Microcrystalline Si thin films by re-crystallization of electrophoretic deposited Si nanoparticle films

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. INTRODUCTIONSi 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 lamp to crystallize a-Si films [13,14], the resulting crystal size is relatively small (< 0.5 μm, on the same order as laser annealing) due to fundamental thermodynamic limitations.This paper presents progress of a new method to produce µc-Si films by flash lamp recrystallization of densely packed Si nanoparticle films, as illustrated in Figure 1. Objectives include (1) developing a stable Si nanoparticle suspension, (2) depositing high quali...