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.

Promotion of Processability in a p-Type Thin-Film Transistor Using a Se-Te Alloying Channel Layer.

p-type thin-film transistors (pTFTs) have proven to be a significant impediment to advancing electronics beyond traditional Si-based technology. A recent study suggests that a thin and highly crystalline Te layer shows promise as a channel for high-performance pTFTs. However, achieving this still requires specific conditions, such as a cryogenic growth temperature and an extremely thin channel thickness on the order of a few nanometers. These conditions critically limit the practical feasibility of the fabrication process. Here, we report a high-performance pTFT incorporating a 60-nm-thick highly crystalline Se-Te alloyed channel layer, produced using pulsed laser ablation at room temperature. The Se0.5Te0.5 alloy system enhances crystalline temperature and widens the band gap compared to a pure Te channel. Consequently, this approach results in a field-effect mobility of 3 cm2/V·s, with an on/off current ratio of 3 × 105, a subthreshold slope of 2.1 V/decade, and a turn-on voltage of 6.5 V, achieved through conventional annealing at 250 °C. To demonstrate its applicability in complementary circuit applications, we integrate a complementary-type inverter using a p-type Se0.5Te0.5 TFT and an n-type Al-doped InZnSnO, demonstrating a high voltage gain of 12 and a low static power consumption of 17 nW. This suggests that the Se-Te alloyed channel approach paves the way to a more straightforward and cost-effective process for Te-based pTFT devices and their applications.

Ferroelectric Hafnia-Based M3D FeTFTs Annealed at Extremely Low Temperatures and TCAM Cells for Computing-in-Memory Applications.

In order to overcome the bottleneck between the central processor unit and memory as well as the issue of energy consumption, computing-in-memory (CIM) is becoming more popular as an alternative to the traditional von Neumann structure. However, as artificial intelligence advances, the networks require CIM devices to store billions of parameters in order to handle huge data traffic demands. Monolithic three-dimensional (M3D) stacked ferroelectric thin-film transistors (FeTFTs) are one of the promising techniques for realizing high-density CIM devices that can store billions of parameters. In particular, oxide channel-based FeTFTs are well suited for these applications due to low-temperature processes, nonvolatility, and 3D integration capability. Nevertheless, the M3D-integrated CIM devices including hafnia ferroelectric films need the high-temperature annealing process to crystallize the ferroelectric layer, making M3D integration difficult. When the FeTFTs are fabricated with an M3D structure, the high-temperature process causes thermal issues in the underlying devices. Here, we present the focused microwave annealed (FMA) oxide FeTFTs with M3D integration at a low temperature of 250 °C. We confirmed that the FeTFTs with metal-ferroelectric-metal-insulator-semiconductor structure exhibited a large memory window of 3.2 V, good endurance over 106 cycles, and a long retention time of 105 s. To understand the different electrical characteristics of FeTFTs in the top and bottom layers, we experimentally analyzed the density of the state of the oxide channel and ferroelectric properties of the ferroelectric gate insulator by using multifrequency capacitance-voltage measurement and nucleation-limited-switching model analysis, respectively. With our approach, we demonstrate for the first time a vertical stacked FeTFTs-based ternary-content-addressable memory (TCAM) cell for CIM application. We believe that the proposed M3D-stacked TCAM cells composed of FeTFTs can be used in high-density memory, energy-efficient memory, and CIM technology.

Effect of hydrogen diffusion in an In-Ga-Zn-O thin film transistor with an aluminum oxide gate insulator on its electrical properties.

We fabricated amorphous InGaZnO thin film transistors (a-IGZO TFTs) with aluminum oxide (Al2O3) as a gate insulator grown through atomic layer deposition (ALD) method at different deposition temperatures (T dep). The Al2O3 gate insulator with a low T dep exhibited a high amount of hydrogen in the film, and the relationship between the hydrogen content and the electrical properties of the TFTs was investigated. The device with the Al2O3 gate insulator having a high H content showed much better transfer parameters and reliabilities than the low H sample. This is attributed to the defect passivation effect of H in the active layer, which is diffused from the Al2O3 layer. In addition, according to the post-annealing temperature (T post-ann), a-IGZO TFTs exhibited two unique changes of properties; the degradation in low T post-ann and the enhancement in high T post-ann, as explained in terms of H diffusion from the gate insulator to an active layer.

Impact of transient currents caused by alternating drain stress in oxide semiconductors.

Reliability issues associated with driving metal-oxide semiconductor thin film transistors (TFTs), which may arise from various sequential drain/gate pulse voltage stresses and/or certain environmental parameters, have not received much attention due to the competing desire to characterise the shift in the transistor characteristics caused by gate charging. In this paper, we report on the reliability of these devices under AC bias stress conditions because this is one of the major sources of failure. In our analysis, we investigate the effects of the driving frequency, pulse shape, strength of the applied electric field, and channel current, and the results are compared with those from a general reliability test in which the devices were subjected to negative/positive bias, temperature, and illumination stresses, which are known to cause the most stress to oxide semiconductor TFTs. We also report on the key factors that affect the sub-gap defect states, and suggest a possible origin of the current degradation observed with an AC drive. Circuit designers should apply a similar discovery and analysis method to ensure the reliable design of integrated circuits with oxide semiconductor devices, such as the gate driver circuits used in display devices.

Defects and Charge-Trapping Mechanisms of Double-Active-Layer In-Zn-O and Al-Sn-Zn-In-O Thin-Film Transistors.

Active matrix organic light-emitting diodes (AMOLEDs) are considered to be a core component of next-generation display technology, which can be used for wearable and flexible devices. Reliable thin-film transistors (TFTs) with high mobility are required to drive AMOLEDs. Recently, amorphous oxide TFTs, due to their high mobility, have been considered as excellent substitutes for driving AMOLEDs. However, the device instabilities of high-mobility oxide TFTs have remained a key issue to be used in production. In this paper, we present the charge-trapping and device instability mechanisms of high-mobility oxide TFTs with double active layers, using In-Zn-O (IZO) and Al-doped Sn-Zn-In-O (ATZIO) with various interfacial IZO thicknesses (0-6 nm). To this end, we employed microsecond fast current-voltage (I-V), single-pulsed I-V, transient current, and discharge current analysis. These alternating-current device characterization methodologies enable the extraction of various trap parameters and defect densities as well as the understanding of dynamic charge transport in double-active-layer TFTs. The results show that the number of defect sites decreases with an increase in the interfacial IZO thickness. From these results, we conclude that the interfacial IZO layer plays a crucial role in minimizing charge trapping in ATZIO TFTs.

Hydrated alizarin complexes: hydrogen bonding and proton transfer.

We investigated the hydrogen bonding structures and proton transfer for the hydration complexes of alizarin (Az) produced in a supersonic jet using fluorescence excitation (FE), dispersed laser induced fluorescence (LIF), visible-visible hole burning (HB), and fluorescence detected infrared (FDIR) spectroscopy. The FDIR spectrum of bare Az with two O-H groups exhibits two vibrational bands at 3092 and 3579 cm(-1), which, respectively, correspond to the stretching vibration of O1-H1 that forms a strong intramolecular hydrogen bond with the C9=O9 carbonyl group and the stretching vibration of O2-H2 that is weakly hydrogen-bonded to O1-H1. For the 1:1 hydration complex Az(H(2)O)(1), we identified three conformers. In the most stable conformer, the water molecule forms hydrogen bonds with the O1-H1 and O2-H2 groups of Az as a proton donor and proton acceptor, respectively. In the other conformers, the water binds to the C10=O10 group in two nearly isoenergetic configurations. In contrast to the sharp vibronic peaks in the FE spectra of Az and Az(H(2)O)(1), only broad, structureless absorption was observed for Az(H(2)O)(n) (n≥ 2), indicating a facile decay process, possibly due to proton transfer in the electronic excited state. The FDIR spectrum with the wavelength of the probe laser fixed at the broad band exhibited a broad vibrational band near the O2-H2 stretching vibration frequency of the most stable conformer of Az(H(2)O)(1). With the help of theoretical calculations, we suggest that the broad vibrational band may represent the occurrence of proton transfer by tunnelling in the electronic ground state of Az(H(2)O)(n) (n≥ 2) upon excitation of the O2-H2 vibration.

Laser induced fluorescence and resonant two-photon ionization spectroscopy of jet-cooled 1-hydroxy-9,10-anthraquinone.

We carried out laser induced fluorescence and resonance enhanced two-color two-photon ionization spectroscopy of jet-cooled 1-hydroxy-9,10-anthraquinone (1-HAQ). The 0-0 band transition to the lowest electronically excited state was found to be at 461.98 nm (21,646 cm(-1)). A well-resolved vibronic structure was observed up to 1100 cm(-1) above the 0-0 band, followed by a rather broad absorption band in the higher frequency region. Dispersed fluorescence spectra were also obtained. Single vibronic level emissions from the 0-0 band showed Stokes-shifted emission spectra. The peak at 2940 cm(-1) to the red of the origin in the emission spectra was assigned as the OH stretching vibration in the ground state, whose combination bands with the C=O bending and stretching vibrations were also seen in the emission spectra. In contrast to the excitation spectrum, no significant vibronic activity was found for low frequency fundamental vibrations of the ground state in the emission spectrum. The spectral features of the fluorescence excitation and emission spectra indicate that a significant change takes place in the intramolecular hydrogen bonding structure upon transition to the excited state, such as often seen in the excited state proton (or hydrogen) transfer. We suggest that the electronically excited state of interest has a double minimum potential of the 9,10-quinone and the 1,10-quinone forms, the latter of which, the proton-transferred form of 1-HAQ, is lower in energy. On the other hand, ab initio calculations at the B3LYP/6-31G(d,p) level predicted that the electronic ground state has a single minimum potential distorted along the reaction coordinate of tautomerization. The 9,10-quinone form of 1-HAQ is the lowest energy structure in the ground state, with the 1,10-quinone form lying approximately 5000 cm(-1) above it. The intramolecular hydrogen bond of the 9,10-quinone was found to be unusually strong, with an estimated bond energy of approximately 13 kcal/mol (approximately 4500 cm(-1)), probably due to the resonance-assisted nature of the hydrogen bonding involved.

Infrared-visible and visible-visible double resonance spectroscopy of 1-hydroxy-9,10-anthraquinone-(H2O)n (n=1,2) complexes.

The structures of hydrated 1-hydroxyanthraquinone complexes (1-HAQ), 1-HAQ(H2O)n=1,2, with intramolecular and intermolecular hydrogen bonding interactions were studied using laser spectroscopic methods such as laser induced fluorescence, fluorescence-detected infrared, infrared-visible hole burning, and visible-visible hole burning spectroscopy. In the 1:1 complex 1-HAQ(H2O)1, the water binds to the free carbonyl group of 1-HAQ not associated with intramolecular hydrogen bond. The second water in the 1:2 complex, 1-HAQ(H2O)2, binds to the first water of the 1:1 complex rather than other hydrogen bonding sites of 1-HAQ. A pair of two geometric isomers was produced in a supersonic jet for each of the 1:1 and 1:2 complexes. Both isomers of each complex have the same vibrational spectra in the region of the OH stretching vibration of water, but have different energies for the 0-0 band of vibronic transition due to the asymmetry of the two phenyl rings in 1-HAQ. The 0-0 bands for all four species of 1-HAQ(H2O)n=1,2 were unambiguously assigned by comparing with the results of ab initio calculations, which yielded the structures, vibrational frequencies, and relative energies of the frontier molecular orbitals.