Microcrystalline and Nanocrystalline Silicon: Simulation of Material Properties
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A24.3.1
MICROCRYSTALLINE AND NANOCRYSTALLINE SILICON: SIMULATION OF MATERIAL PROPERTIES R. BISWAS* AND B. C. PAN*§ AND V. SELVARAJ* *Department of Physics and Astronomy, Microelectronics Research Center and Ames Laboratory-USDOE, Iowa State University, Ames, Iowa 50011 §Department of Physics, University of Science and Technology of China, Hefei 230026, People’s Republic of China ABSTRACT We have simulated nano-crystalline silicon and microcrystalline silicon structures with varying crystallite volume fractions, using molecular dynamics simulations. The crystallite regions reside in an amorphous matrix. We find the amorphous matrix is better ordered in nanocrystalline-Si than in the homogenous amorphous silicon networks, consistent with the observed higher stability of H-diluted films. There is a critical size above which the crystallites are stable and may grow. Sub-nm size crystallites in the protocrystalline phase are found to reduce the strain of the amorphous matrix. We simulated micro-crystalline silicon with a substantial crystallite volume fraction. Microcrystalline structures exhibit a crystalline core surrounded by an amorphous shell with similarities to silicon nanowires. We find a relatively uniform H distribution in the amorphous region and a crystal-amorphous phase boundary that is not welldefined.
INTRODUCTION Mixed phase semiconductors near the phase boundary of amorphous and microcrystalline growth are a very active area of current research. These include two broad classes of new materials. One is microcrystalline silicon (µc), grown on the crystalline side of the phase boundary (Fig. 1) containing a large volume fraction of crystalline grains. The coalesced crystalline grains are separated by amorphous tissue. Microcrystalline silicon is being employed as a stable low bandgap solar cell material [1,2] and is stable to light exposure. The other class of material is grown on the amorphous side of the a-µc phase boundary, in a silane–hydrogen gas mixture with high H dilution ratio (R). Under H-dilution conditions small nano-crystallites with dimensions of a few nm are formed in a background a-Si:H matrix [3,4,5]. As the film grows in thickness, the crystallites grow in size and eventually coalesce at a critical film thickness into the micro-crystalline phase [3,4]. Higher H-dilution decreases the onset of the crystallinity. Most current a-Si:H solar cells employ thin layers of H-diluted materials grown before the onset of microcrystallinity. H-diluted material is markedly more stable to light-induced degradation [6,7,8]. This material is close in electronic properties and band gap to a-Si:H and has a crystallite volume fraction typically less than ~10%. Transmission electron microscopy has imaged the nm size crystallites dispersed in an amorphous matrix [9]. Among the key questions is why such nano-crystalline silicon material has improved stability to light-soaking, and whether the material has superior order over traditional a-Si:H. Fluctuation electron microscopy [10] of sputter-deposited unhydrog
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