ICMCTF2010 Session DP: Symposium D Poster Session

We report the fabrication of self-assembled single-walled carbon nanotube (SWCNT) networks grown by using catalytic-CVD collaborating buffer layer-assisted process with a gas mixture of hydrogen–methane atmosphere. A simple and reproducible method of controlled fabricating self-assembled SWCNT networks is presented in this study. No pre-patterned catalyst process is applied to achieve the lateral-grown SWCNTs during the whole procedure. The electrical and infrared (IR) sensitive properties of the SWCNT network-based detector under cooled and un-cooled modes were both investigated. Results indicate that the variation of resistivity is significantly affected by the factors of network density and surroundings. In addition, the emission energy of the SWCNT networks exhibits around 1.03 eV by using photoluminescence (PL) spectroscopy at room temperature. The finding in our work provides a framework of optical and electrical properties of the self-assembled SWCNT networks for future optical detector applications.

Recently, carbon nanotubes (CNTs) are researched to develop a high resolution x-ray image system for medical applications such as early diagnosis of cancers using the superior properties of CNTs such as chemical stability, thermal conductivity, mechanical strength, and structural aspect ratio. For use of CNT-emitters as electron source for x-ray generation, a large number of investigations have been focused on how to enhance the emission current level and to reduce the turn-on field for electron-emission. In addition, the long-term emission stability should be ensured because the emission capability of CNT-based emitters may be degraded during operation due to several reasons such as weak adhesion and ionic etching of CNTs. In order to achieve desirable performances in emission current and stability, coating of CNTs with various materials have been studied. Among the coating materials, amorphous carbon nitride (a-CN_x) thin films have been proved by our previous work to improve the emission current and stability of CNT-emitters.

In this work, the effects of thermal treatment on CNTs, which were coated with a-CN_x thin films, have been considered for the purpose of enhancing the long-term stability of CNT-emitters. The CNTs were directly grown on metal-tip (tungsten, approximately 500 nm in diameter at the summit part) substrates by inductively coupled plasma-chemical vapor deposition (ICP-CVD). The a-CN_x thin films were coated on CNTs by RF magnetron sputtering. Thermal treatment on a-CN_x coated CNT-emitters was performed using a rapid thermal annealing (RTA) system by varying temperature (300-800°C), atmosphere (N₂, O₂ or none), and process time. Morphologies and microstructures of a-CN_x films were analyzed by field-emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), and x-ray photoelectron spectroscopy (XPS). The stability property of the a-CN_x/CNTs hetero-structured emitters was measured using a high vacuum (below 10^-7 Torr) field-emission measurement system. For characterization of emission stability, the fluctuation and degradation of the emission current were monitored in terms of operation time. The results were compared with a-CN_x coated CNT-emitters that were not thermally heated as well as with the conventional non-coated CNT-emitters.

CrN/Cr bi- layers have been used recently to promote the growth of high quality Hot Filament Chemical Vapor Deposition (HFCVD) diamond coatings onto Co-cemented tungsten carbide (WC- 6wt.% Co) substrates.

Yet, adhesion and wear resistance of diamond coatings on CrN/Cr interlayers can be very poor. In the present investigation, the influence of the crystalline size of the diamond coatings on their adhesion and wear endurance is looked into.

Films of CrN/Cr were deposited onto 10x10x3 mm WC- 6wt.% Co samples using a PVD arc plant. The CrN/Cr interlayer was achieved by progressively decreasing the nitrogen concentration inside the deposition chamber from 100% to 0% of the gas phase, and increasing the argon concentration. This way, when the nitrogen was completely replaced by the argon in the gas phase, a resulting layer of metallic-Cr was superimposed on the underlying graded CrN layer.

Nano - and micro- crystalline Diamond Coatings were deposited by HFCVD onto untreated and Fluidized Bed Machined (FBM) CrN/Cr interlayers. Nanodiamond Coatings (NDC) were characterized by a smoother morphology and they showed improved wear resistance. However, the superimposition of Nanodiamond Coatings (NDC) onto CrN/Cr interlayers corrugated by Fluidized Bed Machining (FBM) was found to be the most promising choice, leading to the formation of highly adherent and wear resistant coatings.

Amorphous hydrogenated carbon (a-C:H or DLC) coatings are widely used in several industrial applications. These coatings are commonly prepared by plasma activated chemical vapor deposition (PACVD). The mainly used technique for deposition a-C:H coating in industrial scale bases on a glow discharge in a hydrocarbon gas like acetylene or methane using a substrate electrode powered with medium frequency (m.f. - some 10 to 300 kHz). A relatively new and promising technique is the deposition of a-C:H coatings by a reactive magnetron sputter deposition with acetylene as reactive gas.

Modified a-C:H-coatings can extend the range of applications. An interesting modification are the Si-modified a-C:H coatings. These a-C:H:Si (Si-DLC) coatings exhibit low coefficients of friction against steel counterpart, good wear resistance and are predicted to extend the operation temperature range to higher temperatures.

Using these two mentioned techniques a-C:H:Si coatings were prepared, characterized and compared. The chemical composition of the coatings was investigated by electron microprobe analysis. The mechanical properties like resistance against abrasive wear, plastic hardness, adhesion and friction coefficients were determined. Cross sectional SEM images showed the growth structure and surface morphology of the coatings.

Copper-doped diamond-like carbon (DLC) films with varying Cu concentrations were deposited on 7075 aluminum alloy substrates using filtered cathodic vacuum arc (FCVA) system. The incorporation of Cu into the DLC thin films have great influence on their microstructure, surface morphology, interfacial qualities, chemical composition and mechanical properties of the resulting films. It is found that the friction coefficient of the thin films is lower than 0.1 and the residual stress between the DLC thin films and aluminum alloy substrates can be substantially decreased after the effective doping of Cu into the films, which implies that the Cu-DLC films are suitable to be used as a protective coating on aluminum alloys.

The diamond-like carbon (DLC) film has been widely used in the cutting and forming industries with its high hardness, low friction coefficient, high wear resistance, and chemical inertness. In this study, Diamond-like carbon (DLC) films were synthesized by the cathodic arc evaporation (CAE) process. The unique process takes advantage of the cathodic arc plasma to trigger the chemical decomposition reaction for DLC thin films. The energetic metal plasma catalyzed the decomposition of hydrocarbon gas (C₂H₂), and induced the formation of the metal doped and hydrogenated DLC thin films. The formation mechanism of DLC was analyzed by the plasma diagnostic. The main features of the plasma diagnostic from the CAE process include Cr、CN and C when CrN transform to DLC.

The oxidation behavior of sp³-rich DLC films was investigated by using thermal gravimetric analysis (TGA) and differential scanning calorimeter (DSC). In this study, a significant weight loss results from the carbon oxidation between 330 and 460^oC. According to the TGA curve, weight gain of specimen changed only slightly as the temperature increased from 500 to 800^oC due to the thermal oxidation of underlying CrN and CrC_xN_y interlayers. The peak temperatures were read from the first derivative curve (DDSC) of the DSC curve, DDSC can observe three change-point of temperatures at 350, 390 and 440 ^oC. Finally, used TGA and DSC calculate the activation energy and the oxidation kinetics.

All the oxidation characteristics of the films were identified by transmission electron microscopy (TEM), X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS).

Since theoretical prediction of tetrahedral β-C₃N₄ by Liu and Cohen, carbon nitride has been of great interest in developing superhard and wide band-gap material. For electronic applications, carbon nitride is a promising low-κ material for multilevel interconnection of ULSI circuits because of the high resistivity and low dielectric constant. Deposition of carbon nitride has been attempted by conventional vapor deposition techniques such as chemical vapor deposition and reactive sputtering. Recently, we have attempted the liquid-phase deposition of amorphous carbon and carbon nitride to develop alternative deposition techniques for carbon related materials. In this work, we have studied electrical properties of liquid–phase deposited carbon nitride films which were deposited by the application of a DC bias voltage to Si substrates immersed in acrylonitrile (CH₂CHCN) . The apparatus used for deposition consists of a glass vessel, two electrodes, and a DC power source. A 20 × 40 mm² n-type Si (100) wafers were mounted on the both of two electrodes. Typical deposition parameters were a bias voltage of 3 kV, a current density of 1 mA/cm², a liquid temperature of 70°C, and a deposition period of 1 h. Continuous and uniform films were grown by the application of both negative and positive bias voltages. X-ray photoelectron spectra of the deposited films show the presence of C, N, and O atoms as major components in the films. Furthermore, sodium is detected for the samples deposited by the negative bias application during the 1st deposition period after changing the reactant liquid. To measure electrical properties of carbon nitride films, Al electrodes with a diameter of 1 mm were formed onto the films by using a high-vacuum evaporation system. Ohmic contact onto the backside of Si was formed to fabricate metal-insulator-semiconductor (MIS) diode. Resistivity of the film was determined to be higher than 10¹¹ Ω cm at 300 K. Capacitance and conductance of MIS diodes were measured as the functions of bias voltage at frequency between 120 Hz and 1 MHz. The samples contaminated by sodium show instable behavior in their capacitance-voltage characteristics. However, the C-V characteristics can be stabilized after bias-aging treatment, indicating that the incorporated sodium acts as mobile ionic charge in the insulating CN_x film. We attempted to prevent the mobile charge contamination of carbon nitride films, and found that sodium concentration of the film decreases rapidly by repeating the deposition period. Based on the C-V results, dielectric constants of the carbon nitride, interface trap densities, and energy diagram of MIS structure are also discussed.

Diamond-like carbon (DLC) films have several advantages in biomedical applications such as high hardness, chemical inertness and low friction. Furthermore, SS 316L substrates coated with DLC films have been reported to have a good biocompatibility and high corrosion resistance. In the present investigation, the coatings were deposited in an industrial facility onto a SS 316L substrate by means of closed field unbalanced magnetron sputter ion plating. The chemical bonding structures of the DLC films were characterized by means of Raman spectroscopy and the electrochemical behavior were evaluated by means of electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization in a 0.89% NaCl solution of pH 7.4 at a temperature of 37ºC, simulating the body fluid environment. The porosity and protective efficiency of DLC coatings were obtained using potentiodynamic polarization tests. Moreover, the delamination area and volume fraction of water uptake of DLC coatings as a function of immersion time were calculated using electrochemical impedance spectroscopy. The DLC films showed high impedance, high polarization resistance and high breakdown potential, which were attributed to the high sp³ content and the uniformity of these films. The excellent chemical inertness of the DLC films makes them promising corrosion resistant coating materials for such applications.

Resistance switching random access memory (RRAM) is one of the promising next generation memory devices. RRAM shows advantages such as highly scalable, low power operation, simple structure, and fast response. The resistive phenomenon is based on the electrically stimulated change of the resistance of metal-insulator-metal (MIM) memory cell. Various models were proposed for the reversible and reproducible resistive switching phenomenon; however, resistance switching mechanisms have not been clearly clarified. The most convincible mechanism is the formation and disruption of conductive filaments which may be composed by plenty of ionic and electronic defects. The resistance switching behavior of zirconium oxide as an insulator film was wildly investigated for resistance memory. Zirconium oxynitrides, being a mixture of ZrN, Zr₂ON₂ and ZrO₂ phases, are expected to have more defects to form the conductive filaments and therefore the mechanism of switching resistivity or the failure of the device can be better studied in this class of materials. In this study, Zr(N,O) films with a thickness of 100 nm were deposited on TiN/SiO₂/Si substrates at 450°C using unbalanced magnetron sputtering. The working gases consisting of argon and nitrogen gases were introduced at fixed flow rates. The reactive gas oxygen ranging from 0 to 1.5 sccm were used to deposit the resistive switching layer Zr(N,O) with different oxygen contents. The I-V characteristics of the TiN/Zr(N,O)/TiN device were analyzed at room temperature. Film thickness and microstructure were observed by a field-emission scanning electron microscope. The crystal structure was characterized by X-ray diffraction. The composition depth profile of the thin film was analyzed by Auger electron spectrometer. Results of this study confirmed that Zr(N,O) used as the insulator film possesses better properties of reversible and reproducible resistance switching than pure ZrO₂ thin film.

We have recently developed a novel catalytic method for synthesizing a wide variety of nanomaterials, such as a carbon nanotubes (CNTs) in organic liquids [1]. The method realized a simple, speedy and high-purity growth of carbon naomaterials in organic liquids. In this method, there is no need for a vacuum reactor. A catalytic decomposition of the organic liquid yields carbon nanomaterials. At the interface between the liquid and the catalyst-supported substrate surface, a large gradient of the temperature exists. This nonequilibrium condition at the substrate surface is a characteristic feature of the liquid phase synthesis method.

It has been interesting for us that the effect of the added sulfur in the gas phase on the structure of a new carbons grown by the chemical method. We have reported that the added H2S in the gas phase enhanced the sp3 formation both in the case of CVD diamond and the carbon nanotube [2-3]. In the case of the liquid phase growth system, how affect the sulfur on the morphology and structure of the carbon nanomaterials is also interesting in the viewpoint of realizing a controllable process for the new carbons.

In this study, we investigated the effect of sulfur on the morphology of the grown materials by using a varied mixture of 1-octanethiol (CH3(CH2)7SH; OcSH) and 1-octanol (CH3(CH2)7OH; OcOH) for a carbon source.

The mixture of OcSH and OcOH was used as an organic liquid source. The cobalt (Co) catalyst-supported silicon (Si) substrate was electrically heated at the temperature in 1073 K in organic liquid. We used thermally oxidized Co-sputtered Si substrate for the growth experiment. The substrate was heated at 1073 K for 10 min in air. The morphology of grown materials was observed by scanning electron microscopy (SEM).

Both in the case of the ratio of OcSH to the mixed organic liquids were 10 % and 50 %, the thick fibriform materials and the film-like materials were obtained. In the case of the ratio was 5 %, the obtained nanomaterials were fibriform materials. Contralry to the former case, the film-like materials was not growned. The density of the fibriform materials was increased with decreasing of the OcSH content in the mixed organic liquid.

The content of OcSH, namely, the amount of sulfur in the liquid phase resulted in a wide variety of morphologies in the grown nanomaterials with different density.

References

[1] M. N.-Gamo, et al., Jpn. J. Appl.Phys. 46 (2007) 632.

[2] M. N.-Gamo, et al., Thin Solid Films 382 (2001) 113.

[3] T. Mitsui, et al., Mat. Res. Soc. Symp. Proc. 706 (2002) 49.