报告题目: Photoelectrochemical Techniques for Fabricating Efficient Vertical Light-emitting Diode

　　报 告 人 :, 楊志忠, Professor，臺灣大學光電所

　　时间: 2013年11月12日 (星期二) 上午10:00

　　地点: 中科院半导体研究所图书馆101会议室

　　Abstract: In this presentation, we discuss the photoelectrochemical (PEC) techniques for sapphire substrate liftoff and surface grating formation for increasing the yield of vertical light-emitting diode (VLED) fabrication and its emission efficiency. Although the laser liftoff technique can produce quite effective sapphire liftoff results for fabricating VLED, it is basically a series process. In other words, this technique needs to process one substrate after another. Also, its high-power laser may damage the active region of an LED, leading to the low yield in fabricating VLED. In this paper, a low-cost sapphire substrate liftoff method based on the PEC process is demonstrated. This method can be used for parallel multiple-wafer liftoff of sapphire substrate. In other words, in one process run, it can remove the sapphire substrates of multiple full-wafer samples simultaneously if the electrolyte container of PEC and the UV light beam for PEC process are large enough. Also, it can be applied to a sample of any wafer size. By preparing patterned sapphire substrate (PSS) with two-dimensional periodic triangular grooves of several microns in width and height and the nitride epitaxial structure with the grooves preserved to form tunnels, PEC electrolyte can flow along the tunnels to etch the bottom of the GaN layer for separating the PSS from the wafer-bonded epitaxial layer. To monitor the formation of such tunnels under the epitaxial layer noninvasively, we use the technique of optical coherence tomography to scan the MOCVD grown samples. In a scanned image, the triangular tunnels can be clearly seen. Based on such a process, the liftoff of the sapphire substrate of a 2-inch full wafer has been accomplished. Also, we demonstrate an inexpensive method for fabricating periodic surface structures, i.e., gratings, on the N-face n-GaN surface of a VLED based on the PEC etching technique and show that such a grating VLED can have significantly higher light extraction efficiency than a similar VLED with a rough surface. The surface gratings of different periods are formed by building a Lloyd’s interferometer within the electrolyte (low-concentration KOH) of the PEC setup. From the observations of scanning electron microscopy and atomic force microscopy, it is found that the surface grating pattern consists of a distribution of grain domains of different sizes. For comparison, rough surfaces are also fabricated with room-temperature high-concentration KOH etching to form a distribution of pyramidal islands. Among the surface grating VLEDs, light extraction is stronger in a sample of a smaller grating period. Compared with the samples of rough surfaces fabricated with PEC or wet etching, the grating VLEDs of short grating periods have the higher light extraction efficiencies. The more effective light extraction in a grating VLED is attributed to the mixture of the diffraction of the grating pattern and the scattering of the distribution of the grain domains of different sizes. Compared with a reference VLED of flat surface, the output intensity of a grating VLED with ~160 nm in grating period and ~220 nm in grating groove depth can be enhanced by >140 %. By adding epoxy to the grating surface, the output intensity can be further increased by >170 %.

　　报告人简介：Professor C. C. Yang received his BS degree in 1976 from the Department of Electrical Engineering, National Taiwan University, Taipei, MS (1981) and Ph.D. (1984) degrees also in electrical engineering from University of Illinois, Urbana-Champaign. Before he joined the Graduate Institute of Electro-Optical Engineering, National Taiwan University, as a full professor in 1993, he has been being a faculty member in the Department of Electrical Engineering, The Pennsylvania State University. During the period from August 2001 to July 2007, he served as the Chairman/Director of the Graduate Institute of Photonics and Optoelectronics, National Taiwan University. His research areas include the MOCVD growth of nitride compounds, MBE growth of nitride and oxide semiconductor nanostructures, white-light light-emitting diodes for solid-state lighting and display, surface plasmonics, photonic crystals, ultrafast optics, and biophotonics (optical coherence tomography). He has published more than 600 journal and conference papers. Professor Yang is a fellow of the Optical Society of America.