Increasing O2 Flow Rate Research Articles

Control of diameter of ZnO nanorods grown by chemical vapor deposition with laser ablation of ZnO

Abstract We made a study of controlling diameters of well-aligned ZnO nanorods grown by low-pressure thermal chemical vapor deposition combined with laser ablation of a sintered ZnO target, which was developed by us. Until now, it has been impossible to control diameters of ZnO nanorods, while the growth orientation was maintained well-aligned. In this study we developed a multi-step growth method to fabricate well-aligned nanorods whose diameters could be controlled. Metal Zn vapor and O2 are used as precursors to grow ZnO nanorods. N2 is used as a carrier gas for the precursors. A substrate is an n -Si (111) wafer. A sintered ZnO target is placed near the substrate and ablated by a Nd–YAG pulsed laser during ZnO nanorod growth. The growth temperature is 530 ∘C and the pressure is 66.5 Pa. A vertical growth orientation of ZnO nanorods to the substrate is realized in the first-step growth although the diameter cannot be controlled in this step. When an O2 flow rate is 1.5 sccm, well-aligned nanorods with 100 nm diameter are grown. Next, the second-step nanorods are grown on only the flat tip of the first-step nanorods. The diameters of the second-step nanorods can be controlled by adjusting the O2 flow rate, and the growth direction is kept the same as that of the first-step nanorods. When the O2 flow rate in second-step growth is smaller than 0.6 sccm, the diameter of the second-step nanorods is 30–50 nm. When the O2 flow rate is between 0.75 and 3.0 sccm, the diameter is almost same as that of the first-step nanorods. When the O2 flow rate is larger than 4.5 sccm, the diameter is increased with increasing O2 flow rate. Further, the third-step ZnO nanorods with gradually increased diameters can be grown on the second-step nanorods with 1.5 sccm O2 flow rate and without laser ablation.

Evolution of structure, magnetic and transport properties of sputtered films from Fe to Fe3O4

Polycrystalline iron oxide films were fabricated using the reactive sputtering method without substrate heating. Structure characterization indicates that the dominant phases in the films evolve from α-Fe to pure Fe3O4 with the increasing O2 flow rate. In polycrystalline Fe3O4 films, disordered atoms exist at the grain boundaries. Magnetic properties analyses reveal that the room-temperature magnetization first decreases and later increases due to the variation of the volume fraction of the paramagnetic FeO phase with a Néel temperature of 198 K. The magnetoresistance MR (= [R(H) − R(0)]/R(0)) of the films increases from 0.1% for pure Fe film to 10.6% for the Fe3O4 film at 80 K under a 90 kOe field. The transport mechanism of FeO–Fe3O4 and Fe3O4 films is suggested to be the tunnelling process, which satisfies the log ρ ∼ T−1/2 relation. The Hall resistivity of the Fe3O4 film decreases with increasing temperature. The ordinary and extraordinary Hall coefficients of the Fe3O4 film at 300 K are about 100 and 420 times larger than those of bulk Fe.

Optical Study of Radicals (OH, O, H, N) in a Needle-plate Negative Pulsed Streamer Corona Discharge

Optical emission spectroscopy has been applied to study the OH radicals and O, H, and N active atoms produced by a high-voltage negative pulsed streamer corona discharge of N2 and H2O mixture gas in a needle-plate reactor at one atmosphere. The relative vibrational populations and the vibrational temperature of N2(C, v′) were determined. The effects of pulsed peak voltage, pulsed repetition rate, and the addition of O2 on the relative populations of OH(A2Σ) radicals, O(3p5P), Hα (3P), and N(3p4P) active atoms were investigated. It was found that the relative populations of those radicals increase with increasing pulsed peak voltage and pulsed repetition rate. The relative population of OH(A2Σ) radicals decreases with increasing O2 flow rate, while the relative populations of O (3p5P), Hα (3P), and N (3p4P) active atoms exhibit a maximum over the studied range of the O2 flow rate. The involved physicochemical processes have also been discussed.

Structure and properties of zirconium oxide thin films prepared by filtered cathodic vacuum arc

Zirconium oxide (ZrO2) thin films deposited at room temperature by the filtered cathodic vacuum arc (FCVA) technique are detailed in terms of the film structure, composition, morphology, and optical and mechanical properties, which are tailored by the oxygen (O2) flow rate during deposition. The relationships between the film structure, composition, morphology, and properties are emphasized. With an increasing O2 flow rate, the film evolves in structure from amorphous, through a pure monoclinic phase with varying preferential orientation, to amorphous again, accompanied by an increase in the O/Zr atomic ratio and a conversion of Zr ions from low oxidation states into Zr4+. Such a structural trend arises from the change in composition, and influences the film morphology and mechanical properties so that the amorphous films exhibit small clusters on the surface and smoother morphology as well as lower hardness compared with the polycrystalline films. The film composition rather than the density dominates the optical properties, where the transmittance and the optical band gap increase with increasing O/Zr values, while the refractive index and extinction coefficients behave conversely with the lowest refractive index (2.16 at 550 nm) approaching the bulk value (2.2) .

MBE growth of ZnSSeO alloy using ZnS as a sulfur source

AbstractWe have grown quaternary alloy semiconductor ZnSSeO by molecular beam epitaxy. Large band gap bowing due to large electronegativity of O is expected in this alloy. The growth of ZnSSeO was proceeded by supplying RF‐excited oxygen during the growth of ZnSSe. The lattice constant of ZnSSeO decreased with increasing O2 flow rate. X‐ray diffraction peak was separated when O2 flow rate was high. However, controllable range of lattice constant without phase separation was expanded compared with ZnSeO ternary alloy. It was shown that S composition parasitically increased with O composition. We have explained the enhancement of S incorporation in terms of reduction of strain energy. The band gap energy decreased with increasing O2 flow rate regardless of phase separation. The amount of band gap bowing was slightly less than that found in ZnSeO. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of O2 Gas on Reaction Mechanisms in the Chemical Vapor Deposition of (Ba, Sr)TiO3 Thin Film

Effects of O2 gas on reaction mechanisms in the chemical vapor deposition (CVD) of (Ba, Sr)TiO3 [BST] film were studied by investigating the atomic incorporation rates of Ba, Sr, and Ti. The atomic incorporation rates were measured using an X-ray fluorescence method for BST film prepared for several molar source supply ratios with different O2 flow rates of 0.1, 0.5, and 1.0 slm. In the experiments, these rates were found to increase monotonically with increasing O2 flow rate in flux regions where incorporation reactions might be dominated kinetically. This suggested that the supplied O2 gas might affect the adsorption of film precursors onto the film surface in the CVD of BST. From the obtained experimental results, we proposed a CVD model, in which some precursors adsorbed on the film surface form adsorptive sites, and that successive precursors are adsorbed thereon. The supplied O2 gas contributes towards the formation of the adsorptive sites; the Ba and Sr precursors release their own oxygen and receive oxygen from the supplied O2 gas, and then the oxidized precursors form adsorptive sites. On the other hand, the Ti precursors form adsorptive sites by holding their own oxygen on the film surface. Then, under the proposed model, atomic incorporation rates and overall sticking coefficients for BST film depositions were numerically calculated. The calculated results were found to be in good agreement with the experimental results for several molar source ratios with different O2 flow rates of 0.1, 0.5, and 1.0 slm.

Low-Temperature Formation of Thin-Gate SiO2 Films by the Ultrahigh-Vacuum Chemical Vapor Deposition with Reduced Subcutaneous Oxidation Using Remote-Plasma-Activated Oxygen and Si2H6

We have investigated the growth condition dependence on the properties of SiO2 films by remote-plasma-enhanced chemical vapor deposition (RPECVD) in an ultrahigh vacuum system with remote-plasma-activated oxygen and Si2H6. During the RPECVD, subcutaneous oxidation of the Si substrate was not negligible, which degraded the controllability of the film thickness and the electrical properties. The amount of the active oxygen species that dominates the reactions of subcutaneous oxidation was considered to be significantly reduced with increasing O2 flow rate, while that of “pure” CVD remained unchanged. As a result, precise control of the RPECVD SiO2 thickness with reduced subcutaneous oxidation has been achieved by setting the O2 flow rate to the maximum and by using higher Si2H6 flow rates. The characteristics of the SiO2 films obtained using optimized RPECVD conditions at temperatures lower than 600°C without a postannealing process have been demonstrated to be comparable to those of thermally grown films.

Titanium oxide films on Si(100) deposited by e-beam evaporation

Titanium oxide films with a thickness of a 400 nm were deposited on p-type Si(100) at room temperature by e-beam evaporation, and titanium dioxide was evaporated in oxygen environment at a pressure of 2×10−6–4×10−5 Torr. Effects of oxygen flow rate (FO2) on the properties of the films, such as surface roughness, composition, and chemical states, have been investigated. The root-mean-square surface roughness of the films increased with increasing FO2 up to 20 sccm, and then decreased over 20 sccm. X-ray diffraction patterns show that the titanium oxide films are amorphous. Oxygen resonance backscattering spectroscopy shows that all films are oxygen rich, i.e., relative atomic ratio (CO/CTi) of the films ranged from 2.05 to 2.25. But the x-ray photoelectron spectroscopy (XPS) analysis shows that the titanium oxide films were oxygen deficient. The ratio of oxygen to titanium (CO2/CTi) of the films calculated by XPS ranged from 1.82 to 1.93. XPS shows that there exist only Ti3+ and Ti4+ charge states in the film. Ti4+ states of the films are nearly constant at 0.47 up to O2 flow rate of 20 sccm, and then slightly increase with increasing O2 flow rate. Although rutile TiO2 is used as a starting material, titanium K-edge x-ray absorption near-edge spectroscopy (XANES) spectra show that the structure of the TiO2−x films grown in oxygen environment are amorphous. The Ti K-edge XANES spectra from the films is the same regardless of FO2.

EXPERIENCE WITH PROPOFOL IN PEDIATRIC SEDATION FOR INVASIVE GASTROINTESTINAL AND OTHER PROCEDURES

Introduction: Propofol is an anesthetic agent being used with increasing frequency to provide deep sedation during invasive procedures in children. We present our experience with this drug, administered and monitored by pediatric intensivists in the ICU following AAP sedation guidelines, and compare it to the use of other medications. Methods: Retrospective chart review of patients who received IV sedation for gastrointestinal (Gl) and other procedures between August 1995 and December 1996. Demographics, ASA status, drugs used, complications, recovery times, and costs were recorded. Results: A total of 277 invasive procedures were performed on 236 patients. Procedures included Gl (EGD, lower endoscopy, PEG, liver biopsy), bronchoscopy, and others (eg. PICC lines). Complete data was available for 268 procedures (208 Gl). 79% of the procedures were performed in outpatients. Propofol was given alone in 127 cases (119 Gl), and other drugs (midazolam, fentanyl or ketamine alone or in combination) in 141 cases (89 Gl). There were 51 complications; 24 in the propofol group and 27 in the non-propofol patients; the complication rate (19%) was equal for both groups. Minor complications occurred in 42 cases (15.6%) and resolved with repositioning, increased O2 flow rate or fluid bolus. Nine complications (3.3%) were of moderate seventy; 5 requiring bag valve mask ventilation, 2 endotracheal intubation, and 2 prolonged recovery times. There were no life-threatening complications. Hypotension was more frequent in the propofol group (14% vs 5%), but the frequency of airway compromise (O2sat<90, EtCO2>45, stridor, apnea) was higher in the non-propofol group (12% vs 7%). Recovery times were significantly shorter in the propofol group (32 min vs 63 min to awakening, 91 min vs 191 min to discharge). A cost analysis showed a potential savings of >U.S. $15,000 in room charges if propofol had been used in all Gl cases during the study period. Conclusions: Propofol can be used safely by non-anesthesiologists under carefully monitored conditions to provide deep sedation for pediatric patients during Gl and other invasive procedures. The complication rate with this drug is not higher than that of other commonly used agents, and its use may result in significant cost savings due to the shorter recovery time.

Preparation of Iron Oxide Thin Films by MOCVD

Iron oxide thin films were deposited from 400 to 600°C by CVD from iron acetylacetonate (Fe(acac)3) using N2 as the transporting gas and separately introduced O2 as oxidation gas. β-Fe2O3 tended to be formed independent of O2 flow rate at 400°C. At 500 and 600°C, spinel-type Fe3O4+x was formed under low O2 flow rates. With increasing O2 flow rate, mixture of β-Fe2O3 and spinel-type Fe3O4+x was formed and the amount of β-Fe2O3 phase increased. Further increase in O2 flow rate resulted in the formation of β-Fe2O3 as a single phase. Fourier transformation infrared spectroscopy (FT-IR) measurement suggested that iron acetyl-acetonate vapor changed the molecular form by decomposition under certain condition. It was supposed that the change in molecular form influences the deposited phase of iron oxide films.

ESR Studies of Bunsen-type Methane-Air Flames. I. Effects of the Entrainment and Diffusion of Secondary Air on the Chemical Reactions in the Flame

Abstract H atoms in a Bunsen-type methane-air flame have been studied by observing their ESR signals at different flow rates of O2 and N2 in a synthetic secondary air. The optical emissions from excited CH, C2, and OH radicals in the same flame have been also studied. It has been found that an increased O2 flow rate produces changes in the intensity of the ESR and emission signals, and that it also increases the burning velocity in the inner-cone flame. An increased N2 flow rate, on the other hand, has almost no effect on the flame. The inner-cone of a Bunsen-type flame is, therefore, not a completely premixed flame; it is affected chemically by the secondary air.