Radio Frequency Co-sputtering Research Articles

Fabrication of Erbium and Ytterbium Co-Doped Tantalum-Oxide Thin Films Using Radio-Frequency Co-Sputtering

An erbium and ytterbium co-doped tantalum-oxide (Ta2 O5:Er, Yb) thin film was fabricated using a simple co-sputtering method for the first time, and its photoluminescence (PL) spectrum was evaluated. Energy transfers between Er3+ and Yb3+ in the Ta2 O5:Er, Yb co-sputtered thin film were discussed by comparing between PL spectra of the Ta2 O5:Er, Yb film and Ta2 O5:Er or Ta2 O5:Yb films reported in our previous works. Such a Ta2 O5:Er, Yb co-sputtered film can be used as a high-refractive- index and light-emitting material of a multilayered photonic crystal that can be applied to a novel light-emitting device, and it will also be used as a multi-functional coating film having both anti-reflection and down-conversion effects for realizing a high-efficiency silicon solar cell.

Transport properties of β-Ga2O3 nanoparticles embedded in Nb thin films

The origin of ferromagnetism in nanoparticles of nonmagnetic oxides is an interesting area of research. In the present work, transport properties of niobium thin films, with β-Ga2O3 nanoparticles embedded within them, are presented. Nanoparticles of β-Ga2O3 embedded in a Nb matrix were prepared at room temperature by radio frequency co-sputtering technique on Si (100) and glass substrates held at room temperature. The thin films deposited on Si substrates were subjected to Ar annealing at a temperature range of 600-650 C for 1 hour. Films were characterized by X-ray diffraction (XRD), Micro-Raman and elemental identification was performed with an Energy Dispersive X-ray Spectroscopy (EDS). Transport measurements were performed down to liquid helium temperatures by four-probe contact technique, showed characteristics analogous to those observed in the context of a Kondo system. A comparison of the experimental data with the theoretical formalism of Kondo and Hamann is presented. It is suggested that this behavior arises from the existence of magnetic moments associated with the oxygen vacancy defects in the nanoparticles of the nonmagnetic oxide Ga2O3.

Fabrication and evaluation of green-light emitting Ta2O5:Er, Ce co-sputtered thin films

Erbium and cerium co-doped tantalum-oxide (Ta2O5:Er, Ce) thin films were fabricated using radio-frequency co-sputtering of Ta2O5, Er2O3, and CeO2 for the first time. Enhanced green-light emission due to Er3+ that seems to be sensitized by Ce3+ was observed from the film annealed at 900°C for 20min. From XRD measurements of the films, the β-Ta2O5 (orthorhombic), δ-Ta2O5 (hexagonal), and (201) Ta2O5 phases seem to be very important for obtaining green PL from them. Such Ta2O5:Er, Ce co-sputtered films can be used as high-refractive-index materials of autocloned photonic crystals that can be applied to novel green-light-emitting devices, and they will also be used as multi-functional coating films that can work both as anti-reflection and down-conversion films for realizing high-efficiency silicon solar cells.

In-situ high resolution transmission electron microscopy observation of silicon nanocrystal nucleation in a SiO2 bilayered matrix

Solid-state nucleation of Si nanocrystals in a SiO2 bilayered matrix was observed at temperatures as low as 450 °C. This was achieved by aberration corrected high-resolution transmission electron microscopy (HRTEM) with real-time in-situ heating up to 600 °C. This technique is a valuable characterization tool especially with the recent interest in Si nanostructures for light emitting devices, non-volatile memories, and third-generation photovoltaics which all typically require a heating step in their fabrication. The control of size, shape, and distribution of the Si nanocrystals are critical for these applications. This experimental study involves in-situ observation of the nucleation of Si nanocrystals in a SiO2 bilayered matrix fabricated through radio frequency co-sputtering. The results show that the shapes of Si nanocrystals in amorphous SiO2 bilayered matrices are irregular and not spherical, in contrast to many claims in the literature. Furthermore, the Si nanocrystals are well confined within their layers by the amorphous SiO2. This study demonstrates the potential of in-situ HRTEM as a tool to observe the real time nucleation of Si nanocrystals in a SiO2 bilayered matrix. Furthermore, ideas for improvements on this in-situ heating HRTEM technique are discussed.

Effect of Mg and Zr co-doping on piezoelectric AlN thin films for bulk acoustic wave resonators

This paper reports the crystal structures and piezoelectric properties of Mg and Zr co-doped AlN (MgZr-doped AlN) thin films. MgZr-doped AlN thin films on Si (100) substrates were fabricated by using a radio-frequency magnetron reactive cosputtering system. The relations between dopant concentration and the crystal structures measured by X-ray diffraction, transmission electron microscopy, and the piezoelectric constant d 33 measured with a piezometer were investigated. Up to total Mg and Zr concentrations of 34.8 at%, the MgZr-doped AlN films maintained the c-axis-oriented wurtzite structure and were likely formed by substituting Mg and Zr atoms into Al atom sites. The d 33 of the MgZr-doped AlN with a total Mg and Zr concentration (Mg + Zr) of 34.8 at% was about three times that of pure AlN. The experimental results on the relation between total Mg and Zr concentration and the crystal structure and the d 33 were in close agreement with the results of first-principles calculations. Finally, thin film bulk acoustic wave resonators (FBARs) that used MgZr-doped AlN as a piezoelectric thin film were fabricated and compared with the AlN-based FBAR. A total Mg and Zr concentration of 13 at% was found to improve the electromechanical coupling coefficient of AlN from 7.1% (for pure AlN) to 8.5%. The results from this study suggest that the MgZr-doped AlN films have potential as piezoelectric thin films for wideband and high-frequency RF applications.

Optical Properties of Amorphous Silicon–Carbon Alloys (a-Si x C1-x ) Deposited by RF Co-Sputtering

Amorphous silicon–carbon alloy (a-SixC1-x) thin films have been deposited by radio frequency (RF) sputter deposition. These films were obtained, from a composite target consisting of silicon fragments regularly distributed on the surface of a pure graphite disc, for different values of silicon surface fraction RSi/C, at an RF power of 250 W. X-ray diffraction diagrams show that all the as-deposited SixC1-x thin films are amorphous. The optical properties of the a-SixC1-x films were investigated by optical transmission measurements in the ultraviolet–visible–near infrared wavelengths range. The optical band gap Eg varies, with RSi/C, from 1.4 to 1.9 eV and presents a wide maximum at an RSi/C value of about 35 %. This maximum value is attributed to amorphous silicon carbide a-SiC as confirmed by theoretical correlation between the molar fraction x and RSi/C. The refractive index n follows well the Cauchy law and the extrapolated value, at infinite wavelengths, increases from 2.1 to 2.65 as the RSi/C fraction increases.

Microstructure Evolution and Enhanced Tribological Properties of Cu-Doped WS2 Films

To improve the tribological properties of WS2 film both in vacuum and in humid air conditions, its microstructure was optimized by doping different concentrations of Cu via radio frequency co-sputtering method. The film microstructure and composition were investigated by field emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, grazing incidence X-ray diffraction and high-resolution transmission electron microscopy. It was verified that Cu was presented in amorphous phase in the WS2 matrix and could also induce amorphization and densification of the composite films gradually. The film microstructure changed from coarse columnar platelet structure at low Cu content (0–5.8 at.%) to transition structure with two separate layers at increased Cu content (11.5–16.2 at.%) and to a featureless structure at high Cu content (above 24.4 at.%). The mechanical and tribological properties of films were evaluated using the scratch tester and ball-on-disk tribometer, respectively. It was found that the incorporation of a suitable content of Cu dopant could significantly improve the film toughness, but excess amount of Cu dopant lead to high brittleness. All the composite films exhibited much lower wear rate and longer wear life than those of pure WS2 film both in vacuum and in humid air conditions. The wear mechanisms were proposed after correlating the mechanical performance with film microstructure.

Infrared and Raman Study of Amorphous Silicon Carbide Thin Films Deposited by Radiofrequency Cosputtering

ABSTRACT Amorphous silicon carbide thin films have been deposited by radiofrequency cosputtering. These films were deposited from a composite target consisting of silicon fragments regularly distributed on the surface of a pure graphite disc for different relative silicon-to-carbon area ratio values (15%, 25%, 35%, 50%, and 65%), with a radiofrequency power of 250 watts. Structural properties of these films were investigated by Fourier transform infrared and Raman spectroscopy techniques. The infrared absorption spectra obtained on as-deposited films show the presence of two absorption bands, which are assigned to vibrational modes of silicon–carbon and silicon–oxygen bonds. The content of silicon–carbon bonds increases with increasing the relative silicon-to-carbon area ratio from 15% to 35% and then decreases for greater values. The maximum value of silicon–carbon bonds’ content, occurring at about the relative silicon-to-carbon area ratio equal to 35%, indicates that this surface ratio value leads to the formation of nearly stoichiometric silicon carbide. The Raman spectroscopy measurements show that all the as-deposited films are amorphous and contain not only silicon–carbon bonds but also silicon–silicon and/or carbon–carbon bonds (sp3and sp2). The silicon-rich films contain mainly silicon bonds, while the carbon-rich ones contain mainly carbon bonds. The maximum content of silicon–carbon bonds for the relative silicon-to-carbon area ratio equal to 35% is confirmed by Raman results.

Structural, optical, and electrical properties of NiO-In composite films deposited by radio frequency cosputtering

In-doped NiO films with indium concentrations ranging from 0 to 30.3 at. % were deposited on glass substrates to investigate corresponding structural, optical, and electrical property variations. The x-ray diffraction patterns show that all films display only NiO peaks. When In atoms were added to NiO films, the NiO peaks shifted to lower angles, indicating that the lattice parameters of the films increased due to the larger In ions substituting for the smaller Ni ions. An electrical resistivity (ρ) too high to be measured occurred when the indium concentration in the NiO film was less than 15.6 at. %. The ρ value dropped significantly to 0.06 Ω·cm as the indium concentration increased to 26.9 at. %. Upon further raising the In to 30.3 at. %, the ρ value decreased further to 0.01 Ω·cm. All the In-doped NiO films showed n-type conduction. The transmittance of undoped NiO film is as high as 96%. On raising the indium concentration to 15.6, 19.9, 26.9, and 30.3 at. %, the transmittances decreased further to 68%, 62%, 57%, and 47%, respectively. Introducing higher In concentrations improved the films’ thermal stability of electrical resistivity.

Pinning effect on the band gap modulation of crystalline BexZn1−xO alloy films grown on Al2O3(0001)

Pinning effect on crystalline BexZn1−xO alloy films on Al2O3(0001).

Morphology evolution of Ag alloyed WS2 films and the significantly enhanced mechanical and tribological properties

The WS2–Ag composite films were prepared by radio frequency co-sputtering method. The effects of alloying Ag content on composition, microstructure, mechanical properties and friction behaviors have been analyzed by X-ray photoelectron spectroscopy (XPS), grazing incidence X-ray diffraction (GIXRD), field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDS) and high resolution transmission electron microscope (HRTEM), scratch tester, nano-indentation tester and ball-on-disk tribo-tester. The Ag addition was in present of nanocrystalline phase in the boundary of the crystalline WS2 matrix and induced morphology change, but could not completely prevent the columnar platelets. The columnar platelet was composed of a great deal of nanocrystalline and a small amount of amorphous WS2 phase. There were no substantial variations in the hardness of the composite films when the Ag content was in the range of 0–20.3at.%. The suitable amount of Ag content was benefited for improving the film adhesive strength and wear resistance both in vacuum and humid air environment. Particularly, the composite film with 9.0at.% Ag exhibited the longest wear life (about 7.6×105cycles in vacuum, 6.7×105cycles in humid air) under high Hertzian contact pressure (as high as 1.0GPa). The wear mechanism was discussed in terms of the rearrangement of wear track and formation of transfer film.

Growth optimization of ZnxCd1 − xS thin films by radio frequency magnetron co-sputtering for solar cell applications

Zinc cadmium sulfide (ZnxCd1−xS) thin films (0≤x≤1) were deposited by the radio frequency (RF) co-sputtering of cadmium sulfide (CdS) and zinc sulfide (ZnS). The RF powers of CdS and ZnS were varied to control the composition of the films, which was confirmed using energy dispersive X-ray analysis. The structural properties of the films were investigated using X-ray diffraction, which showed that the films have a hexagonal (wurtzite) structure with a strong preferential orientation along the (002) plane. The values of lattice constants ‘a’ and ‘c’ decreased as ‘x’ increased. The surface morphology, topology and optical properties were investigated using field emission scanning electron microscopy (FESEM), atomic force microscopy and ultraviolet spectrophotometry. The FESEM studies revealed an increase in grain size for zinc (Zn) contents up to x=0.62, followed by a decrease in the grain size until ‘x’ reached 0.81, above which the films were amorphous. The optical band gaps of the films were obtained from optical absorption measurements and shifted to a higher energy as the content of ‘x’ increased. The presence of a small amount of zinc in CdS strongly influenced the optical band gap and transmittance of ZnxCd1−xS thin films. The electrical sheet resistance of the films was also found to be relatively high. Among the range of ZnxCd1−xS compositions tested, the samples with a Zn content of 0.17 to 0.43 showed a better film quality, making them suitable as the window layer in ZnxCd1−xS/CdTe thin film solar cells.

Optical design and co-sputtering preparation of high performance Mo–SiO2 cermet solar selective absorbing coating

In order to optimize and prepare high performance Mo–SiO2 cermet solar selective absorbing coating, a series of Mo–SiO2 cermet films with different metal volume fraction were deposited on optical glass using mid-frequency (MF) and radio frequency (RF) co-sputtering. The reflectance (R) and transmittance (T) in the wavelength range of 250–2500nm have been simulated using SCOUT software with different dielectric function models. The optical constants, film thickness, metal volume fraction and other parameters have been deduced from the modeling. The fitted optical constants were then used to simulate and optimize the Mo–SiO2 solar selective coating and samples were prepared based on the optimized parameters. The Maxwell Garnett (MG) and Bruggeman (BR) effective-medium theory have been added in the dielectric function models to describe low metal volume fraction cermet layer (LMVF) and high metal volume fraction cermet layer (HMVF), separately. The optical spectra (R and T) of all single films were in a good agreement with the fitted spectra by dielectric function models. The experimental measured reflectance of the solar selective coating was also in rather good agreement with the optimized result. The solar absorptance of theoretically optimized selective coating was 0.945, while the absorptance of the experimental coating was 0.95. The thermal emittance of 0.15 (at 400°C) was obtained.

Sensing Mechanism and Behavior of Sputtered ZnCdO Ozone Sensors Enhanced by Photons for Room- Temperature Operation

ZnCdO oxide thin films have been deposited by using the radiofrequency cosputtering method to prepare ozone sensors for room-temperature operation. The sensors were fabricated in the order: electrode/sensor/glass/illuminant. The mechanisms of photo-assisted oxidation and reduction on the surface of the ZnCdO ozone sensors were investigated. Free electrons and holes were generated by exposure of the surface of the ZnCdO ozone sensor to ultraviolet light, tending to enhance surface absorption of dissociated O2 molecules and dissociation of adsorbed oxygen ions, respectively. Thereby, the sensitivity of the ZnCdO ozone sensor was increased. This photon exposure method can replace the conventional heating mode used in ozone sensors. Moreover, 405-nm light from a light-emitting diode, corresponding to photon energy of 3.061 eV, was found to assist the processes of oxidation and reduction due to the chemical reaction of the ozone gas. We present a possible route for fabrication of portable ZnCdO ozone sensors for room-temperature operation.

Synchrotron radiation based cross-sectional scanning photoelectron microscopy and spectroscopy of n-ZnO:Al/p-GaN:Mg heterojunction

Al-doped ZnO (AZO) deposited by radio frequency co-sputtering is formed on epitaxial Mg-doped GaN template at room temperature to achieve n-AZO/p-GaN heterojunction. Alignment of AZO and GaN bands is investigated using synchrotron radiation based cross-sectional scanning photoelectron microscopy and spectroscopy on the nonpolar side-facet of a vertically c-axis aligned heterostructure. It shows type-II band configuration with valence band offset of 1.63 ± 0.1 eV and conduction band offset of 1.61 ± 0.1 eV, respectively. Rectification behavior is clearly observed, with a ratio of forward-to-reverse current up to six orders of magnitude when the bias is applied across the p-n junction.

Effect of thermal treatments on sputtered silver nanocluster/silica composite coatings on soda-lime glasses: ionic exchange and antibacterial activity

Silver nanocluster/silica composite coatings were deposited on both soda-lime and silica glasses by radio frequency (RF) co-sputtering. The effect of thermal treatments on the microstructure in the range of 150–450 °C were examined by UV–visible spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and Time of Flight-Elastic Recoil Detection Analysis. Sodium/silver ionic exchange was evidenced for coatings sputtered on soda-lime substrates after heating at 450 °C; presence of silver ions and/or silver nanoclusters, nanocluster size and their position inside the sputtered layers will be discussed for as-deposited and heated coatings on both substrates. The antibacterial activity of all coatings was determined against Staphylococcus aureus and Candida albicans by disk diffusion method and colonies forming units count; in agreement with microstructural results, the antibacterial activity present on all coatings was slightly reduced after heating at 450 °C. All coatings have been submitted to humidity plus UV ageing and sterilization by autoclave, gamma ray and ethylene oxide gas. Tape resistance (ASTM D3359-97) tests have been done on each coating before and after ageing and sterilizations, revealing a good adhesion on soda-lime substrates, except for those aged in humidity plus UV and sterilized by autoclave. Scratch tests and nanoindentation tests have been done on each coating, as-deposited and after heating at 450 °C. The coating hardness was improved by heating only when coatings were deposited on silica. The heating of coatings deposited on soda-lime substrates gave opposite effect on their hardness.

Silver nanocluster/silica composite coatings obtained by sputtering for antibacterial applications

Silver nanocluster silica composite coatings were deposited by radio frequency co-sputtering technique on several substrates. This versatile method allows tailoring of silver content and antibacterial behaviour of coatings deposited on glasses, ceramics, metals and polymers for several applications. Coating morphology and composition as well as nanocluster size were analyzed by means of UV-Visible absorption, X-ray diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM), electron dispersive spectroscopy (EDS), X-ray Photoelectron Spectroscopy (XPS) and Atomic Force Microscopy (AFM). The antibacterial effect was verified through the inhibition halo test against standard bacterial strain, Staphylococcus aureus, before and after sterilization process. Tape test demonstrated a good adhesion of the coatings to the substrates.

Antibacterial coating on polymer for space application

The microbiological contamination on board of spacecraft and orbital stations is a relevant problem in prolonged space exploration. For this purpose, an antibacterial silver nanocluster silica composite coating was deposited on a commercial polymer Combitherm®, suitable for aerospace application, using the radio frequency (RF) co-sputtering technique. The presence of metallic silver nanoclusters and silica was confirmed by energy dispersion spectrometry (EDS), x-ray photoelectron spectroscopy (XPS) and localized surface plasmon resonance (LSPR) detected through UV–visible absorption spectrophotometry (UV–Vis). The atomic force microscope (AFM) evidenced the coating morphology. The slight hydrophobicity of both coated and uncoated samples was revealed through the contact angle measurement. The antimicrobial behavior was verified through evaluation of the inhibition halo against several bacterial and fungal species. The coating enhanced the Combitherm® nano-hardness and its resistance to tensile and perforation tests; the coating wear resistance was measured by abrasion test against Kevlar. A folding procedure on the coated Combitherm® and storage in air for three months was also carried out without deterioration of the measured properties. The coating deposition did not influence the air permeability of Combitherm®.

The Effects of Gadolinium Incorporation Into Indium–Zinc–Oxide Thin-Film Transistors

We investigated the effects of gadolinium (Gd) incorporation into indium-zinc-oxide (IZO) thin-film transistors (TFTs) using radio-frequency cosputtering of IZO and Gd. A gadolinium-indium-zinc-oxide (Gd-IZO) TFT with 2.4 at.% Gd content had saturation-mode field-effect mobility, threshold voltage, and switching ratio (on current/off current) of 6.6 cm2V-1s-1, 1.04 V, and on the order of 107, respectively, after thermal annealing at 250 °C. A Gd-IZO TFT with 2.4 at.% Gd content showed better switching performance and thermal stability than pure IZO TFTs due to stable ionic bond between Gd and O.

Low-Temperature Fabrication and Characteristics of Lanthanum Indium Zinc Oxide Thin-Film Transistors

We fabricated lanthanum indium zinc oxide (IZO) (La-IZO) thin-film transistors (TFTs) using radio frequency cosputtering of IZO and lanthanum. The field-effect mobility and switching ratio of the La-IZO TFTs were 4.2 cm2·V-1·s-1 and on the order of 108, respectively, before thermal annealing and 3.02 cm2·V-1·s-1 and on the order of 1010, respectively, after thermal annealing at 150°C in air. Our La-IZO TFTs had better thermal stability than pure-IZO TFTs due to the stable ionic bond between La and O.