Dielectric Hafnium Oxide Research Articles

Large‐Area Electrode Deposition and Patterning for Monolayer Organic Field‐Effect Transistors by Vacuum‐Filtrated MXene

AbstractHigh‐quality organic field‐effect transistors (1L‐OFETs) based on monolayers have made significant progress and are expected to be key components in the development of next‐generation flexible electronics. However, a flexible, low‐cost, damage‐free, and metallic conductance electrode that can accurately demonstrate the exceptional electrical properties of 1L‐OFETs is still in high demand. In this study, the vacuum‐filtrated MXene (Ti3C2Tx) is demonstrated to serve as electrodes without causing chemical or thermal damage to the delicate active layer via a dry‐lithography method. By integrating monolayer 2,9‐didecyldinaphtho[2,3‐b:2,3′‐f]thieno[3,2‐b]thiophene (C10‐DNTT) with MXene, the 1L‐OFETs exhibit a low subthreshold swing of 60.7 mV per decade and high field‐effect mobility of 9.5 cm2 V−1 s−1 on a high‐κ dielectric hafnium oxide. The use of MXene electrodes enables the production of solution‐processed conductors that can achieve uncompromised performance compared to metal contacts. Furthermore, owing to the well‐matched work functions, the contact resistance can be reduced to 165 Ω cm by this printing technique. The 1L‐OFETs fabricated on an ultra‐thin conformal parylene substrate also exhibit uniform electrical properties. It is believed that this processing approach of vacuum‐filtrated MXene conductors is a crucial step toward the application of non‐metal contacts for high‐performance flexible electronics.

Low-voltage polymer transistors on hydrophobic dielectrics and surfaces

A set of unique features, including large-area solution processing on flexible and stretchable substrates, make polymer semiconductors a promising material choice for a range of state-of-the-art applications in electronics, optoelectronics and sensing. Yet, an inherent weakness of polymer semiconductors remains their low dielectric constants, increasing their susceptibility toward unscreened dipoles. These dipoles are particularly prevalent at polymer-dielectric interfaces with high-k dielectrics, which are essential for the operation of devices such as low-voltage field-effect transistors. This shortcoming can be addressed by using self-assembled monolayers (SAMs) to passivate surfaces that impact charge transport. However, SAM-treatment also increases the hydrophobicity of surfaces and therefore poses a challenge for subsequent solution processing steps and complex packaging of devices. Here, we report low-voltage polymer transistors processed by spin coating of the polymer semiconductors on highly hydrophobic SAM-treated aluminum and hafnium oxide dielectrics (contact angles >100) through fine-tuning of the interfacial tension at the polymer-dielectric interface. This approach enables the processing and detailed characterization of near-amorphous (indacenodithiophene-cobenzothiadiazole) as well as semicrystalline (poly(2,5-bis(2-octyldodecyl)-3,6-di(thiophen-2-yl)diketopyrrolo[3,4-c]pyrrole-1,4-dione-alt-thieno[3,2-b]thiophen)) polymer semiconductors. We demonstrate polymer transistors that exhibit high on-currents and field-independent, charge carrier mobilities of 0.8 cm2 V−1s−1 at low operating voltages (<3 V).

Induction and suppression of cell lysis in an electrokinetic microfluidic system.

The ability to strategically induce or suppress cell lysis is critical for many cellular-level diagnostic and therapeutic applications conducted within electrokinetic microfluidic platforms. The chemical and structural integrity of sub-cellular components is important when inducing cell lysis. However, metal electrodes and electrolytes participate in undesirable electrochemical reactions that alter solution composition and potentially damage protein, RNA, and DNA integrity within device microenvironments. For many biomedical applications, cell viability must be maintained even when device-imposed cell-stressing stimuli (e.g., electrochemical reaction byproducts) are present. In this work, we explored a novel and tunable method to accurately induce or suppress device-imposed artifacts on human red blood cell (RBC) lysis in non-uniform AC electric fields. For precise tunability, a dielectric hafnium oxide (HfO2 ) layer was used to prevent electron transfer between the electrodes and the electric double layer and thus reduce harmful electrochemical reactions. Additionally, a low concentration of Triton X-100 surfactant was explored as a tool to stabilize cell membrane integrity. The extent of hemolysis was studied as a function of time, electrode configuration (T-shaped and star-shaped), cell position, applied non-uniform AC electric field, with uncoated and HfO2 coated electrodes (50nm), and absence and presence of Triton X-100 (70µM). Tangible outcomes include a parametric analysis relying upon literature and this work to design, tune, and operate electrokinetic microdevices to intentionally induce or suppress cellular lysis without altering intracellular components. Implications are that devices can be engineered to leverage or minimize device-imposed biological artefacts extending the versatility and utility of electrokinetic diagnostics.

A proton-selective membrane (PSM)-deposited dual-gate ion-sensitive field-effect transistor (DG-ISFET) integrating a microchamber-embedded filter membrane for bacterial enrichment and antimicrobial susceptibility test

The antimicrobial susceptibility test (AST) is one of the most common methods to evaluate bloodstream infections (BSI). Currently, AST methods usually required prolonged sample preparation processes and bulky systems, making it difficult to provide timely information for clinical decisions. In this paper, we integrated a proton-selective membrane (PSM)-deposited dual-gate ion-sensitive field-effect transistor (DG-ISFET) with a microchamber-embedded filter membrane to perform bacterial enrichment and incubation followed by on-chip AST. The DG-ISFET sensor consists of a high-k dielectric hafnium oxide (HfO2) sensing surface and dual gate, enabling high ion sensitivity, and low drifting and hysteresis issues. By depositing a PSM on DG-ISFET, the sensor has higher proton sensitivity and selectivity, preventing interference from other ion signals in the bacterial culture medium. To further shorten the bacterial culture time, we integrated a microchamber-embedded filter membrane on top of the DG-ISFET sensor to enrich the amount of bacteria and place seeded bacteria close to DG-ISFET’s sensing surface. As a proof of concept, we first evaluated the proton sensitivity and selectivity of DG-ISFET under various PSM deposition thicknesses. Then, we demonstrated that the PSM-deposited DG-ISFET system can successfully distinguish kanamycin-susceptible and resistant Escherichia coli strains treated with various kanamycin concentrations in 30 min based on the continuous drain-source current (Ids) pattern. We envision that this fully integrated system can perform rapid, real-time, continuous AST, which can potentially be applied in resource-limited environments or point-of-care settings.

Compact device modelling of interface trap charges with quantum capacitance in MoS2-based field-effect transistors

An effective way to obtain interface trap density in transition metal dichalcogenide field-effect transistors (FETs) via compact device modelling is presented in this study. A computationally efficient model is utilised to evaluate the interface trap charges in a MoS2-based FET device. This model improves the accuracy of the computed surface potential, which is affected by trap charges. The existence of trap states on the interface level can be confirmed by studying the capacitance versus gate voltage () relationship. Most of the previously proposed models ignore the effect of the quantum capacitance when predicting the electrical performance of MoS2-FETs. The electrical performance of a metal-oxide-semiconductor FET with a two-dimensional (2D) molybdenum disulphide (MoS2) channel that introduces a quantum capacitance by including a gate is evaluated. The gate quantum capacitance in parallel with the interface trap capacitance is a second capacitance in series with the gate oxide capacitance and MoS2 capacitance This research explores the process of evaluating the interface trap density from published experimental data of MoS2-based FETs. Using the evaluated trap density values, the device parameters are calculated by considering the relative permittivity of the dielectric hafnium oxide (HfO2) layer, surface potential, interface trap charge and interface trap capacitance Finally, the calculated and experimental data are compared through the normalised root-mean-square deviations (RMSD) to validate the accuracy of the model.

Influence of high energy ion irradiation on structural, morphological and optical properties of high-k dielectric hafnium oxide (HfO2) thin films grown by atomic layer deposition

Abstract Modifications and resulting changes in various properties of nanocrystalline HfO2 high-k dielectric thin films with nominal thickness of 20 nm grown by atomic layer deposition technique on silicon and glass substrates were investigated as a function of swift heavy ions (SHI) irradiation. In the present work, our attention is to study the electronic excitation induced modifications in HfO2 thin films cause of formation of defects by 120 MeV Au9+(SHI) irradiation. For the same purpose, the annealed (500 °C) HfO2 thin films were irradiated with the varying fluence in the range of 1E11 to 1E13 ions/cm2. The virgin and irradiated representative HfO2 thin films were investigated by various techniques. The substantial root mean square (RMS) surface roughness (0.5–1 nm) and variation in grain size (40–52 nm) as a function of Au9+ ions irradiation were determined using atomic force microscope (AFM). The optical properties of HfO2 thin films were analyzed by measuring absorbance and transmission (54–62%) spectra in 200–700 nm wavelength range followed by calculation of optical band gap (Eg). The photoluminescence (PL) spectra obtained at excitation wavelength 220 nm indicate the formation of defects due to SHI irradiation. A change in excitation wavelength (220–260 nm) causes the shift in emission peaks towards higher wavelength. Structural investigations have been carried by Grazing Incidence X-ray Diffraction (GIXRD) techniques which reveal the variation in crystallite size as a function of different fluence. The results of a systematic XPS study of virgin and SHI irradiated samples shows the shifting of Hf 4f and O 1s peaks towards lower binding energy as compared to virgin sample. O K-edge XANES were performed to understand the ion-beam irradiation induced defect formation and their consequences on the electronic structure of HfO2 thin films. The obtained peaks in Rutherford backscattering spectrometry (RBS) affirm the existence of Hf and O elements and width of the peaks determine the sample thickness (20 nm).

Uniform and ultrathin high-κ gate dielectrics for two-dimensional electronic devices

Two-dimensional semiconductors could be used as a channel material in low-power transistors, but the deposition of high-quality, ultrathin high-κ dielectrics on such materials has proved challenging. In particular, atomic layer deposition typically leads to non-uniform nucleation and island formation, creating a porous dielectric layer that suffers from current leakage, particularly when the equivalent oxide thickness is small. Here, we report the atomic layer deposition of high-κ gate dielectrics on two-dimensional semiconductors using a monolayer molecular crystal as a seeding layer. The approach can be used to grow dielectrics with an equivalent oxide thickness of 1 nm on graphene, molybdenum disulfide (MoS2) and tungsten diselenide (WSe2). Compared with dielectrics created using established methods, our dielectrics exhibit a reduced roughness, density of interface states and leakage current, as well as an improved breakdown field. With the technique, we fabricate graphene radio-frequency transistors that operate at 60 GHz, and MoS2 and WSe2 complementary metal–oxide–semiconductor transistors with a supply voltage of 0.8 V and subthreshold swing down to 60 mV dec−1. We also create MoS2 transistors with a channel length of 20 nm, which exhibit an on/off ratio of over 107. Using a monolayer molecular crystal as a seeding layer, hafnium oxide dielectrics with an equivalent oxide thickness of only 1 nm can be deposited on graphene, molybdenum disulfide and tungsten diselenide.

Nanometre-thin indium tin oxide for advanced high-performance electronics.

Although indium tin oxide (ITO) is widely used in optoelectronics due to its high optical transmittance and electrical conductivity, its degenerate doping limits exploitation as a semiconduction material. In this work, we created short-channel active transistors based on an ultra-thin (down to 4 nm) ITO channel and a high-quality, lanthanum-doped hafnium oxide dielectric of equivalent oxide thickness of 0.8 nm, with performance comparative to that of existing metal oxides and emerging two-dimensional materials. Short-channel immunity, with a subthreshold slope of 66 mV per decade, off-state current <100 fA μm-1 and on/off ratio up to 5.5 × 109, was measured for a 40-nm transistor. Logic inverters working in the subthreshold regime exhibit a high gain of 178 at a low-supply voltage of 0.5 V. Moreover, radiofrequency transistors, with as-measured cut-off frequency fT and maximum oscillation frequency fmax both >10 GHz, have been demonstrated. The unique wide bandgap and low dielectric constant of ITO provide prospects for future scaling below the 5-nm regime for advanced low-power electronics.

UV-Cured Hafnium Oxide-Based Gate Dielectrics for Low-Voltage Organic and Amorphous Oxide Thin-Film Transistors.

In this study, we have fabricated hafnium oxide dielectrics for low-voltage organic and amorphous oxide thin-film transistors (TFTs) via a facile low-temperature solution method and investigated the electrical properties of these dielectrics. Hafnium oxide dielectric films can be easily fabricated by a sol-gel solution method and ultraviolet (UV) curing at room temperature. In addition, the surface energy of hafnium oxide films can be easily modified by using phosphonic-acid-based self-assembled monolayers. This modification makes these films compatible with organic semiconductors fabrication methods. These novel dielectrics exhibit excellent insulating properties (leakage current densities of <10-6 A/cm² at 2 V), high capacitances (up to 690 nF/cm²), and smooth surfaces (root-mean-square roughness <0.5 nm). Consequently, hafnium oxide-based dielectrics can be integrated into both pentacene-based and indium oxide-based TFTs, functioning at relatively low voltages (<±3 V) to achieve good performance (hole mobility: 0.31 cm²/V · s, electron mobility: 1.54 cm²/V · s, and on/off current ratios: >105).

Transport in InAs/GaSb quantum spin Hall insulators with high-k dielectrics as the top barrier

We have studied the low temperature transport properties of devices made of InAs/GaSb quantum wells (QWs), where the molecular beam epitaxy (MBE)-grown top AlGaSb barrier was selectively removed and subsequently replaced by a high-k dielectric hafnium oxide layer. From the perspective of quantum spin Hall insulators, the main transport properties of the InAs/GaSb QWs with a hafnium oxide top barrier are similar to those of the InAs/GaSb QWs with MBE-grown barriers. Specifically, the band inversion could be retained after such barrier substitution, and the degree of band inversion could still be tuned by gates. Furthermore, when the Fermi level is tuned into the bulk gap, the bulk conductance vanishes at low temperatures and the transport of the device is dominated by the edge states.

Electronic Transport in Bilayer MoS2 Encapsulated in HfO2.

The exact nature of the interface between a two-dimensional crystal and its environment can have a significant impact on the electronic transport within the crystal, and can place fundamental limitations on transistor performance and long-term functionality. Two-dimensional transition-metal dichalcogenides are a new class of transistor channel material with electronic properties that can be tailored through dielectric engineering of the material/environmental interface. Here, we report electrical transport measurements carried out in the insulating regime of bilayer molybdenum disulfide, which has been encapsulated within a high-κ hafnium oxide dielectric. Temperature- and carrier-density-dependent measurements show that for T < 130 K the transport is governed by resonant tunneling, and at T = 4.2 K the tunneling peak lineshape is well-fitted by a Lorentzian with an amplitude less than e2/h. Estimates of tunneling time give τ ∼ 1.2 ps corresponding to a frequency f ∼ 0.84 THz. The tunneling processes are observable up to T ∼ 190 K (more than a factor of 6 higher than that previously reported for MoS2 on SiO2) despite the onset of variable range hopping at T ∼ 130 K, demonstrating the coexistence of the two transport processes within the same temperature range. At constant temperature, varying the Fermi energy allows experimental access to each transport process. The results are interpreted in terms of an increase in charge carrier screening length and a decrease in electron-phonon coupling induced by the hafnium oxide. Our results represent the first demonstration of the intermediate tunneling-hopping transport regime in a two-dimensional material. The results suggest that interface engineering may be a macroscopic tool for controlling quantum transport within such materials as well as for increasing the operating temperatures for resonant-tunneling devices derived from such materials, with applications in high-frequency electronics and logic devices.

Solid-State Electrolyte-Gated Graphene in Optical Modulators.

The gate-tunable wide-band absorption of graphene makes it suitable for light modulation from terahertz to visible light. The realization of graphene-based modulators, however, faces challenges connected with graphene's low absorption and the high electric fields necessary to change graphene's optical conductivity. Here, a solid-state supercapacitor effect with the high-k dielectric hafnium oxide is demonstrated that allows modulation from the near-infrared to shorter wavelengths close to the visible spectrum with remarkably low voltages (≈3 V). The electroabsorption modulators are based on a Fabry-Perot-resonator geometry that allows modulation depths over 30% for free-space beams.

High mobility thin-film transistor with solution-processed hafnium-oxide dielectric and zinc-indium-tin-oxide semiconductor

In this paper, bottom-gate-top-contact structured thin-film transistors (TFTs) are fabricated by solution-processing of hafnium oxide (HfO2) dielectrics and zinc-indium-tin-oxide (ZITO) semiconductors. Solution-processed HfO2 films are annealed at different temperatures, and the 500℃ annealed HfO2 dielectrics can exhibit optimizing film properties such as smooth surfaces (the RMS value of HfO2 films is less than 1 nm), low leakage current density (1.25×10-7 A/cm2 at 1 MV/cm), high transmittance (above 80% at the wavelength ranging from 400 to 800 nm) and high relative dielectric constant (about 12). The smooth surface of HfO2 dielectrics is attributed to the decreased charge trapping states at the interface between the HfO2 dielectrics and ZITO semiconductors, and thus improves the device electrical performance and stability. Hence, TFT devices of HfO2 dielectrics annealed at 500℃ show a high saturated field effect mobility of more than 100 cm2·V-1·s-1 a low threshold voltage of -0.5 V, an on-to-off current ratio of 5×106 and a small subthreshold swing of 105 mV/dec. An almost negligible threshold voltage shift is observed under a positive bias stress for 1000 s, indicating the excellent stability of HfO2 TFT devices.

Impact of different dopants on the switching properties of ferroelectric hafniumoxide

The wake-up behavior of ferroelectric thin film capacitors based on doped hafnium oxide dielectrics in TiN-based metal–insulator–metal structures is reported. After field cycling a remanent polarization up to 40 µC/cm2 and a high coercive field of about 1 MV/cm was observed. Doping of HfO2 by different dopants with a crystal radius ranging from 54 pm (Si) to 132 pm (Sr) was evaluated. In all cases, an improved polarization–voltage hysteresis after wake-up cycling is visible. For smaller dopant atoms like Si and Al stronger pinching of the polarization hysteresis appeared with increasing dopant concentration and proved to be stable during cycling.

(Invited) Hafnium Oxide Based CMOS Compatible Ferroelectric Materials

The ferroelectric behavior of capacitors based on hafnium oxide dielectrics will be reported. Thin films of 6-30 nm thickness were found to exhibit ferroelectric polarization-voltage hysteresis loops when integrated into TiN-based metal-insulator-metal capacitors. A remanent polarization up to 25 µC/cm2 and a high coercive field of about 1 MV/cm was observed. Doping of HfO2 by different dopants with an atomic radius ranging from 110 nm (Si) to 188 nm (Gd) was evaluated and in all cases ferroelectric behavior was verified by P-V hysteresis, small signal capacitance-voltage, and for Si by piezoelectric measurements. Polarization retention showed no significant decay within a measurement range of up to two days.

Hafnium Oxide Based CMOS Compatible Ferroelectric Materials

The ferroelectric behavior of capacitors based on hafnium oxide dielectrics will be reported. Thin films of 7–30 nm thickness were found to exhibit ferroelectric polarization-voltage hysteresis loops when integrated into TiN-based metal-insulator-metal capacitors. A remanent polarization up to 25 μC/cm2 and a high coercive field of about 1 MV/cm was observed. Doping of HfO2 by different dopants with an atomic radius ranging from 110 pm (Si) to 188 pm (Gd) was evaluated and in all cases ferroelectric behavior was verified by polarization-voltage hysteresis, small signal capacitance-voltage characteristics, and for Si as a dopant also by piezoelectric measurements. Polarization retention and endurance was characterized on capacitor and transistor structures.

EFFECT OF GATE ELECTRODES ON STRUCTURE AND ELECTRICAL PROPERTIES OF SPUTTERED HfO2 THIN FILMS

High dielectric hafnium oxide films were grown by magnetron sputtering and post heat treatment in nitrogen atmosphere at 500°C for 30 min using vacuum annealing furnace. The film keeps amorphous at 500°C and has better interface quality as revealed by X-ray diffraction and transmission electron microscopy. The influence of TiN and Pt electrodes on the electrical property of the film was compared. For the annealed films, TiN electrode was presented as the optimal one. For the conduction mechanism of Pt / HfO 2/p- Si MOS capacitors under gate electron injection, the dominant conduction mechanism at low electric field is Schottky emission. At high electric field, the conversion of current transport mechanism from Schottky emission to trap-assisted tunneling for the annealed HfO 2 film occurs at 0.64 MV/cm.

Silicon nanowires with high-k hafnium oxide dielectrics for sensitive detection of small nucleic acid oligomers.

Nanobiosensors based on silicon nanowire field effect transistors offer advantages of low cost, label-free detection, and potential for massive parallelization. As a result, these sensors have often been suggested as an attractive option for applications in point-of-care (POC) medical diagnostics. Unfortunately, a number of performance issues, such as gate leakage and current instability due to fluid contact, have prevented widespread adoption of the technology for routine use. High-k dielectrics, such as hafnium oxide (HfO(2)), have the known ability to address these challenges by passivating the exposed surfaces against destabilizing concerns of ion transport. With these fundamental stability issues addressed, a promising target for POC diagnostics and SiNWFETs has been small oligonucleotides, more specifically, microRNA (miRNA). MicroRNAs are small RNA oligonucleotides which bind to mRNAs, causing translational repression of proteins, gene silencing, and expressions are typically altered in several forms of cancer. In this paper, we describe a process for fabricating stable HfO(2) dielectric-based silicon nanowires for biosensing applications. Here we demonstrate sensing of single-stranded DNA analogues to their microRNA cousins using miR-10b and miR-21 as templates, both known to be upregulated in breast cancer. We characterize the effect of surface functionalization on device performance using the miR-10b DNA analogue as the target sequence and different molecular weight poly-l-lysine as the functionalization layer. By optimizing the surface functionalization and fabrication protocol, we were able to achieve <100 fM detection levels of the miR-10b DNA analogue, with a theoretical limit of detection of 1 fM. Moreover, the noncomplementary DNA target strand, based on miR-21, showed very little response, indicating a highly sensitive and highly selective biosensing platform.

Dominant Device Instability Mechanism in Scaled Metal–Oxide–Semiconductor Field-Effect Transistors with Hafnium Oxide Dielectric

The negative-bias temperature instability (NBTI) and positive-bias temperature instability (PBTI) of metal–oxide–semiconductor field-effect transistors (MOSFETs) with high dielectric constant (k) gate dielectrics have been studied by varying the physical thickness of high-k dielectric. Both PBTI and NBTI were reduced with decrease in the high-k dielectric thickness, but NBTI reduction saturated in the thin high-k region. Since residual NBTI is sufficiently large to cause device instability, NBTI is still a dominant reliability concern in scaled MOSFETs.

Characteristics and Reliability of Hafnium Oxide Dielectric Stacks With Room Temperature Grown Interfacial Anodic Oxide

Hafnium oxide dielectric stacks with anodic oxide interfacial layer (ANO-IL) were investigated under low-temperature consideration. A tilted-substrate sputtering technique, which provides various film thicknesses in one processing step, was proposed and useful for the characterization of charge distribution. It was found that charges existed in the HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /ANO-IL were smaller than that in HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /rapid-thermal-oxidation IL. The prepared samples exhibit good electrical characteristics, including small electrical hysteresis (< 10 mV), low leakage current, high effective dielectric breakdown field of 12.7 MV/cm, and maximum operating voltages of -2.74 V at 25degC and -2.32 V at 125degC for EOT = 2.3 nm stacks under a ten-year lifetime evaluation. The results suggest that the quality of IL in the dielectric stack is a critical reliability issue and that ANO is provided as a candidate for IL consideration of low-temperature dielectric stacks.