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

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

Simulation-To-Flight (STF-1): A Mission to Enable CubeSat Software-Based Validation and Verification

The Simulation-to-Flight 1 (STF-1) CubeSat mission aims to demonstrate how legacy simulation technologies may be adapted for flexible and effective use on missions using the CubeSat platform. These technologies, named NASA Operational Simulator (NOS), have demonstrated significant value on several missions such as James Webb Space Telescope, Global Precipitation Measurement, Juno, and Deep Space Climate Observatory in the areas of software development, mission operations/training, verification and validation (V&V), test procedure development and software systems check-out. STF-1 will demonstrate a highly portable simulation and test platform that allows seamless transition of mission development artifacts to flight products. This environment will decrease development time of future CubeSat missions by lessening the dependency on hardware resources. In addition, through a partnership between NASA GSFC, the West Virginia Space Grant Consortium and West Virginia University, the STF-1 CubeSat will hosts payloads for three secondary objectives that aim to advance engineering and physical-science research in the areas of navigation systems of small satellites, provide useful data for understanding magnetosphere-ionosphere coupling and space weather, and verify the performance and durability of III-V Nitride-based materials

LOCC: Enabling the Characterization of On-Orbit, Minimally Shielded LEDs

Over the past decade, trends have shown a substantial growth of interest for small satellite solutions, ranging from earth-orbit imaging to cheap global communication networks. Along with space-based applications, several research missions are focused on testing novel sensors and materials, group III-V nitride semiconductors being among them. Small satellite missions provide a unique opportunity to gain an understanding of the reliability and operational characteristics of these materials when exposed to the harsh environment of space. Such insight will lead to unique space applications of these materials in larger missions. While the nature of electronic and optoelectronic devices involving III-V materials under the bombardment of ionizing radiation has been reported, these findings have mostly been established via controlled tests in terrestrial laboratories. In this work, a Low-powered Optoelectronic Characterizer for CubeSat (LOCC) has been developed to perform in- situ current-voltage measurements of III-V nitride based optoelectronic devices on-orbit. Lastly, it is important to note the use of III-V nitride materials in this experiment. Custom InGaN LEDs have been fabricated with a center emission wavelength of 465 nm The LOCC system includes a spectral confirmation module that is used for luminescence characterization of the devices. While most current-voltage measurement instruments are made for laboratory benches and consume higher amounts of power, this system is designed using low-power integrated circuits that are capable of supplying the necessary current while maintaining low-power operation. The system must also operate within the data transfer and storage limitations of the CubeSat platform. LOCC is designed to transmit the experimental results to the satellite’s controlling computer via an I2C bus. The characteristics of low- power consumption and small information storage requirements make LOCC an excellent match for this science mission. This paper details the design and control of the LOCC system. The design includes block diagrams, PCB layout, interfacing, and control. Additionally, the resulting current-voltage measurements, required wattage, and required data storage will be presented to illustrate functionality. This instrumentation will enable the study of III-V nitride based optoelectronic devices in space, as well as parallel advancements of electronics and optical sensors that can be used for short-distance range finding and shape rendering systems for satellite servicing missions

Improvement in the Light Extraction of Blue InGaN/GaN-Based LEDs Using Patterned Metal Contacts

We demonstrate a method to improve the light extraction from an LED using photonic crystal (PhC)-like structures in metal contacts. A patterned metal contact with an array of Silicon Oxide (SiO x ) pillars (440 nm in size) on an InGaN/GaN-based MQW LED has shown to increase output illumination uniformity through experimental characterization. Structural methods of improving light extraction using transparent contacts or dielectric photonic crystals typically require a tradeoff between improving light extraction and optimal electrical characteristics. The method presented here provides an alternate solution to provide a 15% directional improvement (surface normal) in the radiation profile and ~ 30% increase in the respective intensity profile without affecting the electrical characteristics of the device. Electron beam patterning of hydrogen silesquioxane (HSQ), a novel electron beam resist is used in patterning these metal contacts. After patterning, thermal curing of the patterned resist is done to form SiO x pillars. These SiO x pillars aid as a mask for transferring the pattern to the p-metal contact. Electrical and optical characterization results of LEDs fabricated with and without patterned contacts are presented. We present the radiation and intensity profiles of the planar and patterned devices extracted using Matlab-based image analysis technique from 200 μm (diameter) circular unpackaged LEDs

Nanotopographical Modulation of Cell Function through Nuclear Deformation

Although nanotopography has been shown to be a potent modulator of cell behavior, it is unclear how the nanotopographical cue, through focal adhesions, affects the nucleus, eventually influencing cell phenotype and function. Thus, current methods to apply nanotopography to regulate cell behavior are basically empirical. We, herein, engineered nanotopographies of various shapes (gratings and pillars) and dimensions (feature size, spacing and height), and thoroughly investigated cell spreading, focal adhesion organization and nuclear deformation of human primary fibroblasts as the model cell grown on the nanotopographies. We examined the correlation between nuclear deformation and cell functions such as cell proliferation, transfection and extracellular matrix protein type I collagen production. It was found that the nanoscale gratings and pillars could facilitate focal adhesion elongation by providing anchoring sites, and the nanogratings could orient focal adhesions and nuclei along the nanograting direction, depending on not only the feature size but also the spacing of the nanogratings. Compared with continuous nanogratings, discrete nanopillars tended to disrupt the formation and growth of focal adhesions and thus had less profound effects on nuclear deformation. Notably, nuclear volume could be effectively modulated by the height of nanotopography. Further, we demonstrated that cell proliferation, transfection, and type I collagen production were strongly associated with the nuclear volume, indicating that the nucleus serves as a critical mechanosensor for cell regulation. Our study delineated the relationships between focal adhesions, nucleus and cell function and highlighted that the nanotopography could regulate cell phenotype and function by modulating nuclear deformation. This study provides insight into the rational design of nanotopography for new biomaterials and the cell–substrate interfaces of implants and medical devices