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Nanogrids and Beehive-Like Nanostructures Formed by Plasma Etching the Self-Organized SiGe Islands
(2010-09-01)
Chang, Yuan-Ming
;
Jian, Sheng-Rui
;
Juang, Jenh-Yih
;
電子物理學系
;
Department of Electrophysics
A lithography-free method for fabricating the nanogrids and quasi-beehive nanostructures on Si substrates is developed. It combines sequential treatments of thermal annealing with reactive ion etching (RIE) on SiGe thin films grown on (100)-Si substrates. The SiGe thin films deposited by ultrahigh vacuum chemical vapor deposition form self-assembled nanoislands via the strain-induced surface roughening (Asaro-Tiller-Grinfeld instability) during thermal annealing, which, in turn, serve as patterned sacrifice regions for subsequent RIE process carried out for fabricating nanogrids and beehive-like nanostructures on Si substrates. The scanning electron microscopy and atomic force microscopy observations confirmed that the resultant pattern of the obtained structures can be manipulated by tuning the treatment conditions, suggesting an interesting alternative route of producing self-organized nanostructures.
Enhanced visible photoluminescence from ultrathin ZnO films grown on Si-nanowires by atomic layer deposition
(2010-09-24)
Chang, Yuan-Ming
;
Jian, Sheng-Rui
;
Lee, Hsin-Yi
;
Lin, Chih-Ming
;
Juang, Jenh-Yih
Bright room temperature visible emission is obtained in heterostructures consisting of similar to 3.5 nm thick ZnO ultrathin films grown on Si-nanowires produced by means of self-masking dry etching in hydrogen-containing plasma. The ZnO films were deposited on Si-nanowires by using atomic layer deposition (ALD) under an ambient temperature of 25 degrees C. The orders of magnitude enhancement in the intensity of the room temperature photoluminescence peaked around 560 nm in the present ZnO/Si-nanowire heterostructures is presumably due to the high aspect (surface/volume) ratio inherent to the Si-nanowires, which has, in turn, allowed considerably more ZnO material to be grown on the template and led to markedly more efficient visible emission. Moreover, the ordered nanowire structure also features an extremely low reflectance (similar to 0.15%) at 325 nm, which may further enhance the efficiency of emission by effectively trapping the excitation light.
Broadband antireflection and field emission properties of TiN-coated Si-nanopillars
(2014-08-21)
Chang, Yuan-Ming
;
Ravipati, Srikanth
;
Kao, Pin-Hsu
;
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.
Enhanced field emission characteristics in metal-coated Si-nanocones
(2013)
Chang, Yuan-Ming
;
Kao, Pin-Hsu
;
Tai, Hung-Ming
;
Wang, Hau-Wei
;
Lin, Chih-Ming
Metallic gold (Au) and platinum (Pt) thin films were deposited on silicon nanocones (Si-NCs) by sputtering to elucidate the effects of work function and conductivities on the field electron emission characteristics of surface-modified Si-NCs. The results showed that for Pt/Si-NCs and Au/Si-NCs, although the turn-on field defined at a corresponding current density of 10 mu A cm(-2) only improved from 4.20 V mu m(-1) for bare Si-NCs to 3.65 and 2.90 V mu m(-1), respectively, the emission current density measured at 5.00 V mu m(-1) was enhanced by orders of magnitude, reaching 1.82 mA cm(-2) for Au/Si-NCs. Compared to those obtained from various surface-modified Si-nanostructures, such as ZnO/Si-nanopillars and ferroelectrics/Si-nanotips, the current results represent an interesting alternative route for producing surface-modified Si-NCs that might be useful for optical and electronic applications.
Field Emission Properties of Gold Nanoparticle-Decorated ZnO Nanopillars
(2012-12-01)
Chang, Yuan-Ming
;
Lin, Man-Ling
;
Lai, Tung-Yen
;
Lee, Hsin-Yi
;
Lin, Chih-Ming
The structural and optoelectronic properties of ZnO nanopillars (ZnO-NPs) grown on Si substrates by the vapor transport deposition method were investigated. In particular, by varying the deposition duration and hence the morphology of the vertically aligned ZnO-NPs, the resultant field emission characteristics were systematically compared. In addition to identifying the advantageous field emission properties exhibited in the pencil-like ZnO-NPs, we observed that by adhering Au nanoparticles on the surface of the ZnO-NPs the turn-on field and the maximum current density can be drastically improved from 3.15 V/mu m and 0.44 mA/cm(2) at 5 V/mu m for the best ZnO-NPs to 2.65 V/mu m and 2.11 mA/cm(2) at 5 V/mu m for Au/ZnO-NPs, respectively. The enhancement of field emission characteristics that resulted from Au-nanoparticle decoration is discussed on the basis of charge-transfer-induced band structure modifications.
Fabrication and optoelectronic properties of core-shell biomimetic ZnO/Si nanoball arrays
(2012)
Chang, Yuan-Ming
;
Kao, Pin-Hsu
;
Liu, Mao-Chen
;
Lin, Chih-Ming
;
Lee, Hsin-Yi
We report an effective process scheme comprising a single-step Ag sputtering with subsequent dry etching and atomic layer deposition (ALD) process for fabricating biomimetic ZnO/Si nanoball (ZnO/Si-NB) core-shell nanostructures directly on Si substrates. The obtained ZnO/Si-NB core-shell nanostructures consist of similar to 30 nm thick ZnO films grown on Si frustums produced by means of dry etching masked by the self-assembled silver nanodots created by single-step sputtering. The ZnO films were deposited using atomic layer deposition under an ambient temperature of 200 degrees C. The photoluminescence (PL) measurements on these ZnO/Si-NB core-shell nanostructures showed that the visible range emission was almost completely absent and only the ultraviolet emission (3.28 eV peak) resulting from the free excitons was observed, indicating that the films indeed have high crystalline quality. Moreover, a dramatic improvement of the field emission performance was observed for ZnO/Si-NB core-shell nanostructures as compared to the bare Si frustum arrays. The detailed analyses on the field enhancement factor (beta value) based on the Fowler-Nordheim field emission model indicate that the effective work function of the ZnO/Si-NB core-shell nanostructures might be significantly different from that of either ZnO or Si.
Optoelectronic Properties of Density-Controlled ZnO Nanopillar Arrays
(2012-04-12)
Chang, Yuan-Ming
;
Huang, Jheng-Ming
;
Lin, Chih-Ming
;
Lee, Hsin-Yi
;
Chen, San-Yuan
An effective method of controlling the density of ZnO nanopillars (ZnO-NPs) by using the self-assembled silver nanoislands as the growth catalyst is demonstrated. We were able to vary the density of the ZnO-NPs to within the range of similar to 10-30 mu m(-2) by properly manipulating the size distribution of the silver nanoislands partially covering the Si substrates. Continuous silver film can also be used as the catalyst to facilitate the growth of ZnO islands and pillars on Si substrates, albeit with much less control of the resultant density and morphological appearances. The field emission measurements indicate that the performance of field emission for ZnO-NPs can be. improved markedly by reducing the density of the ZnO-NPs. The turn-on field of the low-density ZnO-NPs was as low as 2.39 V/mu m with a corresponding current density of 10 mu A/cm(2) and a field enhancement factor (beta value) of similar to 3500. The enhancement of field emission characteristics is attributed to the much reduced field screen effect by properly controlled density of the ZnO-NPs.
Field Emission in Vertically Aligned ZnO/Si-Nanopillars with Ultra Low Turn-On Field
(2012-03-01)
Chang, Yuan-Ming
;
Liu, Mao-Chen
;
Kao, Pin-Hsu
;
Lin, Chih-Ming
;
Lee, Hsin-Yi
An effective method of fabricating vertically aligned silicon nanopillars (Si-NPs) was realized by using the self-assembled silver (Ag) nanodots as natural metal-nanomask during dry etching process. The obtained Si-NPs were preferentially aligned along the c-axis direction. Ultrathin ZnO films (similar to 9 nm) were subsequently deposited on the Si-NPs by atomic layer deposition (ALD) to enhance the field emission property. The average diameter of the ZnO/Si-NPs is in the order of tens of nanometers, which enables efficient field emission and gives rise to marked improvement in the field enhancement factor, beta. The turn-on field defined by the 10 mu A/cm(2) current density criterion is similar to 0.74 V/mu m with an estimated beta approximate to 1.33x10(4). The low turn-on field and marked enhancement in beta were attributed to the small radius of curvature, high aspect ratio, and perhaps more importantly, proper density distribution of the ZnO/Si-NPs.
Enhanced Free Exciton and Direct Band-Edge Emissions at Room Temperature in Ultrathin ZnO Films Grown on Si Nanopillars by Atomic Layer Deposition
(2011-11-01)
Chang, Yuan-Ming
;
Shieh, Jiann
;
Chu, Pei-Yuan
;
Lee, Hsin-Yi
;
Lin, Chih-Ming
Room-temperature ultraviolet (UV) luminescence was investigated for the atomic layer deposited ZnO films grown on silicon nanopillars (Si-NPs) fabricated by self-masking dry etching in hydrogen-containing plasma. For films deposited at 200 degrees C, an intensive UV emission corresponding to free-exciton recombination (similar to 3.31 eV) was observed with a nearly complete suppression of the defect-associated broad visible range emission peak. On the other hand, for ZnO films grown at 25 degrees C, albeit the appearance of the defect-associated visible emission, the UV emission peak was observed to shift by similar to 60 meV to near the direct band edge (3.37 eV) recombination emission. The high-resolution transmission electron microscopy (HRTEM) showed that the ZnO films obtained at 25 degrees C were consisting of ZnO nanocrystals with a mean radius of 2 nm embedded in a largely amorphous matrix. Because the Bohr radius of free-exictons in bulk ZnO is similar to 2.3 nm, the size confinement effect may have occurred and resulted in the observed direct band edge electron-hole recombination. Additionally, the results also demonstrate order of magnitude enhancement in emission efficiency for the ZnO/Si-NP structure, as compared to that of ZnO directly deposited on Si substrate under the same conditions.
Effect of annealing temperature for Si0.8Ge0.2 epitaxial thin films
(2008-03-15)
Chang, Yuan-Ming
;
Dai, Ching-Liang
;
Cheng, Tsung-Chieh
;
Hsu, Che-Wei
;
機械工程學系
This study investigates the effect of annealing temperature on the Si0.8Ge0.2 epitaxial layers. The Si0.8Ge0.2 epitaxial layers were deposited by using ultrahigh vacuum chemical vapor deposition (UHVCVD) with different annealing temperatures ((400-1000 degrees C). Various measurement technologies, including high-resolution X-ray diffraction ((HRXRD), atomic force microscopy ((AFM) and interfacial adhesion tester, were used to characterize the materials properties of the SiGe epilayers. The experimental results showed that the SiGe epilayers gradually reduced lattice-mismatch to the underlying substrate as annealing temperature increased (from 400 to 800 degrees C), which resulted from a high temperature enhancing interdiffusion between the epilayers and the underlying substrate. In addition, the average grain size of the SiGe films increased from 53.3 to 58 nm with increasing annealing temperature. The surface roughness in thin film annealed at 800 degrees C was 0.46 nm. Moreover, the interfacial adhesion strength increased from 476 +/- 9 to 578 +/- 12 kg/cm(2) with increasing the annealing temperature. (c) 2007 Elsevier B.V. All rights reserved.

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