Pulsed Chemical Vapor Deposition Research Articles

Incremental Growth of Short SWNT Arrays by Pulsed Chemical Vapor Deposition

Very short arrays of continuous single-wall carbon nanotubes (SWNTs) are grown incrementally in steps as small as 25 nm using pulsed chemical vapor deposition (CVD). In-situ optical extinction measurements indicate that over 98% of the nanotubes reinitiate growth on successive gas pulses, and high-resolution transmission electron microscopy (HR-TEM) images show that the SWNTs do not exhibit segments, caps, or noticeable sidewall defects resulting from repeatedly stopping and restarting growth. Time-resolved laser reflectivity (3-ms temporal resolution) is used to record the nucleation and growth kinetics for each fast (0.2 s) gas pulse and to measure the height increase of the array in situ, providing a method to incrementally grow short nanotube arrays to precise heights. Derivatives of the optical reflectivity signal reveal distinct temporal signatures for both nucleation and growth kinetics, with their amplitude ratio on the first gas pulse serving as a good predictor for the evolution of the growth of the nanotube ensemble into a coordinated array. Incremental growth by pulsed CVD is interpreted in the context of autocatalytic kinetic models as a special processing window in which a sufficiently high flux of feedstock gas drives the nucleation and rapid growth phases of a catalyst nanoparticle ensemble to occur within the temporal period of the gas pulse, but without inducing growth termination.

Modelling and Experimentation of Solid Lubrification with Powder MoS<sub>2</sub> through Self-Repairing and Self-Replenishing

Molybdenum disulphides (MoS2), which belong to the family of transition metal dichalcogenides, are well known for their solid lubricating behaviour. Thin films of MoS2 exhibit extremely low coefficient of friction in dry environments, and are typically applied by mixed in oil, grease or impregnated into porous matrix of powdered materials, sputter deposition, pulsed laser ablation, evaporation or chemical vapour deposition and, which are essentially either line-of-sight or high temperature processes. Solid lubricant coatings are attractive because they can reduce friction-generated heat. MoS2 is a common solid lubricant. However, the use of MoS2 can limited by excessive wear, as well as a friction coefficient. Several studies on solid lubricant coatings demonstrated success in lubricating dry sliding contacts over very long periods in tribometer tests or reciprocating sliding experiments.Several pellet-on-disk and pad-on-disk tribometer tests were conducted to study the lubrication characteristics of third-body particles of MoS2 powder. The tests consisted of simultaneous pellet-on-disk and pad-on-disk sliding contacts. Results from the tests show the self-repairing, self-replenishing, oil-free lubrication mechanism of MoS2. A theoretical control volume fractional coverage (CVFC) model was developed to predict: - (1) the friction coefficient at the pad-on-disk interface and - (2) the wear coefficient for the lubricated pellet-on-disk sliding contact. The fractional coverage varies with time and quantifies the amount of third-body film covering the disk asperities. Results from the model capture the tribological behaviour of the experimental sliding contacts reasonably well. The aim of this paper is modeling and experimentation of solid lubrification with MoS2 particles through self-repairing and self-replenishing and through the comparision between theoretical and experimental results obtained in the process of friction and wear by tribological tests.

Effects of AlN interlayer on the transport properties of nearly lattice-matched InAlN/GaN heterostructures grown on sapphire by pulsed metal organic chemical vapor deposition

The effects of AlN interlayer thickness on the transport properties of nearly lattice-matched InAlN/GaN heterostructures grown on sapphire substrates by pulsed metal organic chemical vapor deposition have been studied in detail. A very high electron mobility of approximately 1425cm2/Vs at room temperature and 5308cm2/Vs at 77K together with a two dimensional electron gas (2DEG) density of 1.75×1013cm−2 were obtained for nearly lattice-matched InAlN/GaN heterostructures with an optimum ∼1.2nm thick AlN interlayer. For comparison, InAlN/GaN heterostructure without AlN interlayer exhibited a 2DEG density of 1.61×1013cm−2 with low electron mobility of 949 and 2032cm2/Vs at room temperature and 77K, respectively. This significant enhancement of electron mobility is mainly attributed to an optimized AlN interlayer, which provides a smooth interface between InAlN barrier layer and GaN buffer layer and hence remarkably reduces the alloy disorder scattering by suppressing carrier penetration from the GaN channel into the InAlN barrier layer. Simultaneously, a best surface morphology with a root mean square roughness value of 0.24nm is obtained with the optimized AlN interlayer.

Abnormal behaviors in electrical transport properties of cobalt-doped tin oxide thin films

Electrical transport behaviors of SnO2-based oxides are absolutely essential, either for the understanding of physiochemical properties, or their practical applications. In this paper, an abnormal change in electrical transport is reported upon cobalt doping. A far-from-equilibrium technique—pulsed spray evaporation chemical vapor deposition (PSE-CVD), is investigated for the fabrication of Sn1−xCoxO2−δ (x = 0–0.18) thin films. Upon cobalt doping, the Hall mobility improves gradually and a ten-fold enhancement was noticed for Sn0.82Co0.18O2−δ relative to pure SnO2 films. This unexpected effect induces a dramatic drop in the electrical resistivity. Post-annealing treatment and XPS investigation indicate that the occurrence of surface-stabilized tin interstitials may be the primary reason for the unusual enhancement in conductivity. Cobalt doping not only generates the interstitial tin cations, but also stabilizes to a great extent their presence at the surface. This study may help to illumine new insight for the understanding of doping strategies, and offer a potential route for transport-related applications.

Growth characteristics of uniaxial InGaN/GaN MQW/n-GaN nanowires on Si(111) using MOCVD

We report on the growth of the uniaxial InGaN/GaN multiple quantum well (MQW) on n-GaN nanowires (NWs) on Si(111) substrates by dynamically adjusting the growth parameters using the pulsed flow metalorganic chemical vapor deposition (MOCVD) technique. We carried out a two-step growth process to grow the uniaxial InGaN/GaN MQW/n-GaN NWs structure. In the first step, the n-GaN NWs were grown at 950 °C and in the second stage, we suitably decreased the growth temperature to 630 and 710 °C, respectively, to grow InGaN/GaN MQW NWs. The surface morphology, structural and optical characterization of the grown InGaN/GaN MQW/n-GaN NWs were studied by field-emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), photoluminescence (PL) and cathodoluminescence (CL) measurements. The CL spectrum recorded on an individual InGaN/GaN MQW/n-GaN NW is dominated by the band-edge emission at 366.7 and 413.5 nm corresponding to n-GaN and MQW structures, respectively. The resultant NWs were found to be free of strain. These results indicate that the pulsed MOCVD technique is an effective method to grow uniaxial InGaN/GaN MQW on n-GaN NWs which is advantageous to other growth techniques. These kinds of uniaxial NWs are promising to allow flat band quantum structures that are shown to improve the efficiency of light-emitting diodes.

Realization of transparent and flexible capacitors using reliable graphene electrodes

Reliable graphenes grown by rapid-thermal pulse chemical vapor deposition (CVD) for electrode applications were selectively patterned under optimum conditions for argon rf plasma power and etching time. For the transparent and the flexible capacitors using Bi2Mg2/3Nb4/3O7 (BMNO) dielectric films grown at room temperature, the graphene top and bottom electrodes were integrated onto the polymer substrates. The graphene/BMNO/graphene/Ti/polyethersulfone (PES) capacitors showed typical dielectric and leakage properties for capacitors. The adhesion between substrates and the graphene should be critically considered in order to improve the leakage properties of the capacitors. Graphene that possessed a high bendability was the predominant candidate for application to the top and bottom electrodes of the transparent and flexible capacitors.

Hierarchical TiO2–Si nanowire architecture with photoelectrochemical activity under visible light illumination

Bandgap engineering of TiO2 is a substantial strategy for efficient water splitting in the visible light range. Introducing dopants and hydrogenation have been found effective for that purpose. In this paper, we report the development of a hierarchical three dimensional TiO2–Si nanowire (NW)-based photoelectrochemical (PEC) anode with visible light photochemical activity. The TiO2 NWs were synthesized using a surface reaction-limited pulsed chemical vapor deposition method (SPCVD) with unbalanced TiCl4 and H2O precursors. Dangling Ti–Cl and Ti–OH groups inside TiO2 NW crystals were suggested to be the reason for band narrowing and visible light absorption. The NW structure with a large aspect ratio was formed via the oriented attachment mechanism, which offered a super-high surface area density. This in situ crystal decoration approach opens a new window to tailoring electrical properties of TiO2 for wider spectrum solar energy harvesting and conversion.

Erratum to: Alumina microtubes prepared via template-directed pulsed chemical vapor deposition (pulsed CVD)

Page 4817, Fig. 5 unfortunately comprises data on the residual weight of uncoated fibres that were assigned to incorrect values of the abscissa. The correct diagram is given below. The text passage on page 4818, column 1, paragraph 1, ‘‘The uncoated fibers start to lose weight at approximately 300 C, whereas the coated fibers start to lose weight at approximately 700 C.’’ should read instead: ‘‘The uncoated fibers start to lose weight at approximately 630 C, whereas the coated fibers start to lose weight at approximately 660 C.’’

Preparation and characterisation of chromium-doped cobalt oxide spinel thin films

Thin films of cobalt-based oxide spinel were prepared by pulsed spray evaporation chemical vapour deposition (PSE-CVD), and doping with chromium was systematically investigated up to a Cr/Co ratio of 0.096, corresponding to a stoichiometry of Co3−x Cr x O4 with x = 0.00–0.26. The effect of doping concentration on the structure, assessed by X-ray diffraction and Raman scattering, and on the optical and electrical properties of the oxide films was investigated. Single-phase spinel could be preserved for stoichiometries below x = 0.22. The influence of Cr-doping on the band gap energies and on the electrical conductivity was determined, and the obtained results were exploited to discuss the cationic site occupations. The influence of Cr doping was complemented by the investigation of the surface catalytic reactivity towards the oxidation of dichloromethane.

Numerical investigation of pulsed chemical vapor deposition of aluminum nitride to reduce particle formation

Particles of aluminum nitride (AlN) have been observed to form during epitaxial growth of AlN films by metal organic chemical vapor deposition (MOCVD). Particle formation is undesirable because particles do not contribute to the film growth, and are detrimental to the hydraulic system of the reactor. It is believed that particle formation is triggered by adducts that are formed when the group-III precursor, namely tri-methyl-aluminum (TMAl), and the group-V precursor, namely ammonia (NH 3), come in direct contact in the gas-phase. Thus, one way to eliminate particle formation is to prevent the group-III and the group-V precursors from coming in direct contact at all in the gas-phase. In this article, pulsing of TMAl and NH 3 is numerically investigated as a means to reduce AlN particle formation. The investigations are conducted using computational fluid dynamics (CFD) analysis with the inclusion of detailed chemical reaction mechanisms both in the gas-phase and at the surface. The CFD code is first validated for steady-state (non-pulsed) MOCVD of AlN against published data. Subsequently, it is exercised for pulsed MOCVD with various pulse widths, precursor gas flow rates, wafer temperature, and reactor pressure. It is found that in order to significantly reduce particle formation, the group-III and group-V precursors need to be separated by a carrier gas pulse, and the carrier gas pulse should be at least 5–6 times as long as the precursor gas pulses. The studies also reveal that with the same time-averaged precursor gas flow rates as steady injection (non-pulsed) conditions, pulsed MOCVD can result in higher film growth rates because the precursors are incorporated into the film, rather than being wasted as particles. The improvement in growth rate was noted for both horizontal and vertical reactors, and was found to be most pronounced for intermediate wafer temperature and intermediate reactor pressure.

Pulsed chemical vapor deposition of Cu2S into a porous TiO2 matrix

Chalcocite (Cu2S) has been deposited via pulsed chemical vapor deposition (PCVD) into a porous TiO2 matrix using hydrogen sulfide and a metal-organic precursor. The precursor used is similar to the more common Cu(hfac)(tmvs) precursor, but it is fluorine free and exhibits increased thermal stability. The simultaneous exposure of the substrate to the copper precursor and hydrogen sulfide resulted in nonuniform Cu2S films with a temperature independent deposition rate implying gas phase reaction kinetics. The exposure of mesoporous TiO2 and planar ZnO to alternating cycles of the copper precursor and hydrogen sulfide resulted in a PCVD film that penetrated fully into the porous TiO2 layer with a constant deposition rate of 0.08 nm/cycle over a temperature range of 150–400 °C. The chalcocite (Cu2S) stoichiometry was confirmed with extended x-ray absorption fine structure measurements (EXAFS) and x-ray photoelectron spectroscopy. Calculations of the EXAFS spectrum for different CuxS phases show that EXAFS is sensitive to the different phase stoichiometries. Optical absorption measurements of CVD thin films using photothermal deflection spectroscopy show the presence of a metallic copper-poor phase for gas phase nucleated films less than 100 nm thick and a copper-rich semiconducting phase for thicknesses greater than 100 nm with a direct band gap of 1.8 eV and an indirect bandgap of 1.2 eV.

Thin film silicon nanowire photovoltaic devices produced with gold and tin catalysts

Silicon nanowires produced using pulsed plasma-enhanced chemical vapor deposition have been used as part of a thin film photovoltaic device. Nanowires of differing morphologies were produced by using both gold and tin thin films as a catalyst for growth. A prototype silicon nanowire-based thin-film photovoltaic device was produced by using doped silicon nanowires as the p-layer. Amorphous silicon was used as the intrinsic and n-layers of the device. The nanowires used in the photovoltaic devices had an average diameter of 420 nm after the deposition and coating of amorphous silicon intrinsic and n-layers. The nanowires were deposited in bulk as films of 3 to 42 μm in thickness. The resulting device, although of low efficiency, had a demonstrable photocurrent. Tin-catalyzed nanowires were found to produce a thin-film device with a measurable photocurrent whereas gold-catalyzed silicon nanowires did not. This was attributed to the length of the nanowires and thickness of the p-layer produced when using gold-catalyzed nanowires.

Temperature-dependent photoconductance of heavily doped ZnO nanowires

Ga-doped ZnO nanowires have been synthesized by a pulsed laser chemical vapor deposition method. The crystal structure and photoluminescence spectra indicate that the dopant atoms are well integrated into the ZnO wurtzite lattice. The photocurrent properties at different temperatures have been systematically investigated for nanowires configured as a three-terminal device. Among the experimental highlights, a pronounced semiconductor-to-metal transition occurs upon UV band-to-band excitation. This is a consequence of the reduction in electron mobility arising from the drastically enhanced Coulomb interactions and surface scattering. Another feature is the reproducible presence of two resistance valleys at 220 and 320 K upon light irradiation. This phenomenon originates from the trapping and detrapping processes in the impurity band arising from the native defects as well as the extrinsic Ga dopants. This work demonstrates that due to the dimensional confinement in quasi-one-dimensional structures, enhanced Coulomb interaction, surface scattering, and impurity states can significantly influence charge transport. Open image in new window

Anomalous permittivity and plasmon resonances of copper nanoparticle conformal coatings on optical fibers

The conformal coating of a 50 nm-thick layer of copper nanoparticles deposited with pulse chemical vapor deposition of a copper (I) guanidinate precursor on the cladding of a single mode optical fiber was monitored by using a tilted fiber Bragg grating (TFBG) photo-inscribed in the fiber core. The pulse-per-pulse growth of the copper nanoparticles is readily obtained from the position and amplitudes of resonances in the reflection spectrum of the grating. In particular, we confirm that the real part of the effective complex permittivity of the deposited nano-structured copper layer is an order of magnitude larger than that of a bulk copper film at an optical wavelength of 1550 nm. We further observe a transition in the growth behavior from granular to continuous film (as determined from the complex material permittivity) after approximately 20 pulses (corresponding to an effective thickness of 25 nm). Finally, despite the remaining granularity of the film, the final copper-coated optical fiber is shown to support plasmon waves suitable for sensing, even after the growth of a thin oxide layer on the copper surface.

Duty ratio impact on SiN films deposited in SiH 4-NH 3 plasma at room temperature

Silicon nitride (SiN) films were deposited by using pulsed plasma-enhanced chemical vapor deposition system. The experiments were conducted from SiH 4 and NH 3 plasma at room temperature. The duty ratio was varied in the range 40–90%. Duty ratio-induced ion energy diagnostics was conducted using a non-invasive ion energy analyzer. Ion energy variables collected include a high ion energy ( E h ), a low ion energy ( E l ), a high energy flux ( N h ), a low ion energy flux ( N l ). Their various ratios were calculated to examine relationships between ion energy and film properties, including deposition rate, refractive index, and surface roughness. It is noticeable that high deposition rate of more than 1150 Å/min could be achieved at 60%. The deposition rate and surface roughness highly correlated with N h . From the variation of refractive index, deposition mechanisms were inferred. The deposition rate, refractive index, and surface roughness were varied in a range of 1003–1148 Å/min, 1.64–1.96, and 1.37–3.07 nm, respectively.

Metal–insulator–metal capacitors with MOCVD grown Ce–Al–O as a dielectric

Ce–Al–O thin films were prepared on 70 nm TiN/Si(1 0 0) substrates by pulsed injection metal organic chemical vapor deposition (PI-MOCVD) for metal–insulator–metal (MIM) applications. Depositions were carried out at 400 °C using two separate Ce and Al precursors. In order to get Ce–Al–O films with different stoichiometry, Al 2O 3 and CeO 2 were mixed with different Ce:Al precursors’ ratios. According to the XRD analysis, the as deposited films were amorphous if more aluminum was injected than cerium, and crystalline – if they are cerium rich. Electrical properties have been investigated in MIM capacitors after e-beam evaporation of Au top electrodes. Oxides possess a dielectric constant of 10–20 in combination with leakage current densities as low as 10 −5 A/cm 2 at −2 V. The post deposition annealing (PDA) at 600 °C and 850 °C in N 2 for 5 min lead to the diffusion of Ti from TiN bottom electrode and formation of the rutile TiO 2 phase. Nevertheless, CeAlO 3 films were obtained if the ratio of injected Ce:Al was 1:1. The k values increased to 60 in this case, but the leakage current density worsened to 10 −3 A/cm 2 at −2 V.

Nearly lattice-matched InAlN/GaN high electron mobility transistors grown on SiC substrate by pulsed metal organic chemical vapor deposition

We report on a growth of nearly lattice-matched InAlN/GaN heterostructures on 4H–SiC substrates by pulsed metal organic chemical vapor deposition, and an excellent device characteristic of high electron mobility transistors (HEMTs) fabricated on these InAlN/GaN heterostructures. The electron mobility is 1032 cm2/V s together with a high two-dimensional-electron-gas density of 1.59×1013 cm−2 for the In0.17Al0.83N/AlN heterostructures. HEMTs with gate dimensions of 0.5×50 μm2 and 3 μm source-drain distance exhibits a maximum drain current of 1 A/mm, a maximum extrinsic transconductance of 310 mS/mm, and current gain and maximum oscillation cutoff frequencies of 18 GHz and 39 GHz, respectively.

Alumina microtubes prepared via template-directed pulsed chemical vapor deposition (pulsed CVD)

Bundles of alumina microtubes were prepared by depositing alumina onto bundles of “endless” carbon fibers via pulsed chemical vapor deposition and subsequent removal of the fibers. Thin alumina films were deposited onto “endless” carbon fibers at 77 °C by gas phase exposures to sequential pulses of trimethylaluminum and water vapor, respectively. The carbon fibers were selectively removed using thermal oxidation in air at temperatures exceeding 550 °C. The length of the tubes was primarily limited by the dimension of the used furnace. The longest tubes thus had a length of 30 cm. Scanning electron microscopic (SEM) images of the microtubes revealed that each individual tube was separated from its neighbors and that the tubes had an almost uniform wall thickness. SEM and transmission electron microscopic (TEM) images indicate that the inner side of the wall has the same morphology as the fiber template. As deposited, the alumina films have a predominantly amorphous structure; this is transformed into a polycrystalline structure during thermal oxidation. At low thermal oxidation temperatures, such as 550 °C, the alumina microtubes still comprise a substantial fraction of amorphous structure, at higher oxidation temperatures, 900 °C or above, a dominating polycrystalline structure (with bigger grains) is formed. This transformation gives rise to grain boundaries. These grain boundaries might facilitate oxygen diffusion and thus oxidative removal of the fiber templates.

Growth of Rutile Titanium Dioxide Nanowires by Pulsed Chemical Vapor Deposition

Recently, we developed a surface reaction-limited pulsed chemical vapor deposition (CVD) technique that can grow highly uniform anatase TiO2 nanorods (NRs) over a large area, even inside highly confined submicrometer-sized spaces. Here, we report the growth of rutile TiO2 NWs using this technique by introducing a Au seeding layer at higher deposition temperature, where a small amount of anatase phase was also found. A bifurcated growth of rutile TiO2 NWs was observed. The resulting TiO2 NWs exhibited high-quality crystallinity and large surface areas with exposure of high-index surfaces. Control experiments illustrated the influence of deposition conditions on the TiO2 growth behavior. Higher deposition temperature could convert the TiO2 phase from anatase to rutile. A thin film of Au was able to induce rapid crystal growth resulting in particle formation, while the anisotropic NW growth preferred no Au coating. Shorter purging time could significantly enhance the deposition rate because of the incomplete removal of precursor molecules. This research enriched our knowledge of the surface reaction-limited pulsed CVD technique and demonstrated the capability of using this technique to control the TiO2 phase and morphology.

Rapid-Thermal Pulse 화학증착법에 의해 증착된 그래핀 박막에서 촉매금속 Ni의 두께 및 열처리 조건의 영향

Mono- and few-layer graphenes were grown on Ni thin films by rapid-thermal pulse chemical vapor deposition technique. In the growth steps, the exposure step for 60 s in <TEX>$H_2$</TEX> (a flow rate of 10 sccm (standard cubic centimeters per minute)) atmosphere after graphene growth was specially established to improve the quality of the graphenes. The graphene films grown by exposure alone without <TEX>$H_2$</TEX> showed an intensity ratio of <TEX>$I_G/I_{2D}$</TEX> = 0.47, compared with a value of 0.38 in the films grown by exposure in H2 ambient. The quality of the graphenes can be improved by exposure for 60 s in <TEX>$H_2$</TEX> ambient after the growth of the graphene films. The physical properties of the graphene films were investigated for the graphene films grown on various Ni film thicknesses and on 260-nm thick Ni films annealed at 500 and <TEX>$700^{\circ}C$</TEX>. The graphene films grown on 260-nm thick Ni films at <TEX>$900^{\circ}C$</TEX> showed the lowest <TEX>$I_G/I_{2D}$</TEX> ratio, resulting in the fewest layers. The graphene films grown on Ni films annealed at <TEX>$700^{\circ}C$</TEX> for 2 h showed a decrease of the number of layers. The graphene films were dependent on the thickness and the grain size of the Ni films.