Oxygen Background Pressure Research Articles

Zinc oxide nanostructures grown by pulsed laser deposition

We report the use of PLD to grow different ZnO nanostructures. Very different film morphologies have been observed using different laser wavelengths to ablate the target. The influence of substrate temperature and oxygen background pressure on the film morphology has been investigated too. Smooth and rough films, hexagonal pyramids and columns have been obtained by using a KrF excimer laser (248 nm) for the target ablation, while hexagonal hierarchical structures and pencils have been obtained by using ArF (193 nm). Photoluminescence and X-ray diffraction measurements revealed the good quality of the samples, in particular of those deposited using the ArF laser beam.

Effect of background gas environment on oxygen incorporation in TiN films deposited using UHV reactive magnetron sputtering

The effect of the base pressure on the incorporation of oxygen into reactively magnetron-sputtered metal-nitride films has been investigated. A UHV sputtering system with a base pressure of less than 10 −6 Pa was used to examine the relationship between a deliberately introduced background pressure of oxygen and a measured oxygen content in the sputter-deposited TiN films. The results showed that with an oxygen partial pressure of 10 −4 Pa, the deposited TiN was found to include 10–20 at.% of oxygen when measured by the technique of X-ray photoelectron spectroscopy (XPS). When no oxygen was admitted into the system, no trace of oxygen could be detected in the deposited TiN films. The incorporation mechanism is discussed in terms of the coverage-dependent sticking probabilities of O 2 and N 2 on a Ti metal surface.

Properties of SnO 2 films fabricated using a rectangular filtered vacuum arc plasma source

Transparent and conducting SnO 2 films of 57–200nm thickness were deposited on microscope glass slide substrates, using a rectangular filtered vacuum arc deposition system. The 40 glass slides were equally distributed on a 400 × 420mm substrate carriage, and were exposed to a Sn plasma beam, produced by a rectangular vacuum arc plasma gun with a Sn cathode, and passed through a rectangular magnetic macroparticle filter towards the substrates. The carriage with the substrates was transported past the 94 × 494mm filter outlet. The SnO 2 films were fabricated on the glass substrates at room temperature by maintaining the chamber oxygen background pressure at 0.52Pa. The film composition, and electrical and optical properties were studied as a function of the film thickness. The films were stored under ambient air conditions, and their electrical resistance was measured as a function of storage time over a period of several months. The average resistivity of films was 10–17mΩ cm for films with thickness ( t) less than 100nm, but that of t > 100nm it was 5–9mΩ cm. The resistivity of the films with t > 100nm did not change significantly after 8months of storage in ambient air. The optical transmittance of the films in the visible spectrum was in the range of 75–90%. The optical constants, i.e., the refractive index and the extinction coefficient of the films at wavelength λ = 550nm were in the range of 2.02–2.09 and 0.013–0.023, respectively, and the optical band gap energy was 4.15–4.21eV. Unlike the electrical resistivity, the optical parameters weakly depended on t.

Effect of oxygen concentration on the magnetic properties of La2CoMnO6 thin films

The dependence of the magnetic properties on oxygen concentration in epitaxial La2CoMnO6 thin films deposited on (100)-oriented SrTiO3 substrates has been investigated by varying the oxygen background pressure during growth using pulsed laser deposition. Two distinct ferromagnetic (FM) phases are revealed, and the relative fraction varies with the oxygen concentration. The existence of oxygen vacancies induces the local vibronic Mn3+–O–Co3+ superexchange interactions in direct competition with the static FM Mn4+–O–Co2+ interactions. This results in the appearance of a new low temperature FM phase and suppression of the high-temperature FM phase, creating two distinct magnetic phase transitions.

Effect of incident fluence on the growth of ZnO nanorods by pulsed excimer laser deposition

The incident fluence is shown to be an effective parameter in controlling the diameters of ZnO nanorods grown by pulsed excimer laser deposition following ablation of a ZnO target in low background pressures of oxygen. The evolution in nanorod diameter and the overall sample morphology with incident fluence is rationalised by considering the bombardment effect of high energy species in the plume formed in the ablation event.

Shock front dynamics in the pulsed laser deposition of YBa2Cu3O7−x

The pulsed laser deposition of YBa2Cu3O7−x targets by excimer laser at fluences of 4–10 J cm−2 in low pressure oxygen backgrounds yields emissive plumes with kinetic energies of 50–200 eV, driving the formation of a shock front with Mach numbers of M = 10–50. The propagation of the shock front is independent of atomic species and adequately characterized by the Sedov–Taylor shock model if the dimensionality of the plume is allowed to deviate from ideal spherical expansion. The ideal efficiency of energy conversion from laser pulse to shock expansion is nearly unity at 1 Torr, but decreases rapidly at lower pressures, where the plume expands beyond the laser footprint during ablation. The low oxygen background pressures, 100–1000 mTorr, typically employed for the production of superconducting films is sufficient for the generation of a strong shock front with shock thickness of 5 mm to less than 0.4 mm, but too low to develop three-dimensional flow. Indeed, dimensionality of the expansion ranges from n = 0.8 to 2.4 over the background oxygen pressure range of 25–1000 mTorr. Shock strength is proportional to the Mach number and inversely dependent on pressure, indicating a thickness limited to approximately the collision mean free path.

Ambient Oxygen Effects on the Growth of ZnO Thin Filmsby Pulsed Laser Deposition

ZnO thin films were prepared by pulsed laser deposition on amorphous fused silica substrates at different ambient <TEX>$O_2$</TEX> pressures varying from 0.5 to 500 mTorr, to observe the effect of ambient gas on their crystalline structure, morphology and optical properties. Results of X-ray diffraction, scanning electron microscopy, atomic force microscopy and photoluminescence studies showed that crystallinity, surface features and optical properties of the films significantly depended on the oxygen background pressure during growth. A low oxygen pressure (0.5 mTorr) seems to be suitable for the growth of highly c-axis oriented and smoother films possessing a superior luminescent property. The films grown at the higher <TEX>$O_2$</TEX> pressures (50-500 mTorr) were found to have many defects probably due to an excessive incorporation of oxygen into ZnO lattice. We speculate that the film crystallinity could be affected by the kinetics of atomic arrangement during deposition at the higher oxygen pressures.

Photoluminescence from diameter-selected ZnO nanorod arrays

Arrays of ZnO nanorods with different, user-controlled, diameters were formed on Sisubstrates by 193 nm pulsed laser deposition in a low background pressure ofoxygen. The effect of nanorod diameter on the photoluminescence exhibited bythese arrays was investigated, as a function of incident ultraviolet (UV, 325 nm)laser intensity. Nanorods with large surface to volume ratios can exhibit high UV(∼380 nm) emission efficiencies, particularly when excited at higher incident intensities.Pre-illumination of such nanorods with 325 nm radiation serves to enhance the UV emissionmeasured at lower incident intensities. These observations are rationalized in terms ofphoto-induced desorption of surface-bound oxygen, resulting in a quenching of thedepletion layer at the ZnO surface. These findings serve to reinforce the potential of ZnOnanostructures for optoelectronic device applications.

Large-area production of yttria-stabilized zirconia by pulsed laser deposition

The small size of most of the present-day films produced by PLD (up to few cm2) is not only a general limiting factor for applications in many scientific and technical fields, but is also problematic for studies of samples for which edge effects may play an important role. Yttria-stabilized zirconia (YSZ) films with a uniform thickness have been deposited at different distances from the target and at different oxygen background pressure in the largearea PLD facility at Risø National Laboratory. Films of uniform thickness up to 300 nm and 1200 nm over an area with a diameter of more than 90 mm were achieved. Depending on the oxygen background pressure the YSZ films were found to grow in a highly oriented manner on the Si wafer.

High transmittance–low resistivity cadmium oxide films grown by reactive pulsed laser deposition

High transmittance, low resistivity, and highly oriented cadmium oxide thin films were prepared by pulsed Nd:YAG laser ablation of Cd target in the presence of oxygen as reactive atmosphere. The structural, optical, and electrical properties of CdO films were dependent on the background oxygen pressure PO2. The XRD data show that the grown CdO film at 350 Torr oxygen pressure exhibited preferential orientation along (111) crystal plane. The average transmittance of the CdO films in the visible region was found to be in the range of 65–85% and the corresponding optical energy gap found to be in the range of 2.5–2.8 eV depending on oxygen pressure. The lowest electrical resistivity was found to be 7.56 × 10−3 Ωcm for CdO film prepared at 350 Torr of oxygen ambient without using post-deposition heat treatment.

窒化チタンの反応性スパッタ製膜における真空環境の影響

The effect of background gas environment on the purity of reactively-deposited nitride films has been studied. Especially, the relation between the oxygen background pressure and its incorporation into the TiN film is investigated in this study. We have developed a UHV sputtering system and deposited TiN films under two different base pressure conditions: one was a UHV condition less than 10-6 Pa, and the other was 1×10-4 Pa of oxygen. The oxygen content of the films was examined with X-ray photoelectron spectroscopy. While no trace of oxygen was detected in the TiN film deposited under the UHV condition, 10 at.% of oxygen was observed in that deposited with the O2 introduction. The extent of O2 incorporation into the TiN film is discussed based on the difference in sticking characteristics of oxygen and nitrogen on the titanium surface.

Preparation of La 0.8Sr 0.2Cr 0.97V 0.03O 3 − δ films for solid oxide fuel cell application

La 0.8Sr 0.2Cr 0.97V 0.03O 3 − δ (LSC) is commonly studied as a ceramic interconnect material as well as a coating material for metallic interconnects for solid oxide fuel cell applications. However, it is difficult to sinter this type of material to high density. In order to overcome this problem and to study the material in form of a thin film we have used Pulsed Laser Deposition to obtain a dense, uniform film with the right stochiometry. Investigation of preparation-parameter dependence of the LSC films deposited on a stainless steel substrate during pulsed-laser deposition was carried out. The LSC films were deposited with KrF excimer laser (248 nm) on a stainless steel substrate at different oxygen pressure and substrate temperatures. The substrate temperature (873–1073 K) and the oxygen background pressure (5–20 Pa) were varied in order to obtain optimal growth conditions. The surface morphology and structural information of the films were obtained using scanning electron microscope (SEM) and X-ray diffraction, respectively. Under the optimal preparation-parameter conditions: substrate temperature of 1023 K and an oxygen pressure of 10 Pa the structure of the film agreed with the target structure and the SEM micrographs show that the surfaces are homogeneous, smooth, crack-free and dense.

Photoelectron spectroscopy (XPS) and photoelectron diffraction (XPD) studies on the system hafnium silicide and hafnium oxide on Si(1 0 0)

Continuous down-scaling of silicon based transistors results in device lengths of less than 100 nm. This requires a reduction of the gate dielectric thickness to less than 15Å which is not possible for SiO 2 due to an increasing leakage current. One of the most promising candidates for a replacement material for the gate dielectric is HfO 2 [Wilk GD, Wallace RM, Anthony JM. J Appl Phys 2001; 89:5243]. In this work we applied X-ray photoelectron spectroscopy (XPS) and photoelectron diffraction measurements in order to study the interface of hafnium oxide to Si(1 0 0). The high resolution measurements were performed with synchrotron radiation at beamlines 5 and 11 at DELTA (Dortmund). For the first time, photoelectron diffraction patterns for this system were recorded. The spectral resolution allowed to separate different spectral components. The preparation of hafnium oxide films on Si(1 0 0) was performed by evaporation of hafnium at a partial oxygen background pressure of 1 × 10 - 8 mbar . Three different spectral components were observed in the hafnium 4f photoemission signal by high resolution XPS. The photoelectron signals with binding energies shift of 3.1 and 1.2 eV with respect to signal of hafnium silicide were assigned to hafnium dioxide and hafnium silicate, respectively. The corresponding high-resolution diffraction patterns result from different local environments for each component. The experimental patterns are compared with simulations for a model structure of hafnium silicide.

STRESS CONTROLLED FERROELECTRIC (BA0.5Sr0.5)TiO3 THIN FILMS PREPARED ON GD2O3/SRTiO3(001) BY A PULSED LASER DEPOSITION

ABSTRACT Stress controlled epitaxial ferroelectric Ba0.5Sr0.5TiO3 (BST) films have been deposited on Gd2O3/SrTiO3 by pulsed laser deposition with oxygen background pressure of 200 mTorr at the deposition temperature of 750°C. In order to control the stress in BST films, oxygen pressures for Gd2O3 buffer layers have been varied from 0.1 to 100 mTorr, while that of BST films have been fixed at 200 mTorr. It has been found that the lattice parameters of the BST films deposited on Gd2O3 were changed. Furthermore, microwave properties of co-planar waveguide (CPW) fabricated on BST films were investigated by a HP 8510C vector network analyzer from 1–20 GHz. Large dielectric tunabilities were observed from the CPW's fabricated on BST films deposited on Gd2O3 layers deposited at low and high oxygen pressures, 0.1 and 100 mTorr, respectively.

Pulsed Laser Deposition of High-Quality Manganite Thin Films at Low Background Pressures with in-situ Reflection High Energy Electron Diffraction

We have developed a methodological approach which allows the deposition of high quality fully oxidized manganite thin films at low oxygen background pressure (10-4 - 10-3 mbar) by PLD. In such oxygen conditions the thickness of the deposited layer can be controlled at the level of a single unit cell by the use in situ of a conventional Reflection High Energy Electron Diffraction diagnostic. We show that by decreasing the laser fluence it is possible to improve the oxidation process in thin films under low background oxygen pressure. Films deposited at low laser fluence (about 0.2 J/cm2) show a two-dimensional growth mode and possess very good transport properties without the necessity of any further post-growth annealing treatment. A physical model, based on the plume-background interaction as a primary mechanism of film oxidation during growth, is proposed to explain the experimental findings.

Optical characterization of filtered vacuum arc deposited zinc oxide thin films

ZnO thin films, 100–250 nm thick, were deposited on microscope glass slides and UV fused silica (UVFS) substrates using a filtered vacuum arc deposition (FVAD) system, operating at room temperature (RT) and 200 A for 60 s. The cathode was prepared from 99.9% pure Zn metal and the oxygen background pressure during deposition was in the range 0.67–0.93 Pa. The films were characterized by x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), optical transmission and spectroscopic ellipsometry. As-deposited ZnO films were found to be polycrystalline with c-axis orientation. The atomic concentration ratio of Zn to O in the film as determined by XPS analysis was stoichiometric. Film transmission in the visible was 70–90%. The maximum and minimum values of the refractive index n and the extinction coefficient k in the visible, for all samples, were 2.23 to 1.90 and 0.6 to approximately 0, respectively. The type of inter-band electron transition was found to be direct transition with optical band gap in the range of 3.25–3.42 eV.

High-quality in situ manganite thin films by pulsed laser deposition at low background pressures

We show that by decreasing the laser fluence it is possible to improve the oxidation process in manganite thin films under low background oxygen pressure, allowing the in situ use of conventional Reflection High Energy Electron Diffraction diagnostic. Films deposited at low fluence (corresponding to a deposition rate per pulse lower than 10-2 unit cells per laser shot) show a two-dimensional growth mode and possess very good transport properties without the necessity of any further post-growth annealing treatment. A physical model, based on the plume-background interaction as a primary mechanism of film oxidation during growth, is proposed to explain the experimental findings.

Optical properties of transparent ZnO–SnO2 thin films deposited by filtered vacuum arc

ZnO–SnO2 films were deposited onto glass substrates by filtered vacuum arc deposition. The source was equipped with a 70 at% Zn and 30 at% Sn cathode that was the source of Zn and Sn ion beam. Arc current was 200–300 A. The oxygen background pressure was 4–8 mTorr. The deposition time was 60 or 120 s, resulting in film thickness in the range 100–900 nm. The maximum deposition rate was 7.6 nm s−1. All films were found to be amorphous. The transmission of the film in the VIS was 80%–90%, affected by interference. The refractive index and the extinction coefficient were determined from the measured optical transmission in the range 300–1100 nm by fitting a theoretically calculated film transmission to the measured one, using a single oscillator model. The values of n and k were determined from spectroscopic ellipsometry data and were in the ranges 2.38–1.97 and 0.24–0.013, respectively, depending on wavelengths and deposition parameters. The optical band gap (Eg) was determined by the dependence of the absorption coefficient on the photon energy at short wavelengths. Its values were in the range 3.5–3.62 eV, depending on the deposition conditions.

Structural properties and impedance spectroscopy of excimer laser ablated Zr substituted BaTiO3 thin films

BaZr0.15Ti0.85O3 thin films were deposited on Pt-coated Si substrates using the pulsed excimer laser ablation technique. X-ray diffraction and atomic force microscope techniques were used to study the structural characteristics of the films. Films with good crystalline quality, with an average grain size of 0.5 µm were obtained, under various oxygen background pressures. Ferroelectric hysteresis loops recorded on the films deposited at 26.66 Pa oxygen pressure showed the best properties. To gain a further understanding of the electrical properties of these films, impedance spectroscopy was used and data acquired at several different temperatures. AC conductivity plots showed the presence of space charge conduction at low frequencies; however, at high frequencies, all the curves merged and expectedly showed an almost dc conduction behaviour. The activation energy obtained from ac conductivity data may be attributed to oxygen vacancy motion.

Photoluminescent silicon nanocrystals synthesized by reactive laser ablation

We report the synthesis of Si nanocrystals embedded in a SiO2 matrix using reactive laser ablation in oxygen atmosphere followed by annealing. We observe a strong photoluminescence signal, which is related to the oxygen background pressure used for synthesis. The average nanoparticle size, obtained independently by fitting photoluminescence spectra and from x-ray diffraction patterns, decreases from 16to2nm with increasing oxygen pressure from 0.01to1.1mTorr. The maximum photoluminescence intensity is observed at 0.8mTorr, which corresponds to a crystal size of 2.2±0.4nm. We find that the concentration of nonoxidized Si, which is controlled by the oxygen pressure, determines the final nanocrystal size.