Dc Plasma Chemical Vapor Deposition Research Articles

Preparation of hydrogenated amorphous carbon films using a microsecond-pulsed DC capacitive-coupled plasma chemical vapor deposition system operated at high frequency up to 400 kHz

Hydrogenated amorphous carbon (a-C:H) films are deposited on silicon (Si) substrates using a high-repetition microsecond-pulsed DC plasma chemical vapor deposition (CVD) system from acetylene (C2H2) at a gas pressure of 15 Pa inside a custom-made vacuum chamber. The plasma discharge characteristics, hydrocarbon species, and the microstructure of the resulting films are examined at various pulse repetition rates from 50 to 400 kHz and a fixed duty cycle of 50%. The optical emission spectra confirmed the increase in electron excitation energy from 1.09 to 1.82 eV and the decrease in the intensity ratio of CH/C2 from 1.04 to 0.75 with increasing pulse frequency, indicating the enhanced electron impact dissociation of C2H2 gas. With increasing pulse frequency, the deposition rate gradually increased, reaching a maximum rate of 60 nm/min at 200 kHz, after which a progressive decrease was noted, whereas the deposition area was almost uniform for all the prepared films. Clear trends of increasing sp3 content (amorphization) and decreasing hydrogen (H) content in the films were observed as the pulse repetition rate increased, while most of the hydrogen atoms bonded to carbon atoms by sp3 hybridization rather than by sp2 hybridization.

Pulsed DC plasma CVD system for the deposition of DLC films

Pulsed DC plasma chemical vapour deposition (CVD) system with a compact vacuum chamber has been developed using a simple configured pulsed DC power supply. Pulsed DC discharge was examined with variation of operation parameters as well as source gas materials. Diamond like carbon (DLC) films were deposited on silicon (Si) substrates from acetylene (C2H2) or carbon-monoxide (CO) at two distinct temperatures. The fabricated films were characterized by Raman spectroscopy, FTIR spectroscopy, and scanning electron microscopy (SEM). The results reveal that the films prepared from CO gas exhibited soft conductive DLCs with a large number of graphite like structures whereas mostly insulating hydrogenated amorphous carbon DLCs were obtained from C2H2 gas. The deposition rate from CO was lower than from C2H2 due to etching by oxygen produced from the source gas. As the substrate temperature increased, progressive graphitization was incorporated to the films deposited from both gases. By using the pulsed DC plasma CVD system, the energy efficiency for CVD as the ratio of growth volume to energy consumption was dramatically improved with some distinctive differences in film properties as compared to our previous RF plasma CVD. The results showed many advantageous features of pulsed DC plasma CVD for replacing conventional RF plasma CVD.

Influence of Co Removal for Improving Diamond Films Adhesive on WC-Co Substrates by DC-PCVD

The DC plasma chemical vapor deposition (DC-PCVD) method was a promising technique to fabricate CVD diamond coatings in industry, because it has many good properties, such as high grown rate of diamond films, big grown area, high qualities, and cost effective. In the present work, we synthesized diamond films on cemented carbide (YG6) substrates, and studied the different nitric acid aqueous solution treatment durations influence on diamond nucleation stage. The results indicated that well-chosen nitric acid aqueous solution treatment durations should be selected. For too short treatment times, the residual Co will promote the carbon transform from diamond metastable phase into graphite stable phase. On the other side, too long treatment times will consume too much Co, which will lead to the shortage of cohesive body, and finally the crackers in the bulk propagated and peeled off with the diamond films well grown on WC grains.

Mechanical properties of silicon-doped diamond-like carbon films prepared by pulse-plasma chemical vapor deposition

Silicon-doped diamond-like carbon (Si-DLC) films were prepared by dc pulse-plasma chemical vapor deposition (CVD), using a mixture of acetylene (C 2H 2) and tetramethylsilane (TMS) as the material gas. The pulse voltage was varied from − 2 to − 5 kV, and the TMS flow ratio (TMS/(C 2H 2 + TMS)) was varied from 0 to 40%. At a pulse voltage of − 2 kV, an increase in TMS flow ratio leads to a decrease in hardness. In contrast, at a pulse voltage of − 5 kV, an increase in TMS flow ratio leads to a slight increase in hardness. The high hydrogen concentration in the films due to an increase in TMS flow ratio promotes the formation of polymeric sp 3 C―H bonds, resulting in the fabrication of soft films at a low pulse voltage of − 2 kV. However, an increase in the effect of ion peening on the growth face results in the formation of hard films at a high pulse voltage of − 5 kV. Then, at a pulse voltage of − 5 kV fabricating hard Si-DLC films, an increase in TMS flow ratio leads to an increase in the silicon content in the films, resulting in a decrease in the friction coefficient. Therefore, it is clarified that Si-DLC films fabricated by dc pulse-plasma CVD under a high pulse voltage and high TMS flow ratio exhibit high hardness and a low friction coefficient. Moreover, to investigate the friction coefficient of Si-DLC films fabricated by dc pulse-plasma CVD, films deposited by dc plasma CVD were also evaluated. To obtain the same low friction coefficient, dc pulse-plasma CVD requires less TMS than dc plasma CVD. Hence, it is also clarified that Si-DLC films can be fabricated at a low cost by dc pulse-plasma CVD.

カーボンナノチューブを用いたイオンゲージ用冷陰極電子源の研究

We proposed the cold cathode made of carbon nanotubes (CNTs) for an ionization gauge. The CNTs were grown radially on a Ni wire by DC-plasma chemical vapor deposition (CVD). The radial growth of CNTs was obtained by controlling deposition temperature, C2H2: NH3 ratio and pressure. Pre-heating of the Ni wire in vacuum prior to CVD process resulted in the higher density of CNTs. The field emission characteristics of two types of CNT cathodes were examined.

Fabrication of high-aspect-ratio carbon nanocone probes by electron beam induceddeposition patterning

A high-aspect-ratio cone-shaped carbon nanotube (CNT), which we refer to as a carbonnanocone (CNC), was fabricated for scanning probe microscopy (SPM) by a novel andreliable patterning technique and dc plasma chemical vapour deposition. Carbondots from electron beam induced deposition (EBID) were utilized as convenientchemical-etch masks to create catalyst patterns for the growth of a single CNC probeon a tipless cantilever and an array of CNC probes on a silicon substrate. Thisresist-free EBID process is an efficient way of preparing a patterned catalyst andresultant nanoprobe on the specific edge location of the cantilever. The CNCprobe produces high-resolution images of specimens in air as well as in liquid. Nodegradation in imaging performance was observed after a period of continuousscanning. The CNC bed-of-nails array imaged in contact mode by a commercialSi3N4 probe demonstrates the mechanical toughness/sturdiness of the CNC tip. This alsoindicates the possibility of using the CNC bed-of-nails as a convenient means for thecharacterization of SPM tips.

Heterocrystal and bicrystal structures of ZnS nanowires synthesized by plasma enhancedchemical vapour deposition

ZnS nanowires with heterocrystal and bicrystal structures were successfully synthesizedusing the DC-plasma chemical vapour deposition (CVD) method. The heterocrystallineZnS nanowires have the zinc blende (ZB) and wurtzite (WZ) zones aligned alternately inthe transverse direction but without an obvious period. The bicrystal ZnS nanowires arecomposed of two ZB fractions separated by a clear grain boundary along the length.Significantly, the grain boundaries in both the heterocrystal and bicrystal structures areatomically sharp without any visible lattice distortion. The effects of plasma species, ionbombardment, and silicon impurities in the formation of these distinctive structures arediscussed. A defect-induced red-shift and broadening of the band-gap emission arerevealed in photoluminescence (PL) and cathodoluminescence (CL) measurements.

1 A ∕ cm 2 current density from microgated carbon nanotube field-emitter arrays grown by dc plasma chemical-vapor deposition

We report improved emission performance of microgated carbon nanotube field-emitter arrays. The arrays with 0.1mm2 areas produced anode currents in excess of 1mA (1A∕cm2) and transconductance of up to 174μS (174mS∕cm2) at gate voltages of 75V or less. The carbon nanotubes were grown by dc plasma chemical-vapor deposition, but were not oriented vertically during inspection in the scanning electron microscope. Most of the nanotubes had diameters near 10nm. We suggest that the small tube diameter caused the relatively high emission current and also produced limber tubes, unable to support themselves vertically.

Growth of aligned carbon nanotubes on carbon microfibers by dc plasma-enhanced chemical vapor deposition

It is shown that unidirectionally aligned carbon nanotubes can be grown on electrically conductive network of carbon microfibers via control of buffer layer material and applied electric field during dc plasma chemical vapor deposition growth. Ni catalyst deposition on carbon microfiber produces relatively poorly aligned nanotubes with significantly varying diameters and lengths obtained. The insertion of Ti 5nm thick underlayer between Ni catalyst layer and C microfiber substrate significantly alters the morphology of nanotubes, resulting in much better aligned, finer diameter, and longer array of nanotubes. This beneficial effect is attributed to the reduced reaction between Ni and carbon paper, as well as prevention of plasma etching of carbon paper by inserting a Ti buffer layer. Such a unidirectionally aligned nanotube structure on an open-pore conductive substrate structure may conveniently be utilized as a high-surface-area base electrodes for fuel cells, batteries, and other electrochemical and catalytic reactions.

Control of carbon nanotube morphology by change of applied bias field during growth

Carbon nanotube morphology has been engineered via simple control of applied voltage during dc plasma chemical vapor deposition growth. Below a critical applied voltage, a nanotube configuration of vertically aligned tubes with a constant diameter is obtained. Above the critical voltage, a nanocone-type configuration is obtained. The strongly field-dependent transition in morphology is attributed primarily to the plasma etching and decrease in the size of nanotube-nucleating catalyst particles. A two-step control of applied voltage allows a creation of dual-structured nanotube morphology consisting of a broad base nanocone (∼200nm dia.) with a small diameter nanotube (∼7nm) vertically emanating from the apex of the nanocone, which may be useful for atomic force microscopy.

Tribological properties of nitrogen-containing amorphous carbon film produced by dc plasma chemical vapor deposition

The nitrogen-contained amorphous carbon (a-C:N) films were deposited in a dc plasma chemical vapor deposition system with different substrate bias voltages. The structural, mechanical, and tribological properties of the a-C:N films were investigated. The influence of the bias voltage on the tribological behaviors of the a-C:N films was evaluated under various environments (dry air, O2, N2, and vacuum) using a ball-on-disk friction tester. It showed that the sp3 C and hydrogen concentration of the a-C:N films decreases with increasing the bias voltage. However, the nitrogen concentration increases with increasing the bias voltage. As a result, the hardness and internal stress decrease and the critical load for fracturing increases as the substrate bias increases. For the tribological properties of the a-C:N films, the friction coefficient of the films slightly decreases in the environments of N2, O2, or dry air, but increases slightly in the vacuum environment by increasing the bias voltage. It indicates that the incorporated nitrogen in the a-C:N films would decrease the friction coefficient of the films in N2 or O2 environments, but slightly increases the friction coefficient of the films in a vacuum. The excellent wear resistance of the a-C:N films, in the level of 10−9–10−8 mm3/Nm, can be observed in N2, vacuum, and dry air environments. In addition, the effect of the bias voltage on the wear rate of the a-C:N films becomes less obvious by nitrogen incorporation. So, we suggest the incorporated nitrogen, which bonded to carbon and restrained the increase of the fraction of sp2 C–C, would restrain the wear of the a-C:N films in different environments, especially in dry air.

Vacuum electron emission with low turn-on electric field from hydrogenated amorphous carbon thin films

Hydrogenated amorphous carbon films were deposited with and without hydrogen gas dilution in a small dc bias-assisted plasma chemical vapor deposition system. The field emission characteristics were investigated and compared. It was found that the vacuum electron emission with a very low turn-on electric field (∼0.5 V/μm) could be achieved, which is comparable with the value obtained from nitrogen-doped diamond materials. The improvement of field emission property in the present carbon films was tentatively attributed to the field enhancement effect due to the electronic structure inhomogeneity of the carbon films and the reduction of the surface emission barrier due to the hydrogen termination.

Growth of well-aligned carbon nanotubes on nickel by hot-filament-assisted dc plasma chemical vapor deposition in a CH4/H2 plasma

Well-aligned carbon nanotubes were grown perpendicular to the surface of Ni substrates by hot-filament-assisted dc plasma chemical vapor deposition in a CH4/H2 plasma. It was found, from observation by a scanning electron microscope and a transmission electron microscope, that the nanotubes starting from the side grew first parallel to the plasma boundary and then toward the plasma while those starting on the surface facing the plasma grew straight toward it. It has been suggested that force toward the plasma caused by the local electric field in the sheath and negative charge at the top of nanotubes assist them to grow well-aligned and perpendicular to a substrate in a dc discharge.

LiNiO 2 electrode modified by plasma chemical vapor deposition for higher voltage performance

Surface modification of the electrode improved the performance of the LiNiO 2 electrode charged to 4.6 V. An amorphous carbon thin film was uniformly deposited on the electrode surface using dc plasma chemical vapor deposition. This surface treatment suppressed electrolyte decomposition during the charge process. When a coin-type cell using this electrode was charged to 4.6 V, the charge–discharge efficiency was 84% in the first cycle and 96–98% through the second to fifth cycles. This treatment suggests a new approach to the development of the positive electrode for lithium-ion secondary batteries.

A LiCoO2 Cathode Modified by Plasma Chemical Vapor Deposition for Higher Voltage Performance

Surface modification of the electrode dramatically improved the performance of a electrode charged to 4.6 V. A diamond‐like carbon thin film was deposited uniformly on the electrode surface using dc plasma chemical vapor deposition. This surface treatment suppressed electrolyte decomposition during the charge process. When a coin‐type cell using this electrode was charged to 4.6 V, the discharge capacity in the first cycle was 212 mAh/g and the charge‐discharge efficiency was over 90% through five cycles. This treatment suggests a new approach to the development of cathodes for lithium‐ion secondary batteries. © 2000 The Electrochemical Society. All rights reserved.

Formation of Diamond Films by Intermittent DC Plasma Chemical Vapor Deposition Using Subelectrode

Diamond films were formed by intermittent DC plasma chemical vapor deposition (CVD), wherein the waveform of the power supply was half-wave-rectified and a subelectrode located closely to the cathode was introduced to perform stable discharge at long electrode distance. The crystalline quality of the films deposited on silicon substrate was superior when the electrode distance was about 20 to 25 mm, in comparison with that of films deposited when the electrode distance was 10 or 30 mm. This method was also effective in changing the discharge current widely and in performing a stable discharge at a very low discharge current.

Photoluminescence studies of DC plasma CVD grown a-C : H : N films

Amorphous hydrogenated carbon films with different nitrogen contents (a-C : H : N) were deposited by DC plasma chemical vapour deposition of acetylene + nitrogen (0–62, vol.%) onto glass/Si/fused silica substrates. The deposition temperature was ∼523 K at a system pressure of ∼0.2 mbar and negative bias voltage of −500 V. Photoluminescence of the a-C : H : N films were studied with different excitation energies ( E ex) below and above the band gap. The optical parameters (absorption coefficient, band gap and refractive index) of a-C : H : N films were determined from the transmittance and ellipsometric measurements. Optical properties of the films could be explained by a two phase model which considers the existence of sp 2 clusters (lower band gap region) embedded in a sp 3 matrix (higher band gap region). Carrier confinement within the sp 2 clusters gives rise to radiative recombination, producing strong photoluminescence (PL) at room temperature. The film quality was ascertained from the FTIR and Raman measurements.

Plasma-induced low-temperature growth of graphitic nanofibers on nickel substrates

Graphitic nanofibers have been synthesized on Ni(1 0 0) substrates via DC plasma chemical vapor deposition using a gas mixture of hydrogen and methane. The diameter of the fibers is in the range 50–200 nm, controlled by the size of the nickel tip of each fiber. The substrate temperature during deposition was 350–400°C. No special treatment of the substrate was employed to promote the nucleation of nanofibers, though scratches on the substrate surface were found to be favorable for the nucleation of the fibers. Scanning electron microscopy, transmission electron microscopy, energy dispersive spectroscopy and Raman spectroscopy were used to characterize the fibers obtained. The effect of the plasma on the growth of the fibers low temperature is discussed.

Effects of Alcohol Addition on the Deposition of Diamond Thick Films by dc Plasma Chemical Vapor Deposition Method

Diamond films have been deposited by dc plasma chemical vapor deposition method. The addition of alcohol in the resource gas largely increases the deposition rate. The effects of alcohol addition on deposition rate and film quality are analyzed by scanning electron microscopy and Raman spectrometry. The mechanism of experimental phenomena is discussed.

Diamond synthesis by using very high frequency plasmas in parallel plate electrodes configuration

By using very high frequency (100 MHz) plasma, diamond was synthesized with only rf power in parallel plate electrodes configuration from a mixture gas of methane and hydrogen. Deposition conditions are similar to those used so far in medium pressure methods, i.e., the pressure, methane concentration, substrate temperature, and rf power used are 90–20 Torr, 5–0.5%, 900 °C, and 400–1000 kW, respectively. This plasma is very stable and the power density used is small. Furthermore, diamond formed at a pressure lower than that used in dc plasma chemical vapor deposition. These results suggest the possibility of the application of this method to large area deposition of diamond. However, the quality of the diamonds obtained are not yet very high. Methods of improvement are also discussed.