Growth and Electronic Properties of Nanocrystalline Si
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0910-A13-01
Growth and Electronic Properties of Nanocrystalline Si Vikram L Dalal1, Kamal Muthukrishnan2, Satya Saripalli2, Dan Stieler2, and Max Noack2 1 Electrical and Comp. Engineering, Iowa State University, Ames, Iowa, 50011 2 Iowa State University, Ames, Iowa, 50011 ABSTRACT Nanocrystalline Silicon is an important electronic materials for solar cells, for display devices and for sensors. In this paper, we discuss the influence of ions on the growth and properties of the nanocrystalline Si:H material. Using a unique growth geometry, combination of hot wire and ECR plasma growth, we show that low energy ion bombardment is beneficial for growing high quality materials. Ion bombardment by H is shown to etch the films during growth and also promote crystallinity. The results of film growth are compared with simulations of growth using the SRIM program. The electronic properties measured include mobilities of both electrons and holes in device-type structures, carrier lifetimes, diffusion lengths, defect densities and capture cross-sections for holes. Electron mobility is found to increase with grain size, with a minimum mobility being in the range of 1 cm2/V-s. The hole mobility is also in this range, and three different methods of measuring it give approximately the same value. The capture crosssection for holes is of the order of 1-2 x 10-16 cm2. The lifetime of carriers is found to depend inversely on the defect density, implying that the recombination is trap controlled. INTRODUCTION Nanocrystalline Silicon (nc-Si:H) is widely used for solar cells [1-4] and for thin film transistors [5-7]. The material generally consists of small grains, of the order of 10-20 nm, surrounded by a thin amorphous tissue. The transport properties of electrons and holes are governed by the amorphous tissue. The tissue seems to be quite effective in passivating the grain boundaries, allowing for efficient transfer of electrons and holes from grain to grain. The electronic properties of resulting devices appear to depend strongly upon the degree of amorphous tissue present in the material, with the best solar cell device to-date being produced in materials with significant amount of amorphous tissue, the so-called near-the-edge material. The degree of amorphous tissue also affects the stability of the devices, with some devices degrading under light, some under exposure to moisture, and some not at all. Not much is known about the influence of ions on the growth and properties of nc-Si:H, and even less about nc-Ge:H. Some papers claim that ion bombardment is destructive [8,9]; some others show that it helps in crystallization [10,11] and good properties can be observed in such films and devices [12,13]. Some materials have primarily a orientation, and some others, a . Good device properties are obtained in both classes of materials.There is much left to study in this important material system, particularly what controls grain size and crystallinity; what is the influence of ions and of hydrogen dilution on grain size and orientation;
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