Performance Improvement of InGaN-based Light Emitting Diodes Fabricated by Patterned Sapphire Substrate Technique and Chemical Lift-Off Process

• Main Authors: Hsu-Hung Hsueh, 薛旭宏
• Other Authors: Ray-Hua Horng
• Format: Others
• Language: en_US
• Published: 2016
• Online Access: http://ndltd.ncl.edu.tw/handle/72771861450586136106

博士 === 國立中興大學 === 精密工程學系所 === 104 === Gallium nitride (GaN) is a wide bandgap (3.4 eV) semiconductor with wurtzite structure, and it is suitable to apply for high power devices. In this research, in order to improve the crystal quality of GaN epilayer and the light efficiency of GaN-based blue light emitting diodes (LEDs), two different patterned sapphire substrates (PSSs) have been proposed for the GaN growth. Moreover, a novel technique (Eco-GaN template) was also presented to achieve the reduction in production cost and the substrate reuse. Thus, this dissertation was divided into two parts to discuss. In the first part, the GaN epilayers were grown on various PSSs to reduce the dislocation density of GaN and enhance the internal quantum efficiency of LED. After the GaN growth on PSSs, the threading dislocations can be blocked efficiently by performing the epitaxial lateral overgrowth (ELOG) method. There are two different types of PSSs presented in this study. One is the cone-shaped PSSs and the other is flat-top pyramid PSSs (FTP-PSSs). The cone-shaped PSSs were prepared using dry and wet etching processes. Additionally, to enhance the epilayer quality and LED performance, the wet etching process was used to modify the top-tip shape of the PSSs by changing the etching time. The optimal wet etching time for the cone-shaped PSSs was determined to be 3 min. As the GaN epilayer was prepared on the conventional sapphire substrate (CSS), the FWHM values of XRD rocking curves at (002) and (102) planes were 412 and 593 arcsec, respectively. After depositing the GaN epilayer on the cone-shaped PSS by performing wet etching for 3 min, the FWHM values of XRD rocking curves at (002) and (102) planes were reduced to 272 and 285 arcsec, respectively. Furthermore, compared with the LEDs prepared on the CSS and the cone-shaped PSS without wet etching, the LED fabricated on the cone-shaped PSS by performing wet etching for 3 min showed 55% and 10% enhancements in the light output power (@ 350 mA), respectively. This indicates that the modification in the top-tip shape of cone-shaped PSSs is highly helpful to improve the light efficiency of LED devices. For the fabrication of FTP-PSSs, the sequential wet etching processes were employed. Besides, the effect of pattern size of the FTP-PSS on the epilayer quality and the LED device characteristics were investigated. Three kinds of FTP-PSSs that were denoted as FTP-PSS-A, FTP-PSS-B, and FTP-PSS-C, respectively, were formed for the fabrication of LEDs. The heights of patterns of these three FTP-PSSs were the same as 1.4 um. The diameters of circle areas on the top regions of FTP-PSS-A, FTP-PSS-B, and FTP-PSS-C were 1, 2, and 3 um, respectively. Based on the experimental results, the crystal quality of GaN epilayer and the light extraction of LED can be both enhanced through the use of FTP-PSS, especially for the FTP-PSS-A. The FWHM values of XRD rocking curves at (002) plane for the GaN epilayers grown on CSS and FTP-PSS-A were 412 and 238 arcsec, respectively. Meanwhile, the FWHM values of XRD rocking curves at (102) plane for these two epilayers were 593 and 327, respectively. Moreover, at an injection current of 350 mA, the output powers of LEDs fabricated on CSS and FTP-PSS-A were measured to be 157.38 and 253.94, respectively. In the second part, to achieve the decrease in production cost and the enhancement in LED efficiency, the Eco-GaN template that was defined as the deposition of a thick GaN epilayer with low defect density on sapphire substrate, was presented in this research. There are two different sacrificial layers, i.e., the AlN/patterned-SiO2 and the Ga2O3 layers, were deposited on the Eco-GaN templates, respectively. Then, the GaN-based blue LEDs were prepared on these Eco-GaN templates with various sacrificial layers. Finally, by conducting the chemical lift-off (CLO) process, the LED epitaxial structure was separated from Eco-GaN template that can be used for the reusable applications. According to the electron microscopy observation, the GaN epilayer grown on the AlN/patterned SiO2/Eco-GaN can obtain a high crystal quality through the ELOG process. In addition, to etch the patterned SiO2 and AlN layers, the HF and 80°C-KOH solutions were employed, respectively. By performing the CLO process, the LED device can be transferred to Cu substrate successfully. The output powers (@350 mA) of conventional sapphire-based LED and CLO-LED/Cu were measured to be 114 and 212 mW, respectively. The output power (@350 mA) of CLO-LED/Cu had 86% improvement as compared with that of conventional LED. As mentioned above, the other sacrificial layer prepared on the Eco-GaN template was the Ga2O3 film, which was deposited by MOCVD. To etch the Ga2O3 sacrificial layer, the LED structure/Ga2O3/Eco-GaN sample was immersed into the HF solution, and then the CLO-LED/Cu can be detached from the Eco-GaN template. A highly lateral etching rate of 70 um/hr for the Ga2O3 sacrificial layer was achieved when the CLO process was performed. The output powers (@350 mA) of conventional sapphire-based LED and CLO-LED/Cu were 128 and 187 mW, respectively. It reveals the output power of CLO-LED/Cu possessed 46% enhancement as compared with that of conventional LED. It indicates the good optoelectronic performance of the LED device on Cu substrate can be achieved via the use of CLO technique. On the other hand, the separated Eco-GaN template can be reused, showing a high feasibility in the cost-effective LED fabrication.