Enhancement of GaN-based optoelectronic device efficiency via nano-scale phenomenon

• Main Authors: Tsai, Yu-Lin, 蔡育霖
• Other Authors: Kuo, Hao-Chung
• Format: Others
• Language: en_US
• Published: 2015
• Online Access: http://ndltd.ncl.edu.tw/handle/34fc8a

博士 === 國立交通大學 === 光電工程研究所 === 104 === Recently, III-nitride light-emitting diodes (LEDs) have been regarded as the next generation of solid state lighting (SSL) due to its long lifetime, high efficiency, and energy-saving properties. With the development of LEDs, GaN-based solar cell is also received extensive attention due to the favorable photovoltaic characteristics and the possibility to realize full spectrum multi-junction solar cells. However, the efficiency of GaN-based solar cells and LEDs are still limited by the material properties and the device structure. For example, low light extraction efficiency (LEE), poor directional emission pattern, low internal quantum efficiency (IQE) of green LEDs, termed as “green gap” and the drop of efficiency at high current injection, termed as “efficiency droop” are still the bottlenecks of GaN-based LEDs. For GaN-based solar cells, insufficient light absorption, high threading dislocations (TDs), strain-induced piezoelectric field and high surface reflection are the bottlenecks of GaN-based solar cells. In the past few decades, these issues has been partially addressed by improving the materials quality and innovative device design. Currently, the efficiency enhancement through the material growth and device structure design has almost saturated. In order to further improve the efficiency of GaN-based optoelectronic devices, the next viable way is through the photon management via nano-scaled phenomenon. In the first part of this thesis, we proposed several schemes to enhance the efficiency of GaN-based solar cells. First, the embedded nano-air-void (ENAV) arrays were fabricated by combining the nano-imprint lithography and epitaxial lateral overgrowth technology. The ENAV arrays significantly enhance the IQE by suppressing the TDs and the residual strain in GaN epitaxial layer. Moreover, the ENAV arrays also acts as an efficient scattering back-reflector to increase the photon absorption. Consequently, the short circuit current and power conversion efficiency (PCE) were enhanced by 107% and 130%, respectively. Second, a hybrid InGaN/GaN multiple quantum well (MQW) solar cells were designed with enhanced PCE using colloidal CdS quantum dots (QDs) and back-side distributed Bragg reflectors (DBRs). CdS QDs can absorb ultraviolet (UV) photons, which are strongly absorbed by indium tin oxide (ITO), and they emit photons with a longer wavelength. This process improves the collection of photon-generated carriers and is known as the luminescence down-shifting (LDS). Consequently, an overall PCE that is 20.7% better than that of a reference device without CdS QDs and DBRs. In the second part of this thesis, we proposed several schemes to enhance the efficiency of GaN-based LEDs. First, nano-void arrayss (NVAs) were fabricated to embed within the GaN/InGaN green LEDs by using epitaxial lateral overgrowth (ELO) and nano-sphere lithography techniques. The NVAs act as an efficient scattering back-reflector to outcouple the guided and downward photons, which not only boosting light extraction efficiency of LEDs from 30.65% to 54.56% but also collimating the view angle of LEDs from 131.5o to 114.0o due to the strong guided photon extraction. Second, nano-ring LEDs were fabricated by nano-sphere lithography. The reduced quantum confinement stark effect was demonstrated due to nano-scaled strain relaxation. Therefore, the improved IQE of nano-ring LEDs is obtained. Importantly, the strain in MQW can be tuned by different wall thickness of nano-ring structures, which results in different effective bandgap energy. Consequently, the emission wavelength tuning capability of NRLED through strain engineering can be realized, the color of LEDs can be tuned from green to blue. This result presents the possibility to obtain the different color LEDs on one LED epitaxial wafer, which can be utilized to micor display pixel and multi-channel visible light communication (VLC) system. Finally, we believe that this dissertation points the way towards a promising avenue of highly efficient GaN-based solar cells and LEDs, and should be also beneficial for other types of optoelectronic devices.