Metal-Cavity Used for the Blue-Green GaN-based Vertical Cavity Surface Emitting Lasers (VCSELs)

• Main Authors: Sheng-WenChen, 陳聖文
• Other Authors: Yan-Kuin Su
• Format: Others
• Language: en_US
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/63929584384872492018

碩士 === 國立成功大學 === 奈米科技暨微系統工程研究所 === 100 === The main purpose of this dissertation is to investigate metal-assisted structure and develop the bule-green GaN-based vertical surface emitting lasers (VCSELs) by the simulations and some approaches to improve the performances. The performance improvement will be considered in respect of optical properties including optical confinement factor and optical quality confinement factor, material gain spectra, and efficiency. In respect of research on the metallic-coated cavity under optical pumping conditions at room temperature, we found the structures could efficiently provide good optical confinement. It implied the metal-cavity microlasers provide a great potential choice for constructing GaN-based VCSELs. Due to multiple cavity modes in the structure, the optical gain could be calculated from the micro- photoluminescence (μ-PL) spectrum below the threshold condition by the Hakki-Paoli method. Furthermore, the optical gain along with the size effect of metallic-coated structure was hence analyzed. When diameter of metal-cavity is 5 μm, αMax is enhanced by one order from 10^-4 cm^-1 to 10^-3 cm^-1 due to the microcavity. The effect of size shrinkage provided good performance of metallic GaN-based cavity; nevertheless it increased the difficulty of fabricating process. On account of the small gain volume of VCSELs, it was essential to confine the optical-field intensity as much as possible in the quantum wells in order to achieve maximum modal gain. The optical confinement factor was crucial for the resonant cavity to decide the capability of optical property. Simulation of wave guiding for optimum optical confinement told us the required scales of device to attain the threshold condition. The COMSOL software was chosen for calculating the optical properties including optical confinement factor (Γ), optical quality factor of cavity (Qcavity) and correspondent patterns of electric fields. For fundamental mode, the diameter of the resonant cavity had little influence on Γ and Qcavity in the viewpoint of optics. As for the depth of metal-cavity at 2.5 μm, all values of Qcavity were approximately 99%, which indicated the optical confinement capability was saturated. However, all values of Γ were around 19.3% but not been well improved. Qcavity depended on the depth of the resonant cavity and could be improved by increase the depth of the resonant cavity and the thickness of n-GaN. However, deep etching (long term ICP etching) was necessary to obtain the high depth pillar structures and could lead to damages on the sidewall of pillars. Besides, Γ depended on the position of active region layer. In behalf of improving the optical properties of resonant cavity, the new design structure could be tuned by reducing the thickness of p-GaN from 600 nm to 150 nm and increasing the thickness of active region from 240 nm to 450 nm. As for the depth of new design metal-cavity at 1.5 μm under fundamental mode, all values of Γ were well improved to around 31% and all Qcavity were approximately 99%, which indicated the optical confinement capability was saturated. It was essential to optimize the best optical properties of resonant cavity by tuning the position of active region and epitaxial parameters to avoid the harmful deep-etching process. Finally, Demonstrations of optical pumping laser of our device indicated that fabricating metal-cavity blue-green GaN-based VCSELs is feasible. We successfully demonstrated the metal-cavity GaN-based cavity under optical pumping conditions at 15 K when the diameter and the depth of metal-cavity were 15 μm and 1.5 μm, respectively. The peak material gain of about 1.126 × 10^3 cm^−1 was obtained at the threshold condition about 80 MW/cm^2. Above the threshold condition, only one lasing mode dominated and the obvious change of slope efficiency was found. The peak gain of the lasing mode was at about 521 nm (2.38 eV). The sample with diameter of 20 μm had the peak material gain at 1.007 × 10^3 cm^−1. However, the broadening gain spectrum might reduce the performance of laser devices. This work unequivocally indicates that the metal-assisted structure used on the VCSELs was an important role in lasing action. The results presented here represent a breakthrough in the metal-coated GaN-based VCSELs.