Browsing by Author "Cheng, Hsyi-En"

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Atomic Layer Deposition TiO2 Films and TiO2/SiNx Stacks Applied for Silicon Solar Cells
(2016-08-01)
Yang, Zu-Po
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Cheng, Hsyi-En
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Chang, I-Hsuan
;
Yu, Ing-Song
;
光電系統研究所
Titanium oxide (TiO2) films and TiO2/SiNx stacks have potential in surface passivation, anti-reflection coatings and carrier-selective contact layers for crystalline Si solar cells. A Si wafer, deposited with 8-nm-thick TiO2 film by atomic layer deposition, has a surface recombination velocity as low as 14.93 cm/s at the injection level of 1.0 x 1015 cm(-3). However, the performance of silicon surface passivation of the deposited TiO2 film declines as its thickness increases, probably because of the stress effects, phase transformation, atomic hydrogen and thermal stability of amorphous TiO2 films. For the characterization of 66-nm-thick TiO2 film, the results of transmission electron microscopy show that the anatase TiO2 crystallinity forms close to the surface of the Si. Secondary ion mass spectrometry shows the atomic hydrogen at the interface of TiO2 and Si which serves for chemical passivation. The crystal size of anatase TiO2 and the homogeneity of TiO2 film can be deduced by the measurements of Raman spectroscopy and spectroscopic ellipsometry, respectively. For the passivating contacts of solar cells, in addition, a stack composed of 8-nm-thick TiO2 film and a plasma-enhanced chemical-vapor-deposited 72-nm-thick SiNx layer has been investigated. From the results of the measurement of the reflectivity and effective carrier lifetime, TiO2/SiNx stacks on Si wafers perform with low reflectivity and some degree of surface passivation for the Si wafer.
Direct probe of heterojunction effects upon photoconductive properties of TiO(2) nanotubes fabricated by atomic layer deposition
(2010-06-04)
Chang, Yung-Huang
;
Liu, Chien-Min
;
Tseng, Yuan-Chieh
;
Chen, Chih
;
Chen, Chia-Chuan
This study investigated Schottky- and ohmic-contact effects upon the photoresponses of ITO/TiO(2)/Si and Ti/TiO(2)/Si nanotube-based photodiodes. The TiO(2) tube arrays were fabricated by atomic layer deposition (ALD) and shaped by an anodic aluminum oxide (AAO) template on a p-type Si substrate. The contact area between the electrode (Ti or ITO) and the TiO(2)'s tip was varied by tuning the tube's inner wall thickness with ALD, providing a direct and systematic probe of the heterojunction effects upon the photodiodes' responses. Results show that the Ti/TiO(2)/Si diode exhibits a highly thickness-dependent photoresponse. This is because the photocurrent is driven by the p-n junction at TiO(2)/Si alone and it faces no retarding at the ohmic contact of Ti/TiO(2). For the ITO/TiO(2)/Si diode, the Schottky contact at ITO/TiO(2) regulates photocurrent overriding TiO(2)/Si as a result of higher efficiency in photogeneration, leading to the opposite response compared with the Ti/TiO(2)/Si diode. Respective energy band diagrams are provided to support the statements above, and a consistent picture is obtained for both time response and quantum efficiency measurements.
Direct probe of heterojunction effects upon photoconductive properties of TiO2 nanotubes fabricated by atomic layer deposition
(2010-06-04)
Chang, Yung-Huang
;
Liu, Chien-Min
;
Tseng, Yuan-Chieh
;
Chen, Chih
;
Chen, Chia-Chuan
This study investigated Schottky- and ohmic-contact effects upon the photoresponses of ITO/TiO2/Si and Ti/TiO2/Si nanotube-based photodiodes. The TiO2 tube arrays were fabricated by atomic layer deposition (ALD) and shaped by an anodic aluminum oxide (AAO) template on a p-type Si substrate. The contact area between the electrode (Ti or ITO) and the TiO2's tip was varied by tuning the tube's inner wall thickness with ALD, providing a direct and systematic probe of the heterojunction effects upon the photodiodes' responses. Results show that the Ti/TiO2/Si diode exhibits a highly thickness-dependent photoresponse. This is because the photocurrent is driven by the p-n junction at TiO2/Si alone and it faces no retarding at the ohmic contact of Ti/TiO2. For the ITO/TiO2/Si diode, the Schottky contact at ITO/TiO2 regulates photocurrent overriding TiO2/Si as a result of higher efficiency in photogeneration, leading to the opposite response compared with the Ti/TiO2/Si diode. Respective energy band diagrams are provided to support the statements above, and a consistent picture is obtained for both time response and quantum efficiency measurements.
Effect of Geometric Nanostructures on the Absorption Edges of 1-D and 2-D TiO2 Fabricated by Atomic Layer Deposition
(2013-05-08)
Chang, Yung-Huang
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Liu, Chien-Min
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Cheng, Hsyi-En
;
Chen, Chih
;
材料科學與工程學系
2-Dimensional (2-D) TiO2 thin films and 1-dimensional (1-D) TiO2 nanotube arrays were fabricated on Si and quartz substrates using atomic layer deposition (ALD) with an anodic aluminum oxide (AAO) template at 400 degrees C. The film thickness and the tube wall thickness can be precisely controlled using the ALD approach. The intensities of the absorption spectra were enhanced by an increase in the thickness of the TiO2 thin film and tube walls. A blue-shift was observed for a decrease in the 1-D and 2-D TiO2 nanostructure thicknesses, indicating a change in the energy band gap with the change in the size of the TiO2 nanostructures. Indirect and direct interband transitions were used to investigate the change in the energy band gap. The results indicate that both quantum confinement and interband transitions should be considered when the sizes of 1-D and 2-D TiO2 nanostructures are less than 10 nm.
Enhancement of hydrogen evolution reaction by Pt nanopillar-array electrode in alkaline media and the effect of nanopillar length on the electrode efficiency
(2019-11-15)
Cheng, Hsyi-En
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Li, Wen-Lung
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Yang, Zu-Po
;
光電系統研究所
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Institute of Photonic System
Pt nanopillar-array 3D electrodes with nanopillar length of 150, 450 and 900 nm and nanopillar density of similar to 10(9) cm(-2) were fabricated. Their catalytic activity for hydrogen evolution reaction (HER) was evaluated by linear sweep voltammetry and electrochemical impedance spectroscopy. In comparison with straightly electrodeposited black Pt film and forged Pt sheet electrodes, the HER current density has been significantly improved by the nanopillar-array architecture. The overpotential of HER at current density of 10 mA cm(-2) at 26 degrees C is as low as 78 mV, lower than the black Pt film of 107 mV and the Pt sheet of 174 mV. The improvement of HER is ascribed to the low charge transfer resistance of the 3D electrode and the high desorption capability of hydrogen bubbles at the nanotips. Interestingly, the nanopillar-array 3D electrode has an optimal nanopillar length for HER. The mechanisms for the optimal nanopillar length were investigated here. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Growth Mechanism of TiO(2) Nanotube Arrays in Nanopores of Anodic Aluminum Oxide on Si Substrates by Atomic Layer Deposition
(2011-01-01)
Liu, Chien-Min
;
Chen, Chih
;
Cheng, Hsyi-En
;
材料科學與工程學系
;
Department of Materials Science and Engineering
In this study, we combined atomic layer deposition and anodic aluminum oxide (AAO) on a silicon substrate and developed self-aligned TiO(2) nanotube arrays. We studied the growth mechanism of TiO(2) nanotubes on the inner wall of AAO at 100 and 400 degrees C. We found that at 100 degrees C, TiO(2) grew in a layer-by-layer manner. Therefore, it can be grown into TiO(2) nanotube arrays with very thin walls. However, at 400 degrees C, TiO(2) needs to first form a 2.5-nm amorphous layer, before becoming crystalline TiO(2) via a phase transformation, and growing into crystalline TiO(2) nanotube arrays along the preferred plane {101} by means of a space-limited growth mechanism. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3528939] All rights reserved.
Growth Mechanism of TiO2 Nanotube Arrays in Nanopores of Anodic Aluminum Oxide on Si Substrates by Atomic Layer Deposition
(2011-01-01)
Liu, Chien-Min
;
Chen, Chih
;
Cheng, Hsyi-En
;
材料科學與工程學系
;
Department of Materials Science and Engineering
In this study, we combined atomic layer deposition and anodic aluminum oxide (AAO) on a silicon substrate and developed self-aligned TiO2 nanotube arrays. We studied the growth mechanism of TiO2 nanotubes on the inner wall of AAO at 100 and 400 degrees C. We found that at 100 degrees C, TiO2 grew in a layer-by-layer manner. Therefore, it can be grown into TiO2 nanotube arrays with very thin walls. However, at 400 degrees C, TiO2 needs to first form a 2.5-nm amorphous layer, before becoming crystalline TiO2 via a phase transformation, and growing into crystalline TiO2 nanotube arrays along the preferred plane {101} by means of a space-limited growth mechanism. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3528939] All rights reserved.
Improving color saturation of blue light-emitting electrochemical cells by plasmonic filters
(2017-12-01)
Chiang, Chien-Ming Fan
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Chang, Bo-Ren
;
Lee, Ya-Ju
;
Sarma, Monima
;
Yang, Zu-Po
In consideration of the advantages of light-emitting electrochemical cells (LECs), it is desired to develop the saturated blue LECs for LEC display, which is hindered by the features of broad emission spectrum and emission peak not short enough. In this study, we demonstrated a novel method to improve blue saturation of the sky-blue LECs by engineering its emission spectrum through the plasmonic filters. These plasmonic filters composed of randomly distributed silver nanoparticles (Ag-NPs) can absorb the green and red emission tail of the sky-blue LECs due to localized surface plasmon resonance (LSPR). The LSPR wavelengths of Ag-NPs are tuned by manipulating the effective refractive index of materials around Ag-NPs through the accurate control of the TiO2 thickness using atomic layer deposition technique. By integrating with the plasmonic filters, the CIE1931 coordinate of the blue LECs can approach to (0.14, 0.22), which is comparable to or even better than the reported bluest values of blue LECs. Combination with the green and red LECs, the color gamut increases from 34% (without filters) to 54% of National Television System Committee (NTSC) color gamut, corresponding to 1.6 times enhancement. In addition, the blue LECs integrated with plasmonic filters still have better efficiency than those of the reported bluest LECs. (C) 2017 Elsevier B.V. All rights reserved.
Single material TiO2 thin film by atomic layer deposition for antireflection and surface passivation applications on p-type c-Si
(2018-09-01)
Chen, Tsung-Cheng
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Yang, Tsuo-Chuan
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Cheng, Hsyi-En
;
Yu, Ing-Song
;
Yang, Zu-Po
A thin film deposited on the front surface of solar cell plays an important role in reducing the reflection of incident light and providing surface passivation. Although ultrathin TiO2 films have shown excellent performance of surface passivation, simultaneously serving both functions is hindered by the crystallization issue, which can degrade the passivation quality of TiO2 film, as the film thickness is over certain value (few nanometer only). Here we showed that both functions can be satisfied by single material of titanium oxide film through atomic layer deposition at low temperatures. By varying deposition temperatures from 80 to 200 degrees C, the TiO2 film deposited at 80 degrees C can maintain its amorphous phase up to the thickness of 114 nm. In addition, the optimal thickness for antireflection calculated by OPAL 2 is 61 nm, meaning that amorphous TiO2 film to provide antireflection and surface passivation can be achieved. The passivation quality is verified by minority carrier lifetime measurement using photo-conductance decay method and presented the ultralow surface recombination velocity of 8.7 cm/s. Furthermore, the passivation mechanism is investigated by X-ray diffraction (XRD) and secondary ion mass spectroscopy (SIMS), in which the absence of crystalline issue is confirm and both chemical passivation (due to H termination) and field effect passivation (due to residual Cl atoms) are associated with the improvement of passivation quality. A post deposition anneal is conducted on the TiO2 film deposited at 80 degrees C. The results present that extra thermal budgets can ruin the passivation quality, which is explicated by the TiO2 crystallization as the temperature exceeding 350 degrees C from the XRD measurements, and by the descending of Cl atom (also field effect passivation) as increasing of the annealing temperature from the SIMS measurements. (C) 2018 Elsevier B.V. All rights reserved.
Strongly Enhancing Photocatalytic Activity of TiO2 Thin Films by Multi-Heterojunction Technique
(2018-10-01)
Cheng, Hsyi-En
;
Hung, Chi-Hsiu
;
Yu, Ing-Song
;
Yang, Zu-Po
;
光電系統研究所
The photocatalysts of immobilized TiO(2 )film suffer from high carrier recombination loss when compared to its powder form. Although the TiO2 with rutile-anatase mixed phases has higher carrier separation efficiency than those with pure anatase or rutile phase, the single junction of anatase/rutile cannot avoid the recombination of separated carriers at the interface. In this study, we propose a TiO2/SnO2/Ni multi-heterojunction structure which incorporates both Schottky contact and staggered band alignment to reduce the carrier recombination loss. The low carrier recombination rate of TiO2 film in TiO2/SnO2/Ni multi-heterojunction structure was verified by its low photoluminescence intensity. The faster degradation of methylene blue for TiO2 /SnO2/Ni multi-junctions than for the other fabricated structures, which means that the TiO2 films grown on the SnO2/Ni/Ti coated glass have a much higher photocatalytic activity than those grown on the blank glass, SnO2-coated and Ni/Ti-coated glasses, demonstrated its higher performance of photogenerated carrier separation.
Substrate-insensitive atomic layer deposition of plasmonic titanium nitride films
(2017-03-01)
Yu, Ing-Song
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Cheng, Hsyi-En
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Chang, Chun-Chieh
;
Lin, Yan-Wei
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Chen, Hou-Tong
The plasmonic properties of titanium nitride (TiN) films depend on the type of substrate when using typical deposition methods such as sputtering. Here we show atomic layer deposition (ALD) of TiN films with very weak dependence of plasmonic properties on the substrate, which also suggests the prediction and evaluation of plasmonic performance of TiN nanostructures on arbitrary substrates under a given deposition condition. Our results also observe that substrates with more nitrogen-terminated (N-terminated) surfaces will have significant impact on the deposition rate as well as the film plasmonic properties. We further illustrate that the plasmonic properties of ALD TiN films can be tailored by simply adjusting the deposition and/or post-deposition annealing temperatures. Such characteristics and the capability of conformal coating make ALD TiN films on templates ideal for applications that require the fabrication of complex 3D plasmonic nanostructures. (C) 2017 Optical Society of America
Surface Passivation and Antireflection Behavior of ALD TiO2 on n-Type Silicon for Solar Cells
(2013)
Yu, Ing-Song
;
Wang, Yu-Wun
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Cheng, Hsyi-En
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Yang, Zu-Po
;
Lin, Chun-Tin
Atomic layer deposition, a method of excellent step coverage and conformal deposition, was used to deposit TiO2 thin films for the surface passivation and antireflection coating of silicon solar cells. TiO2 thin films deposited at different temperatures (200 degrees C, 300 degrees C, 400 degrees C, and 500 degrees C) on FZ n-type silicon wafers are in the thickness of 66.4nm +/- 1.1 nm and in the form of self-limiting growth. For the properties of surface passivation, Si surface is effectively passivated by the 200 degrees C deposition TiO2 thin film. Its effective minority carrier lifetime, measured by the photoconductance decay method, is improved 133% at the injection level of 1 x 10(15) cm(-3). Depending on different deposition parameters and annealing processes, we can control the crystallinity of TiO2 and find low-temperature TiO2 phase (anatase) better passivation performance than the high-temperature one (rutile), which is consistent with the results of work function measured by Kelvin probe. In addition, TiO2 thin films on polished Si wafer serve as good ARC layers with refractive index between 2.13 and 2.44 at 632.8 nm. Weighted average reflectance at AM1.5G reduces more than half after the deposition of TiO2. Finally, surface passivation and antireflection properties of TiO2 are stable after the cofire process of conventional crystalline Si solar cells.
The Differences in Optical Characteristics of TiO2 and TiO2/AAO Nanotube Arrays Fabricated by Atomic Layer Deposition
(2012)
Chang, Yung-Huang
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Liu, Chien-Min
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Chen, Chih
;
Cheng, Hsyi-En
;
Lu, Tien-Chang
Self-aligned TiO2 nanotube arrays can be fabricated by atomic layer deposition (ALD) and anodic oxide template (AAO) on Si or quartz substrates. In this study, we compare the hetero-junction effects on optical characteristics of TiO2 nanotube arrays for the cases with AAO stayed intact and after AAO was etched away selectively. Due to the space confinement effect that can occur only in the TiO2/AAO nanotubes, the photoluminance intensity of TiO2 material is enhanced about 5 times for TiO2/AAO nanotubes compared with the TiO2 nanotubes. On the contrary, under 0 V bias, quantum efficiency of the TiO2/AAO nanotubes is 5.8 times lower than the TiO2 nanotubes because of higher probability for the recombination of photon-generated electron-hole pairs from the space confinement effect in TiO2/AAO nanotube arrays. When the bias reached 1 V, the TiO2 nanotubes still revealed more outstanding photon-to-electron transformation properties than the TiO2/AAO nanotubes. However, the quantum efficiency of TiO2/AAO nanotubes began to exceed TiO2 nanotubes when the bias is over 2 V. This phenomenon could result from the restraint of the recombination of photo-generated electron-hole pairs under high field strength and the increase in an amount of photo-generated carriers injected from AAO template. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.045205jes]
The Effect of Geometric Structure on Photoluminescence Characteristics of 1-D TiO2 Nanotubes and 2-D TiO2 Films Fabricated by Atomic Layer Deposition
(2012)
Chang, Yung-Huang
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Liu, Chien-Min
;
Chen, Chih
;
Cheng, Hsyi-En
;
材料科學與工程學系
TiO2 thin films and TiO2 nanotube arrays were fabricated on Si and quartz substrates by atomic layer deposition (ALD) with the application of anodic aluminum oxide (AAO) template at 400 degrees C. In particular, the thickness of the film and the wall thickness of the nanotubes can be controlled precisely using ALD technique. The photoluminescence (PL) characteristic of TiO2 thin films was improved with increased thickness. However, for TiO2 nanotube arrays, with the decrease of the wall thickness, higher PL intensity was obtained. Compared TiO2 thin film with TiO2 nanotube arrays, TiO2 nanotube arrays exhibited a better performance on PL characteristic due to larger surface area. Owing to larger amount of oxygen vacancies on the surface, compared with the volume factor, surface area was a more important factor that affected the performance of PL characteristics. Therefore, when the volume of TiO2 nanotube arrays became smaller, the ratio of surface area to volume was increased and the influence from the surface area became the dominating factor on the performance of the PL characteristics. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.004207jes] All rights reserved.
The heterojunction effects of TiO2 nanotubes fabricated by atomic layer deposition on photocarrier transportation direction
(1970-01-01)
Chang, Yung-Huang
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Liu, Chien-Min
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Chen, Chih
;
Cheng, Hsyi-En
;
材料科學與工程學系
"The heterojunction effects of TiO2 nanotubes on photoconductive characteristics were investigated. For ITO/TiO2/Si diodes, the photocurrent is controlled either by the TiO2/Si heterojunction (p-n junction) or the ITO-TiO2 heterojunction (Schottky contact). In the short circuit (approximately 0 V) condition, the TiO2-Si heterojunction dominates the photocarrier transportation direction due to its larger space-charge region and potential gradient. The detailed transition process of the photocarrier direction was investigated with a time-dependent photoresponse study. The results showed that the diode transitioned from TiO2-Si heterojunction-controlled to ITO-TiO2 heterojunction-controlled as we applied biases from approximately 0 to -1 V on the ITO electrode."
Ultraviolet Photoresponse of TiO(2) Nanotube Arrays Fabricated by Atomic Layer Deposition
(2011-01-01)
Liu, Chien-Min
;
Chen, Chih
;
Cheng, Hsyi-En
;
材料科學與工程學系
;
Department of Materials Science and Engineering
We used atomic layer deposition with anodic aluminum oxide, to fabricate self-aligned TiO(2) nanotube arrays, on a Si substrate, at 400 degrees C. Numerous p-n nanojunctions can be fabricated using this approach. In the absence of the external application of a bias voltage, these crystalline nanotubes make very sensitive ultraviolet sensors. The sensitivity is a result of the built-in voltage, generated by the nanojunction formed between the TiO(2) nanotubes and the Si substrate, which separates electron-hole pairs and produces photocurrents. In addition, the one-dimensional structure of the TiO(2) nanotubes provides a well-defined path for the transportation of the photo-generated carriers. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3576076] All rights reserved.
Ultraviolet Photoresponse of TiO2 Nanotube Arrays Fabricated by Atomic Layer Deposition
(2011-01-01)
Liu, Chien-Min
;
Chen, Chih
;
Cheng, Hsyi-En
;
材料科學與工程學系
;
Department of Materials Science and Engineering
We used atomic layer deposition with anodic aluminum oxide, to fabricate self-aligned TiO2 nanotube arrays, on a Si substrate, at 400 degrees C. Numerous p-n nanojunctions can be fabricated using this approach. In the absence of the external application of a bias voltage, these crystalline nanotubes make very sensitive ultraviolet sensors. The sensitivity is a result of the built-in voltage, generated by the nanojunction formed between the TiO2 nanotubes and the Si substrate, which separates electron-hole pairs and produces photocurrents. In addition, the one-dimensional structure of the TiO2 nanotubes provides a well-defined path for the transportation of the photo-generated carriers. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3576076] All rights reserved.