Optimization of the Optical and Electrical Properties of GaN Vertical Light Emitting Diode with Current Block Layer
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Optimization of the Optical and Electrical Properties of GaN Vertical Light Emitting Diode with Current Block Layer Na Lu1, Zhiqiang Liu3,2, Enqing Guo2, Liancheng Wang2, Andrew Melton3 and Ian Ferguson3 1
Engineering Technology, University of North Carolina at Charlotte, Charlotte, North Carolina, 28213, USA 2 Lighting Research & Development Center, Institute of Semiconductors, Chinese Academy of Science, Beijing, 100083, China 3 Electrical and Computing Engineering, University of North Carolina at Charlotte, Charlotte, North Carolina, 28213, USA ABSTRACT The emission from a light emitting diode (LED) that is emitted under the metal electrode cannot escape into free space. A current blocking layer (CBL) is used to address this issue by forcing the current to flow laterally under the electrode reducing the emission absorbed and hence increasing the overall efficiency of the LED. In this paper a new method to fabricate Schottky and isolating CBLs in GaN LED are investigated. Optical and electrical measurements of these vertical LEDs with and without CBL show different light output powers at identical current densities. The results of this study indicate that CBLs could also be used to suppress the efficiency droop effect for GaN LEDs. INTRODUCTION GaN LEDs have received much attention as a future, green, solid state lighting source and they have already been used in many applications such as automotive applications, back lighting. Research into developing high luminous, power and efficiency LEDs is the key to the continued development of this area. Epitaxial GaN devices grown on sapphire substrate have been the core of this technology and, traditional, lateral LED structures can work with current densities up to 100A/cm2. However, under larger current densities the LED junction temperature rises due to the poor thermal conductivity of the sapphire substrate. This results in a drastic drop of the luminous efficiency and reliability of the device which has been called efficiency droop. A vertical LED structure has many advantages compared to a lateral LED with respect to current spreading, thermal cooling, and luminous efficiency. Therefore, the vertical LED attracted much attention and may become primary chip structure for general lighting applications. However, as discussed above, the external quantum efficiency of GaN LED exhibits large droop when it operating under large injection current densities [1-3]. A number of methods, such as an InGaN barrier, a doped GaN barrier and an AlInGaN quantum well have been used to alleviate the efficiency droop [1-4]. A current blocking layer (CBL) has proved an effective way to improve the external quantum efficiency of LEDs [5-7]. In this work, optical and electrical properties of GaN vertical LED with CBLs are investigated and the results show that the CBL can reduce the efficiency droop of the LED.
EXPERIMENT It is well known that the electrode pattern and the resulting lateral resistance can result in non-uniform current distributions in LEDs. For example, different areas
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