Electron Cyclotron Resonance Chemical Vapor Research Articles

Structural characteristics of carbon nanorods and nanotubes grown using electron cyclotron resonance chemical vapor deposition

Different types of carbonaceous materials were synthesized by electron cyclotron resonance chemical vapor deposition on Ni-coated glass substrates with radio-frequency (rf) self-biasing using a gas mixture of CH4 and Ar. Vertically aligned carbon nanorods and multiwalled carbon nanotubes (MWNTs) were obtained at the rf bias voltages of −100 and −200 V, respectively. High-resolution transmission electron microscopy indicated that the distance between two graphene layers of carbon nanorods is much larger than that of well-graphitized MWNTs having a typical value of graphite. Structural characteristics of carbon nanorods and well-graphitized MWNTs were investigated by electron-energy-loss spectra, in which the energy of π+σ plasmon peak obtained from carbon nanorods shifts to a lower value of 23.8 eV, compared to 25.5 eV from well-graphitized MWNT. Low-energy loss plasmon due to π electrons at 6 eV was not observed in carbon nanorods, but clearly defined for well-graphitized MWNTs. In addition, x-ray photoelectron spectroscopy showed that the delocalization of π electrons becomes more pronounced as the structure of carbonaceous films evolves into more crystalline phase from carbon nanowalls to well-graphitized MWNTs with increasing rf bias voltages. These results can suggest that ionic bombardment energy at high rf bias voltages provides sufficient internal energy for dehydrogenation from hydrocarbon molecules, and thus well-graphitized MWNTs can be synthesized even at low temperatures.

Effects of Deposition Temperature on the Properties of Fluorinated Amorphous Carbon Films

Fluorinated amorphous carbon films (a-C:F) were prepared at different temperatures using a microwave electron cyclotron resonance chemical vapor deposition (ECR-CVD) reactor with CHF3 and C2H2 as source gases. Films were annealed at 500°C in vacuum ambience in order to investigate the relationship of their thermal stability, optical and electrical properties with deposition temperature. Results indicate that the films deposited at high temperature have a less CFx bonding and a more cross-linking structure thus a better thermal stability. They also have a lower bandgap, higher dielectric constant and higher leakage current.

The crystalline properties of carbon nitride nanotubes synthesized by electron cyclotron resonance plasma

Carbon nitride nanotubes (CN-NT) have been synthesized by an electron cyclotron resonance chemical vapor deposition (ECR–CVD) system with a mixture of C 2H 2 and N 2 as precursors without using any catalyst. The carbon nitride nanotubes were synthesized in an anodic alumina membrane as template in which a packed array of parallel, straight and uniform channels with a diameter of approximately 50-nm and 30-μm thick exists. Samples were analyzed by field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Preliminary results showed that the properties of the deposited carbon nitride nanostructures depend on process parameters, such as deposited temperature, ratio of the precursors and microwave power. The aligned nanostructures have been verified by FESEM and HRTEM. The FTIR spectra reveal that some of the carbon atoms may have possibly been substituted by oxygen atom in the carbon nitride nanotubes. From the XPS results, N is either bonded to two C atoms (sp 2 pyridine-like type) or to three (sp 3 urotropine-like type) in the hexagonal sheets. The Raman spectrum showed that carbon nitride nanotubes have a high degree of graphitization.

The role of nitrogen in carbon nanotube formation

To examine the role of nitrogen, Co- and Ni-coated substrates were pretreated with three different gas compositions to compare the pretreated catalyst surfaces; the Fe, Co and Ni foils were subjected to carbon nanotube (CNT) growth experiments with CH 4/H 2 and CH 4/N 2 as source gases; the catalyst pretreatment plus the CNT growth experiments on Co- and Ni-coated Si substrates were carried out using both microwave plasma chemical vapor deposition and electron cyclotron resonance chemical vapor deposition (ECR-CVD) under different nitrogen-containing gases. The results show that the role of nitrogen may be summarized as follows: by comparing with hydrogen plasma, the bombardment energy of nitrogen plasma is greater. Therefore, the presence of nitrogen during CNT growth can keep the front catalyst surface clean and active to prolong surface passivation to enhance carbon bulk diffusion. The higher temperature due to higher bombardment energy of nitrogen plasma can promote agglomeration effects during catalyst pretreatment and the initial stage of CNT growth to produce larger size nano-particles. The presence of nitrogen is a favorable condition for formation of the bamboo-like CNTs, but not a necessary condition. Another favorable condition for formation of the bamboo-like CNTs is to deposit CNTs by ECR-CVD.

Surface and structural properties of ultrathin diamond-like carbon coatings

Nanoscale wear resistance, friction, and electrical conduction tests using atomic force microscope (AFM) have been conducted on ultrathin diamond-like carbon (DLC) coatings, including tetrahedral amorphous carbon (ta-C) deposited using pulsed cathodic arc (PCA) and filtered-PCA, and hydrogenated amorphous carbon (a-C:H) deposited using electron cyclotron resonance—chemical vapor deposition (ECR-CVD). The low-resistant layers at the surfaces of these thin DLC coatings were revealed by AFM-based nanowear tests. Their thickness is mainly determined by the deposition methods and does not show an obvious variation with the coating thickness decreasing from tens of nm to a few nm. The ∼3 nm ta-C coatings from PCA and filtered-PCA deposition were found to have the stable bulk structure beneath the thin (0.3–0.95 nm) surface layers. The ∼3 nm a-C:H coating from ECR-CVD had the extremely low load-carrying capacity and exhibited the evidence of coating delamination, which can be related to the thicker (1.5±0.1 nm) soft surface layers of a-C:H coatings. The results from conducting-AFM measurements indicate that a-C:H coatings have H and sp 3 C enrichment surface layers while the soft surface layers of ta-C coatings have graphite-like structure. The nanoscale friction coefficients of these thin ta-C and a-C:H coatings were compared by AFM-based lateral force microscope. The lower friction coefficient of ta-C coatings can be attributed to the existence of graphite-like surface structure.

Field emission characteristics of multiwalled carbon nanotubes grown at low temperatures using electron cyclotron resonance chemical vapor deposition

Vertically aligned multiwalled carbon nanotubes were synthesized by electron cyclotron resonance chemical vapor deposition on Ni-coated glass substrates at temperatures as low as 400 °C. Negative self-biases were induced to the substrates by radio frequency plasma to give ion bombardment to the growing surface. An increase of self-bias voltages from −50 to −200 V resulted in an evolution of the microstructures from amorphous carbon to nanorods, subsequently to nanotubes. Nanotubes grown above −150 V were more straight in morphology and better in crystallinity than nanorods grown at −100 V. In the field emission (FE) measurements, the electric fields to obtain 1 μA/cm2 were 4.6 and 11.1 V/μm for the nanorods and nanotubes grown at −100 and −200 V, respectively. The emission areas, calculated from the Fowler–Nordheim plots, were much larger in the nanorods than the nanotubes. It is considered that a larger amount of crystalline defects in nanorods plays a major role in improving their FE characteristics.

Optical emission study of CH 4+CHF 3 ECR plasma and properties of a-C:F:H films

a-C:F:H films were prepared with a mixture of CH 4 and CHF 3 gases by microwave electron cyclotron resonance chemical vapor deposition method. An actinometer optical emission spectroscopy was used to reveal the trend in the concentration of some species (H, F, C 2, CH etc.) as a function of flow ratio R {[CHF 3]/([CHF 3]+[CH 4])}. The results from Fourier transform infrared transmission spectroscopy showed the structural evolution of a-C:F:H films with variable gas flow ratios R. This structural evolution had an influence on the film optical band gap. The optical band gap decreased with the increasing flow ratios R in the polymer-like region, while it increased in the PTFE-like region. The correlation between the C 2 species density and the properties of a-C:F:H films is also discussed.

Effects of direct-current bias on the carbonization of Si(100) at low temperatures using electron cyclotron resonance chemical vapor deposition

A thin 3C–SiC layer is formed heteroepitaxially by the carbonization of Si(100) surface at low temperatures by using an ultra-high vacuum electron cyclotron resonance chemical vapor deposition system. Substrate direct-current (DC) bias was an important parameter to affect the crystallinity of the thin films, which could control the flux or total dose of irradiating species, which were involved in the formation of the film. By controlling the substrate bias, the crystallinity changes from amorphous to crystalline. That means the flux or dose of the ions in the plasma affect the crystallinity of growing thin films. In case the substrate was biased positively, Reflection high-energy electron diffraction (RHEED) showed the formation of the heteroepitaxial SiC layer. This implies the importance of the control of total dose of impinging species. Axial plasma potential profiles were measured to investigate the energy of the ions impinging on the surface. RHEED showed the formation of crystalline 3C–SiC when a defect free, clean Si surface was provided by the H 2 plasma cleaning and a positive bias voltage was applied to the substrate during carbonization process. By two-step carbonization using the proper values of bias voltage and CH 4/H 2 flow ratio, crystalline 3C–SiC could be formed at the substrate temperature of 600 °C

Characterization of protective-layer-coated LiMn 2O 4 cathode thin films

We introduced a post-annealing process for the preparation of spinel-phase LiMn 2O 4. The annealed films had proper crystallinity and surface roughness. In order to reduce the interface reaction during operation, we introduced a diamond-like carbon (DLC) film. The DLC films were deposited on LiMn 2O 4 films by using electron cyclotron resonance chemical vapor deposition (ECRCVD). A DLC-coated LiMn 2O 4 film was more stable during the charging–discharging reaction and showed higher discharge capacity over a wide voltage window than the uncoated LiMn 2O 4 film.

Effect of Ion Bombardment on Microstructures of Carbon Nanotubes Grown by Electron Cyclotron Resonance Chemical Vapor Deposition at Low Temperatures

Vertically aligned multi walled carbon nanotubes (MWNTs) were synthesized by electron cyclotron resonance chemical vapor deposition on Ni-coated glass substrates at temperatures as low as 400°C. Negative self-biases were applied to the substrates by radio-frequency (RF) plasma for ion bombardment of the growing surface. It was observed that ion bombardment by RF biasing to the substrates had a great effect upon the growth of carbon nanotubes and their morphologies. High-resolution transmission electron microscopy revealed that the degree of ordering of graphene layers in the synthesized nanotubes increased with RF bias. Raman spectroscopic analyses indicated that the shortening of C–C bonds within the graphene layers of the MWNTs occurred at larger negative biases, which seemed to result from removal of bonded hydrogen from the MWNTs by ion bombardment.

An a-C:F:H Film with High-Thermal Stability by ElectronCyclotron Resonance Chemical Vapour Deposition at Room Temperature

Using CHF3/C6H6 gases, amorphous fluorinated hydrocarbonfilms (a-C:F:H) were deposited by electron cyclotron resonance (ECR)chemical vapour deposition at room temperature. Dependence of the filmproperties on variable flow ratiosR[CHF3]/{ [CHF3]+[C6H6]} was investigated byinfrared absorption, x-ray photoelectron spectroscopy, measurements ofthe film thickness and dielectric constant. Evidence shows that the filmdeposition rate decreases linearly with the increasing flow ratios R andthe adding of C6H6 source gas to ECR plasma enhancesstructural cross-linking in the a-C:F:H films. In order to obtain goodstructural stability of these films, it is necessary to reconcileconfigurations between C-C bond and H or F terminal atoms carefully. Theexperimental results indicate that an a-C:F:H film with low dielectricconstant (k<3) and high thermal stability (>400°C) has beendeposited successfully at R = 56.3%.

Characterization of polycrystalline SiC layers grown by ECR-PECVD for micro-electro-mechanical systems

Large area (up to 4 inch) polycrystalline 3C-SiC films have been deposited by electron cyclotron resonance chemical vapor deposition technique. Crystalline and non-crystalline substrates such as (1 0 0) Si wafers, thermally oxidized Si wafers and Al 2O 3 ceramic sheets have been used, maintaining the same deposition conditions. The structural and morphological properties of the films were analyzed by means of transmission electron microscopy and X-ray diffractometry, while surface morphology was characterized by atomic force microscopy. Preliminary results on technological processes for the realization of polycrystalline SiC based micro-electro-mechanical systems are reported.

Structural Characteristics of Carbon Nanorods and Nanotubes Grown Using Electron Cyclotron Resonance Chemical Vapor Deposition

AbstractVertically aligned carbon nanorods and multi walled carbon nanotubes (MWNTs) were synthesized by electron cyclotron resonance chemical vapor deposition (ECR-CVD) on Ni- coated glass substrates with the RF-self biasing of –100 and –200 V, respectively. High-resolution transmission electron microscopy indicated that the distance between adjacent graphene layers of carbon nanorods is much larger than that of well-graphitized MWNTs. In electron-energy-loss spectra, the energy of π+σ plasmon peak for the carbon nanorod shifts towards lower value of 23.8 eV, by comparison with the well-graphitized MWNT at 25.5 eV. In addition, the π palsmon peak at 6 eV is clearly defined for the well-graphitized MWNT, but not seen for the carbon nanorod. X-ray photoelectron spectroscopy also showed that the delocalization of π electrons gets more pronounced with the structural evolvement from the carbon nanorod to the well-graphitized MWNT. Therefore, it is suggested that ionic bombardment can provide sufficient internal energy for dehydrogenation from hydrocarbon molecules, and thus, well-graphitized MWNTs could be grown even at low temperatures.

Blue photoluminescence of aSiOxNy films prepared by ECR-CVD

Amorphous silicon oxygen nitride (aSiOxNy) films are prepared by microwave electron cyclotron resonance chemical vapor deposition (ECR-CVD) method using 10% SiH4 in N2/O2 mixtures. The Fourier transform infrared absorption spectroscopy shows that the main bonding configuration of the films is SiO and SiN. If O2 is input, the dominant composition of the films is SiOx, while it is SiNx at zero O2 flow rate. A blue photoluminescence (PL) band at 460nm produced by 565eV laser excitation is observed at room temperature from these films, and the emission intensity increases with decreasing O2 flow rate. The blue PL for aSiOxNy films, mainly composed of aSiOx, comes from oxygen deficient defects in the SiOx matrix. For aSiOxNy films, mainly composed of aSiNx, its blue PL is due to the electronhole radiative recombination from the defect states in the SiNx matrix.

The effect of argon on the electron field emission properties of α-C:N thin films

Amorphous carbon nitride (α-C:N) thin films were synthesized on silicon as electron emitters by the electron cyclotron resonance chemical vapor deposition (ECR-CVD) system in which a negative dc bias was applied to the graphite substrate holder and a mixture of C 2H 2 and N 2 was used as precursors. The addition of Ar combined with the application of a negative dc bias can increase nitrogen content (N/C) measured by X-ray photoelectron spectroscopy (XPS), eliminate the dangling bonds in the film determined by Fourier transform infrared (FTIR) spectroscopy, decrease the film thickness measured by field emission scanning electron microscope (FE-SEM), increase the film roughness measured by atomic force microscope (AFM) and raise the graphitic content examined by Raman spectroscopy. The result shows that the onset emission field of α-C:N with Ar addition to the precursors can be as low as 4.5 V μm −1 compared with 9.5 V μm −1 of the film without the addition of Ar.

Feasibility studies of magnetic particle-embedded carbon nanotubes for perpendicular recording media

Nano-sized magnetic particles were successfully used as the catalysts to synthesize magnetic metal-encapsulated carbon nanotubes (CNTs) or nanoparticles on Si wafers in a microwave plasma electron cyclotron resonance chemical vapor deposition (ECR-CVD) system with CH 4 and/or H 2 as source gases. The magnetic catalyst materials, including Fe–Pt, Co–Pt, Nd 2Fe 14B, Fe and Fe–Ni, were first deposited on Si wafers by a physical vapor deposition (PVD) method, with subsequent plasma treatment for nanoparticle transformation. The main process parameters include catalyst materials, hydrogen plasma catalyst pretreatment and deposition temperature. For applications in magnetic media, the process has the following advantages: perpendicularly aligned CNTs or nanoparticles; tip-growth CNTs; well-distributed magnetic particles; detectable magnetic field in each particle; high tube number density (up to 134 Gtubes/inch 2 for Fe-assisted CNTs); favorable catalyst size; higher shape and induced anisotropy; and nanostructures that can be manipulated. The catalyst particle sizes of Fe, Nb 2Fe 14B and Fe–Pt (35–40 nm in diameter) are uniform and greater than but close to the critical optimum size or single domain size, which favor a higher coercive force. The greatest coercive force can reach 750 Oe for Fe-assisted CNTs at a deposition temperature of 715 °C, which is comparable with values reported in the literature. The coercive force difference between the vertical and horizontal directions can reach 300 Oe for Fe-assisted CNTs, and 355 Oe for Nb 2Fe 14B-assisted CNTs.

Effect of bias voltage on the formation of a-C:N nanostructures in ECR plasmas

Amorphous carbon nitride (a-C:N) nanotubes and nanofibers on porous alumina templates were synthesized by an electron cyclotron resonance chemical vapor deposition system in which a variable negative d.c. bias was applied to the substrate holder of graphite to promote the flow of ionic fluxes through the nano-channels of the alumina template in microwave excited plasmas of C 2H 2 or N 2. The aligned structures of a-C:N nanotubes or nanofibers were verified by field emission scanning electron microscopy. Transmission electron microscopy micrographs showed that a-C:N nanotubes and nanofibers were the size with a diameter of ∼100–250 nm and a length of ∼50–80 μm. The amorphous nature of the nanostructures was confirmed by the absence of crystalline phases arising from selected area diffraction patterns. X-ray photoelectron spectroscopy spectra indicated that a-C:N nanotubes and nanofibers were composed of nitrogen and carbon, and the N/C ratios could reach as high as 72%. The absorption bands between 1250 and 1750 cm −1 in Fourier transform infrared spectroscopy provided direct evidence for the presence of nitrogen atoms in the amorphous carbon network. The well-aligned a-C:N nanotubes and nanofibers are expected to have potential applications in optical, electronic and optoelectronic devices.

Ru-doped nanostructured carbon films

Pure and Ru-doped carbon films are deposited on Si (100) substrates by electron cyclotron resonance chemical vapor deposition. The films are characterized by transmission electron microscopy, electron energy loss spectroscopy, energy dispersive X-ray spectroscopy and atomic force microscopy. In both the pure and Ru-doped samples, diamond nanocrystallites are formed in amorphous carbon matrices. The Ru-doped film contains much smaller diamond nanocrystallites (approx. 3 nm) than the pure samples (approx. 11 nm). Lower surface roughnesses are observed in both pure and Ru-doped samples as compared to other reported nanocrystalline diamond films. The conductivity of the Ru-doped film is significantly higher than that of the pure film. The results show that Ru-doped nanocrystalline diamond films have unique structures and properties as compared to pure nanocrystalline diamond films or metal doped diamond-like carbon films, which may offer advantages for electrochemical, optical-window, field emission or tribological applications.

Growth of the large area horizontally-aligned carbon nanotubes by ECR-CVD

For potential applications of carbon nanotubes (CNTs) as connectors in microelectronic devices, the process to synthesize the large area horizontally-aligned CNTs on 100 mm (4 inch) Si wafers was developed, using electron cyclotron resonance chemical vapor deposition, with CH 4 and H 2 as the source gases and Co as the catalyst. The results show that vertical and horizontal CNTs can be obtained by manipulating the electric field applied on the substrate and flow direction of the gases. In the present deposition conditions, the horizontal CNTs show better field emission properties than vertical CNTs. This may be due to the blocking effect of catalysts at the tips and to the diminishment of the effective emission area from defects of vertical CNTs body.

Hydrogenated amorphous silicon carbide deposition using electron cyclotron resonance chemical vapor deposition under high microwave power and strong hydrogen dilution

We have investigated the growth of a-Si1−xCx:H using the electron cyclotron resonance chemical vapor deposition (ECR-CVD) technique, under the conditions of high microwave power and strong hydrogen (H2) dilution. The microwave power used is 900 W and a gas mixture of CH4 and SiH4 diluted in H2 is varied to give carbon (C) fractions x ranging from 0 to 1. We aim to understand the effects of these deposition conditions on the characteristics of ECR-CVD grown a-Si1−xCx:H films at different x. Their microstructure and optical properties are investigated using infrared absorption, Raman scattering, UV-visible spectrophotometry, and photothermal deflection spectroscopy. Information on the atomic fraction x is obtained with Rutherford backscattering spectrometry. The B parameter in the Tauc relation is found to decrease and the Urbach energy Eu increase with x, which are indicative of a higher degree of disorder with C incorporation. At intermediate x, the presence of Si–C bonds can be clearly seen from the IR absorption and Raman scattering results. The T peak around 1200 cm−1 is observed in the Raman spectra of the C-rich samples, with a redshift noted at increasing x. This suggests an increased presence of sp3 C–C bonds in these films, which is attributed to the high microwave power and strong H2 dilution that enhance C sp3 bonding and indirectly limit the number of C sp2 sites. This accounts for the large E04 gaps of more than 3.2 eV observed in such films, which are nearly saturated at large x, instead of exhibiting a maximum at an intermediate x as are commonly reported. Blue photoluminescence (PL) is observed, and the PL peak energies (EPL) are correlated to the E04 gap. The full width at half maximum of the PL are also correlated to the Urbach energy Eu. These results support that the PL broadening is attributed to the disorder broadening arising from the broad band tails.