Browsing by Author "Ke, WC"

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Electrical properties of the free-standing diamond film at high voltages
(2001-05-01)
Huang, BR
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Ke, WC
;
Chen, WK
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電子物理學系
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Department of Electrophysics
Polycrystalline diamond films were deposited on P-type (100) silicon substrates by the microwave plasma chemical vapor deposition system. The free-standing diamond film was then obtained by etching the silicon substrate with a KOH I solution. It was found that more non-diamond components, i.e., SiC and amorphous carbon, existed on the bottom surface of the free-standing diamond film. Two different contact geometries, coplanar contact and sandwich contact geometries, were used to characterize the in-plane and transverse hi.-h-voltage electrical properties of the free-standing diamond film, respectively. The transverse electrical property of the free-standing diamond film showed the asymmetric current-voltage (1-V) characteristic and lower break-down voltage at -220 V and 850 V. However, the in-plane electrical property exhibited the symmetric I-V characteristic in the range of - 1100 V to 1100 V. The electrical properties were successfully represented by the Frenkel-Poole conduction mechanism at high voltages (> 200 V). The simulated results indicated that the breakdown field was strongly related to the Coulombic center density of the free-standing diamond film.
Formation of self-organized GaN dots on Al0.11Ga0.89N by alternating supply of source precursors
(2004-06-15)
Ke, WC
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Huang, HY
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Ku, CS
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Yen, KH
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Lee, L
The self-organized GaN dot structure is successfully grown on a slightly lattice-mismatched Al0.11Ga0.89N epilayer using flow-rate modulation epitaxy (FME) growth technique. From the variation of dot density with growth temperature, we can observe that the GaN dot growth is controlled predominately by the surface diffusion of Ga adatoms at substrate temperatures below 915degreesC and by re-evaporation at higher temperatures. Because of the special alternating gas supply feature in FME, during the Ga source step, it is the Ga metal that is deposited on the underlying Al0.11Ga0.89N layer. This is because of the large lattice mismatch of 41.8% between the Ga metal (4.51 Angstrom) and Al0.11Ga0.89N (3.18 Angstrom). We consider that the GaN dot growth in our study is mainly through the Volmer-Weber growth mode, not the commonly used Stranski-Krastanow growth mode.
Microphotoluminescence spectra of hillocks in Al0.11Ga0.89N films
(2004-10-11)
Ke, WC
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Ku, CS
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Huang, HY
;
Chen, WC
;
Lee, L
The spatial variation of the optical properties of hillocks in Al0.11Ga0.89N films has been studied by using microphotoluminescence (mu-PL) microscopy. The mu-PL spectrum revealed a strong emission (I-H) at 351 nm from the hillock, besides the near-band-edge emission (I-nbe) at 341 nm. Moreover, the I-H intensity increases significantly and its full width at half maximum decreases from similar to76 to similar to53 meV by probing across the hillock center. These indicated that the hillock structure is a strong emission center. The temperature-dependent mu-PL, measurements showed that the I-H also has the S-shape behavior with a transition temperature of similar to120 K which is lower than that of I-nbe. The redshift of I-H is also smaller than I-nbe. Both indicated that the Al composition in hillocks is lower than the surrounding area. (C) 2004 American Institute of Physics.
Near-field optical microscopy and scanning Kelvin microscopy studies of V-defects on AlGaN/GaN films
(2004-10-04)
Ku, CS
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Peng, JM
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Ke, WC
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Huang, HY
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Tang, NE
AlxGa1-xN thin film was grown on undoped GaN/sapphire (0001) substrate by metalorganic chemical vapor deposition. V-defects were directly observed by atomic force microscopy (AFM) with various size of 0.5-2 mum in diameter. In a previous study, the microphotoluminescence spectra showed an extra peak (I-v=350 nm) inside the V-defect besides the near-band-edge emission (I-nbe=335 nm). To achieve better spatial resolution, we used near-field scanning optical microscopy (NSOM) and scanning Kelvin-force microscopy (SKM) to probe the V-defect in detail. The NSOM spectra showed that the intensity of the I-v band increased gradually from V-defect edges to its center, while I-nbe remained unchanged. Besides, the SKM measurements revealed that the Fermi level decreased from the flat region to V-defect center by about 0.2 eV. These results suggest that the I-v band could be related to shallow acceptor levels, likely resulting from V-Ga defects. (C) 2004 American Institute of Physics.
Spatially-resolved photoluminescence studies of V-shaped pits on Al0.16Ga0.84N
(2004-02-15)
Huang, HY
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Ku, CS
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Ke, WC
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Tang, NE
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Peng, JM
We have studied optical properties of V-shaped pits on Al0.16Ga0.84N. The microphotoluminescence spectrum from the pit center shows a broader and stronger emission at 350 nm than the near-band-edge emission at 336 nm from nonpit regions. The results indicated specific defect levels associated with the V-shaped pits. Furthermore, after using atomic force microscopy to probe the surface electrical potential with a conductive tip, the pit's potential was similar to0.2 V lower than its surrounding region. A simplified energy diagram is tentatively proposed to interpret our observation. (C) 2004 American Institute of Physics.
Successive current-voltage measurements of a thick isolated diamond film
(2001-11-01)
Huang, BR
;
Ke, WC
;
Hsu, JF
;
Chen, WK
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電子物理學系
Polycrystalline diamond films were deposited on p-type (100) silicon substrate using a methane/hydrogen gas mixture in a microwave plasma-assisted chemical vapor deposition system. After the back-etched process, the Al contacts were evaporated on both sides of a 150 mum thick isolated diamond film for consecutive high-voltage measurements. It was found that the current-voltage (I-V) characteristics of the Al/diamond/Al structure exhibited two Schottky barrier diodes in a back-to-back configuration. Since the top diamond surface possessed better diamond quality than the bottom surface, the top Schottky diode with a breakdown voltage of 897 V and a lower breakdown voltage of -515 V for the bottom Schottky diode was observed for the first I-V measurement. However, the breakdown voltage was decreased by 37 and 140 V for the top and bottom Schottky diodes after the consecutive sixth repeated measurements. It was found that the oxygenated phenomenon was more prominent; in addition, the quality of the isolated diamond film was also degraded after the consecutive high-voltage measurements. It was indicated that decrease of the breakdown voltage was due to the oxidation layer and the non-diamond components on both surfaces of the isolated diamond film. (C) 2001 Elsevier Science B.V. All rights reserved.