Tuning the Threshold Voltage of an Oxide Thin-Film Transistor by Electron Injection Control Using a p-n Semiconductor Heterojunction Structure

Herein, a heterojunction structure integrating p-type tellurium (Te) and n-type aluminum-doped indium-zinc-tin oxide (Al:IZTO) is shown to precisely modulate the threshold voltage (VT) of the oxide thin-film transistor (TFT). The proposed architecture integrates Te as an electron-blocking layer and Al:IZTO as a charge-carrier transporting layer, thereby enabling controlled electron injection. The effects of incorporating the Te layer onto Al:IZTO are investigated, with a focus on X-ray photoelectron spectroscopy (XPS) analysis, in order to explain the behavior of oxygen vacancies and to depict the energy band structure configurations. By modulating the thickness and employing both single and double deposition methods for the heterojunction Te layer, a remarkable VT shift of up to +20 V is achieved. Furthermore, this study also shows excellent stability to a positive bias stress of +2 MV/cm for 10,000 s without additional passivation layers, demonstrating the robustness of the designed TFT. By a thorough optimization of the Al:IZTO/Te interface, the results demonstrate not only the substantial impact of the introduced heterojunction structure on VT control but also the endurance, durability, and stability of the optimized TFTs under prolonged long-term operating stress, thus offering promising prospects for tailored semiconductor device applications.

Composite Assembling of Oxide-Based Optically Transparent Electrodes for High-Performance Asymmetric Supercapacitors

Simultaneously achieving a transparent and high-energy density supercapacitor is a major challenge because of the trade-off between energy storage capacity and optical transparency of active electrode materials. Herein, we demonstrate a novel approach to construct an optically transparent asymmetric supercapacitor (Trans-ASC) by assembling positive (ZnO-SnO2) and negative (TiO2-SnO2) composite thin-film electrodes on a conductive indium-doped tin oxide substrate via reactive DC magnetron cosputtering. The optical transmittance for both composite thin films is found to be 68% (ZnO-SnO2) and 64% (TiO2-SnO2). Furthermore, electrochemical kinematics of the primed transparent electrodes are scrutinized in 0.5 M KOH electrolyte without affecting the transparency of active electrodes. The structural reliability of the electrodes aids the superb electrochemical performance to construct a Trans-ASC, TiO2-SnO2//ZnO-SnO2, which works at a voltage of +1.2 V and attains a higher areal capacitance of 44.6 mF cm-2 at 2 mA cm-2. The assembled Trans-ASC delivers a maximum areal energy density of 8.75 μW h cm-2 with an optimal areal power density of 570 μW cm-2. Additionally, the capacitance retention of 81.6% and transparency of both electrodes remain almost the same (up to 60% for ZnO-SnO2 and 62% for TiO2-SnO2) even after 10,000 charging-discharging cycles. These remarkable electrochemical properties and outstanding cycling stability of the designed Trans-ASC device make it a potential candidate for storing energy and for further use in transparent electronic devices.

Tail state mediated conduction in zinc tin oxide thinfilm phototransistors under below bandgap optical excitation

We report on the appearance of a strong persistent photoconductivity (PPC) and conductor-like behaviour in zinc tin oxide (ZTO) thinfilm phototransistors. The active ZTO channel layer was prepared by remote plasma reactive sputtering and possesses an amorphous structure. Under sub-bandgap excitation of ZTO with UV light, the photocurrent reaches as high as ~ 10^-4 A (a photo-to-dark current ratio of ~ 10⁷) and remains close to this high value after switching off the light. During this time, the ZTO TFT exhibits strong PPC with long-lasting recovery time, which leads the appearance of the conductor-like behaviour in ZTO semiconductor. In the present case, the conductivity changes over six orders of magnitude, from ~ 10^-7 to 0.92/Ω/cm. After UV exposure, the ZTO compound can potentially remain in the conducting state for up to a month. The underlying physics of the observed PPC effect is investigated by studying defects (deep states and tail states) by employing a discharge current analysis (DCA) technique. Findings from the DCA study reveal direct evidence for the involvement of sub-bandgap tail states of the ZTO in the strong PPC, while deep states contribute to mild PPC.

Optically Induced Rapid Wetting Transition on Zn-Polar and O-Polar Zinc Oxide

The design and fabrication of surfaces that support rapid wetting transition remain technologically challenging. Here, we examine the effects of optical illumination on the wetting behaviors of zinc oxide (ZnO) single crystals. We find that ultraviolet irradiation above the band gap energy promotes a rapid wetting transition, characterized by sliding of the water droplet, within a few seconds. Notably, the transition for Zn-polar (0001) ZnO surfaces is even faster than that for O-polar (0001̅) ZnO surfaces. We confirmed that process is dependent on power, surface polarity, and solution pH and reversible through illumination by near-infrared light, which restores the water contact angle back to its initial value. Surface chemical analysis revealed that the instantaneous photocatalytic formation of surface-terminated hydroxyl (-OH) groups is responsible for the observed rapid wetting transition. Density functional theory calculations with the inclusion of onsite Coulomb interactions revealed that both the Zn-polar and O-polar surfaces can be easily covered with -OH groups through the adsorption of -OH groups or hydrogen atoms, respectively. This study develops a route to fabricate optically active and controllable microfluidic devices that support rapid wetting transitions for water droplet manipulation.

Boosting Carrier Mobility in Zinc Oxynitride Thin-Film Transistors via Tantalum Oxide Encapsulation

Novel TaO _x encapsulation was presented to enhance the field-effect mobility (μ_FE) of ZnON thin-film transistors (TFTs) consisting of a metallic Ta film deposited onto the ZnON surface followed by a modest annealing process. The resulting TaO _x/ZnON film stack exhibited a more uniform distribution of nanoscale ZnON crystallites with increased stoichiometric anion lattices compared to the control ZnON film. The control ZnON TFTs exhibited a reasonable μ_FE, subthreshold gate swing (SS), and I_ON/OFF ratio of 36.2 cm²/V·s, 0.28 V/decade, and 2.9 × 10⁸, respectively. A significantly enhanced μ_FE value of 89.4 cm²/V·s was achieved for ZnON TFTs with a TaO _x encapsulation layer, whereas the SS of 0.33 V/decade and I_ON/OFF ratio of 8.6 × 10⁸ were comparable to those of the control device. This improvement could be explained by scavenging and passivation effects of the TaO _x film on the ZnON channel layer. Density of states (DOS)-based modeling and simulation were performed to obtain greater insight with regard to increasing the performance of the ZnON TFTs with a TaO _x encapsulation layer. A smaller number of subgap states near the conduction band (CB) minimum and a higher net carrier density for the TaO _x-capped device increased the Fermi energy level toward the CB edge under thermal equilibrium conditions, leading to efficient band conduction and fast carrier transport under the on-state condition.

Trap-Assisted Enhanced Bias Illumination Stability of Oxide Thin Film Transistor by Praseodymium Doping

Praseodymium-doped indium zinc oxide (PrIZO) has been employed as the channel layer of thin-film transistors (TFTs). The TFTs with Pr doping exhibited a remarkable suppression of the light-induced instability. A negligible photo-response and remarkable enhancement in negative gate bias stress under illumination (NBIS) were achieved in the PrIZO-TFTs. Meanwhile, the PrIZO-TFTs showed reasonable characteristics with a high-field-effect mobility of 26.3 cm²/V s, SS value of 0.28 V/decade, and I_on/ I_off ratio of 10⁸. X-ray photoelectron spectroscopy, microwave photoconductivity decay, and photoluminescence spectra were employed to analyze the effects of the Pr concentrations on the performance of PrIZO-TFTs. We disclosed that acceptor-like trap states induced by Pr ions might lead to the suppression of photo-induced carrier in conduction band, which is a new strategy for improving illumination stability of amorphous oxide semiconductors. Finally, a prototype of fully transparent AMOLED display was successfully fabricated to demonstrate the potential of Pr-doping TFTs applied in transparent devices.

All-Sputtering, High-Transparency, Good-Stability Coplanar Top-Gate Thin Film Transistors

In this work, transparent, stable coplanar top-gate thin film transistors (TFTs) with an active layer of neodymium-doped indium oxide and zinc oxide (Nd-IZO) were successfully fabricated on a glass substrate by all sputtering processes. The devices with a post-annealing temperature of 400 °C exhibited good electrical performances with a saturation mobility (μsat) of 4.25 cm2·V−1·S−1, Ion/Ioff ratio about 106, Vth of −0.97 V and SS about 0.34 V/decade. Furthermore, the devices exhibited excellent negative and positive bias stability (NBS, PBS) of only a ΔVth shift of about −0.04 V and 0.05 V after 1 h, respectively. In addition, the devices showed high transparency about 96% over the visible-light region of 400–700 nm, which indicates a great potential in transparent displays.

In2Ga2ZnO7 oxide semiconductor based charge trap device for NAND flash memory

The programming characteristics of charge trap flash memory device adopting amorphous In₂Ga₂ZnO₇ (a-IGZO) oxide semiconductors as channel layer were evaluated. Metal-organic chemical vapor deposition (MOCVD) and RF-sputtering processes were used to grow a 45 nm thick a-IGZO layer on a 20 nm thick SiO₂ (blocking oxide)/p⁺⁺-Si (control gate) substrate, where 3 nm thick atomic layer deposited Al₂O₃ (tunneling oxide) and 5 nm thick low-pressure CVD Si₃N₄ (charge trap) layers were intervened between the a-IGZO and substrate. Despite the identical stoichiometry and other physicochemical properties of the MOCVD and sputtered a-IGZO, a much faster programming speed of MOCVD a-IGZO was observed. A comparable amount of oxygen vacancies was found in both MOCVD and sputtered a-IGZO, confirmed by x-ray photoelectron spectroscopy and bias-illumination-instability test measurements. Ultraviolet photoelectron spectroscopy analysis revealed a higher Fermi level (E _F) of the MOCVD a-IGZO (∼0.3 eV) film than that of the sputtered a-IGZO, which could be ascribed to the higher hydrogen concentration in the MOCVD a-IGZO film. Since the programming in a flash memory device is governed by the tunneling of electrons from the channel to charge trapping layer, the faster programming performance could be the result of a higher E _F of MOCVD a-IGZO.

Enhanced Light Stability of InGaZnO Thin-Film Transistors by Atomic-Layer-Deposited Y2O3 with Ozone

We report the effect of Y₂O₃ passivation by atomic layer deposition (ALD) using various oxidants, such as H₂O, O₂ plasma, and O₃, on In-Ga-Zn-O thin-film transistors (IGZO TFTs). A large negative shift in the threshold voltage (V_th) was observed in the case of the TFT subjected to the H₂O-ALD Y₂O₃ process; this shift was caused by a donor effect of negatively charged chemisorbed H₂O molecules. In addition, degradation of the IGZO TFT device performance after the O₂ plasma-ALD Y₂O₃ process (field-effect mobility (μ) = 8.7 cm²/(V·s), subthreshold swing (SS) = 0.77 V/dec, and V_th = 3.7 V) was observed, which was attributed to plasma damage on the IGZO surface adversely affecting the stability of the TFT under light illumination. In contrast, the O₃-ALD Y₂O₃ process led to enhanced device stability under light illumination (ΔV_th = -1 V after 3 h of illumination) by passivating the subgap defect states in the IGZO surface region. In addition, TFTs with a thicker IGZO film (55 nm, which was the optimum thickness under the current investigation) showed more stable device performance than TFTs with a thinner IGZO film (30 nm) (ΔV_th = -0.4 V after 3 h of light illumination) by triggering the recombination of holes diffusing from the IGZO surface to the insulator-channel interface. Therefore, we envisioned that the O₃-ALD Y₂O₃ passivation layer suggested in this paper can improve the photostability of TFTs under light illumination.

Planar n-Si/PEDOT:PSS hybrid heterojunction solar cells utilizing functionalized carbon nanoparticles synthesized via simple pyrolysis route

Herein, we present a facile and simple strategy for in situ synthesis of functionalized carbon nanoparticles (CNPs) via direct pyrolysis of ethylenediaminetetraacetic acid (EDTA) on silicon surface. The CNPs were incorporated in hybrid planar n-Si and poly(3,4-etyhlenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) solar cells to improve device performance. We demonstrate that the CNPs-incorporated devices showed increased electrical conductivity (reduced series resistance) and minority carrier lifetime (better charge carrier collection) than those of the cells without CNPs due to the existence of electrically conductive sp ²-hybridized carbon at the heterojunction interfaces. With an optimal concentration of CNPs, the hybrid solar cells exhibited power conversion efficiency up to 11.95%, with an open-circuit voltage of 614 mV, short-circuit current density of 26.34 mA cm^-2, and fill factor of 73.93%. These results indicate that our approach is promising for the development of highly efficient organic-inorganic hybrid solar cells.

Activation of sputter-processed indium-gallium-zinc oxide films by simultaneous ultraviolet and thermal treatments

Indium–gallium–zinc oxide (IGZO) films, deposited by sputtering at room temperature, still require activation to achieve satisfactory semiconductor characteristics. Thermal treatment is typically carried out at temperatures above 300 °C. Here, we propose activating sputter- processed IGZO films using simultaneous ultraviolet and thermal (SUT) treatments to decrease the required temperature and enhance their electrical characteristics and stability. SUT treatment effectively decreased the amount of carbon residues and the number of defect sites related to oxygen vacancies and increased the number of metal oxide (M–O) bonds through the decomposition-rearrangement of M–O bonds and oxygen radicals. Activation of IGZO TFTs using the SUT treatment reduced the processing temperature to 150 °C and improved various electrical performance metrics including mobility, on-off ratio, and threshold voltage shift (positive bias stress for 10,000 s) from 3.23 to 15.81 cm²/Vs, 3.96 × 10⁷ to 1.03 × 10⁸, and 11.2 to 7.2 V, respectively.

Proton gated oxide electric-double-layer transistors for full-swing low voltage inverter applications

Low voltage full-swing inverters were built by connecting proton gated oxide electric-double-layer transistors in series with resistors.

Dual electrical behavior of multivalent metal cation-based oxide and its application to thin-film transistors with high mobility and excellent photobias stability

The effect of multivalent metal cations, including vanadium(V) and tin (Sn), on the electrical properties of vanadium-doped zinc tin oxide (VZTO) was investigated in the context of the fabrication of thin-film transistors (TFTs) using a single VZTO film and VZTO/ZTO bilayer as channel layers. The single VZTO TFT did not show any response to the gate voltage (insulator-like behavior). On the other hand, the VZTO/ZTO bilayer TFT exhibited a typical TFT transfer characteristic (semiconducting behavior). X-ray photoelectron spectroscopy revealed that, in contrast to what is commonly true in many oxides, oxygen vacancies (V(O)) in VZTO did not provide a dominant contribution to the total carrier concentration, because the V(O) peak area in the single VZTO film was 5.4% and reduced to 4.5% in VZTO/ZTO bilayer. Instead, Sn 3d5/2 and V 2p3/2 spectra suggest that the significant reduction in Sn and V ions is strongly related to the insulator-like behavior of the VZTO film. In negative-bias illumination tests and illumination tests with various photon energies, the VZTO/ZTO bilayer TFT had much better stability than the ZTO TFT. This result is attributed to the reduction of donor-like states ([Formula: see text]O) that can be positively ionized by blue and green illumination.

Impact of the cation composition on the electrical performance of solution-processed zinc tin oxide thin-film transistors

This study examined the structural, chemical, and electrical properties of solution-processed (Zn,Sn)O3 (ZTO) films with various Sn/[Zn+Sn] ratios for potential applications to large-area flat panel displays. ZTO films with a Zn-rich composition had a polycrystalline wurtzite structure. On the other hand, the Sn-rich ZTO films exhibited a rutile structure, where the Zn atom was speculated to replace the Sn site, thereby acting as an acceptor. In the intermediate composition regions (Sn/[Zn+Sn] ratio from 0.28 to 0.48), the ZTO films had an amorphous structure, even after annealing at 450 °C. The electrical transport properties and photobias stability of ZTO thin film transistors (TFTs) were also examined according to the Sn/[Zn+Sn] ratio. The optimal transport property of ZTO TFT was observed for the device with an amorphous structure at a Sn/[Zn+Sn] ratio of 0.48. The mobility, threshold voltage, subthreshold swing, and on/off current ratio were 4.3 cm(2)/(V s), 0 V, 0.4 V/decade, and 4.1 × 10(7), respectively. In contrast, the device performance for the ZTO TFTs with either a higher or lower Sn concentration suffered from low mobility and a high off-state current, respectively. The photoelectrical stress measurements showed that the photobias stability of the ZTO TFTs was improved substantially when the ZTO semiconducting films had a lower oxygen vacancy concentration and an amorphous structure. The relevant rationale is discussed based on the phototransition and subsequent migration mechanism from neutral to positively charged oxygen vacancies.

Improvement in negative bias stress stability of solution-processed amorphous In-Ga-Zn-O thin-film transistors using hydrogen peroxide

We have investigated the effect of hydrogen peroxide (H2O2) on negative bias stress (NBS) stability of solution-processed amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistors (TFTs). The instability of solution-processed a-IGZO TFTs under NBS is attributed to intrinsic oxygen vacancy defects (Vo) and organic chemical-induced defects, such as pores, pin holes, and organic residues. In this respect, we added H2O2 into an indium-gallium-zinc oxide solution to reduce the defects without any degradation of electrical performance. The field-effect mobility and sub-threshold slope of the a-IGZO TFTs were improved from 0.37 cm(2) V(-1) s(-1) and 0.86 V/dec to 0.97 cm(2) V(-1) s(-1) and 0.58 V/dec, respectively. Furthermore, the threshold voltage shift under NBS was dramatically decreased from -3.73 to -0.18 V. These results suggest that H2O2 effectively reduces Vo through strong oxidation and minimizes organic chemical-induced defects by eliminating the organic chemicals at lower temperatures compared to a conventional solution process.