利用奈米尺度現象增強氮化鎵光電元件之效率

Abstract

近年來，三族氮化物光電元件被廣泛地研究與運用。三族氮化物發光二極體由於其壽命長、節能、高效率等特性，逐漸取代傳統照明成為下一世代照明系統。隨著三族氮化物發光二極體的蓬勃發展，以及氮化鎵材料具有高吸收係數與可調能隙等卓越光伏優點，三族氮化物太陽能電池也被期待實現超高效率全頻譜多接面太陽能電池。但三族氮化物光電元件的發展仍然受到本身材料特性與元件結構上之限制，使其效率無法再更進一步增強。在氮化鎵發光二極體方面，存在高缺陷密度、效率下降、量子侷限史塔克效應、低光萃取效率以及過寬的發光光場等問題。在氮化鎵太陽能電池方面則存在，光吸收不足、主動層壓電極化、表面反射率過高等問題。早期眾多研究者，由材料成長以及元件結構設計著手，但現今透過此兩種方式增強效率的幅度已趨於緩和。隨著近年來奈米科技的蓬勃發展，奈米尺度半導體之奈米現象提供了另一條可行的途徑來提升氮化鎵光電元件之效率。本論文運用了新穎的奈米現象來解決目前限制氮化鎵太陽能電池與發光二極體的問題，進而獲得高效率氮化鎵光電元件。 在氮化鎵太陽能電池部份，我們首先結合奈米壓印與磊晶側向成長技術，製作奈米空氣空缺陣列於氮化鎵太陽能電池之底部，由於側向成長機制，空氣空缺陣列可有效抑制缺陷的產生並減少自發極化與壓電極化對於內建電場的影響，有效提升內部量子效應，此外由於奈米空氣空缺與氮化鎵材料存在很大的折射率差異，因此奈米空氣空缺陣列能有效散射與反射入光，在電池達成光捕捉效應，增加光學吸收路徑，解決量子井吸收層吸收不足的問題。接著針對銦錫氧化物(ITO)在紫外光波段吸收過高與量子井吸收層吸收不足的問題進行解決，整合硫化鎘量子點與布拉格反射鏡，光利用量子點光子下轉換機制，將入射紫外光轉換為波長較長之光子，減少銦錫氧化物吸收的問題，此外透過布拉格反射鏡可有效反射入射光子，解決量子井吸收層吸收不足的問題，同時我們發現透過硫化鎘量子點與布拉格反射鏡的結合，硫化鎘量子點的光子下轉換機制也進一步被提升。 在發光二極體部分，首先，我們利用奈米小球微影技術製作大面積奈米環發光二極體，由於奈米尺度應力釋放效應，量子侷限史塔克效應也能被抑制，其應力釋放效應能透過調整奈米環之尺度而改變，伴隨不同應力釋放效果，我們能控制其發光波長，實現多發光色彩於同一磊晶樣品上，同時也提升光子複合效率，此外，奈米環結構也能有效提升光萃取，更進一步提升發光二極體之光輸出。第二部分，我們針對綠光發光二極體效率低落與發光光場過寬的問題進行改善，結合奈米小球微影技術與磊晶側向成長技術，我們成功製作多層空氣空缺陣列於發光二極體底部，由於磊晶側向成長技術，缺陷與內部應力被有效抑制，故內部量子效率被有效提升，同時多層空氣空缺陣列具有超高散射與反射之特性，有效萃取光子並集中發光光場，實現高效率指向性發光二極體。

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.