Hsiao-Chiu Hsu

Laser Scribing of Sapphire Substrate to Increase Side Light Extraction of GaN-Based Light Emitting Diodes

Generally, the laser scribing was done after GaN-based light emitting diodes (LEDs) growth. This study verified that the utilization of laser scribing leads to an increase in the surface roughness of sapphire substrate sidewalls, which reduces the probability of total internal reflection from light striking the sapphire/air interface. Laser scribing also helps increase side light extraction intensity and output power of GaN-based light emitting diodes. Study results indicated that lasers create a laser scribing layer (LSL) at a depth of approximately 30 μ m after GaN-based LEDs grown in the sapphire substrate undergo laser scribing. Scanning electron microscopy was used to observe the rough surface of the LSL, while near-field optical images verified that rough surface LSL contributes to an increase in side wall light extraction intensity of LEDs. Furthermore, changing the depth of focus of the laser beam (from 0 μm to 36 μm) allows the formation of a large quantity of 3 to 5 μm holes on the LSL. Measurement results indicated that these holes caused the LSL surface to be even rougher, which further strengthened LED side wall light extraction intensity. The results after packaging show that LSL with holes increase output power at 20 mA of GaN-based LEDs by approximately 12.2 %.

Enhanced Performance of Nitride-Based Blue LED With Step-Stage MQW Structure

A step-stage InGaN/GaN multiquantum-well (MQW) structure can enhance the efficiency of GaN-based light-emitting diodes (LEDs). Compared to dual-stage MQW LEDs, the step-stage MQW LEDs have lower forward voltage and higher light output. The measured light output power of step-stage LEDs operating at 350 mA shows an increase of approximately 23% with an external quantum efficiency (EQE) increase of 6.6%, when compared to dual-stage LEDs.

Improvement of Light Intensity for Nitride-Based Multi-Quantum Well Light Emitting Diodes by Stepwise-Stage Electron Emitting Layer

We demonstrate the improvement of light intensity for light emitting diodes (LEDs) using an electron emitting layer (EL) constructed by quantum wells (QWs) with stepwise depths. The results show that the stepwise-stage EL can further improve the light intensity, about 20% increment compared with that of a dual-stage LED at 20 mA. It is found that the crystal quality and electron capture rate are higher for a stepwise-stage structure. We also explore the influence of the number of QWs in the stepwise-stage EL on the light intensity. The result shows that the intensity increases with QW number but the saturation starts at about eight QWs.

Direct Growth of a-Plane GaN on r-Plane Sapphire by Metal Organic Chemical Vapor Deposition

In this study, we had demonstrated the direct growth of nonpolar a-plane GaN on an r-plane sapphire by metal organic chemical vapor deposition (MOCVD) without any buffer layer. First, in this experiment, we had determined the optimum temperature for two-step growth, including obtaining three-dimensional (3D) GaN islands in the nucleation layer and coalescing with a further two-dimensional (2D) growth mode. The result shows that the nucleation layer grown under high temperature (1150 °C) leads to large islands with few grain boundaries. Under the same temperature, the effect of the V/III ratio on the growth of the overlaying GaN layer to obtain a flat and void free a-plane GaN layer is also studied. The result indicates one can directly grow a smooth epitaxial layer on an r-plane sapphire by changing the V/III ratio. The rms roughness decreases from 13.61 to 2.02 nm. The GaN crystal quality is verified using a mixed acid to etch the film surface. The etch pit density (EPD) is 3.16 ×107 cm-2.

Dislocation reduction through nucleation and growth selectivity of metal-organic chemical vapor deposition GaN

A novel serpentine channel structure is used to mask the sapphire substrate for the epitaxial growth of dislocation-free GaN. Compared to the existing epitaxial lateral overgrowth methods, the main advantages of this novel technique are: (a) one-step epitaxial growth; (b) up to 4 times wider defect-free regions; and (c) the as-grown GaN film can be transferred easily to any type of substrate. TEM, etch pits and cathodoluminescence experiments are conducted to characterize the quality of as-grown GaN. The results show that the average etch-pit density in the yet-to-be-optimized GaN epi-layers is about 4 × 105 cm−2. The underlying physics of selective nucleation and growth is investigated using the finite element method (COMSOL). It is concluded that the proximity effect dominates the selective growth of GaN on the serpentine channel structure masked sapphire. This novel technique is a promising candidate for the growth of high quality III-nitride and the subsequent high-performance device fabrication inclu...

Enhancement in Light Extraction of GaN-Based Light-Emitting Diodes With High Reflectivity Electrodes

In GaN-based light-emitting diodes (LEDs), using gold-based metals to serve as electrodes, light absorption is a problem that severely restricts the light extraction in LEDs. This study demonstrates the GaN-based LEDs with high reflectivity metals (Ag/Pt) onto an n-type GaN surface and transparent contact layer (indium-tin-oxide), to serve as the n-type electrode and the p-type electrode, respectively. By replacing Cr/Au with Ag/Pt to serve as electrodes of LEDs, the light output power of the LEDs was increased by 15.7%, thereby significantly reducing the manufacturing cost of LEDs.

Enhancement of Optical Polarization Anisotropy of a-Plane InGaN/GaN Multiple Quantum Well Structure from Violet to Blue-Green Light

A nonpolar a-plane (1120) InGaN/GaN epitaxial layer was grown on r-plane (1012) sapphire substrates by metal–organic chemical vapor deposition (MOCVD). In this work, a set of step-stage multiple quantum wells (MQWs) is inserted between underlying GaN and overlying high indium-content MQWs to investigate its influence on the optical properties of the active region. The step-stage MQWs were deposited by varying growth temperature at fixed precursor flow rate. Optical properties were investigated by the measurement of temperature-dependent photoluminescence (TD-PL). The optical polarization ratio, activation energy, and the smile-like curve in full width at half maximum (FWHM) of PL were analyzed in detail.

Improved optical properties of a-plane InGaN/GaN multiple quantum wells with gradient-stages MQW structure

1 Institute of Microelectronics, Department of Electr ical Engineering, and Advanced Optoelectronic Techn ology Center, National Cheng Kung University, No. 1, Dasyue Rd., East District, Tainan 701, Taiwan Phone: +886-919-839392 E-mail: audreyhsu1981@gmail. com 2 Department of Electrical Engineering, Kun Shan Uni versity of Technology, Yung-Kang, Tainan 710, Taiwa n ITRI South Micro Systems Technology Center, Advance d Actuator Systems Department, Tainan 709, Taiwan

Improvement in a-Plane GaN Crystal Quality by Investigating Different Buffer Layer

Non-polar a-plane GaN epitaxial layer was grown on r-plane sapphire substrates by metal–organic chemical vapor deposition (MOCVD). In this paper, we implanted different buffer layer, including the GaN buffer layers under different growth parameters and in situ SiNx layer, to improve the crystal quality. First, high growth temperature (1150 °C) of GaN buffer layer can improve the crystal quality than low temperature (540 °C) by scanning electron microscopy (SEM) images. The inverse pyramidal pits are also reduced by implanting in situ SiNx interlayer, behaving as a nanomask deposited on r-plane sapphire. The GaN film quality is also improved by growing GaN buffer layer under low V/III ratio and confirmed by high-resolution X-ray diffraction (HR-XRD). An improvement of crystal quality through initial surface roughening can be verified by atomic force microscopy (AFM). The rms roughness of the overlaying GaN surface can be reduced from 11.82 to 1.54 nm. The growth mechanism was discussed.