Section 5. Crystallization and polycrystalline silicon Aluminum-induced crystallization: Nucleation and growth process J. Schneider, A. Schneider, A. Sarikov 1 , J. Klein, M. Muske, S. Gall, W. Fuhs * Hahn-Meitner-Institut Berlin, Kekule ´straße 5, D-12489 Berlin, Germany Available online 3 April 2006 Abstract The formation of polycrystalline silicon (poly-Si) films by Al-induced crystallization (ALILE process) was studied in situ by optical microscopy. The characteristic feature of this process is that nucleation is strongly suppressed after an initial nucleation period. The use of cooling periods in the course of annealing leads to enhanced nucleation and reveals that the formation of Si depleted regions around the growing grains prevents further nucleation. The growth mechanism is discussed starting from the phase diagram of the Al/Si system. It turns out that the critical parameter is the actual concentration of Si in Al and the value of supersaturation. Ó 2006 Elsevier B.V. All rights reserved. PACS: 61.66.Bi; 64.70.Kb; 64.75.tg; 68.55.Ac; 81.10.It; 81.30.Mh Keywords: Silicon; Crystallization; Nucleation 1. Introduction It has been known since long that amorphous silicon in contact with certain metals crystallizes at temperatures far below of the temperature of 600 °C normally required for solid phase crystallization [1]. The aluminum-induced layer exchange (ALILE) process is a special form of metal- induced crystallization (MIC) where a stack of glass/Al/ a-Si is transformed into a glass/poly-Si/Al(Si) stack by a simple annealing step at 300–500 °C [2]. Various studies [3–5] showed that this method allows for large grain size (d > 10 lm) and preferential (1 0 0) orientation which sug- gests the use of such films as templates for homoepitaxial growth of silicon at low temperatures. Therefore, such poly-Si layers on low-temperature substrates are of great interest for large area electronic devices such as solar cells [5]. Large grain size can be obtained owing to self-limita- tion of the nucleation rate: once the first nuclei have been formed further nucleation is strongly suppressed and the existing grains can grow to large grain size. In this work in situ optical microscopy is used to study the nucleation process in more detail. It will be demonstrated that the rea- son for the suppression of nucleation is the formation of Si- depleted regions around the growing grains where the Si concentration falls below the critical concentration required for nucleation. 2. Experimental procedure Standard RCA cleaning of the glass substrates (Corning 1737F) was employed prior to the film deposition. Both Al (300 nm) and a-Si (375 nm) layers were deposited by dc magnetron sputtering (target area 4 00 , power 500 W, Ar pressure 6.5 · 10 À3 mbar, 300 K) with deposition rates of 2.7 nm/s and 1.2 nm/s for Al and a-Si, respectively. Prior to the a-Si deposition the Al layer was thermally oxidized (520 °C, 1.5 h in oxygen atmosphere). It has been shown previously that such an Al-oxide layer forms a barrier for mass transport across the Al/a-Si interface [6]. The use of a thick interface layer resulted in long process times, low nucleation rates and large grain sizes [7] which is favorable here for a study of nucleation and growth. 0022-3093/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.09.036 * Corresponding author. Tel.: +49 30 8062 1301/1331; fax: +49 30 8062 1333. E-mail address: fuhs@hmi.de (W. Fuhs). 1 On leave from V. Lashkarev Institute of Semiconductor Physics NAS, 45 Nauki Avenue, Kiev 03028, Ukraine. www.elsevier.com/locate/jnoncrysol Journal of Non-Crystalline Solids 352 (2006) 972–975