Shih-Che Hung

White-light electroluminescence from ZnO nanorods/polyfluorene by solution-based growth

We report bright white-light electroluminescence (EL) from a diode structure consisting of a ZnO nanorod (NR) and a p-type conducting polymer of poly(fluorine) (PF) fabricated using a hydrothermal method. ZnO NRs are successfully grown on an organic layer of PF using a modified seeding layer. The EL spectrum shows a broad emission band covering the entire visible range from 400 to 800 nm. White-light emission is possible because the ZnO-defect-related emission from the ZnO NR/PF heterostructure is enhanced to become over thousand times stronger than that from the usual ZnO NR structure. This strong green-yellow emission associated with the ZnO defects, combined with the blue PF-related emission, results in the white-light emission. Enhancement of the ZnO-defect emission is caused by the presence of Zn(OH)(2) at the interface between the ZnO NRs and PF. Fourier transform infrared spectroscopy reveals that the absorption peaks at 3441, 3502, and 3574 cm(-1) corresponding to the OH group are formed at the ZnO NR/PF heterostructure, which confirms the enhancement of defect emission from the ZnO NR/PF heterostructure. The processing procedure revealed in this work is a convenient and low-cost way to fabricate ZnO-based white-light-emitting devices.

GaAs nanowire/ poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) hybrid solar cells

In this paper, a new type of hybrid solar cell based on a heterojunction between poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and vertically aligned n-type GaAs nanowire (NW) arrays is investigated. The GaAs NW arrays are fabricated by directly performing the nano-etching of GaAs wafer with spun-on SiO(2) nanospheres as the etch mask through inductively coupled plasma reactive ion etching. The PEDOT:PSS adheres to the surface of the GaAs NW arrays to form a p-n junction. The morphology of GaAs NW arrays strongly influences the characteristics of the GaAs NW/PEDOT:PSS hybrid solar cells. The suppression of reflectance and the interpenetrating heterojunction interface of GaAs NW arrays offers great improvements in efficiency relative to a conventional planar cell. Compared to the planar GaAs/PEDOT:PSS cells, the power conversion efficiency under AM 1.5 global one sun illumination is improved from 0.29% to 5.8%.

Silicon Waveguide Sidewall Smoothing by KrF Excimer Laser Reformation

A novel laser-reformation technique is presented for sidewall smoothing of silicon waveguides. A KrF excimer laser is used to melt and reform the sidewalls to reduce the surface roughness. Atomic-force-microscopy measurement shows that the root-mean-square (rms) roughness is reduced from 14 to 0.24 nm. The calculated scattering loss is reduced to 0.033 dB/cm. The waveguide profile after laser illumination at an incident angle of 75deg transforms to a shape of arch. The crystal quality of laser-illuminated silicon wafer characterized by microwave reflection photoconductance-decay carrier lifetimes shows 94% less damage than the furnace-treated wafer.

High Efficiency Flexible Polymer Solar Cells Based on PET Substrates with a Nonannealing Active Layer

The inverted bulk-heterojunction solar cell on the polyester (PET) substrate with a nonannealing active layer is investigated. The atomic force microscope images show that the morphology of the nonannealing active layer of the inverted plastic solar cell evolves with time, which improves the performance of the solar cell. Our investigations show that the grain size of the active layer increases with time, resulting in improvements in the fill factor (from 34.8 to 62.8%) and shunt resistance (from 107 to 505 Ω cm 2 ) as well as a reduction in the series resistance (from 4.82 to 0.96 Ω cm 2 ). The easily processed inverted device with a nonannealing active layer on the indium tin oxide-coated PET substrate exhibits a high power conversion efficiency of ~ 3.66%.

Transfer of aligned single crystal silicon nanowires to transparent substrates

We demonstrate the method of transferring aligned single crystal silicon nanowires (SiNWs) to transparent substrate. The alignment of the transferred nanowires is almost identical to the original one. The density of the transferred SiNWs can achieve 3×107 nanowires/mm2. The low temperature fabrication processes are compatible for a wide range of substrates. The transmission coefficient below 10 % at a wide bandwidth, 400-1100 nm, was found in the transferred SiNWs. The high dense aligned SiNWs are promising for future photovoltaic applications.

Effective energy densities in KrF excimer laser reformation as a sidewall smoothing technique

Profile transformation of sidewalls and roughness reduction by KrF excimer laser reformation at different illumination conditions are presented. The effective energy density derived from a finite element model of heat transfer is used to characterize the molten depth of Si during high energy laser illumination at normal and oblique incidence. The operation range of laser reformation as a sidewall smoothing technique, including incident angles and energy densities, is determined by the effective energy density. Using an energy density higher than 1.4 J/cm2 at the incident angle of 75° is recommended for a minimal residual roughness, acceptable sidewall profile transformation, and a practical interval-height ratio. The upper bound of scattering loss after laser reformation at different effective energy densities is also calculated.

Fabrication of Crystalline Si Waveguides on (1 0 0) Bulk Si Substrate Using Laser Reformation Method

Optical solution has been proposed for short-reach interconnects. A primary concern is the integration of photonics and electronics. A method to fabricate crystalline Si waveguides on insulator from bulk Si substrate using a laser reformation technique is here presented. A high-power laser is used to melt and reshape a Si fin structure. This is followed by an oxidation process to produce oxide as an optical isolation layer beneath the Si and form the waveguide structure. The Si waveguide, using laser reformation method in our experiment, has 140 nm width and 420 nm height, showing a single mode property and an effective refractive index of about 2.09. It represents a viable method for creating crystalline Si waveguides on CMOS-compatible Si substrate and reveals the potential of Si photonic devices integrated with Si electronics.

Fabrication of large-area gallium arsenide nanowires using silicon dioxide nanoparticle mask

Large-area GaAs nanowires are fabricated using SiO2 nanoparticles as the etching mask. SiO2 nanoparticle monolayer is spin coated on the GaAs substrate. To obtain a uniform monolayer of SiO2 nanoparticles across the substrate, raised temperature, adequate solution concentration, and the substrate treated with a solvent for interface activation are required. With the monolayer of SiO2 nanoparticles as the etching mask, the GaAs substrate is etched by induced-coupled plasma reactive ion etcher (ICP-RIE) to form GaAs nanowires with a high aspect ratio. The diameter and length of GaAs nanowires are 70nm and 1.2μm, respectively. The diameter and length of GaAs nanowires can be controlled by the size of SiO2 nanoparticles and etching time of ICP-RIE.

Fabrication of Silicon Nanostructured Thin Film and Its Transfer from Bulk Wafers onto Alien Substrates

Various Si nanostructures can be fabricated using a metal-assisted etching technique, which must be applied on bulk Si wafers, limiting its applications and wasting a significant amount of material. Here, we report a technique to form a Si nanostructured thin film created by metal-assisted chemical etching from bulk Si wafers and to transfer it onto alien substrates. To detach the Si nanostructured thin films completely from bulk Si wafers, a second-step metal-assisted chemical etching made the root of the Si nanostructures become fragile. The transferred Si nanostructures are well-aligned along the normal direction of the receiver substrate. The X-ray diffraction spectrum reveals that the transferred Si nanostructured thin films exhibit good crystal orientation and morphology. A strong light trapping effect between the nanostructures causes such films of 16 μm thickness to exhibit nearly 99% absorption from 400 to 800 nm. This exceeds the theoretically calculated limits of planar Si.

Fabrication of Deep Si Trenches by Self-Assembled Wet Chemical Etching Process

This paper describes a method for fabricating deep Si trenches with only a wet chemical etching process. A typical photolithography process was used to define the etching area. Aqueous HF/AgNO 3 and HF/H 2 O 2 solutions were applied to etch silicon nanowire (SiNW) structures in the selected domains. In the former case, a high selectivity between the bare Si surface and photoresist-covered Si surface can be achieved. The SiNWs with a monolithic 〈100〉 direction can be etched in the selected domain. However, additional (010)- and (001)-oriented etchings were observed on the trench sidewall and wafer surface in the latter case. In addition, deep and highly anisotropic trenches were achieved by removing the SiNWs. The Si wafer was immersed in a concentrated aqueous HF/H 2 O 2 solution. A porous structure was formed in the vicinity of Ag nanoparticles, suggesting that one should remove SiNWs completely to achieve deep anisotropic trenches. This method exhibits high anisotropy and is capable of etching deep Si trenches with depths of about 50 μm without an additional etching mask. It effectively minimizes instrument costs and reveals the potential of large-area fabrication.