Radio Frequency Reactive Sputtering Research Articles

Silicon nitride thin-films deposited by radiofrequency reactive sputtering: Refractive index optimization with substrate cooling in a nitrogen-rich atmosphere

Silicon nitride (SiN) is widely used as a core material in optical waveguides due to its optical properties. The deposition of SiN thin-films by radiofrequency (RF) reactive sputtering is commonly used in low-temperature processes, where the thin-films optical properties can be optimized by controlling the deposition parameters (sputtering power, gases ratio, etc.). This work presents the deposition of several SiN thin-films by RF reactive sputtering with different sputtering powers (ranging from 180 W to 300 W), with a nitrogen-argon ratio of 16:4, and performing substrate cooling in a nitrogen-rich atmosphere immediately after deposition, consisting in keeping the substrate under 16 sccm of nitrogen until it reaches 25 °C. The refractive indices of the SiN thin-films were assessed through ellipsometry, obtaining a maximum refractive index of 1.906 at 400 nm. SiN thin-films were also analyzed by energy-dispersive spectroscopy (EDS) and atomic force microscopy (AFM).

Analog Memory and Synaptic Plasticity in an InGaZnO-Based Memristor by Modifying Intrinsic Oxygen Vacancies.

This study focuses on InGaZnO-based synaptic devices fabricated using reactive radiofrequency sputtering deposition with highly uniform and reliable multilevel memory states. Electron trapping and trap generation behaviors were examined based on current compliance adjustments and constant voltage stressing on the ITO/InGaZnO/ITO memristor. Using O2 + N2 plasma treatment resulted in stable and consistent cycle-to-cycle memory switching with an average memory window of ~95.3. Multilevel resistance states ranging from 0.68 to 140.7 kΩ were achieved by controlling the VRESET within the range of -1.4 to -1.8 V. The modulation of synaptic weight for short-term plasticity was simulated by applying voltage pulses with increasing amplitudes after the formation of a weak conductive filament. To emulate several synaptic behaviors in InGaZnO-based memristors, variations in the pulse interval were used for paired-pulse facilitation and pulse frequency-dependent spike rate-dependent plasticity. Long-term potentiation and depression are also observed after strong conductive filaments form at higher current compliance in the switching layer. Hence, the ITO/InGaZnO/ITO memristor holds promise for high-performance synaptic device applications.

Reliable Accessibility of Intermediate Polarization States in Textured Ferroelectric Al0.66Sc0.34N Thin Film

AbstractFerroelectric materials are promising candidates for neuromorphic computing synaptic devices due to the nonvolatile multiplicity of spontaneous polarization. To ensure a sufficient memory window, ferroelectric materials with a large coercivity are urgently required for practical applications in highly scaled multi‐bit memory devices. Herein, a remarkable reliability of intermediate ferroelectric polarization states is demonstrated in a textured Al0.66Sc0.34N thin film with a coercive field of 2.4 MV cm−1. Al0.66Sc0.34N thin films are prepared at 300 °C on Pt (111)/Ti/SiO2/Si substrates using a radio frequency reactive sputtering method. Al0.66Sc0.34N thin films exhibit viable ferroelectricity with a large remanent polarization value of >100 µC cm−2. Through the conventional current–voltage characteristics, polarization switching kinetics, and temperature dependence of coercivity, the reproducibility of multiple polarization states with apparent accuracy is attributed to a small critical volume (3.7 × 10−28 m3) and a large activation energy (3.3 × 1027 eV m−3) for nucleation of the ferroelectric domain. This study demonstrates the potential of ferroelectric Al1‐xScxN for synaptic weight elements in neural network hardware.

Investigation of the Effect of Vacuum and Air Annealing on the Structural, Optical and Electrical Properties of Radio Frequency Sputtered WO<sub>3</sub> Thin Films

Thin films of tungsten oxide were deposited on glass substrates by the radio frequency (RF) reactive sputtering from a high purity tungsten metal target (99.9%) with a diameter of 10 cm. The reactive sputtering was carried out in an argon-oxygen gas mixture containing 20% of O2 and 80% of Ar. The used RF power is 200 W while fixing the deposition time at 120 min. Finally, the prepared films were annealed at different temperatures (350 °C, 400 °C, 450 °C, 500°C and 550 °C) for 1 hour under air and under vacuum. X-ray diffractograms showed that the deposited thin films crystallized in Hexagonal/Monoclinic WO3 phase. It was found that the crystallite size varies with the annealing temperature and the lattice parameters is a= 7.3064Å, b = 7.5292Å, c = 7.6875Å and a=b= 7.3242Å, c= 7.6624 Å, for h-WO3 and m-WO3 structures, respectively. Scanning Electron Microscopy (SEM), Raman spectra confirmed the formation of WO3 thin films. In addition, optical transmittance data revealed that the optical bandgap of the films decreases with increasing the annealing temperature. Electrical measurements revealed that annealing in air results in more resistive samples, which should be taken into account in future investigations, especially as buffer layers for efficient photovoltaic solar cells. Keywords: Vacuum, Tungsten oxide, Raman spectroscopy, RF Sputtering method, RF Power, Annealing temperature.

Growth of Zn1−xNixO Thin Films and Their Structural, Optical and Magneto-Optical Properties

The radio frequency (RF) reactive sputtering technique has been used to prepare Zn1−xNixO thin films with 0 ≤ x ≤ 0.08. Composite targets were obtained by mixing and pressing NiO and ZnO powders. Sapphire, quartz and glass were used as substrates. X-ray diffraction analysis of Ni-doped ZnO films indicates that all samples are crystalised in a hexagonal wurtzite structure with a preferred orientation along the (002) plane. Any secondary phase, corresponding to metallic nickel clusters or nickel oxides was not observed. High-resolution transmission electron microscopy (HR-TEM) image observed for Zn1−xNixO thin film shows a strong preferred orientation (texture) of crystalline columns in the direction perpendicular to the substrate surface. Different surface morphology was revealed in AFM images depending on the film composition and growth condition. Optical absorption spectra suggest the substitution of Zn2+ ions in the ZnO lattice by Ni2+ ions. The energy bandgap value was also found a complex dependence with an increase in Ni dopant concentration. In photoluminescence spectra, two main peaks were revealed, which are ascribed to near band gap emission and vacancy or defect states. Faraday rotation demonstrates its enhancement and growth of ferromagnetism with the increase in Ni content of Zn1−xNixO thin films at room temperature.

Copper oxide coatings deposited by reactive radio-frequency sputtering for solar absorber applications

Submicron copper oxide coatings were deposited on glass and copper substrates using reactive radio-frequency sputtering of a copper target in argon-oxygen atmospheres. Morphological, structural, and optical properties of the deposited films were examined using X-ray reflectometry and diffraction, optical transmission measurements, Fourier transform infrared spectroscopy, transmission electron microscopy, atomic force microscopy, electrical resistivity, and thermal emissivity measurements. The obtained coatings consist of a major phase of cupric oxide (Cu2O) at 10% oxygen, which transforms into monoclinic CuO through an intermediary tetragonal Cu4O3 phase for greater oxygen flow rates. The morphology of the Cu2O films is interpreted by an original fish-scale-like growth mode based on the nucleation of single crystallites at cluster boundaries of the underlying layer, which promotes the formation of aligned grain strips and yields coalescing clusters of a height similar to the crystallite size. The deposited films behave as p-type semiconductors with a direct bandgap of 1.9 eV and an indirect bandgap of 1.15 eV for Cu2O and CuO, respectively. The oxygen flow rate-dependent crystallite size exhibits a maximum at 30% oxygen for which the CuO film grows in column-shaped structures of continuous sub-100-nm-wide rods with enhanced surface roughness, solar absorptivities of 0.93 and 0.83, and thermal emissivities of 0.07 and 0.17 in the temperature ranges 50–150 °C and 150–250 °C, respectively.

Optical, structural and composition properties of silicon nitride films deposited by reactive radio-frequency sputtering, low pressure and plasma-enhanced chemical vapor deposition

We present a comparative study of optical properties of silicon nitride thin films deposited by reactive radio-frequency (R-RF) sputtering, low pressure chemical vapor deposition (LPCVD) and plasma-enhanced chemical vapor deposition (PECVD). For LPCVD process, two different proportions of mixed gases were used (LPCVD (A) and LPCVD (B) processes) and PECVD deposition were conducted in three regimes: low frequency (LF), mixed frequency and high frequency. Dielectric functions were extracted from ellipsometric measurements for the wavelength range from ultraviolet to near-infrared wavelengths, spanning from 210 nm to 1690 nm. To understand how different deposition parameters affect the optical properties of thin films, additional structures and composite analysis was done by using X-ray photoelectron spectroscopy, grazing incidence X-ray diffraction, X-ray reflectometry, atomic force microscopy, reflection electron energy loss spectroscopy, Fourier-transform infrared spectroscopy and stress measurements. The series of analysis show that the influence of deposition method on optical properties is significant especially for in the range of 200 nm–400 nm. For these UV wavelengths, LPCVD (A)-deposited films give a transparency window at the shortest wavelength up to 275 nm, while R-RF-sputtering and PECVD (LF) lead to transparency windows starting up to 320 nm wavelengths. Hence, appropriate techniques and recipes should be selected to account for various peculiarities in optical and structural properties of silicon nitride films towards their potential applications in photonic and nanostructured systems.

Comparison of the Material Quality of AlxIn1-xN (x-0-0.50) Films Deposited on Si(100) and Si(111) at Low Temperature by Reactive RF Sputtering.

AlxIn1−xN ternary semiconductors have attracted much interest for application in photovoltaic devices. Here, we compare the material quality of AlxIn1−xN layers deposited on Si with different crystallographic orientations, (100) and (111), via radio-frequency (RF) sputtering. To modulate their Al content, the Al RF power was varied from 0 to 225 W, whereas the In RF power and deposition temperature were fixed at 30 W and 300 °C, respectively. X-ray diffraction measurements reveal a c-axis-oriented wurtzite structure with no phase separation regardless of the Al content (x = 0–0.50), which increases with the Al power supply. The surface morphology of the AlxIn1−xN layers improves with increasing Al content (the root-mean-square roughness decreases from ≈12 to 2.5 nm), and it is similar for samples grown on both Si substrates. The amorphous layer (~2.5 nm thick) found at the interface with the substrates explains the weak influence of their orientation on the properties of the AlxIn1−xN films. Simultaneously grown AlxIn1−xN-on-sapphire samples point to a residual n-type carrier concentration in the 1020–1021 cm−3 range. The optical band gap energy of these layers evolves from 1.75 to 2.56 eV with the increase in the Al. PL measurements of AlxIn1−xN show a blue shift in the peak emission when adding the Al, as expected. We also observe an increase in the FWHM of the main peak and a decrease in the integrated emission with the Al content in room-temperature PL measurements. In general, the material quality of the AlxIn1-xN films on Si is similar for both crystallographic orientations.

Cu-Mg-O thin films by RF magnetron co-sputtering: Their band offsets with CdS in heterojunction solar cells

We report on Cu-Mg.O thin films of 100 - 300 nm in thickness produced by radio frequency (RF) reactive sputtering fromCu and Mg-metal targets in an argon-oxygen ambient. The amount of Mg in the thin films was varied through changingthe RF power applied at the Mg target. When the Mg content in the Cu-Mg-O film changed from 0 to 0.3, the opticalbandgap increased from 1.73 eV to 2.13 eV, and the electrical conductivity decreased from 9 x 10-3 Ω-1 cm-1(CuxO) to 6.7x 10-5 Ω-1 cm-1(Cuo7/Mg0.30). Solar cells of these films, FTO/ CdS/Cu0.85Mg0.15O/C-Ag, showed a short circuit current density of 2.86 mA/cm2 an open circuit voltage of 378 mV, and power conversion efficiency of 0.25%. X-ray photo-electron spectroscopy depth profile analyses of the interfaces suggest a negative conduction band offset in CdS/Cu0.85Mg0.15O( — 0.74 eV) as well as in CdS/CuxO (0.9 eV) solar cells.

Influence of the rf power and oxygen content on structural, electrical, and optical properties of V2O5 thin films prepared via reactive radio frequency sputtering

Herein, V2O5 thin films were deposited through O2-reactive radio frequency (RF) magnetron sputtering using a metallic vanadium arget without external heating on a glass substrate. The influence of the RF power and O2 content on phase formation was investigated, and the percentage of the phase volume was related to the electrical and optical properties of the films. These films were composed of a mixture of α and β phases of V2O5, and the coexistence of monoclinic (βм) and tetragonal (βт) symmetries of the β-phase structure was observed. The phase of the film deposited at 100 W RF power with 10% O2 was βт. Increasing the RF power to 150 W led to the development of the βм phase in the film. At 200 W, the obtained film was a mixture of βм- and α-V2O5 phases, and the film produced with an O2 content of more than 10% was a mixture of three phases: βт-, βм-, and α-V2O5. Further increase in the O2 content decreased the βм-phase volume but increased the βт-phase volume. The electrical resistivity and optical properties depended on the phase volume. Furthermore, the relationship between the phase volume and film properties is presented.

Advantageous optical characteristics of tantalum vanadium oxide as counter electrode in electrochromic devices

Smart windows are an important technology in terms of energy saving potential in the building sector due to their ability to control visible light and thermal radiation. The essential component of this type of window glazing is an electrochromic thin film. In addition to the widely established tungsten oxide as the optically active material, in particular the counter electrode offers significant potential for improving the overall device performance. In this study, tantalum vanadium oxide films are prepared by reactive radio-frequency sputtering on fluorine-doped tin oxide substrates and optimized in terms of their spectro-electrochemical properties as ion storage layer. We show that an oxide-based tantalum-vanadium alloy is a promising approach to address the open challenges of pure vanadium pentoxide. The coatings exhibit color neutrality in combination with a high transmittance of up to 80% in the as-prepared state and suitable optical transmittance switching. Additionally, we find both a sufficient stability upon cycling and a suitable charge density of about 35 mC cm−2. Thus, the presented oxide-based alloy offers a beneficial performance as an ion storage layer in electrochromic devices.

Effect of annealing on structural, optical and electrical properties of nickel oxide thin films synthesized by the reactive radio frequency sputtering

In this paper, Nickel oxide (NiO) thin films were synthesized by the reactive radio frequency sputtering. NiO thin films were annealed at different temperatures (350, 400, 450 and 500 °C). Structural characterizations were performed using X-Ray Diffraction, Scanning Electron Microscopy (SEM), Raman spectroscopy, Infrared spectroscopy. Ultraviolet–visible spectrophotometer were used to estimate the energy band-gap and the thickness. Electrical resistivity measurements were carried out using the four-point probe method then analyzed basing on the structural characterization. The XRD patterns showed that the crystallite size increases by increasing the annealing temperature from 6.3 to 15.8 nm. The structure is cubic with lattice parameter of 4.17 nm and the dislocation and micro strain decrease by annealing. The Thin films surfaces were observed to be homogenous and without cracks. The Raman spectra proved the presence of four peaks, related to vibration of Ni–O bands. The FTIR spectra showed the NiO stretching and bending mode. The optical analyses showed that the transmittance and energy band gap augments by increasing the annealing temperature from 3.54 to 3.68 eV. Finally, the resistivity grows by annealing from 0.33 to 1.4 × 104 Ω cm and decreases by increasing the measuring temperature.

Papaverine Infusion Through Aortic Root before Cardiac Self-recovery in Heart Valve Replacement with TiO2 Nanocrystalline Film Material.

This work was to investigate TiO2 nanocrystalline film material in heart valve replacement (HVR) and the effect of papaverine infusion through the aortic root before cardiac self-recovery during the HVR. TiO2 nanocrystalline films were prepared by radio frequency (RF) reactive sputtering. The crystallization characteristics and surface morphology of TiO2 nanocrystalline films were observed by X-ray diffraction and scanning electron microscopy, and the anti-platelet adhesion and anti-coagulation properties of the films were analyzed. 86 patients with heart valve disease were selected and all underwent HVR. They were randomly divided into a control group (routine treatment) and an experimental group (papaverine perfusion through aortic root), with 43 cases in each group. The rate of cardiac self-recovery and the dosage of dopamine were observed. The results showed that the TiO2 nanocrystalline film was composed of a large number of uniform particles, and the average particle size was about 18.97 ± 7.28 nm. The rate of cardiac self-recovery in the experimental group was 97.67%, which was significantly higher than that in the control group (67.44%) (P< 0.05). The dosage of epinephrine, dopamine, and duration of cardiopulmonary bypass (CPB) assistance in the observation group were less than those in the control group (P < 0.05). These results indicated that TiO2 nanocrystalline film could be used in HVR, and papaverine infusion through aortic root before HVR and myocardial protection measures can significantly improve the rate of cardiac self-recovery and promote postoperative recovery.

Enhanced Photocatalytic Activity of TiO2 Thin Film Deposited by Reactive RF Sputtering under Oxygen-Rich Conditions

TiO2 thin films are promising as photocatalysts to decompose organic compounds. In this study, TiO2 thin films were deposited by reactive radio-frequency (RF) magnetron sputtering under various flow rates of oxygen and argon gas. The results show that the photocatalytic activity decreases as the oxygen-gas ratio is increased to 30% or less, while the activity increases under oxygen-rich conditions. It was observed that the crystal structure changed from anatase to a composite of anatase and rutile, where the oxygen-gas ratio during RF sputtering is more than 40%. Interestingly, the oxygen vacancy concentration increased under oxygen-rich conditions, where the oxygen-gas ratio is more than 40%. The sample prepared under the most enriched oxygen condition, 70%, among our experiments exhibited the highest concentration of oxygen vacancy and the highest photocatalytic activity. Both the oxygen vacancies and the composite of anatase and rutile structure in the TiO2 films deposited under oxygen-rich conditions are considered responsible for the enhanced photocatalysis.

Effect of the RF-power and annealing on the structural, optical, morphological and electrical properties of RF-sputtered V2O5 thin films

Thin films of vanadium oxide were deposited on glass substrates by the radio frequency reactive sputtering from a high purity metallic vanadium target (99.7%) with a diameter of 10 cm. The reactive sputtering was carried out in an argon-oxygen gas mixture containing 10% of O2 and 90% of Ar. The films were deposited at different RF powers (150 W, 200 W, 250 W and 300 W) for a fixed deposition time of 150 min. X-ray diffractograms showed that the deposited thin films crystallized in an orthorhombic V2O5 phase. It was found that the crystallite size varies with the RF power and is maximized using 300 W as an RF power. Scanning Electron Microscopy and Raman scattering analyzes have confirmed the formation of V2O5 thin films. In addition, optical transmittance measurements were performed using a Shimadzu UV-PC spectrophotometer in the 220–2000 nm range. It was observed that the optical band gap of the films decreases with increasing the RF power. Electrical resistivity was found to decrease with increasing the RF power from 150 to 250 W, and then it increases.

STUDY OF THE EFFECT OF ANNEALING AND IRRADIATION OF HIGH ENERGY IONS ON VANADIUM OXIDE THIN FILMS

Vanadium due to multiple oxidation states (+2 to +5) has various oxides like vanadium dioxide, vanadium pentoxide, and hexavanadium tredecaoxide. Thin films of vanadium oxide have been fabricated using the reactive radio frequency sputtering technique. Deposited films are then annealed in Argon (Ar) environment at 500°C and then irradiated with 100 MeV Ag8+ high-energy ions. The vibrational properties of the thin films have been analyzed by Raman scattering and Fourier transform infrared spectroscopy (FTIR). Results (Raman and FTIR spectra) of the annealed film show that the V6O13 phase is converted into a mixed phase (V6O13, VO2, and V2O5). The annealed irradiated film shows no change in the Raman lines while a shift towards the lower wave number is noticed in FTIR spectra peaks. This shifting of peaks towards lower wave numbers in FTIR spectra is attributed to the weakening of vanadium oxygen bonds. It is considered that the weakening of bonds could occur when high-temperature cylindrical zones are created along the ion track.

Tuning silicon nitride refractive index through radio-frequency sputtering power

Fabrication of thin-film multilayer structures by sputtering typically requires a multi-cathode deposition machine. This study proposes a simpler approach based on the radio-frequency power modulation; thin silicon nitride (SiNx) thin films were prepared by radio-frequency reactive sputtering in an (Ar + N2) atmosphere at room temperature. The samples were analyzed to highlight the effects of different deposition conditions on the morphological and optical properties of the films in the visible and near/mid-infrared (IR) regions. The refractive index of the films was varied from 1.5 - 2.5 (at λ=800 nm) by tuning the sputtering power. IR reflectance measurements indicated the absence of spurious (oxygen- or hydrogen-based) phases; atomic force microscopy and scanning electron microscopy indicated flat and homogeneous sample surfaces.

Tunable TTB strontium and tantalum based thin films: Influence of the deposition parameters on the structural and dielectric properties

Structural and dielectric properties of thin films produced by reactive radiofrequency sputtering of a (Sr2Ta2O7)100-x (La2Ti2O7)x target with x = 1.65 were studied. The chemical composition characterization shows Sr/Ta ratios ranging between 0.49 and 0.56, thus pointing out the deposition of strontium deficient films, belonging to the tetragonal tungsten bronze family. The highest permittivities and tunabilities, associated with the lowest dielectric losses, are obtained when films are pure and fully textured. This is achieved for a 900 nm-thick film deposited at TS = 850 °C: ε’ = 116, tanδ = 0.007 and tunability T = 14.5% at 340 kV/cm and 100 kHz.

Formation of Mobile Hydridic Defects in Zirconium Nitride Films with n-Type Semiconductor Properties

The incorporation of mobile hydridic defects was demonstrated for low-crystalline zirconium nitride (Zr3N4−δ) films deposited by radio frequency reactive sputtering under a flow of nitrogen-rich reactive gases. Extended X-ray adsorption fine structure analysis confirmed that the local coordination environment around the Zr atoms was very close to that of the orthorhombic Eu3O4 phase. Pristine Zr3N4−δ films exhibited n-type semiconductor behavior due to the presence of nitrogen vacancy donors with a carrier concentration of 1.4 × 1020 cm–3 and an electron mobility of 2.2 × 10–3 cm2 V–1 s–1 at room temperature. Electrochemical and spectroscopic measurements confirmed that Zr3N4−δ was readily hydrogenated upon exposure to H2 gas at 300 °C to form hydridic defects via electron donation to hydrogen adatoms. Hence, the hydrogenated film exhibited H– ion/electron mixed conductor behavior in a H2 atmosphere. These findings open the possibility for exploring new hydride ion conductors based on thermodynamically stable transition metal nitrides.

Double-layer solar absorber coating based on high entropy ceramic AlCrMoTaTiN: Structure, optical properties and failure mechanism

With the rapid advance of science and technology, it is essential to research and develop new intrinsic solar absorber coatings with the simple production process and excellent photo-thermal conversion efficiency. Herein, we have developed a novel AlCrMoTaTiN intrinsic absorber layer by radio frequency (RF) reactive sputtering method via adjusting the deposition time and the flow rate of N2. The AlCrMoTaTiN intrinsic absorber layer exhibits a high absorptance of 0.752 and a low emittance of 0.074 at 82 °C. The AlCrMoTaTiN/Al2O3 solar absorber coating is deposited by using Al2O3 works as an anti-reflective layer and the spectral selectivity (α/ε) is 0.929/0.082. This coating also possesses a good thermal stability, which can be stabilized at 300 °C in air and 500 °C in vacuum for 2 h, respectively. In addition, a prolonged period of thermal stability characterization is carried out, which indicates that this coating could keep good optical performance after annealing at 600 °C for 100 h. The corresponding failure mechanism is obtained by analyzing the Raman spectroscopy and XRD. Obviously, the AlCrMoTaTiN/Al2O3 solar absorber coating has a broad application prospect in large-scale and efficient industrial production.