Thin film transistors by solution-based indium gallium zinc oxide/carbon nanotubes blend

Solution-based indium gallium zinc oxide (IGZO)/single-walled carbon nanotubes (SWNTs) blend have been used to fabricate the channel of thin film transistors (TFTs). The electrical characteristics of the fabricated devices were examined. We found a low leakage current and a higher on/off currents ratio for TFT with SWNTs compared to solution-based TFTs made without SWNTs. The saturation field effect mobility (μ_sat) of about 0.22 cm²/Vs, the current on/off ratio is ~ 10⁵, the subthreshod swing is ~ 2.58 V/decade and the threshold voltage (V_th) is less than − 2.3 V. We demonstrated that the solution-based blend active layer provides the possibility of producing higher performance TFTs for low-cost large area electronic and flexible devices.     

Room‐Temperature Solution‐Synthesized p‐Type Copper(I) Iodide Semiconductors for Transparent Thin‐Film Transistors and Complementary Electronics

Here, room‐temperature solution‐processed inorganic p‐type copper iodide (CuI) thin‐film transistors (TFTs) are reported for the first time. The spin‐coated 5 nm thick CuI film has average hole mobility (µ_FE) of 0.44 cm² V^?1 s^?1 and on/off current ratio of 5 × 10². Furthermore, µ_FE increases to 1.93 cm² V^?1 s^?1 and operating voltage significantly reduces from 60 to 5 V by using a high permittivity ZrO₂ dielectric layer replacing traditional SiO₂. Transparent complementary inverters composed of p‐type CuI and n‐type indium gallium zinc oxide TFTs are demonstrated with clear inverting characteristics and voltage gain over 4. These outcomes provide effective approaches for solution‐processed inorganic p‐type semiconductor inks and related electronics.     

Improved Morphology and Performance of Solution‐Processed Metal‐Oxide Thin‐Film Transistors Due to a Polymer Based Interface Modifier

The influence of a polymer interface modifier on the performance of solution‐processed indium‐based metal‐oxide (MO) thin‐film transistors (TFTs) is investigated. We use the polymer ethoxylated polyethylenimine (PEIE). Compared to a reference sample this modification enhances the mobility by a factor of four, clearly reduces the contact and the sheet resistance, and decreases the charge carrier activation energy by about 20%. The improved electrical performance originates from both a reduced contact and a reduced sheet resistance of the TFTs. The molecular dipole of PEIE reduces the work function of the electrodes. Adversely the dipole enhances the off current and the trap density at the semiconductor/dielectric interface for bottom‐contact transistors with small channel length. The substrate becomes highly polar with a PEIE‐treatment. Accordingly, topographical studies of bottom‐contact TFTs show a very similar MO film morphology on the electrodes and in the channel for modified TFTs, whereas in the untreated samples the film has a higher roughness on the electrodes than in the channel. TFTs in top‐contact configuration with the polymer interface layer at the dielectric/semiconductor interface also show higher mobility compared to the reference MOTFTs which displays that the improved performance is due to the improved morphology of the MO film.     

Inkjet-printed InGaZnO thin film transistor

We report inkjet-printed InGaZnO (IGZO) thin film transistors (TFTs). IGZO ink was prepared by dissolving indium nitrate hydrate, gallium nitrate hydrate and zinc acetate dihydrate into 2-methoxyethanol with additional stabilizers. The resulting films were inkjet-printed with a resolution of 300 dots per inch using droplets with a diameter of 40 µm, and a volume of 35 pl. The films exhibited high optical transparency in the visible range and had a polycrystalline phase of InGaO₃(ZnO)₂ after thermal annealing treatment. The chemical composition of this IGZO sample was also determined, and shown to have high stoichiometric characteristics of low oxygen deficiency. The TFTs with a conventional inverted staggered structure using inkjet-printed IGZO as an active channel layer had a field-effect mobility of ~ 0.03 cm²/Vs in saturation region and an on-to-off current ratio greater than ~ 10⁴.     

Wet etching rates of InGaZnO for the fabrication of transparent thin-film transistors on plastic substrates

The wet etch process for amorphous indium gallium zinc oxide (a-IGZO or a-InGaZnO) by using various etchants is reported. The etch rates of a-IGZO, compared to another indium-based oxides including indium gallium oxide (IGO), indium zinc oxide (IZO), and indium tin oxide (ITO), are measured by using acetic acid, citric acid, hydrochloric acid, perchloric acid, and aqua ammonia as etchants, respectively. In our experimental results, the etch rate of the transparent oxide semiconductor (TOS) films by using acid solutions ranked accordingly from high to low are IZO, IGZO, IGO and ITO. Comparatively, the etch rate of the TOS films by using alkaline ammonia solution ranked from high to low are IGZO, IZO, IGO and ITO, in that order.Using the proposed wet etching process with high etch selectivity, bottom-gate-type thin-film transistors (TFTs) based on a-IGZO channels and Y₂O₃ gate-insulators were fabricated by radio-frequency sputtering on plastic substrates. The wet etch processed TFT with 30 µm gate length and 120 µm gate width exhibits a saturation mobility of 46.25 cm² V^− 1 s^− 1, a threshold voltage of 1.3 V, a drain current on-off ratio > 10⁶ , and subthreshold gate voltage swing of 0.29 V decade^− 1. The performance of the TFTs ensures the applicability of the wet etching process for IGZO to electronic devices on organic polymer substrates.     

Solution Combustion Synthesis: Low‐Temperature Processing for p‐Type Cu:NiO Thin Films for Transparent Electronics

Low‐temperature solution processing opens a new window for the fabrication of oxide semiconductors due to its simple, low cost, and large‐area uniformity. Herein, by using solution combustion synthesis (SCS), p‐type Cu‐doped NiO (Cu:NiO) thin films are fabricated at a temperature lower than 150 °C. The light doping of Cu substitutes the Ni site and disperses the valence band of the NiO matrix, leading to an enhanced p‐type conductivity. Their integration into thin‐film transistors (TFTs) demonstrates typical p‐type semiconducting behavior. The optimized Cu_5%NiO TFT exhibits outstanding electrical performance with a hole mobility of 1.5 cm² V^?1 s^?1, a large on/off current ratio of ≈10⁴, and clear switching characteristics under dynamic measurements. The employment of a high‐k ZrO₂ gate dielectric enables a low operating voltage (≤2 V) of the TFTs, which is critical for portable and battery‐driven devices. The construction of a light‐emitting‐diode driving circuit demonstrates the high current control capability of the resultant TFTs. The achievement of the low‐temperature‐processed Cu:NiO thin films via SCS not only provides a feasible approach for low‐cost flexible p‐type oxide electronics but also represents a significant step toward the development of complementary metal–oxide semiconductor circuits.     

Effect of hafnium addition on the electrical properties of indium zinc oxide thin film transistors

This study reports the performance and stability of hafnium-indium zinc oxide (HfInZnO) thin film transistors (TFTs) with thermally grown SiO2. The HfInZnO channel layer was deposited at room temperature by a co-sputtering system. We examined the effects of hafnium addition on the X-ray photoelectron spectroscopy properties and on the electrical characteristics of the TFTs varying the concentration of the added hafnium. We found that the transistor on-off currents were greatly influenced by the composition of hafnium addition, which suppressed the formation of oxygen vacancies. The field-effect mobility of optimized HfInZnO TFT was 1.34 cm² V⁻¹ s⁻¹, along with an on-off current ratio of 10⁸ and a threshold voltage of 4.54 V. We also investigated the effects of bias stress on HfInZnO TFTs with passivated and non-passivated layers. The threshold voltage change in the passivated device after positive gate bias stress was lower than that in the non-passivated device. This result indicates that HfInZnO TFTs are sensitive to the ambient conditions of the back surface.     

Polymer Doping Enables a Two‐Dimensional Electron Gas for High‐Performance Homojunction Oxide Thin‐Film Transistors

High‐performance solution‐processed metal oxide (MO) thin‐film transistors (TFTs) are realized by fabricating a homojunction of indium oxide (In₂O₃) and polyethylenimine (PEI)‐doped In₂O₃ (In₂O₃:x% PEI, x = 0.5–4.0 wt%) as the channel layer. A two‐dimensional electron gas (2DEG) is thereby achieved by creating a band offset between the In₂O₃ and PEI‐In₂O₃ via work function tuning of the In₂O₃:x% PEI, from 4.00 to 3.62 eV as the PEI content is increased from 0.0 (pristine In₂O₃) to 4.0 wt%, respectively. The resulting devices achieve electron mobilities greater than 10 cm² V^?1 s^?1 on a 300 nm SiO₂ gate dielectric. Importantly, these metrics exceed those of the devices composed of the pristine In₂O₃ materials, which achieve a maximum mobility of ≈4 cm² V^?1 s^?1. Furthermore, a mobility as high as 30 cm² V^?1 s^?1 is achieved on a high‐k ZrO₂ dielectric in the homojunction devices. This is the first demonstration of 2DEG‐based homojunction oxide TFTs via band offset achieved by simple polymer doping of the same MO material.     

Effect of oxygen partial pressure on electrical characteristics of amorphous indium gallium zinc oxide thin-film transistors fabricated by thermal annealing

We report the fabrication and electrical characteristics of high-performance amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs) with a polymer gate dielectric prepared by spin coating on a glass substrate at different oxygen partial pressure values. The transmittance of the deposited polymer film was greater than 90% at 600 nm a-IGZO thin films were deposited on glass substrates using RF magnetron sputtering at different oxygen partial pressure values. The a-IGZO TFTs were prepared by rapid thermal annealing at 350 °C for 10 min at a 0.2% oxygen partial pressure. It was observed that a-IGZO TFTs with an active channel layer exhibited enhanced mode operation, a threshold voltage of 1 V, an on-off current ratio of 10³, and a field-effect mobility of 18 cm²/Vs.     

Effect of oxygen partial pressure on electrical characteristics of amorphous indium gallium zinc oxide thin-film transistors fabricated by thermal annealing

We report the fabrication and electrical characteristics of high-performance amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs) with a polymer gate dielectric prepared by spin coating on a glass substrate at different oxygen partial pressure values. The transmittance of the deposited polymer film was greater than 90% at 600 nm a-IGZO thin films were deposited on glass substrates using RF magnetron sputtering at different oxygen partial pressure values. The a-IGZO TFTs were prepared by rapid thermal annealing at 350 °C for 10 min at a 0.2% oxygen partial pressure. It was observed that a-IGZO TFTs with an active channel layer exhibited enhanced mode operation, a threshold voltage of 1 V, an on-off current ratio of 10³, and a field-effect mobility of 18 cm²/Vs.     

Amorphous oxide channel TFTs

Thin film transistors (TFTs) using amorphous oxides of post-transition metals: indium, gallium, and zinc for the channel materials are fabricated with radio-frequency magnetron sputtering methods for the deposition of the channel and the gate insulator layers, at room temperature with no high-temperature post-deposition annealing process. The TFTs operate as n-channel field-effect transistors with various structures of top/bottom gate and top/bottom source-and-drain contact including the inverse-stagger types, and with various materials for the gate insulators, the electrodes, and the substrates. The TFTs having smoother channel interfaces show the better performance at the saturation mobility beyond 10 cm² V^− 1 s^− 1 and the on-to-off current ratio over 10⁸ than the rough channel interfaces. The ring oscillator circuits operate with five-stage inverters of the top-gate TFTs or the inverse-stagger TFTs. Organic light-emission diode cells are driven by a simple circuit of the TFTs. It is also found by a combinatorial approach to the material exploration that the TFT characteristics can be controlled by the composition ratio of the metals in the channel layers. The amorphous oxide channel TFTs fabricated with sputtering deposition at low temperature could be a candidate for key devices of large-area flexible electronics.     

Effects of the composition of sputtering target on the stability of InGaZnO thin film transistor

In this study, we investigated the electrical characteristics and the stability of amorphous indium gallium zinc oxide (a-IGZO) thin film transistors (TFTs) from the viewpoint of active layer composition. Active layers of TFTs were deposited by r.f. sputtering. Two kinds of sputtering targets, which have different compositional ratios of In:Ga:Zn, were used to make variations in the active layer composition. All the fabricated IGZO TFTs showed more excellent characteristics than conventional amorphous silicon TFTs. However, in accordance with the Ga content, IGZO TFTs showed somewhat different electrical characteristics in values such as the threshold voltage and the field effect mobility. The device stability was also dependent on the Ga content, but had trade-off relation with the electrical characteristics.     

The influence of fabrication process on top-gate thin-film transistors

Amorphous indium zinc oxide (a-IZO) thin-film transistors (TFTs) with bottom- and top-gate structures were fabricated at room temperature by direct current (DC) magnetron sputter in this research. High dielectric constant (κ) hafnium oxide (HfO₂) films and a-IZO were deposited for the gate insulator and the semiconducting channel under a mixture of ambient argon and oxygen gas, respectively. The bottom-gate TFTs showed good TFT characteristics, but the top-gate TFTs did not display the same characteristics as the bottom-gate TFTs despite undergoing the same process of sputtering with identical conditions. The electrical characteristics of the top-gate a-IZO TFTs exhibited strong relationships with sputtering power as gate dielectric layer deposition in this study. The ion bombardment and incorporation of sputtering ions damaged the interface between the active layer and the gate insulator in top-gate TFTs. Hence, the sputtering power was reduced to decrease damage while depositing HfO₂ films. When using 50 W DC magnetron sputtering, the top-gate a-IZO TFTs showed the following results: a saturation mobility of 5.62 cm²/V-s; an on/off current ratio of 1 × 10⁵; a sub-threshold swing (SS) of 0.64 V/decade; and a threshold voltage (V_th) of 2.86 V.     

Improvements in the performances of In–Ga–Zn–O thin-film transistors on glass substrates by annealing treatment

Thin film transistors (TFTs) with amorphous InGaZnO (IGZO) channel layer were fabricated by radio frequency magnetron sputtering technique. The IGZO films show optical transparency over 80 % both before annealing and after annealing. It was found that performances of transistors with IGZO thin films annealed in air at 450 °C were significantly improved. Through annealing treatment, Saturation current of TFTs increased from 2.8 to 181 μA at bias of V_DS = 20, V_GS = 20 V, and saturation mobility is up from 1.49 to 15.8 cm² V^?1 s^?1. In addition, X-ray photoelectric spectroscopy (XPS) was performed to provide elemental information on the surface of the IGZO films before and after annealing. O1s XPS spectra of unannealed and annealed IGZO films indicated oxygen vacancy concentration decreased by annealing treatment.     

The effect of metallization contact resistance on the measurement of the field effect mobility of long-channel unannealed amorphous In-Zn-O thin film transistors

The effect of contact resistance on the measurement of the field effect mobility of compositionally homogeneous channel indium zinc oxide (IZO)/IZO metallization thin film transistors (TFTs) is reported. The TFTs studied in this work operate in depletion mode as n-channel field effect devices with a field effect mobility calculated in the linear regime (μ_FE) of 20 ± 1.9 cm²/Vs and similar of 18 ± 1.3 cm²/Vs when calculated in the saturation regime (μ_FE^sat). These values, however, significantly underestimate the channel mobility since a large part of the applied drain voltage is dropped across the source/drain contact interface. The transmission line method was employed to characterize the contact resistance and it was found that the conducting-IZO/semiconducting-IZO channel contact is highly resistive (specific contact resistance, ρ_C > 100 Ωcm²) and, further, this contact resistance is modulated with applied gate voltage. Accounting for the contact resistance (which is large and modulated by gate voltage), the corrected μ_FE is shown to be 39 ± 2.6 cm²/Vs which is consistent with Hall mobility measurements of high carrier density IZO.     

Field‐Driven Athermal Activation of Amorphous Metal Oxide Semiconductors for Flexible Programmable Logic Circuits and Neuromorphic Electronics

Despite extensive research, large‐scale realization of metal‐oxide electronics is still impeded by high‐temperature fabrication, incompatible with flexible substrates. Ideally, an athermal treatment modifying the electronic structure of amorphous metal oxide semiconductors (AMOS) to generate sufficient carrier concentration would help mitigate such high‐temperature requirements, enabling realization of high‐performance electronics on flexible substrates. Here, a novel field‐driven athermal activation of AMOS channels is demonstrated via an electrolyte‐gating approach. Facilitating migration of charged oxygen species across the semiconductor–dielectric interface, this approach modulates the local electronic structure of the channel, generating sufficient carriers for charge transport and activating oxygen‐compensated thin films. The thin‐film transistors (TFTs) investigated here depict an enhancement of linear mobility from 51 to 105.25 cm² V^?1 s^?1 (ionic‐gated) and from 8.09 to 14.49 cm² V^?1 s^?1 (back‐gated), by creating additional oxygen vacancies. The accompanying stochiometric transformations, monitored via spectroscopic measurements (X‐ray photoelectron spectroscopy) corroborate the detailed electrical (TFT, current evolution) parameter analyses, providing critical insights into the underlying oxygen‐vacancy generation mechanism and clearly demonstrating field‐induced activation as a promising alternative to conventional high‐temperature annealing strategies. Facilitating on‐demand active programing of the operation modes of transistors (enhancement vs depletion), this technique paves way for facile fabrication of logic circuits and neuromorphic transistors for bioinspired computing.     

Effect of gallium content on bias stress stability of solution-deposited Ga-Sn-Zn-O semiconductor transistors

We generated solution-processed thin film transistor (TFTs) using gallium tin zinc oxide (GTZO, Ga-Sn-Zn-O) layers as the channel that exhibit improved bias-stress stability during device operation under ambient conditions. The cause of the bias-stress stability was investigated through comparisons with zinc tin oxide (ZTO, Zn-Sn-O)-based TFTs, which suffer red from bias stress instability. Based on in-depth analysis of the electrical characteristics and chemical structure of both GTZO and ZTO layers, it was discovered that the GTZO layers had a significantly lower oxygen vacancy concentration than did the ZTO layer, which influenced the electrical performance of the GTZO transistors as well as their bias-stress stability. When 5 mol% gallium was added, a bias stress-stable transistor was obtained, exhibiting typical semiconductor behavior with a field-effect mobility of 1.2 cm² V^− 1 s^− 1, on/off ratio of 10⁶, off-current of 1 × 10^− 10 A, and threshold voltage of 19.6 V. Further doping of Ga deteriorated the device performance, which was found to be associated with decreased carrier concentration and segregation of an insulating secondary phase.     

High performance self-aligned top-gate ZnO thin film transistors using sputtered Al2O3 gate dielectric

High performance self-aligned top-gate zinc oxide (ZnO) thin film transistors (TFTs) utilizing high-k Al₂O₃ thin film as gate dielectric are developed in this paper. Good quality Al₂O₃ thin film was deposited by reactive DC magnetron sputtering technique using aluminum target in a mixed argon and oxygen ambient at room temperature. The resulting transistor exhibits a field effect mobility of 27 cm²/V s, a threshold voltage of − 0.5 V, a subthreshold swing of 0.12 V/decade and an on/off current ratio of 9 × 10⁶. The proposed top-gate ZnO TFTs in this paper can act as driving devices in the next generation flat panel displays.     

An investigation of contact resistance between metal electrodes and amorphous gallium-indium-zinc oxide (a-GIZO) thin-film transistors

This paper reports our investigation of different source/drain (S/D) electrode materials in thin-film transistors (TFTs) based on an indium-gallium-zinc oxide (IGZO) semiconductor. Transfer length, contact resistance, channel conductance, and effective resistances between S/D electrodes and amorphous IGZO thin-film transistors were examined. Intrinsic TFT parameters were extracted by the transmission line method (TLM) using a series of TFTs with different channel lengths measured at a low drain voltage. The TFTs fabricated with Cu S/D electrodes showed the lowest contact resistance and transfer length indicating good ohmic characteristics, and good transfer characteristics with intrinsic field-effect mobility (μ_FE-i) of 10.0 cm²/Vs.     

An investigation in InGaO<Subscript>3</Subscript>(ZnO)<Subscript>m</Subscript> pellets as cause of variability in thin film transistor characteristics

Indium–gallium–zinc oxide (IGZO) is a novel amorphous oxide semiconductor, which recently has received much attention for thin film transistors (TFTs) in flat panel displays. Published literature reports significant variations in the properties of thin films and TFTs prepared from IGZO even though the reported process conditions are similar. We demonstrate that these differences could arise from the method for preparation of targets from which the films are made. Accordingly, we also propose simple and appropriate conditions, specifically using much lower sintering temperatures and thus avoiding use of sealed Pt tubes for preparation of IGZO targets in composition range, InGaO₃(ZnO) _m , with 1 ≤ m ≤ 5. These target materials are suitable in physical vapour deposition processes such as pulsed laser deposition and sputtering. In developing the process for sintering, the phase analysis of the target pellets was carried out using X-ray diffraction (XRD). The chemical compositions of the phases are also confirmed with inductively coupled plasma optical emission spectrometry (ICP-OES) and energy dispersive X-ray (EDX) techniques. We also demonstrate successful deposition of amorphous IGZO thin films by pulse laser deposition using the targets prepared by the proposed sintering process. Finally, we demonstrate that unmonitored method of making pellets for films deposition is a cause of variability associated in published literature on IGZO TFTs.