Oxygen Sputtering Research Articles

Electrostatic tuning of the electrical properties of YBa2Cu3O7−x using an ionic liquid

Ultrathin YBa2Cu3O7−x (YBCO) films were grown on SrTiO3 (STO) substrates using the technique of high-pressure oxygen sputtering. Films were then incorporated in a field effect transistor configuration, which facilitated the control of superconductivity by electrostatic charging. While devices using STO as both the substrate and gate dielectric have produced only relatively small shifts in film electrical properties, very large changes can be realized using an electric double layer transistor configuration employing the ionic liquid DEME-TFSI as the dielectric. By depleting holes an electrostatically tuned superconductor insulator transition was studied using a finite size scaling analysis. The breakdown of scaling at the lowest temperatures suggests the presence of a mixed insulator/superconductor phase separating the two ground states. Further depletion of holes resulted in a change of the majority carriers from holes to electrons and the emergence of what appeared to be very weak re-entrant superconductivity. Also by accumulating holes an underdoped film was tuned into the overdoped regime. A two-step mechanism for electrostatic doping was revealed. Hall effect measurements suggested the presence of an electronic phase transition or a change in the Fermi surface as a function of doping near optimal doping.

EFFECTS OF DEPOSITION PARAMETERS AND OXYGEN ADDITION ON PROPERTIES OF SPUTTERED INDIUM TIN OXIDE FILMS

Indium tin oxide (ITO) films were sputtered on corning glass substrate. Oxygen admixture and sputtering deposition parameters were optimized to obtain the highest transparency as well as lowest resistivity. Structural, electrical and optical properties of the films were then examined. Increasing deposition rate and film thickness changed the crystallographic orientation from (222) to (400) and (440), as well as higher surface roughness. It was necessary to apply substrate heating during reposition to get films with better crystallinity. The lowest resistivity of 5.36 x 10-4 Ω•cm was obtained at 750 nm film thickness. The films’ resistivity was increased by addition of oxygen up to 2% in the argon sputtering gas. All films showed over 85% transmittance in the visible wavelength range, possible for applications in photovoltaic and display devices.

Sputtering Effects and Water Formation on an Amorphous Silicate Surface

We studied the formation of deuterated water on an amorphous silicate surface held at low temperature (10 K < T < 40 K). The surface is first characterized by using Ar(+) ion bombardment, and preferential sputtering of oxygen is found. Sputtering creates oxygen vacancies in the surface region that can be filled by deposition of atomic oxygen. The conditions used in the experiment are meant to make it relevant to the study of the initial stages of water formation on dust grains in interstellar space. By changing the D/O ratio of atomic beams of deuterium and oxygen at thermal energy and the temperature of the sample during deposition, we show that the routes to the formation of D2O2 can be untangled and, under certain circumstances, the net yield of D2O2 can be suppressed. The formation efficiency for water and other molecules is then estimated.

Design of p-CuO/n-ZnO heterojunctions by rf magnetron sputtering

Thin film heterojunctions of p-CuO/n-ZnO are prepared by magnetron sputtering and their electrical properties are investigated without and with external illumination. Radio-frequency (rf) sputtering in reactive oxygen–argon atmosphere is optimized to achieve phase-pure p-CuO thin films with mobilities up to 6.6 cm2 Vs−1 and hole concentrations in the order of 1017 cm−3. Intrinsic ZnO and conducting ZnO:Al are deposited by rf magnetron sputtering to form heterojunctions with CuO in both substrate and superstrate configurations with various Ohmic contacts to p-CuO. Heterojunctions in the substrate configuration do not possess diode properties, while devices in the superstrate configuration show clear rectifying characteristics with a threshold voltage (VTH) of 0.66 V and a forward-to-reverse current ratio of ∼450 at VTH. These junctions exhibit a weak photovoltaic effect under illumination, but reasons for the poor photoresponse are still to be identified.

Energy fluxes in a radio-frequency magnetron discharge for the deposition of superhard cubic boron nitride coatings

Energy flux measurements by a calorimetric probe in a rf-magnetron plasma used for the deposition of super-hard c-BN coatings are presented and discussed. Argon as working gas is used for sputtering a h-BN target. Adding a certain amount of N2 is essential for the formation of stoichiometric BN films, since a lack of nitrogen will lead to boron rich films. Subsequently, the contributions of different plasma species, surface reactions, and film growth to the resulting variation of the substrate temperature in dependence on nitrogen admixture are estimated and discussed. In addition, SRIM simulations are performed to estimate the energy influx by sputtered neutral atoms. The influence of magnetron target power and oxygen admixture (for comparison with nitrogen) to the process gas on the total energy flux is determined and discussed qualitatively, too. The results indicate that variation of the energy influx due to additional nitrogen flow, which causes a decrease of electron and ion densities, electron temperature and plasma potential, is negligible, while the admixture of oxygen leads to a drastic increase of the energy influx. The typical hysteresis effect which can be observed during magnetron sputtering in oxygen containing gas mixtures has also been confirmed in the energy influx measurements for the investigated system. However, the underlying mechanism is not understood yet, and will be addressed in further investigations.

ChemInform Abstract: Efficient, Low Cost Synthesis of Sodium Platinum Bronze NaxPt3O4.

AbstractLarge quantities of NaxPt3O4 are prepared with a purity higher than 95% by deposition of PtOy (magnetron sputtering in pure oxygen at a pressure of 4 Pa) on a soda lime glass followed by annealing in air at atmospheric pressure (above 550 °C).

Efficient, Low Cost Synthesis of Sodium Platinum Bronze NaxPt3O4

Large quantities of NaxPt3O4 are prepared with a purity higher than 95% by deposition of PtOy (magnetron sputtering in pure oxygen at a pressure of 4 Pa) on a soda lime glass followed by annealing in air at atmospheric pressure (above 550 °C).

Characterization of single crystal uranium-oxide thin films grown via reactive-gas magnetron sputtering on yttria-stabilized zirconia and sapphire

The microstructure and valence states of three single crystal thin film systems, UO2 on (11¯02) r-plane sapphire, UO2 on (001) yttria-stabilized zirconia, and U3O8 on (11¯02) r-plane sapphire, grown via reactive-gas magnetron sputtering are analyzed primarily with X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS). XRD analysis indicates the growth of single crystal domains with varying degrees of mosaicity. XPS and UPS analyses yield U-4f, U-5f, O-1s, and O-2p electron binding energies consistent with reported bulk values. A change from p-type to n-type semiconductor behavior induced by preferential sputtering of oxygen during depth profile analysis was observed with both XPS and UPS. Trivalent cation impurities (Nd and Al) in UO2 lower the Fermi level, shifting the XPS spectral weight. This observation is consistent with hole-doping of a Mott–Hubbard insulator. The uranium oxide-(11¯02) sapphire system is unstable with respect to Al interdiffusion across the film–substrate interface at elevated temperature.

Dielectric properties of DC reactive magnetron sputtered Al2O3 thin films

Alumina (Al2O3) thin films were sputter deposited over well-cleaned glass and Si <100> substrates by DC reactive magnetron sputtering under various oxygen gas pressures and sputtering powers. The composition of the films was analyzed by X-ray photoelectron spectroscopy and an optimal O/Al atomic ratio of 1.59 was obtained at a reactive gas pressure of 0.03Pa and sputtering power of 70W. X-ray diffraction results revealed that the films were amorphous until 550°C. The surface morphology of the films was studied using scanning electron microscopy and the as-deposited films were found to be smooth. The topography of the as-deposited and annealed films was analyzed by atomic force microscopy and a progressive increase in the rms roughness of the films from 3.2nm to 4.53nm was also observed with increase in the annealing temperature. Al-Al2O3-Al thin film capacitors were then fabricated on glass substrates to study the effect of temperature and frequency on the dielectric property of the films. Temperature coefficient of capacitance, AC conductivity and activation energy were determined and the results are discussed.

Ar ion bombardment modification of Pd–Au/MWCNTs catalyst surfaces studied by electron spectroscopy

AbstractThe Pd–Au multiwall carbon nanotubes (MWCNTs) supported catalyst prepared by polyol method was found to be catalytically active material in direct formic acid fuel cell electrooxidation reaction and during cyclic voltammetry (CV) measurements. The surface of the catalyst after calcination and reduction treatments was investigated previously. The effect of Ar ion bombardment modification on the calcinated catalyst was studied by electron spectroscopy. The chemical and structural changes were deduced from the X‐ray photoelectron spectroscopy (XPS) and X‐ray excited Auger electron spectroscopy (XAES) spectra. The XPS quantitative and qualitative analysis of different chemical forms of atoms indicated that the sputtering causes an increase of Pd surface content, accompanied by decreasing content of N, O, C sp2, and Au (due to preferential sputtering of Au) and significant increase of C sp3 amount. A decrease was also observed in the surface content of (i) carboxyl groups, (ii) water, and (iii) Pd oxide (mainly less stable PdO2). Analysis of XPS inelastic background shape by QUASES indicates that Pd crystallites phase is covered by PdOx + Camorphous overlayer of decreasing thickness after sputtering. Namely, thickness of PdOx overlayer decreased from 8.2 to 3.5 nm and the thickness of amorphous carbon overlayer located on the top increased from 2.3 to 3.5 nm. Significant increase of C sp3 content results from carbon nanotube (CNT) π bonds damaging, whereas decrease of PdO2 content from lower stability of PdO2. Larger thickness of Camorphous overlayer, in contrary to PdOx, may result from higher preferential sputtering of oxygen than carbon atoms and cracking of the MWCNTs structure by Ar+ ions.

Low energy Ar-ion bombardment effects on the CeO2 surface

The structure and electronic properties of epitaxial grown CeO2(111) thin films before and after Ar+ bombardment have been comprehensively studied with synchrotron radiation photoemission spectroscopy (SRPES). Ar+ bombardment of the surface causes a new emission appearing at 1.6eV above the Fermi edge which is related to the localized Ce 4f1 orbital in the reduced oxidation state Ce3+. Under the condition of the energy of Ar ions being 1keV and a constant current density of 0.5μA/cm2, the intensity of the reduced state Ce3+ increases with increasing time of sputtering and reaches a constant value after 15min sputtering, which corresponds to the surface being exposed to 2.8×1015ions/cm2. The reduction of CeO2 is attributed to a preferential sputtering of oxygen from the surface. As a result, Ar+ bombardment leads to a gradual buildup of an, approximately 0.69nm thick, sputtering altered layer. Our studies have demonstrated that Ar+ bombardment is an effective method for reducing CeO2 to CeO2−x and the degree of the reduction is related to the energy and amount of Ar ions been exposed to the CeO2 surface.

Development of Si DRIE process allowing simultaneous etching from narrow and wide mask openings

AbstractWe have developed a new Si DRIE process, which allows simultaneous etching of both narrow and wide mask opening patterns. MEMS devices often contain a wide range of mask opening sizes ranging from micrometer to millimeter scale on the same chip. Conventional DRIE process suffered from a trade‐off between minimizing mask‐undercut for narrow grooves and eliminating black Si on an etched bottom surface for large mask apertures. This was caused by nonuniformity in the deposition of the protective polymeric layer during etching in mask aperture areas having different sizes. Our new DRIE process technique, alternating conventional DRIE with O2 plasma irradiation, adds a SiO2 layer onto the polymeric layer. We have ascertained that the new DRIE process can form a SiO2 protective layer of uniform thickness regardless of the mask aperture size. This uniformity is considered a result of equilibrium between surface oxidation and sputtering of oxygen by plasma irradiation. We have demonstrated that our newly developed process allows simultaneous etching of narrow, deep grooves 1 μm wide and 73 μm deep, and wide aperture areas 500 μm square without black Si on the same chip. © 2011 Wiley Periodicals, Inc. Electron Comm Jpn, 94(4): 36–43, 2011; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ecj.10303

X‐ray photoelectron spectroscopy depth analysis of metal oxides by electrospray droplet impact

The electrospray droplet impact (EDI) method is a newly developed etching method using extremely large charged water cluster ions with masses of several 106 u. This work presents a comparative XPS study of chemical states of the transition metal oxides, TiO2 and Ta2O5, etched by Ar+ and EDI. Selective sputtering of oxygen was observed by Ar+ etching for these samples, but no chemical modification took place by EDI. This finding provided further evidence that EDI has the capability of nonselective etching for both inorganic and organic materials. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Electrical and optical properties of nanostructured VO X thin films prepared by direct current magnetron reactive sputtering and post-annealing in oxygen

Nanostructured vanadium oxide (nano-VO X) thin films were prepared by direct current magnetron reactive sputtering in mixed O 2/Ar discharges and subsequent annealing in oxygen atmosphere. X-ray diffraction and field emission scanning electron microscope were employed to characterize the crystal structures and morphologies, respectively. Fourier transform infrared spectroscopy was applied to analyze the vanadium–oxygen bonds of films. X-ray photoelectron spectroscopy revealed the compositions of the surface and inner portion of nano-VO X thin films. It was shown that the as-deposited films were amorphous, and in-situ annealing of these films in ambient oxygen for 10 min can lead to the growth of nano-VO X thin films. Results of electrical studies indicated that the absolute values of temperature coefficient of resistance and activation energy of films increased significantly after oxygen annealing. Optical investigations carried out in the UV-visible range showed that the absorption edges of nano-VO X films exhibited large red shifts compared with as-deposited film, and that dual optical absorptions were observed in nano-VO X films.

Direct observation of oxygen vacancy and its effect on the microstructure, electronic and transport properties of sputtered LaNiO 3 − δ films on Si substrates

Highly (100)-oriented LaNiO 3 films with different oxygen content were deposited on Si substrates by radio frequency sputtering and post-annealed in oxygen and vacuum conditions. The formation of oxygen vacancies is directly observed by a decrease of lattice oxygen ratio in O 1 s core-level photoelectron spectroscopy. X-ray diffraction measurement indicates that low oxygen pressure during the deposition or annealing process has a significant influence on the lattice constant of LaNiO 3 films. Further valence-band spectra and transport measurements demonstrate that the oxygen vacancies also have a significant influence on the electronic structure and transport behaviors of final LaNiO 3 films.

X-ray photoelectron spectroscopy of nano-multilayered Zr–O/Al–O coatings deposited by cathodic vacuum arc plasma

Nano-multilayered Zr–O/Al–O coatings with alternating Zr–O and Al–O layers having a bi-layer period of 6–7 nm and total coating thickness of 1.0–1.2 μm were deposited using a cathodic vacuum arc plasma process on rotating Si substrates. Plasmas generated from two cathodes, Zr and Al, were deposited simultaneously in a mixture of Ar and O 2 background gases. The Zr–O/Al–O coatings, as well as bulk ZrO 2 and Al 2O 3 reference samples, were studied using X-ray photoelectron spectroscopy (XPS). The XPS spectra were analyzed on the surface and after sputtering with a 4 kV Ar + ion gun. High resolution angle resolved spectra were obtained at three take-off angles: 15°, 45° and 75° relative to the sample surface. It was shown that preferential sputtering of oxygen took place during XPS of bulk reference ZrO 2 samples, producing ZrO and free Zr along with ZrO 2 in the XPS spectra. In contrast, no preferential sputtering was observed with Al 2O 3 reference samples. The Zr–O/Al–O coatings contained a large amount of free metals along with their oxides. Free Zr and Al were observed in the coating spectra both before and after sputtering, and thus cannot be due solely to preferential sputtering. Transmission electron microscopy revealed that the Zr–O/Al–O coatings had a nano-multilayered structure with well distinguished alternating layers. However, both of the alternating layers of the coating contained of a mixture of aluminum and zirconium oxides and free Al and Zr metals. The concentration of Zr and Al changed periodically with distance normal to the coating surface: the Zr maximum coincided with the Al minimum and vice versa. However the concentration of Zr in both alternating layers was significantly larger than that of Al. Despite the large free metal concentration, the Knoop hardness, 21.5 GPa, was relatively high, which might be attributed to super-lattice formation or formation of a metal–oxide nanocomposite within the layers.

Amorphous tungsten-doped In2O3 transparent conductive films deposited at room temperature from metallic target

Abstract Amorphous tungsten-doped In2O3 (IWO) films were deposited from a metallic target by dc magnetron sputtering at room temperature. Both oxygen partial pressure and sputtering power have significant effects on the electrical and optical properties of the films. The as-deposited IWO films with the optimum resistivity of 5.8 × 10−4 Ω·cm and the average optical transmittance of 92.3% from 400 to 700 nm were obtained at a W content of 1 wt%. The average transmittance in the near infrared region (700–2500 nm) is 84.6–92.8% for amorphous IWO prepared under varied oxygen partial pressure. The mobility of the IWO films reaches its highest value of 30.3 cm2 V−1 s−1 with the carrier concentration of 1.6 × 1020 cm−3, confirming their potential application as transparent conductive oxide films in various flexible devices.

A Novel Amorphous InGaZnO Thin Film Transistor Structure without Source/Drain Layer Deposition

In this paper, new structured amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) without conventional source/drain (S/D) layer deposition were introduced. The S/D layers were formed in a-IGZO channel layer by hydrogen (H2) plasma treatments. As the increased plasma treatment time, the resistivity of plasma treated a-IGZO as S/D decreased drastically from 104 to 4.8 ×10-3 Ω cm due to the carrier generation by plasma treatment for 240 s. The carrier concentration mechanism of the H2 plasma treated sample could be attributed to the preferential sputtering of oxygen by H2 bombardment due to formation of an oxygen vacancy and removal of adsorbed O2. The new proposed TFTs exhibited a field-effect mobility of 7.14 cm2 V-1 s-1, an on/off ratio of 7.95 ×106, a threshold voltage of 0.94 V, and a subthreshold swing of 1.06 V/decade. These results demonstrated the possibility of low cost TFT process because S/D electrode deposition was unnecessary.

Effect of initial target surface roughness on the evolution of ripple topography induced by oxygen sputtering of Al films

The effect of pre-existing random roughness on the evolution of ripple structures in O2+ sputtered thin Al films has been investigated. The results show that there is a considerable reduction in initial roughness of the film surface at the early stages of sputtering. For large scale surface structures, angle-dependent first order sputtering is responsible for ion beam smoothening, while for smaller microscopic features, different relaxation mechanisms dominate for smoothing of the surface. At the later stages of sputtering, the curvature dependent erosion instability sets in leading to the development of either coherent ripples or faceted structures depending on the degree of virgin film roughness and bombarding angle. It is found that coating a flat Si surface with ultrathin Al film and subsequent removal of the Al layer by oblique O2+ sputtering leads to the formation of ripple pattern with moderate amplitude in the Si matrix at much lower effective fluence than that would be in bare Si without Al masking.

Amorphous Ge quantum dots embedded in SiO2 formed by low energy ion implantation

Under ultrahigh vacuum conditions, extremely small Ge nanodots embedded in SiO2, i.e., Ge–SiO2 quantum dot composites, have been formed by ion implantation of Ge+74 isotope into (0001) Z-cut quartz at a low kinetic energy of 9keV using varying implantation temperatures. Transmission electron microscopy (TEM) images and micro-Raman scattering show that amorphous Ge nanodots are formed at all temperatures. The formation of amorphous Ge nanodots is different from reported crystalline Ge nanodot formation by high energy ion implantation followed by a necessary high temperature annealing process. At room temperature, a confined spatial distribution of the amorphous Ge nanodots can be obtained. Ge inward diffusion was found to be significantly enhanced by a synergetic effect of high implantation temperature and preferential sputtering of surface oxygen, which induced a much wider and deeper Ge nanodot distribution at elevated implantation temperature. The bimodal size distribution that is often observed in high energy implantation was not observed in the present study. Cross-sectional TEM observation and the depth profile of Ge atoms in SiO2 obtained from x-ray photoelectron spectra revealed a critical Ge concentration for observable amorphous nanodot formation. The mechanism of formation of amorphous Ge nanodots and the change in spatial distribution with implantation temperature are discussed.