Sung-yen Wei
ITRI, Materials and Chemistry Lab, Department Member
Abstract In this study, we controlled the growth of crystalline plane to synthesize the vertical platinum nanopelts onto fluorine doped tin oxide (FTO) glass to construct the platinum nanoroses (PtNRs) by an easy electrochemical... more
Abstract In this study, we controlled the growth of crystalline plane to synthesize the vertical platinum nanopelts onto fluorine doped tin oxide (FTO) glass to construct the platinum nanoroses (PtNRs) by an easy electrochemical deposition (ECD) method at room temperature in the normal atmospheric environment, and applied the PtNRs as a counter electrode (CE) for dye-sensitized solar cell (DSSC). The morphologies and crystalline nanostructures of the ECD PtNRs were examined by field emission scanning electron microscopy and the high-resolution transmission electron microscopy. The electrocatalytic properties of the ECD PtNRs were analyzed by cyclic voltammetry and electrochemical impedance spectrum. The power conversion efficiency (PCE) of the DSSC assembled with the ECD PtNRs CE was examined under the illumination of AM 1.5 (100 mWcm − 2 ). The ECD PtNRs showed well contact behavior with FTO surface and offered a large surface area to promote the redox reaction rates therefore increased the exchange current density. Due to the PtNRs showed the macroporous structure, the electrolyte can easily diffuse through the open space between the nanopelts, resulting in improved kinetics for charge-transfer processes and reduced the charge-transfer resistance. Our findings suggest that the catalytic efficiency of PtNRs with a specific crystal plane was significantly greater than that of a traditional Pt film catalyst. In combination with a N719 dye-sensitized TiO 2 working electrode and an iodine-based electrolyte, the DSSC assembled with the PtNRs CE achieved a PCE of 6.58%, almost 10% higher than that of a cell prepared with a conventional sputtering Pt film CE (6.00%). These results provide a potential strategy for electrochemical catalytic applications.
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ABSTRACT Comparative studies for TiO2-passivated Al0.25Ga0.75N/GaN heterostructure FETs (HFETs) and TiO2-dielectric MOS-HFETs using nonvacuum ultrasonic spray pyrolysis deposition technique are made. Optimum device performances are... more
ABSTRACT Comparative studies for TiO2-passivated Al0.25Ga0.75N/GaN heterostructure FETs (HFETs) and TiO2-dielectric MOS-HFETs using nonvacuum ultrasonic spray pyrolysis deposition technique are made. Optimum device performances are obtained by tuning the layer thickness of TiO2 to 20 nm. High relative permittivity ( $k$ ) of 53.6 and thin effective oxide thickness of 1.45 nm are also obtained. Pulse-IV, Hooge coefficient ( $alpha _{H}$ ), Transmission Electron Microscopy, and atomic force microscope have been performed to characterize the interface, atomic composition, and surface flatness of the TiO2 oxide. Superior improvements for the present TiO2-dielectric MOS-HFET/TiO2-passivated HFETs are obtained, including 47.6%/23.8% in two-terminal gate–drain breakdown voltage (BV $_{rm GD})$ , 111%/22.2% in two-terminal gate-drain turn-ON voltage ( $V_{mathrm{{scriptscriptstyle ON}}})$ , 47.9%/39.4% in ON-state breakdown (BV $_{rm DS})$ , 12.2%/10.2% in drain–source current density ( $I_{rm DS})$ at $V_{rm GS} = 0$ V ( $I_{rm DSS0})$ , 27.2%/11.7% in maximum $I_{rm DS}$ ( $I_{rm DS, max})$ , 3/1-order enhancement in on/off current ratio ( $I_{mathrm{{scriptscriptstyle ON}}} / I_{mathrm{{scriptscriptstyle OFF}}})$ , 58.8%/17.6% in gate-voltage swing linearity, 25.1%/13.2% in unity-gain cutoff frequency ( $f_{T})$ , 40.6%/24.7% in maximum oscillation frequency ( $f_{max })$ , and 33.8%/15.6% in power-added efficiency with respect to a Schottky-gated HFET fabricated on the identical epitaxial structure. The present MOS-HFET has also shown stable electrical performances when the ambient temperature is varied from 300 to 450 K.
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In this paper, a facile and rapid aqueous-based electrochemical technique was used for the phase conversion of Ni into Ni(OH)2 thin film. The Ni(OH)2 thin film was directly converted and coated onto the network surface of Ni foam (NF) via... more
In this paper, a facile and rapid aqueous-based electrochemical technique was used for the phase conversion of Ni into Ni(OH)2 thin film. The Ni(OH)2 thin film was directly converted and coated onto the network surface of Ni foam (NF) via the self-hydroxylation process under alkaline conditions using a simple cyclic voltammetry (CV) strategy. The as-formed and coated Ni(OH)2 thin film on the NF was used as the catalyst layer for the direct growth of carbon nanotubes (CNTs). The self-converted Ni(OH)2 thin film is a good catalytic layer for the growth of CNTs due to the fact that the OH− of the Ni(OH)2 can be reduced to H2O to promote the growth of CNTs during the CVD process, and therefore enabling the dense and uniform CNTs growth on the NF substrate. This binder-free CNTs/NF electrode displayed outstanding behavior as an electric double-layer capacitor (EDLC) due to the large surface area of the CNTs, showing excellent specific capacitance values of 737.4 mF cm−2 in the three-elec...
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A series of new push–pull phenothiazine‐based dyes (HL1, HL2, HL3, HL4) featuring various π spacers (thiophene, 3‐hexylthiophene, 4‐hexyl‐2,2′‐bithiophene) and double acceptors/anchors have been synthesized, characterized, and used as... more
A series of new push–pull phenothiazine‐based dyes (HL1, HL2, HL3, HL4) featuring various π spacers (thiophene, 3‐hexylthiophene, 4‐hexyl‐2,2′‐bithiophene) and double acceptors/anchors have been synthesized, characterized, and used as sensitizers for dye‐sensitized solar cells (DSSCs). Among them, the best conversion efficiency (7.31 %) reaches approximately 99 % of the N719‐based (7.38 %) DSSCs fabricated and measured under similar conditions. The dyes with two anchors have more efficient interfacial charge generation and transport compared with their congeners with only single anchor. Incorporation of hexyl chains into the π‐conjugated spacer of these double‐anchoring dyes can efficiently suppress dye aggregation and reduce charge recombination.
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In this study we use low-cost spray pyrolysis method to produce a AI203:CI/Ti02 bilayer as rear passivation layer to replace the common known AI203/SiNx laminate in PERC structure silicon solar cells. The Ti02 and Ti02/AI203 mixed thin... more
In this study we use low-cost spray pyrolysis method to produce a AI203:CI/Ti02 bilayer as rear passivation layer to replace the common known AI203/SiNx laminate in PERC structure silicon solar cells. The Ti02 and Ti02/AI203 mixed thin film acts a competent barrier from molten aluminum paste during the co-firing process and high refractive index of those thin films demonstrated excellent internal back reflectance (IBR). Very high negative fixed charge concentration is up to 5 X 1012 cm·2 in Ti02 thin film being observed and the interface trap density (Dit) can be controlled via Ti/AI ratio and ultra-thin AI203:CI interfacial layer to maintain r.0odassivation ability and the lowest result of Dit is 8 x 10 0 cm· eV· 1 with a 6nm AI203:CI interfacial layer. Through the use of AI203:CI interfacial layer, the carrier lifetime of bulk wafer increase from 60/ls to 120/ls. All of the results show that this concept is very promising for high efficiency silicon solar cell application.
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Ethylene is one of the most primary and widely used petrochemical products in today’s world and is considered as a chemical building block in the petrochemical industry. In this research, ethylene production from ethane and natural gas is... more
Ethylene is one of the most primary and widely used petrochemical products in today’s world and is considered as a chemical building block in the petrochemical industry. In this research, ethylene production from ethane and natural gas is evaluated in terms of cost and optimum conditions. Also, a comprehensive economic and technical comparison is made to achieve the optimal conditions for ethylene production concerning feed diversity. Nowadays most ethylene production units run with ethane feed. If it is possible to implement gas-ethylene processes with an inexpensive natural gas feed, it will be a significant step for technical and economic optimization. Thus, some methods are introduced and compared, and finally an economic review about best condition for ethylene production from ethane and natural gas/investment with regard to cost and economic efficiency of the methods is provided. The investment cost for Gas to Ethylene (GTE) and Ethane to Ethylene (ETE) processes is 363–701 mi...
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Two dimensional (2D) materials are drawing growing attention for next-generation electronics and optoelectronics owing to its atomic thickness and unique physical properties. One of the challenges posed by 2D materials is the large... more
Two dimensional (2D) materials are drawing growing attention for next-generation electronics and optoelectronics owing to its atomic thickness and unique physical properties. One of the challenges posed by 2D materials is the large source/drain (S/D) series resistance due to their thinness, which may be resolved by thickening the source and drain regions. Recently explored lateral graphene-MoS2 and graphene-WS2 heterostructures shed light on resolving the mentioned issues owing to their superior ohmic contact behaviors. However, recently reported field-effect transistors (FETs) based on graphene-TMD heterostructures have only shown n-type characteristics. The lack of p-type transistor limits their applications in complementary metal-oxide semiconductor (CMOS) electronics. In this work, we demonstrate p-type FETs based on graphene-WSe2 lateral heterojunctions grown with the scalable CVD technique. Few-layer WSe2 is overlapped with the multilayer graphene (MLG) at MLG-WSe2 junctions s...
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This work uses ultrasonic spray pyrolysis deposition to grow titanium dioxide and aluminum oxide (Al<sub>2</sub>O<sub>3</sub>) films, respectively, as an active layer and an insulator layer of the... more
This work uses ultrasonic spray pyrolysis deposition to grow titanium dioxide and aluminum oxide (Al<sub>2</sub>O<sub>3</sub>) films, respectively, as an active layer and an insulator layer of the metal-insulator-semiconductor-insulator-metal (MISIM) photodetector (PD). 10-, 20-, and 30-nm-thick Al<sub>2</sub>O<sub>3</sub> films were deposited and the I-V characteristics in the dark and under illumination were measured and investigated. The dark current was suppressed to 11.6 pA for the MISIM PD with the 30-nm Al<sub>2</sub>O<sub>3</sub>. In addition, the carrier transportation mechanisms of the dark current are analyzed. The photoresponsivity of the MISIM PD with the 10-nm Al<sub>2</sub>O<sub>3</sub> was 8.22 A/W (at 10 V), which is much higher than 0.84 A/W of the metal-semiconductor-metal PD. The noise equivalent power and detectivity of the MISIM PD with the 10-nm Al<sub>2</sub>O<sub>3</sub> were 2.28 × 10<sup>-10</sup> W and 2.4 × 10<sup>9</sup> Jones. The PDs showed a slight degradation when the ambient temperature was up to 450 K.
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This letter demonstrates an integration process of in situ Cl<sup>-</sup> doped Al<sub>2</sub>O<sub>3</sub> and gate recess technique to fabricate the enhancement mode AlGaN/GaN MOSHEMTs. The... more
This letter demonstrates an integration process of in situ Cl<sup>-</sup> doped Al<sub>2</sub>O<sub>3</sub> and gate recess technique to fabricate the enhancement mode AlGaN/GaN MOSHEMTs. The Cl<sup>-</sup> doped Al<sub>2</sub>O<sub>3</sub> thin film is deposited by the ultrasonic spray pyrolysis deposition and characterized by transmission electron microscopy and X-ray photoelectron spectroscopy. The relative permittivity of Cl<sup>-</sup> doped Al<sub>2</sub>O<sub>3</sub> is higher than the pure Al<sub>2</sub>O<sub>3</sub> and the output current is enhanced. The threshold voltage of the enhancement mode AlGaN/GaN MOSHEMT with the Cl<sup>-</sup> doped Al<sub>2</sub>O<sub>3</sub> gate dielectric layer rose from 0.2 to 1.3 V. Furthermore, the breakdown voltage of present enhancement mode AlGaN/GaN MOSHEMT reached 650 V. It was also found that the MOSHEMT with Cl<sup>-</sup> doped Al<sub>2</sub>O<sub>3</sub> has higher gate leakage than that with pure Al<sub>2</sub>O<sub>3</sub>. The thermal stability of threshold voltage and current collapse phenomenon is described in this letter.
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In this letter, non-vacuum ultrasonic spray pyrolysis deposition was used to grow Al<sub>2</sub>O<sub>3</sub>/TiO<sub>2</sub> thin film as the metal-insulator-semiconductor-insulator-metal ultraviolet... more
In this letter, non-vacuum ultrasonic spray pyrolysis deposition was used to grow Al<sub>2</sub>O<sub>3</sub>/TiO<sub>2</sub> thin film as the metal-insulator-semiconductor-insulator-metal ultraviolet photodetector (MISIM UV PD). The anatase TiO<sub>2</sub> with 400 °C annealing was used as the active layer of the UV PD. X-ray diffraction and Raman spectra were used to characterize the crystal phase of the TiO<sub>2</sub>. The Al<sub>2</sub>O<sub>3</sub>/TiO<sub>2</sub> MISIM UV PD has lower dark current (~57.2 pA), higher photoresponse (~24.48 A/W), and higher detectivity (~8.25 × 10<sup>13</sup> Jones), all of which were better than the TiO<sub>2</sub> MSM UV PD. Similar device performance was obtained from the SiO<sub>2</sub>/TiO<sub>2</sub> MISIM UV PD. The external quantum efficiency of the Al<sub>2</sub>O<sub>3</sub>/TiO<sub>2</sub> and SiO<sub>2</sub>/TiO<sub>2</sub> MISIM UV PDs was 8204% and 6840%. Such high external quantum efficiency results from the internal photoconductive gain and the interfacial trap controlled charge injection.
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The mechanism of growth of an epitaxial Si0.5Ge0.5 layer on a single crystalline (sc) Si (100) substrate by aluminum-induced solid phase epitaxy (AI-SPE) at a relatively low temperature (450 °C) has been revealed using in situ heating... more
The mechanism of growth of an epitaxial Si0.5Ge0.5 layer on a single crystalline (sc) Si (100) substrate by aluminum-induced solid phase epitaxy (AI-SPE) at a relatively low temperature (450 °C) has been revealed using in situ heating transmission electron microscopy (TEM). The analysis of the thermodynamics exactly supports the finding from in situ TEM. It evidences that the Si0.5Ge0.5 prefers to nucleate at the interface of the Al layer and the sc-Si (100) substrate due to the lowest critical thickness for nucleation. Based on the results from in situ TEM and thermodynamic analysis, the germanium (Ge) virtual substrate of the compositional gradient can be successfully prepared via a multi-run AI-SPE process at low-temperature.
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This work investigates GaN/Al0.24Ga0.76N/GaN metal-oxide-semiconductor high electron mobility transistors (MOS-HEMTs) grown on a Si substrate with MgO gate dielectric by using the non-vacuum ultrasonic spray pyrolysis deposition (USPD)... more
This work investigates GaN/Al0.24Ga0.76N/GaN metal-oxide-semiconductor high electron mobility transistors (MOS-HEMTs) grown on a Si substrate with MgO gate dielectric by using the non-vacuum ultrasonic spray pyrolysis deposition (USPD) technique. The oxide layer thickness is tuned to be 30 nm with the dielectric constant of 8.8. Electron spectroscopy for chemical analysis (ESCA), secondary ion mass spectrometry (SIMS), atomic force microscopy (AFM), transmission electron microscopy (TEM), C–V, low-frequency noise spectra, and pulsed I–V measurements are performed to characterize the interface and oxide quality for the MOS-gate structure. Improved device performances have been successfully achieved for the present MOS-HEMT (Schottky-gate HEMT) design, consisting of a maximum drain-source current density (I DS, max) of 681 (500) mA/mm at V GS = 4 (2) V, I DS at V GS = 0 V (I DSS0) of 329 (289) mA/mm, gate-voltage swing (GVS) of 2.2 (1.6) V, two-terminal gate-drain breakdown voltage (BV GD) of −123 (−104) V, turn-on voltage (V on) of 1.7 (0.8) V, three-terminal off-state drain-source breakdown voltage (BV DS) of 119 (96) V, and on/off current ratio (I on/I off) of 2.5 × 108 (1.2 × 103) at 300 K. Improved high-frequency and power performances are also achieved in the present MOS-HEMT design.
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A novel process of aluminum-induced solid-phase epitaxy (AI-SPE) is used to fabricate a hetero-epitaxial Si1−xGex (epi-SiGe) film on an sc-Si (100) substrate at relatively low temperatures (lower than 450 °C) using a 1/4 × 4′′ wafer. The... more
A novel process of aluminum-induced solid-phase epitaxy (AI-SPE) is used to fabricate a hetero-epitaxial Si1−xGex (epi-SiGe) film on an sc-Si (100) substrate at relatively low temperatures (lower than 450 °C) using a 1/4 × 4′′ wafer. The stoichiometry of Ge in the epi-SiGe film can be easily tuned in the AI-SPE process by controlling the annealing temperature and the Ge fraction (x) of the initial a-Si1−xGex layer. The stoichiometry and epitaxial relationship of the epi-SiGe film were verified by grazing incidence X-ray diffraction (GI-XRD) and transmission electron microscopy (TEM). The AI-SPE mechanism is directly verified by in situ heating TEM. Based on the experimental results, it is concluded that the AI-SPE mechanism for the epi-SiGe film can be divided into four steps: (a) initial stage; (b) formation of a-Si1−xGex free atoms and diffusion along Al grain boundaries to the sc-(100) substrate surface; (c) nucleation of crystalline Si1−xGex at the surface of the sc-Si (100) substrate; and (d) crystal growth and layer exchange.
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The pulse-mode potentiostatic electrodeposited single-crystal fern-like Pt nanorods enhanced the electrocatalytic activities for the methanol oxidation reaction.
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ABSTRACT Comparative studies for TiO2-passivated Al0.25Ga0.75N/GaN heterostructure FETs (HFETs) and TiO2-dielectric MOS-HFETs using nonvacuum ultrasonic spray pyrolysis deposition technique are made. Optimum device performances are... more
ABSTRACT Comparative studies for TiO2-passivated Al0.25Ga0.75N/GaN heterostructure FETs (HFETs) and TiO2-dielectric MOS-HFETs using nonvacuum ultrasonic spray pyrolysis deposition technique are made. Optimum device performances are obtained by tuning the layer thickness of TiO2 to 20 nm. High relative permittivity ( $k$ ) of 53.6 and thin effective oxide thickness of 1.45 nm are also obtained. Pulse-IV, Hooge coefficient ( $alpha _{H}$ ), Transmission Electron Microscopy, and atomic force microscope have been performed to characterize the interface, atomic composition, and surface flatness of the TiO2 oxide. Superior improvements for the present TiO2-dielectric MOS-HFET/TiO2-passivated HFETs are obtained, including 47.6%/23.8% in two-terminal gate–drain breakdown voltage (BV $_{rm GD})$ , 111%/22.2% in two-terminal gate-drain turn-ON voltage ( $V_{mathrm{{scriptscriptstyle ON}}})$ , 47.9%/39.4% in ON-state breakdown (BV $_{rm DS})$ , 12.2%/10.2% in drain–source current density ( $I_{rm DS})$ at $V_{rm GS} = 0$ V ( $I_{rm DSS0})$ , 27.2%/11.7% in maximum $I_{rm DS}$ ( $I_{rm DS, max})$ , 3/1-order enhancement in on/off current ratio ( $I_{mathrm{{scriptscriptstyle ON}}} / I_{mathrm{{scriptscriptstyle OFF}}})$ , 58.8%/17.6% in gate-voltage swing linearity, 25.1%/13.2% in unity-gain cutoff frequency ( $f_{T})$ , 40.6%/24.7% in maximum oscillation frequency ( $f_{max })$ , and 33.8%/15.6% in power-added efficiency with respect to a Schottky-gated HFET fabricated on the identical epitaxial structure. The present MOS-HFET has also shown stable electrical performances when the ambient temperature is varied from 300 to 450 K.
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ABSTRACT This paper proposed Al2O3 deposition by ultrasonic spray pyrolysis (USP) method as an insulator layer for Al2O3/AlGaN/GaN metal–insulator–semiconductor ultraviolet photodetector (MIS-UV-PD) applications. The composition of... more
ABSTRACT This paper proposed Al2O3 deposition by ultrasonic spray pyrolysis (USP) method as an insulator layer for Al2O3/AlGaN/GaN metal–insulator–semiconductor ultraviolet photodetector (MIS-UV-PD) applications. The composition of USP-grown Al2O3 was confirmed by X-ray photoelectron spectroscopy. The refractive index and transmittance characteristics of USP-grown Al2O3 were also characterized. The Al2O3/AlGaN/GaN MIS-UV-PD performances with different Al2O3 thickness (30, 20, and 15 nm) were investigated. The responsivity was 1.3 × 10−3/7.5 × 10−3/0.83 A/W, UV-to-visible rejection ratio was 2.34 × 103/1.37 × 104/3.18 × 105, and the detectivity was 2.78 × 108/1.26 × 109/1.17 × 1011 cmHz0.5W−1 for the MIS-UV-PD with 30-/20-/15-nm-thick Al2O3. It was found that the performances of MIS-UV-PD with 15-nm Al2O3 as the insulator layer are much better than the MIS-UV-PD with 20- and 30-nm Al2O3.
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ABSTRACT This work investigates Al2O3/AlGaN/GaN metal-oxide-semiconductor high electron mobility transistors (MOS-HEMTs) grown on SiC substrate by using the non-vacuum ultrasonic spray pyrolysis deposition (USPD) method. The Al2O3 was... more
ABSTRACT This work investigates Al2O3/AlGaN/GaN metal-oxide-semiconductor high electron mobility transistors (MOS-HEMTs) grown on SiC substrate by using the non-vacuum ultrasonic spray pyrolysis deposition (USPD) method. The Al2O3 was deposited as gate dielectric and surface passivation simultaneously to effectively suppress gate leakage current, enhance output current density, reduce RF drain current collapse, and improve temperature-dependent stabilities performance. The present MOS-HEMT design has shown improved device performances with respect to a Schottky-gate HEMT, including drain-source saturation current density at zero gate bias (I DSS: 337.6 mA mm−1 → 462.9 mA mm−1), gate-voltage swing (GVS: 1.55 V → 2.92 V), two-terminal gate-drain breakdown voltage (BV GD: −103.8 V → −183.5 V), unity-gain cut-off frequency (f T : 11.3 GHz → 17.7 GHz), maximum oscillation frequency (f max: 14.2 GHz → 19.1 GHz), and power added effective (P.A.E.: 25.1% → 43.6%). The bias conditions for measuring f T and f max of the studied MOS-HEMT (Schottky-gate HEMT) are V GS = −2.5 (−2) V and V DS = 7 V. The corresponding V GS and V DS biases are −2.5 (−2) V and 15 V for measuring the P.A.E. characteristic. Moreover, small capacitance-voltage (C–V) hysteresis is obtained in the Al2O3-MOS structure by using USPD. Temperature-dependent characteristics of the present designs at 300–480 K are also studied.
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ABSTRACT P-type polycrystalline Si film on a foreign substrate can be fabricated at temperatures lower than 773 K by an aluminium-induced crystallisation process. However, the ultimate carrier concentration of the Si film is limited to... more
ABSTRACT P-type polycrystalline Si film on a foreign substrate can be fabricated at temperatures lower than 773 K by an aluminium-induced crystallisation process. However, the ultimate carrier concentration of the Si film is limited to approximately 3 × 1018 cm−3 because of the low solid solubility of Al in Si at temperatures below 773 K. In this study, a process called B-AIC is developed in which boron is co-doped with Al to increase the carrier concentration in Si films to 1019 cm−3 at temperatures as low as 673 K. The carrier concentration can be tuned by the initial thickness of a-Si layer in the B-AIC process. Beside the fabrication of polycrystalline Si film on glass, the epitaxial growth of this heavily doped p++-Si film can also be realized on a single crystalline Si wafer via a solid phase epitaxy mechanism.
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We directly synthesized a platinum-nanoparticles/graphitic-nanofibers (PtNPs/GNFs) hybrid nanostructure on FTO glass. We applied this structure as a three-dimensional counter electrode in dye-sensitized solar cells (DSSCs), and... more
We directly synthesized a platinum-nanoparticles/graphitic-nanofibers (PtNPs/GNFs) hybrid nanostructure on FTO glass. We applied this structure as a three-dimensional counter electrode in dye-sensitized solar cells (DSSCs), and investigated the cells&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39; photoconversion performance.