Chemical Deposition Research Articles

Ultra‐Efficient Heat Transport Across a “2.5D” All‐Carbon sp2/sp3 Hybrid Interface

AbstractSingle‐ and few‐layer graphene‐based thermal interface materials (TIMs) with extraordinary high‐temperature resistance and ultra‐high thermal conductivity are very essential to develop the next‐generation integrated circuits. However, the function of the as‐prepared graphene‐based TIMs would undergo severe degradation when being transferred to chips, as the interface between the TIMs and chips possesses a very small interfacial thermal conductance. Here, a “2.5D” all‐carbon interface containing rich covalent bonding, namely a sp2/sp3 hybrid interfaces is designed and realized by a plasma‐assisted chemical vapor deposition with a function of ultra‐rapid quenching. The interfacial thermal conductance of the 2.5D interface is excitingly very high, up to 110–117 MWm−2 K−1 at graphene thickness of 12–25 nm, which is even more than 30 % higher than various metal/diamond contacts, and orders of magnitude higher than the existing all‐carbon contacts. Atomic‐level simulation confirm the key role of the efficient heat conduction via covalent C−C bonds, and reveal that the covalent‐based heat transport could contribute 85 % to the total interfacial conduction at a hybridization degree of 22 at %. This study provides an efficient strategy to design and construct 2.5D all‐carbon interfaces, which can be used to develop high performance all‐carbon devices and circuits.

Ultra-Efficient Heat Transport Across a "2.5D" All-Carbon sp2/sp3 Hybrid Interface.

Single- and few-layer graphene-based thermal interface materials (TIMs) with extraordinary high-temperature resistance and ultra-high thermal conductivity are very essential to develop the next-generation integrated circuits. However, the function of the as-prepared graphene-based TIMs would undergo severe degradation when being transferred to chips, as the interface between the TIMs and chips possesses a very small interfacial thermal conductance. Here, a "2.5D" all-carbon interface containing rich covalent bonding, namely a sp2/sp3 hybrid interfaces is designed and realized by a plasma-assisted chemical vapor deposition with a function of ultra-rapid quenching. The interfacial thermal conductance of the 2.5D interface is excitingly very high, up to 110-117 MWm-2 K-1 at graphene thickness of 12-25 nm, which is even more than 30 % higher than various metal/diamond contacts, and orders of magnitude higher than the existing all-carbon contacts. Atomic-level simulation confirm the key role of the efficient heat conduction via covalent C-C bonds, and reveal that the covalent-based heat transport could contribute 85 % to the total interfacial conduction at a hybridization degree of 22 at %. This study provides an efficient strategy to design and construct 2.5D all-carbon interfaces, which can be used to develop high performance all-carbon devices and circuits.

A multi-method study of femtosecond laser modification and ablation of amorphous hydrogenated carbon coatings

AbstractWe present a study on femtosecond laser treatment of amorphous hydrogen-containing carbon coatings (a-C:H). The coatings were deposited on silicon wafers by a plasma-assisted chemical vapour deposition (PA-CVD), resulting in two different types of material with distinct properties (referred to as “absorbing” and “semi-transparent” coatings in the following). The samples were laser-treated with single fs-laser pulses (800 nm center wavelength, 35 fs pulse duration) in the ablative regime. Through a multi-method approach using topometry, Raman spectroscopy, and spectroscopic imaging ellipsometry, we can identify zones and thresholds of different fluence dependent effects and have access to the local dielectric function. The two coating materials react significantly different upon laser treatment. We determined the (non-ablative) modification threshold fluence for the absorbing coating as $$3.6\times {10}^{-2}$$ 3.6 × 10 - 2 Jcm−2 and its ablation threshold as 0.22 Jcm−2. The semi-transparent coating does not show such a low-fluence modification but exhibits a characteristic interference-based intra-film ablation mechanism with two distinguishable ablation thresholds at 0.25 and 0.28 Jcm−2, respectively. The combination of tailored layer materials and correlative imaging spectroscopic methods delivers new insights into the behaviour of materials when treated with ultrashort-pulse laser radiation. Graphical abstract

Comparison of the Sensitivity of Various Fibers in Distributed Acoustic Sensing

Standard single-mode telecommunication optical fiber is still one of the most popular in distributed acoustic sensing. Understanding the acoustic, mechanical and optical features of various fibers available currently can lead to a better optimization of distributed acoustic sensors, cost reduction and adaptation for specific needs. In this paper, a study of the performances of seven fibers with different coatings and production methods in a distributed acoustic sensor setup is presented. The main results include the amplitude–frequency characteristic for each of the investigated fibers in the range of acoustic frequencies from 100 to 7000 Hz. A single-mode fiber fabricated using the modified chemical vapor deposition technique together with a polyimide coating has shown the best sensitivity to acoustic events in the investigated range of frequencies. All of this allows us to both compare the studied specialty fibers with the standard single-mode fiber and choose the most suitable fiber for a specific application, providing an enhancement for the performance of distributed acoustic sensors and better adaptation for the newly aroused potential applications.

A Single-Crystal Antimony Trioxide Dielectric for 2D Field-Effect Transistors.

The remarkable potential of two-dimensional (2D) materials in sustaining Moore's law has sparked a research frenzy. Extensive efforts have been made in the research of utilizing 2D semiconductors as channel materials in field-effect transistors. However, the next generation of integrated devices requires the integration of gate dielectrics with wider bandgaps and higher dielectric constants. Here, insulating α-Sb2O3 single-crystal nanosheets are synthesized by one-step chemical vapor deposition method. Importantly, the α-Sb2O3 single-crystal dielectric exhibits a high dielectric constant of 11.8 and a wide bandgap of 3.78 eV. Besides, the atomically smooth interface between α-Sb2O3 and MoS2 enables the fabrication of dual-gated field-effect transistors with the top gate dielectric of α-Sb2O3 nanosheets. The field-effect transistors exhibit a switching ratio of exceeding 108, which achieves the manipulation of field-effect transistors by using 2D dielectric materials. These results hold significant implications for optimizing the performances of 2D devices and innovating microelectronics.

ZnS Thin Films Prepared Using Spray Pyrolysis Technique

It is well known that spray pyrolysis of an aqueous solution of thiourea and zinc acetate onto a glass substrate at spray rates of (3 mlmin-1) may successfully synthesize thin coatings of zinc sulfide. The impact of several substrate temperatures (150, 200, 300, and 400 oC) on the structural and optical characteristics was also discussed in this research. Field emission scanning electron microscopy (FESEM), UV-Vis-NIR spectrometry, and X-ray diffraction (XRD) were used to characterize the films. On each one of the deposited ZnS sheets, a cubic structure was seen. On the other hand, the temperature of the substrate appeared to affect the crystallinity and morphology. It was discovered that as the substrate temperature was lowered, the bandgap value of ZnS films somewhat rose. Furthermore, it is most likely that the Zn(S,O) production inside the solid solution is responsible for the fluctuations seen in the 3.5-3.8 eV region.

Quantitative Defect Analysis in CVD‐Grown Monolayer MoS2 via In‐Plane Raman Vibration

ABSTRACTThe synthesis of two‐dimensional transition metal dichalcogenide (2D‐TMD) materials gives rise to inherent defects, specifically chalcogen vacancies, due to thermodynamic equilibrium. Techniques such as chemical vapor deposition (CVD), metal‐organic chemical vapor deposition (MOCVD), atomic layer deposition (ALD), flux growth method, and mechanical exfoliation produce large‐scale, uniform 2D TMD films, either in bulk or monolayers. However, defects on the film surface impact its quality, and it is necessary to measure defect density. The phonon confinement model indicates that the first‐order Raman band frequency shift depends on defect density. Monolayer Molybdenum disulfide (MoS2) exhibits three phonon dispersions at the Brillouin zone edge (M point): out‐of‐plane optical phonon vibration (ZO), in‐plane longitudinal optical phonon vibration (LO), and in‐plane transverse optical phonon vibration (TO). The LO and ZO modes overlap with Raman in‐plane vibration (𝐸12g) and Raman out‐of‐plane vibration (𝐴1g), respectively, causing peak broadening. In the presence of defects, the Raman 𝐸12g vibration energy decreases due to a reduced restoring force constant. The Raman 𝐴1g vibration trend is random, influenced by both restoring force constant and mass. The study introduces a quantitative defect measurement technique for CVD‐grown monolayer MoS2 using Raman 𝐸12g mode, employing sequential data processing algorithms to reveal defect density on the film surface.

Study of the effect of forced precipitation of working fluid on a vineyard sprayer for weed control

BACKGROUND: A distinctive feature of the sprayer presented is the possibility of more efficient penetration of the working solution into the middle and lower tiers of weeds, as well as on the lower (abaxial) side of the leaf. This is ensured by the use of an air-liquid jet, which significantly increases the effectiveness of the applied chemical method of weed control. AIM: Analysis of the effect of air jets on the deposition of chemicals on the surface of leaves of weeds. The penetration of the drug into the middle and lower tiers of plants, as well as deposition on the adaxial and abaxial sides of the leaves, is taken into account. METHODS: To combat unwanted vegetation, a special device was applied, which was used in field research both with and without air support. With the help of the developed method, spraying was carried out on indicator cards fixed on the upper, middle and lower tiers of the plant RESULTS: For the first time, an innovative technology based on the use of a flexible air distribution sleeve as part of a herbicide sprayer was proposed to combat weeds in vineyards. This sleeve ensures the precise supply of airflow from the fan to the working fluid spraying area. The result of such an air-liquid flow is improved efficiency and monodispersity of herbicide droplets, which, in turn, ensures uniform coverage of the lower tiers and the abaxial surface of the leaves of weeds. This innovative solution opens up new prospects in the fight against unwanted vegetation in the vineyards. CONCLUSION: The practical value of the proposed weed control technology lies is increasing the quality of the technological operation due to the forced precipitation of the working solution.

Structural and Electrical Characterization of Solution‐Deposited β‐Ga2O3:Al

The wide bandgap oxide semiconductor thin films are synthesized using tetrahydroxogallate (III) ammonium {NH4Ga(OH)4} precursor at a concentration of 10 at% Ga and varying amounts of hydrated aluminum nitrate between 0.6 and 3.2 at%. Thin films of β‐Ga2O3:Al are synthesized by spin coating and spray pyrolysis with postannealing in nitrogen ambient at 930 °C. The structural properties of the thin films are investigated using XRD and Raman spectroscopy, while the electrical characteristics are determined using 4‐point probe, current–voltage (I–V), and capacitance–voltage (C–V) measurements with Ti/Al/Ni/Au Ohmic contacts and Pd/Au Schottky contacts. The β‐Ga2O3 with 2.2 at% Al is found to be the optimal concentration in this study, resulting in ideality factors of 1.10 and 1.09, saturation currents of 3.17 × 10−6 and 3.10 × 10−6 A, Schottky barrier heights of 0.73 and 0.88 eV, and series resistances of 948 and 955 Ω, for the spin‐coated and pyrolytically sprayed samples respectively.

Study on the Mechanism of Bubble Defects in the PECVD Amorphous Silicon Films on Dielectric Substrate

The amorphous silicon (a-Si) grown by plasma enhanced chemical vapor deposition (PECVD) has been widely applied in advanced semiconductor devices. However, it still suffers from the bubble defects when the deposition temperature goes above 450 °C. In this work, we have investigated the influence of underlying materials on the formation of bubbles of a-Si. The a-Si was deposited on different dielectric substrates, including silicon nitrides (SiN) and silicon dioxide (SiO2), using PECVD technique at a substrate temperature of 500 °C. A large number of bubbles of the a-Si has been observed on the thermal ALD deposited SiN underlayer, and some of them even burst. In contrast, no bubble defects were observed at the a-Si grown on PECVD SiN and PECVD SiO2 films. Such deviation may be attributed to the quality of the underlying material, which induces the H/H2 diffusion during the growth of a-Si and results in bubbles. A solution based on the model has been used to suppress the formation of such bubbles. An inserting layer of SiO2 was introduced in between SiN and a-Si to improve the density of the lower layer material and the adhesion between the two materials. As a result, there is no bubble defects at the surface of a-Si observed using optical microscope. Our work reveals the mechanism of the formation of bubble defects and paves a new method to eliminate the bubbles defects and to form high-quality a-Si, which shows potential in the manufacture of semiconductor devices.

Synthesis and characterization of p-CuO/n-ZnO heterostructured composite thin films for the detection of formaldehyde gas.

We report the synthesis and characterization of pure CuO and CuO-ZnO nanostructured composite thin films sprayed on particle-free glass substrates using chemical spray pyrolysis method. The films were systematically analyzed through microstructural, morphological, chemical, and gas-sensing studies. X-ray diffraction (XRD) studies confirmed the polycrystalline nature of the films, with a predominant monoclinic phase along the (002) direction. Key structural parameters, such as crystallite size, dislocation density, strain, and the number of crystallites per unit area, were reported from XRD analysis. Field emission scanning electron microscopy (FESEM) revealed a bundled-like morphology with uniform particle distribution, enhancing the surface area for effective gas interaction. X-ray photoelectron spectroscopy (XPS) results indicated that Cu and Zn ions existed predominantly in the 2+ oxidation state, contributing to the films' reactivity. Significantly, the gas sensing studies were investigated with static liquid distribution method, highlighting the remarkable performance of the 30 wt.% CuO-ZnO composite thin film. This composite exhibited a substantial response to 5 ppm formaldehyde at ambient conditions, showing a recovery time of 22 seconds and a response time of 15 seconds. These findings underscore the potential of CuO-ZnO composites for efficient formaldehyde gas sensing applications, marking a notable advancement in the field of environmental monitoring.

Foams-to-Films: A Facile Approach Towards Space-Confined CVD Growth of MoS2.

2D materials have rapidly become the building blocks for the next generation of semiconducting materials and devices, with Chemical vapor deposition (CVD) emerging as a prefered method for their synthesis. However, the predictable and reproducible growth of high quality, large 2D monolayers remains challenging. An important facet is controlling the local environment at the surface of the substrate - here, space-confinement techniques have emerged as promising candidates. We demonstrate that space-confined CVD growth using microstructured MoOx grown on Ni foam is an appealing approach for rapid growth of high quality MoS2 monolayers; a very important subset of 2D materials. This method eschews the use of powders which can be more difficult to control. By incorporation of a porous barrier in the Ni foam support, the rate of delivery of both the Mo and S source to the substrate is dampened, leading to coverage of large, high quality, mono-to-few layer triangular domains as confirmed by Raman and photoluminescence (PL) spectroscopies together with atomic force microscopy (AFM) height measurements.

Stability and reversibility of organic molecule modifications of CVD-synthesized monolayer MoS2.

We investigated the stability of monolayer MoS2samples synthesized using chemical vapor deposition (CVD) and subsequently modified with organic molecules under ambient conditions. By analyzing the optical signatures of the samples using photoluminescence spectroscopy (PL), Raman spectroscopy, and surface quality using atomic force microscopy (AFM), we observed that this modification of monolayer MoS2with organic molecules is stable and retains its optical signature over time under ambient conditions. Furthermore, we show the reversibility of the effects induced by the organic molecules, as heating the modified samples restores their original optical signatures, indicating the re-establishment of the optical properties of the pristine monolayer MoS2.

The Effect of DLC Surface Coatings on Microabrasive Wear of Ti-22Nb-6Zr Obtained by Powder Metallurgy

Titanium alloys have a high cost of production and exhibit low resistance to abrasive wear. The objective of this work was to carry out diamond-like carbon (DLC) coating, with dissimilar thicknesses, on Ti-22Nb-6Zr titanium alloys produced by powder metallurgy, and to evaluate its microabrasive wear resistance. The samples were compacted, cold pressed, and sintered, producing substrates for coating. The DLC coatings were carried out by PECVD (plasma-enhanced chemical vapor deposition). Free sphere microabrasive wear tests were performed using alumina (Al2O3) abrasive suspension. The DLC-coated samples were characterized by scanning electron microscopy (SEM), Vickers microhardness, coatings adhesion tests, confocal laser microscopy, atomic force microscopy (AFM), and Raman spectroscopy. The coatings did not show peeling-off or delamination in adhesion tests. The PECVD deposition was effective, producing sp2 and sp3 mixed carbon compounds characteristic of diamond-like carbon. The coatings provided good structural quality, homogeneity in surface roughness, excellent coating-to-substrate adhesion, and good tribological performance in microabrasive wear tests. The low wear coefficients obtained in this work demonstrate the excellent potential of DLC coatings to improve the tribological behavior of biocompatible titanium alloy parts (Ti-22Nb-6Zr) produced with a low modulus of elasticity (closer to the bone) and with near net shape, given by powder metallurgy processing.

Deep Ultraviolet Excitation Photoluminescence Characteristics and Correlative Investigation of Al-Rich AlGaN Films on Sapphire

AlGaN is attractive for fabricating deep ultraviolet (DUV) optoelectronic and electronic devices of light-emitting diodes (LEDs), photodetectors, high-electron-mobility field-effect transistors (HEMTs), etc. We investigated the quality and optical properties of AlxGa1−xN films with high Al fractions (60–87%) grown on sapphire substrates, including AlN nucleation and buffer layers, by metal–organic chemical vapor deposition (MOCVD). They were initially investigated by high-resolution X-ray diffraction (HR-XRD) and Raman scattering (RS). A set of formulas was deduced to precisely determine x(Al) from HR-XRD data. Screw dislocation densities in AlGaN and AlN layers were deduced. DUV (266 nm) excitation RS clearly exhibits AlGaN Raman features far superior to visible RS. The simulation on the AlGaN longitudinal optical (LO) phonon modes determined the carrier concentrations in the AlGaN layers. The spatial correlation model (SCM) analyses on E2(high) modes examined the AlGaN and AlN layer properties. These high-x(Al) AlxGa1−xN films possess large energy gaps Eg in the range of 5.0–5.6 eV and are excited by a DUV 213 nm (5.8 eV) laser for room temperature (RT) photoluminescence (PL) and temperature-dependent photoluminescence (TDPL) studies. The obtained RTPL bands were deconvoluted with two Gaussian bands, indicating cross-bandgap emission, phonon replicas, and variation with x(Al). TDPL spectra at 20–300 K of Al0.87Ga0.13N exhibit the T-dependences of the band-edge luminescence near 5.6 eV and the phonon replicas. According to the Arrhenius fitting diagram of the TDPL spectra, the activation energy (19.6 meV) associated with the luminescence process is acquired. In addition, the combined PL and time-resolved photoluminescence (TRPL) spectroscopic system with DUV 213 nm pulse excitation was applied to measure a typical AlGaN multiple-quantum well (MQW). The RT TRPL decay spectra were obtained at four wavelengths and fitted by two exponentials with fast and slow decay times of ~0.2 ns and 1–2 ns, respectively. Comprehensive studies on these Al-rich AlGaN epi-films and a typical AlGaN MQW are achieved with unique and significant results, which are useful to researchers in the field.

Smoothening Perfluoroalkylated Surfaces: Liquid‐Like Despite Molecular Rigidity?

AbstractThe rational design of surfaces at the molecular level is essential toward realizing many engineering applications. However, molecular‐scale defects affect processes such as triboelectrification, scaling, and condensation. These defects are often detectable via contact angle hysteresis (CAH) measurements. Liquid‐like surfaces exhibit extremely low CAH (≤5°) and rely on the use of highly flexible molecular species such as long‐chain alkyls or siloxanes. Their low glass transition temperatures lead to the so‐termed self‐smoothing behavior, reducing sensitivity to defects formed during fabrication. However, utilizing rigid molecular species such as perfluoroalkyl chains often results in higher hysteresis (10° to 60°) as defects are not self‐smoothed after fabrication. Consequently, state‐of‐the‐art perfluoroalkylated surfaces often show sub‐optimal interfacial properties. Here, a customizable chemical vapor deposition process creates molecularly‐thick, low‐defect surfaces from trichloro(1H,1H,2H,2H‐perfluorooctyl)silane. By implementing moisture‐exposure controls, highly homogenous surfaces with root‐mean‐square roughness below 1 nm are fabricated. CAH is achieved down to ≈4° (average: 6°), surpassing the state‐of‐the‐art by ≈5°. Reduction of CAH (26° to 6°) results in condensation suppression, decreasing surface droplet density by one order and surface droplet coverage by 40%. This work guides the synthesis of high‐quality surfaces from tri‐functional perfluoroalkylsilanes with liquid‐like properties despite their molecular rigidity.

Phase Transformation on Two-Dimensional MoTe2 Films for Surface-Enhanced Raman Spectroscopy

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have recently become attractive candidate substrates for surface-enhanced Raman spectroscopy (SERS) owing to their atomically flat surfaces and adjustable electronic properties. Herein, large-scale 2D 1T′- and 2H-MoTe2 films were prepared using a chemical vapor deposition method. We found that phase structure plays an important role in the enhancement of the SERS performances of MoTe2 films. 1T′-MoTe2 films showed a strong SERS effect with a detection limit of 1 × 10−9 M for the R6G molecule, which is one order of magnitude lower than that of 2H-MoTe2 films. We demonstrated that the SERS sensitivity of MoTe2 films is derived from the efficient photoinduced charge transfer process between MoTe2 and adsorbed molecules. Moreover, a prohibited fish drug could be detected by using 1T′-MoTe2 films as SERS substrates. Our study paves the way to the development and application of high-performance SERS substrates based on TMD phase engineering.

Passively generated Q-switched pulses fiber laser in 2µm wavelength using MXene Ti3C2Tx and Ti2CTx saturable absorber

Abstract In this paper, Q-switched Thulium-doped fiber lasers (TDFLs) operating near the 2 µm region were demonstrated, utilizing a newly developed Thulium doped fiber as the gain medium and titanium carbide MXene as the saturable absorber (SA). The Thulium-doped fiber with a composition of SiO2-GeO2-Al2O3-HfO2-Y2O3-Bi2O3, was produced from a preform prepared using the modified chemical vapor deposition (MCVD) process. Two variations of titanium carbide MXene-based SA, Ti3C2Tx and Ti2CTx, were tested and compared, both successfully generating Q-switched pulses. When Ti3C2Tx was used, the Q-switched pulse had a center wavelength of 1934.223 nm, a repetition rate of 55.39 kHz and a pulse width and 1.942 µs. For Ti2CTx, the generated Q-switched pulse has a center wavelength of 1932.229 nm, with a repetition rate of 77.28 kHz and a pulse width of 1.608 µs. The signal-to-noise ratio (SNR) values were 45.6 dB and 50.2 dB for Ti3C2Tx and Ti2CTx respectively. Based on the experimental results, both Ti3C2Tx and Ti2CTx show promise as SAs for generating Q-switched pulses in the 2 µm region within the TDFL cavity.

Features of current flow in the n-CoFe2O4/n-CdTe heterojunction

n-CoFe2O4/n-CdTe heterojunctions with a current rectification ratio of 3·105 at voltages |V| = 1.5 V were made by spray pyrolysis of aqueous solutions of cobalt and iron salts on n-CdTe substrates. Based on the analysis of the temperature dependence of I-V-characteristics in the forward voltage range, a change in the mechanisms of current flow in n-CoFe2O4/n-CdTe heterojunctions was established from overbarrier at voltages of 3kT/q < V < 0.3 V to tunneling at voltages of 0.4 V < V < 1 V. The role of surface states in the formation of the energy profile of the heterojunction and the participation of energy levels in the band gap of n-CdTe in the formation of the tunnel current have been clarified. The reasons for the occurrence of negative differential resistance at the forward biases of the structure have been clarified. The current at reverse biases in the range −3 V < V < -3kT/q was analyzed. According to the analysis of the C-V-characteristics, an inversion layer was found in the heterojunction and its behavior from voltage was explained.

Tailored Growth of Transition Metal Dichalcogenides' Monolayers by Chemical Vapor Deposition.

Here, results on the tailored growth of monolayers (MLs) of transition metal dichalcogenides (TMDs) are presented using chemical vapor deposition (CVD) techniques. To enable reproducible growth, the flow of chalcogen precursors is controlled by Knudsen cells providing an advantage in comparison to the commonly used open crucible techniques. It is demonstrated that TMD MLs can be grown by CVD on large scale with structural, and therefore electronic, photonic and optoelectronic properties similar to TMD MLs are obtained by exfoliating bulk crystals. It is shown that besides the growth of the "standard" TMD MLs also the growth of MLs that are not available by the exfoliation is possible including examples like lateral TMD1-TMD2 ML heterostructures and Janus TMDs. Moreover, the CVD technique enables the growth of TMD MLs on various 3D substrates on large scale and with high quality. The intrinsic properties of the grown MLs are analyzed by complementary microscopy and spectroscopy techniques down to the nanoscale with a particular focus on the influence of structural defects. Their functional properties are studied in devices including field-effect transistors, photodetectors, wave guides and excitonic diodes. Finally, an outlook of the developed methodology in both applied and fundamental research is given.