Ratio In Gas Phase Research Articles

MOCVD growth of InAsN for infrared applications

The ternary alloy InAsN has been grown by MOCVD for the first time. The growth was performed at 70 Torr using TMI, AsH 3 and NH 3 as source gases and GaAs(100) as substrate. The growth temperature and AsH 3 to NH 3 gas phase ratio, NH 3 AsH 3 , were changed from 550 to 750°C and 100 to 1000, respectively. InAs was also grown at 100 Torr and 500°C. All the samples were characterized by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) at room temperature. The increase in growth temperature and gas phase ratio of NH 3 made the nitrogen solid composition larger. All the samples have a direct band gap which decreases with increasing nitrogen content. The smallest band gap energy of 0.12 eV at room temperature was obtained for InAs 0.939N 0.061.

Energetic Fuel Droplet Gasification with Liquid-Phase Reaction

Abstract An exploratory analytical and computational study of gasifying energetic liquid fuel droplets is presented. The liquid-fuel reaction, heating, and breakup are modelled and characterized. A spherically-symmetric geometry is considered, and the problem is described by considering a general formulation capable of resolving the solution domain consisting of a liquid-phase region, a bubbly two-phase region, and a gas-phase region. The transient, two-phase, governing equations are solved numerically for various values of the nondimensional rate constant, heat of decomposition, activation energy, number of bubbles per unit mass, and ratio of gas-phase to liquid-phase thermal conductivities.

Enlarged Parameter Space by Use of VHF-GD for Deposition of Thin <p> Type μc-Si:H Films

AbstractIn this paper new results on thin p - type μc-Si:H films deposited at low temperatures of 170 °C by the Very High Frequency - Glow Discharge technique (VHF-GD) are presented. The “tolerated” amount of diborane added in the gas phase ratio as well as the influence of three different plasma excitation frequencies (70, 100 and 130 MHz) in obtaining high electrical conductivity are investigated. The goal is to optimise very thin (< 400 Å) and hence optically transparent films by maintaining high conductivities for the application as window layers of solar cells.

Gas Transport in Sol-Gel Derived Porous Carbon Aerogels

ABSTRACTDue to their high electrical conductivity, their large specific surface area and their high porosity sol-gel derived nanoporous carbons are promising materials for electrodes, e.g. in water desalination systems or fuel cells. In order to optimize their properties with respect to these applications information is needed about transient and steady state transport through the interconnected pores.Dynamic gas expansion and time resolved permeation measurements allow to determine the relevantquantities, i.e. the permeability, the ratio of gas phase to surface diffusion and the volume of dead end pores along with the tortuosity.Experimental data on nanoporous carbons of different density are presented. All samples investigated were prepared via pyrolysis of resorcinol formaldehyde aerogels. The measurements were performed with different gases below 0.1 MPa.

Transport and photoluminescence of hydrogenated amorphous silicon–carbon alloys

The optoelectronic properties of hydrogenated amorphous silicon–carbon alloys (a-SiC:H) are studied over the entire compositional range of carbon content. The films are prepared using radio-frequency glow discharge and optimization was made with respect to deposition power and pressure, hydrogen dilution, and methane (or ethylene) -to-silane gas phase ratio. Regardless of the deposition conditions and source gases used, the optical, structural, and transport properties of the a-SiC:H alloys followed simple universal dependencies related to changes in the density of states associated with their structural disorder. The Urbach tail energy Eu and the B factor of the Tauc plot correlate with E04 (defined as the energy at which the absorption coefficient is equal to 104 cm−1) taken from photothermal deflection spectroscopy measurements. Up to E04pds≊2.6 eV, Eu increases monotonically from 50 up to ≊200 meV, while the B factor decreases from ≊800 down to ≊200 cm−1/2 eV−1/2. Above E04pds≊2.6 eV, both Eu and B remain approximately constant. The photoconductivity decreases exponentially with E04pds and is below 10−10 Ω−1 cm−1 for E04pds≥2.6 eV. Room-temperature photoluminescence is observed when E04pds≥2.6 eV. The photoluminescence peak position lies an average of 0.6 eV below the value of E04pds and increases linearly with decreasing value of the B factor of the Tauc plot.

Three‐dimensional model interpretation of NOx measurements from the lower stratosphere

A three‐dimensional off‐line chemistry transport model, driven by European Centre for Medium‐Range Weather Forecasts winds and temperatures, is used to interpret measurements of NO and NO2 taken from the DC‐8 during the second Airborne Arctic Stratospheric Expedition. The model was run in three configurations: gas phase chemistry alone, inclusion of the N2O5 aerosol reaction, and inclusion of both N2O5 and ClONO2 aerosol reactions. The run including the N2O5 aerosol reaction alone usually agreed best with measured NOx/NOy ratios in midlatitude air masses. The NOx/NOy ratios of the run with both aerosol reactions were always too low, while the gas phase ratios were usually too high, especially during March. All three simulations generated extremely low NO2/NOy ratios in air parcels that had spent several days or more in the polar night. Measured NO2/NOy ratios in these types of air masses were sometimes equally low but could also be considerably higher. Observed NO/NO2 ratios differed strongly from known theory.

Doping Properties of Zinc in InAlGaAs Grown by Low-Pressure Metal-Organic Vapor-Phase Epitaxy

The electrical and optical properties of Zn-doped InAlGaAs layers grown by low-pressure metal-organic vapor-phase epitaxy are studied with different Al contents. Hole concentration of InAlGaAs decreases with increasing Al content, which is due to the relationship between the Al content and the Zn acceptor binding energy derived from the low-temperature 2K photoluminescence spectra. The deterioration of Zn-doped InAlAs layers is observed in layers grown with a lower V/III gas-phase ratio, which is the result of an oxygen-related deep level.

Structural, Optical and Electrical Properties of μc-SiC:H Thin Films Deposited by the VHF-GD

ABSTRACTIn the present paper the authors present new results on thin (μc-SiC:H films deposited at low temperatures by the Very High Frequency - Glow Discharge technique (VHF-GD) at 70 MHz. The individual effects of each of the deposition parameters (methane and diborane gas phase ratios, input power, deposition temperature and pressure) are investigated with respect to the structural, optical and electrical properties of the films; the goal being the growth of optimised, thin μc-SiC:H layers for use as highly conductive and high gap window layers in solar cells.

Dark and photoconductivity of TBP doped n-type a-Si:H

In the present paper dark and photoconductivity of n-type a-Si:H films, obtained by using tertiarybutylphosphine (TBP) as dopant instead of phosphine, which is difficult to handle because of its high toxicity and pyrophoricity, have been reported. The films were obtained by adding TBP vapor to Silane gas in ratio varying from 10 −1% to 2% in an rf glow discharge process using He as carrier gas. The activation energy and pre-exponential factor have been determined as a function of TBP/SiH 4 gas phase ratio by measuring the conductivity as a function of temperature. The conductivity is found to be singly activated in the temperature range 350 to 450 K for samples having TBP/SiH 4 ratio as 1% and 2%. In the present work the activation energy is found to increase from 0.21 to 0.37 eV as the TBP/SiH 4 ratio increases from 10 −1% to 2%. The optical bandgap (1.98 eV) has been found to be constant for samples with different TBP/SiH 4 ratio. Intensity variation of photoconductivity indicates bimolecular recombination to be the dominant mechanism. Photosensitivity is found to decrease with in TBP/SiH 4 ratio.

Preparation and purification of dimethylzinc and the effect of iodine doping on the opto-electronic properties of epitaxial ZnSe layers grown by metalorganic chemical vapour deposition

The effect of iodine concentration in dimethylzinc (DMZn) precursors and the variation of the group VI/group II gas phase ratio on the electrical and optical properties of epitaxial ZnSe grown by atmospheric pressure MOCVD on (100) GaAs substrates have been investigated. Three purified DMZn sources were used which contained known concentrations of iodine, together with hydrogen selenide as a source of selenium. The highest carrier concentrations were recorded in ZnSe grown using the DMZn source containing the highest iodine concentration (0.9 ppm). In all cases the carrier concentration showed a decrease with the increase of the VI/II ratio. Strong features due to donor bound exciton transitions are observed in the photoluminescence spectra recorded at 10 K of samples grown at a low VI/II ratio, whereas at high VI/II ratios the spectrum decreases in overall intensity. In the case of the ZnSe grown from the DMZn containing a high iodine concentration, the I2 emission is replaced by the Ix emission attributable to the same impurity species in a state of different polarization. At high VI/II ratios, emissions such as Iv and Ex due to intrinsic features in the epilayer are more intense. The impurity responsible for the change in carrier concentration with VI/II ratio is considered to be mainly iodine in very low concentration occupying vacant selenium sites, VSe. The VSe concentration is lower at high VI/II ratios. ZnSe grown from the DMZn source containing the least iodine (0.2 ppm) gave photoluminescence spectra with the near band edge linewidths as narrow as 1.9 meV and carrier concentrations down to 1014 cm−3.

Theoretical interpretation of the gas phase equilibrium of 2-hydroxypyridine/2(1 H)-pyridinone

Theoretical ab initio calculations are performed to provide support for our hypothesis that the gas phase ratio of the 2-hydroxypyridine/2(1 H)-pyridinone tautomers results from the thermodynamic equilibrium which exists between cyclic hydrogen-bonded oxo-oxo and hydroxy-hydroxy dimers. We therefore, disprove the monomolecular mechanism of the tautomerization process, which has frequently been assumed in the past to estimate the free energy difference between the monomeric tautomers based on the experimentally determined tautomer ratio.

In situ B-doped Si epitaxial films grown at 450‡ C by remote plasma-enhanced chemical vapor deposition: Physical and electrical characterization

In situ boron doping of Si epitaxial films grown at 450‡ C by remote plasma-enhanced chemical vapor deposition (RPCVD) has been studied using secondary ion mass spectroscopy (SIMS), Hall effect measurements, defect etching in conjunction with Nomarski microscopy, cross-sectional transmission electron microscopy (XTEM), and current-voltage measurements. Boron incorporation is shown to be controllable and electrically active from 7 × 1017 to over 1020 cm-3, with no dependence on process parameters (temperature, rf power, and substrate bias) in the ranges studied, other than the B2H6/SiH4 gas-phase ratio. No change in deposition rate upon introduction of B2H6 dopant gas is seen, contrary to what has been observed in several higher-temperature CVD processes. No defects such as stacking faults are seen under Nomarski microscopy, but a visible haze covers some areas ofin situ B-doped wafers. This haze appears to consist of amorphous cone-shaped structures with their apexes at the substrate-epilayer interface. The origin of the conical defects is believed to be related to some phenomenon at the initiation of growth. In order to evaluate the electrical quality ofin situ B-doped epilayers,P +/N mesa diodes have been fabricated using both homoepitaxial and heteroepitaxial (GexSi1-x)p-type epitaxial films. The electrical junction in these diodes coincides with the (epi-substrate)—interface in the grown films. To avoid interdiffusion or annealing effects during diode fabrication, all processing temperatures were kept at or below 450‡ C. Ideality factors are 1.2-1.3 for all diodes, indicating diffusion-limited transport rather than recombination in the depletion region.

Boron doped hydrogenated amorphous silicon films prepared by photo-CVD

Abstract Boron doped hydrogenated amorphous silicon (a-Si:H) films have been grown from a gas mixture of diborane and silane utilizing mercury sensitized photo-CVD. Film growth characteristics along with their structural, optical and electrical properties have been studied as functions of diborane to silane gas phase ratio and total chamber pressures. Hydrogen bonding configuration and total content in the doped films have been correlated with boron related radicals, which essentially decide film growth mechanism and are related to variation in process parameters. Amongst these process parameters, chamber pressure above 1.0 Torr has been observed to influence boron doping efficiency in the film. This in turn enables films to be more ordered and conducting (p-type) when grown at higher pressures.

High Channel Mobility a-Si:H Thin Film Transistors with Oxide/Nitride Dielectrics

ABSTRACTWe describe: i) nitride-layer optimization; ii) device fabrication: and iii) electrical properties of a-Si:H thin film transistors, TFTs, that integrate oxide/nitride dielectrics into an inverted, staggered gate structure. We have systematically changed the concentrations of Si-Si, Si-H and Si-NH bonding groups within the deposited nitride layers by varying the source gas flow ratio, R = NH3/SiH4 from 2.5 to 12.5. The electrical characteristics of the TFTs improve significantly as the gas phase ratio R is increased from 2.5 to approximately 10, and then decrease as R is further increased. The performance of the TFTs peaks for a source gas ratio of -10, where the channel mobility is ∼1.4 cm2/V-s, the threshold voltage is 2.3 V; and the Ion to Ioff current ratio is > 105.These increases in performance can only realized in devices in which the back of the Si channel region is passivated with an oxy-nitride interfacial region.

Precursors for wide band gap II–VI semiconductors: Growth of the Zinc-based chalcogenides — ZnSe and ZnS

Zinc selenide and zinc sulphide, which have direct bandgaps of 2.7eV and 3.7eV respectively, have potential applications in optoelectronic, OE, devices operating in the blue region of the visible spectrum. Traditionally, thin film single crystal layers of these compounds have been grown by elemental vapour transport at high temperatures (600–900°C) which results in material containing many defects and defect impurity complexes. However, since the early work of Manasevit [1], MOVPE has been increasingly used to fabricate II– VI semiconductors. The lower growth temperatures involved (<500°C) and improved control of gas phase ratios have resulted in material with correspondingly lower impurity incorporation.

Hydrogen in oxidized silicon oxynitride thin films

Silicon oxynitride films with O/N ratios ranging between 0 and 1 have been oxidized in dry and wet ambients at 1000°C for times ranging from 0.5 up to 9 h. Since it is believed that hydrogen plays a crucial role in the oxidation of silicon oxynitrides the H depth distribution in the oxide/(oxy)nitride layer structures was investigated. For the analysis we applied high energy ion beam methods, viz. nuclear reaction analysis and elastic recoil detection. The hydrogen depth profiles are characterized by a hydrogen pile-up at the oxide/(oxy)nitride interface; the H concentrations in the oxide as well as in the remaining oxynitrides are below 1 at%. In this paper we report the amount of H in the interfacial region ( H int) as a function of the oxidation parameters. H int increases with increasing O/N and with increasing H 2O/O 2 gas-phase ratio up to H 2O/O 2 = 75/25. At low O/N ratios, H int increases with oxidation time, whereas at large O/N values it decreases. The origin of the H pile-up is briefly discussed.

Vaporization of ices containing S 2—implications for comets

The detection of S 2 in Comet IRAS-Araki-Alcock 1983d and the suggestion that it was a parent molecule has raised many interesting astrochemical questions concerning its origin and eventual release into the coma. Experimentally, we have examined the following two questions: (1) Can S 2 be stored in ices during warming, and (2) is S 2 released during vaporization of its water ice matrix? We have recorded the ultraviolet spectrum of S 2 in argon and krypton matrices and water ice in which the ratio of matric or molecular ice to S 2 was the order of 500:1. The intensity of S 2 absorptions decreased in argon during warming. In water the weak S 2 signal was detected from 12 to 140°K during short annealing periods, but was not detected after annealing at 140°K for 17hr. S 3 and S 4 were also detected in some ices along with seven unidentified absorption features centered near 6250 Å. S 2 was not detected with a mass spectrometer during vaporization of Ar: S 2, Kr: S 2, Kr: 34S 2, or H 2O: 34S 2 (each ∼ 500:1) mixtures. Our experiments support the idea that the S 2:H 2O ratio in the solid phase may not be a predictor of the gas phase ratio. After warming, a room temperature residue was always formed. Sulfur was detected in that residue. We conclude that similar vaporization from a cometary nucleus would not release the bulk of any S 2 trapped in the ice and that an alternate mechanism for releasing this diradical is required.

Critical current densities of Nb 3Ge tapes prepared by chemical vapour deposition on resistively heated tape substrates

Superconducting Nb 3Ge tapes were prepared continuously by the CVD process, changing the deposition temperature and the composition. Nb 3Ge films were deposited on 2.5 mm wide and 0.1 mm thick Hastelloy tapes, resistively heated by electric currents. The substrate tape temperatures were measured during continuous depositions. The maximum value of J c 2 × 10 4 A cm −2 at 20 T and 4.2 K, was obtained for the Nb 3Ge tape with a Nb 3Ge layer thickness of 11 μm per side, prepared at the Nb Ge ratio in gas phase of 3.6 and at the substrate temperature of 834°C. A 17 m long Nb 3Ge tape was prepared uniformly at a tape pull rate of 4 cm min −1. The utilization efficiency of Nb and Ge particles as starting materials was 42%. Merits and demerits of resistively heating the substrate tape in the CVD process for the fabrication of long Nb 3Ge tapes are presented. Possible improvements on our method are also described.

Theoretically predicted and experimentally determined effects of the Si/(Si+C) gas phase ratio on the growth and character of monocrystalline beta silicon carbide films

The effects of the Si/(Si+C) ratio in the reaction gas stream on the growth and properties of monocrystalline β-SiC films grown on Si(100) substrates via chemical vapor deposition have been theoretically and experimentally studied. The amounts of condensed phases of β-SiC and Si, and the partial pressures of the remaining Si and C-containing gases as a function of the Si/(Si+C) ratio in the source gases have been initially obtained from thermodynamic calculations using the ‘‘solgasmix-pv’’ computer program. Complementary and comparative experimental growth studies have shown that inclusion-free films having maximum values in growth rate and carrier concentration and a minimum value of resistivity were obtained near Si/(Si+C)=0.5.

Characterization of plasma-enhanced chemically-vapor-deposited silicon-rich silicon dioxide/thermal silicon dioxide dual dielectric systems

Plasma enhanced chemically-vapor-deposited silicon-rich oxides (200 Å and 500 Å in thickness) of various excess silicon content were deposited onto thermal silicon dioxide (SiO2) layers (103, 207, and 530 Å in thickness) grown on a p-type silicon (Si) substrate. The dielectric constant, electron injection efficiency, current-voltage (I-V) reproducibility, and breakdown property of these composite structures were examined. The dielectric constants of Si-rich oxide were observed to increase with Si content from 3.8 for films deposited at a gas phase ratio (R0) of the concentration of nitrous oxide (N2O) to silane (SiH4) of 150 to ∼10 for films deposited with R0=0. The Si-rich oxides with R0≤5 were found to work as electron injectors. The average oxide field needed to induce a current of 4.8×10−7 A/cm2 through the SiO2 (530 Å in thickness) decreased about 40% in magnitude by adding a Si-rich oxide layer with the optimized R0(=1) compared to that of a control sample which had no Si-rich oxide layer. For thin SiO2 (103 Å and 207 Å in thickness) samples, the decrease of the average field was only 2% and 10% in magnitude with the optimized R0 (=2) layer, respectively, due to the relatively large voltage drop (≊−1 V) across the Si-rich oxide compared to that across the thermal oxide layer. The voltage drop across the oxide is discussed in terms of a dual dielectric model. The yeild, which was defined as the percentage of capacitors that required a field larger than 2 MV/cm to obtain a current of 9.6×10−4 A/cm2, on as-fabricated samples was larger than 90% for all samples with Si-rich oxide. The samples were not destroyed by the passage of a relatively high current density (1.21×10−2 A/cm2) through the oxide and subsequent measurements resulted in approximately the same field to produce the specified current as for the first measurement. The yield was found to have a maximum at R0=1–10 depending on the thickness of Si-rich oxides and SiO2. Current-voltage reproducibility was also found to be improved by the deposition of Si-rich oxide.