Increasing Substrate Temperature Research Articles

Microstructure evolution of carbon films, grown by physical vapor deposition, when substrate temperature is increased

We study how the increases of substrate temperature influence the structural properties of carbon films deposited by physical vapor deposition (PVD) techniques. We installed and adapted a heating system inside the deposition chamber, which can reach temperatures as high as 700 °C. Here we develop an experimental setup that allows us to obtain large films of the desired material on any substrate, with deposition times of the order of a minute. The characterization is based mainly on the analysis of the Raman spectra, where the evolution of the G and D peaks corresponding to the material in its amorphous phase is observed. With the increase of substrate temperature, the sp2 zones grow. A displacement to the right of the G peak and an increase in the ID/IG ratio is seen. At 700 °C a 2D zone at a frequency greater than 2000 cm−1 appears. Four Lorentzian-shaped bands are necessary to account for the peaks at this zone, whose centers correspond to different combinations of first-order ones. This shows that we are transitioning from an amorphous to a carbon phase with more ordered or graphitic zones and that we are near the crystallization point. This opens the path to produce structures similar to graphene using this method of deposition. The Tauc gap energy ratio decreases as temperature increases indicating that there is a graphitization of the sample. Transmission FTIR study is carried out at some of the intermediate temperatures, determining the type of bond at low frequencies. These bonds are consistent with the ones of the hydrogenated amorphous carbon (a-C:H) structure.

Substrate temperature effects on PEALD HfAlO dielectric films for IGZO-TFT applications

Aluminum incorporated hafnium oxide (HfAlO) has garnered significant attention due to its high dielectric constant. The present study employs supercycle plasma-enhanced atomic layer deposition (PEALD) to prepare HfAlO films, switching between HfO2 and Al2O3 deposition. The effects of substate temperature on the HfAlO films are systematically investigated. The experimental results indicate that increased substrate temperatures favor Al2O3 growth on HfO2, but excessively high temperatures result in deviated PEALD growth and deteriorated film properties. The PEALD window can be reached at substrate temperatures of 300 to 350 °C. The Al content is substantially affected by the substrate temperature, despite a constant Al2O3 PEALD cycle ratio used. The film deposited at 350 °C exhibits the highest Al content, density, and (111) crystalline orientation ratio, leading to a high breakdown field of 6 MV/cm and a considerable dielectric constant of 27. The HfAlO films are applied to indium gallium zinc oxide transistor as the dielectric layer, and the 350 °C-deposited HfAlO leads to the best device performance with a remarkably high on/off current ratio of 3.5 × 109, low threshold voltage of −0.1 V, saturated mobility of 6.1 cm2 V−1 s−1, and a low subthreshold swing of 76 mV/dec.

Effect of zirconia coating on Ti6Al4V at elevated substrate temperature on its wear, corrosion and hemocompatibility properties

Titanium and its alloys have been recognized as biocompatible materials since decades although these materials can at times show wear, corrosion and thrombosis leading to various health complications. Surface coating has been found to be one of the effective techniques to overcome this issue in-vitro as well as in-vivo. In this paper, thin film coating of zirconia (ZrO2) on Ti6Al4V is reported using pulsed laser deposition (PLD) technique at elevated substrate temperatures ranging from room temperature to 800 °C (TiZr800). The morphology, composition, wettability and average surface roughness of the samples were analyzed by scanning electron microscope, Raman spectrometer, water contact angle (WCA) and profilometer, respectively. Adhesion between film and substrate was qualified using Elcometer. The tribological studies were performed in ball-on-disc against ZrO2 counterpart, the electrochemical potential analysis was done in simulated body fluid to replicate body environment and the hemocompatibility test was evaluated on the basis of human whole blood adsorption and distribution of Red Blood Cells (RBCs) on the samples. The results revealed an increase in WCA and film adhesion with increased substrate temperature. At 800 °C, formation of zirconium carbide, a relatively harder material, was observed due to diffusion of substrate carbon into ZrO2 film, as confirmed by the Raman analysis. In comparison to pristine, TiZr800 sample showed a reduction in wear rate from 3.5 × 10−3 mm3/Nm to 1.1 × 10−3 mm3/Nm indicating improved resistance of the sample against abrasive wear. Further, drop in Icorr from 4.96 × 10−7 A/cm2 to 2.75 × 10−7 A/cm2 resulted in a 44 % increase in corrosion protective efficiency of the sample coated at 800 °C. Additionally, a 4.5 fold increase in RBC counts was observed on TiZr800 sample. Therefore, it is inferred that while ZrO2 coating can improve overall surface properties of Ti–6Al–4V, additional benefits can be obtained by performing PLD at higher substrate temperatures, more so when the temperature is such as to promote formation of ZrC in the coating.

Highly oriented epitaxial Cu2O (011) thin film grown on MgO (001) substrate by dynamic aurora PLD method

The epitaxial Cu2O films are grown on MgO (001) substrates by the dynamic aurora pulsed laser deposition (PLD) method. The structure and epitaxial growth of the as-prepared thin films at different substrate temperatures (25 °C to 600 °C) were investigated. The thin film prepared at room temperature (RT, 25 °C) exhibits (111) plane of monoclinic CuO phase with poor crystallinity. The thin films deposited at 200 °C to 600 °C show single-phase cubic Cu2O with good crystallinity. The highly oriented epitaxial Cu2O thin film on the MgO substrate is achieved at low temperatures of 200 °C by applying an induced electromagnetic field during thin film growth. The quality and crystallinity of epitaxial Cu2O thin films remarkably improved with increasing temperature from 200 °C to 600 °C and all these thin films exhibited an out-of-plane epitaxial relationship of Cu2O (011) ‖ MgO (001). The CuO thin film grown on MgO substrate at RT shows a single domain structure, as observed by RHEED. In constrast, Cu2O films grown at 200 °C to 600 °C exhibit two different kinds of domain structure with corresponding in-plane epitaxial relationship of Cu2O [100] ‖ MgO [110] and Cu2O [100] ‖ MgO [11¯0]. This report reveals, for the first time, the feasibility of epitaxial growth of highly oriented Cu2O (011) thin film achieved at the low temperature of 200 °C by the dynamic aurora PLD method. The quality of epitaxial Cu2O (011) is significantly enhanced with increasing substrate temperature from 200 °C to 600 °C.

Role of substrate temperature on the performance of BaTiO3/Si photodetector prepared by pulsed laser deposition

In this study, the pulsed laser deposition (PLD) method was employed to fabricate nanostructured BaTiO3 films on glass and silicon substrates at varying temperatures. The structural analysis confirmed the formation of crystalline nanostructured BaTiO3 with mixed tetragonal and hexagonal phases, and the film deposited at 150 °C has the best crystallinity and largest particle size. The optical energy gap of the BaTiO3 nanostructure decreases from 3.94 to 3.84 eV, with increasing substrate temperature from 60 to 150 °C. Photoluminescence spectra of BaTiO3 films deposited at 25, 60, 100, and 150 °C exhibit emission peaks centered at 450, 512, 474, and 531 nm, respectively. Raman spectra of BaTiO3 films show E (LO), A (TO), E (LO) + TO, and B1 vibration modes. Hall measurements reveal that the mobility of the BaTiO3 film increases with temperature up to 100 °C and then decreases at 150 °C. The current–voltage characteristics of the BaTiO3/p-Si heterojunction, deposited over a temperature range of 25 to 150 °C, were investigated in the dark and under illumination. The heterojunctions exhibit rectifying properties, with the best rectification factor observed for the heterojunction prepared at 100 °C. The values of the ideality factor for the heterojunctions fabricated at 25, 60, 100, and 150 °C were 4.3, 3.8, 2.8, and 5, respectively. The study reveals an improvement in both the figures of merit and the photodetector performance with increased substrate temperature. The responsivity increases from 2.2 to 9.25 A/W as the deposition temperature rises from 25 to 100 °C. The detectivity (D*) and external quantum efficiency (EQE) of the photodetector prepared at the optimum substrate temperature of 100 °C, were found to be 4.62 × 1012 Jones and 114%, respectively, at 500 nm.

Modulation of electrical properties of sputtered Ta2O5 films by variation of RF power and substrate temperature

Dielectric thin films are important building blocks of microelectronic devices, and hence, research on the development of high-k dielectric thin films has drawn tremendous research interest. In this research, thin films of tantalum oxide (Ta2O5), a high-k dielectric material, are deposited on the Si substrate by the radio frequency (RF) magnetron sputtering technique. During the deposition of Ta2O5 thin film, the sputtering parameters such as sputtering power and substrate temperature were systematically varied, and post-deposition structural, morphological, and electrical properties of sputtered Ta2O5 films are studied by x-ray diffraction, Fourier transform infrared spectroscopy, atomic force microscope, capacitance–voltage (C-V) and current–voltage (I-V) measurement techniques. The annealed Ta2O5 thin film at the temperature of 900 °C for 1 h possesses polycrystalline nature with β—phase orthorhombic crystal structure. The film deposited at 150 W and substrate temperature at room temperature has shown comparatively lower surface roughness, which depicts the energy, and mobility of adatoms greatly influenced by RF power and substrate temperature. With the increase in sputtering power, the oxide charge density (Qox) is found to increase. On the other hand, Qox is found to decrease with the increase in substrate temperature. The film deposited at RF power of 150 W and substrate temperature of 300 °C is found to be of high dielectric constant, low oxide and interface charge density, and lower leakage current.

In2S3 thin films with potential use as window layers in photovoltaic devices

In this study, we use the Haacke’s figure of Merit (FOMH) as a valuable parameter for determining suitable Indium Sulphide thin films among a set of samples processed with different deposition parameters, in order to use them as window layers in the usual configuration of photovoltaic devices. Two sets of In2S3 thin films were deposited by radiofrequency (RF) magnetron sputtering technique with an average thickness around 90 nm and using different deposition parameters. One set of samples was processed using different RF power in the range of 75–250 W at room temperature, the second set was obtained using different substrate temperatures 24 °C–350 °C keeping a constant RF power at 75 W. The FOMH consider optical and electrical properties of In2S3 thin films in order to evaluate the feasibility of the thin film to be used as a window layer in photovoltaics devices. The In2S3 thin films processed by RF-Sputtering were analyzed by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), Ultraviolet-Visible spectroscopy, Raman spectroscopy, x-ray diffraction (XRD) and four-probe method. The In2S3 thin films had a high optical transmittance of 84% and the energy band gap values of the films decreased from 2.48 to 2.38 with the increase in substrate temperature and deposition power. The estimated grain size decreasing from 65 to 45 nm as the RF power increased. Additionally, the crystallinity of the selected In2S3 thin films was analyzed by Raman and XRD and showing that the cubic and tetragonal β-In2S3 phases can coexist in the films. The In2S3 thin films showed resistivity vales around Higher values of FOMH were obtained for In2S3 thin films processed at 75 W, 250 W, 100 °C and 300 °C, therefore these layer are feasible candidates to be used as window layers in photovoltaic devices.

Investigation of electrical, corporeal, ocular, and aquaphobic properties of zirconia thin-films by varying substrate temperature for high voltage insulators

ABSTRACT The addition of ZrO2 nanoparticles to porcelain insulators and its influence on their electrical and corporeal characteristics are studied throughout a wide range of sintering temperatures. The DC magnetron sputtering with a 99.99% pure zirconium target is utilized to create zirconium oxide thin films on glass and silicon substrates in a 12-inch-diameter chamber. Different temperatures were used to sinter the produced samples (room temperature (RT) − 400°C) for 1 h with gas pressure of 15 mTorr for all depositions. Scanning electron microscopy (SEM) was used to characterize the microstructures of a subset of the samples. In order to assess the physical and microphysical changes brought on by an increase in substrate temperature, the phase composition of various nanocomposites samples was determined using X-ray diffraction (XRD). The breakdown strength, electrical resistivity, and dielectric constant were measured to assess the electrical attributes of the various samples. The obtained findings showed that the electrical and corporeal characteristics of samples sintered at RT were the best. In addition, the sample of nanocomposite porcelain sintered at room temperature has excellent insulating qualities, confirming the possibility of electro-technical porcelain manufacture (water contact angle = 103.5º), dielectric constant = 24.4, refractive index = 2.15, and bandgap = 5.33.

Substrate surface processing in gas vacuum arc discharge

Problem. Preparation of the surface before the deposition of vacuum arc coatings is of great importance, as it affects not only the adhesive properties of the obtained coatings, but also the physical and mechanical properties of the substrate surface layer. The processes of ions interaction with the substrate surface during its ionic cleaning, before the deposition of coatings, have been relatively little studied and are almost not described in the literature. Goal. In this work, some of their features are considered in relation to the surface preparation of materials before the deposition of vacuum arc coatings. Methodology. The nature of the plasma interaction with the substrate material is determined by the ions energy and their flow density, other things being equal. The energy of the ions treating the substrate consists of their initial energy and the energy acquired in the Debye layer adjacent to the substrate when a negative potential is applied to it. The value of the potential at which the processes of condensation or sputtering are balanced is determined mainly by the nature of the vaporized material. When the value of the substrate potential increases, the deposited particles and its material are sputtered. At the same time, atoms on the substrate that are weakly connected to it, various impurities are removed, substrate temperature increases, especially in its surface layers. Results. The work investigates the influence of various metals and gases used to clean the surface of substrates made of cubic boron nitride PcBN and steels on the surface morphology. It was established that the use of gases (argon, nitrogen) for cleaning the surface of dielectric materials, in particular PcBN, showed the best results, which subsequently leads to an increase in the adhesion of the vacuum arc coating to the substrate. Practical value. The obtained results of investigation are of great practical importance. They can be used for the increasing of tool life and productivity of mechanical engineering.

Effect of substrate temperature on structural, morphological, and optical properties of Gallium Oxide thin films deposited by RF-Sputtering

In this paper, gallium oxide thin films at different substrate temperatures were deposited on Indium TinOxide (ITO), glass and p-type silicon wafers by radio-frequency magnetron sputtering (RFMS). The effect of substrate temperature at 8 mTorr deposition pressure on the structural, morphological and optical properties of films was investigated. X-ray diffraction (XRD) analysis of thin films deposited on p-Si substrate suggests that crystal structure characteristics of synthesized thin films depend on the substrate temperature. From SEM analyzes of Ga2O3 thin films grown on p-type silicon substrate, It was observed that when the temperature increased, a porous structure appeared and the particle size changed depending on the substrate temperature. Morover, optained results from the absorption measurements, the band gap energy of the Gallium Oxide thin films deposited on the p-Si substrate decreased with increasing substrate temperature.

Effects of deposition temperature on the mechanical and structural properties of amorphous Al–Si–O thin films prepared by radio frequency magnetron sputtering

Aluminosilicate (Al–Si–O) thin films containing up to 31 at.% Al and 23 at.% Si were prepared by reactive RF magnetron co-sputtering. Mechanical and structural properties were measured by indentation and specular reflectance infrared spectroscopy at varying Si sputtering target power and substrate temperature in the range 100 to 500 °C. It was found that an increased substrate temperature and Al/Si ratio give denser structure and consequently higher hardness (7.4 to 9.5 GPa) and higher reduced elastic modulus (85 to 93 GPa) while at the same time lower crack resistance (2.6 to 0.9 N). The intensity of the infrared Si-O-Si/Al asymmetric stretching vibrations shows a linear dependence with respect to Al concentration. The Al–O–Al vibrational band (at 1050 cm−1) shifts towards higher wavenumbers with increasing Al concentration which indicates a decrease of the bond length, evidencing denser structure and higher residual stress, which is supported by the increased hardness. The same Al–O–Al vibrational band (at 1050 cm−1) shifts towards lower wavenumber with increasing substrate temperature indicating an increase in the average coordination number of Al.

Influence of Substrate Temperature and Deposition Rate on the Texture and Surface Morphology of ISD–MgO

In this study, MgO thin films were grown on Hastelloy metal tapes by the inclined substrate deposition (ISD) method for coated conductors. The effects of substrate temperature and deposition rate on the biaxial texture alignment, texture angle (defined as the tilt angle of the MgO (002) plane from the substrate normal) as well as surface morphology of MgO films were investigated systematically. For the substrate inclination angle of 30°, as the increase of substrate temperature from room temperature to 600 °C, the texture quality of the MgO films degrades significantly, meanwhile the texture angle decreases from 27° to 19°. For depositions under substrate inclination angles of 30°, 35° and 55°, the full-width-at-half-maximum (FWHM) value of the in-plane and out-of-plane alignment of MgO films exhibits a “V” shape relationship with the deposition rate. Scanning electron microscopy (SEM) images show that enhanced surface random diffusion causes the film to become dense and reduces the intergrain voids in the columnar structure.

Impact of temperature difference on the features of spray deposited yttrium doped cobalt selenide (YCoSe) thin films for photovoltaic application

In this investigation, spray deposited yttrium doped cobalt selenide (YCoSe) thin materials were synthesized in soda lime glass and the impact of substrate temperature (140 oC, 160 oC, 180 oC and 200 oC) on their elemental composition, surface morphology, structural, electrical and optical properties were investigated using scanning electron microscopy-SEM X-ray diffraction –XRD, four-point probe and UV-VIS spectrophotometer respectively. The EDX plots of the deposited undoped and Y-doped cobalt selenide revealed the major elements: cobalt, selenium and yttrium. This confirms the deposition of CoSe and Y-doped CoSe thin materials. The morphology of undoped CoSe thin materials was very rough containing randomly oriented non-uniform thin particles while addition of Y dopant (0.1 mol%) at substrate temperature of 140 oC gave a homogenous distribution of compact rectangular-like nanograins. The XRD result shows that the films are cubic polycrystalline in nature and the film grown at substrate temperature of 180 oC was seen to give the most excellent crystalline quality and a preferential orientation along (111) direction. From electrical results it was observed that increase in substrate temperature increased the film thickness with decreased resistivity and increased conductivity. The optical properties were found to vary with substrate temperature despite the fact that the variation was not totally linear, as the film deposited at substrate temperature of 160 oC deviated from the linearity in all the optical properties. The energy band gap of the deposited samples ranges from 1.25 eV–1.75 eV. The materials produced could be used in the production of photovoltaic devices.

Growth rate and substrate temperature dependent magnetic and magneto-microstructural properties of sputtered Fe-Co-Al thin films

This work reports the effect of growth rate and substrate temperature on the structural, magnetic and magneto-microstructural properties of Fe-Co-Al alloy thin films. 75 nm thick Fe-Co-Al films were deposited on Si 〈100〉 substrates at room temperature by high power impulse magnetron sputtering at different growth rates viz., 0.67–1.6 Å/s. All the films exhibited a uniform composition of about Fe60Co25Al15 irrespective of growth rates. In addition to this, all the films crystallized in single phase with bcc type structure. Growth rate was found to have a considerable impact on the magnetic properties of Fe-Co-Al thin films. Film grown at 1.3 Å/s exhibited an in-plane magnetic anisotropy with large saturation magnetization combined with low coercivity when compared with other films. To improve the magnetic properties further, Fe-Co-Al films were also processed at different substrate temperatures (Ts) viz., 300, 400 and 500 °C. Substrate temperature of about 300 °C was found to have beneficial effect on the soft magnetic characteristics of Fe-Co-Al films. While the crystal structure did not show any perceptible change with processing conditions, magnetic microstructures traced from magneto optic Kerr microscopy displayed substantial change. Upon increase in substrate temperature, during magnetization reversal process, magnetic domain images traced along the easy magnetization direction magnetization showed transformation from branched band domains (as-deposited) → wide band domains (Ts = 300 °C) → less periodic branched band domains (Ts = 400 °C) → ripple domains (Ts = 500 °C). Attempts were made to correlate the achieved magnetic properties in Fe-Co-Al with magnetic microstructures.

The evolution of preferred orientation and morphology of AlN films under various sputtering parameters

Wurtzite aluminum nitride (AlN) films with (100) and (002) preferred orientations are obtained on Si (100) wafers by radio frequency magnetron sputtering. The effects of target to substrate distance (TSD), RF power, deposition pressure and substrate temperature on crystal orientation and morphology of AlN films are analyzed. It is demonstrated that the TSD is the key factor affecting the preferred orientation of AlN films. The preferred orientation transforms from (002) to (100) with the increase of TSD. Further increase of substrate temperature and RF power causes the coexistence of (002) and (100) orientation. The evolution of orientation is explained by the change in kinetic energy of sputtered particles. The correlation between the crystal orientation and the surface morphology is investigated.

Nonmonotonic dependence of adhesion between liquid aluminum and silicon surface on the temperature of the surface

In this paper, the effect of temperature on the adhesion properties between liquid aluminum (Al) and solid silicon (Si) in the presence and absence of vacancy defects is elucidated. Firstly, the perfect defect-free and vacancy defect models consist of crystalline Al probe and Si substrate are established by classical molecular dynamics simulation method. Then, the melting and adhesion process of probe Al are simulated, and the adhesion performance and microscopic permeation evolution of liquid Al/solid Si are analyzed. The results show that the adhesion force changes nonmonotonically with increasing substrate temperature T without vacancy defects. Specifically, when the substrate temperature varies at relatively low values smaller than the melting point of Al, that is, 100 K < T < 933 K, the thermal excitation provides more energy to the substrate Si atoms, which intensifies the aggregation of the substrate atoms, makes the interfacial atoms more dense and the number of atoms permeating into the substrate decreases, resulting in a decrease in adhesion force. On the contrary, when 933 K < T < 1500 K, due to the thermal effect, higher temperatures intensify the thermal vibration of the substrate atoms, resulting in violent collisions between the interfacial atoms, and the space for free movement increases, thus making the distance between the atoms larger. And the number of Al atoms permeating into the substrate Si increases, leading to an increase in interfacial adhesion. Furthermore, the adhesion force shows an upward trend with the elevated temperature in the presence of vacancy defects at low temperatures, this is attributed to the fact that more atoms are broken away from the equilibrium lattice structure, and the number of permeating atoms increases by increasing temperature. In particular, the interfacial adhesion is the largest when the vacancy defects of the substrate are the most serious.

Enhanced photovoltaic performance of silicon-based solar cell through optimization of Ga-doped ZnO layer

In the present study, the impact of deposition pressure and substrate temperature of Ga-doped Zinc Oxide (GZO) thin film and the photovoltaic performance of this structure as a transparent conductive oxide (TCE) layer in silicon-based solar cell were investigated. Implementing a single target of GZO, the structural, optical, and electrical properties of 350 nm thick GZO thin films with various deposition pressure (5 mTorr, 10 mTorr, 15 mTorr and 20 mTorr) at room temperature (RT) and substrate temperature (RT, 150 °C, 200 °C, 250 °C) at 15 mTorr deposition pressure were fabricated using RF magnetron sputtering technique. The aim here was to find out the GZO films with the optimum pressure and substrate temperature to incorporate them into solar cell as a TCE layer. The X-ray diffraction (XRD) and atomic force microscopy (AFM) techniques were used to determine the structural properties of all samples. The optical transmission measurements were performed using spectroscopic Ellipsometer and the band gap values were calculated by Tauc plot using optical transmission data. In addition, the electrical characterization of the GZO samples were analyzed by the Van der Pauw method and Hall measurements. Finally, the most promising GZO thin film was determined based on the structural and optoelectrical characterization. The findings indicated that the XRD pattern of all the prepared films was dominated by (002) preferential orientation irrespective of the deposition pressure and substrate temperature. The AFM measurements showed that all the samples had a dense surface morphology regardless of the deposition pressures, but the surface morphology of the samples was clearly changed upon increasing substrate temperatures. The transmission values of the film did not significantly alter (∼82%) when the deposition pressures except for the substrate temperature of 200 °C (86%) were changed. The band gap values were calculated between 3.30 eV and 3.36 eV, which can be associated with enhancement of crystalline quality of the films. The lowest resistivity and the highest carrier concentration values belonged to the film fabricated at 15 mTorr@200 °C by 2.0 × 10−3 Ω.cm and 1.6 × 1020 cm−3, respectively. Both increasing the deposition pressure (up to 15 mTorr) and substrate temperature (up to 200 °C) contributes to improving the crystallite size, widening the optical band gap, lowering the resistivity, and increasing the carrier concentration. In order to evaluate and compare the effect of both deposition pressure and substrate temperature, Silicon-based solar cells were fabricated using the most promising layers (15 mTorr@RT, 15 mTorr@200 °C). The cell performance with the GZO thin film as a TCE layer showed that varying both the pressure and substrate temperature of the GZO film contributed to enhancing the solar cell parameters. Thus, the conversion efficiency increased from 9.24% to 12.6% with the sequential optimization of pressure and temperature. It can be concluded that the pressure applied during the deposition and substrate temperature had a significant impact on the properties of GZO thin films and its photovoltaic performance of solar cell used as TCE layer.

The microstructural evolution of sputtered ZnO epitaxial films to stress-relaxed nanorods

Epitaxial ZnO thin films and nanorods were grown on c-sapphire substrate by reactive sputtering of Zn in Ar-O2 atmosphere at substrate temperatures ranging from near room temperature to 700 °C. Scanning electron microscopy showed that with increase in substrate temperature, the morphology transformed from columnar films to vertically aligned ZnO nanorods. High resolution x-ray diffraction revealed improvement in crystalline and epitaxial quality with increase in substrate temperature, along with the decrease of edge dislocation density from 5 × 1012 cm−2 to 8 × 1010 cm−2. The films grown near room temperature showed large hydrostatic strain (∼6 × 10−3) and compressive intrinsic biaxial stress (-0.8 GPa), which decreased substantially with increase in substrate temperature, due to the desorption of excess oxygen and reduction in edge dislocation density. Above the substrate temperature of 500 °C, the intrinsic biaxial stress reversed from compressive to mildly tensile in the case of nanorods, due to the intrinsic tensile stress, originating from crystallite coalescence. Raman measurements correlate well with the changes in biaxial stress and confirm the improvements in crystallinity and epitaxial quality of nanorods grown at higher temperatures. Room temperature photoluminescence of the ZnO films showed weak near-band-edge emission, which enhanced drastically in the case of nanorods due to their superior microstructure and epitaxial quality.

Devising technological solutions for gas sensors based on zinc oxide for use at critical infrastructure facilities

This paper reports a study of a gas sensor based on nanostructured zinc oxide in order to establish the conditions for its production and operating characteristics under the influence of the target gas ethanol. The studied samples were produced by magnetron sputtering on direct current. The method of forming the device structure was chosen among others due to the fact that it has a high rate of deposition at low values of the working gas pressure, there is no overheating of the substrate, a low degree of contamination of the obtained films, the possibility of obtaining samples of uniform thickness on a large area of the substrate. A VUP-5M vacuum unit with an original material-saving magnetron was used to obtain the films. Studies of the effect of temperature on the resistance of a gas sensor based on ZnO have been carried out. It was established that the change in the resistance of the tested sample depends on the temperature of the substrate. The resistance of the gas sensor in atmospheric air decreases with increasing substrate temperature from room temperature (25 °C) to 200 °C. A further increase in temperature from 200 °C leads to an increase in the resistance of the structure until it stabilizes in the temperature range of 300–400 °C. It was established that the operating temperature range of the gas sensor based on ZnO is within 300–400 °C. The characteristics of the gas sensor based on ZnO were studied and the working temperature of the sensor was determined for the rapid identification of ethanol in atmospheric air at a target gas concentration of 500 ppm. It was established that for rapid operation of the instrument structure, the temperature of the substrate should be 400 °C, a decrease or increase in temperature leads to a decrease in the sensitivity of the sensor to the target gas. It was established that the gas sensor demonstrates stability and a consistent sensitivity response upon repeated exposure to the target gas

High Entropy Oxides Synthesis by Rapid Plasma Generation with Applications Towards Electrocatalytic Hydrogen Generation

It has been challenging over the past three decades to find viable energy alternatives with higher efficiency and lower cost to beat the non-friendly and commercially dominant hydrocarbon energy sources. Hydrogen is deemed to be one of the most prominent candidates, as a green fuel. Hydrogen can be generated in abundance from water splitting. Utilization of hydrogen in fuel cells, with no doubt, produces no harmful gases. Nevertheless, the two parts of the technology: hydrogen production and reduction are cumbersome. Both processes rely on kinetically sluggish electrocatalytic reactions. Enormous research efforts have been spent, with thousands of studies presented in literature, despite the technology left with a dim light shed on from commercial and practical point of views. Many materials-related challenges are hindering the flourishing of green hydrogen technology, where poor catalyst durability is a major challenge, despite being made from very expensive elements (e.g., Ir, ru or Pt). Therefore, gigantic effort of research has been devoted to find inexpensive alternatives. Transition metals showed interesting performance towards water splitting (e.g., oxygen evolution (OER)).Recent research showed that electrocatalysts designed by alloying two or more elements (e.g., transition metals) showed promising enhancement in catalytic activity and durability due to synergistic effects from alloying, such as strain engineering or electron exchange. The potential of the alloying approach is constrained by standard synthesis methods (e.g., wet chemistry and thermal decomposition techniques) due to the governing thermodynamics rules (e.g., miscibility of the different alloying elements). That renders the synthesis of high entropy alloys (HEA), with more than 3 elements, to be complicated and tedious to achieve by those conventional methods.In the presented talk we are presenting the utilization of microwaves to generate plasma to synthesize metastable high entropy oxides (HEA) and their outstanding catalytic performance towards OER, in correlation to their unique chemistries and structures. Plasma generated enable temperature increase of the precursor substrate to elevated levels (~1000s °C), in conjunction with rapid cooling, within seconds range. Our results show successful synthesis of non-noble metals HEA NPs, possessing higher catalytic activity than IrO2 catalyst for OER. The nature of the alloying elements dictates OER activity by promoting different oxidation states of the catalytically active transition metals (e.g., Fe, Ni and Co). Our developed techniques resulted in formation of super-strong metal-carbide bonds “chemically welding” HEA NP to the catalyst support.