Improving GaN-based light-emitting diodes
Date of Issue2017-02-08
School of Electrical and Electronic Engineering
Due to the superior advantages of high energy conversion efficiency, high brightness, high reliability, controllable color properties, long lifetime, and ease of miniature and digitalization, GaN-based light-emitting diodes (LEDs) are regarded as the next generation solid state lighting sources to replace the conventional lighting devices of fluorescent and incandescent lamps. Tremendous effort has been devoted to LED research, development and commercialization, thus drastic advances have been made in the past decades. However, despite these advantages and achievements, GaN-based LEDs still suffer from several technical issues, including insufficient heat dissipation, current crowding effect, low light extraction efficiency, and efficiency droop. All these problems, especially efficiency droop delay LED’s further expansion in the general lighting market. Hence, a large amount of research work focusing on improving the efficiency droop has been carried out to achieve better LED performance. In this dissertation, the InGaN/GaN multiple quantum well (MQW) LEDs have been studied from multiple aspects, including material quality, light extraction efficiency, p-type doping influence, current crowding effect, electron overflow, and carrier transport. The LED epitaxial wafers studied are grown on c-plane patterned sapphire substrate using metal-organic chemical vapor deposition (MOCVD) system. Standard fabrication processes for flip-chip LEDs are developed and electrical and optical characterizations are conducted to evaluate the performance. Difficulty in p-type doping and low hole injection efficiency lead to non-uniform carrier distribution within MQWs. Consequently, the quantum well (QW) number is crucial to the LED performance considering the recombination volume and carrier distribution. On the other hand, from the perspective of epitaxial growth, the strain will accumulate due to lattice mismatch between III-nitride layers. When the effective thickness of the lattice-mismatched MQW active region is beyond its critical layer thickness, the misfit strain relaxation accompanied with the generation of defects occurs during the epitaxial growth. As a result, the choice of QW number is critical to LED performance. In this thesis, the influence of QW number on LED performance is systematically investigated both numerically and experimentally. By investigating the carrier concentration distribution, electron overflow, and current spreading for LEDs with varied QW numbers, the improvement mechanism is uncovered. The deep analysis of the measured external quantum efficiency (EQE) curves in conjunction with carrier lifetime measurement reveals the limitations of increasing QW number from the perspective of strain accumulated and defects generated during epitaxial growth. Moreover, the study of p-type doping is conducted due to its vital impact on the hole injection for GaN-based LEDs. We propose a depletion-region Mg-doping method to systematically analyze the effectiveness of different Mg-doping profiles ranging from the electron blocking layer to the active region. Numerical computations show that the Mg-doping decreases the valence band barrier for holes and thus enhancing the hole transportation. Meanwhile, the depletion-region Mg-doping approach also increases the barrier height for electrons, which leads to a reduced electron overflow. More importantly, the depletion-region Mg-doping increases the hole concentration in p-GaN layer. Experimentally measured EQE indicates that Mg-doping position is vitally important, i.e., the doping in or adjacent to QW degrades the LED performance resulting from Mg diffusion and the corresponding nonradiative recombination. One potential approach to improve the power and luminous flux of a micro-LED is to optimize its size and emitting surface area. Nevertheless, the size of GaN-based LEDs is limited by the insufficient current spreading due to the low conductivity of the p-GaN layer. Thus micro-wall geometry with different mesa dimensions for InGaN/GaN LEDs is designed and fabricated to investigate the size effect on the LED performance. Through building a model for current paths, the underlying physics and mechanism of the improvement of the current spreading effect are revealed by numerically calculating the average current density in the vertical direction. In addition, in order to meet the requirement for high output power and improve the light extraction efficiency, a novel LED architecture with three micro-walls is proposed and the corresponding improvement mechanism is explored as well. In summary, this dissertation studies the InGaN/GaN MQW LEDs both numerically and experimentally. Several key factors are investigated including the QW number, p-type doping, current spreading, and light extraction efficiency. The underlying physics and mechanism of the influence of these factors are revealed. This work provides insightful knowledge of LED physics and also useful guidelines on the critical QW number, p-type doing, and light extraction architecture when designing the high performance GaN-based LEDs.