Impact of Layer Configuration and Doping on Electron Transport and Bias Stability in Heterojunction and Superlattice Metal Oxide Transistors

The astonishing recent progress in the field of metal oxide thin‐film transistors (TFTs) and their debut in commercial displays is accomplished using vacuum‐processed multicomponent oxide semiconductors. However, emulating this success with their solution‐processable counterparts poses numerous scientific challenges. Here, the development of high mobility n‐channel TFTs based on ultrathin (<10 nm) alternating layers of In₂O₃ and ZnO that are sequentially deposited to form heterojunction and superlattice channels is reported. The resulting TFTs exhibit high electron saturation mobility (13 cm² V^?1 s^?1), excellent current on/off ratios (>10⁸) with nearly zero onset voltages and hysteresis‐free operation despite the low temperature processing (≤200 °C). The enhanced performance is attributed to the formation of a quasi‐2D electron gas‐like system at the In₂O₃/ZnO heterointerface due to the conduction band offset. It is shown that altering the oxide deposition sequence has an adverse effect on electron transport due to formation of trap states. Optimized multilayer TFTs are shown to exhibit improved bias‐stress stability compared to single‐layer TFTs. Modulating the electron concentration within the superlattice channel via selective n‐doping of the ZnO interlayers leads to almost 100% saturation mobility increase (≈25 cm² V^?1 s^?1) even when the TFTs are fabricated on flexible plastic substrates.     

Metal Oxide Transistors via Polyethylenimine Doping of the Channel Layer: Interplay of Doping,Microstructure, and Charge Transport

Polymer doping of solution‐processed In₂O₃ with small amounts of the electron‐rich polymer, polyethylenimine (PEI), affords superior transistor performance, including higher electron mobility than that of the pristine In₂O₃ matrix. PEI doping of In₂O₃ films not only frustrates crystallization and controls the carrier concentration but, more importantly, acts as electron dopant and/or scattering center depending on the polymer doping concentration. The electron donating capacity of PEI combined with charge trapping and variation in the matrix film microstructure yields, for optimum PEI doping concentrations of 1.0%–1.5%, electron mobilities as high as ≈9 cm² V^?1 s^?1 on a 300 nm SiO₂ gate dielectric, an excellent on/off ratio of ≈10⁷, and an application optimal V _T. Importantly, these metrics exceed those of the pure In₂O₃ matrix with a maximum mobility ≈4 cm² V^?1 s^?1. Furthermore, we show that this approach is extendible to other oxide compositions such as IZO and the technologically relevant IGZO. This work opens a new means to fabricate amorphous semiconductors via solution processing at low temperatures, while preserving or enhancing the mobility of the pristine polycrystalline semiconductor.     

Doping Indium Oxide Films with Amino-Polymers of Varying Nitrogen Content Markedly Affects Charge Transport and Mechanical Flexibility

Here, correlations between polymer structure and charge transport in solution-processed indium oxide, In₂O₃:polymer blend flexible thin film transistors (TFTs) are investigated using four polymers having electron-donating amine functionalities (polyethyleneimine (PEI), poly(allylamine), polyethyleneimine ethoxylated (PEIE), and PVP-NH₂ (PVP; poly(4-vinylphenol)), and two PEI-PEIE mixtures) with varied atomic amine nitrogen content (N%) of 12.6, 9.1, 6.9, 2.6, respectively. These amino-polymers influence the semiconducting oxide film TFT electron mobilities via a delicate interplay of electron transfer/doping, charge generation/trap-filling, film morphological/microstructural variations, which depend on the polymer structure, thermal stability, and N%, as well as the polymer content of the In₂O₃ precursor and the carbon residue content in In₂O₃. Thus, increasing the N% from 0.0% in the control PVP to 12.6% in PEI increases the electron doping capacity, the polymer content of the blend formulation, and the blend TFT field-effect mobility. Optimal polymer incorporation invariably enhances charge transport by as much as ≈2×, leading to a maximum carrier mobility of 8.47 ± 0.73 cm² V⁻¹ s⁻¹ on rigid Si/SiO_x substrates and a remarkable 31.24 ± 0.41 cm² V⁻¹ s⁻¹ on mechanically flexible polyimide/Au/F:AlO_x substrates with Al contacts. Furthermore, all of the polymers equally enhance the mechanical durability of the corresponding In₂O₃:polymer blend TFTs with respect to mechanical stress.     

Water‐Induced Scandium Oxide Dielectric for Low‐Operating Voltage n‐ and p‐Type Metal‐Oxide Thin‐Film Transistors

Solution‐processed metal‐oxide thin films based on high dielectric constant (k) materials have been extensively studied for use in low‐cost and high‐performance thin‐film transistors (TFTs). Here, scandium oxide (ScO_x) is fabricated as a TFT dielectric with excellent electrical properties using a novel water‐inducement method. The thin films are annealed at various temperatures and characterized by using X‐ray diffraction, atomic‐force microscopy, X‐ray photoelectron spectroscopy, optical spectroscopy, and a series of electrical measurements. The optimized ScO_x thin film exhibits a low‐leakage current density of 0.2 nA cm^?2 at 2 MV cm^?1, a large areal capacitance of 460 nF cm^?2 at 20 Hz and a permittivity of 12.1. To verify the possible applications of ScO_x thin films as the gate dielectric in complementary metal oxide semiconductor (CMOS) electronics, they were integrated in both n‐type InZnO (IZO) and p‐type CuO TFTs for testing. The water‐induced full oxide IZO/ScO_x TFTs exhibit an excellent performance, including a high electron mobility of 27.7 cm² V^?1 s^?1, a large current ratio (I_on/I_off) of 2.7 × 10⁷ and high stability. Moreover, as far as we know it is the first time that solution‐processed p‐type oxide TFTs based on a high‐k dielectric are achieved. The as‐fabricated p‐type CuO/ScO_x TFTs exhibit a large I_on/I_off of around 10⁵ and a hole mobility of 0.8 cm² V^?1 at an operating voltage of 3 V. To the best of our knowledge, these electrical parameters are among the highest performances for solution‐processed p‐type TFTs, which represents a great step towards the achievement of low‐cost, all‐oxide, and low‐power consumption CMOS logics.     

Low‐Temperature,Nontoxic Water‐Induced Metal‐Oxide Thin Films and Their Application in Thin‐Film Transistors

Here, a simple, nontoxic, and inexpensive “water‐inducement” technique for the fabrication of oxide thin films at low annealing temperatures is reported. For water‐induced (WI) precursor solution, the solvent is composed of water without additional organic additives and catalysts. The thermogravimetric analysis indicates that the annealing temperature can be lowered by prolonging the annealing time. A systematic study is carried out to reveal the annealing condition dependence on the performance of the thin‐film transistors (TFTs). The WI indium‐zinc oxide (IZO) TFT integrated on SiO₂ dielectric, annealed at 300 °C for 2 h, exhibits a saturation mobility of 3.35 cm² V^?1 s^?1 and an on‐to‐off current ratio of ≈10⁸. Interestingly, through prolonging the annealing time to 4 h, the electrical parameters of IZO TFTs annealed at 230 °C are comparable with the TFTs annealed at 300 °C. Finally, fully WI IZO TFT based on YO_x dielectric is integrated and investigated. This TFT device can be regarded as “green electronics” in a true sense, because no organic‐related additives are used during the whole device fabrication process. The as‐fabricated IZO/YO_x TFT exhibits excellent electron transport characteristics with low operating voltage (≈1.5 V), small subthreshold swing voltage of 65 mV dec^?1 and the mobility in excess of 25 cm² V^?1 s^?1.     

Fully Patterned Low‐Voltage Transparent Metal Oxide Transistors Deposited Solely by Chemical Spray Pyrolysis

All‐inorganic transparent thin‐film transistors deposited solely by the solution processing method of spray pyrolysis are reported. Different precursor materials are employed to create conducting and semiconducting species of ZnO acting as electrodes and active channel material, respectively, as well as zirconium oxide as gate dielectric layer. Additionally, a simple stencil mask system provides sufficient resolution to realize the necessary geometric patterns. As a result, fully functional low‐voltage n‐type transistors with a mobility of 0.18 cm² V^?1 s^?1 can be demonstrated via a technique that bears the potential for upscaling. A detailed microscopic evaluation of the channel region by electron diffraction, high‐resolution and analytical TEM confirms the layer stacking and provides detailed information on the chemical composition and nanocrystalline nature of the individual layers.     

Effects of Interfacial Dielectric Layers on the Electrical Performance of Top‐Gate In‐Ga‐Zn‐Oxide Thin‐Film Transistors

We investigate the effects of interfacial dielectric layers (IDLs) on the electrical properties of top‐gate In‐Ga‐Zn‐oxide (IGZO) thin film transistors (TFTs) fabricated at low temperatures below 200°C, using a target composition of In:Ga:Zn = 2:1:2 (atomic ratio). Using four types of TFT structures combined with such dielectric materials as Si₃N₄ and Al₂O₃, the electrical properties are analyzed. After post‐annealing at 200°C for 1 hour in an O₂ ambient, the sub‐threshold swing is improved in all TFT types, which indicates a reduction of the interfacial trap sites. During negative‐bias stress tests on TFTs with a Si3N4 IDL, the degradation sources are closely related to unstable bond states, such as Si‐based broken bonds and hydrogen‐based bonds. From constant‐current stress tests of Id = 3 µA, an IGZO‐TFT with heat‐treated Si₃N₄ IDL shows a good stability performance, which is attributed to the compensation effect of the original charge‐injection and electron‐trapping behavior.     

High‐Mobility ZnO Thin Film Transistors Based on Solution‐processed Hafnium Oxide Gate Dielectrics

The properties of metal oxides with high dielectric constant (k) are being extensively studied for use as gate dielectric alternatives to silicon dioxide (SiO₂). Despite their attractive properties, these high‐k dielectrics are usually manufactured using costly vacuum‐based techniques. In that respect, recent research has been focused on the development of alternative deposition methods based on solution‐processable metal oxides. Here, the application of the spray pyrolysis (SP) technique for processing high‐quality hafnium oxide (HfO₂) gate dielectrics and their implementation in thin film transistors employing spray‐coated zinc oxide (ZnO) semiconducting channels are reported. The films are studied by means of admittance spectroscopy, atomic force microscopy, X‐ray diffraction, UV–Visible absorption spectroscopy, FTIR, spectroscopic ellipsometry, and field‐effect measurements. Analyses reveal polycrystalline HfO₂ layers of monoclinic structure that exhibit wide band gap (≈5.7 eV), low roughness (≈0.8 nm), high dielectric constant (k ≈ 18.8), and high breakdown voltage (≈2.7 MV/cm). Thin film transistors based on HfO₂/ZnO stacks exhibit excellent electron transport characteristics with low operating voltages (≈6 V), high on/off current modulation ratio (～10⁷) and electron mobility in excess of 40 cm² V^?1 s^?1.     

Properties of In2O3 films obtained by thermal oxidation of sprayed In2S3

In₂S₃ thin films were grown by the chemical spray pyrolysis (CSP) method using indium chloride and thiourea as precursors at a molar ratio of S:In=2.5. The deposition was carried out at 350 °C on quartz substrates. The film thickness is about 1 µm. The films were then annealed for 2 h at 550, 600, 650 and 700 °C in oxygen flow. This process allows the transformation of nanocrystal In₂O₃ from In₂S₃ and the reaction is complete at 600 °C. X-ray diffraction spectra show that In₂O₃ films are polycrystalline with a cubic phase and preferentially oriented towards (222). The film grain size increases from 19 to 25 nm and RMS values increase from 9 to 30 nm. In₂O₃ films exhibit transparency over 70–85% in the visible and infrared regions due to the thickness and crystalline properties of the films. The optical band gap is found to vary in the range 3.87–3.95 eV for direct transitions. Hall effect measurements at room temperature show that resistivity is decreased from 117 to 27 Ω cm. A carrier concentration of 1×10¹⁶ cm^?3 and mobility of about 117 cm² V^?1 s^?1 are obtained at 700 °C.     

Energy Quantization in Solution‐Processed Layers of Indium Oxide and Their Application in Resonant Tunneling Diodes

The formation of quantized energy states in ultrathin layers of indium oxide (In₂O₃) grown via spin coating and thermally annealed at 200 °C in air is studied. Optical absorption measurements reveal a characteristic widening of the optical band gap with reducing In₂O₃ layer thickness from ≈43 to ≈3 nm in agreement with theoretical predictions for an infinite quantum well. Through sequential deposition of In₂O₃ and gallium oxide (Ga­₂O₃) layers, superlattice‐like structures with controlled dimensionality and spatially varying conduction band characteristics are demonstrated. This simple method is then explored for the fabrication of functional double‐barrier resonant tunneling diodes. Nanoscale current mapping analysis using conductive atomic force microscopy reveals that resonant tunneling is not uniform but localized in specific regions of the apparent device area. The latter observation is attributed to variation in the layer(s) thickness of the In₂O₃ quantum well and/or the Ga₂O₃ barrier layers. Despite the nonidealities, the tremendous potential of solution‐processable oxide semiconductors for the development of quantum effect devices that have so far been demonstrated only via sophisticated growth techniques is demonstrated.     

Core‐Brominated Tetraazaperopyrenes as n‐Channel Semiconductors for Organic Complementary Circuits on Flexible Substrates

Organic thin‐film transistors (TFTs) are prepared by vacuum deposition and solution shearing of 2,9‐bis(perfluoroalkyl)‐substituted tetraazaperopyrenes (TAPPs) with bromine substituents at the aromatic core. The TAPP derivatives are synthesized by reacting known unsubstituted TAPPs with bromine in fuming sulphuric acid, and their electrochemical properties are studied in detail by cyclic voltammetry and modelled with density functional theory (DFT) methods. Lowest unoccupied molecular orbital (LUMO) energies and electron affinities indicate that the core‐brominated TAPPs should exhibit n‐channel semiconducting properties. Current‐voltage characteristics of the TFTs established electron mobilities of up to μ_n = 0.032 cm² V^?1 s^?1 for a derivative which was subsequently processed in the fabrication of a complementary ring oscillator on a flexible plastic substrate (PEN).     

A Review of Low‐Temperature Solution‐Processed Metal Oxide Thin‐Film Transistors for Flexible Electronics

Solution processing, including printing technology, is a promising technique for oxide thin‐film transistor (TFTs) fabrication because it tends to be a cost‐effective process with high composition controllability and high throughput. However, solution‐processed oxide TFTs are limited by low‐performance and stability issues, which require high‐temperature annealing. This high thermal budget in the fabrication process inhibits oxide TFTs from being applied to flexible electronics. There have been numerous attempts to promote the desired electrical characteristics of solution‐processed oxide TFTs at lower fabrication temperatures. Recent techniques for achieving low‐temperature (<350 °C) solution‐processed and printed oxide TFTs, in terms of the materials, processes, and structural engineering methods currently in use are reviewed. Moreover, the core techniques for both n‐type and p‐type oxide‐based channel layers, gate dielectric layers, and electrode layers in oxide TFTs are addressed. Finally, various multifunctional and emerging applications based on low‐temperature solution‐processed oxide TFTs are introduced and future outlooks for this highly promising research are suggested.     

An Easily Sintered,Chemically Stable,Barium Zirconate‐Based Proton Conductor for High‐Performance Proton‐Conducting Solid Oxide Fuel Cells

Yttrium and indium co‐doped barium zirconate is investigated to develop a chemically stable and sintering active proton conductor for solid oxide fuel cells (SOFCs). BaZr_0.8Y_0.2‐xIn_xO_{3‐ δ} possesses a pure cubic perovskite structure. The sintering activity of BaZr_0.8Y_0.2‐xIn_xO_{3‐ δ} increases significantly with In concentration. BaZr_0.8Y_0.15In_0.05O_{3‐ δ} (BZYI5) exhibits the highest total electrical conductivity among the sintered oxides. BZYI5 also retains high chemical stability against CO₂, vapor, and reduction of H₂. The good sintering activity, high conductivity, and chemical stability of BZYI5 facilitate the fabrication of durable SOFCs based on a highly conductive BZYI5 electrolyte film by cost‐effective ceramic processes. Fully dense BZYI5 electrolyte film is successfully prepared on the anode substrate by a facile drop‐coating technique followed by co‐firing at 1400 °C for 5 h in air. The BZYI5 film exhibits one of the highest conductivity among the BaZrO₃‐based electrolyte films with various sintering aids. BZYI5‐based single cells output very encouraging and by far the highest peak power density for BaZrO₃‐based proton‐conducting SOFCs, reaching as high as 379 mW cm^?2 at 700 °C. The results demonstrate that Y and In co‐doping is an effective strategy for exploring sintering active and chemically stable BaZrO₃‐based proton conductors for high performance proton‐conducting SOFCs.     

Thermally deposited Ag-doped CdS thin film transistors with high-k rare-earth oxide Nd2O3 as gate dielectric

The performance of thermally deposited CdS thin film transistors doped with Ag has been reported. Ag-doped CdS thin films have been prepared using chemical method. High dielectric constant rare earth oxide Nd₂O₃ has been used as gate insulator. The thin film trasistors are fabricated in coplanar electrode structure on ultrasonically cleaned glass substrates with a channel length of 50 μm. The thin film transistors exhibit a high mobility of 4.3 cm² V^?1 s^?1 and low threshold voltage of 1 V. The ON-OFF ratio of the thin film transistors is found as 10⁵. The TFTs also exhibit good transconductance and gain band-width product of 1.15 × 10^?3 mho and 71 kHz respectively.     

Zinc Oxide Thin-Film Transistors Fabricated at Low Temperature by Chemical Spray Pyrolysis

We report the electrical behavior of undoped zinc oxide thin-film transistors (TFTs) fabricated by low-temperature chemical spray pyrolysis. An aerosol system utilizing aerodynamic focusing was used to deposit the ZnO. Polycrystalline films were subsequently formed by annealing at the relatively low temperature of 140°C. The saturation mobility of the TFTs was 2 cm²/Vs, which is the highest reported for undoped ZnO TFTs manufactured below 150°C. The devices also had an on/off ratio of 10⁴ and a threshold voltage of ?3.5 V. These values were found to depend reversibly on measurement conditions.     

Skin‐Like Oxide Thin‐Film Transistors for Transparent Displays

Flexible transparent display is a promising candidate to visually communicate with each other in the future Internet of Things era. The flexible oxide thin‐film transistors (TFTs) have attracted attention as a component for transparent display by its high performance and high transparency. The critical issue of flexible oxide TFTs for practical display applications, however, is the realization on transparent and flexible substrate without any damage and characteristic degradation. Here, the ultrathin, flexible, and transparent oxide TFTs for skin‐like displays are demonstrated on an ultrathin flexible substrate using an inorganic‐based laser liftoff process. In this way, skin‐like ultrathin oxide TFTs are conformally attached onto various fabrics and human skin surface without any structural damage. Ultrathin flexible transparent oxide TFTs show high optical transparency of 83% and mobility of 40 cm² V^?1 s^?1. The skin‐like oxide TFTs show reliable performance under the electrical/optical stress tests and mechanical bending tests due to advanced device materials and systematic mechanical designs. Moreover, skin‐like oxide logic inverter circuits composed of n‐channel metal oxide semiconductor TFTs on ultrathin, transparent polyethylene terephthalate film have been realized.     

Structural and Electrical Characterization of ZnO Films Grown by Spray Pyrolysis and Their Application in Thin‐Film Transistors

The role of the substrate temperature on the structural, optical, and electronic properties of ZnO thin films deposited by spray pyrolysis using a zinc acetate precursor solution is reported. Analysis of the precursor compound using thermogravimentry and differential scanning calorimetry indicates complete decomposition of the precursor at around 350 °C. Film characterization using Fourier Transform Infrared Spectroscopy (FTIR), photoluminescence spectroscopy (PL), and ultraviolet–visible (UV–Vis) optical transmission spectroscopy suggests the onset of ZnO growth at temperatures as low as 100 °C as well as the transformation to a polycrystalline phase at deposition temperatures >200 °C. Atomic force microscopy (AFM) and X‐ray diffraction (XRD) reveal that as‐deposited films exhibit low surface roughness (rms ≈ 2.9 nm at 500 °C) and a crystal size that is monotonously increasing from 8 to 32 nm for deposition temperatures in the range of 200–500 °C. The latter appears to have a direct impact on the field‐effect electron mobility, which is found to increase with increasing ZnO crystal size. The maximum mobility and current on/off ratio is obtained from thin‐film transistors fabricated using ZnO films deposited at >400 °C yielding values on the order of 25 cm² V^?1s^?1 and 10⁶, respectively.     

Low‐Temperature‐Processed Printed Metal Oxide Transistors Based on Pure Aqueous Inks

Additive patterning of transparent conducting metal oxides at low temperatures is a critical step in realizing low‐cost transparent electronics for display technology and photovoltaics. In this work, inkjet‐printed metal oxide transistors based on pure aqueous chemistries are presented. These inks readily convert to functional thin films at lower processing temperatures (T ≤ 250 °C) relative to organic solvent‐based oxide inks, facilitating the fabrication of high‐performance transistors with both inkjet‐printed transparent electrodes of aluminum‐doped cadmium oxide (ACO) and semiconductor (InO_x ). The intrinsic fluid properties of these water‐based solutions enable the printing of fine features with coffee‐ring free line profiles and smoother line edges than those formed from organic solvent‐based inks. The influence of low‐temperature annealing on the optical, electrical, and crystallographic properties of the ACO electrodes is investigated, as well as the role of aluminum doping in improving these properties. Finally, the all‐aqueous‐printed thin film transistors (TFTs) with inkjet‐patterned semiconductor (InO_x ) and source/drain (ACO) layers are characterized, which show ideal low contact resistance (R _c < 160 Ω cm) and competitive transistor performance (µ _lin up to 19 cm² V^?1 s^?1, Subthreshold Slope (SS) ≤150 mV dec^?1) with only low‐temperature processing (T ≤ 250 °C).     

Influence of annealing on the structural,optical and electrical properties of indium oxide films deposited on c-sapphire substrate

Indium oxide (In₂O₃) films were prepared on Al₂O₃ (0001) substrates at 700 °C by metal-organic chemical vapor deposition (MOCVD). Then the samples were annealed at 800 °C, 900 °C and 1 000 °C, respectively. The X-ray diffraction (XRD) analysis reveals that the samples were polycrystalline films before and after annealing treatment. Triangle or quadrangle grains can be observed, and the corner angle of the grains becomes smooth after annealing. The highest Hall mobility is obtained for the sample annealed at 900 °C with the value about 24.74 cm²·V^-1·s^-1. The average transmittance for the films in the visible range is over 90%. The optical band gaps of the samples are about 3.73 eV, 3.71 eV, 3.70 eV and 3.69 eV corresponding to the In₂O₃ films deposited at 700 °C and annealed at 800 °C, 900 °C and 1 000 °C, respectively.     

3D MoS2–Graphene Microspheres Consisting of Multiple Nanospheres with Superior Sodium Ion Storage Properties

A novel anode material for sodium‐ion batteries consisting of 3D graphene microspheres divided into several tens of uniform nanospheres coated with few‐layered MoS₂ by a one‐pot spray pyrolysis process is prepared. The first discharge/charge capacities of the composite microspheres are 797 and 573 mA h g^?1 at a current density of 0.2 A g^?1. The 600th discharge capacity of the composite microspheres at a current density of 1.5 A g^?1 is 322 mA h g^?1. The Coulombic efficiency during the 600 cycles is as high as 99.98%. The outstanding Na ion storage properties of the 3D MoS₂–graphene composite microspheres may be attributed to the reduced stacking of the MoS₂ layers and to the 3D structure of the porous graphene microspheres. The reduced stacking of the MoS₂ layers relaxes the strain and lowers the barrier for Na⁺ insertion. The empty nanospheres of the graphene offer voids for volume expansion and pathways for fast electron transfer during repeated cycling.