Young-Joon Baik

Growth of carbon nanotubes by chemical vapor deposition

Abstract The growth behavior of carbon nanotubes (CNT) deposited from C2H2 by thermal CVD method was investigated. Nickel particles of diameter ranging from 15 to 90 nm were used as the catalyst. CNTs were deposited in various environments of N2, H2, Ar, NH3 and their mixtures to investigate the effect of the environment on the CNT growth behavior. The deposition was performed at 850°C in atmospheric pressure. In pure N2 environment, thick carbon layer deposition occurred on the substrate without CNT growth. The Ni particles encapsulated by the carbon deposition could not work as the catalyst in this condition. However, the growth of CNT was enhanced as the H2 concentration increased in the mixture of N2 and H2 environment. In pure H2 environment, randomly tangled CNTs could be obtained. The growth of CNT was much enhanced when using NH3 as the environment gas. Vertically aligned CNTs could be deposited in NH3 environment, whereas the CNT growth could not be obtained in the mixture of N2 and H2 environment of the same ratio of N/H. These results were discussed in terms of the passivation of the catalyst caused by the excessive deposition of carbon on the catalyst surface. For the deposition of the CNT, the decomposition rate of C2H2 should be controlled to supply carbon for nanotube growth without passivation of the catalyst surface by excessive carbon deposition. The present work showed that the composition of environment gas significantly affects the reaction kinetics in the CNT growth. It is also noted that nitride surface layer formation on Ni catalyst in NH3 environment can affect the CNT growth behavior.

Aligned Diamond Nanowhiskers

We investigated the formation of nanowhiskers by means of air plasma dry etching using diamond films of two different kinds: as-grown diamond films and films with molybdenum (Mo) deposits. As for the as-grown diamond films, nanowhiskers were found to form preferentially at grain boundaries of diamond crystals. Auger depth profile analysis of the etched films revealed a progressive enrichment by Mo toward the whisker tip, resulting from accidental sputtering of Mo substrate holder. With dry etching of diamond films with preformed Mo deposits, well-aligned whiskers 100 nm in diameter were found to form uniformly over the entire film surface with a population density of 30/μm 2 . From these findings, it follows that Mo deposits serve as micromasks for the formation of the nanowhiskers. It was also confirmed that these whiskers showed excellent field-emission behavior.

Effect of fluorine addition on transparent and conducting Al doped ZnO films

Al doped ZnO (AZO) films with varying fluorine content were prepared by radio frequency magnetron sputtering at a room temperature to investigate doping effects of fluorine on the structural, the optical, and the electrical properties. The small amount of fluorine addition to AZO films resulted in beneficial effect on the electrical conductivity by improving the direct current (dc) Hall mobility, and the minimum specific resistivity was as low as 5.9×10−4Ωcm. With increasing fluorine content in AZO films, the optical absorption loss in the visible range decreased regardless of carrier concentration in the films. X-ray diffraction and scanning electron micrograph analyses showed that the crystallinity of AZO films was deteriorated by addition of fluorine. Small amount of fluorine addition to AZO film resulted in decrease of absorption loss as well as increase in Hall mobility, and the beneficial effects of fluorine addition was deduced to be caused by killing in-grain point defects. From the comparison bet...

Behaviour of Co binder phase during diamond deposition on WCCo substrate

Abstract The movement of the Co-rich binder phase and its interaction with growing diamond particles during deposition were investigated by repeated observations of the same site on a WC-Co substrate surface. Both etched and unetched (as-polished) specimens of WC-5Co were used for deposition. The diamond deposition was carried out using the tungsten filament chemical vapour deposition method. Raman spectra have shown that the quality of the diamond deposited on the etched substrate was better than that on the as-polished substrate. The facet size of the diamond film surface on the as-polished substrate was smaller than that on the etched specimen. These effects were caused by the atomic-scale interaction of Co atoms in the binder phase. A special feature of the diamond film on the as-polished specimen was a very rough diamond film surface. This phenomenon was observed to result from the abnormal movement of the Co-rich binder phase to the deposited diamond particle surface and the subsequent non-uniform growth of particles during deposition. The phenomenological characteristics of the Co-rich binder phase movement were also explained.

Effect of NH3 environmental gas on the growth of aligned carbon nanotube in catalystically pyrolizing C2H2

Abstract It has been well known that vertically aligned carbon nanotubes (CNTs) can be grown by thermal CVD in NH 3 environment. However, the mechanism of the vertically aligned CNT growth is yet to be clarified. In the present work, we investigated the effect of NH 3 gas on the CNT growth in the viewpoint of catalyst passivation. The particles of Ni or Co of diameter ranging from 17 to 58 nm were used as the catalyst. CNTs were deposited at 950°C using C 2 H 2 in various environments of NH 3 , H 2 , or their mixtures. In H 2 environment, significant catalyst passivation was observed at the C 2 H 2 concentration of 2.4 vol.% due to the excessive supply of carbon. However, vertically aligned CNTs were deposited in NH 3 environment even when the C 2 H 2 concentration was 16.7 vol.%. From the composition analysis of the catalyst surface, we could show that activated nitrogen atoms were generated by the decomposition of NH 3 . Two possible roles of the nitrogen were suggested based on the bamboo growth model. The nitrogen atoms enhance the formation of graphitic layer and/or improve the separation kinetics of the graphitic layer from the catalyst. The growth behaviors without the pre-treatment for 1 h in NH 3 environment showed that the role of the nitrogen appeared in different ways depending on the catalyst materials. In the case of Co catalyst, where vertically aligned CNT growth was observed without the pre-treatment, enhanced formation of the graphitic layer might be the significant role of the nitrogen. However, when using Ni catalyst, the pre-treatment in NH 3 environment was required for the CNT growth, which implies that both the formation and the separation of the graphitic layer were essential.

Fabrication of diamond nano-whiskers

Aligned diamond whiskers were formed by air plasma etching of polycrystalline diamond films. A small amount of Mo was deposited on the diamond substrate for use as an etch-resistant mask, and the negatively biased substrate was etched by RF or DC plasma. The shape and the density of the whiskers depended on the etching parameters, such as substrate temperature, chamber pressure, bias voltage, etching power, and the amount of Mo. If the substrate temperature was high, for example, the whiskers became thick. With the optimum conditions, we could fabricate diamond whiskers as thin as 60 nm with a density of 50/μm 2 .

Precursor gas effect on the structure and properties of diamond-like carbon films

Abstract The effect of precursor gases on the diamond-like carbon (DLC) film deposition was investigated in the r.f. plasma-assisted CVD method. DLC films were deposited using methane or benzene as the precursor gas. The residual stresses, hardnesses, total hydrogen concentrations and electron energy loss spectra were compared at the same value of V b P 1 2 . (Here, Vb is the self-bias voltage of cathode and P the deposition pressure.) The values of V b P 1 2 ranges from 33 to 250 V mTorr −1 2 for methane and from 33 to 900 V mTorr −1 2 for benzene by changing the negative bias voltage from −100 to −900 V and deposition pressure from 1 to 100 mTorr. We observed significant differences between the structures and properties of these films. In the same range of V b P 1 2 the structure and properties of films deposited from benzene show characteristic behaviors of lower energy deposition than those from methane. The present observations are discussed in terms of the difference in the ion energy per carbon atom at the growth surface. The total hydrogen concentration in the films deposited from benzene is smaller by about 7 at.% in this experimental range.

Morphology variation of diamond with increasing pressure up to 400 torr during deposition using hot filament CVD

Abstract The effect of the growth pressure and substrate temperature on diamond crystallite size was investigated during deposition by hot filament chemical vapor deposition (CVD). A methane–hydrogen gas mixture was used as the precursor gas. The gas flow rates of methane and hydrogen and the deposition time were kept constant at 4 and 100 sccm and 10 h, respectively. The growth pressure and substrate temperature were varied between 40 and 400 torr and between 1020 and 1250°C, respectively. The structure of the films was characterized by high-resolution scanning electron microscopy (HRSEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and micro-Raman spectroscopy. Typically, the diamond crystallite size decreased with increasing pressure and decreasing substrate temperature. For example, with increasing growth pressure at 1100°C, the structure of the film gradually changed from microcrystalline to nanocrystalline diamond and the non-diamond defects increased. As the substrate temperature increased at 200 torr, the structure of the film gradually changed from nanocrystalline to microcrystalline diamond.

Optical analysis of doped ZnO thin films using nonparabolic conduction-band parameters

The optical properties of impurity doped ZnO thin films were analyzed by taking into account the nonparabolicity in the conduction-band and the optically determined carrier concentration and mobility were correlated with those measured by Hall measurement. The Drude parameters obtained by applying a simple Drude model combined with the Lorentz oscillator model for the optical transmittance and reflectance spectrum were analyzed by using the carrier density dependent bare band effective mass determined by the first-order nonparabolicity approximation. The squared plasma energy multiplied by the carrier density dependent effective mass yielded fairly linear relationship with respect to the carrier concentration in wide carrier density range of 1019 − 1021 cm−3, verifying the applicability of the nonparabolicity parameter for various types of impurity doped ZnO thin films. The correlation between the optical and Hall analyses was examined by taking the ratios of optical to Hall measurements for carrier densi...

Mechanical analysis for crack-free release of chemical-vapor-deposited diamond wafers

Chemical-vapor-deposited (CVD) diamond thick films for electronic applications must be released without cracks from the substrate as freestanding wafers. In this study, the mechanism of cracking in the CVD films was investigated experimentally and theoretically. Experimental observations showed that cracks initiated at the edge of the diamond wafer and then propagated towards the center. Finite-element analysis (FEA) reveals that, during cooling, compressive thermal stresses concentrate at the thick films edge and additional tensile stress acts circumferentially. This was verified by the experimental analysis of diamond films deposited on Si, Mo and W substrates. Observations on low interfacial adhesion and crack-free film on the W substrate indicated that, in addition to the high thermal stress, low interfacial adhesion plays an important role in cracking. Thus, film cracking depends on the fracture strength of the film and its relative magnitude with respect to interfacial adhesion. Methods of crack suppression were suggested on the basis of this cracking mechanism: increase of film thickness and minimization of the substrates CTE and interfacial adhesion. The analysis was confirmed by successful suppression of cracking by application of a low-adhesion interlayer prior to deposition of diamond film.