Carrier Conduction Research Articles

Impact of Al2O3 spacers on the improvement in short-channel effects for the mesa-shaped vertical-channel In-Ga-Zn-O thin-film transistors with a channel length below 100 nm

To achieve a channel length scaling and an improvement in short-channel effect (SCE) for the mesa-shaped vertical thin-film transistors (VTFTs) using In-Ga-Zn-O (IGZO) channels prepared by atomic layer deposition (ALD), the impacts of two strategic approaches were investigated: the introduction of an ALD Al2O3 spacer and the modulation of ALD oxidants. The Al2O3 spacers were patterned with sound profiles by means of a hybrid-etching technique combining a wet- and plasma dry-etching processes to form vertical sidewalls corresponding to a channel length. For the cases when the Al2O3 spacers were prepared with H2O oxidant, the IGZO VTFT with an In/Ga ratio of 1.7 showed the degradations in device characteristics including a turn-on voltage (VON) of −8.0 V and a subthreshold swing (SS) of 1.50 V/dec. When the In/Ga ratio was increased to 2.9, the device failed to be operated with conductive properties. The excess hydrogen contained within the Al2O3 spacer was suggested to be thermally diffused into the IGZO from the back-channel interface, leading to marked increase in the number of conduction carriers within the channel. To suppress the doping effect of hydrogen from the spacer, the ALD oxidant was strategically modulated to O3. As results, the VON and SS of the IGZO VTFT were markedly improved to be −2.0 V and 0.483 V/dec, respectively. Additionally, the drain induced barrier lowering coefficient was also significantly enhanced from 2.9 to 0.13 V/V, demonstrating the excellent immunity against the SCE for the IGZO VTFTs with nanoscale channel lengths. It was concluded from these findings that the spacer engineering can be one of the most important guides for the implementation of IGZO VTFTs with channel lengths below 100 nm.

Synthesis and characterization of TiO2-graphene heterostructured quantum dots by electrochemical process

Titanium dioxide (TiO2) is a trendily attractive semiconductor owing to its optical and electrical properties within the UV-visible range. Nevertheless, limitations such as instability in the UV range and rapid recombination of photogenerated charge carriers leading to poor efficiency in photoactivity are major limiting factors that hold back the application of the materials on large scale. In order to tackle the stated issues, graphene quantum dots (GQDs), with their high carrier conductivity, UV-stability and thermal conductivity, can be used to improve TiO2’s flaws as optoelectronic materials. As a suitable edge transporter and modifier, GQDs can be grown as heterostructure well-bonded with TiO2 quantum dots. This material modification can lead to combined benefits of using TiO2 QDs’ preferable optoelectronic properties in UV-visible range and GQDs’ high conductivity and stability. This allows the synthesis of heterostructured quantum dots that can be tailored towards wide range of applications such as electron transport layer materials in perovskite solar cells or highly stable colloidal-based photocatalysts. In this study, TiO2/GQDs nanocomposites were synthesized using a solution-based electrochemical process with citric acid and potassium chloride (KCl) acting as electrolytes. Our proposed recipe yielded heterostructured QDs with average size of 3.44 nm well below the exciton Bohr radius of both TiO2 and graphene and hydrodynamic sizes around 45-59 nm with zeta potential in range of -24 to 30 mV. This yielded colloidal solution with stability and surface ion tunability based on KCl concentration with 0.3 M being the most stable recipe. Additionally, absorption band edge can also be tuned within the range of 2.8-3.9 eV which can be adjusted with concentration of KCl based on the desired applications.

Two-dimensional heavy fermion in a monoatomic-layer Kondo lattice YbCu2

The Kondo effect between localized f-electrons and conductive carriers leads to exotic physical phenomena. Among them, heavy-fermion (HF) systems, in which massive effective carriers appear due to the Kondo effect, have fascinated many researchers. Dimensionality is also an important characteristic of the HF system, especially because it is strongly related to quantum criticality. However, the realization of the perfect two-dimensional (2D) HF materials is still a challenging topic. Here, we report the surface electronic structure of the monoatomic-layer Kondo lattice YbCu2 on a Cu(111) surface observed by synchrotron-based angle-resolved photoemission spectroscopy. The 2D conducting band and the Yb 4f state, located very close to the Fermi level, are observed. These bands are hybridized at low-temperature, forming the 2D HF state, with an evaluated coherence temperature of about 30 K. The effective mass of the 2D state is enhanced by a factor of 100 by the development of the HF state. Furthermore, clear evidence of the hybridization gap formation in the temperature dependence of the Kondo-resonance peak has been observed below the coherence temperature. Our study provides a new candidate as an ideal 2D HF material for understanding the Kondo effect at low dimensions.

Optical, Dielectric, and Electrical Properties of Tungsten-Based Materials with the Formula Li(2−x)NaxWO4 (x = 0, 0.5, and 1.5)

In the present study, three chemical compounds, Li2WO4, Li0.5Na1.5WO4, and Li1.5Na0.5WO4, are produced using the solid–solid method. Unlike the compound Li0.5Na1.5WO4, which crystallizes in the orthorhombic system with the space group Pmmm, both compounds Li2WO4 and Li1.5Na0.5WO4 crystallize in the monoclinic system with the space group P2/m. A morphological analysis reveals that all three compounds have a compact structure with some porosity present. An EDX analysis confirms the chemical composition of the three samples. The optical measurements provide information on the optical gaps and Urbach energies of the materials under consideration. Their dielectric characteristics are investigated in a frequency range of 100–106 Hz and at temperatures ranging from 300 to 600 K. Moreover, this research enables us to determine the ferroelectric transition as well as the type of dielectric material. In this study, an investigation of electrical conductivity was conducted for well-defined temperature and frequency values; which provided us with information about the mechanism of conduction and charge carrier transport models.

Hybrid Transverse Magneto‐Thermoelectric Cooling in Artificially Tilted Multilayers

AbstractIn artificially tilted multilayers comprising two different conductors that are alternately and obliquely stacked, transverse thermoelectric conversion occurs, in which charge and heat currents are interconverted in the orthogonal direction. Although transverse thermoelectric conversion also occurs in homogeneous materials as an intrinsic transport phenomenon owing to the effects of magnetic fields, magnetization, and spins on conduction carriers, such magneto‐thermoelectric effects are investigated independently of thermoelectrics for artificially tilted multilayers. Here, this study shows that the synergy of these different principles improves the performance of transverse thermoelectric conversion. Using lock‐in thermography techniques, transverse thermoelectric conversion processes are visualized in artificially tilted multilayers and the experiments clarify how nonuniform charge currents are converted into orthogonal heat currents. Through the measurements of temperature change under magnetic fields, the contributions of the magneto‐thermoelectric effects are quantified in the artificially tilted multilayers and magnetically enhanced hybrid transverse thermoelectric cooling is demonstrated. By replacing one of the conductors in the multilayer with permanent magnets, the same functionality is obtained even in the absence of magnetic fields, paving the way for the creation of “thermoelectric permanent magnets” that exhibit efficient transverse thermoelectric conversion together with spontaneous magnetization. This study provides a new material design guideline for transverse thermoelectrics.

In-situ growth of CuI/Porphyrin-based graphdiyne (PDY) p-n heterojunction for high-performance cathodic photoelectrochemical sensing

Cathodic photoelectrochemical (PEC) bioanalysis has been attracting great interest due to its excellent anti-interference capability. As a p-type wide energy band gap semiconductor, CuI is regarded as an ideal material for the construction of photocathodes, while its poor stability and narrow light absorption range limit its application as a photoelectrode in PEC sensing. Herein, we report a simple in-situ growth strategy to synthesize CuI/Porphyrin-based graphdiyne (PDY) p-n heterojunction as a photocathode. CuI crystals were firstly deposited on the ITO glass surface via the electrochemical deposition, and CuI could catalyze the in-situ Glaser coupling reaction of 5,10,15,20-tetra(4-ethynylphenyl) porphyrin (TEPP), allowing the growth of PDY film on the CuI crystal surface and forming the p-n heterojunction photocathode. The as-prepared PDY film plays a crucial role in the significant enhancement of PEC performance because of its intense light absorption, and the formation of CuI/PDY p-n heterojunction can efficiently promote the transportation of photogenerated carriers between CuI and PDY thanks to the excellent electrical conductivity and high carrier mobility of highly π-conjugated PDY. After the growth of PDY, the photocurrent response of PDY/CuI/ITO is almost seven folds higher compared to that of CuI/ITO. Owing to the selective Cu–S bond interaction between l-cysteine (L-Cys) and Cu+, a PEC sensor for the L-Cys determination is developed with a broad linear range from 0.05 to 1.2 μM and a limit of detection (LOD) of 0.03 μM.

Y and La Doping in CaMnO3 Compounds: Effects of Dopant Identity and Amount on Charge Transport Kinetics

Enhancing electronic transport properties of thermoelectric oxides is of great technological importance. Oxides are promising candidates for waste heat harvesting at elevated temperatures as well as for electricity generation in low‐power applications. To this purpose, fundamental understanding of their electrical and thermal conduction mechanisms is essential. Herein, the conduction mechanism of CaMnO3 materials is focused on and how dopant identity and amount alter the kinetic properties of charge transport is investigated. Ca1−xRxMnO3 compounds with R = Y and La are synthesized, where 0 ≤ x ≤ 0.13, and the electrical conductivity and Seebeck coefficient for temperatures ranging from 300 to 1050 K, indicating that Y‐doped compounds are usually more conductive than their La‐doped counterparts, are measured. Analysis of both in terms of the small polaron hopping model reveals that Y doping reduces conduction activation energies, resulting in higher electrical conductivity and charge carrier mobility. Remarkably high values of thermoelectric power factor for the Ca0.97La0.03MnO3 compound, for example, 300 μWm−1 K−2 at 1050 K are observed; furthermore, these values are preserved for a wide temperature range, rendering this compound a good candidate for heat‐to‐electrical power generation at elevated temperatures.

Symmetric Supercapacitors based on Reduced Graphene Oxide/Multi-walled Carbon Nanotubes/Cobalt Oxide Ternary Composites

Ternary nanocomposites of reduced graphene oxide/multi-walled carbon nanotubes/cobalt oxide nanoparticles (rGO/MWCNT/Co3O4) were synthesized employing a facile hydrolysis method with subsequent heat treatment. The good electrical conductivity and remarkable carrier mobility of rGO and MWCNTs make them a suitable matrix for hybrid supercapacitors, and their composites with metal oxides exhibit enhanced electrochemical properties due to the advantages of the synergistic contribution and the integration of different dimensionalities. The binary counterparts of Co3O4 with GO or MWCNTs were also produced using the same technique to gain additional insight into the characteristics of the individual components. The structural and morphological properties of the composites were analyzed using various analytical techniques. The electrochemical behaviors of the prepared composites were investigated using cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS) in 1 M H2SO4. A platinum electrode modified with the rGO/MWCNT/Co3O4 composite displayed a remarkable specific capacitance of 922 F g−1 at a current density of 1 A g−1 composite with a negligible capacitance drop after 2000 cycles. The symmetric supercapacitor fabricated using the rGO/MWCNT/Co3O4 composite showed an energy density of 32.2 Wh Kg−1 at 1 A g−1, and the corresponding power density was 2000 W Kg−1. The supercapacitor fabricated using the composite displayed 83% capacitance retention after 2000 cycles at 3 A g−1 composite.

Preparation and properties of shape-stable phase change material with enhanced thermal conductivity based on SiC porous ceramic carrier made of iron tailings

SiC porous ceramic carriers with adjustable porosity of 79.9%–90.7% were prepared by foam gel-casting forming and carbothermal reduction reaction sintering, using industrial waste fine-grained iron tailings and graphite powder as raw materials. Then paraffin/SiC shape-stable phase change materials were prepared by spontaneous infiltration employing paraffin as phase change material. Thermal conductivity of the SiC carriers (0.17–0.31 W/m·K) is 2.7–3.7 times higher than that of iron tailings porous ceramics with the same porosity. A thermal conductivity model of the SiC carrier is established and verified by experimental data. Phase composition, microstructure, molecular structure, mechanical properties and thermal properties of the shape-stable phase change materials were characterized by XRD, SEM, FT-IR, WDW, Hot Disk and DSC, respectively. There is only a physical combination between paraffin and SiC carriers. The compressive strength of the shape-stable phase change materials (2.0–2.3 MPa), weight loss and latent heat loss are less than 5% and 4.7% after 100 thermal cycles, respectively, which can meet the application requirements as a functional material. Thermal conductivity of the shape-stable phase change materials (0.7–0.73 W/m·K) is significantly improved, a latent heat of 138.5 J/g, and the efficiency of energy storage and release is 2.3–3.3 times as good as than that of paraffin, suggesting that this material is capable of recycling and reusing waste heat as a highly efficient thermal energy storage system.

Modulation of carrier conduction in CsPbBr3 perovskite quantum dots with band-aligned electron and hole acceptors.

The lead halide perovskites have emerged as promising materials with intriguing photo-physical properties and have immense potential for photovoltaic applications. A comprehensive study on the kinetics of charge carrier (electron/hole) generation and transfer across the interface is key to realizing their future scope for efficient device engineering. Herein, we investigate the interfacial charge transfer (CT) dynamics in cesium lead halide (CsPbBr3) perovskite quantum dots (PQDs) with energetically favorable electron acceptors, anthraquinone (AQ) and p-benzoquinone (BQ), and hole acceptors such as pyrene and 4-(dimethylamino)pyridine (DMAP). With various steady-state and time-resolved spectroscopic and microscopic measurements, a faster electron transfer rate is estimated for CsPbBr3 PQDs with BQ compared to that of AQ, while a superior hole transfer for DMAP is divulged compared to pyrene. In concurrence with the spectroscopic measurements, conducting atomic force microscopic studies across the electrode-PQD-electrode junction reveals an increment in the conductance of the PQD in the presence of both the electron and hole acceptors. The variation of the density of states calculation in the presence of the hole acceptors offers strong support and validation for faster CT efficiency. The above findings suggest that a careful selection of simple yet efficient molecular arrangements can facilitate rapid carrier transfer, which can be designed as auxiliary layers for smooth CT and help in the engineering of cost-effective photovoltaic devices.

Morphology matters: Investigating the influence of granules and nanofibers on the physicochemical properties of TiO2 for optoelectronic applications

This study investigated the influence of morphology on the physiochemical properties of TiO2, specifically comparing granule-like structures and nanofibers. Both morphologies were fabricated using different methods and characterized comprehensively. After annealing, X-ray diffraction confirmed their crystalline nature with peaks corresponding to the anatase phase of titania. Scanning electron microscopy revealed long nanofibers with diameters ranging from 62 nm to 95 nm, while the granules appeared as spherical or nearly spherical particles with sizes between 43 nm and 59 nm. Energy-dispersive X-ray analysis confirmed the presence of Ti and O in both structures. The Fourier-transform infrared and raman spectra exhibited characteristic bands related to the Ti–O–Ti stretching, vibrational, binding and anatase phase of titania, respectively. Raman peaks' intensity and position were influenced by crystal size, quality, and strain. The nanofibers showed higher raman mode peak intensities due to their larger surface area to volume ratio and high aspect ratio. In UV–Vis–NIR spectra, the nanofibers displayed higher absorption and a narrower bandgap than the granules. Hall Effect measurements indicated that the nanofibers had a higher carrier concentration and conductivity than the granules. Additionally, the nanofibers exhibited hydrophobic behavior with a water contact angle of 95°, while the granules were relatively hydrophilic with a contact angle of 58o. The findings could potentially facilitate the creation of customized TiO2 materials that are better suited for specific optoelectronic devices. This advancement would result in improved overall efficiency and performance across a range of technological uses.

Enhancing long-term stability and energy/power density of sodium ion battery through fluorination-induced electron density engineering in covalent organic nanosheets

The structure-related performance of covalent organic nanosheets (CONs) as anode materials in sodium-ion batteries is investigated by the introduction of an electron-withdrawing atom such as fluorine in the unit structure of CON. Then, strong permanent dipoles induced by fluorine are well preserved through the CON framework; these dipoles can provide a large number of accessible sites for Na+ on the self-assembled frameworks. The effect of the dipole on the Na+ storage capacity of fluorinated CON-37 is demonstrated by subsequently comparing that of non-fluorinated CON-35 with respect to various electrochemical behaviors. As a result, the interaction between Na+ and the π framework of CON-37 is reduced with respect to CON-35 as fluorine could withdraw the electron density of the framework. Therefore, the charging/discharging capacity of CON-37 can be improved as a result of better ion and charge carrier conductivity with high stability for up to 2500 subsequent cycles. Furthermore, its chemical structure is extremely well maintained for up to 2500 cycles. The CON-37 electrode exhibits stable cycling performance and a rate capability, maintaining a highly improved reversible discharge capacity of ∼ 350 mA h g−1 for up to 2500 cycles at a current density of 100 mA g−1.

Computational Modeling, Simulation and Tight–Binding Analysis of Electronic Characteristics of Defective Germanene Nanoribbons for Future Applications of High Sensitivity Ge-Based Nanosensors

In this research, the electronic characteristics of germanene sheet and nanoribbons using the computational modeling, simulation and tight binding approximation are investigated. Our analysis is focused on the pristine sheet of germanene as well as defective monolayer. The obtained results show that applying the Stone–Wales defect into the germanene monolayer changes the energy band structure. The E-k curves around the Dirac point are no longer linear, in which a band gap is opened, and the Fermi velocity is reduced. Furthermore, the main parameters such as density of states, carrier concentration in degenerate and non-degenerate limits, carrier effective mass, conductance and AC conductivity of germanene are analytically modeled with the inclusion of the spin–orbit coupling effect, temperature and ribbon width. Obtained results demonstrate that the inclusion of the spin–orbit coupling makes a small splitting of the energy levels and creating a small band gap. Finally, the Tight binding and computational values are compared with our simulation results and available data, and a rational agreement is reported in terms of trend and value. The findings of this study provide theoretical reference for the design of germanene-based nanosensors and optoelectronic devices.

Optical Absorption, Photocarrier Recombination Dynamics and Terahertz Dielectric Properties of Electron-Irradiated GaSe Crystals

Optical absorption spectra of 9 MeV electron-irradiated GaSe crystals were studied. Two absorption bands with the low-photon-energy threshold at 1.35 and 1.73 eV (T = 300 K) appeared in the transparency region of GaSe after the high-energy-electron irradiation. The observed absorption bands were attributed to the defect states induced by Ga vacancies in two charge states, having the energy positions at 0.23 and 0.61 eV above the valence band maximum at T = 300 K. The optical pump-terahertz probe technique (OPTP) was employed to study the dark and photoexcited terahertz conductivity and charge carrier recombination dynamics at two-photon excitation of as-grown and 9 MeV electron-irradiated GaSe crystals. The measured values of the differential terahertz transmission at a specified photoexcitation condition were used to extract the terahertz charge carrier mobilities. The determined terahertz charge carrier mobility values were ~46 cm2/V·s and ~14 cm2/V·s for as-grown and heavily electron-irradiated GaSe crystals, respectively. These are quite close to the values determined from the Lorentz–Drude–Smith fitting of the measured dielectric constant spectra. The photo-injection-level-dependent charge carrier lifetimes were determined from the measured OPTP data, bearing in mind the model injection-level dependencies of the recombination rates governed by interband and trap-assisted Auger recombination, bulk and surface Shockley–Read–Hall (SRH) recombination and interband radiative transitions in the limit of a high injection level. It was found that GaSe possesses a long charge carrier lifetime (a~1.9 × 10−6 ps−1, b~2.7 × 10−21 cm3ps−1 and c~1.3 × 10−37 cm6ps−1), i.e., τ~0.53 μs in the limit of a relatively low injection, when the contribution from SRH recombination is dominant. The electron irradiation of as-grown GaSe crystals reduced the charge carrier lifetime at a high injection level due to Auger recombination through radiation-induced defects. It was found that the terahertz spectra of the dielectric constants of as-grown and electron-irradiated GaSe crystals can be fitted with acceptable accuracy using the Lorentz model with the Drude–Smith term accounting for the free-carrier conductivity.

Electrochemical performance of rGO/MWCNT/CoS ternary composite for supercapacitor applications

Renewable energy systems and energy storage technologies gain significant attention in the present scenario due to the increasing global energy demand. The intriguing properties of supercapacitors make them a promising up-and-comer in electrochemical energy storage applications. The fabrication of hybrid capacitors, integrating the electric double layer and the pseudocapacitive materials, improves the performance of the supercapacitors. Herein, we report the synthesis of a ternary nanocomposite of reduced graphene oxide, multi-walled carbon nanotubes, and cobalt sulfide (rGO/MWCNT/CoS), employing a one-pot hydrothermal process. Both rGO and MWCNTs constitute an excellent matrix for hybrid supercapacitors owing to their exceptional conductivity and extraordinary carrier mobility. Their composites with metal sulfides demonstrate superior electrochemical properties due to the synergistic contributions. The pseudocapacitive CoS boosts up the specific capacitance of the system and functions as a spacing material to mitigate the graphene layer aggregation. The incorporation of MWCNTs improves the conductivity and cyclability of the composites by linking the graphene layers into a network structure. The electrochemical investigation of the rGO/MWCNT/CoS reveals that the ternary composite is a good choice as a supercapacitor electrode with a remarkable specific capacitance of 612 Fg−1 at 1 A/g and an appreciable cyclability of 93 % across 2000 cycles.

Surface Roughness-Induced Changes in Important Physical Features of CoFeSm Thin Films on Glass Substrates during Annealing.

Co60Fe20Sm20 thin films were deposited onto glass substrates in a high vacuum setting. The films varied in thickness from 10 to 50 nm and underwent annealing processes at different temperatures: room temperature (RT), 100, 200, and 300 °C. Our analysis encompassed structural, magnetic, electrical, nanomechanical, adhesive, and optical properties in relation to film thickness and annealing temperature. X-ray diffraction (XRD) analysis did not reveal characteristic peaks in Co60Fe20Sm20 thin films due to insufficient growth-driving forces. Electrical measurements indicated reduced resistivity and sheet resistance with increasing film thickness and higher annealing temperatures, owing to hindered current-carrier transport resulting from the amorphous structure. Atomic force microscope (AFM) analysis showed a decrease in surface roughness with increased thickness and annealing temperature. The low-frequency alternating current magnetic susceptibility (χac) values increased with film thickness and annealing temperature. Nanoindentation analysis demonstrated reduced film hardness and Young's modulus with thicker films. Contact angle measurements suggested a hydrophilic film. Surface energy increased with greater film thickness, particularly in annealed films, indicating a decrease in contact angle contributing to this increase. Transmittance measurements have revealed intensified absorption and reduced transmittance with thicker films. In summary, the surface roughness of CoFeSm films at different annealing temperatures significantly influenced their magnetic, electrical, adhesive, and optical properties. A smoother surface reduced the pinning effect on the domain walls, enhancing the χac value. Additionally, diminished surface roughness led to a lower contact angle and higher surface energy. Additionally, smoother surfaces exhibited higher carrier conductivity, resulting in reduced electrical resistance. The optical transparency decreased due to the smoother surface of Co60Fe20Sm20 films.

Charge transport mechanism, dielectric relaxations and relaxor ferroelectric properties of Sm2MgMnO6 double perovskite

This study investigates the charge transport mechanism, dielectric relaxation and relaxor ferroelectric properties of Sm2MgMnO6 which exhibits monoclinic crystal structure with space group P21/n. The charge carriers conduction mechanism in the temperature range 150 K–239 K and 244 K–304 K are correlated barrier hopping (CBH) and overlapping large polaron tunneling (OLPT), respectively. The maximum barrier height associated to CBH model is 0.26 meV. The dc activation energy associated to CBH and OLPT model are 0.11 eV and 0.91 eV respectively. The polaron hopping energy associated to Mott's variable range hopping model is 0.11 eV at 244 K. The value of tanδ varies from 0.004 to 3.329 in temperature range 151.07 K–306.87 K and frequency range 1 kHz–800 kHz. The temperature variation of ε′, ε″ and tanδ shows a single relaxation which follows Vogel–Fulcher (VF) law. The VF law and the shape of P–E hysteresis loop confirms the relaxor ferroelectric nature of the studied material. The saturated polarization (PMax), remnant polarization (Pr) and coercive field (Ec) depend on both the electric field (E0) and frequency (f). The loop area (A′) and dielectric breakdown field (EBD) depend on E0 as A′∝E02 and EBD∝E0, respectively. The PMax, Pr and Ec depend on f as PMax∝f−0.13, Pr∝f−0.48 and Ec∝f−0.48, respectively. The energy storage efficiency (η) varies with frequency as: η%=20.44f0.21.

Resistive switching behavior and mechanism of HfO x films with large on/off ratio by structure design

Different bilayer structures of HfO x /Ti(TiO x ) are designed for hafnium-based memory to investigate the switching characteristics. The chemical states in the films and near the interface are characterized by x-ray photoelectron spectroscopy, and the oxygen vacancies are analyzed. Highly improved on/off ratio (∼104) and much uniform switching parameters are observed for bilayer structures compared to single layer HfO x sample, which can be attributed to the modulation of oxygen vacancies at the interface and better control of the growth of filaments. Furthermore, the reliability of the prepared samples is investigated. The carrier conduction behaviors of HfO x -based samples can be attributed to the trapping and de-trapping process of oxygen vacancies and a filamentary model is proposed. In addition, the rupture of filaments during the reset process for the bilayer structures occur at the weak points near the interface by the recovery of oxygen vacancies accompanied by the variation of barrier height. The re-formation of fixed filaments due to the residual filaments as lightning rods results in the better switching performance of the bilayer structure.

Ultrahigh Thermoelectric Performance of ZnO-CdO Thin Films

Zinc oxide (ZnO) is emerging as a promising n-type thermoelectric material (TE) for power harvesting due to its high melting point and large Seebeck coefficient. However, the TE performance of ZnO is limited by high thermal conductivity and low carrier mobility. Adding or doping a divalent element such as cadmium oxide (CdO) can lower the thermal conductivity and enhance the carrier concentration of ZnO. In this paper, the thermoelectric transport properties of ZnO-CdO nanocrystalline thin films are investigated by varying the Zn/Cd ratio at temperatures ranging from room temperature (RT) to 423 K. The electrical conductivity, carrier concentration and mobility of ZnO were enhanced by increasing the Cd concentration. The maximum power factor of 2.75 × 10−4 W m−1 K−2 was obtained at 423 K for the Zn/Cd = 1:3 sample. The thermal conductivity was dominated by lattice thermal conductivity in which Umklapp scattering occurs between anharmonic phonons. The thermal conductivity of ZnO decreased significantly with increasing Cd concentration. The highest estimated figure of merit (ZT) of 0.59 was found at 413 K for the Zn/Cd = 1:3 sample, which is 223 times greater than for ZnO, indicating that the film is efficient in energy generation.

Studying the Effects of Annealing and Surface Roughness on Both the Magnetic Property and Surface Energy of Co60Fe20Sm20 Thin Films on Si(100) Substrate

In this study, Co60Fe20Sm20 alloy was employed for sputter deposition onto Si(100) substrate within a high vacuum environment, and subsequent thermal treatment was conducted using a vacuum annealing furnace. Thorough measurements and analyses were carried out to evaluate how various film thicknesses and annealing temperatures affect the material. The investigations encompassed observations of structural and physical properties, magnetic traits, mechanical behavior, and material adhesion. The results from the four-point probe measurements clearly demonstrate a trend of decreasing resistivity and sheet resistance with increasing film thickness and higher annealing temperature. Analysis through atomic force microscopy (AFM) shows that heightened annealing temperature corresponds to decreased surface roughness. Furthermore, when analyzing low-frequency alternating current magnetic susceptibility (χac), it became evident that the maximum magnetic susceptibility value consistently rises with increased film thickness, regardless of the annealing temperature. Through magnetic force microscopy (MFM) observations of magnetic domain images in the films, it became apparent that there was a noticeable reduction in the brightness contrast of the magnetic domains. Furthermore, nanoindentation analysis reveals a clear trend. Elevating the film thickness leads to a reduction in both hardness and Young’s modulus. Contact angles range between 67.7° and 83.3°, consistently under 90°, highlighting the hydrophilic aspect. Analysis of surface energy demonstrates an escalation with increasing film thickness, and notably, annealed films exhibit a substantial surge in surface energy. This signifies a connection between the reduction in contact angle and the observed elevation in surface energy. Raising the annealing temperature causes a decline in surface roughness. To summarize, the surface roughness of CoFeSm films at different annealing temperatures significantly impacts their magnetic, electrical, and adhesive properties. A smoother surface reduces the pinning effect on domain walls, thus enhancing the χac value. Furthermore, diminished surface roughness leads to a decline in the contact angle and a rise in surface energy. Conversely, rougher surfaces exhibit higher carrier conductivity, contributing to a reduction in electrical resistance.