Glow Discharge Chemical Vapor Deposition Research Articles

Special Issue on Plenary and Invited Papers From ICOPS 2006

Thirteen referred papers from the plenary and invited talks are in this Special Issue. The papers are from a range of plasma science topics including high-energy density plasmas, atmospheric pressure plasmas, microwave generation, thermal plasma torches, and applications of plasmas to biomedical, nanotechnology, displays, lighting, and plasma-assisted com- bustion. In the area of Z-pinch and high-energy density plas- mas, Bott et al. present an experimental study using imaging and X-ray diagnostics of the precursor column formation in wire array Z-pinches. Ottinger and Schumer present a model of the power flow in a Z-pinch-driven inertial fusion energy sys- tem that utilizes a recyclable transmission line. Drake presents a study of radiative shocks in optical thick media by looking at the fluid theory and using a model for the radiative transfer that yields a description of the shock structure. In the area of atmospheric pressure plasma discharges, the paper by Hopfe and Sheel examines the use of atmospheric plasmas for coating and etching applications. Plasma sources they consider include microwave chemical-vapor deposition (CVD), linear-shaped plasma dc ArcJet CVD, and dielectric barrier glow discharge CVD. Shi, Liu, and Kong examine the operation and proper- ties of dielectric-barrier radio-frequency atmospheric pressure glow discharges. Roth et al. examine several applications of atmospheric pressure discharges based on the One Atmosphere Uniform Glow Discharge Plasma technology. In the area of thermal plasmas, Tanaka et al. present a study of independently controlling the atomic nitrogen flux and the enthalpy flow, from an induction thermal Ar-N2 plasma torch, by modulating the coil current. In the area of microwave generation, Thumm et al. present experimental results on a new megawatt class design for a gyrotron operating at 140 GHz. This gyrotron is designed for use as the millimeter-wave power source for heating plasmas to high temperatures for fusion reactions. Several of the papers in this Special Issue present studies of the application of plasmas. Chu presents a paper on plasma- treated biomaterials that looks at the use of atmospheric pres- sure plasma spraying and plasma immersion ion implantation. Applications discussed include orthopedic implants, biocom- patible coatings, and antibacterial coatings. Ostrikov overviews the ways that plasmas are used in nanoscience areas. Examples from nature and nanofabrication in the laboratory are discussed. Park et al. present work on making 20 × 20 and 50 × 50 addressable arrays of microcavity discharges that are built based on dielectric barrier designs. Paul et al. developed a 3-D model for high-intensity discharge lamps. The model results are validated by comparison to experimental temperature mea- surements of a test lamp. Vincent-Randonnier et al. describe a study and experimental test bench for plasma-assisted com- bustion using a dielectric barrier discharge to assist a methane diffusion flame. We would like to thank all the referees for the papers in this Special Issue. A special thanks is also due to Dr. S. Gitomer, Editor-in-Chief of the IEEE TRANSACTIONS ON PLASMA SCIENCE, for his guidance and support during the formation of this Special Issue.

Hydrogen in nano-diamond films

The distribution, content and bonding of hydrogen in nano-crystalline carbon films possessing a prevailing diamond character are investigated by secondary ion mass spectroscopy (SIMS), high resolution electron energy loss spectroscopy (HREELS) and Raman spectroscopy. The films were deposited by DC glow discharge chemical vapor deposition from a methane–hydrogen mixture. Following the formation of a thin oriented precursor graphitic film, diamond nucleation occurs and a nano-diamond film grows. The hydrogen content in the precursor oriented graphitic film is ∼5 at.%. Concurrently with the nucleation and growth of nano-diamond a considerable increase of the hydrogen concentration in the films occurs reaching a value of 15–20 at.%. This is accompanied by appearance of ∼1150 and ~1450 cm −1 Raman peaks, attributed to distinctive bonding of hydrogen in nano-diamond films. In this work the origin of the Raman peaks was investigated by means of isotopic shift on the films deposited from deuterized and C 13 gas mixtures. The HREELS revealed a broad band associated with C–H vibration centered at 362 meV. Concurrently with film evolution the full width half maximum of this band broadens from 33 to 42 meV and it shifts from 362 meV to 367 meV. From EELS measurements it was determined that the hydrogen plasma at the conditions applied for nano-diamond growth disrupts the crystalline structure of diamond resulting in an amorphous surface layer. In this work we further corroborate the role of hydrogen as a bonding and displacing agent during diamond nucleation and nano-diamond growth as was recently proposed by us.

Morphological and phase evolution of nano-crystalline carbon films with a predominant diamond character

Abstract In the present work, the evolution of nano-diamond films deposited by the glow discharge chemical vapour deposition method onto silicon substrates as a function of time were studied by various complementary techniques. Our analysis showed that prior to formation and growth of continuous films of a predominantly nano-diamond character, a graphitic phase is formed. This graphitic film displays a preferred alignrnent of its basal planes perpendicular to the silicon substrate interface. The nano-diamond character of the films gradually evolves reaching a maximum value of ∼80% for film thickness larger than ∼0.5 μm. It was found that the appearance of the nano-diamond phase is initially accompanied with an increase in surface roughness which decreases with film growth. It was determined that the continuous nano-diamond films are composed of diamond nano-crystallites, ∼5 nm in diameter alongside a sp2 bonded amorphous carbon phase. Most likely the amorphous carbon component in the films exists at the nano-diamond grain boundaries. The phase and morphological evolution of the films were investigated, with nanoscopic resolution, by a number of complementary spectroscopy and microscopy techniques: near edge X-ray adsorption fine structure, scanning electron microscopy and atomic force microscopy, high-resolution transmission electron microscopy and X-ray diffraction.

Effects of spatial separation on the growth of vertically aligned carbon nanofibers produced by plasma-enhanced chemical vapor deposition

Vertically aligned carbon nanofibers (VACNFs) with vastly different spacing were grown by catalytically controlled dc glow discharge chemical vapor deposition. Both densely packed VACNFs and essentially isolated VACNFs were studied using scanning electron microscopy and x-ray energy dispersive spectroscopy. The morphology and chemical composition of isolated VACNFs were found to have a strong dependence upon the growth conditions, in particular on the C2H2/NH3 gas mixture used. This is attributed to the sidewalls of isolated VACNFs being exposed to reactive species during growth. In contrast, the sidewalls of densely packed VACNFs were shielded by the neighboring VACNFs, so that their growth occurred mainly in the vertical direction, by diffusion of carbon through the catalyst nanoparticle and subsequent precipitation at the nanofiber/nanoparticle interface. These striking differences in the growth process result in the formation of flattened carbon nanostructures (carbon nanotriangles) and also are quite important for the realization of VACNF-based devices.

Evolution and properties of nanodiamond films deposited by direct current glow discharge

Nanocrystalline carbon films possessing a prevailing diamond character are deposited by a direct current glow discharge chemical vapor deposition method using a 9:91 vol % methane to hydrogen gas mixture. In the present work the evolution and properties of nanodiamond films deposited by this method onto silicon substrates as a function of time were studied by various complementary techniques. Our analysis showed that prior to formation and growth of continuous films of a predominantly nanodiamond character, a graphitic phase is formed. After the nanodiamond phase is stabilized, near edge x-ray adsorption fine structure measurements proved the predominant diamond character of the film to be about 80%. By electron energy loss spectroscopy analysis the sp2-like character of the nanodiamond grain boundaries has been determined. The nanodiamond films were found to be thermally stable up to temperatures of ∼950 °C as established by vacuum heating. By scanning electron microscopy and atomic force microscopy the morphology of the films was examined showing that the formation of the nanodiamond phase is initially accompanied with an increase in surface roughness which decreases with film growth. By high-resolution transmission electron microscopy it was determined that the continous nanodiamond films are composed of diamond nanocrystallites, 3–5 nm in diameter.

Light-induced structural changes and their correlation to metastable defect creation in intrinsic hydrogenated amorphous silicon films

Device-quality intrinsic a-Si:H films were prepared by three methods, hot-wire (HW) chemical vapor deposition (CVD), and glow-discharge (GD) CVD with and without H dilution, and show varied light-induced metastable defect creation [Staebler-Wronski effect (SWE)]. We found the following: (a) In addition to the nonuniform H distribution, the a-Si network is inhomogeneous, and the film prepared by GD is more homogeneous than the HW film. (b) The light-induced increase of Si-H stretching absorption at $\ensuremath{\sim}2000 {\mathrm{cm}}^{\ensuremath{-}1}$ is on the order of ${10}^{\ensuremath{-}2}$ in all the films, and an additional decrease at $\ensuremath{\sim}2025 {\mathrm{cm}}^{\ensuremath{-}1}$ is found in films with larger SWE. (c) The change of the compressive stress is on the order of ${10}^{\ensuremath{-}4}$ of the initial value in the HW films, which is the same order of magnitude as in GD films. Both the initial stress and light-induced volume expansion decrease with decreasing Si-H concentration. No simple correlation between the light-induced structural changes and the conductivity changes was found in the HW a-Si:H films. We describe the light-induced structural changes in conjunction with the creation of metastable defects by a two-phase model.

Photothermal deflection spectroscopy and transmission measurements of a-C:H films

Absorption coefficient of a-C:H film prepared by saddle-field glow-discharge chemical vapor deposition was measured by means of photothermal deflection spectroscopy and transmission measurements. The absorption spectra of these films are divided into a larger absorption region and a smaller absorption region. The larger absorption region can be interpreted in terms of the Tauc model and an optical gap is obtained for the a-C:H film. The smaller absorption region can be analyzed in terms of the Urbach tail and yields information about the band tails of the a-C:H film. The effect of doping on the absorption tails and the optical gap is also reported. We found that both the optical gap and the breadth of the band tail decreased with the concentration of the impurities, phosphorous or hydrogen. Also there is a correlation between the band tail spread and the optical gap. The breadth of the band tail increases with the optical gap.

Fieldmap of morphology of diamond films grown by use of d.c. plasma glow discharge chemical vapour deposition

We have studied the morphology and texture of CVD diamond films grown by d.c. plasma glow discharge CVD by means of SEM analysis and X-ray diffraction. This works follows a preliminary work on morphology changes from disordered to textured structure with a thorough investigation in a wide interval of substrate temperature, 955–1150°C, and methane into hydrogen volume concentration, 0.9–3.8%. The other deposition parameters have been kept constant: overall pressure, 210 Torr, gas flow rate 300 sccm, power density, 38 W mm −2. Care has been taken to prepare the substrate with a standard procedure in order to avoid unreproducible effects on nucleation and growth. In-plane texture has been determined by calculating the average percentage of oriented grains from the area of the Bragg peaks and the powder diffraction data. The results of this quantitative analysis confirm the SEM morphology observation. A fieldmap has been developed for the film morphology and texture as functions of substrate temperature and methane concentration in order to give an useful information for directing the CVD process by means of this particular method. Moreover a qualitative agreement with previously reported data supports the hypothesis that the morphology does not depend on the particular CVD method but only on the deposition conditions, i.e. the deposition parameters and the activation of the reactant gases. The density of nucleation has been also studied by SEM analysis, leading to conclude that there is no competition between grains and evolutionary growth for the used deposition conditions, resulting in grain sizes within the same order of magnitude of the film thicknesses. A clear correlation has been found between the rate of growth of the films and their texture, in the sense that maxima and minima of the rate of growth always correspond to maxima in the diamond texture. A thorough explanation of this correlation will require further ad hoc investigations.

Synthesis of low leakage current chemical vapour deposited (CVD) diamond films for particle detection

We report on synthesis of diamond films by direct current glow discharge chemical vapour deposition (CVD) prepared at different deposition conditions, for application in high energy physics. The syntesis apparatus is briefly described. Continuous undoped diamond samples have been grown onto Mo substrates with a deposition area up to 1 cm 2 and an electrical resistivity as high as 10 13 Ωcm. The deposition parameters are related to the material properties of the diamonds, investigated by optical spectroscopy, electron microscopy and diffraction analysis. Decreasing the linear growth rate results in good quality films with small remnants of graphite-like phases. The high crystalline quality and phase purity of the films are related to very low values of leakage currents. The particle induced conductivity of these samples is also studied and preliminary results on charge collection efficiency are presented.

High performance glow discharge a-Si1−xGex:H of large x

Radio frequency glow discharge chemical vapor deposition has been used to deposit thin films of a-Si1−xGex:H which possess optoelectronic properties that are greatly improved over any yet reported in the range of x⩾0.6. These films were deposited on the cathode (cathodic deposition) of an rf discharge. Their properties are assessed using a large variety of measurements and by comparison to the properties of alloys conventionally prepared on the anode (anodic deposition). Steady state photoconductivity measurements yield a quantum-efficiency-mobility-lifetime product, ημτ, of (1–3)×10−7 cm2 V−1 for 1.00⩾x⩾0.75 and (6–10)×10−8 cm2 V−1 for 0.75⩾x⩾0.50, and photocarrier grating measurements yield ambipolar diffusion lengths several times greater than previously obtained for alloys of large x. It is confirmed that the improvements in phototransport are not due to a shift in the Fermi level. In fact, results of recent measurements on lightly doped samples strongly suggest that for these cathodic alloys neither photocarrier is dominant [(μτ)e≈(μτ)h]. The improvements are attributed in large part to the reduction of long range structural heterogeneity observed in x-ray scattering and electron microscopy, and partly to the reduction in midgap state density. In spite of the superior properties, an assessment of the data of the cathodic alloys suggests that alloying introduces mechanisms detrimental to transport which are not present in a-Si:H or a-Ge:H. The Urbach tail width is 42±2 meV for cathodic a-Ge:H and 45±2 meV for cathodic a-Si1−xGex:H and is constant with x. From differences in the band edges and tails we infer that the atomic bond ordering is different between the cathodic and anodic alloys. For a given composition the cathodic alloys have roughly an order of magnitude lower midgap state density than do the anodic alloys, and both midgap densities increase exponentially with x, consistent with defect creation models from which the lower midgap density can be attributed to a larger band gap and decreased valence band tail width. A photoluminescence peak is observed with an intensity roughly an order of magnitude greater than for the anodic alloys, and a significantly different peak energy. Section VII E provides an overview of the results and conclusions. The improved properties of these alloys have significant implications for current and future device applications.

Epitaxial Growth of High Quality Diamond Film on the Cubic Boron Nitride Surface by Chemical Vapor Deposition

High quality diamond epitaxial film has been obtained on high pressure synthesized cubic boron nitride crystal surface by dc glow discharge chemical vapor deposition. The growth characteristics of the epitaxial film have been investigated by scanning electron microscope. The quality of the film has been confirmed by Raman spectroscope to be close to that of natural diamond.

Examination of (crystallized a-Ge:H)/a-SiN x:H multilayers which display photoluminescence

A study has been conducted of (crystallized a-Ge:H)/a-SiN x:H multilayers which display room temperature photoluminescence in the visible range. The -Ge:H/a-SiN x:H multilayers were prepared by glow discharge chemical vapor deposition by changing gas flows in a continuous deposition using a computer. Crystallization of the a-Ge:H layers was achieved by either thermal annealing in vacuum or exposure to a scanning laser light. The multilayer structure was confirmed by glancing angle X-ray diffraction. Crystallization was confirmed by Raman scattering. When crystallization of the a-Ge:H is achieved using the laser-scanning technique, strong visible photoluminescence with a peak centered around 625 nm, independent of the a-Ge:H layer thickness, is observed. The crystallization is accompanied by severe cracking and pitting of the film. A study of several a-Ge:H/a-SiN x:H layered structures reveals that this laser-induced crystallization only occurs when the a-Ge:H is in a state of high stress. Further study reveals that a-SiN x :H also photoluminesces with a similarly broad peak in the same wavelength range as observed from the crystallized multilayers. Using slow thermal annealing, it was possible to crystallize the a-Ge:H with minimal physical damage to the film. Photoluminescence measurements of these quantum-well structures yield a low intensity broad peak in the visible range before crystallization which does not change after crystallization. One is led to conclude that the photoluminescence observed in the laser-crystallized a-Ge:H/a-SiN x:H is from the a-SiN x :H and cannot be attributed to effects of quantum confinement.

Deposition of thick diamond films by pulsed d.c. glow discharge CVD

Polycrystalline diamond films were deposited on molybdenum substrates by pulsed d.c. glow discharge chemical vapour deposition. A stable discharge with a methane in hydrogen mixture (0.5–7 vol.%) was maintained by the plasma generator over a wide range of power density (1–7 A cm −2). Deposition areas from 0.4 to 3 cm 2 were used and growth rates of more than 40 μm h −1 were achieved. The well-faceted diamond layers were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), micro-Raman and Fourier transform IR (FTIR) spectroscopy.

The effect of biological fluids on the response of DLC films to a novel erosion durability test

The durability of diamond-like carbon (DLC) films, deposited by an r.f. plasma-assisted glow discharge chemical vapour deposition (CVD) technique onto metallic substrates (304 steel and Ti-6Al-4V), has been measured by a novel method in which the coating is bombarded with small solid particles in a carefully controlled manner. Two different types of particle, soft rounded glass beads and hard angular silica particles, were used to examine the effect of particle type on the wear behaviour. The durability was measured both before and after exposure of the specimens to various biological fluids, including distilled water, phosphate-buffered saline (PBS) solution and 40% bovine serum in sterile PBS. The specimens were exposed for 7 and 28 days at 37 °C in a humidified atmosphere containing 5% carbon dioxide. One set of experiments was carried out at 60 °C to determine the effect of temperature on the film stability. Environmental exposure had little direct effect on the film properties, but the fluids tended to degrade the interfacial adhesion when penetration to the interface was possible. This can occur via microscopic penetrations in the film or via the edges. The buffered saline solution appeared to be the most aggressive in this respect. A correlation between the damage mechanism during solid particle erosion and the type of particle used was found.

Diamond film deposition by downstream d.c. glow discharge plasma chemical vapour deposition

A new d.c. glow discharge chemical vapour deposition (CVD) method was developed for diamond film (DF) deposition onto insulator substrates. This method uses ordinary d.c. plasma CVD equipment, but the anode system was modified both to insulate the substrate holder from the anode and to set the substrate downstream of the plasma. This is achieved by two methods. First, the anode was made from the tungsten grid and the insulated substrate was placed under the grid and out of the discharge area. Second, the anode was made from the thick-wall molibden tube and the substrates were placed on the insulator inside the tube. The experiments were carried out under a hydrogen pressure of 50–150 Torr and methane concentration of 1%–2%. The substrates (Mo or Si) were set on the silica substrate holder. The temperature was approximately 1000 °C. Diamond films were grown by both methods. Scanning electron microscopy, cathodoluminescence microscopy and spectroscopy, Raman scattering and X-ray diffractometry were used to study and compare the diamond films.

Diamond Nuclei on the Silicon Mirror Surface by Direct-Current Glow Discharge Chemical Vapor Deposition

A new method is proposed for generation of high density diamond nuclei in the dc glow discharge chemical vapor deposition. Application of dc negative valtage together with high methane content has been found to generate diamond nuclei as high as 1010/cm2 on mirror-polished Si wafer. It has been indicated that carbon over-saturation, negative voltage and hydrogen play an important role in this new pretreatment process.

Compamson in the Growth and Properties of rf Sputtered μc-Si:H and Glow Discharge-Chemical Vapor Deposited μc-Si:H Films

ABSTRACTProperties of μc-Si:H films grown by rf sputtering and by glow discharge-chemical vapor deposition (GD-CVD) using diluted-hydrogen and hydrogen-atom-treatment method were compared employing TEM, X-ray diffraction, Raman scattering and FT-IR. The films deposited by both methods all exhibited comparable grain sizes in the range of 10–18 nm. and showed the same tendency in almost all the Measurements.

The influence of unsteady state plasma on the properties of a-Si:H films formed by glow discharge-chemical vapour deposition

An unsteady state plasma is known to exist during the initial stage of the glow discharge decomposition of silane (SiH 4) gas. The properties of the i layer deposited in this unsteady state plasma and p-i-n stacked hydrogenated amorphous silicon solar cells having such an i layer were investigated. It was found that this layer had an undesirably wide optical band gap and low conductivity because of the large number of SiH 2 sites.

Synthesis of electrically conductive organic thin film by plasma polymerization of 3,4,9,10-perylenetetracarboxylic dianhydride

The first synthesis of electrically conductive organic thin film by plasma polymerization (glow-discharge chemical vapour deposition) is reported. The monomer specifically employed is 3,4,9,10-perylenetracarboxylic dianhydride consisting of five-condensed aromatic rings attached by carboxylic groups outside the rings. The film is air-stable and hard enough not to be scratched out. Electrical conductivity of the samples measured is more than 1.0 × 10 0 S cm -1 with n-type carriers at room temperature. Preliminary spectroscopic data are also to be shown.

Laser Optogalvanic Monitoring of SiH in a Chemical Vapor Deposition Glow Discharge

A demountable hollow cathode discharge cell that is useful for studies of plasmas relevant to chemical vapor deposition (CVD) was assembled and demonstrated. Direct current glow discharge decomposition of 10% silane in helium in this cell at 20 Torr produced silicon-containing cathodic thin films that were subsequently examined with the use of Raman and x-ray fluorescence spectroscopy. A pulsed dye laser-excited optogalvanic detection technique was used to monitor the presence of a transient molecular intermediate—silicon hydride—in this decomposition plasma. An optogalvanic spectrum of the SiH A2δ -X2τ transition near 420 nm is presented. This electronic absorption technique complements other spectroscopic methods for mechanistic studies and optimization of glow discharge CVD plasmas that are currently used for the commercial preparation of technologically important thin film materials such as hydrogenated amorphous silicon. It will be particularly useful for detection of nonluminescent plasma intermediates.