Microcrystalline Si thin films by re-crystallization of electrophoretic deposited Si nanoparticle films
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Microcrystalline Si thin films by re-crystallization of electrophoretic deposited Si nanoparticle films Braden Bills1,2, Mukul Dubey2, Baojie Yan3, David Stevenson3, Qi Hua Fan2,1 1
Applied NanoFilms, 2301 Research Park Way STE 217, Brookings, SD 57006, USA Electrical Engineering department, South Dakota State University, Brookings, SD 57006, USA 3 Wintek Electro-Optical Corp., 1665 Highland Dr. STE E, Ann Arbor, MI 48108, USA 2
ABSTRACT A new route to producing microcrystalline silicon (µc-Si) thin films by re-crystallizing Si nanoparticle films by flash lamp method is presented. High quality Si nanoparticle films with high uniformity and high particle packing density were obtained using a stable non-aqueous Si nanoparticle suspension and the electrophoretic deposition (EPD) method. Morphology and crystallinity of as-deposited and flash lamp re-crystallized Si nanoparticle films were studied. INTRODUCTION Si nanoparticles are increasing in interest recently for their high surface area to volume ratio, quantum properties, and re-crystallization potential. Promising applications of Si nanoparticle films include Li ion battery electrode [1], spontaneous hydrogen generation [2], photon downshifting for solid state lighting [3] and solar cells [4], quantum energy devices [5], and re-crystallized µc-Si. The latter is the present focus area. Among a variety of applications, µc -Si thin film is the critical material that leads to advanced flat panel displays with superior performance and high-efficiency solar cells with low-cost [6, 7]. Flat panel liquid crystal displays with µc-Si thin film transistors (TFTs) achieve higher display brightness and faster dynamic response while consuming less power compared to amorphous Si (a-Si) TFTs due to higher carrier mobility, better stability and lower recombination loss. Multi-junction thin film Si solar cells use µc-Si as bottom sub-cells to absorb red and infrared light and improve the stability and efficiency of the solar cell. Commercial grade µc-Si films have high uniformity in terms of thickness, composition, and structure. Further, low-cost devices require high deposition rates of µc-Si for high throughput and low processing temperatures to use inexpensive substrates, such as glass and polymers. There is currently no efficient method to produce large area µc-Si thin films at low temperatures. Conventional approaches include crystallization of a-Si films through laser annealing and high-temperature (600-1000 ºC) vacuum annealing [8, 9]. Laser annealing, commercially known as low temperature poly silicon (LTPS), is accomplished by scanning a laser beam across the a-Si film surface [10, 11]. This process is very slow and accounts for 80% loss of yield compared to conventional a-Si TFT [12]. Laser annealing is especially not suitable for large area devices, such as solar panels and TV screens. High-temperature vacuum annealing is not suitable for common substrate materials, e.g. glass and polymers, and is also not economic. Although recently there have been efforts using flash
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