Negative Bias Illumination Stress Research Articles

Abnormal Threshold Voltage Shift and Sub-channel Generation in Top-Gate InGaZnO TFTs under Backlight Negative Bias Illumination Stress

Previous reports on the top-gate IGZO thin-film transistors (TFTs) under negative bias illumination stress (NBIS) show a threshold voltage (VTH) shift to the left. However, in the present study, a right shift in VTH was observed in the reverse-sweep current–voltage (I–V) curves after applying NBIS to top-gate IGZO TFTs by a UV backlight. The hysteresis window of the forward and reverse sweep I–V curves widened with the increase in the stress time. The abnormal VTH shift and hysteresis-window increment are explained using an energy-band diagram. In addition, an abnormal gm peak appeared in the forward I–V curve after 2000 s of the backlight NBIS, which was verified by testing the devices with different dimensions. Silvaco TCAD simulation results indicated a strong electric field in the sidewall, confirming the generation of sub-channels. The comparison of the ΔVTH after UV backlight NBIS in three different devices suggested that the higher N2O/SiH4 flow rate and lower power deposition can produce a good quality buffer layer. A rapid examination of a buffer layer is possible using the backlight NBIS. In addition, the presence of H2 in the buffer layer was confirmed by the thermal desorption spectroscopy. Finally, the reliability of the devices can be enhanced by improving the modulation of the existing deposition process.

Robustness of Passivated ALD Zinc Tin Oxide TFTs to Aging and Bias Stress

Thin-film electronics fabricated with amorphous oxide semiconductors (AOS) are being studied for beyond-display technologies because of their superior electron transport. To widen the commercial applications, robust devices fabricated using scalable deposition techniques are required. Here, we fabricate bottom-gate, top contact thin-film transistors (TFTs) using zinc tin oxide, an AOS deposited by atomic layer deposition (ALD), and study robustness to aging and bias stress for devices stored in a dark air ambient with and without Al2O3 passivation. All samples exhibit excellent mobility ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu _{\text {FE}}$ </tex-math></inline-formula> ) and subthreshold swing (SS) robustness to aging. Most of the device aging occurs within the first 25 days, after which the performance stabilizes. Changes due to positive bias stress, which are dominated by interactions with ambient molecules, are greatly reduced by passivation. Passivation, however, results in an increase in contact resistance ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${R}_{C}$ </tex-math></inline-formula> ), SS, and negative bias illumination stress (NBIS) instability. The increase in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${R}_{C}$ </tex-math></inline-formula> is attributed to the diffusion of oxygen vacancies that occurs during the passivation process, while the increase in SS and NBIS instability is attributed to subgap interface defects. The experimental results are compared to Silvaco ATLAS simulations to confirm the proposed physical mechanisms and allow quantification of the associated defects and non-idealities.

A Simple Doping Process Achieved by Modifying the Passivation Layer for Self-Aligned Top-Gate In-Ga-Zn-O Thin-Film Transistors at 200 °C.

In this paper, a facile modifying technique of source/drain regions conductivity was proposed for self-aligned top-gate In-Ga-Zn-O (IGZO) thin-film transistors (TFTs) by controlling the process parameter of the passivation layer at relatively low temperatures. The sheet resistance of the source and drain regions of IGZO was approximately 365 Ω/□, and there was no significant change within a month. The device parameters of mobility, threshold voltage, subthreshold swing, and current switching ratio of the fabricated device were 15.15 cm2V-1s-1, 0.09 V, 0.15 V/dec, and higher than 109, respectively. The threshold voltage drift under negative bias illumination stress was -0.34 V. In addition, a lower channel width-normalized contact resistance of 9.86 Ω·cm was obtained.

Developing Subthreshold-Swing Limit of PEALD In–Sn–Ga–O Transistor via Atomic-Scaled Sn Control

The In–Sn–Ga–O (ITGO) thin-film transistor (TFT) is promising in that it possesses enhanced electrical characteristics and stability because the tin (Sn) has large spherical s orbitals and a high binding energy with oxygen (O). Recently, there have been several reports of ITGO material fabricated via sputtering. Therefore, studies that control Sn composition to achieve unique characteristics and obtain conformal films have been limited. For these reasons, we evaluated plasma-enhanced atomic layer deposition (PEALD)-derived ITGO materials with varied Sn composition. The electrical characteristics are enhanced with the Sn subcycle ratio from 1 to 5 in the ITGO films (carrier concentration: 6.4 × 1020 to 2.2 × 1021 cm–3, resistivity: 8.6 × 10–4 to 6.1 × 10–4 Ω cm). These results are due to carrier generation by the Sn, a decrease in oxygen-related defects (22.9% to 19.5%), and suppressed indium-oxide (In2O3) crystallinity. To demonstrate their applicability to practical devices, the ITGO films were applied to TFTs as active channel layers. The ITGO TFTs with increasing Sn content show a decrease then increase of subthreshold swing (S.S.) characteristics, while the field-effect mobility (μFE) slightly increases. In addition, the ITGO TFTs show improved negative bias illumination stress (NBIS) stability with increasing Sn content, while the turnaround point is observed in the Sn 5 subcycle. As a result, the ITGO TFT with the Sn 3 subcycle shows an extremely low S.S. of 64.8 ± 1.9 mV/decade and an NBIS result of −0.5 V. The outstanding performance of the ITGO TFT with Sn 3 subcycle is attributed to its low number of oxygen-related defects, amorphous structure, and low valence band maximum (VBM) level. Therefore, the ITGO TFTs with optimized Sn content have the advantages of low power consumption and resistance to an environment with illumination.

Enhanced stability and mobility of solution-processed oxide thin-film transistors with bilayer terbium-incorporated indium oxide channel

The trade-off between mobility and stability in oxide thin-film transistors (TFTs) hinders further advances of an active-matrix flat panel display. Herein, a solution-processed bilayer active channel is designed to improve the stability and mobility simultaneously. The optical bandgap and work function of Tb:In2O3 films are modulated by tuning the film thickness and Tb concentration of Tb-doped indium oxide (Tb:In2O3) films. Large conduction band offset is achieved in a Tb:In2O3 bilayer channel, which induces accumulation of abundant electrons at the interface. The mobility is significantly improved to 38.2 cm2/V s, and the photoinduced stability of bilayer Tb:In2O3 TFTs is improved with low threshold voltage shift of 0.26 and −0.38 V under negative-bias illumination stress and negative-bias temperature illumination stress, respectively.

The Mechanism of the Photostability Enhancement of Thin-Film Transistors Based on Solution-Processed Oxide Semiconductors Doped with Tetravalent Lanthanides.

The applications of thin-film transistors (TFTs) based on oxide semiconductors are limited due to instability under negative bias illumination stress (NBIS). Here, we report TFTs based on solution-processed In2O3 semiconductors doped with Pr4+ or Tb4+, which can effectively improve the NBIS stability. The differences between the Pr4+-doped In2O3 (Pr:In2O3) and Tb4+-doped In2O3 (Tb:In2O3) are investigated in detail. The undoped In2O3 TFTs with different annealing temperatures exhibit poor NBIS stability with serious turn-on voltage shift (ΔVon). After doping with Pr4+/Tb4+, the TFTs show greatly improved NBIS stability. As the annealing temperature increases, the Pr:In2O3 TFTs have poorer NBIS stability (ΔVon are -3.2, -4.8, and -4.8 V for annealing temperature of 300, 350, and 400 °C, respectively), while the Tb:In2O3 TFTs have better NBIS stability (ΔVon are -3.6, -3.6, and -1.2 V for annealing temperature of 300, 350, and 400 ℃, respectively). Further studies reveal that the improvement of the NBIS stability of the Pr4+/Tb4+:In2O3 TFTs is attributed to the absorption of the illuminated light by the Pr/Tb4fn-O2p6 to Pr/Tb 4fn+1-O2p5 charge transfer (CT) transition and downconversion of the light to nonradiative transition with a relatively short relaxation time compared to the ionization process of the oxygen vacancies. The higher NBIS stability of Tb:In2O3 TFTs compared to Pr:In2O3 TFTs is ascribed to the smaller ion radius of Tb4+ and the lower energy level of Tb 4f7 with a isotropic half-full configuration compared to that of Pr 4f1, which would make it easier for the Tb4+ to absorb the visible light than the Pr4+.

Effects of Source/Drain Electrodes on the Performance of InSnO Thin-Film Transistors.

Oxide thin-film transistors (TFTs) are of increasing interest in the field of advanced displays. In this work, we explore Al, InSnO (ITO), Ti, and Mo as source/drain electrodes of ITO TFTs. A comparison study is conducted on the electrical properties of ITO TFTs with the four categories of source/drain electrodes. Interestingly, the ITO TFT with an Al source/drain electrode exhibits better device performance, such as a field-effect mobility (μFE) of 26.45 cm2/Vs, a reasonable turn-on voltage (VON) of 2.7 V, and a steep subthreshold swing (SS) of 201.50 mV/decade. The contact properties of ITO TFTs are further analyzed, and the results show that the device with an Al electrode exhibits lower contact resistance than the other devices. However, the devices with the four electrode materials all reveal excellent stability under negative bias illumination stress (NBIS) with |ΔVTH| &lt; 1 V. This work paves the way for the practical applications of ITO TFTs in next-generation displays.

Enhanced Electrical and Mechanical Performance of InSnZnO TFTs With Multifunctional Laminated Organic Passivation Layer

To realize high-performance and stable electrical characteristics for ZnO-based thin-film transistors (TFTs) under various environments, we propose InSnZnO (ITZO) TFTs with multifunctional laminated organic (MLO) passivation layers (PVLs). The MLO PVLs were constructed from self-assembled monolayers (SAMs) and an ultraviolet (UV) light absorber-modified polydimethylsiloxane (PDMS) layer. The SAMs can passivate the ITZO surface and suppress the chemical reaction during the spin-coating process, and the modified PDMS layer can block UV light. The MLO-treated ITZO TFTs exhibited a high-performance and robust stability with high mobility (~21.18 cm2 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\cdot $ </tex-math></inline-formula> V−1 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\cdot $ </tex-math></inline-formula> s−1), a considerable ON– OFF current ratio (~ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$6.65\times10$ </tex-math></inline-formula> 9), and a steep subthreshold slope (~100 mV/dec). The shift of the threshold voltage ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\textit V}_{{\mathrm {th}}}$ </tex-math></inline-formula> ) under gate bias stress and UV light illumination stress was investigated for the devices with different PVLs. The ITZO TFTs with MLO PVLs showed improved stability under positive gate bias stress (PBS), positive gate bias illumination stress (PBIS), and negative gate bias illumination stress (NBIS) compared to untreated and SAM-treated ITZO TFTs. Furthermore, the MLO-treated ITZO TFTs achieved a weak response to 365-nm UV light and showed superb stability with the negligible change of electrical performance, even after a 1-h water soaking test and a 100-day storing test. In addition, the MLO PVL exhibited high flexibility without signs of cracking even after 10 000 bending cycles and was proven suitable for fabricating flexible oxide TFTs.

(Digital Presentation) Negative Bias Illumination Stress on a-Igzo TFT with a Top Barrier

The double-layer a-IGZO thin film transistors (DL-TFTs) using a quantum well channel and a top barrier can improve not only the electrical properties and the positive bias temperature instability as compared to the single-layer TFT (SL-TFTs) [1][2] but also the negative bias illumination stress (NBIS). In this work, DL-TFTs have less negative VT shifts than that of SL-TFTs under NBIS. Moreover, the negative VT shift of DL-TFTs decreases with increasing top barrier thickness due to the energy barrier formed by the valence band discontinuity at the interface between the channel layer and the top barrier. Experimental Process: The 100 nm Mo/Al layers were first deposited on a glass substrate and patterned to form the bottom gate (BG) metal, followed by the deposition of 320 nm thick SiNx/SiOx gate insulator (GI) using plasma-enhanced chemical vapor deposition (PECVD). For the active layer of SL-TFT, 15 nm a-IGZO with high indium content was deposited to form a high mobility channel (HM-IGZO) using radio frequency magnetron sputtering with Ar plasma. As for the DL-TFTs, an extra standard a-IGZO (STD-IGZO, the atomic composition of the target is In2O3:Ga2O3:ZnO=1:1:2 mol%) was deposited as a top barrier. Two different top barrier thicknesses were deposited (10 nm and 30 nm) to investigate the thickness effect on the NBIS. The active layer was then patterned by photolithography. A 260 nm Ti was deposited and patterned by photolithography and reactive ion etch (RIE) as the S/D metal, followed by SiOx/SiNx top gate insulator deposition and the transparent indium tin oxide (ITO) top gate (TG) metal formation. To conduct the NBIS test, -30 V and 0 V were applied to the bottom gate and the top gate, respectively, and a light source with a wavelength of 365 nm was used as the illumination stress. Additional 100 nm films of HM and STD IGZO were deposited to investigate the band alignment. The bandgap (Eg), valence band offsets (EF-EV, where EF is the Fermi level and EV is the valence band edge), and work function can be extracted by Tauc method, X-Ray photoelectron spectroscopy (XPS), and Kelvin probe force microscopy (KPFM), respectively, to obtain the band alignment of the HM (channel layer) and the STD (top barrier) IGZO [3]. Results and Discussions: The evolution of VT shifts under NBIS are shown in Fig. 1. DL-TFTs have less VT shift than that of SL-TFT. Since the energy of the photon (3.4 eV) is larger than the bandgap of the a-IGZO (~3 eV), electron-hole pairs are generated by the light source. Since a negative voltage is applied to the BG, the generated holes are trapped into the BG GI, resulting in the negative VT shift. In DL-TFTs, the holes generated in the top barrier cannot be trapped into the BG GI due to the energy barrier formed by the valence band discontinuity. With the band alignment analysis [1], the valence band discontinuity is found to be 0.39 eV, which is high enough to stop the holes generated in the top barrier from trapping into the BG GI. The electrons generated in the top barrier can still be trapped into the TG GI, compensating the negative VT shift (Fig. 2(a)), resulting in a smaller VT shift in DL-TFTs than that of SL-TFTs. Reportedly, the VT shift increases with increasing a-IGZO thickness in SL-TFTs [4]. However, the VT shift decreases with increasing top barrier thickness in DL-TFTs (Fig. 1). Although the generated electron-hole pairs in the top barrier increase with increasing top barrier thickness, the additional holes cannot be trapped into the BG GI due to the valence band discontinuity, while the additional electrons can easily be trapped into the TG GI, leading to more VT shift compensation and thus less negative VT shift in the DL-TFT (Fig. 2(b)). Conclusion: The DL-TFTs have less VT shift under NBIS as compared to SL-TFTs. The NBIS can be further improved by increasing the top barrier thickness. Acknowledgment: This work was supported in part by the Ministry of Science and Technology, Taiwan, under Grant MOST 110-2218-E-A49-013-MBK, Grant MOST 110-2218-E-002-030, and Grant MOST 110-2622-8-002-014, and in part by the Ministry of Education, Taiwan, under Grant NTU-CC-110L892601. Reference: [1] J.-C. Chiu et al., J-EDS (2021) [2] J.-C. Chiu et al., IEDMS (2021) [3] A.-H. Tai et al., TED (2019) [4] M. M. Billah et al., EDL (2016) Figure 1

(Invited, Digital Presentation) Oxide TFTs Based on Semiconductors and Semimetals

Oxide semiconductors have opened a new era for large-area, flexible and transparent applications. Despite the progresses, a bottleneck issue of oxide thin-film transistors (TFTs) is the instability either under bias stress or when used as a current source. Furthermore, the carrier mobility and current driving capability need to be improved for high-spec displays. It is still hugely challenging to overcome both issues using the conventional device structure and oxide semiconductor materials.Here, we review our recent work on novel oxide TFTs that show a few desirable properties. Rather than using an ohmic metal contact as the source electrode, a high work-function Schottky source contact enables depletion around the TFT source region, which results in intrinsic immunity to the negative bias illumination stress, no obvious short channel effect, and superb current saturation over a wide range of drain voltage1. The flat saturation current gives rise to an extremely high voltage gain reaching 23,000, which is, to the best of our knowledge, the highest gain ever achieved by a solid-state transistor to date. The threshold voltage is also found to remain stable under different drain voltages, in contrast to standard TFTs, which may be useful in larger-area displays where the drain voltages of the drive TFTs can differ2. Furthermore, the depletion provided by the Schottky source electrode allows utilizing semi-metal ITO to replace IGZO as the TFT channel layer, which significantly enhances the carrier mobility and current driving capability. Other related work may also be discussed in the talk including oxide Schottky diodes operating beyond 10 GHz3, oxide TFTs operating beyond 1 GHz4, significantly enhanced carrier mobility by self-assembled monolayer treatment5,6, and CMOS-like oxide-based logic circuits7,8. References Extremely high-gain source-gated transistors, J Zhang, J Wilson, G Auton, Y Wang, M Xu, Q Xin, A Song, Proceedings of the National Academy of Sciences 116 (11), 4843-4848 (2019)Comparative Study of Short-Channel Effects Between Source-Gated Transistors and Standard Thin-Film Transistors, Zhenze Wang, Li Luo, Yiming Wang, Jiawei Zhang, and Aimin Song, IEEE Transactions on Electron Devices, 69(2), 561 - 566 (2022).Flexible indium–gallium–zinc–oxide Schottky diode operating beyond 2.45 GHz, J Zhang, Y Li, B Zhang, H Wang, Q Xin, A Song, Nature communications 6 (1), 1-7 (2015).Amorphous-InGaZnO thin-film transistors operating beyond 1 GHz achieved by optimizing the channel and gate dimensions, Y Wang, J Yang, H Wang, J Zhang, H Li, G Zhu, Y Shi, Y Li, Q Wang, Qian Xin, Zhongchao Fan, Fuhua Yang, Aimin Song, IEEE Transactions on Electron Devices 65 (4), 1377-1382 (2018).Significant Performance Improvement of Oxide Thin‐Film Transistors by a Self‐Assembled Monolayer Treatment, W Cai, J Zhang, J Wilson, J Brownless, S Park, L Majewski, A Song, Advanced Electronic Materials 6 (5), 1901421 (2020).Significant performance enhancement of very thin InGaZnO thin-film transistors by a self-assembled monolayer treatment, W Cai, J Wilson, J Zhang, J Brownless, X Zhang, LA Majewski, A Song, ACS Applied Electronic Materials 2 (1), 301-308 (2020).Complementary integrated circuits based on p-type SnO and n-type IGZO thin-film transistors, Y Li, J Yang, Y Wang, P Ma, Y Yuan, J Zhang, Z Lin, L Zhou, Q Xin, Aimin Song, IEEE Electron Device Letters 39 (2), 208-211 (2017).Thin Film Sequential Circuits: Flip-Flops and a Counter Based on p-SnO and n-InGaZnO, Y Yuan, J Yang, Y Wang, Z Hu, L Zhou, Q Xin, A Song, IEEE Electron Device Letters 42 (1), 62-65 (2020).

(Invited) Approach to Oxide Tfts with High Mobility & Stability

Thin film transistors (TFTs) with the channel of amorphous oxide semiconductors (AOSs) represented by amorphous Indium-Gallium-Zinc Oxide (a-IGZO) are now applied to drive pixels of the state-of the art display based on high mobility, good homogeneity, easy fabrication and optical transparency [1]. Recently, larger mobility TFTs are highly demanded for emerging applications such as ultra-high vision and memories. However, there is a critical obstacle to be overcome, i.e., an empirical trade-off rule in oxide TFTs between mobility and stability. Higher mobility TFTs possess less stability to various types of stress (negative bias-thermal stress instability, NBTS, positive bias-thermal stress instability, PBTS, and negative bias-illumination stress instability, NBIS). The Elucidation of origin for this trade-off rule and new approach are required to realize high mobility and high stability TFTs. We clarify that the origin of this trade-off through comparison in TFTs between IGZO with low mobility and high stability and ITZO with high mobility and low stability. A critical phenomenon was noted during the conventional TFT fabrication; CO-related impurity donates electrons to Indium-Tin-Zinc Oxide (ITZO) with high mobility under the negative bias stress condition but such CO-related impurity effect is negligible in IGZO with lower mobility than ITZO [2]. This finding may be understood by considering that large difference in ECBM determines the transfer of electron generated by adsorbed CO-related species on the back surface of ITZO [3]. Based on this model and this finding, we successfully fabricated the most stable ITZO TFT with a high mobility of 70 cm2/Vs by eliminating CO-related impurity and further passivation; NBTS (-20V, 60 oC, 3600 s, ΔVth: -0.02 V), PBTS (+20 V, 60 oC, 3600 s, ΔVth: 0.12 V) and NBIS (-20V, 15000 lx, 3600s, ΔVth: -1.33 V).[1] Hosono, H., & Kumomi,K., (ed), Amorphous Oxide Semiconductors: IGZO and Related Materials for Display and Memory (Wiley, 2022); Hosono, H. (2018). How we made the IGZO transistor. Nature Electronics, 1(7), 428-428.[2]Shiah, Y. S., Sim, K., Ueda, S., Kim, J., & Hosono, H. (2021). Unintended Carbon-Related Impurity and Negative Bias Instability in High-Mobility Oxide TFTs. IEEE Electron Device Letters, 42(9), 1319-1322.[3] Shiah, Y. S., Sim, K., Shi, Y., Abe, K., Ueda, S., Sasase, M., Kim,J., & Hosono, H. (2021). Mobility–stability trade-off in oxide thin-film transistors. Nature Electronics, 4(11), 800-807.

(Invited, Digital Presentation) Oxide TFTs Based on Semiconductors and Semimetals

A bottleneck issue of oxide TFTs is the instability under bias stresses and illuminations. Furthermore, the carrier mobility needs improvement for high pixel density and high brightness displays. Here, we use a high work-function Schottky source contact to enable depletion around the TFT source region. This results in a high intrinsic immunity to negative bias illumination stress, no obvious short channel effect, and extremely flat current saturation at low drain voltages. A voltage gain reaching 23,000 V/V is obtained due to the flat saturation current, which is the highest ever achieved with a solid-state transistor. Furthermore, the Schottky barrier depletion allows to utilize ITO to replace IGZO as the channel layer, which significantly enhances carrier mobility. Because ITO is used for both channel and drain electrode, the TFT appears to have no channel, enabling aggressive scaling for a significant increase in flat-panel display pixel aperture ratio, brightness and pixel density.

Investigation on Stability in Solution-Processed In-Zn-Sn-O TFT Array Under Various Intensity of Illumination

The effect of light illumination intensity on the stability of In-Zn-Sn-O (IZTO) thin-film transistors (TFTs) array fabricated by solution processed is investigated. Comparison with positive bias stress, the light illumination suppresses the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {th}}$ </tex-math></inline-formula> shift under positive bias illumination stress because of the increase in the free electrons induced by the ionized oxygen vacancies in the IZTO channel layer, but transfer characteristics show two-stage degradation under negative bias illumination stress (NBIS). The hump phenomenon is induced after NBIS, and the light illumination intensity is higher, the hump phenomenon becomes more serious. The hump phenomenon is relevant to ionized oxygen vacancies, which act as shallow donor-like states near the conduction-band minimum in IZTO.

The Effect of the Charge Transfer Transition of the Tetravalent Terbium on the Photostability of Oxide Thin‐Film Transistors

AbstractThe development of the next‐generation display technologies requires thin‐film transistors (TFTs) with high mobility and good negative‐bias‐illumination stress (NBIS) stability. Here, a tetravalent‐terbium‐doped indium oxide (Tb:In2O3) semiconductor is reported, which can effectively improve the NBIS stability of the TFT while ensuring high mobility. The TFT with Tb:In2O3 channel layer exhibited remarkable performance with a saturation mobility of 45.0 cm2 V–1 s–1 (with average mobility of 38.6 cm2 V–1 s–1), a turn‐on voltage (Von) of −1.1 V, and an on‐off current ratio of 108. In addition, the Tb:In2O3 TFT showed greatly improved NBIS stability with Von shift (ΔVon) of −3.9 V (with average ΔVon of 4.0 V) under 3600 s stress with −20 V gate voltage and white light illumination (compared to ΔVon of −11.7 V for the pure In2O3 TFT). Comprehensive studies reveal that the effective improvement of NBIS stability after Tb4+ doping is mainly attributed to the wide‐band absorption of the incident blue light by the Tb4f 7—O2p6 to Tb4f 8—O2p5 charge transfer (CT) transition that has smaller overall lattice expansion/contraction and shorter relaxation time compared to VO ionization.

Improvement in performance of indium gallium oxide thin film transistor via oxygen mediated crystallization at a low temperature of 200 °C

We report the fabrication of high-performance polycrystalline indium gallium oxide (IGO) thin film transistors (TFTs) at a low temperature of 200 °C. Growth of a highly aligned cubic phase with a bixbyite structure was accelerated at a certain proportion of oxygen plasma density during deposition of the IGO thin film, which leads to outstanding electrical characteristics. The resulting polycrystalline IGO TFT exhibited a high field-effect mobility of 56.0 cm2/V, a threshold voltage (VTH) of 0.10 V, a low subthreshold gate swing of 0.10 V/decade, and a current modulation ratio of >108. Moreover, the crystalline IGO TFTs have highly stable behaviors with a small VTH shift of +0.8 and −1.0 V against a positive bias stress (VGS,ST−VTH = 20 V) and negative bias illumination stress (VGS,ST−VTH = −20 V) for 3,600 s, which is attributed to the high quality of the bixbyite crystalline structure.

Excessive Oxygen Peroxide Model‐Based Analysis of Positive‐Bias‐Stress and Negative‐Bias‐Illumination‐Stress Instabilities in Self‐Aligned Top‐Gate Coplanar In–Ga–Zn–O Thin‐Film Transistors

AbstractAn excess oxygen‐peroxide‐based model that can simultaneously analyze the positive‐bias‐stress (PBS) and negative‐bias‐illumination‐stress (NBIS) instabilities in commercial self‐aligned top‐gate (SA‐TG) coplanar indium–gallium–zinc oxide (IGZO) thin‐film transistors (TFTs) is proposed herein. Existing studies have reported that the transition of oxygen vacancy (VO) charge states from VO0 to VO2+ is the dominant physical mechanism responsible for the negative shift of threshold voltage (VTH) under NBIS. However, in this study, it is observed that both the PBS and the NBIS stabilities of IGZO TFTs deteriorate at a faster rate as the amount of oxygen increases within the channel layer, implying that the conventional VO‐related defect model is inappropriate in elucidating the PBS and NBIS instabilities of commercial SA‐TG coplanar IGZO TFTs, where the channel layers are formed under high oxygen flow rates (OFRs) to make VTH positive. On the basis of the full‐energy range subgap density of states extracted before and after each stress from IGZO TFTs with different OFRs, it is determined that the generation and annihilation of the subgap states in the excess oxygen peroxide configuration are the dominant physical mechanisms for PBS and NBIS instabilities in commercial SA‐TG coplanar IGZO TFTs, respectively.

Sol-gel indium-zinc-tin-oxide thin film transistor pixel array with superior stabilityunder negative bias illumination stress

In this paper, we fabricate a back channel etched structure thin film transistor (TFT) pixel array with hafnium-aluminum oxide dielectric and indium-zinc-tin-oxide (IZTO) semiconductor using a solution process. The electrical characteristics of IZTO TFT are modified by N&lt;sub&gt;2&lt;/sub&gt;O plasma treatment. In comparison with the subthreshold swing and saturation mobility of the device untreated by plasma , the subthreshold swing decreases from 204 to 137 mV·dec&lt;sup&gt;–1&lt;/sup&gt;, and the saturation mobility increases from 29.12 to 51.52 cm&lt;sup&gt;2&lt;/sup&gt;·V&lt;sup&gt;–1&lt;/sup&gt;·s&lt;sup&gt;–1&lt;/sup&gt;. Improvement in the mobility and the subthreshold swing (SS) demonstrate that interface states may be passivated by reactive O radicals that are generated by N&lt;sub&gt;2&lt;/sub&gt;O plasma, which is confirmed by the result of X-ray photoelectron spectrum analysis. In addition, the stability of negative bias illumination stress (NBIS) shift is only 0.1V for 3600 s with an illumination intensity of 10000 lux. This result indicates that its superior stability meets the requirements for the display driver. Therefore, N&lt;sub&gt;2&lt;/sub&gt;O plasma treatment is verified to be an effective method to improve device performance and light stability for IZTO TFT pixel array.

Highly stable Ba-addition InZnSnO channels of light emitting transistors and thin film transistors

In this study, we investigate the characteristics of barium-addition indium-zinc-tin-oxide (B–IZTO) channels fabricated by the co-sputtering of barium-tin-oxide and indium-zinc-tin-oxide (IZTO) targets for the operation of thin film transistors (TFTs) and light emitting transistors (LETs). The X-ray photoelectron spectroscopy (XPS) analysis verifies that increasing the BaSnO3 sputtering power significantly decreases the concentration of oxygen vacancy from 35.0% to 16.3%. In addition, increasing the Ba concentration in the B–IZTO films increases the average transmittance of the B–IZTO channel in the visible light region (400–800 nm) from 86.7% to 88.3%, and the electrical resistivity from 84.3 to 5408 Ω∙cm, respectively. We find that the appropriate composition of Ba in the IZTO channels greatly improves the subthreshold swing (SS), the stability against the negative bias illumination stress (NBIS) of the B–IZTO channels-based TFTs. As the Ba content of the IZTO channel layer increases, SS decreases from 0.49 to 0.13 V/decade and the threshold voltage shift (ΔVTH) by NBIS reduces from − 13.1 to − 5.07 V, respectively. In the case of the B–IZTO-based LET with the highest Ba content, the device has the lowest light turn on voltage (VON) of 1.84 V, and shows the maximum brightness of 1.53 × 104 cd/m2. In this study, we confirm that the addition of Ba in the IZTO effectively suppresses the oxygen vacancies in the IZTO channel, resulting in the enhanced stability of the B–IZTO-based TFTs, and improves the performance of the B–IZTO-based LETs.

Steep Subthreshold Swing and Enhanced Illumination Stability InGaZnO Thin-Film Transistor by Plasma Oxidation on Silicon Nitride Gate Dielectric.

In this paper, an InGaZnO thin-film transistor (TFT) based on plasma oxidation of silicon nitride (SiNx) gate dielectric with small subthreshold swing (SS) and enhanced stability under negative bias illumination stress (NBIS) have been investigated in detail. The mechanism of the high-performance InGaZnO TFT with plasma-oxidized SiNx gate dielectric was also explored. The X-ray photoelectron spectroscopy (XPS) results confirmed that an oxygen-rich layer formed on the surface of the SiNx layer and the amount of oxygen vacancy near the interface between SiNx and InGaZnO layer was suppressed via pre-implanted oxygen on SiNx gate dielectric before deposition of the InGaZnO channel layer. Moreover, the conductance method was employed to directly extract the density of the interface trap (Dit) in InGaZnO TFT to verify the reduction in oxygen vacancy after plasma oxidation. The proposed InGaZnO TFT with plasma oxidation exhibited a field-effect mobility of 16.46 cm2/V·s, threshold voltage (Vth) of −0.10 V, Ion/Ioff over 108, SS of 97 mV/decade, and Vth shift of −0.37 V after NBIS. The plasma oxidation on SiNx gate dielectric provides a novel approach for suppressing the interface trap for high-performance InGaZnO TFT.

Photo-induced Reactive Oxygen Species Activation for Amorphous Indium-Gallium-Zinc Oxide Thin-Film Transistors Using Sodium Hypochlorite.

We proposed a novel material named sodium hypochlorite (NaClO) solution as a source of activation for amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistors (TFTs). We reduced the activation temperature from 300 to 150 °C using NaClO solution (concentration: 50%) and obtained satisfactory electrical characteristics of a-IGZO TFTs. The field-effect mobility, threshold voltage, on/off ratio, subthreshold swing, and threshold voltage (Vth) shift under negative bias illumination stress (VG = -20 V and VD = 10.1 V for 10,000 s) of NaClO (50%)-activated a-IGZO TFTs were 10.41 cm2/V·s, 1.51 V, 2.78 × 108, 0.37 V/dec, and -5.43 V, respectively. Also, the Vth shifts of the NaClO (50%)-activated a-IGZO TFTs (150 °C) under the positive bias stress test decreased from 5.01 to 1.87 V (VG = 20 V and VD = 10.1 V for 10,000 s) compared with that of only-annealed (300 °C) a-IGZO TFTs. Also, the mechanism of NaClO activation for a-IGZO TFTs is clarified through photo-assisted oxygen radical (POR) and heat-driven oxygen radical (HOR) effects. The POR and HOR effects generated the reactive oxygen species (ROS) from NaClO solution (50%), which activated a-IGZO TFTs at a low temperature (150 °C). When the NaClO solution (50%) was exposed to external energy, it generated ROS such as hydroxyl radicals (OH•), hydroperoxyl radicals (HO2•), and oxygen radicals (O•), which promoted the formation of strong metal-oxide bonds in a-IGZO TFTs. Furthermore, NaClO solution (50%) was applied to a-IGZO TFTs on a flexible polyimide substrate and electrohydrodynamic printing process for selective deposition.