Browsing by Author "Wang, W. H."

Now showing 1 - 7 of 7

Effect of substrate bias on growth and properties of carbon nanotubes deposited under no hydrogen introduction by MPCVD
(2006-04-01)
Chuang, P. K.
;
Teng, I. J.
;
Wang, W. H.
;
Kuo, C. T.
;
材料科學與工程學系
In order to examine the consequence of replacing hydrogen gas with high substrate bias, processes were successfully demonstrated to fabricate various carbon catalyst-assisted nanostructures, including carbon nanotubes (CNTs), by microwave plasma chemical vapor deposition (MPCVD), under the pure CH4 as the only source gas and without additional H-2 gas introduction. The Si wafers were first deposited with catalyst by physical vapor deposition (PVD), and then followed by H-plasma pretreatment to obtain the well-distributed catalyst particles on wafer. The pretreated specimens were then deposited in MPCVD to manipulate the various carbon nanostructures. The main process parameters include catalyst material (Co, Ni and Fe), the substrate bias (0 to - 320 V), and deposition time (1 to 20 min). The as-deposited nanostructures were characterized by SEM, HRTEM, Raman spectroscopy, and field emission I-V measurement. The results indicate that the various nanostructures on Si wafers can be manipulated under sufficient high negative substrate bias (>-200 V) without the additional hydrogen introduction. These results also improve our understanding on growth mechanisms of CNTs or other nanostrucrures, where the most of the proposed mechanisms in the literature were too much emphasis on the role of hydrogen. On important process parameters, except the substrate bias, it is interesting to note that effect of deposition time on nanostructures is essentially to extend the more CNTs into the plasma zone, which can result in different purification effect on CNTs, vary the alignment and sizes of CNTs, and expose the more nanostructures near the substrate surface. Under the present deposition 2 conditions, the best nanostructures with the best field emission properties (Turn-on field = 5 V/mu m, maximum current density > 35 mA/cm(2)) are the well-aligned CNTs with proper tube number densities, purities and sizes. (c) 2005 Elsevier B.V. All rights reserved.
Effects of sputtering pressure and Al buffer layer thickness on properties of AZO films grown by rf magnetron sputtering
(2010-08-20)
Tseng, C. H.
;
Wang, W. H.
;
Chang, H. C.
;
Chou, C. P.
;
Hsu, C. Y.
Transparent and conductive Al-doped (2 wt.%) zinc oxide (AZO) films were deposited on inexpensive soda-lime glass substrates by using rf magnetron sputtering at room temperature. This study analyzed the effects of argon sputtering pressure, which varied in the range from 0.46 to 2.0 Pa, on the morphological, electrical and optical properties of AZO films. The only (0 0 2) diffraction peak of the film were observed at 20 similar to 34.45 degrees, exhibiting that the AZO films had hexagonal ZnO wurtzite structure, and a preferred orientation with the c-axis perpendicular to the substrate. By applying a very thin aluminum buffer layer with the thickness of 2 nm, findings show that the electrical resistivity was 9.46 x 10(-4) Omega-cm, and the average optical transmittance in the visible part of the spectra was approximately 81%. Furthermore, as for 10 nm thick buffer layer, the electrical resistivity was lower, but the transmittance was decreased. (C) 2010 Elsevier Ltd. All rights reserved.
High-temperature magnetoresistance study of a magnetic tunnel junction
(2006-09-01)
Chen, D. C.
;
Yao, Y. D.
;
Chen, C. M.
;
Hung, James
;
Chen, Y. S.
The thermal stability and the spin transportation phenomenon at room temperature and 140 degrees C of a series of magnetic tunneling junctions with the structure of bottom electrode/PtMn/Pinned layer/ AlO(x)/CoFe/NiFe/top electrode have been investigated. The MR ratio decreases from 33.5% at room temperature to 29% at 140 degrees C. The MR ratio at room temperature increases roughly 0.8% after thermal treatment at temperatures above 60 degrees C. This is related to the thermal relaxation of the strains existing in the samples. (C) 2006 Elsevier B.V. All rights reserved.
Low-temperature growth mechanism of SW-NTs networks by buffer layer-assisted MPCVD
(2006-04-01)
Wang, W. H.
;
Peng, Y. R.
;
Chuang, P. K.
;
Kuo, C. T.
;
材料科學與工程學系
We present the microstructures and growth mechanism of networks of single-walled carbon nanotubes (SWNTs) fabricated by buffer layer-assisted microwave plasma chemical vapor deposition (MPCVD) at relatively low temperatures. The morphologies and bonding structures of carbon nanostructures were characterized by field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), and Raman Spectroscopy. Additionally, the surface roughness of buffer layer and its bonding structures with catalyst atom are analyzed by atomic force microscope (AFM) and X-ray photoelectron spectroscopy (XPS), respectively. Analytical results demonstrate that networks of CNTs are formed by bundles or individual SWNTs. We suggest that the formation of SWNTs networks at low temperature may follow the root-growth mechanism. The differences comparing with previous growth mechanism are that the nuclei formation resulted from the rough surface of the buffer layer, and dissolution of carbon in catalyst will be enhanced due to the surface diffusion of the buffer layer. These unique effects from the buffer layer can provide favorable conditions to grow SWNTs at relatively low process temperatures. (c) 2005 Elsevier B.V. All rights reserved.
Magnetophotoluminescence properties of Co-doped ZnO nanorods
(2009-04-13)
Lin, C. Y.
;
Wang, W. H.
;
Lee, C. -S.
;
Sun, K. W.
;
Suen, Y. W.
We present the detailed experimental results of the magnetic and optical properties of cobalt doped ZnO nanorods, especially the temperature and magnetic field dependence of photoluminescence up to 14 T. The Raman measurements indicate that our Co-doped ZnO nanorods have the same lattice constant as crystalline bulk ZnO. Sharp luminescence peaks centered at around 670 nm were observed at low temperature and their intensity decreased with increasing magnetic field. The luminescence peaks were attributed to d-d transitions in the Ligand field from the doped Co ions. We also observed a diamagnetic shift at a temperature of 1.5 K when the magnetic field was scanned from 0 to 14 T. The exciton radius of the Co-doped ZnO nanorods was deduced from the magnetophotoluminescence results.
Super growth of vertically aligned SWCNTs using self-assembled nanoparticles from CoCrPtO(x) ultra-thin film
(2007-01-01)
Wang, W. H.
;
Hong, T. H.
;
Kuo, C. T.
;
材料科學與工程學系
;
Department of Materials Science and Engineering
The self-assembly of catalytic nanoparticles by the decomposition of an as-deposited oxidized CoCrPt thin film is investigated, and the feasibility of its use in fabricating vertically aligned SWCNT films at a low synthesis temperature (similar to 600 degrees C) by microwave plasma CVD is described. The XPS results indicate that small nanoparticles with the diameters of 3-3.5 nm were formed in the explosion associated with the reduction of PtO(2) in the CoCrPtO(x) film. Cr(2)O(3) is employed to inhibit the agglomeration of nanoparticles and Co is typically involved in the dissolution and precipitation of carbon species for SWCNT growth. These small, self-assembled catalytic nanoparticles obtained from the CoCrPtO(x) ultra-thin film can be used to fabricate an extremely dense and highly oriented SWCNT film oil a silicon wafer at a temperature of similar to 600 degrees C. (c) 2006 Elsevier Ltd. All rights reserved.
Super growth of vertically aligned SWCNTs using self-assembled nanoparticles from CoCrPtOx ultra-thin film
(2007-01-01)
Wang, W. H.
;
Hong, T. H.
;
Kuo, C. T.
;
材料科學與工程學系
;
Department of Materials Science and Engineering
The self-assembly of catalytic nanoparticles by the decomposition of an as-deposited oxidized CoCrPt thin film is investigated, and the feasibility of its use in fabricating vertically aligned SWCNT films at a low synthesis temperature (similar to 600 degrees C) by microwave plasma CVD is described. The XPS results indicate that small nanoparticles with the diameters of 3-3.5 nm were formed in the explosion associated with the reduction of PtO2 in the CoCrPtOx film. Cr2O3 is employed to inhibit the agglomeration of nanoparticles and Co is typically involved in the dissolution and precipitation of carbon species for SWCNT growth. These small, self-assembled catalytic nanoparticles obtained from the CoCrPtOx ultra-thin film can be used to fabricate an extremely dense and highly oriented SWCNT film oil a silicon wafer at a temperature of similar to 600 degrees C. (c) 2006 Elsevier Ltd. All rights reserved.