High Base Pressure Research Articles

Crystallinity control of sputtered ZnO films by utilizing buffer layers fabricated via nitrogen mediated crystallization: Effects of nitrogen flow rate

High quality ZnO:Al (AZO) films have been obtained by utilizing buffer layers fabricated via nitrogen mediated crystallization (NMC), where sputtering method is employed for preparation of both buffer layers and AZO films. Introduction of small amount of N2 (N2/(Ar+N2) = 16%) to the sputtering atmosphere of NMC-ZnO buffer layers drastically improves the crystallinity of buffer layers and thus AZO films. The most remarkable effect of the buffer layers is a significant reduction in the resistivity at high base pressure of background gases. The resistivity of conventional AZO films increases from 2.0 m O•cm to 70.0 O•cm with increasing the base pressure from 3×10-5 Pa to 1×10-3 Pa, while the resistivity of AZO films with NMC buffer layers increases from 0.5 mO•cm to 2.0 mO•cm, where the thickness of AZO film is 88 nm. Furthermore, AZO films with a sheet resistance of 10O/? and an optical transmittance higher than 80% in a wide wavelength range of 400–1100 nm have been obtained.

Nanostructured Titanium/Diamond-Like Carbon Multilayer Films: Deposition, Characterization, and Applications

Titanium/diamond-like carbon multilayer (TDML) films were deposited using a hybrid system combining radio frequency (RF)-sputtering and RF-plasma enhanced chemical vapor deposition (PECVD) techniques under a varied number of Ti/diamond-like carbon (DLC) bilayers from 1 to 4, at high base pressure of 1 × 10(-3) Torr. The multilayer approach was used to create unique structures such as nanospheres and nanorods in TDML films, which is confirmed by scanning electron microscopy (SEM) analysis and explained by a hypothetical model. Surface composition was evaluated by X-ray photoelectron spectroscopy (XPS), whereas energy dispersive X-ray analysis (EDAX) and time-of-flight secondary ion mass spectrometer (ToF-SIMS) measurements were performed to investigate the bulk composition. X-ray diffraction (XRD) was used to evaluate the phase and crystallinity of the deposited TDML films. Residual stress in these films was found to be significantly low. These TDML films were found to have excellent nanomechanical properties with maximum hardness of 41.2 GPa. In addition, various nanomechanical parameters were calculated and correlated with each other. Owing to metallic interfacial layer of Ti in multilayer films, the optical properties, electrical properties, and photoluminescence were improved significantly. Due to versatile nanomechanical properties and biocompatibility of DLC and DLC based films, these TDML films may also find applications in biomedical science.

Incorporation and critical concentration of oxygen in a-Si:H solar cells

For different process conditions, series of hydrogenated amorphous silicon p-i-n solar cells with various oxygen concentrations in the intrinsic absorber layer were fabricated by plasma-enhanced chemical vapor deposition at 13.56 MHz using process gas mixtures of SiH 4 and H 2. Oxygen was introduced into the gas phase during the deposition process by a controllable leak in the chamber wall and the amount of oxygen supply is characterized by the oxygen base pressure p b . It is found that for a certain deposition regime defined by silane and H 2 flows, deposition pressure and substrate temperature the oxygen incorporation follows an expected dependence on the ratio p b / r d with r d the deposition rate. This relation is not valid for the comparison of different deposition regimes. A high hydrogen flow is found to reduce the oxygen incorporation strongly. The photovoltaic parameters of the solar cells were measured in the initial state as well as after 1000 h of light-soaking. The critical oxygen concentration (i.e. the upper limit of incorporated oxygen not leading to a decay of the solar cell performance) was determined for each regime in the initial and light-soaked state. For all deposition regimes, the results show no difference in these critical oxygen concentrations for the initial and light-soaked state. The critical oxygen concentration, is found to differ for the different process regimes and turns out to be the highest (approximately 1×10 20 cm −3) for the deposition regime with the highest hydrogen flow rate, which interestingly is the regime with the lowest oxygen incorporation at a given p b / r d ratio. This combination makes the regime of high hydrogen gas flow suitable for depositing high-efficiency solar cells at high base pressure.

Air-Based Deposition of Conductive Nitride Thin Films by Sputtering

Conductive nitride films are conventionally prepared by reactive sputtering using nitrogen as a reactive gas. Replacing nitrogen with air as a reactive gas allows the process to be conducted at high base pressures, i.e., low vacuum, which can reduce substantially the processing time. In this study, TiN films were selected as a model system for the investigation. As the air/Ar flow ratio reached 0.10–0.15, the films revealed the characteristic rock-salt structured TiN. The N/Ti of the films was 0.80–0.83 with 9–13 atom % of oxygen. The determined resistivities and hardnesses of the films were in the range of 110–130 μΩ·cm and 26–27 GPa, respectively. All obtained data fulfill the characteristics of TiN films. Kinetic formation apparently prevails over thermodynamic predictions at low air/Ar ratios. In such plasma, the dissociation energy of nitrogen is much smaller than that of oxygen, leading to the high impingement rate of nitrogen. The dissolution of oxygen in the TiN films seems also to stabilize the nitride structure.

Correlation of sp 3 and sp 2 fraction of carbon with electrical, optical and nano-mechanical properties of argon-diluted diamond-like carbon films

In the present work the correlation of electrical, optical and nano-mechanical properties of argon-diluted diamond-like carbon (Ar-DLC) thin films with sp 3 and sp 2 fractions of carbon have been explored. These Ar-DLC thin films have been deposited, under varying C 2H 2 gas pressures from 25 to 75 mTorr, by radio frequency-plasma enhanced chemical vapor deposition technique. X-ray photoelectron spectroscopy studies are performed to estimate the sp 3 and sp 2 fractions of carbon by deconvoluting C 1s core level spectra. Various electrical, optical and nano-mechanical parameters such as conductivity, I– V characteristics, optical band gap, stress, hardness, elastic modulus, plastic resistance parameter, elastic recovery and plastic deformation energy have been estimated and then correlated with calculated sp 3 and sp 2 fractions of carbon and sp 3/sp 2 ratios. Observed tremendous electrical, optical and nano-mechanical properties in Ar-DLC films deposited under high base pressure conditions made it a cost effective material for not only hard and protective coating applications but also for electronic and optoelectronic applications.

Base Pressure Effects on the Structure, Surface Roughness and Electrical Resistivity of Mo Thin Films by RF Magnetron Sputtering

Molybdenum (Mo) thin films were deposited on soda-lime glass substrate by using RF magnetron sputtering. As is well known, base pressure in sputtering is a processing parameter for preparing high quality Mo films, in addition to RF power and working pressure. The structure, surface roughness and electrical resistivity of Mo thin films were investigated at three base pressures of 1.8×10-7, 5.8×10-7and 2.7×10-6 torr. Experimental results indicate that the Mo films deposited at a low base pressure of 1.8×10-7 torr exhibit a low electrical resistivity of 8.910-6 –cm, a low root mean square value of roughness of 4.92nm and a stronger peak intensity of (110) plane than that of the other two pressures. This finding suggests that Mo films sputtered at a low base pressure have higher crystallinity, larger grain size, smoother surface morphology and lower electrical resistivity than those sputtered at a high base pressure.

Preparation of ZrN xO y films by magnetron sputtering using air as a reactive gas

Zirconium oxynitride (ZrN x O y ) thin films were prepared by d.c. magnetron sputtering using air as a reactive gas. Replacing conventionally used N 2/O 2 with air as a reactive gas allows the process to perform at high base pressures (low vacuum), which could drastically reduce the processing time. The color of the obtained films changed from light golden and dark golden for low air/Ar flow ratios, to dark violet and bright cyan for high air/Ar ratios. X-ray diffraction patterns show that the films transformed from ZrN and Zr 2ON 2 mixed phases to a Zr 2ON 2 phase, and then an m-ZrO 2 phase. The thickness of the films decreased slightly with increasing the air/Ar flow ratio. ZrN x O y films possessed a mixture of Zr–N–O, Zr–N and Zr–O chemical binding states determined from X-ray photoelectron spectroscopy. ZrN x O y films with mainly a Zr 2ON 2 phase exhibited the band gap of 1.96–2.26 eV, while the m-ZrO 2 films with slight nitrogen incorporation had a band gap of 2.32 eV, evaluated from transmittance spectra. By varying the air/Ar ratio during deposition, the nitrogen/oxygen content of the films could be controlled and hence, the color, crystal structure, atomic composition, and band gap of the films could be tailored.

Influences of oxygen contamination on evaporated poly-Si thin-film solar cells by solid-phase epitaxy

The influences of the oxygen contaminations on the crystal quality and performances of the evaporated polycrystalline silicon (poly-Si) thin-film solar cells prepared by solid-phase epitaxy were investigated by applying different deposition rates and base pressures. The experimental results show that although the evaporated poly-Si thin-film solar cell obtained at high base pressures (9.33 × 10 − 5 Pa) and high deposition rate (300 nm/min) has small amount of SiO 2 precipitations, it still shows the similar good material quality and performances as the cell prepared at low base pressure (1.33 × 10 − 6 Pa) and high deposition rate (300 nm/min) with oxygen interstitials. On the other hand, the poly-Si thin-film solar cell deposited at low base pressure (1.33 × 10 − 6 Pa) and low deposition rate (50 nm/min) has large amount of SiO 2 precipitations and resulting worse material quality and hence cell performances. Therefore, the high deposition rate is desirable to maximize the solar cell performance, as well as the throughput. It is a more influential factor than the base pressure.

Coexistence of epitaxial Ta(111) and Ta(110) oriented magnetron sputtered thin film on c-cut sapphire

Epitaxial Ta films were grown on c-cut and r-cut sapphire substrates using magnetron sputtering. X-ray diffraction measurements indicate that the Ta films grown on c-cut sapphire substrates have two different crystal orientations. Low vacuum base pressure produced films with the (111) plane parallel to the basal plane after an initial (110) plane growth phase, whereas high base pressure resulted in the growth of (110) plane parallel to the basal plane with three azimuthally oriented domains coexisting. The superconducting critical temperatures Tc of the films grown on c-cut substrates were found to be slightly higher than those for the films grown on r-cut substrates. The x-ray photoelectron spectroscopic data show oxygen content in the high vacuum base pressure deposited films and a difference in the valance band with different Ta surface planes.

Characterization of N-doped TiO2 films prepared by reactive sputtering using air/Ar mixtures

Nitrogen-doped titanium dioxide (TiO2−xNx) thin films desirable for visible light photocatalysts were prepared by reactive sputtering using air/Ar mixtures. Using air as the reactive gas allows the process to conduct at high base pressures (low vacuum), which reduces substantially the processing time. The obtained films transformed from mixed phases to anatase phase as the air/Ar flow ratio increased. Substitutional doping of nitrogen verified by X-ray photoelectron spectroscopy accounts for the red-shift of absorption edge in the absorption spectra. Anatase TiO2−xNx films could incorporate up to about 7.5at.% substitutional nitrogen and a maximum of 23at.% nitrogen was determined in the films with mixed phases. The optical band gaps of the TiO2−xNx films calculated from Tauc plots varied from 3.05 to 3.11eV and those of the mixed phase ranged from 2.77 to 3.00eV, which are all lower than that for pure anatase TiO2 and fall into the visible light regime.

Growth of ${\rm MgB}_{2}$ Thin Films In Situ by RF Magnetron Sputtering With a Pocket Heater

We have grown MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> thin films using RF magnetron sputtering combined with a pocket heater. This technique relies on a low-pressure environment for sputter deposition of boron and a high-pressure environment for thermal evaporation of Mg. We have obtained superconducting MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> thin films using substrate temperatures of 480-540degC and Mg furnace temperatures of 730-750degC. The T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> onset of the thin films increased from 21.6 K to 35 K with increasing substrate temperature due to better crystallization. Higher boron deposition rates also increase T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> . The highest J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of the films at 5 K and near zero magnetic field is 1.5 MA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> which is comparatively lower than the films grown using a pocket heater with boron deposition by chemical vapor deposition or electron beam evaporation. The chemical composition analysis by WDS exhibits a high concentration of oxygen and carbon in the MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films, which is due to a high background base pressure and an impure B sputtering target. These results suggest that the T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> , J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> , and resistivity of MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films are mainly determined by an inhomogeneous microstructure and superconducting percolation paths through impurity phases such as MgO. By comparison with carbon or oxygen doped films, the high impurity content in the sputtered MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films might act as method to achieve high H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c2</sub> .

X-ray photoelectron spectroscopy analyses of titanium oxynitride films prepared by magnetron sputtering using air/Ar mixtures

X-ray photoelectron spectroscopy (XPS) has been employed to investigate titanium oxynitride (TiN xO y) films prepared by d.c. magnetron sputtering using air/Ar mixtures, which allows one to perform the deposition at a high base pressure (1.3 × 10 − 2 Pa) and can reduce substantially the processing time. XPS analyses revealed that all the prepared TiN xO y films comprised Ti–N, Ti–N–O, and Ti–O chemical states. When the air/Ar ratio was below 0.3, nitrogen-rich TiN xO y films were obtained. As the air/Ar ratio was above 0.4, oxygen-rich TiN xO y films were formed. XPS depth profile analyses were also performed in selected specimens. It has been found that at relatively low air/Ar ratios, such as 0.5, the oxygen content of the films increased toward the film/substrate interface and when the air/Ar ratio was higher, TiN xO y films with large oxygen content with uniform concentrations were then formed.

Cellular vortex shedding in the wake of a tapered plate

Direct numerical simulation (DNS) of vortex shedding behind a tapered plate with the taper ratio 20 placed normal to the inflow has been performed. The Reynolds numbers based on the uniform inflow velocity and the width of the plate at the wide and narrow ends were 1000 and 250, respectively. For the first time ever cellular vortex shedding was observed behind a tapered plate in a numerical experiment (DNS). Multiple cells of constant shedding frequency were found along the span of the plate. This is in contrast to apparent lack of cellular vortex shedding found in the high-Reynolds-number experiments by Gaster &amp; Ponsford (Aero. J., vol. 88, 1984, p. 206). However, the present DNS data is in good qualitative agreement with similar high-Reynolds-number experimental data produced by Castro &amp; Watson (Exp. Fluids, vol. 37, 2004, p. 159). It was observed that a tapered plate creates longer formation length coupled with higher base pressure as compared to non-tapered (i.e. uniform) plates. The three-dimensional recirculation bubble was nearly conical in shape. A significant base pressure reduction towards the narrow end of the plate, which results in a corresponding increase in Strouhal number, was noticed. This observation is consistent with the experimental data of Castro &amp; Rogers (Exp. Fluids, vol. 33, 2002, p. 66). Pressure-driven spanwise secondary motion was observed, both in the front stagnation zone and also in the wake, thereby reflecting the three-dimensionality induced by the tapering.

Microstructural and electrical characteristics of reactively sputtered ZrN x thin films

The effects of nitrogen flow rate on the microstructure, composition and electrical properties of ZrN x films prepared by reactive sputtering were investigated by X-ray diffraction, field-emission electron probe micro-analyzer, transmission electron microscopy, X-ray photoelectron spectroscopy and four-point probe method. Results indicated that the ZrN x films were crystallized in NaCl-type structure and showed a near-stoichiometric composition at nitrogen flow rate of 3 SCCM. The existence of residual oxygen in the ZrN x films was ascribed to the relatively high base pressure attained in the present work. The residual oxygen atoms in the ZrN x films may possibly substitute the nitrogen atoms or occupy the interstitial positions in the ZrN lattice, causing that the so-called near-stoichiometric films and the delay of occurrence for over-stoichiometric films in the N/Zr ratio evolution with nitrogen flow rate. The slightly larger lattice constant of ZrN x films compared with the ideal value of ZrN compound demonstrated the presence of residual oxygen atoms in the films. The electron scattering effects due to impurity atoms may be enhanced by the incorporation of residual oxygen atoms and were responsible for the overall raise in the resistivity of ZrN x films as a function of nitrogen flow rate, comparing with the value reported by previous studies.

Influence of base pressure and atmospheric contaminants on a-Si:H solar cell properties

The influence of atmospheric contaminants oxygen and nitrogen on the performance of thin-film hydrogenated amorphous silicon (a-Si:H) solar cells grown by plasma-enhanced chemical vapor deposition at 13.56 MHz was systematically investigated. The question is addressed as to what degree of high base pressures (up to 10−4 Torr) are compatible with the preparation of good quality amorphous silicon based solar cells. The data show that for the intrinsic a-Si:H absorber layer exists critical oxygen and nitrogen contamination levels (about 2×1019 atoms/cm3 and 4×1018 atoms/cm3, respectively). These levels define the minimum impurity concentration that causes a deterioration in solar cell performance. This critical concentration is found to depend little on the applied deposition regime. By enhancing, for example, the flow of process gases, a higher base pressure (and leak rate) can be tolerated before reaching the critical contamination level. The electrical properties of the corresponding films show that increasing oxygen and nitrogen contamination results in an increase in dark conductivity and photoconductivity, while activation energy and photosensitivity are decreased. These effects are attributed to nitrogen and oxygen induced donor states, which cause a shift of the Fermi level toward the conduction band and presumably deteriorate the built-in electric field in the solar cells. Higher doping efficiencies are observed for nitrogen compared to oxygen. Alloying effects (formation of SiOx) are observed for oxygen contaminations above 1020 atoms/cm3, leading to an increase in the band gap.

Effect of annealing on the structural, optical and electrical properties of ITO films by RF sputtering under low vacuum level

Indium tin oxide (ITO) thin films were prepared by RF sputtering of ceramic ITO target in pure argon atmosphere at a high base pressure of 3×10−4mbar without substrate heating and oxygen admittance. The use of pure argon during deposition resulted in films with high transparency (80–85%) in the visible and IR wavelength region. The films were subsequently annealed in air in the temperature range 100–400°C. The annealed films show decreased transmittance in the IR region and decreased resistivity. The films were characterized by electron microscopy, spectrophotometry and XRD. The predominant orientation of the films is (222) instead of (400). The transmission and reflection spectra in the wavelength range 300–2500nm are used to study the optical behaviour of the films. The optical transmittance and reflectance spectra of the films were simultaneously simulated with different dielectric function models. The best fit of the spectrophotometric data was obtained using the frequency-dependent damping constant in the Drude model coupled with the Bruggeman effective medium theory for the surface roughness. It has been found that the sputtering power and the chamber residual pressure play a key role in the resulting optical properties. This paper presents the refractive index profile, the structure determined from the XRD and the electrical properties of ITO films. It has been found from the electrical measurement that films sputtered at 200W power and subsequently annealed at 400°C have a sheet resistance of 80Ω/□ and resistivity of 1.9×10−3Ωcm.

Preparation of titanium oxynitride thin films by reactive sputtering using air/Ar mixtures

In the literature titanium oxynitride (TiN x O y ) films have often been prepared by controlling N 2/O 2/Ar mixing gases in physical vapor deposition. In this study, TiN x O y films were prepared by d.c. magnetron sputtering using air/Ar mixtures. Replacing N 2/O 2 with air as a reactive gas allowed the process performed at high base pressures, i.e., low vacuum, which could drastically reduce processing time while achieving similar quality of the films prepared at a low base pressure. When the air/Ar flow ratio increased from 0.15 to 0.30, the color of the films changed from light golden to dark golden and X-ray diffraction patterns show that the preferred orientation of films with rock-salt structure changed from (111) into (200). The oxygen content in the TiN x O y films increased with increasing the air/Ar ratio, as determined by X-ray photoelectron spectroscopy. The thickness of the films decreased with increasing the air/Ar ratio, revealing the target poisoning effect especially at high air/Ar ratios. Electrical resistivities of the films increased with the ratio owing to the increase of oxygen content in the films.

Application of glow discharge butadiene coatings on plasticized cornstarch substrates

In the present work, cast thermoplastic starch films were successfully modified by plasma of 1,3-butadiene to reduce water sensitivity. To overcome the difficulties involved in plasma coating of substrates with high water content, high base pressure was used before the introduction of the working gas and mild deposition parameters were employed. The coatings produced at 6 Pa base pressure were smooth, oxygen free and presented low internal stress. After being coated under these mild conditions, cornstarch films were submitted to wettability measurements and atomic force microscopy (AFM) morphological evaluation. Results showed that the coating process improved significantly the cornstarch films' physical properties; water absorption was reduced up to 80%, while water contact angle increased more than 100%. AFM images indicated that the produced coatings cover homogeneously all the different regions of the cornstarch film.

Properties of titanium thin films deposited by dc magnetron sputtering

Titanium (Ti) thin films were deposited by dc magnetron sputtering at conventional conditions with different cathode power (75–150 W), sputtering pressure (1.1–3.3 Pa) and base vacuum ((4–13) × 10 −4 Pa). The compositional properties of the films were studied using four point probe, XPS and spectroscopic ellipsometry techniques. Additionally, the structural properties of the Ti films were obtained using XRD, SEM and AFM techniques and also derived from ellipsometry data. Deposition conditions with low cathode power (75 W) and high base pressure (13 × 10 −4 Pa), independent of the other parameters, yielded significant nitridation in the films and that condition, together with high sputtering pressure (3.3 Pa), resulted in transparent Ti oxide film. The films prepared under these conditions were amorphous. Otherwise, under deposition conditions of higher cathode power and lower sputtering and base pressures, only oxidation in the films was observed. The concentration of the oxide phase decreased with an increase in the cathode power and decrease in the sputtering and base pressures. Furthermore, under these conditions the films crystallized. The crystallinity increased with increase in cathode power (100–150 W) and sputtering pressure (1.1–2 Pa) and eventually was found to be amorphous with a slight increase in the base vacuum from the lowest of 4 × 10 −4 Pa. However, the morphological properties of the Ti films were least influenced by the preparation conditions. The films were uniform and void free exhibiting densely packed morphological characteristics with similar smooth surface roughness. A strong relation existed between the composition and structural properties in the films. From this dependence the optimum deposition conditions is obtained to prepare metallic and crystalline Ti films under conventional conditions.

Study of argon additive in a beam injection type negative ion source using VUV emission spectroscopy

Effects of Ar addition are studied by using a beam injection type negative ion source. With adding Ar, I H − increases at low base H 2 pressure. At high base H 2 pressure, however, I H − decreases. VUV emission intensities also decrease at high base pressure. In other words, Ar addition is adverse effect for production of H 2 (v″). Therefore, decrease in I H − is caused by decrease in H 2 ( v″). In D 2 plasmas, variation patterns of plasma parameters and VUV intensities by Ar addition are nearly the same as ones in H 2 plasmas. Even in low base pressure, however, enhancement of I D − is not observed.