Microwave Chemical Vapor Deposition Research Articles

Photoconductivity effects of nitrogenated carbonaceous films grown on various types of substrates by surface wave microwave plasma CVD

The n-type conductivity nitrogen-doped amorphous carbon (n-C) films have been grown by surface wave microwave chemical vapor deposition (SWP-CVD) on plastic and quartz substrates. The bonding, electrical and photovoltaic properties have been studied. The Tauc plot of UV–visible measurement has shown that n-C films deposited on plastic have smaller optical band gaps compared with the films grown on quartz substrate using the same parameters. Temperature dependence and photoresponse measurements have also shown that the n-C thin films deposited on plastic have higher photoelectrical response. Our experimental results have shown that flexible polytetrafluoroethene plastic substrate is a reliable candidate for future photovoltaic applications.

Extreme ultraviolet single-crystal diamond detectors by chemical vapor deposition

High-quality single-crystal diamond films, homoepitaxially grown by microwave chemical vapor deposition, have been used to produce diamond-based photodetectors. Such devices were tested over a very wide spectral range, from the extreme ultraviolet (UV) (20 nm) up to the near IR region (2400 nm). An optical parametric oscillator tunable laser was used to investigate the 210–2400 nm spectral range in pulse mode. In this region, the spectral response shows a UV to visible contrast of about 6 orders of magnitude. A time response shorter than 5 ns, i.e., the laser pulse duration, was observed. By integrating the pulse shape, a minor slow component was evidenced, which can be explained in terms of trapping–detrapping effects. Extreme UV gas sources and a toroidal grating vacuum monochromator were used to measure the device response down to 20 nm in continuous mode. In particular, the extreme UV He spectrum was measured and the He II m, 30.4 nmand He I 58.4 nm emission lines were clearly detected. The measured time response of 0.2 s is totally due to the instrumental readout time constants. In both experimental setups an extremely good stability and reproducibility of the device response were obtained, whereas no persistent photoconductivity nor undesirable pumping effects were observed.

Effects of deposition gas pressure on the properties of hydrogenated amorphous carbon nitride films grown by surface wave microwave plasma chemical vapor deposition

The hydrogenated amorphous carbon nitride films (a-CN:H) films have been grown by a surface wave microwave chemical vapor deposition (SWMP-CVD) method at various deposition gas pressures. The effects of deposition gas pressure on the bonding, optical and electrical properties were studied through X-ray photoelectron spectroscopy, UV-visible spectroscopy and temperature dependence of electrical measurements of as-grown and annealed of the a-CN:H films at 500 °C in Argon gas ambient for 30 min. The optical absorption coefficients were obtained for the as-grown a-CN:H films, which is found to increase upon increase of deposition gas pressures and annealing temperatures. The temperature dependence measurement has shown the electrical conductivity is observed not to follow the simple activated form. Study of activation energy suggests that the Fermi level of the a-CN:H films is moved from valence band edge to nearer to the conduction band edge through mid gap. The variation of conductivity is in good agreement with the activation energy analyses. The XPS measurement also has shown the deposition gas pressure and annealing temperature has affected the properties of a-CN:H films. The results obtained are discussed and also compared with carbon films prepared by other deposition techniques in the literature.

PHOTOCONDUCTIVITY EFFECTS OF NITROGENATED CARBONACEOUS FILMS GROWN ON VARIOUS TYPES OF SUBSTRATES BY SURFACE WAVE MICROWAVE PLASMA CVD

The n-type conductivity nitrogen-doped amorphous carbon (n-C) films have been grown by surface wave microwave chemical vapor deposition (SWP-CVD) on plastic and quartz substrates. The bonding, electrical and photovoltaic properties have been studied. The Tauc plot of UV-Visible measurement have shown that n-C films deposited on plastic have smaller optical band gaps compared to the films grown on quartz substrate at the same parameters. Temperature dependence and photoresponse measurements also have shown that the properties of n-C thin films deposited on plastic have higher photoelectrical response. Our experimental results have shown that the flexible polytetrafluoroethene plastic substrate is a reliable candidate for future photovoltaic applications.

A preliminary dosimetric characterization of chemical vapor deposition diamond detector prototypes in photon and electron radiotherapy beams

Three radiation detectors based on polycrystalline diamond films with different thickness and resistivity, obtained by microwave chemical vapor deposition, were tested to assess their suitability for relative dosimetry of photon and electron beams supplied by clinical linear accelerators. All samples showed a linear response as a function of the absorbed dose. The sensitivity per unit of detector sensitive volume spanned between 7 and 43 nC Gy(-1) mm(-3) with an applied electric field of 40 kV/cm. The dose rate dependence was evaluated following the Fowler theory and delta coefficient values between 0.95 and 1.00 were found for the three samples when polarized at 40 kV/cm. Percentage depth dose curves, output factors, and normalized dose profiles were determined for 6 and 10 MV photon beams and for 6 and 15 MeV electron beams. The results obtained with the diamond detectors were in good agreement with those obtained by reference detector measurements [all the data were within the experimental uncertainty of 1% (1sigma)].

Carbon nanotube synthesis, characteristics, and microbattery applications

The stabilities and efficiencies of cathodes of microbatteries are important for their superior performance. The performance of cathode of microbattery systems can be improved by applying new materials or combining components with unique properties into cathode materials. Individual carbon nanotubes exhibit extraordinary mechanical, thermal and electrical properties. Composite materials, using carbon nanotubes as fillers are, expected to show similar superior properties. This study reports synthesis of carbon nanotubes by microwave chemical vapor deposition (MACVD) and applying them in the cathode of microbatteries to improve battery performance. Carbon nanotubes were grown on Ni deposited porous alumina substrates in a flowing gas mixture of methane and hydrogen. Characterization of the carbon nanotubes was carried out using scanning electronic microscopy, transmission electron microscopy, and Raman spectroscopy. It was found that the carbon nanotubes were randomly oriented, spaghetti-like, and with a high aspect ratio. The diameters of CNTs, as determined by transmission electron microscopy (TEM) were between 10 and 100 nm. TEM also showed that the nanotubes had a central hollow with amorphous carbons covering outside wall. The CNTs were nanocoated with a V 2O 5 sol–gel, which upon curing formed a xeorgel about the carbon nanotubes. This formed a molecular composite cathode. Microbatteries/microcapacitors were made and subjected to charge/discharge cycles with a consistent maximum charge of 4.60 V.

Role of thin Fe catalyst in the synthesis of double- and single-wall carbon nanotubes via microwave chemical vapor deposition

Synthesis of vertically aligned small diameter (single- and double-wall) carbon nanotube films on thermally oxidized n+-Si(001) wafers, with acetylene diluted with ammonia gas mixture using a microwave plasma-assisted chemical vapor deposition technique, is reported. Experiments show that by continuous reduction in the thickness of the iron catalyst film to ∼0.3–0.5nm, or alternately, smaller catalyst particles produces hollow concentric tubes with a fewer number of walls. Double- and single-wall carbon nanotubes with diameters ranging from 1 to 5nm were identified using transmission electron microscopy and Raman spectroscopy. A relatively higher deposition temperature (∼850°C) in conjunction with a controlled catalyst and rapid growth (<40s) allowed for the growth of well-graphitized, high areal density (∼1012-1013∕cm2) nanotubes with reduced amorphous carbon and iron. Our results also indicate that the base growth is the most appropriate model to describe the growth mechanism for the nanotube films.

Growth and morphology of aligned carbon nanotube layers

The time dependence of the growth of aligned multi-walled carbon nanotubes (MWNT), obtained by bias-assisted microwave chemical vapour deposition (CVD), has been investigated. Short deposition times up to about 15 min favour the tubular tube structure being predominantly observed in the layers, whereas the bamboo-like structure was dominant after longer times (>60 min). It was clarified that this behaviour is related to the different growth rates of the structures, which are governed by the particles size and by the particle size distribution, respectively. Owe to an excess of large particles, the bamboo-like structure is dominant at long deposition time. The experimental findings confirm that both structures are formed by diffusion controlled processes. In the case of the tubular grown tubes, the diffusion coefficient of C in the Co particle was estimated to be about D≈1.8 10 −11 m 2/s, which is in agreement with the assumption of a liquid particle state. For the bamboo-like structure, bulk diffusion through solid particles is likely.

Growth of diamond film on single crystal lithium niobate for surface acoustic wave devices

Single crystalline lithium niobate thin films on diamond are useful for surface acoustic wave devices that operate at gigahertz frequency. The differences in the atomic bonding and crystalline structure between the lithium niobate and diamond and the high interfacial energy were responsible for formation of microcrystalline films of lithium niobate deposited on polycrystalline diamond. We have carried out growth of continuous diamond layer on single crystal lithium niobate by microwave chemical vapor deposition to overcome the above deficiencies. The temperature of the substrate during deposition has been maintained near 700 °C to reduce the loss of lithium from lithium niobate and to reduce the thermal stresses. Characterization by x-ray diffraction and Raman spectroscopy showed that the lithium niobate lattice is preserved without incorporation of carbon and the diamond layer is free from graphitic carbon. The integrated lithium niobate-diamond layered structure is shown to be suitable for surface acoustic wave devices.

Preparation and field electron emission of microcrystalline diamond deposited on a porous silicon substrate

In this paper, the preparation of discontinuous microcrystalline diamond with special structure, deposited on a porous silicon (PS) substrate by microwave chemical vapor deposition (MW-CVD) technology was reported, and the field electron emission of the diamonds were studied. Our experimental results indicated that the diamonds have a lower field emission threshold voltage (<0.8 V/μm) and a higher field emission current density (>17.5A/cm 2). A preliminary explanation of this special field electron emission was given in our paper.

Thermal detrapping analysis of pumping-related defects in diamond

A method is presented to selectively characterize the traps involved in the pumping procedure of diamond films. The pumping process strongly reduces the concentration of active carrier trapping centers, leading to an enhancement of electronic properties of such material, and is obtained by irradiating the diamond films with ionizing radiation. Since the improved transport properties lead to an increased efficiency when diamond films are used as radiation detectors, valuable information on this process can be obtained by analyzing the response of diamond based particle detectors. For this purpose a high-quality diamond film was grown by microwave chemical vapor deposition and a particle detector was realized. Its response to a 5.5-MeV Am241 α-particles was measured after successive annealing steps performed at different temperatures in the 180–228 °C range. Before each annealing curve at a given temperature, the detector was driven to the pumped state through β-particle irradiation. The efficiency versus annealing time curves evidence a thermally induced detrapping, confirming the pumping mechanism as a filling and consequent passivation of defects. The analysis of the decay time of the detector efficiency as a function of temperature allows the determination of the activation energy of these defects, which is Ea=1.62±0.15 eV.

SiC nanofibers grown by high power microwave plasma chemical vapor deposition

Silicon carbide (SiC) nanofibers have been synthesized on Si substrates covered by Ni thin films using high power microwave chemical vapor deposition (CVD). Characterization using transmission electron microscopy (TEM) combined with electron energy-dispersive X-ray spectroscopy (EDX) revealed that the resultant fibrous nanostructures were assigned to β-SiC with high crystallinity. The formation of SiC nanofibers can be explained by the vapor liquid solid (VLS) mechanism in which precipitation of SiC occurs from the supersaturated Ni nanoparticle containing Si and C.

Synthesis of carbon nanocoils by microwave CVD

The carbon nanocoil is an interesting kind of nanomaterial, which is expected tohave various novel applications in microelectronics, microelectromechanicalsystems (MEMS) and bioMEMS due to its nanosize coil morphology. Carbonnanocoils were synthesized by catalytic pyrolysis of acetylene over 2.2–3.0 µm Niparticles at 600°C in a microwave chemical vapour deposition (CVD) system.Microwave CVD derived nanocoils are rather uniform with an average coildiameter of 110–170 nm, fibre diameter of 80–120 nm and pitch of around 130–200nm. The effects of reaction temperature, the size of Ni catalyst and the flow rateof acetylene were examined. Additionally, the growth mechanism of carbonnanocoils was studied. It was found that two individual carbon nanocoils extrudefrom the same Ni particle in the same direction as straight fibres at thebeginning and then bend to grow into nanocoils in opposite directions.

Synthesis and Electrochemical Characteristics of a Nanocomposite Diamond Electrode

A nanocomposite material with novel electrochemical properties was synthesized by immersing a platinum wire into the dense zone of the 1-2% CH 4 /H 2 plasma in a microwave chemical vapor deposition reactor. Raman analysisof the deposit indicated the presence of nanocrystalline diamond, graphite, and amorphous sp 3 carbon. Notably, the characteristic broad shoulder between 1350 and 1550 cm - 1 attributable to amorphous sp 2 carbon was absent. Growth of freestanding submicrometer-thick whiskers containing diamond and graphite nanocomposite was observed. The nanocomposite material, without any prior surface pretreatment, showed high reversibility (ΔE p < 100 mV) for dopamine redox reactions in 0.1 M KCl solution.

Diffusion of hydrogen from a microwave plasma into diamond and its interaction with dopants and defects

The diffusion of hydrogen (deuterium) into various kinds of single crystal diamonds (defect free, boron doped or containing defects) is studied by electrical (Hall effect) and secondary ion mass spectroscopy (SIMS) measurements. Hydrogen is incorporated into the samples from a microwave hydrogen (deuterium) plasma at 820 K. Post-diffusion annealings are also performed, when required, up to 1700 K. The defects are introduced into the diamonds either by ion implantation or unintentionally during the homoepitaxial microwave chemical vapor deposition growth of the diamond layer. The diffusion profile of hydrogen is found to be affected by the presence of dopants and defects (impurities as well as native defects). Hydrogen diffusion is already observed at 850 K in B-doped or defect containing diamonds. This result is in contrast to the case where hydrogen is implanted into a B implantation doped diamond layer in which only a slight diffusion of hydrogen is observed above 1200 K. The electrical characteristics of the initially B doped layer are found deteriorated or improved by the diffusion of hydrogen, depending on the characteristics of the B-doped layer. The experimental results are interpreted in the light of published theoretical predictions.

B–C–N coatings prepared by microwave chemical vapor deposition

Ternary B–C–N amorphous and crystalline coatings were deposited by microwave chemical vapor deposition using a mixture of NH 3, CH 4, B 2H 6 and H 2 gases at substrate temperatures in the range 800–1350°C, and a gas pressure in the range (9.0–12)×10 3 Pa. We studied the influence of deposition conditions (i.e. gas composition, substrate temperature and material) on the coating structure, chemical composition and surface morphology. Both amorphous and polycrystalline coatings were deposited. Amorphous coatings were usually formed at lower substrate temperatures and were non-homogeneous across the coating thickness. Nanocrystalline inclusions of boron carbide, boron nitride and diamond were observed in amorphous B–C–N compounds. Polycrystalline coatings were generally represented by both diamond and boron nitride phases. In one case, a polycrystalline coating with the composition of B 2CN 4 was fabricated. The experimental results are discussed in detail.

Trapping and detrapping effects in high-quality chemical-vapor-deposition diamond films: Pulse shape analysis of diamond particle detectors

An analysis of the time evolution of the response of diamond particle detectors is carried out, using as a probe 5.5 MeV \ensuremath{\alpha} particles impinging on high-quality diamond films grown by microwave chemical vapor deposition (CVD). Both the amplitude and the time evolution of the pulses are shown to change drastically when the detector is preirradiated with \ensuremath{\beta} particles (pumping), a slow component developing after pumping, indicating carriers trapping and releasing (detrapping). Pulse shapes obtained for positive and negative detector polarities are compared in both the as-grown and pumped states. The presence of at least two trapping centers for holes is necessary to explain the results, the shallower having an activation energy of about 0.3 eV. The effects of pumping are clarified, and the different role played by electrons and holes is evidenced. We modify a previous model for trapping-detrapping behavior originally applied to Si(Li) detectors to describe the more complex behavior of CVD diamond detectors, and develop a computer simulation based on it. The simulated pulse shapes agree very well with experiment with reasonable values of the physical parameters involved, making this technique helpful for studying and identifying defects which are responsible for limitation of the efficiency of CVD diamond particle detectors. Field-assisted detrapping seems to take place for fields of about ${10}^{4}\mathrm{V}/\mathrm{c}\mathrm{m}.$

Trapping–detrapping behavior in CVD diamond particle detectors

Diamond based particle detectors were built up using high quality diamond films grown by microwave chemical vapor deposition (CVD). The efficiency (η) and charge collection distance (CCD) of such devices were tested by a 5.5 Mev 241Am α-particle source. Their response times were then carefully investigated both in the as-grown normal state and after irradiation with β-particles for approximately 60 h, in order to bring the detectors in the so called pumped state. A drastic change in the time evolution, the signal amplitude and the symmetry of the pulse shapes recorded with positive and negative polarization of the detector is observed as soon as the priming procedure takes place. This behavior is explained in the framework of a model in which a trapping–detrapping mechanism for CVD diamond is accounted for. Two different kinds of trapping centers for electrons and holes are proposed as the limiting factor in the diamond detection performance. A very good agreement between the simulation and the experimental pulse shapes is observed, thus allowing a better understanding of the priming procedure and the possible identification of the crystal defects limiting the efficiency of diamond based particle detectors.

CN x /carbon nanotube junctions synthesized by microwave chemical vapor deposition

The CNx/carbon nanotube junctions were successfully synthesized by microwave plasma-assisted chemical vapor deposition method from the mixture of N2/CH4 and H2/CH4 gases in a continuous growth process. High resolution transmission electron microscopy revealed that these junctions were of heterostructure between CNx nanotubes with polymerized nanobells and cylindrical carbon nanotubes. The growth process is quite simple and can be easily scaled up. The intimate correlation between the electronic structure and the chemical composition at the both sides of the junction indicate some interesting properties and offers potential applications for future nanodevices.

Erosion damage and optical transmittance of diamond films

Diamond films on silicon wafers were fabricated by microwave chemical vapor deposition to investigate the effect of erosion damage on optical transmittance. After deposition, the growth side was mechanically polished and the silicon substrate was removed by chemical etching with hydrofluoric acid to make an optically flat infra-red (IR) window. With prolonged erosive impacts by SiC particles, the two sides of free-standing diamond windows showed significantly different damage resistance. The optical transmittance of the substrate side, which has smaller diamond grains, was maintained to be the same as that of the non-eroded diamond even after 400 times of repeated erosive impacts. However, optical transmittance of the nucleation side, which has larger diamond grain, was too low to be used as an IR window. The difference in the damage resistance between the substrate side and growth side is discussed based on scanning electron microscopy of the eroded region to explain the different optical behavior of transmittance.