Study on size-dependent characteristics of group III-nitride semiconductor micro light emitting diodes for micro display and visible light communication

Abstract

Group III-nitride semiconductor based micro light emitting diodes (LEDs) have attracted much interest due to their superior properties such as high efficiency and stability. Based on the advantages, the micro LEDs are widely being utilized in various fields such as micro display, wireless communication, medical indicator, etc. However, it has been studied that reducing the size of the micro LEDs accompanies changes in electrical and optical properties such as wall-plug efficiency. To fully utilize the advantages of the III-nitride micro LEDs, comprehension on these chip size-dependent properties is highly desirable. In this dissertation, we studied chip size dependent optical and electrical properties of III-nitride micro LEDs fabricated by top-down and bottom-up processes, respectively. Physical mechanisms on the changes of properties with reducing chip size are studied and suggested. Finally, proper application fields as micro display and visible light communication are suggested. Firstly, for micro display application, blue and green micro LEDs with various chip size were fabricated by top-down process. From photoluminescence (PL) experiments, all the investigated micro LEDs showed decreasing PL efficiency with reducing size. However, the ratio of efficiency degradation to the efficiency of the bare LED wafer depended on the properties of the bare LED wafer, such as recombination rate and surface recombination velocity. From temperature dependent current density-voltage measurements, two different major current components are distinguished, and size-dependency of each current components are compared between blue and green micro LEDs. In addition, microscopic PL investigation on a single blue micro LED was carried out to investigate the physical origin of the chip size-dependent properties. Based on these investigations, we expect that more precise design on efficient micro LEDs is available. Secondly from bottom-up approach, 3-dimensional (3D) structures based phosphor-free white LEDs are grown by selective area growth technique. From cathode-luminescence and transmission electron microscope images, the broad emission in visible range was correlated to the structural properties. With chip fabrication optimized for the 3D structures, the 3D structure based LEDs showed much reduced leakage current with reliable electroluminescence from low injection current. The phosphor-free white LEDs showed much faster electrical-to-optical modulation properties compared to conventional phosphor-based white LEDs. Based on these investigations, we suggested the 3D structure based phosphor-free white micro LEDs as a new candidate for indoor lighting as well as visible light communication transceiver. From two approaches, we investigated chip size dependent optical and electrical properties of group III-nitride semiconductor based micro LEDs and their optimized application fields. Based on these investigations, we expect more precise design and fabrication of micro LEDs are available, to be utilized more effectively in variety of application fields.