Chia-Hua Chan

Efficiency enhancement in GaAs solar cells using self-assembled microspheres

In this study we develop an efficient light harvesting scheme that can enhance the efficiency of GaAs solar cells using self-assembled microspheres. Based on the scattering of the microspheres and the theory of photonic crystals, the path length can be increased. In addition, the self-assembly of microspheres is one of the simplest and the fastest methods with which to build a 2D periodic structure. The experimental results are confirmed by the use of a simulation in which a finite-difference time-domain (FDTD) method is used to analyze the absorption and electric field of the 2D periodic structure. Both the results of the numerical simulations and the experimental results show an increase in the conversion power efficiency of GaAs solar cell of about 25% when 1 microm microspheres were assembled on the surface of GaAs solar cells.

Improved output power of GaN-based light-emitting diodes grown on a nanopatterned sapphire substrate

This letter describes the improved output power of GaN-based light-emitting diodes (LEDs) formed on a nanopatterned sapphire substrate (NPSS) prepared through etching with a self-assembled monolayer of 750-nm-diameter SiO2 nanospheres used as the mask. The output power of NPSS LEDs was 76% greater than that of LEDs on a flat sapphire substrate. Three-dimensional finite-difference time-domain calculation predicted a 40% enhancement in light extraction efficiency of NPSS LEDs. In addition, the reduction of full widths at half maximum in the ω-scan rocking curves for the (0 0 2) and (1 0 2) planes of GaN on NPSS suggested improved crystal quality.

Output power enhancement of light-emitting diodes via two-dimensional hole arrays generated by a monolayer of microspheres

The output power enhancement of the GaN-based light-emitting diodes (LEDs) featuring two-dimensional (2D) hole arrays is demonstrated. The 2D air hole arrays were first generated in the photoresist by utilizing the focusing nature of microspheres, and then transferred onto the GaN surface through dry etching. The maximum output power of the surface-textured LEDs was enhanced by 45% compared with the LEDs without surface texturing. The finite-difference time-domain calculation was performed and revealed that the light extraction efficiency of the textured LEDs increased with increasing etching depth.

Self-assembled free-standing colloidal crystals

We propose a novel technique to fabricate a free-standing three-dimensional colloidal crystal by self-assembling the colloidal microspheres with controllable thickness from the air–liquid interface. Highly ordered three-dimensional colloidal crystals are formed by polymethylmethacrylate or polystyrene monodisperse microspheres. We also demonstrate the fabrication technique of the free-standing inversed opals by removing the microspheres using calcination. The free-standing colloidal crystal structures can be used for nano-photonic circuits, white-light LEDs or as a photocatalyst.

Light enhancement by the formation of an Al oxide honeycomb nanostructure on the n-GaN surface of thin-GaN light-emitting diodes

By using a micron polystyrene ball array as a template, an Al oxide honeycomb structure was produced on the n-GaN surface of a thin-GaN light-emitting diode (LED). The Al oxide honeycomb structure consists of the networking hexagonal Al oxide nanowall. With the Al oxide honeycomb nanostructure, the total lighting output of thin-GaN LED was enhanced by 35%. The authors believe that the networking nanowall of the Al oxide honeycomb structure acted as a waveguide to extract the light emitted to the outer medium effectively.

Patterning Periodical Motif on Substrates Using Monolayer of Microspheres: Application in GaN Light-Emitting Diodes

In this article, we present a method of patterning a GaN substrate using polystyrene spheres and its inversed structure as the mask. By dry etching, different surface morphologies and depths of the GaN substrate can be obtained by changing the etching time. We used such a sphere-patterned GaN substrate to fabricate GaN-based light-emitting diodes (LED). It was found that the total lighting output of the sphere-patterned GaN LED was increased by 37%.

Compact and low-loss bent hollow waveguides with distributed Bragg reflector

In this study, a hollow bent waveguide with distributed Bragg reflectors (DBR) in silicon substrate was presented theoretically and experimentally. We used the two-dimensional finite-difference time-domain method to simulate bending transmission efficiencies for arc- and cut-type 90 degrees -bent waveguides. The air core was embedded by Si(3)N(4)/SiO(2) multilayer. The multilayer stacks were deposited by using plasma-enhanced chemical vapor deposition on the top and bottom of air core. The lowest 90 degree bending loss is around 3.9dB for the arc-type bending waveguides and 0.8dB for cut-type bending waveguides, respectively. This waveguide demonstrates a possibility for higher density of integration in planar light wave circuits.

Crack-free GaN grown on AlGaN∕(111)Si micropillar array fabricated by polystyrene microsphere lithography

The authors report on the growth of GaN on AlGaN∕(111)Si micropillar array by metal-organic chemical vapor deposition. Using the substrates with micropillar array, 2-μm-thick GaN films without cracks can be achieved. Transmission electron microscopy, atomic force microscopy, and micro-Raman studies indicate that the dislocation density and residual stress of the GaN grown on micropillar array are also reduced. The results reveal the potential of this type of substrates for growing GaN-based devices as well as preparing GaN freestanding substrates.

Rapid fabrication of 2D and 3D photonic crystals and their inversed structures.

In this paper a new technique is proposed for the fabrication of two-dimensional (2D) and three-dimensional (3D) photonic crystals using monodisperse polystyrene microspheres as the templates. In addition, the approaches toward the creation of their corresponding inversed structures are described. The inversed structures were prepared by subjecting an introduced silica source to a sol-gel process; programmed heating was then performed to remove the template without spoiling the inversed structures. Utilizing these approaches, 2D and 3D photonic crystals and their highly ordered inversed hexagonal multilayer or monolayer structures were obtained on the substrate.

Low power consumption and high-contrast light scattering based on polymer-dispersed liquid crystals doped with silver-coated polystyrene microspheres

Polymer-dispersed liquid crystals (PDLCs) have attracted considerable attention for optical device applications in recent years. However, the high operating voltage of PDLCs limits their applications. This study reports a simple approach used for the first time to decrease the operating voltage of PDLCs by means of doping 3 μm-diameter silver-coated polystyrene microspheres (Ag-coated PSMSs) into PDLCs. Ag-coated PSMSs construct an induced electric field between each other when an external electric field is applied. This induced electric field can enhance the effective electric field so the operating voltage can be actively reduced from 77 V to 40 V. Such PDLCs also possess a high contrast ratio of >50 and a high on-state transmittance of ~73%. Therefore, PDLCs doped with Ag-coated PSMSs maintain a high contrast ratio and improve their electro-optical properties.