Browsing by Author "Chang, Jung-Hung"

Now showing 1 - 3 of 3

Catalytically solid-phase self-organization of nanoporous SnS with optical depolarizability
(2016)
Cheng, Chih-Hsien
;
Chi, Yu-Chieh
;
Wu, Chung-Lun
;
Lin, Chun-Jung
;
Tsai, Ling-Hsuan
The catalytic solid-phase synthesis of self-organized nanoporous tin sulfide (SnS) with enhanced absorption, manipulative transmittance and depolarization features is demonstrated. Using an ultralow radio-frequency (RF) sputtering power, the variation of the orientation angle between the anodized aluminum oxide (AAO) membrane and the axis of the sputtered ion beam detunes the catalytically synthesized SnS from nanorod to nanoporous morphology, along the sidewall of the AAO membrane. The ultraslow catalytic sputtering synthesis on the AAO at the RF plasma power of 20 W and the orientation angle of 0 degrees regulates the porosity and integrality of nanoporous SnS, with average pore diameter of 80-150 nm. When transferring from planar to nanoporous structure, the phase composition changes from SnS to SnS2-Sn2S3, and the optical bandgap shrinks from 1.43 to 1.16 eV, due to the preferred crystalline orientation, which also contributes to an ultralow reflectance of <1% at 200-500 nm when both the transmittance and the surface scattering remain at their maxima. The absorption coefficient is enhanced by nearly one order of magnitude with its minimum of >5 x 10(4) cm(-1) at the wavelength between 200 and 700 nm, due to the red-shifting of the absorption spectrum to at least 100 nm. The catalytically self-organized nanoporous SnS causes strong haze and beam divergence of 20 degrees-30 degrees by depolarized nonlinear scattering at the surface, which favors the solar energy conversion with reduced surface reflection and enhanced photon scattering under preserved transmittance.
Growing GaN LEDs on amorphous SiC buffer with variable C/Si compositions
(2016-01-22)
Cheng, Chih-Hsien
;
Tzou, An-Jye
;
Chang, Jung-Hung
;
Chi, Yu-Chieh
;
Lin, Yung-Hsiang
The epitaxy of high-power gallium nitride (GaN) light-emitting diode (LED) on amorphous silicon carbide (a-SixC1-x) buffer is demonstrated. The a-SixC1-x buffers with different nonstoichiometric C/Si composition ratios are synthesized on SiO2/Si substrate by using a low-temperature plasma enhanced chemical vapor deposition. The GaN LEDs on different SixC1-x buffers exhibit different EL and C-V characteristics because of the extended strain induced interfacial defects. The EL power decays when increasing the Si content of SixC1-x buffer. The C-rich SixC1-x favors the GaN epitaxy and enables the strain relaxation to suppress the probability of Auger recombination. When the SixC1-x buffer changes from Si-rich to C-rich condition, the EL peak wavelengh shifts from 446 nm to 450 nm. Moreover, the uniform distribution contour of EL intensity spreads between the anode and the cathode because the traping density of the interfacial defect gradually reduces. In comparison with the GaN LED grown on Sirich SixC1-x buffer, the device deposited on C-rich SixC1-x buffer shows a lower turn-on voltage, a higher output power, an external quantum efficiency, and an efficiency droop of 2.48 V, 106 mW, 42.3%, and 7%, respectively.
Strong optical nonlinearity of the nonstoichiometric silicon carbide
(2015-01-01)
Cheng, Chih-Hsien
;
Wu, Chung-Lun
;
Lin, Yung-Hsiang
;
Yan, Wen-Long
;
Shih, Min-Hsiung
Enhanced nonlinear refractive indices and absorption coefficients of nonstoichiometric SixC1-x with varying C/Si composition ratios from 0.51 to 1.83 grown by low-temperature plasma-enhanced chemical vapor deposition are demonstrated. When increasing the [CH4]/[CH4 + SiH4] fluence ratio from 70% to 92%, two Raman scattering signals at 795 and 970 cm(-1) contributed by transverse and longitudinal optical modes of Si-C bonds are simultaneously enhanced in stoichiometric and C-rich SixC1-x samples, whereas the Si-associated Raman peak at 520 cm(-1) decreases with increasing C/Si composition ratio. The C-rich SixC1-x film shows additional Raman scattering peaks at 1330 and 1580 cm(-1), which are individually contributed by the diamond-like C-C bonds and the G peak of the graphite-like C-C bonds, respectively. The nonlinear refractive index increases to 1 +/- 0.1 x 10(-11) cm(2) W-1 upon increasing the C/Si composition ratio up to 1.83, which is four orders of magnitude higher than that of bulk SiC. The increased C/Si composition ratio reduces the lattice constant and the effective mass of the SixC1-x film and enhances its nonlinear refractive index effectively. In comparison with the sp(3)-orbital C-C bond, the sp(2)-orbital C-C bond in SixC1-x films dominates the enhancement of nonlinear refractive index. By contrast, the nonlinear absorption coefficient changes sign and decreases magnitude when transferring the SixC1-x film from Si-rich to C-rich conditions. This sign reversal is primarily attributed to the conversion in the absorption mechanism from two-photon absorption to nonlinear saturable absorption. Unlike Si-Si bonds which favor two-photon absorption, the sp(2)-orbital C-C bonds with delocalized pi-electrons essentially enhance the saturable absorbance. Hence, a higher nonlinear refractive index and a higher saturable absorption coefficient can be obtained by enriching the sp(2)-orbital C-C bonds in C-rich SixC1-x films.