Design, Fabrication and Characterization of Photonic Crystal Light-Emitting Diodes for Solid-State Lighting.

摘要

Residential, commercial, and industrial lighting applications contribute to ∼19% of total energy consumption worldwide. The application of more efficient sources of lighting, such as solid-state lighting (SSL) sources, could result in potential energy savings of about 65%. Current technologies employ semiconductor-based light-emitting diodes (LEDs) as the core elements of SSL devices to provide general-purpose light in a wide range of color temperatures. However, there still exists several device level issues, such as poor material quality, low quantum efficiencies, large percentage of light being trapped, etc. These non-idealities are barriers for SSL sources replacing incandescent and compact fluorescent sources on an equivalent lumens-per-watt basis.;WVU SSL research interests involve addressing device-level issues associated with III-V nitride materials, as well as optimizing the growth of materials and performance of fabricated devices. One major goal of research efforts is to provide solutions for improvement in light extraction in III-nitride-based devices through the use of integrated, device-level optical elements such as photonic crystals. Photonic Crystals (PhCs) are periodic dielectric structures that possess unique optical properties. PhCs are known for possessing an optical band gap that enables blocking of certain range of wavelengths based on their feature sizes. Additionally, they can also be utilized as diffractive elements when placed in the path of the photons. PhC structures in LEDs are commonly utilized for light extraction improvement and the integration process into the device structure often results in sub-optimal electrical characteristics. The work presented here provides the details of novel processes to add nanophotonic structures to metal and transparent conducting contacts (like indium tin oxide (ITO)) for indium gallium nitride/gallium nitride (InGaN/GaN) based multi-quantum well blue LEDs with emission wavelength in range of lambda=440--470 nm. The developed integration processes will enable improvement in the light extraction of the devices while reducing damage to the active regions of the device and maintaining optimal electrical characteristics. Novel electron beam resist like hydrogen silsesquioxane (HSQ) was utilized to achieve integration of PhCs with minimal degradation. Due to its unique chemical properties, a new classification of PhC structures were realized, that involves cured form of HSQ and named hybrid PhCs. Applying this process, hybrid PhC structures with features of 150 nm in diameter with a pitch of 500 nm in triangular and square lattice configurations fabricated in ITO contacts were integrated into the LEDs. As a result, the devices with hybrid PhC structures showed an improvement of ∼5x in intensity when compared to the unpatterned device.;This work also involved the development of novel bilayer methods using HSQ and sacrificial polymer layers for successful integration of PhCs with holes in transparent conducting layer contacts like ITO. The bilayer process developed will enable in realizing the more traditional PhC structures without the aforementioned process induced sub-optimal electrical characteristics. Additionally, nanosphere lithography (NSL) techniques like spin coating and thermal evaporation were explored as alternative patterning methodologies to enable integration of PhC structures on a large-scale. Utilizing thermal evaporation method, a 98.5% coverage of uniform single layer of polystyrene beads was achieved over a 1.5 x 1.5 cm2 area. This approach to device fabrication will allow PhCs to be integrated into commercial devices inducing less structural damage.