Sang-Wan Ryu

The effect of strain on the interdiffusion in InGaAs/GaAs quantum wells

The effect of strain on the cation interdiffusion in InGaAs/GaAs quantum wells is described. It is found that the Fick’s diffusion equation does not properly describe the interdiffusion in the heterostructure with strained layers. It is believed that the strain changes crystal defect concentration and thus diffusivity is influenced by strain. Diffusion equation including the strain effect is formulated and solved numerically. The experimental photoluminescence peak shifts as a function of annealing time are well‐fitted by this analysis and useful parameters such as diffusivity describing InGaAs/GaAs quantum well interdiffusion are extracted.

Doping selective lateral electrochemical etching of GaN for chemical lift-off

An electrochemical etching based on oxalic acid was developed for use in the chemical lift-off of GaN epitaxial structures. It was shown that only the Si-doped n-GaN layer was etched away, while the p-type and undoped GaN layers were not etched at all. The etch rate and the remaining structure were analyzed for various doping concentrations and etching voltages. A lateral etch rate of 12 μm/min was achieved under 60 V for n-type doping concentration of 8×1018 cm−3. This doping selective etching was used to lift-off a GaN epitaxial layer patterned into 300×300 μm2 squares.

Design Optimization of Photonic Crystal Structure for Improved Light Extraction of GaN LED

We performed a theoretical analysis on improved light extraction efficiency of LEDs with photonic crystals (PCs). The light propagation and extraction of PC LEDs were simulated using the finite-difference time-domain method for various PC LED structures. LEDs with both top and bottom PCs and PC LEDs grown on patterned substrates were considered to maximize light extraction efficiency. The design parameters of the PC were varied, and optimized values were obtained. A disordered PC was simulated, and we showed that the light extraction efficiency of a disordered PC was nearly equivalent to that of an ordered PC with the same pattern periodicity. This result revealed that the increased light extraction of PC LEDs was mainly due to scattering. Moreover, by comparing the enhancement of PC LEDs with different shapes of air holes, we showed that only the density of holes and the area occupied by holes play important roles in light extraction. The shapes of the holes have no strong effect on the enhancement of light extraction.

Improved Light Output of Photonic Crystal Light-Emitting Diode Fabricated by Anodized Aluminum Oxide Nano-Patterns

A photonic crystal (PC) light-emitting diode (LED) was fabricated using an anodized aluminum oxide (AAO) nano-pattern technique, which has potential applications to mass production. The AAO nano-pattern, which was anodized in 0.3-M oxalic acid at 70 V, was transferred to the n-GaN bottom cladding through reactive ion etching and covered during the regrowth of GaN to form periodic air holes. The PC was embedded into the bottom cladding, which resulted in a low operating voltage. For the PC LED, the light output was enhanced by more than 20%, while the operating voltage increased only slightly. The radiation pattern of the PC LED showed two lobes at approximately plusmn30deg, which originated from the diffraction by PC.

Determination of interdiffusion coefficients of cations and anions in InGaAs/InP superlattice

The interdiffusion coefficients of cations and anions in InGaAs/InP superlattices (SLs) on their respective sublattices were analyzed quantitatively. Double crystal X-ray diffraction and simulation of the rocking curves based on dynamical diffraction theory were used to measure the interface strain that develops during rapid thermal annealing. Low temperature photoluminescence (PL) measurements were also done to assess the interdiffusion through the change in ground state transition energy of the SL. Simulation with the proper selection of the interdiffusion coefficients results in proper fitting of the interface strain profile and PL transition energies. Using this method, interdiffusion behaviors of InGaAs/InP SLs with and without SiO2:P capping were analyzed. Interdiffusion coefficients of 5.8×10−17 and 2.9×10−17 cm2/s were obtained for the anion and cation sublattices respectively, when the SL without SiO2:P was annealed at 800 °C.

New modification of the HPM for numerical solutions of the sine-Gordon and coupled sine-Gordon equations

We present an efficient modification of the Hes homotopy perturbation method that will facilitate the calculations. We apply this modification to solve sine-Gordon and coupled sine-Gordon equations, numerically. The new modification introduces a promising tool for many non-linear problems. The present method performs extremely well in terms of accuracy, efficiency, simplicity and reliability.

Monolithic integration of a multiwavelength laser array associated with asymmetric sampled grating lasers

We present a novel approach for a monolithically integrated multiwavelength laser array based on asymmetric sampled grating lasers. The asymmetric sampled grating laser combines sampled gratings of different periods with an index shifter to utilize the first-order reflection for lasing operation. With this structure, a simple fabrication procedure as well as high yield could be achieved without using complex and time-consuming e-beam lithography for multiperiod gratings. With numerical analysis based on a transfer matrix method, the effect of grating strength and mirror coating was analyzed to improve single-mode and power extraction performance. By using high-reflection/antireflection coatings on both facets, it was shown theoretically that high-power extraction efficiency as well as high single-mode yield was achieved. A four-channel laser array with 400-GHz wavelength spacing was fabricated and its operation at designed wavelengths was demonstrated. The individual laser showed a threshold current of 9-13 mA and a slope efficiency of around 0.21 W/A. A high sidemode suppression ratio over 44 dB was observed as well.

Improved photoelectrochemical water splitting efficiency of nanoporous GaN photoanode

Nanoporous (NP) GaN was used as a photoanode during photoelectrochemical water splitting and its efficiency and stability were compared with those of planar GaN. The NP GaN showed larger photocurrent density under illumination, which indicated more hydrogen generation. The enhancement was higher for a large applied voltage where the water splitting process was limited by charge transfer between semiconductor and electrolyte. Photocurrents decreased with time during the water splitting; however, the photocurrent degradation was less severe for NP GaN. It was attributed to efficient hole transport from photoanode to electrolyte in NP GaN due to its large surface area.

Analysis of Disordered Photonic Crystal Implemented in Light-Emitting Diode for High Light Extraction Efficiency

The efficiency of a light-emitting diode (LED) is limited because a large amount of generated light is confined inside due to total internal reflection. It is well known that light extraction from LEDs can be enhanced by using photonic crystal. However, the underlying physical mechanisms for increased emission are still largely unknown. In this work, we used the finite difference time domain method to analyze the photonic crystal (PC) LED with disordered air holes. The light extraction efficiency of disordered PC was nearly equivalent to that of ordered PC with the same pattern periodicity. This result supported the idea that the increased light extraction of PC LED was mainly due to the scattering, which is irrelevant to photonic band structure.

Towards highly efficient photoanodes: the role of carrier dynamics on the photoelectrochemical performance of InGaN/GaN multiple quantum well coaxial nanowires

The carrier dynamics in highly active InGaN/GaN coaxial nanowire photoanodes were studied for photoelectrochemical water splitting applications that can provide deeper insight to enhance the photon-to-electron conversion efficiency. The carrier dynamics in InGaN/GaN multiple quantum well coaxial nanowires (MQW-CNWs) with three different quantum well (QW) thicknesses and the same barrier thickness were studied optically using temperature-dependent and time-resolved photoluminescence spectroscopies. The role of the carrier dynamics on the photoelectrochemical water splitting (PEC-WS) performance of the MQW-CNWs was also investigated. The dependence of the PEC-WS performance and carrier dynamics on the QW thickness provided results indicative of the impact of the exciton localization and the defect states in the photoanodic performance of the MQW-CNWs. Strong localization effects and defect-induced recombination have been shown using samples with a thin QW with thicknesses up to 3 nm. During the PEC-WS, the samples showed a large onset potential and a low photocurrent density that led to low incident-photon-to-current conversion efficiency (IPCE). As the QW thickness approached 6 nm, negligible localization as well as improved photoemission quality were achieved, which lead to a small overpotential and a high IPCE of approximately 15%. The result demonstrated that an efficient photoanode requires a high crystal quality and weak localization, which can be achieved through careful structural optimization.