Radio-frequency Sputtering Research Articles

Competing phases in epitaxial SnO2 thin films deposited on sapphire(0001) substrates using radio-frequency powder sputtering

Herein, the competitive crystalline phase between orthorhombic columbite (C–SnO2) and tetragonal rutile (R–SnO2) phases in SnO2 thin films sputter-deposited on sapphire(0001) substrates was investigated. Off-specular X-ray diffraction analysis revealed that only the C–SnO2 was formed initially; however, the growth of the R–SnO2 dominated with increasing thickness. Both phases were epitaxial to the sapphire, and the directional relationship can be summarized as R[100]//C[100]//sapphire[0001] along the surface normal direction and R[010]//C[001]//sapphire [112‾0] along the in-plane direction. In particular, a supercell structure was proposed to accommodate the lattice and symmetry mismatch between C–SnO2 and c-plane sapphire. The electrical conductance of C–SnO2 behaves like a metal as a function of annealing temperature. As the film thickness increased, the optical bandgap gradually decreased from ∼4.68 to ∼3.56 eV owing to the increased amount of the R–SnO2 in the SnO2 thin films. Our results support the crossover of the dominant crystalline phase from the initial C–SnO2 to the later R–SnO2 during the sputter deposition of epitaxial SnO2/sapphire(0001) thin films, which significantly altered the electrical and optical properties.

ZINC SULFIDE ANTI-REFLECTIVE THIN FILM COATING FOR GERMANIUM OPTICAL WINDOWS

In this work, Anti-reflective thin film is made on Germanium (Ge) optical window, which is one of the most used materials in thermal imaging systems. ZnS material was used its optical transmittance between 2-14 m and due to the fact that it has a refractive index proportional to the refractive index of Ge. ZnS thin films have been prepared by Radio Frequency (RF) magnetron sputtering on Germanium (Ge) optical windows for anti-reflection coating (ARC). ZnS films were produced at different thicknesses using RF sputtering system working pressures under 3, 20 and 30 mTorr. The other RF systems parameters such as RF power, deposition temperature were kept constant for all depositions. Crystal structures, optical and surface properties of ZnS thin films were characterized with X-ray diffraction (XRD), Atomic force microscopy (AFM), Fourier transform ınfrared (FTIR) and UV-VIS transmission spectrometer. The characterization results of Ge optical windows coated ZnS thin films grown at 3 mTorr pressure show that high optical transmission and good crystallinity in ınfrared wavelength region (2-14 um).

Production of high-transparent MgO films by radio-frequency sputtering method

Magnesium oxide (MgO) thin films were deposited on silicon and glass substrates by the radio frequency (RF) sputtering method. The MgO films were annealed at 400 ⁰C for 4h. The effect of working pressure on the structure and optical properties of MgO films was investigated. Structural characterization of thin films was determined using the X-ray diffraction (XRD) method. XRD results showed the presence of dominant peaks corresponding to the (200) and (220) lattice planes of MgO. However, peaks corresponding to (111), (311) and (222) lattice planes of the MgO also appeared in the films deposited at low pressure. It was determined that the average crystal size decreased as the working pressure reduced, while the deposition rate increased. SEM analysis showed that the microstructure of the nano-spherical MgO film transformed into a coarse-grained nano-pyramidal shape after annealing. The optical properties of MgO films were investigated by UV-Vis spectroscopy. Accordingly, it was determined that the absorption threshold of the films was around 310 nm wavelength and the optical band gap of the films varied between 4.07 and 4.14 eV. As a result, MgO films with high transmittance reaching an average of 95% in the visible region were obtained.

Effects of substrate temperature and sputtering gas composition on physical properties and photocatalytic activities of WOx thin films deposited via radiofrequency sputtering

Tungsten oxide (WOx) is expected to act as a photocatalytic material under visible light. We have deposited WOx thin films using radiofrequency sputtering and evaluated the photocatalytic activities of the films via degradation of methylene blue solution. The optical absorbance, crystallization, and surface morphology of the WOx thin films were also investigated. The absorption edge of the WOx thin films was shifted to the long-wavelength region when the substrate temperature was high and the O2 gas flow rate was low. Crystallization proceeded when the substrate temperature was high and additional WOx diffraction peaks were appearing with low O2 gas flow during growth. Furthermore, the grain size of the WOx thin films was smaller when the O2 gas flow was high. The photocatalytic activity was higher when the substrate temperature and O2 gas flow rate were low.

ZnO piezoelectric coatings used for the ultrasonic excitation of longitudinal-transverse waves and their application for smart bolts

The accurate measurement of the prestress for bolt has been intensively investigated in recent years. Many research studies have been conducted to improve the accuracy of prestress measurement using ultrasound techniques; in particular, the real-time prestress of bolt can be obtained without the requirements to calibrate the original load when ultrasonic longitudinal and transverse waves are applied simultaneously. However, there are few studies that have focused on ultrasonic transducers that can generate longitudinal and transverse waves simultaneously. Furthermore, research on coatings deposited directly on the top of the bolts for the excitation of longitudinal and transverse waves has not been reported. ZnO coating is widely used for the ultrasonic transducer due to its excellent piezoelectric property and stable structure in high temperatures. Therefore, it is an outstanding candidate for the excitation of both longitudinal and transverse waves and has a good application prospect in the field of bolt prestress measurement by ultrasonic sound wave with high accuracy. In this paper, ZnO piezoelectric coatings were deposited on (100)-oriented Si substrates by radio frequency sputtering techniques. The morphology, structure, and echo signal characteristics of the coatings were characterized by SEM, AFM, XRD, and ultrasonic measurement instruments. The experimental results showed that Ar/O2 ratio and deposition position exhibited significant effects on the excitation waveform of the ZnO piezoelectric coating. When the Ar/O2 ratio reduced to 1:3, the coating could excite both transverse and longitudinal waves. However, when the deposition position gradually moved away from the sputtering center, the transverse wave gradually enhanced. The coating could excite a pure transverse wave when the sample was at the edge of the effective sputtering area, which showed that the type of the excitation wave and the relative intensity of each wave could be well controlled by the Ar/O2 ratio and deposition position. These research results have a good application prospect in bolt stress measurement.

Enhanced Ga2O3-Based RRAM via Stacked Bilayer ZnO/Ga2O3

The stability and endurance of resistive random-access memory (RRAM) devices over long-term use has been widely acknowledged as a concern. Therefore, different top electrodes and oxygen concentration flows were used with stacked ZnO/Ga2O3 as the switching layer to enhance the performance of Ga2O3-based RRAM. All switching layers were deposited by radio frequency sputtering in this study, and the oxygen vacancies were well controlled by controlling the oxygen concentration flow. When a stacked structure was formed, the gradients in the concentration of oxygen vacancies and mobility influenced the set and reset processes. With the stacked structure, the average set voltage was 1.5 V, and the average reset voltage was −0.7 V. In addition, under DC sweeps, the stacked RRAM demonstrated a high operating life of more than 300 cycles. In conclusion, the performance and stability of RRAM can be enhanced by adjusting the concentration of oxygen vacancies using different compositions of elements.

Experimental investigations of plasma dynamics in the hysteresis regime of reactive RF sputter processes

Reactive radio frequency (RF) sputter processes are highly relevant for thin film deposition, but there is no complete understanding of the fundamentals of their operation. While the Berg model describes the hysteresis regime considering the oxygen coverage of the boundary surfaces, a complete fundamental understanding of the plasma–surface interactions and their effects on the discharge is still missing. In this work, we provide such fundamental insights based on an extensive experimental analysis of the physics in the hysteresis regime of magnetized reactive sputter processes, where the same reactive gas admixture can lead to different discharge characteristics depending on the previous state of the plasma. A variety of plasma and surface diagnostics is used to reveal these insights. A low pressure capacitively coupled RF discharge (CCP, 13.56 MHz) with a magnetron-like magnetic field topology adjacent to the target is operated in argon gas with a variable admixture of O2. The applied RF power, the gas flows/pumping speed, as well as the neutral gas pressure are changed systematically to understand the effects of these external control parameters on the hysteresis regime. The magnetic asymmetry effect is found to play an important role, since an axially non-uniform magnetic field is used to realize a local electron confinement at the target. Similar to process control in applications, the DC self-bias is measured to stabilize the surface composition using a feedback controller with the oxygen gas flow as the manipulated variable.

Fabrication of InGaZnO-SnO2/PCBM hybrid electron transfer layer for high-performance Perovskite solar cell and X-ray detector

We developed heterojunction electron-transfer layers (ETLs) comprising layers of metal oxides and phenyl-C71-butyric acid methyl ester (PCBM) for use in glass/indium tin oxide/hole transport layer/Perovskite/ETL/LiF/Al solar cells and X-ray detectors. Indium gallium zinc oxide (IGZO), tin oxide (SnO2), or IGZO/SnO2 layers were stacked on a PCBM layer via radio frequency (RF) magnetron sputtering at various temperatures. The formation of the metal-oxide layers and on a PCBM film were confirmed by conducting compositional and elemental mapping studies. Current-voltage experimental results show that the heterojunction fabricated by forming a IGZO/SnO2 layer on a PCBM film possessed higher charge carrier capacity and exciton dissociation properties compared with using either IGZO or SnO2 separately. The Perovskite solar cell with an IGZO/SnO2@100/PCBM (100 refers 100oC of RF sputtering temperature) ETL attained a power conversion efficiency (PCE) of 12.56 ± 0.15%, which was 36% more efficient than a device with a pure PCBM ETL (PCE = 9.22 ± 0.09%). Moreover, an X-ray detector fabricated with an IGZO/SnO2@100/PCBM ETL obtained a maximum sensitivity of 3.98 mA/Gy·cm2 and collected charge density (CCD)-dark current density (DCD) of 13.29 μA/cm2.

(Digital Presentation) Optimization of MIM Rectifiers for Terahertz Rectennas

Metal-Insulator-Metal (MIM) rectifiers comprising thin films of Al2O3, ZnO, NiO and Nb2O5 and metal configurations of Au/Au, Au/Zn and AuCr/AuCr, have been fabricated using atomic layer deposition and radio-frequency sputtering. The effect of device area scaling from 104 µm2 to 1 µm2 on rectification properties, in particular zero-bias dynamic resistance (R0) and zero-bias responsivity (β0) has been studied and found to be of critical importance in improving diode coupling efficiency. A significant increase of current has been found for Au/3.3 nm ZnO/Au diode when compared to the reference Au/3 nm Al2O3/Au diode, that resulted in obtaining the lowest R0 of 540 W for a device area of 104 µm2. The best performing device is found to be 1 µm2 AuCr/6.77 nm NiO/AuCr featuring (R0, b0) = (461 kW, 0.76 A/W) and a coupling efficiency of 1.5 ´ 10-5 %.

Study of the properties of a PZT thin film formed on a highly doped silicon substrate

Thin films of lead zirconate titanate were formed by plasma radiofrequency sputtering on a low-resistance silicon substrate. The properties of the films were studied by X-ray phase analysis, scanning electron microscopy, atomic force microscopy and, piezoresponse force microscopy. The domain structure was determined by the morphology of the film surface, which had a high surface potential having an average value of +10 V. The average value of the piezoelectric modulus was 15.5 ÷ 17.9 pC/V, and the magnitude of the coercive field was (3.05 ± 0.25)·105 V/cm. The formed PZT films can be used to manufacture the energy harvesters.

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.

Facet passivation process of high-power laser diodes by plasma cleaning and ZnO film

Passivation of dangling bonds at the cleaved mirror facet and its durability are fundamental features of semiconductor lasers to obtain reliable operation with a long device lifetime. The high non-radiative recombination activity of the surface states needs to be controlled to prevent the Fermi level pinning before the deposition of mirror coating materials. Here, we report the incorporation of plasma cleaning of the facet and ZnO film as a passivation layer for the fabrication of high-power semiconductor lasers. The Argon plasma cleaning process was investigated to eliminate surface contamination without damaging the cavity surface. The ZnO passivation films were systematically studied by varying the chamber pressure and sputtering power of the radio frequency (RF) sputter coating process. We obtained homogeneous and dense ZnO films with high surface quality and optical absorption coefficient of zero. By incorporating the optimum plasma cleaning and passivation layer parameters, GaAs-based laser devices with significantly improved catastrophic optical mirror damage (COMD) power were achieved. COMD threshold was increased from 11.9 W to 20.7 W. The life test results demonstrate no failure for facet cleaned and passivated devices for more than 500 h, confirming the long-term effectiveness of the process for actual device integration.

Bandgap modulation and electrical characteristics of (AlxGa1−x)2O3/4H-SiC thin film heterostructures

In this work, (AlxGa1−x)2O3 thin films were grown on a 4H-SiC substrate by radio frequency (RF) sputtering. (AlxGa1−x)2O3 targets with different nominal Al content xt: 0, 0.1, 0.4, 0.8, and 1 were used for RF sputtering. Samples were subsequently annealed at 800 °C to enhance film crystallinity. X-ray diffraction analysis revealed the improved crystallinity for increased Al content (xt), ranging from a ratio of 0 to 0.4 (in relation to Al+Ga content). The films with Al content of 0.8 and 1 exhibited poor crystallinity. The chemical compositions of the Al, Ga, and O atoms were consistent with those of the sputtering targets. Furthermore, the analysis of the inelastic energy loss from X-ray photoelectron spectroscopy confirmed that bandgap tuning is possible for (AlxGa1−x)2O3 using the RF sputtering method. Hall mobility improved up to 17.1%, from 21.90 (xt: 0) to 25.65 (xt: 0.4) cm2V−1s−1. I−V characteristics corresponded well with the Hall measurement. Therefore this study effectively demonstrated the tuning of the bandgap of (AlxGa1−x)2O3 by varying the Al composition. Further, the electrical properties of the (AlxGa1−x)2O3 can be improved or maintained from that of conventional Ga2O3 devices by optimizing the growth ambient.

Cerium-tricalcium phosphate coating for 316L stainless steel in simulated human fluid: Experimental, biological, theoretical, and electrochemical investigations

Tricalcium phosphate (TCP) has many advantages in biomedical applications especially in teeth and bones, and therefore many researches focused on enhancing the properties of this material by different methods. Because of importance of the cerium in repairing the performance of cells in human body, this work aims to substitute the cerium in TCP structure to give better properties. Coating of Ce/TCP on SS 316L was applied by radio frequency (RF) sputtering technique. This coating was characterized by XRD, FESEM/EDS with EDS mapping, AFM, and electrochemical analysis. These techniques confirm the presence of Ce with TCP in deposited layer and the SEM gave more compact layer with higher roughness and lower average diameter. EDS mapping shows the presence of suggested metal ions in the coating. Theoretical study by DFM was done to illustrate the substitution of Ce in the lattice structure and stable sites for accommodation. Electrochemical studies showed that presence of the coating layer improves corrosion resistance with 91% protection efficiency.

A surface plasmon resonance temperature sensor using TiO2 nanoparticles on hetero-core fiber optic structure with Au thin film

Abstract A surface plasmon resonance (SPR)-based temperature sensor was developed based on hetero-core structured fiber optics with multilayer films of titanium dioxide (TiO2) nanoparticles (NPs) and poly-L-lysine (PLL) formed by electrostatic interaction as a simple wet process on gold (Au) film onto a cylindrical cladding surface. We experimentally observed that the resonant wavelength shifted 135.6 nm toward a shorter wavelength for a temperature change from 100 °C to 300 °C. In light intensity-based measurement, the sensitivity of the transmitted loss change was 1.8 × 10−2 dB °C−1 at a wavelength of 700 nm when 19 layers of TiO2 NPs and PLL was annealed onto a 40 nm thick Au film, which improved by over 12 times higher sensitivity than in the conventional radiofrequency (RF) sputtering fabrication method. The proposed sensor successfully detected temperature changes with high sensitivity and linearity as well as simplified the fabrication process compared with the conventional RF sputtering fabrication method.

Spatial profile of Al-ZnO thin film on polycarbonate deposited by ring-shaped magnetized rf plasma sputtering with two facing cylindrical Al2O3 – ZnO targets

Radial profiles of the ion saturation current are measured in a ring-shaped magnetized radio-frequency plasma sputtering process with two facing cylindrical ZnO targets including Al2O3 (2% wt.). The profile has a non-uniform shape with a peak whose position corresponds to the target near the electrode due to the effect of the magnetic field distribution. It becomes uniform at large distances between the substrate and a target (d st ≥ 50 mm). The radial profile of the resistivity of the Al-ZnO (AZO) films deposited on a polycarbonate plate at Ar gas pressure of 0.27 Pa is uniform at about 10−3 Ω · cm for d st ≥ 50 mm. The films deposited at various positions and room-substrate-temperature also show a good crystallinity based on an X-ray diffraction peak of about 33.95–34.44°. The grains exhibit a preferential orientation along the [002] axis with its size ranging from 18.15 to 28.17 nm. A higher transmittance of 95.6% in the visible region is also obtained.

Fractal and inertia moment analyses for thin-film quality monitoring

The widespread applications of thin films in optronics demand innovative techniques for its characterizations. The work reported here proposes electronic speckle pattern interferometry and fractal-based methods for assessing the quality of thin films taking the industrially relevant molybdenum oxide (MoO3) incorporated niobium oxide (Nb2O5) films. The films with different levels of MoO3 incorporation (1, 2, 3, 5, and 10 wt. %) are prepared by radio frequency sputtering. The study reveals the structure modifications of Nb2O5 from the orthorhombic to monoclinic phases with an associated morphological variation revealed through atomic force microscopy and field-emission scanning electron microscopy analyses. The films’ specklegrams are recorded under thermal stress; the inertia moment (IM) and fractal analyses are computed and compared with the root-mean-square surface roughness of the films. The lacunarity analysis of the AFM films agrees well with the specklegrams. Thus, the lower IM and lacunarity values of the specklegrams can be regarded as indicators of the good quality of thin films.

High electron mobility in randomly oriented polycrystalline BaSi2 films formed through radio-frequency sputtering

Semiconducting barium disilicide (BaSi2) is a promising material for solar cell and thermoelectric applications; hence, high-mobility films are of great importance. In this study, we achieved substantially high electron mobilities exceeding 103 cm2 V−1 s−1 at 300 K in randomly oriented polycrystalline BaSi2 films formed on Si3N4 insulating films at 600 °C through radio-frequency sputtering. The BaSi2 films consisted of small grains (<0.5 µm in diameter), and the electron concentration was in the order of 1015–1016 cm−3. Kelvin probe force microscopy revealed that the root-mean-square surface potential values were lower than 31 mV, indicating that the grain boundaries did not hinder electron transport. The potential barrier height across positively charged cracks on the surface of the BaSi2 films, wherein oxidation proceeded, was as small as 30–40 mV. These results indicate that polycrystalline BaSi2 films/insulating films with high electron mobilities are useful for various electronic device applications.

Optical Modulation and Phase Distribution in LiCoO2 upon Li-Ion De/Intercalation

Modulation of reflectance resulting from the change in optical constants in Li x CoO2 during lithium de/intercalation is studied and quantified by in-operando and ex situ optical spectroscopy. To this aim, the LiCoO2 (LCO) thin films are sputter deposited using radio-frequency ion-beam sputtering. The films are structurally characterized by X-ray diffraction and transmission electron microscopy. The reversible electrochemical and electrochromic performance is determined by in-operando optical reflectance. Ex-situ reflectance, at particular charge states, is used to determine the optical constants by modeling the optical spectrum using the Clausius-Mossotti relation. The model reveals a dominant resonant wavelength at 646 nm for the fully intercalated state of LCO. For the delithiated state or Li0.5CoO2, a much broader and significantly larger absorption peak is obtained by the model description. This significantly broad and intense absorption peak can be associated with the conducting nature of the films upon lithium removal. Furthermore, the observed complex refractive index (CRI), evolving with the lithium content, is justified by the prior reported density of states calculations. With the CRI, the corresponding variation of the real and imaginary part of the dielectric function reveals that the intercalation of lithium and the consequent phase propagation follows a layer-like reaction.

Modeling and investigation of the effect of annealing on the electrothermal properties of Ga2O3/SiC heterojunction diodes

In this study, the effects of annealing on the electrothermal properties of Gallium Oxide (Ga2O3) thin films were analyzed. Ga2O3 films were deposited by radio frequency sputtering on 4H-silicon carbide (4H-SiC) substrates. After deposition, one set of the devices was annealed at 900°C for 1 h under a N2 atmosphere. X-ray photoelectron spectroscopy (XPS) was performed to measure the change in the bandgap after annealing. Numerical simulations were performed using the parameters obtained from various measurements, such as XPS and Hall measurements, to investigate the electrothermal properties of Ga2O3. The thermal stability of the devices was derived from the current-voltage characteristics under different ambient temperatures. The relationship between the self-heating effect and annealing was investigated by studying the temperature distribution and power density. We confirmed that the annealed sample has the high current performance with a slight temperature rise.