Browsing by Author "Ravipati, Srikanth"

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Broadband and wide angle antireflection of sub-20 nm GaAs nanograss
(2012-06-01)
Ravipati, Srikanth
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Shieh, Jiann
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Ko, Fu-Hsiang
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Yu, Chen-Chieh
;
Chen, Hsuen-Li
GaAs nanograss with diameters of less than 20 nm has been fabricated using a simple, one-step, maskless plasma etching-based approach. GaAs nanograss exhibits remarkable broadband antireflection properties, which arise from the graded refractive index between air and the GaAs substrate by the nanograss layer. Moreover, GaAs nanograss shows less sensitivity to transverse electric polarized light and transverse magnetic polarized light over a wide range of incident angles compared to the strong variation in a bare GaAs substrate. These effects show that sub-20 nm GaAs nanograss with lengths of approximately 200 nm has an enhanced absorption of 98-100% when the incident energy is larger than the GaAs bandgap (lambda = 240-873 nm) and an enhanced absorption of 72-98% when the incident energy is less than the bandgap (lambda = 873-2400 nm). Our simple, one-step, and maskless plasma etching method opens a promising approach for the direct implementation of broad omnidirectional antireflective surfaces on solar cells and various optoelectronic devices to improve device performance.
Broadband antireflection and field emission properties of TiN-coated Si-nanopillars
(2014-08-21)
Chang, Yuan-Ming
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Ravipati, Srikanth
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Kao, Pin-Hsu
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Shieh, Jiann
;
Ko, Fu-Hsiang
Broadband antireflection and field emission characteristics of silicon nanopillars (Si-NPs) fabricated by self-masking dry etching in hydrogen-containing plasma were systematically investigated. In particular, the effects of ultrathin (5-20 nm) titanium nitride (TiN) films deposited on Si-NPs by atomic layer deposition (ALD) on the optoelectronic properties were explored. The results showed that by coating the Si-NPs with a thin layer of TiN the antirefiection capability of pristine Si-NPs can be significantly improved, especially in the wavelength range of 1000-1500 nm. The enhanced field emission characteristics of these TiN/Si-NP heterostructures suggest that, in addition to the reflectance suppression in the long wavelength range arising from the strong wavelength-dependent refractive index of TiN, the TiN-coating may have also significantly modified the effective work function at the TiN/Si interface as well.
Environmentally Stable Flexible Metal-Insulator-Metal Capacitors Using Zirconium-Silicate and Hafnium-Silicate Thin Film Composite Materials as Gate Dielectrics
(2011-08-01)
Meena, Jagan Singh
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Chu, Min-Ching
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Wu, Chung-Shu
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Ravipati, Srikanth
;
Ko, Fu-Hsiang
Fully flexible metal-insulator-metal (MIM) capacitors fabricated on 25 mu m thin polyimide (PI) substrates via the surface sol gel process using 10-nm-thick zirconium-silicate (ZrSi(x)O(y)) and hafnium-silicate (HfSi(m)O(n)) films as gate dielectrics. The surface morphology of the ZrSi(x)O(y) and HfSi(m)O(n) films were investigated using atomic force microscopy and scanning electron microscopy, which confirmed that continuous and crack-free surface growth had occurred on the PI. Both the films treated with oxygen (O(2)) plasma and annealing (ca. 250 degrees C) consisted of amorphous phase; confirmed by X-ray diffraction. We employed X-ray photoelectron spectroscopy (XPS) at high resolution to examine the chemical composition of the films subjected to various treatment conditions. The shift of the XPS peaks towards higher binding energy revealed the O(2) plasma-pretreatment followed by annealing was the most effective process to the surface oxidation at relatively low-temperature, for further passivate the grease traps and making dielectric films thermally stable. The ZrSi(x)O(y) and HfSi(m)O(n) films in sandwich-like MIM configuration on the PI substrates exhibited the low leakage current densities of 7.1 x 10(-9) and 8.4 x 10(-9) A/cm(2) at applied electric field of 10 MV/cm and maximum capacitance densities of 7.5 and 5.3 fF/mu m(2) at 1 MHz, respectively. In addition, the ZrSi(x)O(y) and HfSi(m)O(n), films in MIM capacitors showed the estimated dielectric constants of 8.2 and 6.0, respectively. Prior to use of flexible MIM capacitors in advanced flexible electronic devices; the reliability test was studied by applying day-dependent leakage current density measurements up to 30 days. These films of silicate-surfactant mesostructured materials have special interest to be used as gate dielectrics in future for flexible metal-oxide-semiconductor devices.
Fabrication and enhanced field emission properties of novel silicon nanostructures
(2010-12-01)
Ravipati, Srikanth
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Kuo, Chang-Jung
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Shieh, Jiann
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Chou, Cheng-Tung
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Ko, Fu-Hsiang
We reported the fabrication and the field emission properties of two-tier novel silicon nanostructures. First, silicon nanopillars with ordered high aspect ratio were achieved by using conventional lithographic techniques to act as the field emission sources. Second, sharp-edged well-aligned silicon nanograss was fabricated on top of the nanopillars by means of hydrogen plasma dry etching to induce the field emission characteristics. The turn-on fields were obtained as 10.5 and 14.4 V/mu m under current density of 0.01 mA/cm(2) for two-tier patterns separated by respective 5 mu m and 2 mu m spaces. The excellent field emission property from these novel nanostructures exhibited a great potential as high-performance field emitter arrays towards future nanoelectronic devices. (C) 2010 Elsevier Ltd. All rights reserved.
Facile synthetic route to implement a fully bendable organic metal-insulator-semiconductor device on polyimide sheet
(2012-05-01)
Meena, Jagan Singh
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Chu, Min-Ching
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Wu, Chung-Shu
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Liang, Jie-Chian
;
Chang, Yu-Cheng
Triblock copolymer surfactant, HO(CH2CH2O)(20)(CH2CH(CH3)O)(70)(CH2CH2O)(20)H (i.e. P123)-based nanocrystalline (nc)-TiO2 thin film had been synthesized on organic flexible polyimide (PI) sheet for their application in organic metal-insulator-semiconductor (MIS) device. The nc-TiO2 film over PI was successfully deposited for the first time by a systematic solution proceeds dip-coating method and by the assistance of triblock copolymer surfactant. The effect of annealing temperature (270 degrees C, 5 h) on the texture, morphology and time-induced hydrophilicity was studied by X-ray diffraction (XRD), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and contact angle system, respectively, to examine the chemical composition of the film and the contact angle. The surface morphology of the semiconducting layer of organic pentacene was also investigated by using AFM and XRD, and confirmed that continuous crystalline film growth had occurred on the nc-TiO2 surface over flexible PI sheet. The semiconductor-dielectric interface of pentacene and nc-TiO2 films was characterized by current-voltage and capacitance-voltage measurements. This interface measurement in cross-link MIS structured device yielded a low leakage current density of 8.7 x 10(-12) A cm(-2) at 0 to -5V, maximum capacitance of 102.3 pF at 1 MHz and estimated dielectric constant value of 28.8. Furthermore, assessment of quality study of nc-TiO2 film in real-life flexibility tests for different types of bending settings with high durability (c. a. 30 days) demonstrated a better comprehension of dielectric properties over flexible PI sheet. We expected them to have a keen interest in the scientific study, which could be an alternate opportunity to the excellent dielectric-semiconductor interface at economic and low temperature processing for large-area flexible field-effect transistors and sensors. (C) 2012 Elsevier B. V. All rights reserved.
Plasma-made silicon nanograss and related nanostructures
(2011-05-04)
Shieh, Jiann
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Ravipati, Srikanth
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Ko, Fu-Hsiang
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Ostrikov, Kostya (Ken)
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材料科學與工程學系
Plasma-made nanostructures show outstanding potential for applications in nanotechnology. This paper provides a concise overview on the progress of plasma-based synthesis and applications of silicon nanograss and related nanostructures. The materials described here include black silicon, Si nanotips produced using a self-masking technique as well as self-organized silicon nanocones and nanograss. The distinctive features of the Si nanograss, two-tier hierarchical and tilted nanograss structures are discussed. Specific applications based on the unique features of the silicon nanograss are also presented.
Ultralow Reflection from a-Si Nanograss/Si Nanofrustum Double Layers
(2013-03-25)
Ravipati, Srikanth
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Shieh, Jiann
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Ko, Fu-Hsiang
;
Yu, Chen-Chieh
;
Chen, Hsuen-Li
A double-layer nanostructure comprising amorphous Si nanograss on top of Si nanofrustums (NFs) with a total height of 680 nm exhibits ultralow reflection. Almost near-unity absorption and near-zero reflectance result in this layered nanostructure, over a broad range of wavelengths and a wide range of angles of incidence, due to the low packing density of a-Si and the smooth transition of the refractive index from the air to the Si substrate across both the nanograss and NF layers.