In solar applications, traditional crystalline silicon photovoltaic (PV) cells are the most commonly used technology to harvest solar energy. The efficiency of Si PV is fundamentally limited to around 33% and in practice, these cells have an outdoor efficiency of less than 22%. Concentrated PV technology uses multi-junction PV cells that collect a broader spectrum of the sun with high efficiency (>40% has been reported). However, due to the different semiconductors used, multi-junction cell costs are higher than traditional PV cells. Increasing the solar concentration not only reduces the cost of electricity produced by multi-junction cells, by reducing the required area, but can also maximize the IV efficiency of the cells. There exist different methods to concentrate solar energy such as large parabolic mirrors, which have tracking challenges due their size and weight; or spherical lens arrays, which have limited optical geometrical concentration ratios. In this respect, freeform optical devices can be used to enhance the optical throughput for multi-junction cells and reduce the space required to achieve large concentration ratios. In this work, we discuss a novel optical design combining aspherical lens arrays and arrays of optical waveguides, which constitute broadband, freeform non-imaging optical devices. We compare different waveguide designs which have been optimized using non-sequential ray tracing software. The relationship between the optical surface quality and the optical efficiency is also investigated. Finally, we present the results of the experimental characterization of these waveguides under laboratory conditions using different techniques to measure optical throughput and stray light losses.
Because of exceptional mechanical, chemical, and tribological properties, diamond has a great potential to be used as a material for the development of high-performance MEMS and NEMS such as resonators and switches compatible with harsh environments, which involve mechanical motion and intermittent contact. Integration of such MEMS/NEMS devices with complementary metal oxide semiconductor (CMOS) microelectronics will provide a unique platform for CMOS-driven commercial MEMS/NEMS. The main hurdle to achieve diamond-CMOS integration is the relatively high substrate temperatures (600-800°C) required for depositing conventional diamond thin films, which are well above the CMOS operating thermal budget (400 °C). Additionally, a materials integration strategy has to be developed to enable diamond-CMOS integration. Ultrananocrystalline diamond (UNCD), a novel material developed in thin film form at Argonne, is currently the only microwave plasma chemical vapor deposition (MPCVD) grown diamond film that can be grown at 400 °C, and still retain exceptional mechanical, chemical, and tribological properties comparable to that of single crystal diamond. We have developed a process based on MPCVD to synthesize UNCD films on up to 200 mm in diameter CMOS wafers, which will open new avenues for the fabrication of monolithically integrated CMOS-driven MEMS/NEMS based on UNCD. UNCD films were grown successfully on individual Si-based CMOS chips and on 200 mm CMOS wafers at 400 °C in a MPCVD system, using Ar-rich/CH4 gas mixture. The CMOS devices on the wafers were characterized before and after UNCD deposition. All devices were performing to specifications with very small degradation after UNCD deposition and processing. A threshold voltage degradation in the range of 0.08-0.44V and transconductance degradation in the range of 1.5-9% were observed.
We report a novel method of fabricating self-assembled carbon nanotube (CNT) on Si nanocrystals and the photocurrent from this network. Silicon-on-insulator (SOI) substrate with 10nm thin top silicon layer is annealed at elevate temperature in an ultra-high vacuum environment. The Si layer dewets and aggregates into Si nanocrystal islands with dimensions about 90 nm high, 100-150 nm wide, and 200nm apart. 1nm thin Fe film is deposited on the decomposed SOI as catalyst for CNT growth. The growth is done by chemical vapor deposition (CVD) at 900 °C with a flow of CH4 at 400sccm and H2 at 20sccm. The CVD grown CNTs show strong preferential growth on the top portion of the Si nanocrystals and form a suspended network connecting the nanocrystals. No photolithographic process is needed to create this self-assembled CNT network. We find that the reason that few CNT are found on the oxide surface is because of the influence of the island topography on the CH4 gas flow pattern, with feedstock unable to reach the oxide surface when the islands are close to each other. We demonstrate that, by shining a low power 650nm wavelength commercial red laser pointer on this network, it generates photocurrent on the level of 20nA photocurrent under 1 volt bias condition. Since a 100 mW 1.175 μm wavelength IR laser does not generate any distinguishable photocurrent in our measurement setup, we believe the photocurrent generated by 650 nm red laser mainly comes from the Si nanocrystals instead of the CNTs. We demonstrate that a dense, self-assembled CNT network can be formed on the decomposed Si nanocrystals and can be used as conducting media for electric measurement.
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