Papers by Keyword: Chemical Vapor Deposition (CVD)

Abstract: SiC growth on as-received and striated Si(001) substrates was studied. SiC films were grown by pulsed-jet chemical vapor deposition using monomethylsilane as a gas source at 780°C. Two kinds of Si surfaces were prepared. One was an as-received Si(001) surface and the other was an striated (scratched) Si(001) surface. It was found that nucleation rate of SiC is quite different between these two kinds of surfaces. The film growth rate was very low for the as-received Si(001) surface compared with the striated surface, and after 8 hours of growth hardly any film was grown and only square-shaped islands were observed. On the other hand, for the undulant substrate about 100nm thick 3C-SiC film was grown after 8 hours of deposition. This film growth rate difference appears to be due to the difference in density of nucleation sites. For the as-received Si(001) surface, nucleation site density appears to be quite small due to the atomically flat surface. On the other hand, for the undulant surface, nucleation site density was large enough for the film to grow faster.

Abstract: The influence of in situ etching of Si-face n-4H-SiC wafers in H2 and propane on the surface morphology of the grown epi-layers were examined using differential interference contrast (DIC) optical microscopy and atomic force microscope (AFM). Two defect-selective etching techniques were applied in order to reveal the type and spatial distribution of defects in the substrates and epi-layers. It was found that for the flow applied in this experiment propane plays a significant role for the etching process. Depending on temperature and etching time we obtained completely different picture of substrate surface morphology. The propane etching was verified as a tool for substrate surface improvement.

Abstract: Basal Plane Dislocations (BPD) in SiC are thought to cause degradation of bipolar devices as they can trigger the formation and expansion of stacking faults during device operation. Therefore, epilayers without any BPD are strongly recommended for the achievement of long-term reliable bipolar devices. Such epilayers can be achieved by supporting the conversion of BPD into Threading Dislocations (TD), which depends on the epitaxial growth mode (as described in literature). In this work, the influence of several pre-treatments of the SiC substrate prior to epitaxial growth and different epitaxial growth parameters on the reduction of the BPDs in the SiC epilayers was investigated on 4° off-axis substrates. The dislocation content in substrates and epilayers was determined by Defect Selective Etching (DSE) in molten KOH. The averaged BPD density in epitaxial layers can be reduced to < 100 cm-2 for substrate preparation techniques and to < 30 cm-2 for well-suited epitaxial growth parameters. A certain combination of epitaxial growth parameters leads to < 3 BPD/cm2 in the epitaxial layer.

Abstract: Triangular defects and inverted pyramid type defects formed during homoepitaxial growth on 4H-SiC Si face, 4° off-cut towards [11-20] direction have been investigated. Growth parameters responsible for triangular defect formation were identified and optimized for its reduction in this study. It was found that although the high temperature reduces the density of inverted pyramid type defects, it is not the only remedy for reducing their density and cleanliness of susceptor along with the initial growth condition plays a major role in the formation of these defects.

Abstract: Diamond-like carbon (DLC) films have been deposited on silicon substrates using microwave plasma enhanced chemical vapor deposition (PE-CVD) process of CH4 in H2 gas mixture. The well-faceted good quality DLC film with distinct diamond Raman spectroscopic characteristics are found at low CH4 concentration. Schottky barrier diode (SBD) structures are fabricated onto the grown DLC films using Ti/Au and Al as ohmic and rectifying contacts, respectively. The responses of DLC-SBD to DC and time varying signals have been studied. The frequency dependent response results are compared to models, which includes as input parameters the depletion and bulk regions resistances and capacitances trap effects, and SBD parameters of which extracted from the DC I-V characteristics. It is found that the frequency dependent properties of DLC-SBD can be associated with deep trap states inside the DLC material rather than with only the SBD geometrical structure.

Abstract: Carbon nanotubes (CNTs) and helical nanofibers (HNFs) were selectively grown on copper substrates by chemical vapor deposition using acetylene as a carbon source. The experiments were carried out by using Ni, Fe and Co as single and co-catalysts which were deposited onto the substrates by a sparking method. The catalyst-coated copper substrates were heated at 750°C in a mixed-gas-flowing tube furnace, at an argon flow rate of 100 ml/min and various acetylene flow rates of 3, 5 and 10 ml/min. The larger diameter of HNFs was grown only on Ni and Ni-Fe catalysts at the acetylene flow rates of 5 and 10 ml/min whereas the uniform smaller diameter of CNTs was preferentially grown on Fe-Co and Ni-Fe catalysts at the flow rate of 3 ml/min. We suggest that Co likely prevents the formation of HNFs whereas Ni promotes.

Abstract: The coil-like shape carbon nanotubes (CNTs) were synthesized on stainless steel substrates using acetylene gas (C2H2) at 750oC under pressure of 4110−× bar by thermal chemical vapor deposition (CVD) method. The formation of catalytic nanoparticles used hydrogen (H2) as the environment gas. The scanning electron microscope (SEM) image shown the average coil diameter of CNTs is 0.8µm and 0.1 nm for the average coil pitch of CNTs. The Raman spectrum shown the defect of CNTs and indicate that the carbon samples were tubes.

Abstract: Commercially, multi-walled carbon nanotubes (MWCNTs) production methods are based on the use of transition metal catalysts such as chemical vapor deposition (CVD). Raw MWCNTs usually contain metal catalysts and other carbon impurities. A common route to eliminate these impurities is an acid treatment. In addition, this route induces cutting of MWCNTs which can control the aspect ratio of MWCNTs. The aspect ratio controlled MWCNTs can use many applications such as the electrode material, biological imaging and sensing, etc. In this study, the aspect ratio of MWCNTs was controlled using an acid treatment with a 3:1 concentrated H2SO4/HNO3 mixture by varying the treatment time. Results show that an acid treatment can control the aspect ratio of MWCNTs. The aspect ratio controlled MWCNTs were observed by TEM and Raman spectra.

Abstract: High-quality single-walled carbon nanotubes (SWNTs) are a key aspect in the emerging field nanotechnology. Although many approaches have been developed, the research on the synthesis of SWNTs is still needed. In this study, we report the synthesis of high-quality SWNTs by floating catalyst chemical vapor deposition, which employs ferrocene as the catalyst precursors. We obtained massive deposits at low temperature region. The deposits were characterized by scanning electron microscopy, transmission electron microscopy, and visual laser Raman spectroscopy. The Raman spectrum obtained from raw deposits shows clear radial breathing mode at the range from 180cm-1 to 300cm-1 and high-intensity graphite mode at 1577.7cm-1 with a shoulder at 1550.5cm-1, and almost no detectable peak around at 1545cm-1, which is induced by defects, is observed. These results indicate that the deposits are high-quality SWNTs.

Abstract: The straight silica nano-rods with a diameter of about 200nm and smooth surface were synthesized by chemical vapor deposition method at 1300°C. The as-synthesized samples were characterized by means of scanning electron microscopy, energy dispersive x-ray, and transmission electron microscopy. The results show that as-synthesized silica nano-rods have a uniform size, well-defined shape, and smooth surface. However, the morphologies and microstructures of silica nano-rods are affected by synthesis conditions, such as the concentration of the SiOx and the the deposition temperature. On the basis of these experimental results, a possible growth mechanism of silica nano-rods in this process is proposed.