Thin-film Transistor Devices Research Articles

P‐30: Thermally Activated and Field‐Enhanced Diffusion of Dopants in a‐InGaZnO TFTs Under Circuit Operations and Correlation to Device Stabilities

The spatial distribution of the donor dopant and the source‐drain resistance (RSD) in oxide Thin Film Transistor (TFT) for display backplane is usually considered to be determined by the device structure and the process. This work shows that the dopant concentration of lateral distribution of thermal equilibrium carrier concentration n0(y) may vary dynamically depending on the operation conditions during circuit operation and thermally‐activated and field‐enhanced diffusion of dopants in InGaZnO (IGZO) was found as a physical cause. In actual Organic Light‐Emitting Diode (OLED) backplane circuit operation, the reliability degradation conditions of IGZO TFTs were summarized into three conditions: Negative Bias Temperature Stress (NBTS), Constant Voltage Stress (CVS), and Drain‐to‐Source Voltage (VDS)‐sweep, and as a result of analyzing TFT device reliability, dopant diffusion occurred as the temperature increased, and the lateral field increased, resulting in RSD increase and channel resistance decrease. This causes degradation of device characteristics accumulated during circuit operation and shows various degradation patterns. It is connected to the existing Positive Bias Stress (PBS) and Negative Bias Stress (NBS) degradation mechanisms and other hot carrier degradation and self‐heating. In particular, it has been reported that dopant diffusion and source‐drain series resistance (RSD) change occur under high VGS, high VDS, and short channel conditions, resulting in permanent failure of the IGZO TFT channel by forming positive feedback.

Machine Learning–Assisted Thin-Film Transistor Characterization: A Case Study of Amorphous Indium Gallium Zinc Oxide (IGZO) Thin-Film Transistors

Machine learning was applied to classify the device characteristics of indium gallium zinc oxide (IGZO) thin-film transistors (TFTs). A K-means approach was employed for initial clustering of IGZO transfer curves into three of four grades (high, medium-high, medium, and low) of TFT performance according to qualitative features. A 2-layered artificial neural network (ANN) and 4-layered deep neural network (DNN) were used to extract mobility, threshold voltage, on/off current ratio, and sub-threshold slope device parameters from high-grade and medium-high-grade oxide TFTs. Ground-truth device parameters were calculated using in-house codes based on a rules-based approach consistent with the definitions employed to train the ANN and DNN. The DNN-predicted parameters were in closer agreement with manual and macro-based calculations than were those obtained from the ANN. Synergistic integration of K-means clustering and DNN effectively extracted TFT device parameters encountered in processing high volumes of data in industrial and academic domains of the microelectronics field.

Characteristics of ALD‐ZnO Thin Film Transistor Using H2O and H2O2 as Oxygen Sources

AbstractZnO thin films are deposited by atomic layer deposition (ALD) using diethylzinc as the Zn source and H2O and H2O2 as oxygen sources. The oxidant‐ and temperature‐dependent electrical properties and growth characteristics are systematically investigated. Materials analysis results suggest that H2O2 provides an oxygen‐rich environment so that the oxygen vacancies (VO) is suppressed, implying a lower carrier concentration and a higher resistivity. The lower growth rate makes it possible for the ZnO thin films to grow along the lower surface energy direction of <002>, leading to a lower Hall mobility. Furthermore, the ZnO semiconductor is integrated into thin film transistor (TFT) devices and the electrical properties are analyzed. The TFT with H2O2‐ZnO grown at 150 °C shows good electrical properties, such as a high field‐effect mobility of 10.7 cm2 V–1 s–1, a high ratio Ion/Ioff of 2 × 107, a sharp subthreshold swing of 0.25 V dec–1, and a low trapping state (Ntrap) of 2.77 × 1012 eV–1 cm–2, which provides a new pathway to optimize the performance of metal‐oxide electronics.

Powder X-ray diffraction as a powerful tool to exploit in organic electronics: shedding light on the first N,N',N''-trialkyldiindolocarbazole.

The first crystal structure of a fully N-alkylated diindolocarbazole derivative, namely, 5,8,14-tributyldiindolo[3,2-b;2',3'-h]carbazole (1, C36H39N3), has been determined from laboratory powder X-ray diffraction (PXRD) data. A complex trigonal structure with a high-volume unit cell of 12987 Å3 was found, with a very long a(=b) [52.8790 (14) Å] and a very short c [5.36308 (13) Å] unit-cell parameter (hexagonal setting). The detailed analysis of the intermolecular interactions observed in the crystal structure of 1 highlights its potential towards the implementation of this core as a semiconductor in organic thin-film transistor (OTFT) devices. Since the molecule has a flat configuration reflecting its π-conjugated system, neighbouring molecules are found to stack atop each other in a slipped parallel fashion via π-π stacking interactions between planes of ca 3.30 Å, with a centroid-centroid distance between the aromatic rings corresponding to the shortest axis of the unit cell (i.e. c). The alkylation of the three N atoms proves to be a decisive feature since it favours the presence of C-H...π interactions in all directions, which strengthens the crystal packing. As a whole, PXRD proves to be a valuable option for the resolution of otherwise inaccessible organic crystal structures of interest in different areas.

Electrical and Hysteresis Characteristics of Top-Gate InGaZnO Thin-Film Transistors with Oxygen Plasma Treatment Prior to TEOS Oxide Gate Dielectrics

We report the impact of oxygen (O2) plasma time on an amorphous indium–gallium–zinc oxide (a-IGZO) thin-film surface that was carried out before TEOS deposition in order to optimize the performance of thin-film transistors (TFTs). TheO2 plasma time of 60 s possessed the largest on/off current ratio of >108, with a field-effect mobility (µFE) of 8.14 cm2 V−1 s−1, and the lowest subthreshold swing (S.S.) of 0.395 V/decade, with a threshold voltage (Vth) of −0.14 V. However, increases in Ioff and S.S. and decreases in the µFE were observed for the longer O2 plasma time of 120 s. As the O2 plasma time increased, the reduction in the carrier concentration in the IGZO channel layer may have resulted in an increase in Vth for the IGZO TFT devices. With an increase in the O2 plasma time, the surface roughness of the IGZO channel layer was increased, the carbon content in the TEOS oxide film was reduced, and the film stoichiometry was improved. The SIMS depth profile results showed that the O/Si ratio of TEOS oxide for the sample with the O2 plasma time of 60 s was 2.64, and its IGZO TFT device had the best electrical characteristics. In addition, in comparison to the IGZO TFT device without O2 annealing, larger clockwise hysteresis in the transfer characteristics revealed that a greater number of electrons were trapped at the interface between TEOS oxide and the a-IGZO channel layer. However, hysteresis curves of the O2-annealed IGZO TFTs with various O2 plasma times were greatly reduced, meaning that the electron traps were reduced by O2 annealing.

Nonvolatile High-Speed Switching Zn-O-N Thin-Film Transistors with a Bilayer Structure.

Zinc oxynitride (ZnON) has the potential to overcome the performance and stability limitations of current amorphous oxide semiconductors because ZnON-based thin-film transistors (TFTs) have a high field-effect mobility of 50 cm2/Vs and exceptional stability under bias and light illumination. However, due to the weak zinc-nitrogen interaction, ZnON is chemically unstable─N is rapidly volatilized in air. As a result, recent research on ZnON TFTs has focused on improving air stability. We demonstrate through experimental and first-principles studies that the ZnF2/ZnON bilayer structure provides a facile way to achieve air stability with carrier controllability. This increase in air stability (e.g., nitrogen non-volatilization) occurs because the ZnF2 layer effectively protects the atomic mixing between ZnON and air, and the decrease in the ZnON carrier concentration is caused by a shallow-to-deep electronic transition of nitrogen deficiency diffused from ZnON into the interface. Further, the TFT based on the ZnF2/ZnON bilayer structure enables long-term air stability while retaining an optimal switching property of high field-effect mobility (∼100 cm2/Vs) even at a relatively low post-annealing temperature. The ZnF2/ZnON-bilayer TFT device exhibits fast switching behavior between 1 kHz and 0.1 MHz while maintaining a stable and clear switching response, paving the way for next-generation high-speed electronic applications.

Conjugated Metal-Organic Macrocycles: Synthesis, Characterization, and Electrical Conductivity.

The dimensional reduction of solids into smaller fragments provides a route to achieve new physical properties and gain deeper insight into the extended parent structures. Here, we report the synthesis of CuTOTP-OR (TOTPn- = 2,3,6,7-tetraoxidotriphenylene), a family of copper-based macrocycles that resemble truncated fragments of the conductive two-dimensional (2D) metal-organic framework Cu3(HHTP)2 (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene). The planar metal-organic macrocycles self-assemble into ordered nanotubes with internal diameters of ∼2 nm and short interlayer distances of ∼3.20 Å. Strong π-π stacking interactions between macrocycles facilitate out-of-plane charge transport, and pressed pellet conductivities as high as 2(1) × 10-3 S cm-1 are observed. Peripheral alkyl functionalization enhances solution processability and enables the fabrication of thin-film field-effect transistor devices. Ambipolar charge transport is observed, suggesting that similar behavior may be operative in Cu3(HHTP)2. By coupling the attractive features of metal-organic frameworks with greater processability, these macrocycles enable facile device integration and a more nuanced understanding of out-of-plane charge transport in 2D conductive metal-organic frameworks.

Low‐Temperature Atomic Layer Deposition of High‐k SbOx for Thin Film Transistors

AbstractSbOx thin films are deposited by atomic layer deposition (ALD) using SbCl5 and Sb(NMe2)3 as antimony reactants and H2O and H2O2 as oxidizers at low temperatures. SbCl5 can react with both oxidizers, while no deposition is found to occur using Sb(NMe2)3 and H2O. For the first time, the reaction mechanism and dielectric properties of ALD‐SbOx thin films are systematically studied, which exhibit a high breakdown field of ≈4 MV cm−1 and high areal capacitance ranging from 150 to 200 nF cm−2, corresponding to a dielectric constant ranging from 10 to 13. The ZnO semiconductor layer is integrated into a SbOx dielectric layer, and thin film transistors (TFTs) are successfully fabricated. A TFT with a SbOx dielectric layer deposited at 200 °C from Sb(NMe2)3 and H2O2 presents excellent performance, such as a field effect mobility (µ) of 12.4 cm2 V−1 s−1, Ion/Ioff ratio of 4 × 108, subthreshold swing of 0.22 V dec−1, and a trapping state (Ntrap) of 1.1 × 1012 eV−1 cm−2. The amorphous structure and high areal capacitance of SbOx boosts the interface between the semiconductor and dielectric layer of TFT devices and provide a strong electric field for electrons to improve the device mobility.

The effect of passivation-layer process to amorphous InGaZnO thin-film transistors using back-channel etch method

In this work, based on the channel damage caused by source/drain etching and passivation-layer deposition, the effects of the passivation-layer process on amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) devices were studied by combining experimental investigation with simulation verification. In terms of experimental exploration, it was found that the back-channel N2O plasma treatment had a significant impact on the performance of the device, which was difficult to control. Hence, to achieve a low cost, the entire back-channel process was directly carried out as two steps of SiO x passivation-layer deposition and final thermal annealing. In the aspect of simulation verification, the influence of the passivation-layer deposition radio-frequency (RF) power and the annealing effect on the internal mechanism of the device was studied based on a high-concentration doped defect density of states (DOS) model (doping level was N D = 1020 cm−3). The experimental results demonstrated that the high-performance of an a-IGZO TFT device can be achieved by adjusting the RF power of SiO x passivation-layer deposition. It was more important that annealing after passivation-layer deposition was a critical step in the manufacture of high-performance TFTs. The device exhibited the ideal performance after annealing under 1000 W RF power, with a threshold voltage of 5.65 V, a saturation mobility of 12.87 cm2 V−1s−1, a subthreshold swing of 0.88 V dec−1, and a current on-off ratio of 2.62 × 10°8. In addition, using the DOS model, it was found that the SiO x passivation-layer process had a significant impact on the DOS distribution and the carrier distribution in the channel, which in turn caused the threshold voltage to drift. At last, the high uniformity and stability of an a-IGZO TFTs array on glass were characterized.

Benchmarking contact quality in N-type organic thin film transistors through an improved virtual-source emission-diffusion model

Due to nonideal behavior, current organic thin film transistor technologies lack the proper models for essential characterization and thus suffer from a poorly estimated parameter extraction critical for circuit design and integration. Organic thin film transistors are often plagued by contact resistance, which is often less problematic in inorganic transistors; consequently, common models used for describing inorganic devices do not properly work with organic thin film transistors. In this work, we fabricate poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} based organic thin film transistors with reduced contact resistance through the introduction of metallic interlayers between the semiconductor and gold contacts. The addition of 10 nm thick manganese interlayer provides optimal organic thin film transistor device performance with the lowest level of contact resistance. Improved organic thin film transistors were characterized using an improved organic virtual-source emission diffusion model, which provides a simple and effective method to extract the critical device parameters. The organic virtual-source emission diffusion model led to nearly perfect prediction using effective gate voltages and a gate dependant contact resistance, providing a significant improvement over common metal–oxide–semiconductor field-effect transistor models such as the Shichman–Hodges model.

Migration of weakly bonded oxygen atoms in a-IGZO thin films and the positive shift of threshold voltage in TFTs

Amorphous indium-gallium-zinc oxide (a-IGZO) thin films are prepared by pulsed laser deposition and fabricated into thin-film transistor (TFT) devices. In-situ x-ray photoelectron spectroscopy (XPS) illustrates that weakly bonded oxygen (O) atoms exist in a-IGZO thin films deposited at high O2 pressures, but these can be eliminated by vacuum annealing. The threshold voltage (V th) of the a-IGZO TFTs is shifted under positive gate bias, and the V th shift is positively related to the deposition pressure. A temperature variation experiment in the range of 20 K–300 K demonstrates that an activation energy of 144 meV is required for the V th shift, which is close to the activation energy required for the migration of weakly bonded O atoms in a-IGZO thin films. Accordingly, the V th shift is attributed to the acceptor-like states induced by the accumulation of weakly bonded O atoms at the a-IGZO/SiO2 interface under positive gate bias. These results provide an insight into the mechanism responsible for the V th shift of the a-IGZO TFTs and help in the production of reliable designs.

Vacuum-Free Liquid-Metal-Printed 2D Indium-Tin Oxide Thin-Film Transistor for Oxide Inverters.

A cost-effective, vacuum-free, liquid-metal-printed two-dimensional (2D) (∼1.9 nm-thick) tin-doped indium oxide (ITO) thin-film transistor (TFT) was developed at the maximum process temperature of 200 °C. A large-sized 2D-ITO channel layer with an electron density of ∼1.2 × 1019 cm-3 was prepared in an ambient atmosphere. The 2D-ITO-TFT operated in full depletion with a threshold voltage of -2.1 V and demonstrated good TFT device characteristics such as a high saturation mobility of ∼27 cm2 V-1 s-1, a small subthreshold slope of <382 mV decade-1, and a large on/off-current ratio of >109. The TFT device simulation analysis found that the 2D-ITO-TFT performances were controlled by the shallow acceptor-like in-gap defects spreading in the midgap region of over 1.0 eV below the conduction band minimum. Post-thermal annealing tuned the electron density of the 2D-ITO channel and enabled it to produce enhancement and depletion-mode 2D-ITO-TFTs. A full signal swing zero-VGS-load n-type metal-oxide semiconductor (NMOS) inverter composed of depletion-load/enhancement-driver 2D-ITO-TFTs and a complementary inverter with p-channel 2D-SnO-TFT were successfully demonstrated using all 2D-oxide-TFTs.

Printed carbon nanotube thin film transistors based on perhydropolysilazane-derived dielectrics for low power flexible electronics

Electric double layer (EDL) electrolyte dielectrics have attracted great interest for low voltage and low power consumption portable thin film transistor (TFT) devices and circuits due to their high gate modulation efficiency. In this work, fully printed single-walled carbon nanotube (SWCNT) TFTs are fabricated using proton conducting inorganic perhydropolysilazane (PHPS) derivatives as the EDL dielectrics at low frequencies (<1 kHz) that can be processed in humidity even at quite low temperature (<100 °C) and relatively short reaction time (∼10 min). Fabricated fully printed SWCNT TFTs exhibit excellent electrical properties with low operating voltage (±1 V), high on/off ratio (∼106), small subthreshold swing (70–110 mV/dec), good stability and the average carrier mobility of 6.3 cm2 V−1 s−1. In addition, the precise control of the SWCNT TFTs’ threshold voltage and polarity (from depleted p-type to enhanced p and n-type, and well-balanced ambipolar) is successful demonstrated by adjusting the printable sc-SWCNT ink concentrations and electron doping technology (using ethanolamine as the electron doping agent). Furthermore, fully printed flexible complementary metal oxide semiconductor (CMOS) and CMOS-like inverters based on as-prepared SWCNT TFTs represent extremely low static power consumption (0.1 pW/μm and 0.05 pW/μm, respectively), high voltage gain (up to 45 at VDD = 0.7 V) and relatively large noise margins.

Solution processed low-voltage metal-oxide transistor by using TiO2/Li–Al2O3 stacked gate dielectric

A solution processed top-contact bottom-gated SnO2 thin-film transistor (TFT) has been fabricated using a TiO2/Li–Al2O3 bilayer stacked gate dielectric that show operating voltage of this TFT within 2.0 V. It is observed that the bilayer dielectric has much higher areal capacitance with lower leakage current density that significantly improve the overall device performance of TFT. The TFT with bilayer gate dielectric shows an effective carrier mobility (μsat) of 9.2 cm2 V−1 s−1 with an on/off ratio of 7.1 × 103 which are significantly higher with respect to the TFT with a single layer Li–Al2O3 gate dielectric. The origin of this improvement is due to the Schottky junction between the highly doped silicon (p++-Si) and TiO2 of bilayer stacked dielectric that induced electrons to the channel which reduces the dielectric/semiconductor interface trap-state. This investigation opens a new path to develop TFT device performance using a suitable bilayer stack of gate dielectric.

(La,Ba)SnO3 -based Thin-Film Transistors: Large-Signal Model and Scaling Projections

Perovskite oxides are extremely interesting for their possible use in high-mobility thin-film transistors (TFTs) suitable for high-performance large-area circuits. Here we present a semianalytical model of a recently fabricated TFT based on ${\mathrm{La}\text{\ensuremath{-}}\mathrm{Ba}\mathrm{Sn}\mathrm{O}}_{3}$, and explore the possibilities for technology optimization and the intrinsic potential of the device concept for applications in transparent and flexible electronics. We show that ${\mathrm{La}\text{\ensuremath{-}}\mathrm{Ba}\mathrm{Sn}\mathrm{O}}_{3}$ TFTs can outperform existing TFT device technologies through a detailed benchmarking exercise, and that these devices can be promising for display electronics and for a broader range of applications.

Trends in performance characteristics and modelling of oxide based TFT

This paper reviews Thin Film Transistors (TFTs) and its performance characteristics. Specific focus has been given on the performance characteristics of oxide based TFT because of the wide range of applications of such TFT’s. The oxide TFT devices are having excellent potential in the near future as active matrix displays. Due to fast expansion of electronics industry, modelling of TFT’s and ability to simulate the circuit behavior prior to production is important. The parameters such as grain boundary defect, mobility, on-off current ratio, thermal stability etc. are important factors need to consider and capture while modeling. This paper also finally discusses the importance of programming voltage, process temperature charge leakage and storage capacity of memory devices.

From stretchable and healable to self-healing semiconducting polymers: design and their TFT devices

This review concisely elaborates the advancement from stretchable toward autonomous self-healable semiconducting polymers owing to their long-term use and enhanced reliability, maintenance, and durability for modern thin-film transistor devices.

N-type polymer semiconductors incorporating para, meta, and ortho-carborane in the conjugated backbone

We report on three novel n-type conjugated polymer semiconductors incorporating carborane in the polymer backbone and demonstrate their applicability in optoelectronic devices. Comparing the optoelectronic properties of para-, meta-, and ortho-carborane isomers revealed similar energetic characteristics between the different polymers, with the carborane unit acting as a “conjugation breaker”, confining electron delocalisation to the conjugated moieties. The fabrication of all-polymer organic photovoltaic (OPV) devices and thin-film transistors (TFTs) revealed some differences in device performance between the polymers, with the meta-carborane based polymer exhibiting superior performance in both OPV and TFT devices.

Interface scattering dominated carrier transport in hysteresis-free amorphous InGaZnO thin film transistors with high-k HfAlO gate dielectrics by atom layer deposition

In this work, we systematically investigated the carrier transport of hysteresis-free amorphous InGaZnO (a-IGZO) thin film transistors (TFTs) incorporating high-k (HfO2) x (Al2O3) y gate dielectrics with different composition and permittivity by atomic layer deposition. A dielectric surface morphology dominated interface scattering carrier transport mechanism is demonstrated, and the effect of the dielectric polarization and the interface states on the carrier mobility is discovered in TFT devices gated by high quality dielectrics with negligible charge trap effect. Accordingly, an a-IGZO TFT gated by (HfO2)0.5(Al2O3)0.5 dielectric with the smoothest surface exhibits the best performance in terms of a preferable field-effect mobility of 18.35 cm2 V−1 s−1, a small subthreshold swing of 0.105 V decade−1, a high on/off current ratio of 4.6 × 106, and excellent stability under positive bias stress.

Advances in environmentally-friendly organic thermoelectric and organic thin-film transistor materials

<p indent=0mm>Organic semiconductors (OSCs) are significant systems of next-generation optoelectronic materials, which are widely applied in flexible devices, power generation and sensors. However, most of the organic semiconductors are prepared in chlorinated solvents which are harmful to human health and ecological environment. Nowadays, accompanied with the trends of environmentally-friendly optoelectronics, the OSCs processed with green solvents attracted extensive attention and developed rapidly. In this article, we reviewed the advances in environmentally-friendly OSCs in molecular design, solvent selection and processing method. The application of those materials in organic thin-film transistor and organic thermoelectric devices was highlighted. Finally, challenges and perspectives of the environmentally-friendly OSCs are also summarized.