bluuebunny/crossref_metadata_2025_split huggingface dataset:
(Invited) Highly Reliable Metal Oxide Thin Film Transistors for Flexible Devices
Authors: Uraoka Yukiharu , Corsino Dianne , Bermundo Juan Paolo , Fujii Mami N. , Uenuma Mutsunori 
Published: December 2020

Amorphous oxide semiconductors (AOS), such as amorphous InZnO (IZO), have been widely explored as channel material for thin-film transistors (TFTs) due to its high mobility (μ) and excellent uniformity.Its solution processability allows for application to cheap and flexible electronics. To achieve cheaper manufacturing cost and large-area production, successful integration of solution-based layers is necessary. In fact, other popular solution-based conductive films, such as carbon nanotubes and polymeric materials have been previously used as electrodes. However, their conductivity is generally lower and their integration requires complex processes. Meanwhile, AOS show promising optoeletronic properties as well as good contact with oxide channels. In this work, solution-based IZO semiconductor was functionalized for electrode applications using photo-assisted treatments, namely UV and excimer laser irradiation, and was employed in an all-solution oxide TFT fabrication.
IZO functionalization was achieved through either UV (λ=254 nm) irradiation for 15 to 60 mins or with additional excimer laser (λ=248 nm) irradiation in vacuum, air, or N2 at a fluence from 100 to 140 mJ cm-2. The results of X-ray photoelectron spectroscopy O 1s analysis suggests oxygen vacancy generation which effectively increases the carrier concentration in the irradiated IZO film. Moreover, IZO was crystallized from the amorphous as-deposited film after irradiation as obtained from X-ray diffraction and scanning transmission electron microscopy analyses. The laser-induced heating likely caused the IZO crystallization which provided the enhanced charge transport in the oxide film. Finally, the metal-oxide functionalization was employed in an all-solution processed oxide TFT fabrication using IZO for the channel and the electrodes, and fluorinated polysiloxane for the gate insulator. The self-aligned TFT structure that was employed, enabled the selective functionalization of the IZO gate, source, and drain electrodes. The metal oxide functionalization using UV and excimer laser irradiation yielded oxide TFTs with excellent transfer characteristics and good stability even after three months of storage in air. The all-solution processed fabrication demonstrated in this work is relevant for high throughput fabrication such as roll-to-roll processing.
(Invited) Heterojunction Channel Engineering for in-Ga-Zn-O Thin-Film Transistors
Authors: Furuta Mamoru , Koretomo Daichi , Magari Yusaku 
Published: December 2020

Heterojunction channel engineering approach is presented as a means of enhancing the performance and reliability of In–Ga–Zn–O thin-film transistors (IGZO TFTs). A bottom-gate IGZO TFT with the heterojunction channel was formed by depositing an In-rich IGZO on a typical composition of IGZO111. A potential well of 0.4 eV was expected to form for electrons at the heterojunction interface. Field effect mobilities (μFE) of the TFTs were 12 cm2/Vs and 23 cm2/Vs for homogeneous IGZO111 and high-In IGZO channels, respectively. In addition, threshold voltage stability (ΔVth) of the TFTs showed big a difference between homogeneous IGZO111 and high-In IGZO channels under positive bias and temperature stress (PBTS). The IGZO111 TFT exhibited good Vth stability (ΔVth of +0.4 V) after the PBTS of +20 V for 104 sec at 60 °C. In contrast, although the homogeneous high-In IGZO TFT exhibited a higher μFE than the IGZO111 TFT, the PBTS stability noticeably degraded (ΔVth of +7.0 V) from the IGZO111 TFT.
For the heterojunction channel TFTs, when the high-In IGZO layer was deposited on the IGZO layer, μFE increased from the IGZO111 TFT especially in low gate voltage (VGS) region. In addition, the PBTS stability of the heterojunction channel TFT was clearly improved from the high-In IGZO TFT by inserting IGZO111 layer at the interface between HI-IGZO and the GI. A maximum μFE of 24.7 cm2/Vs and improved PBTS stability (ΔVth of +0.3 V) were achieved by the heterojunction channel TFT with a high-In IGZO/IGZO111 thicknesses of 10/5 nm.
Device simulation results revealed that quantum confinement effect in the heterojunction channel played an important role in determining carrier transport properties in the TFT, result in improving μFE and PBTS stability. A channel engineering approach will provide a method of overcoming the trade-off between μFE and the PBTS stability of oxide TFTs.
(Invited) Amorphous Zinc Oxysulfide Thin Films: Synthesis, Physical Properties, and TFT Application
Authors: Hirose Yasushi , Chen Zhen , Hasegawa Tetsuya 
Published: December 2020

Amorphous oxide semiconductors (AOSs) have been under extensive research during recent years. Because of their high electron mobility even under amorphous state, they have been widely used as key materials for flat panel displays and flexible electronics devices, especially as channel layer of thin film transistor (TFT). Recently, it has been reported that an amorphous mixed-anion semiconductor consisting of only earth-abundant elements, zinc oxynitride (a-ZnON), shows high electron mobility and capability to TFT application [1]. However, a-ZnON is unstable in air due to weak chemical bonds between Zn and N [2]. To overcome this drawback, we synthesized another earth-abundant amorphous mixed-anion semiconductor, amorphous zinc oxysulfide (a-ZnOS) thin films and found that their electron mobility is comparable to those of conventional AOSs (~15 cm2V-1s-1), in addition to good chemical stability under ambient conditions [3]. Successively, a-ZnOS based TFTs were fabricated with bottom-gate and top contact configuration. a-ZnOS thin films with S/(S+O) = ~0.30 shows obvious TFT performance in the pristine states, although their field effect mobility (μ
FE) was moderate (μ
FE <1 cm2V−1s−1). μ
FE increased with the decrease of S/(S+O), but the carrier density became too large for TFT operation. By annealing these films in vacuum at 150 °C for 10 min, their TFT performance was significantly improved; on/off ration over 104 and μ
FE over 10 cm2V−1s−1.
[1] Kim et al., Sci. Rep. 3, 1459 (2013).
[2] Lee et al. Appl. Phys. Lett. 107, 122105 (2015)
[3] Y.Zhu et al., Adv. Electron. Mater. 6, 1900602 (2019).
High Mobility and Air-Stable Zinc Oxynitride Thin-Film Transistors Via ZnF2 Interlayer Insertion
Authors: Kim Hyoung-Do , Kim Hyun-Suk 
Published: December 2020

Increasing demands for high-performance thin-film transistors (TFT) in the display application have led to the development of amorphous oxide semiconductors (AOS) such as In-Ga-Zn-O (IGZO), In-Zn-O (IZO), etc. The AOS based TFTs show high field-effect mobility as high as 10 cm2/Vs, and are suitable for the fabrication of large size liquid crystal display (LCD) with high resolution. However, the displays based on organic light-emitting diodes (OLEDs) or micro light-emitting diodes (micro-LED) with very high resolution (super-high vision, 8000 X 4000) require relatively high driving currents compared to the LCD display. Therefore, the AOS based TFTs with higher field-effect mobility (> 50 cm2/Vs) are strongly required into the display backplane array.
Recently, zinc oxynitride (ZnON) have researched intensively as an active material for high-performance TFTs due to its high field-effect mobility (> 50 cm2/Vs), excellent stability under bias or light irradiation, and easily tunable bandgap energy. However, some studies reported that the nitrogen-related defects such as nitrogen-deficient site (VN) in ZnON act as carrier trap sites, so that degrade the electrical characteristics of ZnON TFTs. Moreover, the zinc nitride (Zn3N2) is unstable to air due to its low heat of formation, thus the Zn3N2 into ZnON is easily converted to zinc oxide. Therefore, reducing VNs and preventing the degradation of Zn3N2 is necessary for obtaining high-performance ZnON TFTs.
In this study, the effects of a thin ZnF2 interlayer between source/drain electrode and ZnON active layer are investigated. The properties of ZnON TFTs were evaluated by the different thicknesses of ZnF2 and the change of deposition conditions. The inserted ZnF2 interlayer acts as i) carrier modulation between source/drain and active layer due to high resistive ZnF2, and ii) passivation layer which prevents Zn3N2 volatilization. Furthermore, in our previous study demonstrated that the incorporated fluorine in ZnON acts as a structural stabilizer, so that reduces VN sites by increasing the effective coordination number of zinc. Moreover, incorporated fluorine which substitutes nitrogen also shown acceptor-like behavior rather than carrier donor. Thus, the VNs near the interface between ZnF2 and ZnON would be reduced, therefore the net density of VNs is decreased.
Experimental results showed that an optimized ZnON/ZnF2 TFTs showed high field-effect mobility (> 50 cm2/Vs) and good transfer characteristics (high on/off ratio and low subthreshold swing), whereas ZnON TFTs without ZnF2 interlayer showed completely conductive behavior. In addition, the air stability of ZnON TFTs was improved via the ZnF2 interlayer, the transfer characteristics were almost unchanged even a few months later. Further analysis such as bias and illumination stability and charge transport mechanisms were discussed in detail.
Effects of Tantalum Doping on Electrical Characteristics of Indium-Gallium-Zinc Oxide Thin-Film Transistors
Authors: Baek Seung-Ryum , Seong Tae-Yeon 
Published: December 2020

Thin-film transistors (TFTs) that utilize oxide semiconductors as channel materials have been widely used in various electronic device applications, such as displays, sensors, and memory devices. Among the oxide semiconductors, transparent amorphous indium-gallium-zinc oxides (a-IGZO) have superior characteristics, namely, high field-effect mobility (~10cm2/Vs), good uniformity, low processing temperatures, optical transparency, and low-cost fabrication processability compared with conventional amorphous/poly-silicon TFTs. However, oxide TFTs including a-IGZO TFTs have bias stress-induced instabilities, which are still critical issues in launching commercial oxide TFT products. These issues can be attributed to the oxygen vacancy-related defects, which are known to act as a major source of electrons and as carrier traps. To suppress such oxygen vacancy-related defects, many researchers reported various techniques, such as ozone treatment, oxygen annealing on oxide TFTs, passivation, and metal-doped oxide semiconductors.
In this study, we investigated the device performance of tantalum (Ta)-doped IGZO TFTs as a function of Ta content. Ta was used as a dopant because of its bond-dissociation-energy (BDE, 805kJ/mol) with oxygen higher than those of In, Ga, and Zn. The Ta-doped IGZO, which was adopted as a channel layer of bottom-gate TFTs, was fabricated using a magnetron co-sputtering method. Heavily doped p-type Si wafer with thermally grown SiO2 (300nm) was used as substrates. The RF power for the a-IGZO (In2O3 :Ga2O3 : ZnO = 1:1:1)target was fixed at 100 W while the power for the Ta target increased from 0 to 50 W. Aluminum (Al) was deposited as source/drain electrodes using an electron beam evaporator. With increasing Ta content in the IGZO films, the field-effect mobility (μFE) of the Ta-doped IGZO TFTs decreased, the threshold voltage (Vth) increased from negative to positive values, and the subthreshold swing (SS) became smaller with higher on/off ratio. Especially, Ta (10W)-doped IGZO TFTs exhibited μFE of 7.11 cm2/Vs, a SS of 0.195V/decade, and an on/off ratio ~1x108. These results show that the Ta-doping improves the switching characteristics of the Ta-IGZO TFTs. Furthermore, to evaluate the bias-stress stability, positive bias stress (PBS), negative bias stress (NBS) tests were conducted.
Photoconductive Solution Processed ZnO Quasi-superlattice Films
Authors: Buckley Darragh , Inguva Saikumar , McNulty David , Zubialevich Vitaly Z. , Parbrook Peter , Gity Farzan , Hurley P. K. , O'Dwyer Colm 
Published: December 2020

Zinc oxide (ZnO) and Al-doped ZnO are important optoelectronic materials. ZnO in particular has a wide band gap (Eg
~ 3.3 eV at 300 K), large exciton binding energy (~66 meV) and especially for the variety of methods by which it can be processed. Moreover, ZnO is readily able to alloy with other metals in the oxide form and has a lattice that can facilitate interstitial doping using several metals, including Al and Sn. This gives ZnO a key role in optoelectronics, metal oxide thin films and thin film transistor (TFT) technologies.
We demonstrate that crystalline, epitaxial-like and highly ordered ZnO and Al:ZnO (AZO) quasisuperlattice thin films can be achieved from a precursor liquid at relatively low temperature via spin-coating, which crystallize with near epitaxy with a pronounced c-plane texture. We also show the visible and near-infra red (VIS-NIR) light spectroscopy of ZnO and Al:ZnO multi-layered thin film structures grown on oxidized silicon substrates.
We show measurements demonstrating sub-band optical pumping of AZO QSLs that enable significant photoconductivity. In AZO QSLs, conduction band electrons from valence band pumping at incident irradiation > Eg, are not required to initiate or enhance photoconductivity, and PL data indicate a donor level that enable enhance sub-band photoconductivity even up to bias voltages of 200 V. Photoexcitation with a 532 nm source efficiently pumps the AZO conduction band without increasing the density of electron-holes in the valence band, minimizing field-driven recombination and enabling significant photoconductance over wide voltage ranges. Density functional theory analysis on ZnO systems with Al-doping configurations of ~2% indicate that interstitial doping is required to generate a density of states that corroborates the proposed donor level to enable the photoconductive properties seen in this work.

References

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(Invited) Percolation Carbon Nanotube Thin Film Transistors
Authors: Shur Michael , Park Junsung , Zhang Yuhui , Liu Xueqing , Ytterdal Trond 
Published: December 2020

The decrease of the gate voltage swing switching a field effect transistor from on to off state and, consequently, the decrease of the power supply voltage is the key challenge of the transistor design. Gate voltage dependent tunneling has been used to achieve this goal by decreasing the slope of the exponential current decay in the subthreshold regime below 60 mV per decade. We describe a new Carbon Nanotube (CNT) Percolating Thin Film Transistor (PTFT) that should allow achieving a steep subthreshold slope. CNT TFTs use mats of CNTs deposited on a substrate and forming a random percolating network. The conduction contacts between the individual CNTs is determined by tunneling and, therefore, could be strongly affected by external agents, such as gases or biological fluids. The asymmetry of the contacts to each CNT enables the rectification of electromagnetic radiation impacted on the CNT mat changing the drain-to-source conduction, with giant relative changes near the percolation point. Our experimental data show that sub-THz radiation shifts the percolation point at high intensity, when the rectified radiation changes the shape of the potential barrier between the CNT in a proximity contact. The CNT PTFT operates near the percolation point. In this regime, the gate bias shifts the percolation point by (a) depleting the CNTs in the subthreshold regime and (b) changing the shape of the tunneling barrier. This ensures the metal-insulator transition resulting in a sharp decrease of the subthreshold current slope. We present a compact model of the percolated CNT PTFT based on our Unified SPICE/ADS model that shows that this effect could reduce the subthreshold slope of the CNT TFs from 340 meV per decade to about 190 meV per decade.
Acknowledgments
This work at RPI was supported by the U.S. Army Research Laboratory under the Cooperative Research Agreement (Project Monitor Dr. Meredith Reed) and by the US ONR (Project Monitor Dr. Paul Maki).
Phase Control of MoTe2 Synthesized Via Laser Annealing Process: A Non-Transfer, Fast and Patternable Approach
Authors: Kuo Yao-Zen , Medina Henry , Chueh Yu-Lun , Chen Yu-Ze 
Published: December 2020

Layared materials such as graphite and transition metal dichalcogenides have attracted lots of attention owing to their unusal nature of reacing atomic thickness. These so-called two-dimensional materials reveal outstanding physical properties when their thickness reduce to mono layer, such as indirect to direct bandgap transition for TMDs, which is ideal for different kinds of potential applications in optoelectronics, energy harvesting, Li-ion batteries and supercapacitors. Among the numorous TMDs, MoTe2 is the only TMD which can be synthesized in two phases because of the small energy difference between them. However, the synthesis approach to control these two phases is not refined enough. Most reliable strategy is CVD process, but its require high temperature annealing for a long time while control the Te atomphere at the same time. These complicated process is hard to applied and high temperature also restrict the use of substrates. Therefore, further improvement of the synthesis methods is an important issue.
Here, we use a laser approach to induce the MoTe2 growth directly on different substrates such as Quartz and sapphire and so on. We control the two phases by tuning the laser power and irradiation time. TEM supported by Raman and X-ray Photoelectron Spectroscopies confirm the existence and quality of the synthesized films. Furtheremore, this approach can be extended to the synthesis of other TMDs. The laser annealing assisted method develops a new approach that is fast, cheap, patternable and does not require transfer giving a step forward on the technology development for practical applications.
GaN Single Crystalline Domain Growth on Graphene Substrate
Authors: Park Jonghyurk , Lee H. , Kim D. , Jang D. , Kim C , Yoon S 
Published: December 2020

Graphene is very well known 2D nano-material which have novel physical and chemical properties even in a few layer thick form. Recently, there have been extensive studies on hetero-structures based on these Van-der-Waals 2D crystals like graphene, transition metal dichalcogenide (TMD), boron nitride (BN) and so on. In the case of graphene, this 2D layer can be stable and can be transferred to other target substrate with ease. Therefore, graphene can be a good candidate materials for growing other active materials on it. In this study, we tried to grow GaN micro-domain on graphene substrate using hydride vapor phase epitaxy (HVPE) method. GaN epitaxial behavior can be affected by the condition of graphene substrate and we compared the growth behavior of GaN micro-domain on graphene changing the preparation condition. And, several experimental parameters could be controlled like flow rate of Ga and N source with carrier gas, temperature and time to get the proper morphology of GaN micro-crystals. Despite of lattice mis-match of hexagonal GaN on graphene, we could grow single crystalline GaN micro-domain having the lateral size of a few or lager microns. We characterized these micro-crystalline GaN domains using optical microscopy, secondary electron microscope (SEM), micro-Raman spectroscopy and x-ray diffraction (XRD) which confirmed the crystalline phase of each domain. As a typical Van-der-Waals crystal, pristine non-defective graphene may have lacking dangling bonds leading to very low nucleation density for GaN growth and can be used even as a mask layer. When grown on the transferred graphene, however, we expect to have enough nucleation sites originating from the physical defects like grain boundary, vacancy and ripples of poly-crystalline graphene layer. This experimental finding is very interesting to further studies of graphene applications for hetero-structures of graphene and other complementary active materials because graphene has disadvantage in the applications of switching devices, optical elements and can be a potential candidate to overcome these weak points by utilizing very efficiently active GaN together.
[1] Y. Kim et al., Nature, 544, p.340 (2017)
[2] V. Kumaresan et al,, Nano Letters, 16, p.4895 (2016)
(Invited) Ion-Conductive Metal-Organic Frameworks
Authors: Sadakiyo Masaaki 
Published: December 2020

Proton conductivity in solids is of importance not only for useful applications such as fuel cells but also for better understanding of the nature of proton-transport phenomena. Recently, metal–organic frameworks (MOFs) have been emerged as a novel class of proton-materials because the ability to rationally design and chemically tune their architecture allows chemists to establish various methods to control the proton conductivity. We have proposed three types (types I–III) of concepts to give high proton conductivity to MOFs. In type I, proton carriers are introduced as counterions into the pores (e.g., H3O+). In type II, acidic groups are placed on the framework (e.g., –COOH). In type III, charge-neutral proton-conducting species are incorporated within the pores (e.g., H3PO4).1 We have synthesized (NH4)2(adp)[Zn2(ox)3]·3H2O (adp = adipic acid; ox = oxalate), having both type I and type III features; (NH4)2(adp)[Zn2(ox)3]·3H2O shows high proton conductivity because of the existence of two-dimensional (2-D) hydrogen-bonding networks which act as a crystalline proton-conducting pathways.2 Intentional control of proton conductivity and the proton dynamics in this MOF has been also studied.
On another front, hydroxide-ion conductors have received a great deal of interest as electrolytes for alkaline fuel cells that can operate without precious metal catalysts such as Pt. We have also focused on creating novel hydroxide ion conductors using the MOFs. We proposed a new strategy to introduce hydroxide ions into MOFs, i.e., which is “hydroxide salt inclusion into alkaline-stable MOFs”. We succeeded in synthesizing a novel MOF including alkylammonium hydroxide salt by employing an alkaline-stable ZIF-85 framework (NBu4-ZIF-8-OH) (Figure 1).6
,7 Ionic conductivity of NBu4-ZIF-8-OH was found to be 2.3 × 10–8 S cm–1 at maximum, which is four orders of magnitude higher than that of blank ZIF-8. Considering that the framework structure of NBu4-ZIF-8-OH is the same as that of ZIF-8, the enhancement in the ionic conductivity can be attributed to the included salt.
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Figure 1