Amorphous Indium Gallium Zinc Oxide Thin-film Transistors Research Articles

Specific contact resistivity reduction in amorphous IGZO thin-film transistors through a TiN/IGTO heterogeneous interlayer

Oxide semiconductors have gained significant attention in electronic device industry due to their high potential for emerging thin-film transistor (TFT) applications. However, electrical contact properties such as specific contact resistivity (ρC) and width-normalized contact resistance (RCW) are significantly inferior in oxide TFTs compared to conventional silicon metal oxide semiconductor field-effect transistors. In this study, a multi-stack interlayer (IL) consisting of titanium nitride (TiN) and indium-gallium-tin-oxide (IGTO) is inserted between source/drain electrodes and amorphous indium-gallium-zinc-oxide (IGZO). The TiN is introduced to increase conductivity of the underlying layer, while IGTO acts as an n+-layer. Our findings reveal IGTO thickness (tIGTO)-dependent electrical contact properties of IGZO TFT, where ρC and RCW decrease as tIGTO increases to 8 nm. However, at tIGTO > 8 nm, they increase mainly due to IGTO crystallization-induced contact interface aggravation. Consequently, the IGZO TFTs with a TiN/IGTO (3/8 nm) IL reveal the lowest ρC and RCW of 9.0 × 10−6 Ω·cm2 and 0.7 Ω·cm, significantly lower than 8.0 × 10−4 Ω·cm2 and 6.9 Ω·cm in the TFTs without the IL, respectively. This improved electrical contact properties increases field-effect mobility from 39.9 to 45.0 cm2/Vs. This study demonstrates the effectiveness of this multi-stack IL approach in oxide TFTs.

Rapid thermal annealing effect on performance variations of solution processed indium–gallium–zinc-oxide thin-film transistors

In this paper, the 1 min annealing effect on the electrical characteristics of solution-processed amorphous indium–gallium–zinc-oxide thin-film transistors (a-IGZO TFTs) was analyzed in an ambient gas environment comprising He, Ar, N2, and O2 and an annealing temperature range from 400 to 600 °C for different active layer thicknesses. Even for the short annealing time of 1 min, the He-annealed TFTs show good performance with a threshold voltage of −1.27 V, a subthreshold slope of 605 mV/dec, and a field-effect mobility of 7.19 cm2/Vs under the thick active layer condition. The resulting good performance of the He-annealed TFT originates from the high thermal velocity of the He atom, which can be confirmed from the x-ray photoelectron spectroscopic measurement by the sharp definition of the active layer near the a-IGZO/gate insulator interface.

Extraction Technique for the Conduction Band Minimum Energy in Amorphous Indium–Gallium–Zinc–Oxide Thin Film Transistors

The conduction band minimum energy in amorphous oxide semiconductor-based thin film transistors (AOS TFTs) is a key parameter governing the accurate extraction of energy distribution for the subgap density-of-states (DOSs) and carrier mobility. We report a technique for extraction of the gate voltage ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {CBM}}{)}$ </tex-math></inline-formula> and corresponding energy ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${E}_{\text {F},{\text {CBM}}}$ </tex-math></inline-formula> = <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${E}_{\text {C}-{\text {EREF}}}{)}$ </tex-math></inline-formula> for the quasi-Fermi level ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${E}_{\text {F}}{)}$ </tex-math></inline-formula> equal to the conduction band minimum ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${E}_{\text {C}}{)}$ </tex-math></inline-formula> as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {CBM}}$ </tex-math></inline-formula> = <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {GS}}$ </tex-math></inline-formula> ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${E}_{\text {F}}$ </tex-math></inline-formula> = <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${E}_{\text {C}}{)}$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${E}_{\text {F},{\text {CBM}}}$ </tex-math></inline-formula> = <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${E}_{\text {F}}$ </tex-math></inline-formula> ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {GS}}$ </tex-math></inline-formula> = <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {CBM}}{)}$ </tex-math></inline-formula> . In order to confirm this technique through optoelectronic experimental data, amorphous indium–gallium–zinc–oxide (a-IGZO)-based thin film transistor was irradiated with various wavelengths and power, and obtained <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {CBM}}$ </tex-math></inline-formula> = 7.1 V and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${E}_{\text {F} {\text {CBM}}}$ </tex-math></inline-formula> = 71 meV in the dark state. This technique is expected to be useful in the accurate characterization of the subgap DOS and the effective mobility in AOS TFTs through a simple and effective extraction process.

Control of Hydrogen Concentration in Ingazno Thin Film Using Cryopumping System

Recently, amorphous metal-oxide semiconductors (AOSs) such as indium gallium zinc oxide (IGZO) have attracted great attention with their lower power consumption and higher mobility than amorphous silicon in the display field. However, the threshold voltage (Vth) shift of IGZO thin film transistor (TFT) caused by their poor reliability leads to the drop panel luminance. Therefore, improvement of IGZO TFT reliability is necessary, which is largely influenced by oxygen vacancy and hydrogen. In particular, hydrogen has shown different roles inside the IGZO active layer by hydrogen concentration. At lower concentration, the hydrogen passivation of oxygen deficiency region induces the improvement of initial electrical characteristics and reliability [2]. Meanwhile, Vth shift and degradation reliability are generated at higher concentration owing to the hydrogen related defect states [3]. Therefore, controlling excess hydrogen is necessary for reliability enhancement.In this work, IGZO thin film was fabricated by cryopumping system instead of a turbo molecular pump (TMP) for radio frequency sputtering to reduce hydrogen concentration. Cryopump is an adsorption type that enables excellent evacuation ability of hydrogen and moisture taking up most of the partial pressure in high vacuum. As a result, deposition of IGZO thin film with lower hydrogen impurity is available. The hydrogen concentration in the IGZO thin film using both cryopump and TMP was quantitatively analyzed by elastic recoil detection (ERD) analysis. Then, electrical characteristics of the self-aligned top-gate structure TFTs fabricated by both pumps were measured with bias stress conditions.To sum up, we showed the improved reliability of IGZO TFT by controlling excess hydrogen using cryopumping system. Fig.1. (a) Schematic of RF magnetron sputtering process with cryopump. (b) TOF-SIMS depth profiles of hydrogen concentration in IGZO film using TMP and cryopump Acknowledgment This work was supported by the Korea Evaluation Institute of Industrial Technology(KEIT) grant funded by the Korea government (MOTIE) (No. 2021-11-1283) References Chung, Ui-Jin, et al. "15‐1: Invited Paper: Manufacturing Technology of LTPO TFT." SID Symposium Digest of Technical Papers. Vol. 51. No. 1. 2020.Hanyu, Yuichiro, et al. "Hydrogen passivation of electron trap in amorphous In-Ga-Zn-O thin-film transistors." Applied Physics Letters 103.20 (2013): 202114.Mativenga, Mallory, et al. "Origin of light instability in amorphous IGZO thin-film transistors and its suppression." Scientific reports 11.1 (2021): 1-12 Figure 1

Impact of electrode materials on the performance of amorphous IGZO thin-film transistors

This study reports on the fabrication and characterization of thin-film transistors (TFTs) based on indium–gallium–zinc–oxide (IGZO) with various source- and drain-region metals (Pt, W and Ti). The performance of the IGZO transistors is compared to TFTs based on hydrogenated amorphous silicon (a-Si:H) with Pt source- and drain-regions. From the output characteristics maximum saturation mobilities of µ = 0.45 cm2/Vs for a-Si:H, and µ = 24 to 50 cm2/Vs for IGZO TFTs are extracted, which are competitive to high-performance thin-film transistors. The study reveals a general influence of the source- and drain-electrode material on the maximum saturation mobility and inverse sub-threshold slope.Graphical abstract

Performance Improvement of Self-Aligned Coplanar Amorphous Indium–Gallium–Zinc Oxide Thin-Film Transistors by Boron Implantation

The electrical properties and device stability of a self-aligned (SA) coplanar amorphous indium–gallium–zinc oxide (a-IGZO) thin-film transistor (TFT) were investigated by implanting boron (B) into the source/drain (SD) n+ region. To evaluate the effect according to the depth profile of B in the a-IGZO film, various implantation energies were applied. The electrical properties were optimized when the projection range of B was in the central vertical region of the a-IGZO film. B implantation decreased the resistivity of the a-IGZO film from 3.1 × 102 to 2.1 × 10–3 Ω·cm compared to an untreated a-IGZO film, while the field-effect mobility (μfe) improved from 2.96 to 17.22 cm2/(V·s). Moreover, the fabricated SA coplanar a-IGZO TFTs with a B-doped n+ region exhibited excellent stability, with a threshold voltage shift (ΔVth) of <0.2 V during a 3000 s thermal stability test performed at 200 °C and a bias stress test under a gate voltage of ±20 V. During the implantation process, B ions with high kinetic energy collide with IGZO atoms, resulting in the formation of an oxygen vacancy (VO) and an oxygen interstitial (Oi) simultaneously. The implanted B ions and Oi are bonded such that the VO sites are maintained by the B–O reaction and can contribute to an increase in the carrier concentration in a-IGZO films, thereby increasing the conductivity of the n+ region.

Performance Enhancement of Transparent Amorphous IGZO Thin-Film Transistor Realized by Sputtered Amorphous AlOx Passivation Layer

We report a high-mobility transparent Indium-Gallium-Zinc-Oxide (IGZO) thin-film transistor (TFT) with sputtered AlOx passivation layer. The interfacial region between the IGZO layer and the AlOx layer played a crucial role in improving the field-effect mobility (the maximum field-effect mobility increased from 6.292 cm2 Vs−1 for the TFT without the AlOx layer to 69.01 cm2 Vs−1 for the TFT with the passivation layer) and the on/off current ratio (from ∼107 without the layer to ∼108 with the layer). The driving current of IGZO TFT was also significantly enhanced. The formation of the interfacial layer has been investigated and verified. The ion bombardment during the AlOx deposition broke the In-O bond in IGZO, generating oxygen ions (O2−). The segregation of the O2− was facilitated by the sputtered amorphous AlOx. A metallic In-rich layer with high oxygen vacancy concentration was formed at the interface, leading to an increase in the carrier concentration in the interfacial layer. Besides the electrical performance, the reliability tests, including long-term exposure in the ambient environment and positive bias illumination stress (PBIS), showed improved results as well.

Amorphous InGaZnO Thin-Film Transistors With Sub-10-nm Channel Thickness and Ultrascaled Channel Length

We investigate the effect of channel layer thickness on effective mobility ( $\mu _{\text {eff}}$ ) in the sub-10-nm regime of amorphous indium–gallium–zinc–oxide thin-film transistors ( $\alpha $ -IGZO TFTs). TFT devices with extremely scaled channel thickness ${t} _{{\alpha}-{\text {IGZO}}}$ of 3.6 nm were realized, exhibiting low subthreshold swing (SS) of 74.4 mV/decade and the highest effective mobility $\mu _{\text {eff}}$ of 34 cm2/ $V\cdot s $ at a carrier density ${N} _{Carrier}$ of $\sim 5 \times 10^{12}$ cm−2 for any kind of $\alpha $ -IGZO TFTs having sub-10-nm ${t} _{{\alpha}-{\text {IGZO}}}$ . No significant degradation of $\mu _{\text {eff}}$ is observed as $\alpha $ -IGZO thickness reduced from 6 to 3.6 nm. By scaling down the channel length ${L} _{\text {CH}}$ to 38 nm, high extrinsic transconductance ( ${G} _{m, \max}$ ) of $125~\mu \text{s}/\mu \text{m}$ (at ${V}_{\text {DS}}$ of 1 V) and ON-state current ${I} _{\text {ON}}$ of $350~\mu \text{A}/\mu \text{m}$ at ${V}_{\text {GS}}$ – ${V} _{T}$ of 3 V with ${V}_{\text {DS}}$ of 2.5 V are achieved.

Drain-Bias Transient Instability of Amorphous Indium–Gallium–Zinc Oxide Thin-Film Transistors

High-voltage amorphous indium–gallium–zinc oxide thin-film transistors can be monolithically integrated into the system-on-chip platform as input–output bridges. However, a transient instability showing substantial ON-current degradation under high drain bias is discovered. This drain-bias transient instability depends on the stress time of less than 10 s and both drain and gate stress voltages. It is attributed to the charge trapping in local oxygen vacancies with shallow energy levels and the migration of oxygen ions near the drain. We found that oxygen vacancies are induced by metal contacts. An elevated-metal structure suppresses the transient instability by separating the metal contact region farther away from the channel compared with the conventional top-contact structure.

Hybrid integration of carbon nanotube and amorphous IGZO thin-film transistors

Solution-processed carbon nanotubes (CNTs) have recently attracted significant attention as p-type thin-film transistor (TFT) channels due to their high carrier mobility, high uniformity, and low process temperature. However, implementing sophisticated macroelectronics with a combination of single CNT-TFTs has been challenging because it is difficult to fabricate n-type CNT-TFTs. Therefore, in combination with indium-gallium-zinc-oxide (IGZO), which has excellent electrical performance and has been commercialized as an n-type oxide TFT, we demonstrated various hybrid complementary metal-oxide semiconductor integrated circuits, such as inverters and nor and nand gates. This hybrid integration approach allows us to combine the strength of p-type CNT- and n-type IGZO-TFTs, thus offering a significant improvement for macroelectronic applications.

Multilevel memory and synaptic characteristics of a-IGZO thin-film transistor with atomic layer–deposited Al2O3/ZnO/Al2O3 stack layers

Abstract

Novel Top-Anode OLED/a-IGZO TFTs Pixel Circuit for 8K4K AM-OLEDs

This paper proposes amorphous indium–gallium–zinc oxide thin-film transistors (TFTs) four-transistor-two-capacitor (4T2C) pixel circuit in combination with a top-anode organic light-emitting diode (OLED) for the use in 8K4K active-matrix organic light-emitting diode displays (AM-OLEDs). The proposed pixel circuit compensates for driving TFTs threshold voltage ( ${V} _{\textsf {TH}}$ ) shifts and mobility variations, ${V} _{\textsf {DD}}$ current–resistance ( ${I}$ – ${R}$ ) drops, and ${V} _{\textsf {SS}}~{I}$ – ${R}$ -induced rises. Both the positive and negative ${V} _{\textsf {TH}}$ of the driving TFT can be sensed by the proposed circuit. In this paper, we analyze the impact of the driving TFT compensation time for low gray levels. The current error rates are calculated when TFT mobility variations are considered. The proposed pixel circuit allows an increase in the compensation time to reduce the impact of driving TFT ${V} _{\textsf {TH}}$ and mobility variations on the performance of 4T2C pixel circuit. Conventional two-transistor-one-capacitor, five-transistor-two-capacitor, and proposed 4T2C pixel circuits are simulated and compared in this paper. The proposed pixel 4T2C circuit reduces the current error rates to below 5.79% when the ${V} _{\textsf {TH}}$ shifts of ±2 V, mobility variations of ±10%, ${V} _{\textsf {DD}}~{I}$ – ${R}$ drops of 1 V, and ${V} _{\textsf {SS}}~{I}$ – ${R}$ rises of 1 V are all considered. We show that the proposed 4T2C pixel circuit is suitable for 8K4K AM-OLEDs.

The Vacancy Pool Model for Amorphous In-Ga-Zn-O Thin-Film Transistors

In this paper, the reaction rate of oxygen vacancy ( $\text{V}_{\mathrm{ O}}$ ) by the derivatives of threshold voltage ( ${V}$ th) in the amorphous indium–gallium–zinc oxide thin-film transistors under light pulses with altering duty ratios is investigated. More importantly, after collecting and analyzing a lot of experimental results, a comprehensive model named $\text{V}_{\mathrm{ O}}$ pool is proposed. The proposed model can more universally describe the characteristic of $\text{V}_{\mathrm{ O}}$ reacting to the light and its degradation behavior under various kinds of stress condition.

Electrical Stability of Solution-Processed a-IGZO TFTs Exposed to High-Humidity Ambient for Long Periods

The variations in the electrical and mechanical properties of solution-processed amorphous indium–gallium–zinc–oxide thin-film transistors exposed to high-humidity ambient conditions for long periods were analyzed. When the TFT was exposed to high-humidity conditions, field-effect mobility severely decreased, while ON/OFF current ratio improved and subthreshold slope value remained nearly constant, which is different from that exposed to low-humidity condition. We found that the H2O molecules induce mechanical peeling of the active layer such that they act as acceptor-like deep states, which is very different from the prior results under low humidity condition. The variations in electrical characteristics were systematically analyzed using a technology-CAD simulation before and after exposure to highhumidity conditions.

Suppressed Degradation of Elevated-Metal Metal–Oxide Thin-Film Transistors Under Bipolar Gate Pulse Stress

In this letter, investigations on the reliability of elevated-metal metal–oxide (EMMO) thin-film transistors (TFTs) under bipolar gate pulse stress are reported. Different from conventional amorphous indium–gallium–zinc oxide TFTs, EMMO TFTs exhibit negligible dynamic degradation during the pulse transitions and the device degradation is dominated by the DC mechanism. Though negative threshold voltage $({V}_{\textsf {th}})$ shift is observed under both positive bias stress (PBS) and negative bias stress, the ${V}_{\textsf {th}}$ shift caused by the bipolar pulse stress is smaller than that induced by the PBS with the same positive gate bias as in the bipolar gate pulse and the same effective PBS time, where the ${V}_{\textsf {th}}$ shift introduced during the positive bias stage in the bipolar pulse stress is believed to be partially recovered during the negative bias stage. These degradation phenomena are proposed to be due to the improved quality of the channel and the gate oxide with the oxidizing thermal heat-treatment during the fabrication of EMMO TFTs, which eventually lead to a reliable performance for the EMMO TFTs under bipolar gate pulse stress.

Highly stable field emission from ZnO nanowire field emitters controlled by an amorphous indium–gallium–zinc-oxide thin film transistor

Lowering the driving voltage and improving the stability of nanowire field emitters are essential for them to be applied in devices. In this study the characteristics of zinc oxide (ZnO) nanowire field emitter arrays (FEAs) controlled by an amorphous indium–gallium–zinc-oxide thin film transistor (a-IGZO TFT) were studied. A low driving voltage along with stabilization of the field emission current were achieved. Modulation of field emission currents up to three orders of magnitude was achieved at a gate voltage of 0–32 V for a constant anode voltage. Additionally, a-IGZO TFT control can dramatically reduce the emission current fluctuation (i.e., from 46.11 to 1.79% at an emission current of ∼3.7 µA). Both the a-IGZO TFT and ZnO nanowire FEAs were prepared on glass substrates in our research, demonstrating the feasibility of realizing large area a-IGZO TFT-controlled ZnO nanowire FEAs.

Amorphous IGZO TFT-Based Pixel Buffer to Suppress Blue-Phase Liquid Crystal High-Frequency Effect

This letter discusses the frequency effect of polymer-stabilized blue-phase liquid crystal (BPLC) on the data voltages of a conventional pixel circuit and develops a new pixel buffer that exploits amorphous indium–gallium–zinc-oxide thin-film transistors (TFTs). During high-speed operations, BPLC seriously distorts data voltages because of the highly frequency-dependent dielectric anisotropy. To lessen the frequency-dependent effect of BPLC, a source-follower structure is integrated into the proposed pixel buffer to prolong the charging time of the BPLC cell. The experimental results show that the source-follower yields linear output characteristics. Further compensation for TFT ${{\text{V}}}_{{\mathrm{TH}}}$ shifts and suppressing the leakage current are confirmed to make the data voltages on the BPLC cell highly uniform and stable.

The Mobility Enhancement of Indium Gallium Zinc Oxide Transistors via Low-temperature Crystallization using a Tantalum Catalytic Layer

High-mobility indium gallium zinc oxide (IGZO) thin-film transistors (TFTs) are achieved through low-temperature crystallization enabled via a reaction with a transition metal catalytic layer. For conventional amorphous IGZO TFTs, the active layer crystallizes at thermal annealing temperatures of 600 °C or higher, which is not suitable for displays using a glass substrate. The crystallization temperature is reduced when in contact with a Ta layer, where partial crystallization at the IGZO back-channel occurs with annealing at 300 °C, while complete crystallization of the active layer occurs at 400 °C. The field-effect mobility is significantly boosted to 54.0 cm2/V·s for the IGZO device with a metal-induced polycrystalline channel formed at 300 °C compared to 18.1 cm2/V·s for an amorphous IGZO TFT without a catalytic layer. This work proposes a facile and effective route to enhance device performance by crystallizing the IGZO layer with standard annealing temperatures, without the introduction of expensive laser irradiation processes.

IGZO TFT-based circuit with tunable threshold voltage by laser annealing

In this work, a high-performance inverter based on amorphous indium–gallium–zinc oxide thin-film transistors (TFTs) has been fabricated, which consists of a driver TFT and a load TFT. The threshold voltage (Vth) of the load TFT can be tuned by applying an area-selective laser annealing. The transfer curve of the load TFT shows a parallel shift into the negative bias direction upon laser annealing. Based on x-ray photoelectron spectroscopy analyses, the negative Vth shift can be attributed to the increase of oxygen vacancy concentration within the device channel upon laser irradiation. Compared to the untreated inverter, the laser annealed inverter shows much improved switching characteristics, including a large output swing range which is close to full swing, as well as an enhanced output voltage gain. Furthermore, the dynamic performance of ring oscillator based on the laser-annealed inverter is improved.

Two-stage unified stretched-exponential model for time-dependence of threshold voltage shift under positive-bias-stresses in amorphous indium–gallium–zinc oxide thin-film transistors

In this study, we show that the two-stage unified stretched-exponential model can more exactly describe the time-dependence of threshold voltage shift (ΔVTH) under long-term positive-bias-stresses compared to the traditional stretched-exponential model in amorphous indium–gallium–zinc oxide (a-IGZO) thin-film transistors (TFTs). ΔVTH is mainly dominated by electron trapping at short stress times, and the contribution of trap state generation becomes significant with an increase in the stress time. The two-stage unified stretched-exponential model can provide useful information not only for evaluating the long-term electrical stability and lifetime of the a-IGZO TFT but also for understanding the stress-induced degradation mechanism in a-IGZO TFTs.