Bau-Tong Dai

Fabrication and Characterizations of Black Hybrid Silicon Nanomaterials as Light-Trapping Textures for Silicon Solar Cells

The effects of antireflective coatings on solar cell surfaces on photovoltaic characteristics are important issues. A comparative study of black hybrid silicon (Si) nanomaterials (BHSNMs) is demonstrated via the vapor-liquid-solid reaction using gold as the mediating catalyst and silane as the Si source ambient. By developing proper growth conditions, the study demonstrates BHSNMs with excellent antireflective characteristics. Results of this study reveal that the BHSNMs with a crisscrossed silicon rod microstructure and nanostructured texture provide excellent light trapping. Raman spectra show that the crystal structure of BHSNMs transits from single-crystalline Si to mixed amorphous Si with polycrystalline Si with increasing SiH 4 flow time. In addition, BHSNM reflections were lower than 0.4% for the 200-1100 nm wavelength range by suitably adjusting growth time. A conversion efficiency of around 1.34% with a 4.75 mA/cm 2 photocurrent V oc of 400 mV and fill factor of 70.37% for the BHSNM solar cells was demonstrated. Experimental results indicate that the BHSNMs provide excellent light trapping and can be used as a promising antireflective material for solar cell applications.

Photovoltaic Characterizations of Crisscrossed-Silicon-Nanorod Solar Cells

A comparative study on photovoltaic characterizations in crisscrossed-silicon-nanorod (CSNR) solar cells with an n-type-CSNR/intrinsic-CSNR/polycrystalline-silicon/p-type-Si(100)-stacked structure was demonstrated by means of the vapor-liquid-solid technique. Results reveal that an improved conversion efficiency of ~1.66% and an external quantum efficiency (EQE) of ~29% at 690 nm were obtained. The CSNR reflectance was lower than 2% for the 200-1100-nm wavelength range. By developing a proper growth time, the CSNR solar cell has a larger EQE. The mechanisms related to excellent light trapping and a larger EQE were mainly due to the CSNR microstructure.

Bimetallic oxide nanoparticles CoxMoyO as charge trapping layer for nonvolatile memory device applications

The reduced CoxMoyO bimetallic oxide nanoparticles (BONs) embedded in the hafnium oxynitride high-k dielectric have been developed by means of the chemical vapor deposition method. Capacitance-voltage (C-V) measurements estimate that a charge trap states density of 1.1×1012cm−2 and a flatband voltage shift of 700mV were achieved during the C-V hysteresis sweep at ±5V. Scanning electron microscopy image displays that the CoxMoyO BONs with a diameter of ∼4–20nm and a surface density of ∼1×1011cm−2 were obtained. The writing characteristics measurements illustrate that the memory effect is mainly due to the holes trapping.

Fabrication and Photovoltaic Characteristics of Coaxial Silicon Nanowire Solar Cells Prepared by Wet Chemical Etching

Nanostructured solar cells with coaxial p-n junction structures have strong potential to enhance the performances of the silicon-based solar cells. This study demonstrates a radial junction silicon nanowire (RJSNW) solar cell that was fabricated simply and at low cost using wet chemical etching. Experimental results reveal that the reflectance of the silicon nanowires (SNWs) declines as their length increases. The excellent light trapping was mainly associated with high aspect ratio of the SNW arrays. A conversion efficiency of ∼7.1% and an external quantum efficiency of ∼64.6% at 700 nm were demonstrated. Control of etching time and diffusion conditions holds great promise for the development of future RJSNW solar cells. Improving the electrode/RJSNW contact will promote the collection of carries in coaxial core-shell SNW array solar cells.

Physical and electrical characteristics of carbon nanotube network field-effect transistors synthesized by alcohol catalytic chemical vapor deposition

Carbon nanotubes (CNTs) have been explored in nanoelectronics to realize desirable device performances. Thus, carbon nanotube network field-effect transistors (CNTNFETs) have been developed directly by means of alcohol catalytic chemical vapor deposition (ACCVD) method using Co-Mo catalysts in this work. Various treated temperatures, growth time, and Co/Mo catalysts were employed to explore various surface morphologies of carbon nanotube networks (CNTNs) formed on the SiO2/n-type Si(100) stacked substrate. Experimental results show that most semiconducting single-walled carbon nanotube networks with 5-7 nm in diameter and low disorder-induced mode (D-band) were grown. A bipolar property of CNTNFETs synthesized by ACCVD and using HfO2 as top-gate dielectric was demonstrated. Various electrical characteristics, including drain current versus drain voltage (Id-Vd), drain current versus gate voltage (Id-Vg), mobility, subthreshold slope (SS), and transconductance (Gm), were obtained.

Characterization of co x ni y o bimetallic oxide nanoparticles as charge trapping nodes in nonvolatile memory devices

The nanoparticles (NPs) have been widely used to supersede the conventional charge trapping layers (CTLs) of the silicon-oxide-nitride-oxide-silicon devices for reducing the charge loss due to a local leakage path. Moreover, to reduce the gate tunneling leakage current, the high-dielectric-constant (high-k) gate oxides with the identical equivalent-oxide thickness (EOT) are promising for the advanced metal-oxide-semiconductor (MOS) devices applications. Therefore, the NPs embedded in the high-k gate dielectric would be an attractive technology option for the nonvolatile memory device (NVM) applications.

Fabrication challenges for a complicated micro-flow channel system at low temperature process

This paper will present fabrication challenges for a complicated micro channel system at low temperature process by MEMS techniques. This channel will be integrated with an array of temperature sensors and a set of heaters for the purpose of study on the micro-scale heat transfer inside. The heat transfer results may provide a clue whether the microchannel cooling process can be used to solve the future cooling problem encountered in an extremely high power density CPU chip. Design and fabrication challenges encountered in this processes are discussed. A final measurement for the validation of the heaters and the sensors fabricated and a study of the heat transfer coefficient distributions inside the micro channel are also presented.

Synthesis of well aligned silicon nanowire arrays by reflow of photoresist techniques

Well aligned Si nanowires (SiNWs) by Vapor-Liquid-Solid growth process are presented. Instead of using H2, the current work uses N2 as carrier gas. The growth conditions of SiNWs are controlled by the ratio of nitrogen versus silane gas. Tapering of nanowires was found at T=620degC and P=333 m torr, and the tapering parameter was reduced by increasing the N2 gas. Tapering of nanowires is attributed to volume reduction of Au catalyst which diffuses onto the side wall of nanowires. However, it is found that the N2 gas has a similar effect of O2 gas which can reduce diffusion of Au catalyst so that volume reduction of Au catalyst is not so significant and nanowire growth becomes untapered. By adopting reflow of photoresist techniques, the size of metal catalyst for SiNWs growth can be significantly reduced, and the growth location of SiNWs can be defined. Well aligned Si nanowires can be obtained. However, aspect ratio of the nano size hole in the photoresist after reflow has a significant effect on the amount of the Au in the nanoparticles and thus the quality and growth of the nanowires.

Selective growth of high purity single-walled carbon nanotubes network from alcohol

This study presents a novel selective growth method to pattern the high purity and single-walled carbon nanotubes (SWNTs) network by alcohol catalytic chemical vapor deposition (ACCVD). A hydrophilic surface with a contact angle of 44.22deg or even lower can disperse the Co-Mo catalysts easily and uniformly. Therefore, the SWNTs networks can be only formed and grown on the surface of SiO2 layer after a highly hydrophilic surface treatment. This novel method can be applied in the process of SWNTs electronics devices such as nanosensors or transistors. Finally, a SWNTs networks field effect transistor was also fabricated and demonstrated.