Precursor Partial Pressures Research Articles

On the growth kinetics, texture, microstructure, and mechanical properties of tungsten carbonitride deposited by chemical vapor deposition

Tungsten carbonitride [W(C,N)] was deposited on cemented carbide substrates by chemical vapor deposition (CVD) in a hot-wall reactor using tungsten hexafluoride (WF6), acetonitrile (CH3CN), and hydrogen (H2) as precursors. Tungsten carbides and nitrides with a hexagonal δ-WC type structure are generally difficult to obtain by CVD. Here, it was found that the combination of WF6 and CH3CN precursors enabled the deposition of W(C,N) coatings with a δ-WC type structure and columnar grains. A process window as a function of the deposition temperature and precursor partial pressures was determined to establish the conditions for the deposition of such coatings. Scanning electron microscopy, x-ray diffraction, electron backscatter diffraction, and elastic recoil detection analysis were used for the investigation of the coating thickness, microstructure, texture, and composition. From the investigation of the kinetics, it was concluded that the growth was mainly controlled by surface kinetics with an apparent activation energy of 77 kJ/mol, yielding an excellent step coverage. The partial reaction orders of the reactants together with their influence on the microstructure and coating composition was further used to gain a deeper understanding of the growth mechanism. Within the process window, the microstructure and the texture of the W(C,N) coatings could be tailored by the process parameters, enabling microstructural engineering with tuning of the mechanical properties of the W(C,N) coatings. The nanoindentation hardness (36.6–45.7 GPa) and elastic modulus (564–761 GPa) were found to be closely related to the microstructure.

Epitaxy of Pseudomorphic GeSn Layers with Germane (GeH4) or Digermane (Ge2H6) as Ge Precursors and Tin Tetrachloride (SnCl4) as the Sn Precursor

In CEA-LETI, GeSn/SiGeSn stacks are typically grown at 100 Torr with Ge2H6, Si2H6 and SnCl4 and temperatures in the 301°C-363°C range. Ge2H6 has however a reputation of being unstable for high concentrations (10% in H2). The short shelf time can be adressed by switching over to lower concentrations and regularly changing bottles, which is complex as Ge2H6 is costly and complicated to order. One solution could be to switch from digermane to germane, which was shown by several R&D institutes to yield high quality GeSn layers. The epitaxy tools used, then, were capable of Atmospheric Pressure CVD (i. e. @ 760 Torr), while we have Reduced Pressure CVD chambers that can operate only up to a few hundreds of Torr.We have thus investigated the growth of tens of nm thick GeSn layers on 2.5 µm thick Ge Strain-Relaxed Buffers, themselves on Si(001). First GeSn growth trials at 349°C, 100 Torr with the same GeH4 flow than Ge2H6 did not yield any peaks in X-Ray Diffraction (XRD). Changing the Mass-Flow Controller and using a GeH4 flow 4 times higher resulted in modest Sn content layers, with some Sn surface segregation, however. Increasing, for fixed GeH4 and SnCl4 flows, the precursor partial pressures by (i) reducing the H2 carrier flow or (ii) increasing the chamber pressure resulted in linear GeSn Growth Rate (GR) and Sn content increases. Sn droplets were present on the surface up to 300 Torr, however. By contrast, GeSn layers grown at 400 Torr were high quality in XRD (well defined, intense peaks with thickness fringes, as shown in Fig. 1), with cross-hatched surfaces as the Ge SRBs underneath (Fig. 2).For a fixed GeH4 partial pressure (1.28 Torr), we have then evidenced a linear increase of the GeSn GR with the SnCl4 flow at 349°C. This increase was sub-linear at 325°C, with a slope change for P(SnCl4) = 0.008 Torr (Fig. 3). By contrast, the Sn content x was almost steady with the SnCl4 flow at 349°C. At 325°C, we had first of all an increase of the Sn content up to P(SnCl4) = 0.008 Torr, then a decrease, as shown in Fig. 4. Two SnCl4 partial pressures were therefore selected for the investigation of the impact of temperature on GeSn growth kinetics: 0.008 and 0.016 Torr (i.e. F(SnCl4)/F(H2) = 2E-05 or 4E-05).We had exponential increases, over the 301°C - 349°C range, of the GeSn GR with the temperature, with similar activation energies for both chemistries: 8.6 – 11.8 kcal mol.-1 for GeH4 ó 10.4 kcal. mol.-1 for Ge2H6 (Fig. 5). Meanwhile, the Sn content linearly decreased as the growth temperature increased, with a slope which was less for GeH4 than for Ge2H6 (- 1.0 or -1.1 % / 10°C, to be compared with - 1.7% / 10°C) (Fig. 6). Although the GeH4 flow was 4 times the Ge2H6 one and the chamber pressure 4 times higher (400 Torr ó 100 Torr), the GeSn GR was almost the same for a given SnCl4 flow. Sn contents were by contrast significantly lower with GeH4 than with Ge2H6. Halving, for the same GeH4 flow, the SnCl4 flow resulted in GeSn GR 40% lower but slighly higher Sn contents, which was once again counterintuitive!The Ge GR component (= (1-x)*GeSn GR) increased exponentially with the temperature, with similar values and activation energies for both precursors. Meanwhile, we had, over the 301°C-349°C range, a slight decrease of the Sn GR component (= x*GeSn GR) as the temperature increased. This was likely due to Sn sublimation. Sn GR components were 50% higher, for the same SnCl4 flow, with Ge2H6 than with GeH4. Complex interplays between precursors thus govern the GeSn growh kinetics with both precursors. Higher Sn contents and GeSn GR are nevertheless feasible with Ge2H6 + SnCl4 instead of GeH4 + SnCl4. Finally, while the GeSn GR and Sn contents were rather uniform over the 200mm wafer surface with Ge2H6 + SnCl4, the situation was definitely worse with GeH4 + SnCl4. GR were then significantly higher at the wafer edges than close to the center (although the Sn content was fairly uniform). Figure 1

Growth mode control for direct-gap core/shell Ge/GeSn nanowire light emission

Germanium-tin is a promising semiconductor alloy system for novel light emitting devices and optical sensors in the mid-IR region. For sufficiently high Sn compositions, the material has a direct band-gap near 0.5 eV, and could have applications either as a detector or an emitter. Although high Sn compositions have been achieved in Ge1−xSnx through a variety of growth strategies, the understanding of how chemical vapor deposition conditions affect Sn composition and optical properties in core–shell Ge/Ge1−xSnx nanowires is still lacking. In this study, gas precursor partial pressures and shell growth temperatures are systematically varied to provide guiding principles to overcome obstacles for higher Sn incorporation. We achieve a direct band-gap material using an elastically compliant Ge core nanowire substrate. In the course of the growth study, we demonstrate several findings regarding the Ge1−xSnx shell growth mechanism. First, we observe an H2 passivation effect in which higher H2 to SnCl4 partial pressure ratio results in a concurrent increase in axial wire growth and decrease in radial growth. Second, we find that Ge1−xSnx shell growth in the studied CVD process is mass transport limited. Third, our results suggest that low shell growth temperature and high shell growth rate facilitate high Sn composition through metastable Sn solute trapping due to suppressed surface diffusion relative to the velocity of advancing shell surface steps. In this work, we demonstrate single nanowire photoluminescence at room temperature from core-shell Ge/Ge0.88Sn0.12 nanowires. Understanding the Ge1−xSnx shell growth mechanism via chemical vapor deposition (CVD) facilitates achieving minimal residual strain in the shell and the high crystalline quality and large Sn composition necessary for the observed optical properties. The results are universally applicable to Ge1−xSnx thin film epitaxy on compliant substrates including grown or etched nanowires, nanosheets, or free-standing 2D crystals.

In situ metal-organic chemical vapour deposition growth of III–V semiconductor nanowires in the Lund environmental transmission electron microscope

A new environmental transmission electron microscope has been installed in Lund in order to investigate the growth of III–V semiconductor nanowires by metal-organic chemical vapour deposition. We report here on the concepts behind the design of the facility and on details of the operation, and we refer to early results to highlight the new information that can be accessed from in situ studies. The installation includes a gas handling system that delivers the precursors to III–V semiconductor growth under controlled conditions. The core microscope is a Hitachi HF-3300S 300 kV transmission electron microscope with additional pumping that can handle up to 6 Pa of gas injected into the specimen area, or up to 400 Pa if an apertured lid is fitted to the holder. Various custom specimen holders incorporate precursor gas lines, a heating chip or a double tilt mechanism. The polepiece gap has been expanded to accommodate the holders, while the combination of an imaging aberration corrector and a cold field emission gun delivers a point resolution of 86 pm. Single images with atomic level detail are collected by one camera while another camera provides real-time video recording. A scanning unit offers high angle annular dark field and secondary electron images, and compositional microanalysis is performed with energy dispersive spectroscopy. In summary, III–V nanowires have been grown successfully in situ across a range of controlled conditions such as substrate temperature and precursor partial pressures. Atomic resolution images and movies, and spectroscopy data taken during this growth allow detailed measurements of structures, compositions and growth rates—data that are otherwise hard or impossible to obtain from ex situ studies—and further our understanding of the mechanisms of crystal growth.

SiO2 microstructure evolution during plasma deposition analyzed via ellipsometric porosimetry

AbstractThe evolution of microstructures, deposited from hexamethyldisiloxane (HMDSO) and oxygen gas mixtures by two different low pressure plasma sources, namely an inductively coupled plasma (ICP process) at 3 Pa and a microwave plasma (MW process) at 100 Pa, is evaluated and compared. The microstructure is monitored using ellipsometric porosimetry (EP) applying three different solvent molecules (water, ethanol, and toluene) to probe the different adsorption and absorption mechanisms as well as the pore sizes. Both plasma processes are adjusted so that an equivalent oxygen atom contribution to the growth flux is established and that an equivalent specific energy per molecule is dissipated in the process. The major difference is the partial pressure of the HMDSO precursor molecules, which is 0.04 Pa in the ICP process and 1 Pa in the MW process. The porosimetry analysis indicates that the films originating from the MW process are more porous than those from the ICP process. The pore sizes are typically in the range of 0.3 nm for films deposited from both plasma processes. This is explained by assuming that the gas phase polymerization in the MW process is much stronger due to the higher HMDSO partial pressure and, therefore, the films are deposited from larger HMDSO fragments in the MW process compared with smaller HMDSO fragments in the ICP process. This difference in the main growth species becomes visible in the different microstructures. Consequently, a plasma process using smaller precursor partial pressures seems to be optimal.

The Effect of Plasma Gas Composition on the Nanostructures and Optical Properties of TiO2 Films Prepared by Helicon-PECVD

TiO2 films were deposited from oxygen/titanium tetraisopropoxide (TTIP) plasmas at low temperature by Helicon-PECVD at floating potential ([Formula: see text] or substrate self-bias of [Formula: see text]50[Formula: see text]V. The influence of titanium precursor partial pressure on the morphology, nanostructure and optical properties was investigated. Low titanium partial pressure ([TTIP] [Formula: see text] 0.013[Formula: see text]Pa) was applied by controlling the TTIP flow rate which is introduced by its own vapor pressure, whereas higher titanium partial pressure was formed through increasing the flow rate by using a carrier gas (CG). Then the precursor partial pressures [TTIP[Formula: see text]CG] [Formula: see text][Formula: see text]Pa and 0.093[Formula: see text]Pa were obtained. At [Formula: see text], all the films exhibit a columnar structure, but the degree of inhomogeneity is decreased with the precursor partial pressure. Phase transformation from anatase ([TTIP] [Formula: see text] 0.013[Formula: see text]Pa) to amorphous ([TTIP[Formula: see text]CG] [Formula: see text][Formula: see text]Pa) has been evidenced since the O[Formula: see text] ion to neutral flux ratio in the plasma was decreased and more carbon contained in the film. However, in the case of [Formula: see text]50[Formula: see text]V, the related growth rate for different precursor partial pressures is slightly ([Formula: see text] 15%) decreased. The columnar morphology at [TTIP] [Formula: see text] 0.013[Formula: see text]Pa has been changed into a granular structure, but still homogeneous columns are observed for [TTIP[Formula: see text]CG] [Formula: see text][Formula: see text]Pa and 0.093[Formula: see text]Pa. Rutile phase has been generated at [TTIP] [Formula: see text][Formula: see text]Pa. Ellipsometry measurements were performed on the films deposited at [Formula: see text]50[Formula: see text]V; results show that the precursor addition from low to high levels leads to a decrease in refractive index.

Chemical vapor deposition of magnetic iron-cobalt alloy thin films: Use of ammonia to stabilize growth from carbonyl precursors

In previous work, it was demonstrated that FexCo(1−x) alloy thin films with near ideal magnetic properties can be grown by chemical vapor deposition (CVD) from the precursors Fe(CO)5 and Co2(CO)8; previous attempts to grow such films by CVD, using these or other precursors, had not been able to afford high saturation magnetization. However, it was found that the morphology and composition were extremely sensitive to small variations in the deposition temperature and the precursor partial pressures. In a second work, it was showed that the CVD of pure iron films from Fe(CO)5 is subject to a self-poisoning effect in which the growth surface accumulates carbon, which causes the growth rate to decline progressively to zero. Then it was shown that the poisoning effect can be eliminated by adding a coflow of NH3 during CVD, which does not introduce measurable quantities of nitrogen into the film. In the current work, the authors return to the compositional instabilities in FexCo(1−x) alloy growth and show that, as seen for pure Fe growth, these instabilities can be as attributed to a surface poisoning effect involving dissociative chemisorption of carbon monoxide. It was found that a coflow of ammonia, which inhibits CO adsorption, enables the growth of FexCo(1−x) films over a wide temperature window with highly reproducible morphology and stoichiometry. Alloys that were grown under the NH3 coflow with suitable compositions (x ∼ 0.6) achieve near ideal values of the saturation magnetization.

(Invited) Epitaxy of 2D Transition Metal Dichalcogenide Monolayers and Heterostructures

There is growing interest in monolayer transition metal dichalcogenides (TMDs) such as MoS2 and WSe2 and related 2D heterostructures due to their compelling electronic and photonic properties. The majority of this work has been carried out using ultra-thin flakes exfoliated from bulk chalcogenide crystals but future device development requires the ability to synthesize large area single crystal films and heterostructures. Our research is aimed at the development of an epitaxial growth technology for layered dichalcogenides, similar to that which exists for III-V and other compound semiconductors, based on gas source chemical vapor deposition (CVD). This approach provides a high overpressure of chalcogen species needed to maintain stable growth at elevated temperature and excellent control of the precursor partial pressures to achieve layer-by-layer and heterostructure growth. Our recent studies have focused on the epitaxial growth of WSe2 and WS2 monolayer films and heterostructures using metal hexacarbonyl and hydride chalcogen precursors on substrates including sapphire and hexagonal boron nitride (hBN). A multi-step precursor modulation growth method was developed to independently control nucleation density and the lateral growth rate of WSe2 and WS2 monolayer domains on the substrate. This approach also enables measurement of W-species surface diffusivity and domain growth rate as a function of growth conditions providing insight into the fundamental mechanisms of monolayer growth. Using this approach, coalesced monolayer and few-layer TMD films were obtained on sapphire substrates up to 2” in diameter at growth rates on the order of ~ 1 monolayer/hour. In-plane X-ray diffraction demonstrates that the films are epitaxially oriented with respect to the sapphire resulting from a merging of an equal mixture of 0o and 60o oriented domains. WSe2 also grows epitaxially on hBN single crystal flakes but in this case the domains exhibit a predominant direction resulting in a reduced density of anti-phase boundaries. Applications and challenges of this approach in the growth of 2D heterostructures will also be discussed.

Effect of reactor pressure on the conformal coating inside porous substrates by atomic layer deposition

Atomic layer deposition (ALD) is renowned for its step coverage in porous substrates. Several emerging applications require a combination of this high step coverage with high throughput ALD, like spatial ALD. Often, high throughput ALD is performed at atmospheric pressure, and therefore, the effect of reactor pressure on the saturation dose is investigated. ALD inside porous substrates is governed by three key parameters: the reaction probability, the pore aspect ratio, and the precursor diffusion coefficient, of which the latter one contains the reactor pressure dependency. The effect of these parameters on the saturation dose is validated using Monte Carlo modeling, where the reactor pressure dependency is included through the mean free path. A reaction-limited and a diffusion-limited regime can be identified, and it is shown that for many realistic experimental conditions, even at low reactor pressures, the saturation dose is in the diffusion-limited regime. An expression for the pressure dependent saturation dose in the diffusion-limited regime is derived. For small pore diameters, the saturation dose is pressure independent, but for larger pores, higher saturation doses are required for atmospheric reactor pressures than for low reactor pressures. However, as high reactor pressures enable much higher precursor partial pressures than low reactor pressures, the resulting saturation times can be much shorter at atmospheric pressure than low pressure. Often, high surface area porous substrates will lead to supply limited conditions, and increased saturation times have to be taken into account. These results show that the atmospheric pressure ALD can be used for high throughput ALD inside porous substrates, as long as high precursor partial pressures and molar flows can be applied. This is experimentally demonstrated by a near 100% step coverage obtained by atmospheric spatial ALD of alumina in high aspect ratio pores.

In situ preparation of Si p-n junctions and subsequent surface preparation for III-V heteroepitaxy in MOCVD ambient

III-V integration on active Si-bottom cells promises not only high-efficiency multi-junction solar cells but also lower production costs. In situ preparation of an adequate Si p-n junction in metalorganic chemical vapor deposition ambient is challenging, particularly since the final Si surface should be atomically well-ordered to enable low-defect III-V nucleation. Precisely, a single-domain Si(100) surface with double layer steps needs to be prepared in order to suppress antiphase disorder in subsequently grown III-V layer structures on top of the Si p-n junction. We first investigate the formation of a n+-type collector in Si(100) as a result of annealing in tertiarybutylphosphine (TBP) or tertiarybutylarsine (TBAs) ambient. We illustrate how the n-type doping concentrations and their depth profiles depend on the essential preparation parameters, such as precursor partial pressures, exposure and annealing time, as well as reactor pressure. Subsequently, by applying in situ reflectance anisotropy spectroscopy, we find that exposure of Si(100) to TBP or TBAs leads to atomic disorder on the surface. Further, we apply an additional annealing step without precursor supply leading to predominantly (1×2) reconstructed Si(100) surfaces, which are suitable for subsequent low-defect III-V growth.

Vapor-liquid-solid epitaxial growth of Si1−xGex alloy nanowires: Composition dependence on precursor reactivity and morphology control for vertical forests

Growth of high-density group IV alloy nanowire forests is critical for exploiting their unique functionalities in many applications. Here, the compositional dependence on precursor reactivity and optimized conditions for vertical growth are studied for Si1−xGex alloy nanowires grown by the vapor-liquid-solid method. The nanowire composition versus gas partial-pressure ratio for germane-silane and germane-disilane precursor combinations is obtained at 350 °C over a wide composition range (0.05 ≤ x ≤ 0.98) and a generalized model to predict composition for alloy nanowires is developed based on the relative precursor partial pressures and reactivity ratio. In combination with germane, silane provides more precise compositional control at high Ge concentrations (x > 0.7), whereas disilane greatly increases the Si concentration for a given gas ratio and enables more precise alloy compositional control at small Ge concentrations (x < 0.3). Vertically oriented, non-kinking nanowire forest growth on Si (111) substrates is then discussed for silane/germane over a wide range of compositions, with temperature and precursor partial pressure optimized by monitoring the nanowire growth front using in-situ optical reflectance. For high Ge compositions (x ≈ 0.9), a “two-step” growth approach with nucleation at higher temperatures results in nanowires with high-density and uniform vertical orientation. With increasing Si content (x ≈ 0.8), the optimal growth window is shifted to higher temperatures, which minimizes nanowire kinking morphologies. For Si-rich Si1−xGex alloys (x ≈ 0.25), vertical nanowire growth is enhanced by single-step, higher-temperature growth at reduced pressures.

Kinetics of terbium oxide film growth from Tb(dpm)3 vapor

We have studied the kinetics of terbium oxide chemical vapor deposition using the Tb(dpm)3 com� plex. At substrate temperatures in the range 465-560°C, films ranging in thickness from 25 to 360 nm have been obtained. At precursor partial pressures below 13 Pa in an argon atmosphere, the films had smooth sur� faces with separate islands. Higher precursor pressures led to the formation of dense island films. At deposi� tion temperatures in the range 465-560°C, the process was found to switch from kinetically limited, with an effective activation energy of 190 ± 15 kJ/mol, to adsorptionlimited, with an activation energy of 38 ± 5 kJ/mol. The order of the reaction with respect to the precursor was determined to be 0.8.

Selective Area Spatial Atomic Layer Deposition of ZnO, Al2O3, and Aluminum-Doped ZnO Using Poly(vinyl pyrrolidone)

Spatial atomic layer deposition (SALD) is gaining traction in the thin film electronics field because of its ability to produce quality films at a fraction of the time typically associated with ALD processes. Here, we explore the process space for the fabrication of thin film patterned-by-printing electronics using the combination of SALD and selective area patterning. First, a study of SALD growth conditions for the three primary components of our metal oxide thin film electronics, namely alumina (Al2O3) dielectric, zinc oxide (ZnO) semiconductor, and aluminum doped ZnO (AZO) conductor, provides insight into the potential trade-offs in performance, substrate latitude (temperature), and process speed. At constant precursor partial pressures, the precursor exposure times and substrate temperatures were varied from 25 to 400 ms and from 100 to 300 °C, respectively. The very short gas exposure and purge times obtainable only with a spatial implementation of ALD are shown always to be advantageous for through...

Origins of unintentional incorporation of gallium in InAlN layers during epitaxial growth, part II: Effects of underlying layers and growth chamber conditions

We systematically study the origins and mechanisms for unintentional incorporation of gallium (Ga) during epitaxial growth of ternary InAlN thin-film layers. The origins of auto-incorporation of Ga have been investigated by using different underlying layers, regrown layers, and growth chamber conditions. It is shown that Ga-containing deposition on a wafer susceptor/carrier and on surrounding surfaces of uncooled parts in a growth chamber can be responsible for Ga in the InAl(Ga)N layers, while a GaN underlying layer below an InAl(Ga)N layer does not contribute to the auto-incorporation of Ga in the InAl(Ga)N layers. Especially, the Ga-containing deposition on the surfaces inside the chamber is believed to be the dominant source of auto-incorporated Ga, possibly due to the high vapor pressure of a liquid phase as a result of eutectic system formation between indium (In) and Ga. The pressure of liquid-phase Ga, pGa=~3.67×10−4Torr, can be significant as compared to precursor partial pressures with pTMAl=3.7×10−4Torr and pTMIn=2.4×10−5Torr. In addition, magnesium (Mg) or magnesium precursor (Cp2Mg) in the growth chamber is shown to promote the auto-incorporation of Ga in the InAl(Ga)N layers.

Diameter Limitation in Growth of III-Sb-Containing Nanowire Heterostructures

The nanowire geometry offers significant advantages for exploiting the potential of III-Sb materials. Strain due to lattice mismatch is efficiently accommodated, and carrier confinement effects can be utilized in tunneling and quantum devices for which the III-Sb materials are of particular interest. It has however proven difficult to grow thin (below a few tens of nanometers), epitaxial III-Sb nanowires, as commonly no growth is observed below some critical diameter. Here we explore the processes limiting the diameter of III-Sb nanowires in a model system, in order to develop procedures to control this effect. The InAs-GaSb heterostructure system was chosen due to its great potential for tunneling devices in future low-power electronics. We find that with increasing growth temperature or precursor partial pressures, the critical diameter for GaSb growth on InAs decreases. To explain this trend we propose a model where the Gibbs-Thomson effect limits the Sb supersaturation in the catalyst particle. This understanding enabled us to further reduce the nanowire diameter down to 32 nm for GaSb grown on 21 nm InAs stems. Finally, we show that growth conditions must be carefully optimized for these small diameters, since radial growth increases for increased precursor partial pressures beyond the critical values required for nucleation.

Exploring SiC Growth Limitation of Vapor-Liquid-Solid Mechanism when Using Two Different Carbon Precursors

Abstract. In this paper, conditions for obtaining high growth rate during epitaxial growth of SiC by vapor-liquid-solid mechanism are investigated. The alloys studied were Ge-Si, Al-Si and Al-Ge-Si with various compositions. Temperature was varied between 1100 and 1300°C and the carbon precursor was either propane or methane. The variation of layers thickness was studied at low and high precursor partial pressure. It was found that growth rates obtained with both methane and propane are rather similar at low precursor partial pressures. However, when using Ge based melts, the use of high propane flux leads to the formation of a SiC crust on top of the liquid, which limits the growth by VLS. But when methane is used, even at extremely high flux (up to 100 sccm), no crust could be detected on top of the liquid while the deposit thickness was still rather small (between 1.12 μm and 1.30 μm). When using Al-Si alloys, no crust was also observed under 100 sccm methane but the thickness was as high as 11.5 µm after 30 min growth. It is proposed that the upper limitation of VLS growth rate depends mainly on C solubility of the liquid phase.

Morphology transitions in ZnO nanorods grown by MOCVD

Morphology transitions (nanorods–nanowalls and nanorods–nanotubes-layer) were induced in the growth of ZnO nanostructures by metal organic chemical vapor deposition (MOCVD) on c-sapphire, using helium as carrier gas, and dimethylzinc–triethylamine and nitrous oxide as zinc and oxygen sources, respectively. A systematic study of the influence of the VI/II ratio and precursor flow-rates on the morphology of ZnO nanorod arrays has been carried out, taking advantage of the ability of MOCVD to individually control the precursor partial pressures. Growth mechanisms are discussed to understand the evolution of the nanostructures morphology for different growth conditions. In particular, the influence of the gas phase supersaturation on the vertical alignment and aspect ratio of the nanorods is emphasized.

Estimating transport-shifted acid dew-point surface temperatures and conditions for the avoidance of acid mists in energy recovery operations

The availability of acid dew-point correlations (for flue gases containing the precursors of sulfuric-, nitric-, hydrochloric acids, etc.) that directly relate Tw,dp to the participating species partial pressures (e.g., Banchero and Verhoff, 1975; Verhoff and Banchero, 1974) without the explicit need for liquid phase thermodynamic properties has led to their popularity for preliminary design estimates of onset surface temperatures for corrosive deposits. Despite the fact that these correlations cannot be used when either or both precursor partial pressures become too small, when they are invoked within their intended domains of validity, we show here that they can be conveniently used to not only quickly estimate acid dew-point temperatures in the presence of unavoidable gas phase precursor species Soret separation effects, but also to provide preliminary estimates of the surface temperatures, Tw,AMO, associated with the local onset of acid mists in the vicinity of cooled surfaces. Our present simple theoretical/numerical methods are illustrated for the most familiar case of sulfuric acid deposits. While we indicate that increased accuracy and generality will require a more fundamental approach in which condensate thermodynamic non-ideality is explicitly introduced, we show here that prudent use of these earlier engineering correlations can lead to immediately useful preliminary estimates of both transport-shifted Tw,dp- and Tw,AMO-values.

Liquid droplet dynamics and complex morphologies in vapor–liquid–solid nanowire growth

Abstract

Study of the effect of gas pressure and catalyst droplets number density on silicon nanowires growth, tapering, and gold coverage

We investigated the growth of silicon nanowires from Au-rich catalyst droplets by two different methods: chemical vapor deposition (CVD) and molecular beam epitaxy (MBE). The growth rate is found to be diameter-dependent and increases with increasing precursor partial pressures. The comparison of the experimental results with models shows that the contribution of Si atoms that diffuses from the substrate and the NW sidewalls toward the catalyst droplet can be neglected in CVD for the different pressures used in this study, whereas it is the major source of Si supply for the MBE growth. In addition, by decreasing the number density of catalyst droplet prior to the NW growth in CVD, it is also found that this parameter affects the NWs morphology, increasing the tapering effect when the silane partial pressure is small enough to allow gold atom diffusion from the catalyst droplet.