The epitaxial growth of ZnO on sapphire and MgAl spinel using the vapor phase reaction of Zn and H2O

The gas phase reaction of Zn and H₂O in a He carrier gas has been used as the basis for the chemical vapor deposition of ZnO on sapphire and MgAl spinel. Deposit characteristics were studied as a function of reactor linear gas stream velocity, Zn/H₂O vapor phase ratio, temperature and substrate preparation. It was round that the substrate support can influence the surface morphology significantly and that in situ pretreatment can affect epitaxial relationships between deposit and substrate. Transparent, visually smooth deposits of (11–24) ZnO can be obtained on chemically polished (0001) sapphire at 815°C using average linear gas stream velocities, ν, of 6-12 cm/sec referenced to room temperature in conjunction with a Zn/H₂O reactor input-pressure ratio of 0.02–0.09 (using Zn reactor pressures of 1.2–5.0 × 10⁻³ atm) . The substrates are given an H₂O in situ pretreatment at 900°C prior to deposition at ν = 3 cm/sec with the partial pressure of H₂O in He = 5.7 × 10 atm.     

C2H2 sensing at V1+V3 band with a hollow-core photonic bandgap fiber

In this paper, we demonstrate the acetylehe (C2H2) sensor with high sensitivity using a hollow-core photonic bandgap fiber (HC-PCF). Experiments for measuring C2H2 concentrations in gas mixture are performed. Using a 2 m-long HC-PCF as gas cell, the spectrum of acetylene at n1+n3 band has been measured, and the P11-branch has been selected for the purpose of sensing. A minimum detectivity of 143 parts per million by volume (ppmv) for the system configuration is estimated.     

Heteroepitaxial silicon-carbide nitride films with different carbon sources on silicon substrates prepared by rapid-thermal chemical-vapor deposition

Cubic crystalline silicon-carbon nitride (Si_1−x−yC_xN_y) films have been grown successfully using various carbon sources by rapid-thermal chemical-vapor deposition (RTCVD). The characteristics of the Si_1−x−yC_xN_y films grown with SiH₃CH₃, C₂H₄, and C₃H₈ are examined and compared by x-ray photoelectron spectroscopy (XPS) spectra, scanning electron microscopy (SEM) images, and transmission electron microscopy (TEM) patterns. The XPS spectra show that the differences of chemical composition and chemical-bonding state are co-related to the C bonding type of the different carbon source. The SEM images and TEM analysis indicate that the better Si_1−x−yC_xN_y film can be obtained using C₃H₈ gas as the carbon source. In addition, correlations between the growing stages to the microstructure of the cubic-crystalline Si_1−x−yC_xN_y films have been illustrated in detail.     

Field Emission and Electric Discharge of Nanocrystalline Diamond Films

Nanocrystalline diamond (NCD) films were produced by microwave plasma-enhanced chemical vapor deposition (MPECVD) using gas mixtures of Ar, H₂, and CH₄. The structural properties, electron emission, and electric discharge behaviors of the NCD films varied with H₂ flow rates during MPECVD. The turn-on field for electron emission at a pressure of 2.66 × 10⁻⁴ Pa increased from 4.2 V μm⁻¹ for the NCD films that were deposited using a H₂ flow rate of 10 cm³ min⁻¹ to 7 V μm⁻¹ for films deposited at a H₂ flow rate of 20 cm³ min⁻¹. The NCD film with a low turn-on field also induced low breakdown voltages in N₂. The grain size and roughness of the NCD films may influence both the electron emission and the electric discharge behaviors of the NCD cathodes.     

Low temperature heteroepitaxial growth of Si1−xGexon-Si by photo-enhanced ultra high vacuum chemical vapor deposition using Si2H6 and Ge2H6

Strained-layer Si_1×Ge_x-on-Si heteroepitaxy has been achieved by photolytic decomposition of disilane (Si₂H₆) and digermane (G e₂H₆) in an ultra high vacuum (UHV) chamber at substrate temperatures as low as 275°C. An ArF excimer laser (193 nm) shining parallel to the Si substrate was used as the UV light source to avoid surface damage and substrate heating. The partial pressures of the source gases in the reactor were chosen to vary the Ge mole fraction x from 0.06 to 0.5 in the alloy. The Si₂H₆ partial pressure was kept at 10 mTorr and the Ge₂H₆ partial pressure was varied from 0.13 to 2 mTorr with the laser intensity fixed at 2.75 × 10¹⁵ photons/cm²·pulse. To fit the Si_1−xGe_x growth rate and Ge mole fraction data, the absorption cross section of Ge₂H₆ at 193 nm was set to 1 × 10⁻¹⁶ cm², which is 30 times larger than that of Si₂H₆ (3.4 × 10⁻¹⁸ cm2). For Si_1−xGe_x alloy growth, the deposition rate of Si increases with Ge mole fraction, resulting in increased Si_1−xGe_x alloy growth rates for higher Ge content. The increase of the Si growth rate was attributed to the enhanced adsorption rate of Si₂H₆ pyrolytically in the presence of Ge, rather than due to photolytic decomposition reaction. The Ge mole fraction in Si_1−xGe_x alloys can be predicted by a new model for Si and Ge pyrolytic and photolytic growth. The model describes the increased growth rate of Si_1−xGe_x alloys due to a Ge₂H₆ catalytic effect during photo-enhanced chemical vapor deposition.     

Selective and nonselective wet etching of Zn0.9Mg0.1O/ZnO

Wet etch rates at 25°C for Zn_0.9Mg_0.1O grown on sapphire substrates by pulsed laser deposition (PLD) were in the range 300–1100 nm · min⁻¹ with HCl/H₂O (5×10⁻³−2×10⁻² M) and 120–300 nm · min⁻¹ with H₃PO₄/H₂O (5×10⁻³−2×10⁻² M). Both of these dilute mixtures exhibited diffusion-limited etching, with thermal activation energies of 2–3 kCal · mol⁻¹. By sharp contrast, the etch rates for ZnO also grown on sapphire by PLD were much slower in similar solutions, with rates of 1.2–50 nm · min⁻¹ in HCl/H₂O (0.01–1.2 M) and 12–54 nm · min⁻¹ in H₃PO₄/H₂O (0.02–0.15 M). The etching was reaction limited over the temperature range 25–75°C, with activation energies close to 6 kCal · mol⁻¹. The resulting selectivity of Zn_0.9Mg_0.1O over ZnO can be a high as ∼400 with HCl and ∼30 with H₃PO₄.     

Concentrations of native and gold defects in HgCdTe from first principles calculations

We have studied the defect formation energies of the various native (vacancies, interstitials, and antisites) and Au defects in Hg_1−xCd_xTe using density functional-based total energy calculations with ultrasoft pseudo-potentials. These studies are important for infrared (IR) detection technology where the device performance can be severely degraded because of defects. To calculate formation energies, we modeled the neutral and charged defects using supercells containing 64 atoms. From the formation energies, we have determined the defect concentrations as a function of stoichiometry and temperature. We find the prevalent neutral defects to be Au at the Hg site (Au_Hg ), Hg vacancies (V_Hg ), and Te antisites (Te_Hg ). We have also explicitly studied charged defects and have found Te _Hg ²⁺ , Au _Hg ¹⁻ , V _Hg ¹⁻ , V _Hg ²⁻ , and V _Te ²⁺ to have low formation energies. We have identified Au_Hg to be the prevalent Au defect, having concentrations several orders of magnitude greater than the other Au defects. We find that the charge state of V_Hg is primarily (1−) or (2−) depending on the electronic chemical potential.     

The chemical control of high-Tc superconductivity: Metal-superconductor-insulator transition in (Tl1−yPby)Sr2(Ca1−xYx)Cu2O7

We have carried out a detailed study of the relationship between the semiconducting, the superconducting, and the metallic region in the system (Tl_0.5Pb_0.5)Sr₂(Ca_1−xY_x)Cu₂O₇. The critical density (or concentration), n_c, derived from Hall measurement of the titled series compounds at which the transition from insulator to metal place can be given by the well-known Mott formula: n_c ^1/3a_H ^*=0.26 where a_H ^* is the effective Bohr orbit radius of the isolated donor or acceptor state wavefunction. We propose that the insulator-metal transition in the cuprates generally occurs at a very high hole density (n_c∼10²⁰−10²¹cm⁻³) as compared to conventional doped semiconductors (n_c∼10¹⁷−10¹⁸cm⁻³) but rather lower than the critical densities for elemental metals (n_c>10²¹cm⁻³).     

Multiphoton ionization of acetone-water clusters at 355 nm

Ti me of flight mass spectrometer(TOF-MS) is verysuccessful apparatus to detect clusterions[1-3].It isknown protonatedions are easy to formedin multipho-tonionization( MPI)[4 ,5].Acetoneis the si mplest molecule in the ketones ,it isvery popularinvestigat…     

Growth of quaternary InP-based materials by LP-MOVPE using TBA and TBP in N2 ambient

We have investigated the growth of quaternary In_1−xGa_xAs_yP_1−y/InP materials using TBA and TBP in a N₂ ambient. This process improves significantly the uniformity of In_1−xGa_xAs/InP QWs whereas it does not improve the quaternary Q(1.3)/InP uniformity compared to the conventional process utilizing AsH₃ and PH₃ in H₂. The effect on the x and y uniformity for different combinations of the group-V precursors TBA, TBP, PH₃, and AsH₃ with the carrier gases H₂ and N₂ is evaluated. Advantages with the TBA/TBP/N₂ process are discussed.     

Electrical properties of carbon nitride thin films: Role of morphology and hydrogen content

The influence of hydrogen content and ambient humidity on the electrical properties of carbon nitride (CN_x) films deposited by reactive magnetron sputtering from a graphite target in Ar discharges mixed with N₂ and H₂ at a substrate temperature of 350°C have been investigated. Carbon films deposited in pure Ar exhibit a dark resistivity at room temperature of ∼4 × 10⁻² Ωcm, while the resistivity is one order of magnitude lower for CN_0.25 films deposited in pure N₂, due to their denser morphology. The increasing H₂ fraction in the discharge gas leads to an increased resistivity for all gas mixtures. This is most pronounced for the nitrogen-free films deposited in an Ar/H₂ mixture, where the resistivity increases by over four orders of magnitude. This can be related to a decreased electron mobility as H inhibits the formation of double bonds. After exposure to air, the resistivity increases with time through two different diffusion regimes. The measured electrical properties of the films are related to the apparent film microstructure, bonding nature, and ambient humidity.     

High Surface Area MoS2/Graphene Hybrid Aerogel for Ultrasensitive NO2 Detection

A MoS₂/graphene hybrid aerogel synthesized with two‐dimensional MoS₂ sheets coating a high surface area graphene aerogel scaffold is characterized and used for ultrasensitive NO₂ detection. The combination of graphene and MoS₂ leads to improved sensing properties with the graphene scaffold providing high specific surface area and high electrical and thermal conductivity and the single to few‐layer MoS₂ sheets providing high sensitivity and selectivity to NO₂. The hybrid aerogel is integrated onto a low‐power microheater platform to probe the gas sensing performance. At room temperature, the sensor exhibits an ultralow detection limit of 50 ppb NO₂. By heating the material to 200 °C, the response and recovery times to reach 90% of the final signal decrease to <1 min, while retaining the low detection limit. The MoS₂/graphene hybrid also shows good selectivity for NO₂ against H₂ and CO, especially when compared to bare graphene aerogel. The unique structure of the hybrid aerogel is responsible for the ultrasensitive, selective, and fast NO₂ sensing. The improved sensing performance of this hybrid aerogel also suggests the possibility of other 2D material combinations for further sensing applications.     

Modeling the hydrogen distribution accompanying electron injection in SiO2 films in strong electric fields

A new theoretical model is proposed for describing the behavior of hydrogen when electrons are injected from contacts in thin-film SiO₂ in strong fields. Hot electrons dislodge hydrogen from dangling SiO_ and Si_ bonds, creating traps for electrons and holes. Computations have been carried out for hydrogen redistribution and charge accumulation in SiO₂. The experimental data of D.A. Buchanan et al. [J. Appl. Phys. 76, 3595 (1994)] are used to determine the hydrogen-formation reaction cross section σ _H⋍6×10⁻²⁰ cm² and the hydrogen-photogeneration frequency ν _ph=2×10⁻⁶ sec⁻¹. An explanation is given for the accumulation of an anomalous positive charge in the SiO₂ film. Fiz. Tekh. Poluprovodn. 31, 257–263 (March 1997)     

(AlGa)As grown by low pressure metalorganic vapor phase epitaxy using a N2 carrier

We have studied the growth of Al_xGa_1−xAs (0.24<x<0.34) using a N₂ carrier in low pressure metalorganic vapor phase epitaxy. Growth temperature, gas velocity, and V/III ratio were varied to achieve optimum growth conditions. Layers with excellent morphology and electrical and optical properties comparable to samples grown using standard conditions (with a H₂ carrier) can be deposited in a nitrogen ambient. Al_0.24Ga_0.76As bulk material grown on an AlAs buffer layer with a background doping of 1.3×10¹⁶ cm⁻³ showed Hall mobilities of 4500 and 2300 cm²/Vs at 77 and 300K. Photoluminescence studies at 2K revealed strong bound exciton transitions with a full width at half maximum of 5.2 meV for Al_0.29Ga_0.71AS.     

The electrical conductivity of polycrystalline SnO2(Cu) films and their sensitivity to hydrogen sulfide

The effect of doping with copper on the sensor properties and the electrical conductivity of polycrystalline SnO₂(Cu) films has been investigated. It has been found that at room temperature the residual conductivity is observed after the films are exposed to H₂S. This made it possible to determine the character of the low-temperature conductivity of the films for different degrees of saturation with hydrogen sulfide. A comparison of the obtained data with the results of layerwise elemental analysis suggested a model that explains the mechanism of the gas sensitivity of SnO₂(Cu) to hydrogen sulfide. In contrast to the mechanisms, which are associated with the work done by the surface and which are standard for gas sensors, in the present case the change in the conductivity is due to the chemical reaction of the electrically active copper with sulfur in the entire volume of the film. This reaction determines the selectivity and high sensitivity of SnO₂(Cu) to H₂S. Fiz. Tekh. Poluprovodn. 31, 400–404 (April 1997)     

The equilibrium constant for the reaction of ZnO + H2 and the chemical vapor transport of zno via the Zn + H2O reaction

Using a gas transpiration method, the equilibrium constant for the reaction ZnO(s) + H₂(v)⇄ Zn(v) + H₂O(v) was determined directly in the temperature interval 592–950°C. The data are described by the equation log₁₀K = [-11.794 × 10³/T] + 8.040 with ΔH° = 53.97 Kcal/mole and ΔS° = 36.79 e.u. Based on this Information, vapor-solid gas concentration curves useful for defining conditions for the chemical vapor transport of ZnO via the reverse of the above reaction have been computer calculated.     

Electrical properties of Pb1−xCdxS epitaxial films

We have measured the resistivity ρ and Hall coefficient R_H at 300, 77, and 4.2 K of p-type Pb_1−XCd_XS epitaxial films as a function of substrate temperature T_s, film thickness d, and composition x. The films were vapor deposited on cleaved (111) BaF₂ (111) SrF₂ , and (001) NaCl and polished (001) BaF₂ substrates. The Hall mobility μ_H at 77 K of p-type PbS films increased approximately linearly from 1 × 10⁴ to 2 × 10⁴ cm² V⁻¹ sec⁻¹ as T_s was varied from 400 to 500°C, respectively. Both μ_H and R_H increased with d due to the presence of a strong p-type surface layer on the exposed surface. The x of the films was controlled by the x of the source material and T_s. The mole fraction of CdS could be varied between 0.002 < x < 0.06 by varying T between 513 and 410°C, respectively, and using source material with x = 0.06. The electrical properties of samples grown on freshly cleaved (111) BaF₂ and (111) SrF₂ were essentially identical even though the lattice constant of SrF₂ is a better match to Pb_1−XCd_XS than BaF₂. The R_H and μ_H at 77 K were independent of thickness for low substrate temperatures and were observed to increase with increasing thickness for high substrate temperatures. The μ_H increased with decreasing temperature and became temperature independent below about 30 K, which is similar to the behavior observed in other lead salt compounds. However, the magnitude of μ_H was considerable lower throughout the 300 to 4.2 K temperature range than for PbS films. The R_H showed little temperature variation, which is typical lead salt behavior. Supported by Naval Surface Weapons Center Independent Research Funds.     

CO-laser photoacoustic detection of phosgene (COCl2)

Detection of COCl₂, a highly toxic gas in chemical industry, using laser photoacoustic spectroscopy is presented. The spectrophone system used has a broad band LN₂ cooled CW CO laser as a source of radiation, which operates from 4.8–8.4 um in the mid-infrared. Using an extracavity open longitudinal resonant cell, absorption signals to about 30 CO laser lines in the 5.45 um region could be observed. detection sensitivity has been estimated to be ppb order.     

Low temperature growth and planar doping of ZnSe in a plasma-stimulated LP-MOVPE system

In a low-pressure metalorganic vapor phase epitaxy process, we used dc-plasma activated nitrogen to dope ZnSe, grown with ditertiarybutylselenide and dimethylzinc-triethylamine. The nitrogen concentration of up to 2 × 10¹⁸ cm⁻³ in the doped layers can be adjusted by the growth temperature, the dc-plasma power, and the N₂ dopant flow. Due to the high n-type background carrier concentration of the order of 10¹⁷ cm⁻³ in undoped samples, the doped layers show n-type conductivity or were semi-insulating because of an additional compensation by hydrogen incorporated with a concentration of the order of 10¹⁸ cm⁻³. A planar doping scheme was applied to reduce this hydrogen incorporation by one order of magnitude, although H₂ was used as carrier gas.     

Study of sensitivity and selectivity of α-Fe2O3 thin films for different toxic gases and alcohols

This paper presents a detailed study on the sensitivity and selectivity of α-Fe₂O₃ thin films produced by deposition of Fe and post-deposition annealed at two temperatures of 600 °C and 800 °C with flow of oxygen for application as a sensor for toxic gases including CO, H₂S, NH₃ and NO₂ and alcohols such as C₃H₇OH, CH₃OH, and C₂H₅OH. The crystallographic structure of the samples was studied by X-ray diffraction (XRD) method while an atomic force microscope (AFM) was employed for surface morphology investigation. The electrical response of the films was measured while they were exposed to various toxic gases and alcohols in the temperature range of 50–300 °C. The sample annealed at higher temperature showed higher response for different gases and alcohols tested in this work which can be due to the higher resistance of this sample. Results also indicated that the α-Fe₂O₃ thin films show higher selectivity to NO₂ gas relative to the other gases and alcohols while the best sensitivity is obtained at 200 °C. The α-Fe₂O₃ thin film post-deposition annealed at 800 °C also showed a good stability and reproducibility and a detection limit of 10 ppm for NO₂ gas at the operating temperature of 200 °C.