Radio-frequency Sputtering Research Articles

Effects of change of oxygen vacancy on hysteresis voltage and stability under time-temperature dependence positive bias stress in amorphous SZTO transistors

The study on voltage hysteresis (VHys) and instability under the time dependence positive bias stress at a fixed temperature (PBTS) in amorphous silicon zinc tin oxide (a-SZTO) thin-film transistors (TFTs) has been investigated by varying the oxygen flow rate (OFR) during the a-SZTO channel material growth via radio frequency (RF) sputtering. Current-voltage (I-V) measurement was initially taken to study the VHys, which revealed that the VHys was widened by increasing the OFR. The threshold voltage (Vth) shift in a-SZTO TFTs and the impact of OFR on electrical properties and electrical stability of a-SZTO TFTs were analyzed by the PBTS measurement. From PBTS, we observed that the threshold voltage shift difference (∆Vth) increases as the OFR increases. We conclude that a plausible reason for these results may be due to the acceptor-like trap states induced by OFR. The density of Gaussian-like states, excess or weakly bonded oxygen acting as acceptor-like states, is increased by increasing OFR during the deposition of channel material. These results are expected to be of general importance for future works of other amorphous oxide based TFTs.

Study of a Highly Sensitive Formaldehyde Sensor Prepared With a Tungsten Trioxide Thin Film and Gold Nanoparticles

A new gold (Au) nanoparticle (NP)/tungsten trioxide (WO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> )-based chemoresistive formaldehyde gas sensor is fabricated and reported. The WO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> thin film and Au NPs are prepared by radio frequency (RF) sputtering and rapid thermal evaporation (RTE) approaches, respectively. The utilized Au NPs successfully elevate the surface area/volume ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${S}_{A}/{V}$ </tex-math></inline-formula> ) ratio and improve corresponding sensing performance. Experimentally, the proposed Au NP/WO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> sensor shows a very high sensing response of 1303.5 and a response (recovery) time of 16 s (16 s) under introduced 20 ppm formaldehyde (HCHO)/air gas at 225 °C. This value is remarkably higher than those of previous reports. In addition, a very low detection content of 40 ppb HCHO/air is obtained, revealing that a widespread detection range of formaldehyde concentration can be acquired for the proposed sensor. To expand the application ability in wireless sensing, a gray algorithm GM(1, 1) model incorporating the first-order differential approach is used to substantially diminish the transmission data, increase the transmission efficiency, and improve the accuracy of the response data. The formaldehyde sensor in this study exhibits advantages including a simple structure, ease of manufacturing, relatively lower cost, practical smart sensing ability, and wireless transmission applications.

Properties of Indium Tin Oxide Films Grown on Microtextured Glass Substrates

Indium tin oxide (ITO) films were grown by radio frequency (RF) sputtering on plain and textured glass substrates that are kept at room temperature. Periodical texturing on the substrate surface wa...

Optical properties of La1−xSrxVO3 (0 ≤ x ≤ 1) films grown on LSAT substrates using radio frequency sputtering deposition

La1−xSrxVO3 (0 ≤ x ≤ 1) films can be transparent conducting oxides such as n-type SrVO3 and p-type La2/3Sr1/3VO3 films. They also show a semiconductor-to-metal transition (SMT) near x = 0.2. The optical and electrical properties of La1−xSrxVO3 (0 ≤ x ≤ 1) films grown on (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) substrates using RF sputtering deposition were investigated. Resistivities of La1−xSrxVO3 showed SMT near x = 0.2. La1−xSrxVO3 films grown on LSAT substrates were found to have high crystallinity using θ−2θ scan and grazing incidence x-ray diffraction measurements. Dielectric functions of La1−xSrxVO3 films were measured using spectroscopic ellipsometry. The evolution of optical spectra of La1−xSrxVO3 films from LaVO3 to SrVO3 was compared to the electronic band structures as a function of Sr composition, which was either experimentally observed or theoretically calculated in the literature. With increasing Sr composition, the interband transition energies of O 2p to V 3d states near 4 eV changed linearly as a function of Sr content.

Impact of samarium on the structural and physical properties of sputtered ZnO thin films

Zinc oxide (ZnO) thin films containing samarium (Sm) were prepared by radio frequency (RF) sputtering. The Sm was introduced to ZnO films at different RF sputtering powers. The structure of the prepared films has a hexagonal ZnO phase, and the presence of the Sm in the ZnO lattice led to a decrease in the crystalline nature. The average crystallite size values along the major plane (002) decreased with the increase in sputtering power on the Sm. Spherical grains were observed and their values increased from 74.1 nm for the base sample to 82.57 nm when the Sm sputtered at 70 W. After intercalating Sm in the ZnO lattice, the surface roughness was improved. The absorption edge of the samples shifted to a higher photon energy when the Sm increased from 2.51 to 4.33 at%. The linear and non-linear optical parameters and photoluminescence are discussed in terms of the Sm content.

The crystalline nature and samarium substitution improves the nanomechanical and microwave dielectric properties of SBTi thin films

Samarium substituted SBTi (SrBi3.25Sm0.75Ti4O15) thin films were deposited in the temperature range of 675–750 °C on amorphous substrates using radio frequency (RF) sputtering technique. Amorphous to preferred crystalline nature and single phase formation was identified by X-ray diffraction analysis. Atomic force microscopy was used to capture the surface morphology and calculate the grain size. Microwave dielectric properties of SrBi3.25Sm0.75Ti4O15 thin films were investigated at 10 & 20 GHz. The highest dielectric constant (∼118 @ 10 GHz, 104 @ 20 GHz) was obtained for films deposited at 725 °C with preferred orientation. Nanoindentation and scratch tests were performed using nanoindentation technique in order to evaluate the nanomechanical properties of the SrBi3.25Sm0.75Ti4O15 thin films. The best hardness ∼9.6 GPa and Young's modulus ∼120 GPa was observed for the films deposited at 750 °C. This study explored that the films with preferred orientation have potential applications in ferroelectric random access memories and actuators.

A Comparative Study of the Mechanical and Tribological Properties of Thin Al2O3Coatings Fabricated by Atomic Layer Deposition and Radio Frequency Sputtering

Thin Al2O3films (150 nm thick) are deposited by atomic layer deposition (ALD) and radio frequency sputtering on Si substrates and submitted to annealing in N2atmosphere at 900 °C for 90 min. X‐ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy and energy dispersive X‐ray spectroscopy (SEM–EDS), nanohardness, and fretting wear measurements are used to infer the structural, morphological, mechanical, and wear properties of the as‐deposited and annealed films. Results show a higher hardness for the annealed coatings, being the hardness of the annealed ALD coating the highest (18.8 GPa). The measured mechanical properties convey clear trends of stiffening and hardening associated with selected process (ALD versus sputtering) and postprocessing (annealed versus unannealed).

Sputter-grown Eu-doped WO3-Eu2(WO4)3 composite red phosphor thin films

Eu3+-activated WO3-Eu2(WO4)3 luminescent composite thin films are grown on low-cost slide glass substrates by radio frequency sputtering and then annealing in an air atmosphere furnace. The films obtained by sputtering 12 at.% of Eu-added WO3 target afforded a two-phase composite structure comprising Eu-doped WO3 and Eu2(WO4)3 after annealing. The composite thin films annealed at 600 °C exhibited strong red emission after growing at room temperature compared to those annealed at 500 °C under the excitation of 254, 394, and 465 nm corresponding to ultraviolet, near-ultraviolet, and blue light, respectively. The thin films grown at low temperatures without annealing did not emit under ultraviolet light. The crystalline structure, surface morphology, and optical properties, such as transmittance and photoluminescence, of the films grown at 500 °C were almost similar to those of the thin films grown at room temperature when they were annealed at the same temperature. Pure WO3 thin films without Eu doping showed similar crystallization behavior under the same annealing conditions as Eu-added composite films; however, they did not exhibit photoluminescence properties. Our report on the integration of thin-film red phosphors on glass substrates is promising for the development of low-cost and large-area optical devices activated at various excitation wavelengths.

Fabrication and characterization of a porous ZnO flexible UV photodetector on fiberglass and the electrode dimensions effect

In this study, three flexible metal-semiconductor-metal ultraviolet photodetectors are fabricated on the FR4 fiberglass substrate based on the porous zinc oxide by radiofrequency sputtering. Their copper electrodes were created by printed circuit board, a simple and cheap method. Here, the effect of porosity on the photodetector parameters is discussed. The results show that the difference in the finger size in the photodetectors leads to differences in the parameters. In these photodetectors, as the fingers increase in length, dark current increases from 0.25 to 67.3 µA, photocurrent from 1.98 to 208 µA, and response time from 430 to 570 ms. Also, with expanding finger width, these parameters changed 18.48 µA, 26.8 µA, and 440 ms, respectively.

Role of Interfacial Oxide in the Preferred Orientation of Ga2O3 on Si for Deep Ultraviolet Photodetectors.

It is generally known that a layer of amorphous silicon oxide (SiO2) naturally exists on the surface of silicon, resulting in the growth of gallium oxide (Ga2O3) that is no longer affected by substrate crystallinity during sputtering. This work highlights the formation energy between the native amorphous nano-oxide film formed on the Si substrate and monoclinic β-Ga2O3 dominating the preferred orientation prepared for deep ultraviolet photodetectors. The latter were deposited on p-type silicon (p-Si) with (111) orientation using radio frequency sputtering at 600 °C and post rapid thermal annealing (RTA). The X-ray diffraction (XRD) results indicate both as-deposited and postannealing films with the (400) preferred orientation for a layer thickness of 100 nm. However, slight random orientation with the amorphous structure is mixed in the preferred one for the as-deposited film with a thickness of 200 nm and reduced after being annealed at 800 °C, which is observed by XRD and transmission electron microscopy. Meanwhile, thermal-induced massive twin boundaries (TBs) and stacking faults (SFs) were generated when annealed at 1000 °C, owing to the relaxation of lattice strain by the coherent interface. The interfacial bonding energy per unit area (Ei) between β-Ga2O3 films with various facets ((001), (010), (100), and (2̅01)) and amorphous SiO2 was calculated using density functional theory. The Ei of β-Ga2O3 (100)/SiO2 reveals the highest value (0.289 eV/Å2), which is consistent with the (100) preferred orientation of deposited films. The (100) preferred orientation is the driving force for TBs and SFs. The discrimination of responsivities and the photo/dark current contrast ratio (Iph/Idark) are inversely proportional to the amorphous structure, grain boundaries, TBs, and SFs. Therefore, optimum metal–semiconductor–metal photodetector performance is achieved for RTA-treated samples at 800 °C with an Iph/Idark of 3.91 × 102 and a responsivity of 0.702 A/W (λpeak = 230 nm) at 5 V bias for a 200 nm thin film.

Fe2O3-WO3 and Fe2O3-CuO nanoheterostructures by XPS

The detrimental effects of gaseous nitrogen oxides (NOx, with x = 1 and 2) on both human health and the environment have triggered efforts aimed at the development of solar-activated photocatalysts for their efficient removal. In this regard, Fe2O3-WO3 and Fe2O3-CuO nanoheterostructures were prepared by a two-step vapor phase route. In particular, chemical vapor deposition (CVD) of β-Fe2O3, a scarcely investigated iron(III) oxide polymorph, was followed by radio frequency-sputtering of WO3 or CuO under mild conditions. The adopted strategy enabled the obtainment of nanoheterostructures with a peculiar pyramidal morphology and a uniform dispersion of CuO or WO3 onto the pristine iron(III) oxide. In this work, the chemical composition of the target systems was investigated by means of x-ray photoelectron and x-ray excited-Auger electron spectroscopies. In addition to the identification of elemental chemical states, the reported results confirmed the formation of pure and O-deficient systems, in which the direct interplay between the single components opens the door to air purification using the developed systems as photocatalysts.

Origin of magnetism in low energy Ni ion implanted ZnO thin films

ZnO films grown on Si(100) substrate through radio frequency (RF) sputtering are subjected to 100 keV Ni ion beam at different fluence (1×1016, 2.5×1016 and 5×1016 ions/cm2). High resolution X-ray diffraction revealed the increase in crystallite size and development of stress gradient in the ZnO thin films with the increasing Ni ion fluence. Ni implanted ZnO/Si(100) films exhibits room temperature ferromagnetism (RTFM) which is discussed via defect induced bound magnetic polaron model. Near-edge X-ray absorption fine-structure (NEXAFS) measurements at Ni L3,2-edge confirm the nature of implanted Ni ion inside the ZnO matrix. Further, NEXAFS at O K-edge revealed the onset of O(2p) with Ni(3d) orbital states, which is correlated with the observed RTFM.

(INVITED) Tungsten oxide films by radio-frequency magnetron sputtering for near-infrared photonics

Tungsten oxide WO3-x is a transition metal oxide and a wide bandgap semiconductor, with a wide range of possible optical and photonic applications. In dependence on the fabrication techniques different stoichiometric ratios (x) and crystalline phases are obtained, which end up with an overall polymorph and extremely versatile material, characterized by tailorable dielectric properties. In particular, WO3-x thin film deposition by Radio-Frequency (RF) sputtering techniques provides a precise control of thickness, composition and nanostructure. In this work we introduce and discuss a specific process of deposition, that is magnetron RF-sputtering as a suitable way to grow WO3-x thin films with selected properties. Possibility of integrating WO3-x thin film on to one-dimensional (1D) photonic crystal structures is also explored. Films are transparent in the near and short-wavelength infrared optical spectral range. Their quality is assessed by morphological, structural and compositional characterizations. Dielectric properties are characterized by optical spectroscopy and ellipsometry, the latter also evaluates the degree of optical anisotropy of thin films in their crystalline phase. An 1D photonics bandgap structure is designed, formed by a SiO2–TiO2 multilayer and capped with a 450 nm-thick transparent WO3-x film, so that surface confinement and local enhancement of the optical field at 1416 nm in the topmost WO3-x layer is obtained.

Ripple patterns over oblique Ar+ sputtered SiC/Si(1 1 1) surfaces: Role of preferential sputtering

Controlled surface structuring of amorphous silicon carbide (a-SiC) thin films as grown on silicon substrates Si(111) by Radio Frequency (RF) sputtering and irradiation of obliquely incident 80 keV Ar+ ion beam has been investigated. Sub-wavelength ripple patterns with wave-vector parallel to the ion beam projection are found to evolve on argon sputtered surfaces. Studies reveal that the temporal parameters such as ripple wavelength and amplitude, ordering and homogeneity of these patterns vary non-linearly with argon ion fluence. The formation of such surface structures is attributed to the preferential sputtering of silicon in comparison to carbon. Our results reveal that ion irradiation can be exploited to fabricate controlled surface nanostructures by optimizing the irradiation parameters.

A Study on the ZnO Thin Film Deposited by RF Sputtering Method as an Electron Transport Layer in Quantum Dot Light-Emitting Diodes

We report a highly efficient quantum dot light emitting diode (QLEDs) with a radio frequency (RF) sputtered ZnO thin film as an electron transport layer (ETL) instead of the conventional ZnO nanoparticles (NPs) by solution process. ZnO NPs have been used as a key material to improve the performance of QLEDs, but the charge imbalance in ZnO NPs resulting from fast electron injection, and their limited uniformity are significant disadvantages. In this study, ZnO layers were deposited by RF sputtering with various O2 partial pressures. All of the ZnO films showed preferential growth along the (002) direction, smooth morphology, and good optical transmittance. To test their feasibility for QLEDs, we fabricated devices with RF sputtered ZnO layers as an ETL, which has the inverted structure of ITO/RF sputtered ZnO/QDs/CBP/MoO3/Al. The optical/electrical characteristics of two devices, comprised of RF sputtered ZnO and ZnO NPs, were compared with each other. QLEDs with the sputtered ZnO ETL achieved a current efficiency of 11.32 cd/A, which was higher than the 8.23 cd/A of the QLEDs with ZnO NPs ETL. Next, to find the optimum ZnO thin film for highly efficient QLEDs, deposition conditions with various O2 partial pressures were tested, and device performance was investigated. The maximum current efficiency was 13.33 cd/A when the ratio of Ar/O2 was 4:3. Additional oxygen gas reduced the O vacancies in the ZnO thin film, which resulted in a decrease in electrical conductivity, thereby improving charge balance in the emission layer of the QLEDs. As a result, we provide a way to control the ZnO ETL properties and to improve device performance by controlling O2 partial pressure.

A Study on the ZnO Thin Film Deposited by RF Sputtering Method as an Electron Transport Layer in Quantum Dot Light-Emitting Diodes

We report a highly efficient quantum dot light emitting diode (QLEDs) with a radio frequency (RF) sputtered ZnO thin film as an electron transport layer (ETL) instead of the conventional ZnO nanoparticles (NPs) by solution process. ZnO NPs have been used as a key material to improve the performance of QLEDs, but the charge imbalance in ZnO NPs resulting from fast electron injection, and their limited uniformity are significant disadvantages. In this study, ZnO layers were deposited by RF sputtering with various O2 partial pressures. All of the ZnO films showed preferential growth along the (002) direction, smooth morphology, and good optical transmittance. To test their feasibility for QLEDs, we fabricated devices with RF sputtered ZnO layers as an ETL, which has the inverted structure of ITO/RF sputtered ZnO/QDs/CBP/MoO3/Al. The optical/electrical characteristics of two devices, comprised of RF sputtered ZnO and ZnO NPs, were compared with each other. QLEDs with the sputtered ZnO ETL achieved a current efficiency of 11.32 cd/A, which was higher than the 8.23 cd/A of the QLEDs with ZnO NPs ETL. Next, to find the optimum ZnO thin film for highly efficient QLEDs, deposition conditions with various O2 partial pressures were tested, and device performance was investigated. The maximum current efficiency was 13.33 cd/A when the ratio of Ar/O2 was 4:3. Additional oxygen gas reduced the O vacancies in the ZnO thin film, which resulted in a decrease in electrical conductivity, thereby improving charge balance in the emission layer of the QLEDs. As a result, we provide a way to control the ZnO ETL properties and to improve device performance by controlling O2 partial pressure.

Large Area Deposition by Radio Frequency Sputtering of Gd0.1Ce0.9O1.95 Buffer Layers in Solid Oxide Fuel Cells: Structural, Morphological and Electrochemical Investigation.

We investigate the influence of position, under large circular sputtering targets, on the final electrochemical performance of 35 mm diameter button solid oxide fuel cells with sputter-deposited Gadolinium doped Ceria barrier layers, positioned in order to almost cover the entirety of the area associated with a 120 × 80 mm2 industrial cell. We compare the results obtained via structural and morphological analysis to the Electrochemical Impedance Spectroscopy (EIS) measurements performed on the button cells, disentangling the role of different parameters. The Atomic Force Microscopy analysis makes it possible to observe a decrease in the roughness values from the peripheral to the central zones under the sputtering target, with peak-to-valley roughness values, respectively, decreasing from 380 nm to 300 nm, while Scanning Electron Microscopy and Energy Dispersive Spectroscopy show a dependence of the layer coverage from the position. The electrochemical performances of button cells with buffer layers of only 200 nm in thickness, and with negligible thickness gradients across them, show current density values of up to 478 mA/cm2 at 0.8 V and 650 °C, with an improvement of more than 67% with respect to button cells with standard (screen printed) buffer layers. These results point out the major influence exerted by parameters such as the thickness gradient and the coverage of the sputtered buffer layers in determining the final electrochemical performances.

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.

Probing of Pd4+ Species in a PdOx–CeO2 System by X-Ray Photoelectron Spectroscopy

The oxidized palladium nanoparticles comprising Pd4+ species were prepared by radio frequency (RF) discharge in an O2 atmosphere and analyzed with X-ray photoelectron spectroscopy. PdOx particles were deposited on CeO2 or the reference support (Ta2O5) with variation of the RF sputtering time. Regardless of the used support, small PdOx particles (d < 1 nm) contained only Pd2+ species, while an increase of the particle size led to the appearance of the additional oxidized Pd state—Pd4+. The stabilization of Pd4+ on the surface of defect PdO particles was proposed. The Pd4+ species in the PdOx/CeO2 system was stable during heating in ultra-high vacuum conditions up to 250 °C. Pd4+ species demonstrated a high reaction probability toward CO oxidation at room temperature. However, a transition from the relatively inert support (Ta2O5) to the reducible oxide (CeO2) did not lead to a significant improvement of the Pd4+ reaction probability. Pd4+ species could not be recovered by the exposure of the reduced systems to molecular oxygen at room temperature. The obtained results bring new insights into consideration of Pd4+ species as active sites for oxidation processes at low temperatures.