Eun Kyu Kang

Increased Light Extraction From GaN Light-Emitting Diodes by

We demonstrate GaN-based blue light-emitting diodes (GaN LEDs) with artificial compound eye structures (CESs). The GaN LEDs consist of microlens arrays (MLAs) and antireflective subwavelength structures (SWSs). The CESs, formed on the LED surface by two-step pattern transfer of reflowed photoresist micro patterns and Ag nanoparticles, play an important role in enhancing the light extraction efficiency by reducing both the Fresnel and the total internal reflection. The CES integrated GaN LEDs show an output power enhancement of 93% compared to that of conventional GaN LEDs with flat surface, without any serious degradation of electrical characteristics. Optical simulations by ray-tracing and the rigorous coupled-wave analysis method provide design guidelines for MLAs and SWSs, respectively.

{\rm SiN}_{{\rm x}}

We present bifunctional nanostructured (NS) coverglasses with enhanced transmittance and self-cleaning function for improving the efficiency of a photovoltaic (PV) module with an InGaP/GaAs/Ge triple-junction solar cell. Prior to the fabrication of NS coverglasses, theoretical investigations were carried out using a rigorous coupled-wave analysis method to determine the desirable glass nanostructures that can effectively enhance the light absorption of the PV module. The transmission properties of the fabricated NS coverglasses with different glass nanostructures were systematically analyzed by considering the absorption spectrum of three subcells of the solar cell in order to find the most effective NS coverglass for enhancing the photocurrent of the PV module and thereby improving the conversion efficiency. The PV module with the most effective NS coverglass showed 4.2% enhanced short-circuit current density and 4.3% enhanced efficiency compared with the PV module with a bare coverglass measured under one sun of the air mass 1.5 global, showing the necessity of integrating a finely designed NS coverglass to effectively improve the efficiency of various PV systems with multijunction solar cells.

Compound Eyes

Although recently developed bio-inspired nanostructures exhibit superior optic performance, their practical applications are limited due to cost issues. We present highly transparent glasses with grassy surface fabricated with self-masked dry etch process. Simultaneously generated nanoclusters during reactive ion etch process with simple gas mixture (i.e., CF4/O2) enables lithography-free, one-step nanostructure fabrication. The resulting grassy surfaces, composed of tapered subwavelength structures, exhibit antireflective (AR) properties in 300 to 1,800-nm wavelength ranges as well as improved hydrophilicity for antifogging. Rigorous coupled-wave analysis calculation provides design guidelines for AR surface on glass substrates.

Efficiency Enhancement of III-V Triple-Junction Solar Cell Using Nanostructured Bifunctional Coverglass With Enhanced Transmittance and Self-Cleaning Property

We demonstrate the distinctive optical properties of disordered nanostructures on glass substrates in accordance with changes in the average size of the nanostructures. Dissimilar sizes of nanostructures were fabricated by using different thicknesses of thermally dewetted Ag nanoparticles as etch masks. Unlike a flat glass substrate, the nanostructured glasses (NSGs) show a changed optical characteristic. By increasing the size of the nanostructures, the wavelength of the peak transmittance of about 99% gradually moved from 730 to 2000 nm. To clearly discern the effect of the different sizes of nanostructures, the normalized angle-dependent transmittance spectra of the NSGs were analyzed. Only if the size becomes relatively larger than the wavelength of the incident light are the transmittance spectra more strongly affected by the incident angle as well as by the relative size, rather than by the Fresnel reflection.

Antireflective grassy surface on glass substrates with self-masked dry etching

We demonstrate the hierarchical micro- and subwavelength-structures on the surface of GaN based green vertical light-emitting diodes (VLEDs) to enhance the light extraction efficiency. Hierarchical structures (HSs), which were fabricated by simple two-step process consisting of pattern transfer of reflowed photoresist for micro structures (MSs) and self-masked dry etching method for subwavelength structures (SWSs), play an important role in enhancing the light extraction efficiency by reducing both the total internal reflection and the Fresnel reflection. HSs lead to an enhancement of optical output power of ~200% compared to the conventional VLEDs with flat surface as a result of reflection suppression and transmittance enhancement, without any serious degradation of electrical characteristics and a change of peak emission wavelength as function of injection current. Furthermore, optical simulations by ray-tracing and the rigorous coupled-wave analysis methods provide design guidelines for MSs and SWSs, respectively.

Size-dependent optical behavior of disordered nanostructures on glass substrates

We demonstrate an advanced structure for optical interconnect consisting of 4 channel × 10 Gb/s bidirectional optical subassembly (BOSA) formed using silicon optical bench (SiOB) with tapered fiber guiding holes (TFGHs) for precise and passive optical alignment of vertical-cavity surface-emitting laser (VCSEL)-to-multi mode fiber (MMF) and MMF-to-photodiode (PD). The co-planar waveguide (CPW) transmission line (Tline) was formed on the backside of silicon substrate to reduce the insertion loss of electrical data signal. The 4 channel VCSEL and PD array are attached at the end of CPW Tline using a flip-chip bonder and solder pad. The 12-channel ribbon fiber is simply inserted into the TFGHs of SiOB and is passively aligned to the VCSEL and PD in which no additional coupling optics are required. The fabricated BOSA shows high coupling efficiency and good performance with the clearly open eye patterns and a very low bit error rate of less than 10-12 order at a data rate of 10 Gb/s with a PRBS pattern of 231-1.

Improved light extraction efficiency of GaN-based vertical LEDs using hierarchical micro/subwavelength structures

We report on advanced types of coverglasses, which include the hierarchical micro- and subwavelength-structured surfaces for improved absorption efficiency of photovoltaic modules. Prism-shaped microstructures (PSMSs) help eliminate the optical shading caused by the metal grid of solar cells. The subwavelength structures (SWSs) with a tapered shape behave as a refractive index matching layer to reduce the surface reflection at the interface of air and coverglass. The geometries of the PSMSs and SWSs were designed by ray-tracing and the rigorous coupled-wave analysis method simulations, respectively, which provided the design guidelines. The PSMSs and SWSs were fabricated by a simple two-step process consisting of an isotropic wet etching process with hydrofluoric acid solution using a SiNx mask and a self-masked dry etching process, respectively. A hybrid patterned coverglass incorporating the PSMSs with SWSs were mounted on GaInP/(In)GaAs/Ge triple-junction solar cells with a precise alignment process. The measured power conversion efficiency of the subreceiver module with the patterned glass reached 32.97% for 1 sun, which is 11.19% higher compared with a subreceiver module with a flat coverglass.

4 channel × 10 Gb/s bidirectional optical subassembly using silicon optical bench with precise passive optical alignment.

This Letter reports a novel method for the simple fabrication of microlens arrays with a controlled shape and diameter on glass substrates. Multilayer stacks of silicon dioxide deposited by oblique angle deposition with hole mask patterns enable microlens formation. Precise control of mask height and distance, as well as oblique angle steps between deposited layers, supports the controllability of microlens geometry. The fabricated microlens arrays with designed geometry exhibit uniform optical properties.

Improved Light Absorption of GaInP/GaAs/Ge Solar Cell Modules With Micro/Nanoengineered Coverglasses

We demonstrated a new structure of optical interconnect module based on double side perforated silicon optical platform. This structure can eliminate the use of micro mirrors, lens, and guide pin, leading to a reduced packaging cost and volume. Simulation results show that the proposed structure can result in high coupling efficiency with relaxed permissible error for assembly.