Chemical Vapor Deposition Environment Research Articles

Non-equilibrium fractal growth of MoS2 for electrocatalytic hydrogen evolution

Non-equilibrium fractal growth of MoS2 was induced by establishing an extremely Mo rich chemical vapor deposition (CVD) environment using a rapid heating rate in a confined reaction space.

Urchin-like artificial gallium oxide nanowires grown by a novel MOCVD/CVD-based route for random laser application

A novel procedure based on a two-step method was developed to obtain β-Ga2O3 nanowires by the chemical vapor deposition (CVD) method. The first step consists in the gallium micro-spheres growth inside a metal-organic chemical vapor deposition environment, using an organometallic precursor. Nanoscale spheres covering the microspheres were obtained. The second step involves the CVD oxidization of the gallium micro-spheres, which allow the formation of β-Ga2O3 nanowires on the micro-sphere surface, with the final result being a nanostructure mimicking nature's sea urchin morphology. The grown nanomaterial is characterized by several techniques, including X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray, transmission electron microscopy, and photoluminescence. A discussion about the growth mechanism and the optical properties of the β-Ga2O3 material is presented considering its unknown true bandgap value (extending from 4.4 to 5.68 eV). As an application, the scattering properties of the nanomaterial are exploited to demonstrate random laser emission (around 570 nm) when it is permeated with a laser dye liquid solution.

Electroluminescence from Ge1−ySny diodes with degenerate pn junctions

The light emission properties of GeSn pn diodes were investigated as a function of alloy composition and doping levels. Very sharp interfaces between contiguous ultra-highly doped p- and n-layers were obtained using in situ doping with B2H6 and P(SiH3)3 in a chemical vapor deposition environment, yielding nearly ideal model systems for systematic studies. Changes in the doping levels and layer Sn concentrations are shown to greatly affect the electroluminescence spectra. This sensitivity should make it possible to optimize the emission efficiency for these structures in the interesting quasi-direct regime, for which direct gap luminescence is observed due to the proximity of the conduction band quasi-Fermi level to the minimum of the conduction band at the center of the Brillouin zone. Such structures represent the basic building block of Ge-based electrically pumped lasers.

Rhenium Alloys as Ductile Substrates for Diamond Thin-Film Electrodes.

Molybdenum-rhenium (Mo/Re) and tungsten-rhenium (W/Re) alloys were investigated as substrates for thin-film, polycrystalline boron-doped diamond electrodes. Traditional, carbide-forming metal substrates adhere strongly to diamond but lose their ductility during exposure to the high-temperature (1000°C) diamond, chemical vapor deposition environment. Boron-doped semi-metallic diamond was selectively deposited for up to 20 hours on one end of Mo/Re (47.5/52.5 wt.%) and W/Re (75/25 wt.%) alloy wires. Conformal diamond films on the alloys displayed grain sizes and Raman signatures similar to films grown on tungsten; in all cases, the morphology and Raman spectra were consistent with well-faceted, microcrystalline diamond with minimal sp2 carbon content. Cyclic voltammograms of dopamine in phosphate-buffered saline (PBS) showed the wide window and low baseline current of high-quality diamond electrodes. In addition, the films showed consistently well-defined, dopamine electrochemical redox activity. The Mo/Re substrate regions that were uncoated but still exposed to the diamond-growth environment remained substantially more flexible than tungsten in a bend-to-fracture rotation test, bending to the test maximum of 90° and not fracturing. The W/Re substrates fractured after a 27° bend, and the tungsten fractured after a 21° bend. Brittle, transgranular cleavage fracture surfaces were observed for tungsten and W/Re. A tension-induced fracture of the Mo/Re after the prior bend test showed a dimple fracture with a visible ductile core. Overall, the Mo/Re and W/Re alloys were suitable substrates for diamond growth. The Mo/Re alloy remained significantly more ductile than traditional tungsten substrates after diamond growth, and thus may be an attractive metal substrate for more ductile, thin-film diamond electrodes.

Effects of Reduced Chemical Vapor Deposition Environment on Growth and Optical Characteristics of TiO2 Nanobelts

TiO2-delta nanobelts were self-catalytically grown at 510 degrees C on bare Si (100) substrates using metallorganic chemical vapor deposition. The nanobelt formation was critically affected by the partial pressure of oxygen. The nanobelts were grown when supplying only Ar or a mixed gas of Ar (90%) and H2 (10%), while thin films were formed with an O2 gas flow of more than 50 cm3 min(-1). The nanobelts consisted of approximately 20-30 nm size rutile-dominant nanocrystallites. A vapor-solid growth mechanism excluding a liquid phase appeared to control the nanobelt formation. The grown TiO2-delta nanobelts showed a strong photoluminescence (PL) spectra peak at approximately 550 nm due to oxygen vacancies. The nanobelt surface possessed significant amount of oxygen vacancies contributing PL and actively reacting with the environment, indicating promise for photocatalytic and gas sensor applications in a visible light regime.

Growth of Ge Nanowires from Au−Cu Alloy Nanoparticle Catalysts Synthesized from Aqueous Solution

Elemental metal nanoparticles are widely used to catalyze semiconductor nanowire growth, but size-controlled alloy nanoparticles have not been explored in this context. We present a simple aqueous synthesis of Au−Cu2O core−shell nanoparticles to produce Au−Cu alloy nanoparticles of controlled size and composition by vacuum annealing. Colloidal Au nanoparticles were used as size-controlled seeds, and fine control of the Cu2O shell thickness enabled tuning of the average Au−Cu alloy composition. The alloy nanoparticles were found to catalyze Ge nanowire growth in a low-pressure chemical vapor deposition environment. The nanowire growth rate for Au−Cu nanoparticles was intermediate to that of Cu (slowest) and Au (fastest) nanoparticles under identical conditions, suggesting a vapor−solid−solid growth process. These catalysts provide a useful platform to explore the influence of catalyst phase and chemistry on nanowire growth mechanisms that determine important variables including the doping rate and junction...

Environmental stability of candidate dielectrics for GaN-based device applications

The thermal and chemical stability of potential dielectrics for GaN-based devices in a GaN metal organic chemical vapor deposition (MOCVD) environment is investigated, and their suitability for use as a gate dielectric and as a regrowth mask for the selective area growth of GaN is discussed. Thin films of MgO, Sc2O3, and Sc2O3/MgO were grown by molecular beam epitaxy on GaN/sapphire substrates and then annealed in a MOCVD reactor under GaN growth conditions except for the lack of trimethylgallium. All films were processed into metal-oxide-semiconductor diodes and were characterized before and after being annealed using atomic force microscopy, x-ray reflectivity, x-ray photoelectron spectroscopy, current-voltage (I-V), and capacitance-voltage (C-V) measurements. The Gibbs free energy of all possible reactions was calculated, and their probability and possible influence on the characterization results is examined. After being annealed, the atomic force microscopy of the oxide films showed some degree of roughening for all of them. Despite the surface roughening, all the oxide films examined showed potential for use as a regrowth mask. The MOCVD anneal caused the electrical properties of the MgO film to degrade considerably, and the Sc2O3 films were unable to be electrically characterized after annealing due to shorting, which is believed to be caused by the formation of a ScN layer on the surface. The effect of the thickness of the Sc2O3 cap for MgO films was investigated. The characterization results indicate that the Sc2O3 film dissolved into MgO during annealing and that a ScN film did not form on the surface. Of all the oxide films examined in this study, the Sc2O3/MgO stack with the thinner Sc2O3 cap exhibited the greatest stability with respect to its electrical properties.

Chemical Vapor Deposition of Diamond Films on Patterned GaN Substrates via a Thin Silicon Nitride Protective Layer

Integrating diamond films with GaN-based devices may enhance heat dissipation and thus improve device performance for high power loading. Direct deposition of diamond films on GaN layers has been hampered by GaN degradation in the chemical vapor deposition environment for diamond growth. In this work, three approaches were introduced to grow high-quality diamond films on patterned GaN substrates via a thin silicon nitride protective layer, that is, (i) a two-step process involving an initial rapid growth step, (ii) addition of nitrogen to hydrogen-based plasma to suppress reactions between GaN and hydrogen, and (iii) deposition in argon-based plasma. Continuous, adherent, and high-quality micro- and nanocrystalline diamond films were successfully deposited. All three approaches were effective in reducing plasma-induced GaN decomposition and etching, and in eliminating film cracks and delamination.

Detection of SH and CS radicals by cavity ringdown spectroscopy in a hot filament chemical vapor deposition environment

The SH and CS radicals produced during sulfur-assisted hot filament chemical vapor deposition (HFCVD) were studied by cavity ringdown spectroscopy (CRDS) by detecting the A(0) ← X(0) transition near 323 and 259 nm, for SH and CS, respectively. CRDS is a non-intrusive spectroscopic diagnostic tool suitable for measuring trace amounts of gas species in hostile, reactive environments such as that of chemical vapor deposition (CVD). The SH and CS radicals are thought to play important roles in the gas-phase and heterogeneous chemistry of sulfur-assisted HFCVD synthesis of nano-structured carbon-based materials. A detailed analysis of the rotationally resolved SH and CS spectra led to an estimate of their effective rotational temperature and number density.

The Generation of Domain Boundaries in Catalytically-Grown Carbon Nanotubes

Arrays of catalytically-grown multi-wall carbon nanotubes were grown using identical conditions in a chemical vapor deposition environment, but cooled at different cooling rates, to identify the influence of cooling rate on the structural properties of the nanotube at the catalyst-wall interface. Ex-situ transmission electron microscopy led to the identification of twist, twin, and tilt domain boundaries in all samples irrespective of cooling rate. In addition, the relative position of twist, twin, and tilt domain boundaries in nanotubes cooled at different rates was maintained uniformly across all samples cooled at different rates. The results are interpreted in light of the concurrence of base- and tip-growth for the catalytic synthesis of nanotubes, suggesting a rather steady position occupied by the domain boundaries coupled to the catalytic particles.

All‐Dry Synthesis and Coating of Methacrylic Acid Copolymers for Controlled Release

Initiated chemical vapor deposition (iCVD) is presented as an all-dry synthesis and coating method for applying methacrylic acid copolymers as pH-responsive controlled release layers. iCVD combines the strengths of liquid-phase chemical synthesis with a precision solvent-free chemical vapor deposition environment. Copolymers of methacrylic acid and ethyl acrylate were confirmed by a systematic shift in the carbonyl bond stretching mode with a shift in the comonomer ratio within the copolymer and by the ability to apply the Fineman-Ross copolymerization equation to describe copolymerization kinetics. Copolymers of methacrylic acid and ethylene dimethacrylate showed pH-dependent swelling behavior that was applied to the enteric release of fluorescein and ibuprofen.

Interface broadening due to ion mixing during thin film growth at the radio-frequency-biased electrode in a plasma-enhanced chemical vapor deposition environment

The authors show that ion bombardment in the range of tens to a few hundreds of eV, used in ion- and plasma-assisted deposition processes, can lead to thin film growth dominated by subsurface deposition due to subplantation (shallow implantation). This can cause significant interface broadening during the initial stages of film deposition as a result of ion mixing. First, by studying the modifications of a c-Si(100) target exposed to an O2 plasma at the radio-frequency (rf)-biased electrode using in situ real-time spectroscopic ellipsometry (RTSE), the authors detect implantation, damage, and oxidation to a depth of up to ∼10nm. They validate these results using high resolution transmission electron microscopy and simulate the effects of ion-surface interactions at the rf-biased electrode by using Monte Carlo TRIDYN simulations. The simulation code, which was modified specifically to consider a broadband ion energy source, enabled the authors to reproduce depth and time relevant experimental results with good agreement. In situ RTSE was then used to monitor TiO2 deposition on SiO2 under similar ion bombardment conditions. The authors observed the formation of a 2-to4-nm-thick interfacial layer, depending on the ion-to-neutral flux ratio (ϕi∕ϕn), which was controlled by varying the deposition rate. TRIDYN simulations revealed that oxygen subplantation causes interfacial broadening during the growth through ballistic mixing of Ti and Si atoms at the interface; the interface width scales as ∼(ϕi∕ϕn)1∕2. Intensive ion mixing at ϕi∕ϕn>1 is also shown to be responsible for the ballistic displacement of the majority of surface-deposited Ti atoms into the bulk, so that the growth appears to be dominated by subsurface deposition under conditions of intense ion bombardment.

Synthesis, structure, and field emission properties of sulfur-doped nanocrystalline diamond

The synthesis and properties of sulfur-doped nanocrystalline diamond films were investigated. The films were deposited by hot-filament chemical vapor deposition on Mo substrates using methane, hydrogen, and hydrogen disulfide. The nanocrystalline nature of the material arises from the induction of continuous secondary nucleation in the chemical vapor deposition environment. Complementary characterization tools were employed in order to obtain a comprehensive and coherent understanding of the correlations between the structural and electronic properties. In particular, sulfur-doped nanocrystalline diamond films show n-type Hall conductivity, enhanced field emission properties, and insensitivity to ion radiation. It was found that n-type doping of the tetragonally-bonded carbon matrix together with a nano network of trigonally-bonded carbon are crucial elements for enhanced field emission from nanocrystalline diamond. These conclusions and the corresponding supporting evidence are discussed.

Initiated Chemical Vapor Deposition (iCVD) of Poly(alkyl acrylates): An Experimental Study

In a two-part investigation, an experimental study and a kinetic model analysis of the initiated chemical vapor deposition (iCVD) of alkyl acrylate polymers are described. In this first part, an experimental study was performed to look at the effect of process parameters on iCVD polymerization. A homologous series of alkyl acrylates, from ethyl up to hexyl acrylate, were iCVD polymerized. The resulting polymers matched well spectroscopically with those from liquid-phase polymerization, demonstrating that stoichiometric polymers with no observable cross-linking can be achieved in a chemical vapor deposition environment. Deposition rate and molecular weight increased by a factor of over 300 and 60, respectively, when monomer saturated vapor pressure, Psat, was reduced from 42.6 to 0.584 Torr at equal gas pressures, PM. Over three times increase in deposition rate was observed for ethyl acrylate when substrate temperature was reduced from 29 to 17 °C. These trends are attributed to an increase in PM/Psat or, equivalently, monomer surface concentration in Henry's law limit at low PM/Psat. Evidence for adsorption-limited iCVD kinetics came from an apparent negative activation energy of −79.4 kJ/mol obtained experimentally that agreed well with a mathematically derived activation energy of −81.8 kJ/mol equal to twice the heat of desorption in the negative sense. Adsorption measurements found Henry's law limit to be valid and, when fitted to a BET equation, allowed the heat of desorption to be calculated. On the basis of this experimental study, process guidelines were made to define the appropriate parameter space for future iCVD polymerization, with PM/Psat in the range of 0.4−0.7 recommended as an optimal iCVD window.

Effect of the chemical vapor deposition environment on the faceted surface of α-(0001) sapphire

Abstract The terrace-and-step structure on the α-(0 0 0 1) sapphire surface induced by annealing has been known to be stable unlike the 7 × 7 reconstructed surface of silicon. However, the behavior of the terrace-and-step structure in an actual growth environment is relatively unknown. This article investigates the effect of the H2–CH4 chemical vapor deposition environment on the faceted surface of α-(0 0 0 1) sapphire using atomic force microscopy, Auger electron spectroscopy, X-ray photoelectron spectroscopy and Raman spectroscopy.

Temperature dependence of the optical functions of amorphous silicon-based materials: application to in situ temperature measurements by spectroscopic ellipsometry

The optical functions of amorphous silicon thin films have been studied with spectroscopic ellipsometry (SE) in the temperature range from 290 to 520 K. The ellipsometry data were modeled using Tauc–Lorentz dispersion law for amorphous materials. It has been found that the temperature coefficients of Tauc–Lorentz parameters, such as the optical gap, are rather similar for four different materials, which suggests that the obtained values are valid for a broad range of amorphous silicon-based materials and can be used to determine the surface temperature by ellipsometry. A practical example of using spectroscopic ellipsometry for in situ temperature measurements in the plasma enhanced chemical vapor deposition environment is given.

Indium versus hydrogen-terminated GaN(0001) surfaces: Surfactant effect of indium in a chemical vapor deposition environment

First-principles pseudopotential density functional calculations of the relative stability of H- and In-terminated GaN(0001) surfaces are reported. These total energy calculations show that surfaces terminated by one or two monolayers of In are more stable under typical metalorganic vapor deposition conditions than the H-terminated surface structures that have been proposed. Indium may act as a surfactant to improve the growth morphology of GaN films grown by metalorganic vapor deposition via a mechanism similar to that operative in molecular beam epitaxy.

The role of atomic surface structure in the metalorganic chemical vapor deposition of III-V compound semiconductors

The atomic structure of gallium arsenide and indium phosphide (001) surfaces in the metalorganic chemical vapor deposition (MOCVD) environment has been characterized in situ by scanning tunneling microscopy and infrared spectroscopy. During growth at V/III ratios above 10, these films exhibit similar surface structures. They are terminated with alkyl groups, hydrogen atoms and group V dimers (As or P) adsorbed on top of a complete layer of group V atoms. As the V/III ratio decreases, the exposed arsenic and phosphorous atoms desorb from the surface. On gallium arsenide, this occurs through a phase transition, involving gallium out-diffusion and surface roughening. By contrast, on InP, the underlying phosphorous atoms simply form rows of dimers, yielding a smooth, continuous layer.

Investigations of Chemical Vapor Deposition of GaN Using Synchrotron Radiation

We apply synchrotron X-ray analysis techniques to probe the surface structure of GaN films during synthesis by metallorganic chemical vapor deposition (MOCVD). Our approach is to observe the evolution of surface structure and morphology in real-time using grazing incidence X-ray scattering (GIXS). This technique combines the ability of X-rays to penetrate the chemical vapor deposition environment for in situ measurements, with the sensitivity of GIXS to atomic scale structure. Examples are presented from some of our studies of growth modes and surface evolution as a function of process conditions that illustrate the capabilities of synchrotron X-ray analysis during MOCVD growth. We focus on studies of the homoepitaxial growth mode, island coarsening dynamics, and effects of impurities. © 2001 The Electrochemical Society. All rights reserved.

Step bunching on the vicinal GaN(0001) surface

Nominally $2\ifmmode^\circ\else\textdegree\fi{}$ vicinal GaN(0001) surfaces exhibit monolayer-height steps at $990\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ in the metal-organic chemical vapor deposition environment. Real-time x-ray scattering observations at $715--990\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ indicate that there is a tendency for step bunching during growth. Below $850\ifmmode^\circ\else\textdegree\fi{}\mathrm{C},$ step bunches nucleated during growth remain and coarsen after growth, while above $850\ifmmode^\circ\else\textdegree\fi{}\mathrm{C},$ the surface reverts to monolayer-height steps after growth. Surfaces vicinal toward the ${11\ifmmode\bar\else\textasciimacron\fi{}00}$ and the ${112\ifmmode\bar\else\textasciimacron\fi{}0}$ planes exhibit similar behavior. We suggest a simple equilibrium surface orientational phase diagram for vicinal GaN(0001) that is consistent with these observations.