permeation measurement (H₂ gas diffusion) for applications as passivation layers in various electronic devices.

Role ofin-situhydrogen plasma treatment on gate bias stability and performance of a-IGZO thin-film transistors

This work investigates the effect of anin situhydrogen plasma treatment on gate bias stability and performance of amorphous InGaZnO thin-film transistors (TFTs) deposited by using atmospheric-pressure PECVD. The H₂plasma-treateda-IGZO channel has shown significant improvement in bias stress induced instability with a minuscule threshold voltage shift (ΔV_th) of 0.31 and -0.17 V under positive gate bias stress (PBS) and negative gate bias stress (NBS), respectively. With the aid of the energy band diagram, the proposed work demonstrates the formation of negative species O₂^-and positive species H₂O⁺in the backchannel under PBS and NBS in addition to ionized oxygen vacancy (V_o) defects ata-IGZO/ZrO₂interfaces are the reason for gate bias instability which could be effectively suppressed within situH₂plasma treatment. From the experimental result, it is observed that the electrical performance such as field-effect mobility (μ_FE), on-off current ratio (I_on/I_off), and subthreshold swing improved significantly byin situH₂plasma treatment with passivation of interface trap density and bulk trap defects.

Hydrogen Barriers Based on Chemical Trapping Using Chemically Modulated Al2O3 Grown by Atomic Layer Deposition for InGaZnO Thin-Film Transistors

In this study, the excellent hydrogen barrier properties of the atomic-layer-deposition-grown Al₂O₃ (ALD Al₂O₃) are first reported for improving the stability of amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs). Chemical species in Al₂O₃ were artificially modulated during the ALD process using different oxidants, such as H₂O and O₃ (H₂O-Al₂O₃ and O₃-Al₂O₃, respectively). When hydrogen was incorporated into the H₂O-Al₂O₃-passivated TFT, a large negative shift in V_th (ca. -12 V) was observed. In contrast, when hydrogen was incorporated into the O₃-Al₂O₃-passivated TFT, there was a negligible shift in V_th (ca. -0.66 V), which indicates that the O₃-Al₂O₃ has a remarkable hydrogen barrier property. We presented a mechanism for trapping hydrogen in a O₃-Al₂O₃ via various chemical and electrical analyses and revealed that hydrogen molecules were trapped by C-O bonds in the O₃-Al₂O₃, preventing the inflow of hydrogen to the a-IGZO. Additionally, to minimize the deterioration of the pristine device that occurs after a barrier deposition, a bi-layered hydrogen barrier by stacking H₂O- and O₃-Al₂O₃ is adopted. Such a barrier can provide ultrastable performance without degradation. Therefore, we envisioned that the excellent hydrogen barrier suggested in this paper can provide the possibility of improving the stability of devices in various fields by effectively blocking hydrogen inflows.

Interface tailoring through the supply of optimized oxygen and hydrogen to semiconductors for highly stable top-gate-structured high-mobility oxide thin-film transistors

Self-aligned structured oxide thin-film transistors (TFTs) are appropriate candidates for use in the backplanes of high-end displays. Although SiN_x is an appropriate candidate for use in the gate insulators (GIs) of high-performance driving TFTs, direct deposition of SiN_x on top of high-mobility oxide semiconductors is impossible due to significant hydrogen (H) incorporation. In this study, we used AlO_x deposited by thermal atomic layer deposition (T-ALD) as the first GI, as it has good H barrier characteristics. During the T-ALD, however, a small amount of H from H₂O can also be incorporated into the adjacent active layer. In here, we performed O₂ or N₂O plasma treatment just prior to the T-ALD process to control the carrier density, and utilized H to passivate the defects rather than generate free carriers. While the TFT fabricated without plasma treatment exhibited conductive characteristics, both O₂ and N₂O plasma-treated TFTs exhibited good transfer characteristics, with a V_th of 2 V and high mobility (∼30 cm² V⁻¹ s⁻¹). Although the TFT with a plasma-enhanced atomic layer deposited (PE-ALD) GI exhibited reasonable on/off characteristics, even without any plasma treatment, it exhibited poor stability. In contrast, the O₂ plasma-treated TFT with T-ALD GI exhibited outstanding stability, i.e., a V_th shift of 0.23 V under positive-bias temperature stress for 10 ks and a current decay of 1.2% under current stress for 3 ks. Therefore, the T-ALD process for GI deposition can be adopted to yield high-mobility, high-stability top-gate-structured oxide TFTs under O₂ or N₂O plasma treatment.

By supplying optimized oxygen and hydrogen, the highly stable and high mobility oxide TFTs with the top-gate structure were fabricated.

Wearable 1 V operating thin-film transistors with solution-processed metal-oxide semiconductor and dielectric films fabricated by deep ultra-violet photo annealing at low temperature

Amorphous metal-oxide semiconductors (AOSs) such as indium-gallium-zinc-oxide (IGZO) as an active channel have attracted substantial interests with regard to high-performance thin-film transistors (TFTs). Recently, intensive and extensive studies of flexible and/or wearable AOS-based TFTs fabricated by solution-process have been reported for emerging approaches based on device configuration and fabrication process. However, several challenges pertaining to practical and effective solution-process technologies remain to be resolved before low-power consuming AOS-based TFTs for wearable electronics can be realized. In this paper, we investigate the non-thermal annealing processes for sol-gel based metal-oxide semiconductor and dielectric films fabricated by deep ultraviolet (DUV) photo and microwave annealing at low temperature, compared to the conventional thermal annealing at high temperature. A comprehensive investigation including a comparative analysis of the effects of DUV photo and microwave annealing on the degree of metal-oxide-metal networks in amorphous IGZO and high-dielectric-constant (high-k) aluminum oxide (Al2O3) films and device performance of IGZO-TFTs in a comparison with conventional thermal annealing at 400 °C was conducted. We also demonstrate the feasibility of wearable IGZO-TFTs with Al2O3 dielectrics on solution-processed polyimide films exhibiting a high on/off current ratio of 5 × 104 and field effect mobility up to 1.5 cm2/V-s operating at 1 V. In order to reduce the health risk and power consumption during the operation of wearable electronics, the operating voltage of IGZO-TFTs fabricated by non-thermal annealing at low temperature was set below ~1 V. The mechanical stability of wearable IGZO-TFTs fabricated by an all-solution-process except metal electrodes, against cyclic bending tests with diverse radius of curvatures in real-time was investigated. Highly stable and robust flexible IGZO-TFTs without passivation films were achieved even under continuous flexing with a curvature radius of 12 mm.

Role of Hydrogen in Active Layer of Oxide-Semiconductor-Based Thin Film Transistors

Hydrogen in oxide systems plays a very important role in determining the major physical characteristics of such systems. In this study, we investigated the effect of hydrogen in oxide host systems for various oxygen environments that acted as amorphous oxide semiconductors. The oxygen environment in the sample was controlled by the oxygen gas partial pressure in the radio-frequency-sputtering process. It was confirmed that the hydrogen introduced by the passivation layer not only acted as a “killer” of oxygen deficiencies but also as the “creator” of the defects depending on the density of oxide states. Even if hydrogen is not injected, its role can change owing to unintentionally injected hydrogen, which leads to conflicting results. We discuss herein the correlation with hydrogen in the oxide semiconductor with excess or lack of oxygen through device simulation and elemental analysis.

Negative threshold voltage shift in an a-IGZO thin film transistor under X-ray irradiation

We investigated the effects of X-ray irradiation on the electrical characteristics of an amorphous In–Ga–Zn–O (a-IGZO) thin film transistor (TFT). The a-IGZO TFT showed a negative threshold voltage (V_TH) shift of −6.2 V after 100 Gy X-ray irradiation. Based on spectroscopic ellipsometry (SE) and X-ray photoelectron spectroscopy (XPS) analysis, we found that the Fermi energy (E_F) changes from 2.73 eV to 3.01 eV and that the sub-gap state of D1 and D2 changes near the conduction band minimum (CBM) of the a-IGZO film after X-ray irradiation. These results imply that the negative V_TH shift after X-ray irradiation is related to the increase in electron concentration of the a-IGZO TFT active layer. We confirmed that the sources for electron generation during X-ray irradiation are hydrogen incorporation from the adjacent layer or from ambient air to the active layer in the TFT, and the oxygen vacancy dependent persistent photocurrent (PPC) effect. Since both causes are reversible processes involving an activation energy, we demonstrate the V_TH shift recovery by thermal annealing.

We studied the effect of X-ray irradiation on the negative threshold voltage shift of bottom-gate a-IGZO TFT. Based on spectroscopic analyses, we found that this behavior was caused by hydrogen incorporation and oxygen vacancy ionization.

Investigations on the bias temperature stabilities of oxide thin film transistors using In–Ga–Zn–O channels prepared by atomic layer deposition

Bias temperature stress stabilities of thin-film transistors (TFTs) using In–Ga–Zn–O (IGZO) channels prepared by the atomic layer deposition process were investigated with varying channel thicknesses (10 and 6 nm). Even when the IGZO channel thickness was reduced to 6 nm, the device exhibited good characteristics with a high saturation mobility of 15.1 cm² V⁻¹ s⁻¹ and low sub-threshold swing of 0.12 V dec⁻¹. Excellent positive and negative bias stress stabilities were also obtained. When positive bias temperature stress (PBTS) stability was tested from 40 to 80 °C for 10⁴ s, the threshold voltages (V_TH) of the device using the 6 nm-thick IGZO channel shifted negatively, and the V_TH shifts increased from −0.5 to −6.9 V with the increasing temperature. Time-dependent PBTS instabilities could be explained by a stretched-exponential equation, representing a charge-trapping mechanism.

Bias temperature stress stabilities of thin-film transistors (TFTs) using In–Ga–Zn–O (IGZO) channels prepared by the atomic layer deposition process were investigated with varying channel thicknesses (10 and 6 nm).