Enhanced Light Stability of InGaZnO Thin-Film Transistors by Atomic-Layer-Deposited Y2O3 with Ozone

Al2O3/HfO2 Nanolaminate Dielectric Boosting IGZO-Based Flexible Thin-Film Transistors

HIGHLIGHTS

A stable laminated Al₂O₃/HfO₂ insulator is developed by atomic layer deposition at a relatively lower temperature of 150 °C. The flexible thin-film transistors (TFTs) with bottom-gate top-contacted configuration are fabricated on a flexible substrate with the Al₂O₃/HfO₂ insulator. The flexible TFTs present the carrier mobilities of 9.7 cm² V⁻¹ s⁻¹, ON/OFF ratio of ~ 1.3 × 10⁶, subthreshold voltage of 0.1 V, saturated current up to 0.83 mA, and subthreshold swing of 0.256 V dec⁻¹.

ABSTRACT

Flexible thin-film transistors (TFTs) have attracted wide interest in the development of flexible and wearable displays or sensors. However, the conventional high processing temperatures hinder the preparation of stable and reliable dielectric materials on flexible substrates. Here, we develop a stable laminated Al₂O₃/HfO₂ insulator by atomic layer deposition at a relatively lower temperature of 150 °C. A sputtered amorphous indium-gallium-zinc oxide (IGZO) with the stoichiometry of In_0.37Ga_0.20Zn_0.18O_0.25 is used as the active channel material. The flexible TFTs with bottom-gate top-contacted configuration are further fabricated on a flexible polyimide substrate with the Al₂O₃/HfO₂ nanolaminates. Benefited from the unique structural and compositional configuration in the nanolaminates consisting of amorphous Al₂O₃, crystallized HfO₂, and the aluminate Al–Hf–O phase, the as-prepared TFTs present the carrier mobilities of 9.7 cm² V⁻¹ s⁻¹, ON/OFF ratio of ~ 1.3 × 10⁶, subthreshold voltage of 0.1 V, saturated current up to 0.83 mA, and subthreshold swing of 0.256 V dec⁻¹, signifying a high-performance flexible TFT, which simultaneously able to withstand the bending radius of 40 mm. The TFTs with nanolaminate insulator possess satisfactory humidity stability and hysteresis behavior in a relative humidity of 60–70%, a temperature of 25–30 °C environment. The yield of IGZO-based TFTs with the nanolaminate insulator reaches 95%. [Image: see text]

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s40820-022-00929-y.

Roles of Oxygen Interstitial Defects in Atomic-Layer Deposited InGaZnO Thin Films with Controlling the Cationic Compositions and Gate-Stack Processes for the Devices with Subμm Channel Lengths

Roles of oxygen interstitial defects located in the In-Ga-Zn-O (IGZO) thin films prepared by atomic layer deposition were investigated with controlling the cationic compositions and gate-stack process conditions. It was found from the spectroscopic ellipsometry analysis that the excess oxygens increased with increasing the In contents within the IGZO channels. While the device using the IGZO channel with an In/Ga ratio of 0.2 did not show marked differences with the variations in the oxidant types during the gate-stack formation, the device characteristics were severely deteriorated with increasing the In/Ga ratio to 1.4, when the Al₂O₃ gate insulator (GI) was prepared with the H₂O oxidants (H₂O-Al₂O₃) due to a higher amount of excess oxygen in the channel. Additionally, during the deposition process of the Al-doped ZnO (AZO) gate electrode (GE) replacing from the indium-tin oxide (ITO) GE, the thermal annealing effect at 180 °C facilitated the passivation of oxygen vacancy and the strengthening of metal-oxygen bonding, which could stabilize the TFT operations. From these results, the gate-stack structure employing O₃-processed Al₂O₃ GI (O_3-Al₂O₃) and AZO GE (OA) was suggested to be most suitable for the device using IGZO channel with a higher In content. On the other hand, the device employing H₂O-Al₂O₃ GI and AZO GE exhibited larger negative shifts of threshold voltage (V_TH) under positive-bias-temperature stress (PBTS) condition than the device employing O_3- Al₂O₃ GI and ITO GE due to larger hydrogen contents within the gate stacks. Anomalous negative shifts of V_TH were accelerated with increasing the In contents of the IGZO channel. When the channel length of the fabricated device were scaled down to submicrometer regime, the OA gate stacks successfully alleviated the short-channel effects.

Ultraviolet-Assisted Low-Thermal-Budget-Driven α-InGaZnO Thin Films for High-Performance Transistors and Logic Circuits

Developing a low-temperature fabrication strategy of an amorphous InGaZnO (α-IGZO) channel layer is a prerequisite for high-performance oxide-based thin film transistor (TFT) flexible device applications. Herein, an ultraviolet-assisted oxygen ambient rapid thermal annealing method (UV-ORTA), which combines ultraviolet irradiation with rapid annealing treatment in an oxygen atmosphere, was proposed to realize the achievement of high-performance α-IGZO TFTs at low temperature. Experimental results have confirmed that UV-ORTA treatment has the ability to suppress defects and obtain high-quality films similar to high-temperature-annealing-treated samples. α-IGZO/HfAlO TFTs with high-performance and low-voltage operating have been achieved at a low temperature of 180 °C for 200 s, including a high μ_sat of 23.12 cm² V^-1 S^-1, large I_on/off of 1.1 × 10⁸, small subthreshold swing of 0.08 V/decade, and reliable stability under bias stress, respectively. As a demonstration of complex logic applications, a low-voltage resistor-loaded unipolar inverter based on an α-IGZO/HfAlO TFT has been built, demonstrating full swing characteristics and a high gain of 13.8. Low-frequency noise (LFN) characteristics of α-IGZO/HfAlO TFTs have been presented and concluded that the noise source tended to a carrier number fluctuation (ΔN) model from a carrier number and correlated mobility fluctuation (ΔN-Δμ) model. As a result, it can be inferred that the low-temperature UV-ORTA technique to improve α-IGZO thin film quality provides a facile and designable process for the integration of α-IGZO TFTs into a flexible electronic system.

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.

Graded Channel Junctionless InGaZnO Thin-Film Transistors with Both High Transporting Properties and Good Bias Stress Stability

InGaZnO (IGZO) is currently the most prominent oxide semiconductor complement to low-temperature polysilicon for thin-film transistor (TFT) applications in flat panel displays. However, the compromised transport performance and bias stress instability are critical issues inhibiting its application in ultrahigh-resolution optoelectronic displays. Here, we report the fabrication of graded channel junctionless IGZO:O|N TFTs with both high transporting properties and good bias stress stability by systematic manipulation of oxygen vacancy (V_O) defects through sequential O antidoping and O/N codoping of the continuous IGZO framework. The transporting properties and bias stress stability of the graded channel IGZO:O|N TFTs, which exhibited high field-effect mobilities close to 100 cm² V^-1 s^-1, negligible performance degradations, and trivial threshold voltage shifts against gate bias stress and photobias stress, are simultaneously improved compared to those of the controlled single-channel uniformly doped IGZO:O TFTs, IGZO:N TFTs, and double-channel barrier-confined IGZO:O/IGZO:N TFTs. The synergistic improvements are attributed to the sequential mobility and stability enhancement effects of O antidoping and O/N codoping where triple saturation currents are induced by O antidoping of the front-channel regime while the trapped electrons and photoexcited holes in the back-channel bulk and surface regions are suppressed by O/N codoping. More importantly, fast accumulation and barrier-free full depletion are rationally realized by eliminating the junction interface within the graded channel layer. Our observation identifies that graded channel doping could be a powerful way to synergistically boost up the transport performance and bias stress stability of oxide TFTs for new-generation ultrahigh-definition display applications.

Light response behaviors of amorphous In–Ga–Zn–O thin-film transistors via in situ interfacial hydrogen doping modulation

Thin-film transistors (TFTs) based on amorphous In–Ga–Zn–O (a-IGZO) channels present high mobility, large-area uniformity, mechanical flexibility and photosensitivity, and thus have extensive applicability in photodetectors, wearable devices, etc. However, pure a-IGZO based photosensors only exhibit a UV light response with limited sensitivity performance. By utilizing in situ interfacial hydrogen doping, it is demonstrated that the a-IGZO TFTs with the Al₂O₃ dielectric deposited by plasma-enhanced atomic layer deposition at room temperature (RT) have excellent photosensing performance, such as a photoresponsivity of over 6 × 10⁵ A W⁻¹ and a light to dark current ratio up to 10⁷. This is attributed to spontaneous interfacial hydrogen doping into the a-IGZO channel during sputtering deposition of a-IGZO on hydrogen-rich Al₂O₃ films, thus generating subgap states in the band gap of IGZO. Further, color pattern imaging was achieved by employing an array of the color distinguishable devices, and flexibility was demonstrated by fabricating the TFTs onto polymer substrates. Moreover, it is also found that both the RT and 150 °C Al₂O₃ a-IGZO TFTs exhibit typical light-stimulated synaptic behaviors, including excitatory post-synaptic current and pair-pules facilitation, etc., and the memory time of the synaptic devices can be easily modulated by the degree of the interfacial hydrogen doping.

Thin-film transistors (TFTs) based on amorphous In–Ga–Zn–O (a-IGZO) channels present high mobility, large-area uniformity, mechanical flexibility and photosensitivity, and thus have extensive applicability in photodetectors, wearable devices, etc.

Printed flexible and transparent electronics: enhancing low-temperature processed metal oxides with 0D and 1D nanomaterials

Metal oxides have broad multifunctionality and important applications to energy, sensing, and information display. Printed electronics have recently adopted metal oxides to push the limits of performance and stability for flexible thin film systems. However, a grand challenge in this field is to achieve these properties while balancing the thermal budget, which critically determines the applicability, flexibility, and cost of these systems. This paper presents a focused review of printed metal oxide electronics, highlighting our recent work developing high-performance, printed transistors processed at low temperatures via aqueous precursor chemistries, nanomaterial hybrid inks, and ultraviolet annealing. These results reveal the potential for printing uniquely high-performance active devices (electronic mobility >10 cm² V^-1 s^-1) but also illustrates the utility of nanocomposites that integrate nanomaterials within a metal oxide matrix for improving device performance.

All-Sputtering, High-Transparency, Good-Stability Coplanar Top-Gate Thin Film Transistors

In this work, transparent, stable coplanar top-gate thin film transistors (TFTs) with an active layer of neodymium-doped indium oxide and zinc oxide (Nd-IZO) were successfully fabricated on a glass substrate by all sputtering processes. The devices with a post-annealing temperature of 400 °C exhibited good electrical performances with a saturation mobility (μsat) of 4.25 cm2·V−1·S−1, Ion/Ioff ratio about 106, Vth of −0.97 V and SS about 0.34 V/decade. Furthermore, the devices exhibited excellent negative and positive bias stability (NBS, PBS) of only a ΔVth shift of about −0.04 V and 0.05 V after 1 h, respectively. In addition, the devices showed high transparency about 96% over the visible-light region of 400–700 nm, which indicates a great potential in transparent displays.

Effects of Ar Addition to O2 Plasma on Plasma-Enhanced Atomic Layer Deposition of Oxide Thin Films

A method for significantly increasing the growth rates (GRs) of high- k oxide thin films grown via plasma-enhanced atomic layer deposition (PE-ALD) by enhancing the plasma density through the addition of Ar gas to the O₂ plasma oxidant was developed. This approach led to improvements of ∼60% in the saturation GRs of PE-ALD ZrO₂, HfO₂, and SiO₂. Furthermore, despite the significantly higher GR enabled by PE-ALD, the mechanical and dielectric properties of the PE-ALD oxide films were similar or even superior to those of films grown via the conventional O₂ plasma process. Optical emission spectroscopy analyses in conjunction with theoretical calculation of the electron energy distribution function revealed that adding Ar gas to the O₂ plasma increased the density of high-energy electrons, thereby generating more O₂ plasma species, such as ions and radicals, which played a key role in improving the GRs and the properties of the films. This promising approach is expected to facilitate the high-volume manufacturing of films via PE-ALD, especially for use as gate insulators in thin-film transistor-based devices in the display industry.