High-resolution transmission-electron-microscopy study of ultrathin Al-induced crystallization of amorphous Si

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The process of ultrathin Al-induced crystallization of amorphous Si (a-Si) has been investigated by using high-resolution transmission electron microscopy and Auger electron spectroscopic depth profiling. Ultrathin Al overlayers, with thicknesses of 2.0 and 4.5 nm, have been shown to be capable of inducing full crystallization of an a-Si bottom layer as thick as 40 nm at temperatures as low as 320  C. After full crystallization of a-Si, the Al of the original 2.0-nm Al overlayer completely moved through the Si layer, leaving a high-purity, large-grained crystalline Si layer above it. Such movement of Al also occurs for the originally 4.5-nm Al overlayer, but in this case the crystallized Si layer is relatively fine-grained and contains 5.0 at.% of residual Al nanocrystals distributed throughout the layer. The observations have been interpreted on the basis of sites available for nucleation of crystalline Si in the microstructure of the Al/Si layer system upon annealing.

I. INTRODUCTION

Thin-film crystalline silicon (c-Si) is the key material for fabricating high-performance thin film transistors in advanced active matrix flat-panel displays,1 and for manufacturing low-cost, high-efficiency solar cells.2,3 For both applications, the use of low-cost, lightweight, flexible or even rollable substrates as supporting media is of great interest.4,5 However, these new types of substrates, such as polymers, are often heat-sensitive.4 As a result, traditional technologies developed for preparing c-Si thin films, which typically require high process temperatures (above 700  C), cannot meet these technological demands. Therefore, various techniques have been investigated in recent years for preparing c-Si thin films at (much) reduced temperatures (compared to 700  C; see above discussion), such as excimer-laser annealing,1 sequential lateral solidification,1 and metal-induced crystallization (MIC).6–16 MIC involves that the temperature for crystallization of amorphous Si (a-Si) is greatly reduced if it is put in direct contact with a metal, such as Ni,8,9 Cu,10 and Al.6,11–16 In comparison with laser-induced crystallization techniques, MIC is a much more economic approach and can also be more easily integrated into current semiconductor-manufacturing processes on an industrial scale. However, a main drawback of MIC is that some metal may occur within the crystallized Si film after MIC. These metal remnants could act as recombination centers for free carriers in c-Si and consequently deteriorate the device performance. As a result, there is a pronounced need to a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0404

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J. Mater. Res., Vol. 24, No. 11, Nov 2009

understand and control the microstructure of Si films crystallized by MIC and, in particular, the formation of these metal remnants in the crystallized Si films. One potential solution to reduce the amount of metal within the crystallized Si film is by using ultrathin metal layers to catalyze the crystallization. This

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