Yongjo Park

Alternating-current Light Emitting Diodes with a Diode Bridge Circuitry

Most solid-state light emitting devices operate under direct current (DC) condition now. We report the alternating current (AC) light emitting devices fabricated with a diode bridge circuitry which is also made of light emitting diodes (LEDs). The LED bridge circuitry which is flipped on a silicon submount is composed of 4 branches with 7 LED chips and participates as a light emitting component as well. The AC LED can be operated with radiant flux of 0.83 W at an electric power of 8.5 W. This concept could be applied to fabricate compact and economical AC LEDs for a solid-state illumination.

Microstructured Air Cavities as High-Index Contrast Substrates with Strong Diffraction for Light-Emitting Diodes

Two-dimensional high-index-contrast dielectric gratings exhibit unconventional transmission and reflection due to their morphologies. For light-emitting devices, these characteristics help guided modes defeat total internal reflections, thereby enhancing the outcoupling efficiency into an ambient medium. However, the outcoupling ability is typically impeded by the limited index contrast given by pattern media. Here, we report strong-diffraction, high-index-contrast cavity engineered substrates (CESs) in which hexagonally arranged hemispherical air cavities are covered with a 80 nm thick crystallized alumina shell. Wavelength-resolved diffraction measurements and Fourier analysis on GaN-grown CESs reveal that the high-index-contrast air/alumina core/shell patterns lead to dramatic excitation of the low-order diffraction modes. Large-area (1075 × 750 μm(2)) blue-emitting InGaN/GaN light-emitting diodes (LEDs) fabricated on a 3 μm pitch CES exhibit ∼39% enhancement in the optical power compared to state-of-the-art, patterned-sapphire-substrate LEDs, while preserving all of the electrical metrics that are relevant to LED devices. Full-vectorial simulations quantitatively demonstrate the enhanced optical power of CES LEDs and show a progressive increase in the extraction efficiency as the air cavity volume is expanded. This trend in light extraction is observed for both lateral- and flip-chip-geometry LEDs. Measurements of far-field profiles indicate a substantial beaming effect for CES LEDs, despite their few-micron-pitch pattern. Near-to-far-field transformation simulations and polarization analysis demonstrate that the improved extraction efficiency of CES LEDs is ascribed to the increase in emissions via the top escape route and to the extraction of transverse-magnetic polarized light.

Partial Polarization Matching in GaInN-Based Multiple Quantum Well Blue LEDs Using Ternary GaInN Barriers for a Reduced Efficiency Droop

GaInN-based multiple-quantum-well (MQW) blue LEDs with ternary GaInN barriers polarization-matched to GaInN wells are fabricated. Single-layered Ga0.9In0.1N and Ga0.9In0.1N/GaN multiple-layered quantum barriers (MLQBs) are used for 50% polarization matching. Compared to conventional GaInN/GaN MQW LEDs, the polarization-matched LED with GaInN/GaN MLQBs shows a higher light output power in a high injection current regime, resulting in reduced efficiency droop, along with a minimal blue-shift of emission with injection current, reduced ideality factor, and reduced forward voltage. These results are attributed to a reduced magnitude of polarization sheet charges at heterointerfaces between the GaInN well and the GaInN barrier, and the resultant reduced internal polarization field in the MQWs, thereby minimizing electron leakage current and efficiency droop.

Strong light-extraction enhancement in GaInN light-emitting diodes patterned with TiO2 micro-pillars with tapered sidewalls

An effective method to enhance the light extraction for GaInN light-emitting diodes (LEDs) is reported. The method employs TiO2 micro-pillars with tapered sidewalls, which are refractive-index-matched to the underlying GaN. The tapered micro-pillars are fabricated by using reflowed photoresist as mask during CHF3-based dry etch, with O2 added in order to precisely control the taper angle. LEDs patterned with TiO2 micro-pillars with tapered sidewalls show a 100% enhancement in light-output power over planar reference LEDs. The measured results are in good agreement with ray-tracing simulations, showing strong potential of optical surfaces that are controlled in terms of refractive index and lateral structure.

Improved performance of AlGaN-based deep ultraviolet light-emitting diodes with nano-patterned AlN/sapphire substrates

We demonstrated AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs) with periodic air-voids-incorporated nanoscale patterns enabled by nanosphere lithography and epitaxial lateral overgrowth (ELO) on a 4-in. sapphire substrate. The nanoscale ELO improved the crystal quality of overgrown epitaxial layers at a relatively low growth temperature of 1050 °C and at small coalescence thickness less than 2 μm. The light output power of the DUV LED was enhanced significantly by 67% at an injection current of 20 mA. We attribute such a remarkable enhancement to the formation of embedded periodic air voids which cause simultaneous improvements in the crystal quality of epitaxial layers by ELO and light extraction efficiency enabled by breaking the predominant in-plane guided propagation of DUV photons.

Local electroluminescence and time-resolved photoluminescence study of InGaN light-emitting diodes

Uniformity of luminescence properties in blue InGaN light-emitting diodes has been studied and analyzed by local time-resolved photoluminescence and microelectroluminescence measurements at different biasing. For studied structures, some nonuniform distribution of photoluminescence properties has been observed at reverse biasing conditions. This nonuniformity revealed inhomogeneous distribution of electric field over the active region. It is supposed that nonuniform distribution of acceptors concentration in p-GaN is a source of electric field fluctuations. Microelectroluminescence measurements showed that areas with locally lower acceptor concentration in p-GaN layer emit lower electroluminescence intensity. This was caused by limited hole injection efficiency into multiple quantum wells region at high current.

Extraction efficiency enhancement in GaN-based light emitters grown on a holographically nano-patterned sapphire substrate

We observed over two-fold enhancement in light extraction efficiency from GaN-based light emitters grown on a nano-patterned sapphire substrate. Nano-hole array patterns were generated by laser holography, which enabled a large area process with high throughput.

Multi-step plasma etching process for development of highly photosensitive InSb mid-IR FPAs

Reactive ion beam etching (RIBE) with CH4/H2/Ar or Cl2/Ar and ion beam etching (IBE) with Ar has been widely used for indium-contained compound semiconductors such as InAs, InP and InSb. To improve the performance of InSb FPAs, reduction of the ion-induced defects and the surface roughness is one of the key issues. To find the optimized plasma etching method for the fabrication of InSb devices, conventional plasma etching processes were comparatively investigated. RIBE of InSb was observed to generate residual by-products such as carbide and chloride causing the degradation of devices. On the other hand, very smooth surface was obtained by etching with N2. However, the etch rate of the N2 etching was too slow for the application to the device fabrication. As an alternative way to solve these problems, a multi-step plasma etching process, a combination of the Ar etching and the N2 etching, for InSb was developed. As gradually increasing the amount of N2 gas flow during the etching process, the plasma damage causing the surface roughen decreased and consequently smoother surface close to that of N2 RIE could be obtained. Furthermore, Raman analysis of the InSb surface after the plasma etching indicated clearly that the multi-step etching process was an effective approach in reducing the ion-induced damages on the surface.

High-power packages for phosphor-based white-light-emitting diode lamps

With the rapid development of high-power white light-emitting diodes (LEDs), advances in packaging are required to further improve the device performance. In this work, an optimized packaging configuration for high power LED lamps with enhanced phosphorescence efficiency is presented based on ray-tracing simulations and experimental results

Linearly polarized photoluminescence of anisotropically strained c-plane GaN layers on stripe-shaped cavity-engineered sapphire substrate

Anisotropic in-plane strain and resultant linearly polarized photoluminescence (PL) of c-plane GaN layers were realized by using a stripe-shaped cavity-engineered sapphire substrate (SCES). High resolution X-ray reciprocal space mapping measurements revealed that the GaN layers on the SCES were under significant anisotropic in-plane strain of −0.0140% and −0.1351% along the directions perpendicular and parallel to the stripe pattern, respectively. The anisotropic in-plane strain in the GaN layers was attributed to the anisotropic strain relaxation due to the anisotropic arrangement of cavity-incorporated membranes. Linearly polarized PL behavior such as the observed angle-dependent shift in PL peak position and intensity comparable with the calculated value based on k·p perturbation theory. It was found that the polarized PL behavior was attributed to the modification of valence band structures induced by anisotropic in-plane strain in the GaN layers on the SCES.Anisotropic in-plane strain and resultant linearly polarized photoluminescence (PL) of c-plane GaN layers were realized by using a stripe-shaped cavity-engineered sapphire substrate (SCES). High resolution X-ray reciprocal space mapping measurements revealed that the GaN layers on the SCES were under significant anisotropic in-plane strain of −0.0140% and −0.1351% along the directions perpendicular and parallel to the stripe pattern, respectively. The anisotropic in-plane strain in the GaN layers was attributed to the anisotropic strain relaxation due to the anisotropic arrangement of cavity-incorporated membranes. Linearly polarized PL behavior such as the observed angle-dependent shift in PL peak position and intensity comparable with the calculated value based on k·p perturbation theory. It was found that the polarized PL behavior was attributed to the modification of valence band structures induced by anisotropic in-plane strain in the GaN layers on the SCES.