Analytical Studies of the Capping Layer Effect on Aluminum Induced Crystallization of Amorphous Silicon

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0910-A21-09

Analytical Studies of the Capping Layer Effect on Aluminum Induced Crystallization of Amorphous Silicon Husam H. Abu-Safe, Abul-Khair M. Sajjadul-Islam, Hameed A. Naseem, and William D. Brown Arkansas Photovoltaic Research Center, Department of Electrical Engineering, University of Arkansas, Fayetteville, AR, 72701 ABSTRACT The effect of capping layer on metal induced crystallization of amorphous silicon was studied. Three sets of samples were prepared in this study. All samples had the basic layer structure of amorphous silicon layer deposited on a glass substrate. This was followed by a thin aluminum layer deposition. The second and third sets, however, had a third layer of amorphous silicon with thicknesses of 20 and 50 nm, respectively. These layers were deposited on top of the aluminum. The samples were annealed at 400°C for 15, 30 and 45 minutes. The resultant films were characterized using X-Ray diffraction, scanning electron microscopy, energy dispersive xray spectroscopy, and atomic force microscopy. It was observed that the capping layer reduces protrusion formation and therefore improving the smoothness of the crystallized polysilicon films. INTRODUCTION Amorphous silicon (a-Si) can be transformed into polysilicon (poly-Si) at low temperatures using metal induced crystallization (MIC). In this technique, a-Si is deposited on a substrate followed by a thin layer of aluminum deposited on top. The transformation occurs when the samples are annealed at temperatures higher than 150°C [1-3]. However, if the film deposition sequence was reversed (i.e. aluminum first and then a-Si), the temperature required to obtain the transformation increases to about 500°C [4]. In the first configuration silicon protrusions are typically observed on top of the aluminum surface affecting the smoothness of the aluminum surface. The source of these protrusions is believed to be the silicon atoms coming from the a-Si layer below [2]. The silicon atoms diffuse through aluminum and appear on top of the upper layer leaving behind a rough poly-Si surface. The resulting poly-Si film although continuous is not smooth. Jenq et al. [5] have reported that the final poly-Si obtained from the second configuration mentioned above (comprising of substrate/aluminum/a-Si structures) is smoother than that from the first configuration (comprising of substrate/a-Si/aluminum structures.) Crystallization mechanism in the second configuration is dominated by amorphous silicon uptake and diffusion through aluminum to deposit as poly-Si resulting in layer inversion [4]. In the current study we propose a method to reduce the protrusions and the accompanying roughness in the crystallized films at low temperature by introducing a thin amorphous silicon layer on top of the aluminum layer. The aluminum mixes with the a-Si near the top thereby reducing the motive force for the diffusion of silicon atoms from the lower silicon layer. This is expected to yield a smooth surface as a-Si is transformed into poly-Si thin films.

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