On the growth mechanism of polycrystalline silicon thin film by Al-induced layer exchange process Noritaka Usami a,b,n , Mina Jung a , Takashi Suemasu b,c a Institute for Materials Research (IMR), Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan b CREST, Japan Science and Technology Agency, Sanbancho, Tokyo 102-0075, Japan c Institute of Applied Physics, University of Tsukuba, 1-1-1 Tennohdai, Tsukuba, Ibaraki 305-8573, Japan article info Available online 6 August 2012 Keywords: A1. Solid solutions A1. Optical microscopy A3. Al-induced layer exchange process B2. Silicon abstract We attempted to clarify controlling mechanisms of Al-induced layer exchange process of Al and amorphous silicon (a-Si) and microstructures in resultant polycrystalline silicon (poly-Si) thin film by utilizing in situ observation of the growth process. Introduction of a few nm-thick germanium adlayer remarkably reduces crystallization time and affects the grain size of poly-Si films. This is likely to be accompanied by the growth mode transition as suggested by change of Avrami constant. Control of the Al/a-Si interface is of crucial importance to control the growth process. & 2012 Elsevier B.V. All rights reserved. 1. Introduction The polycrystalline silicon (poly-Si) thin films on inexpensive glass substrates are attractive for large area electronics and solar cell applications [1,2]. Especially, it would be ideal if one could realize polycrystalline films with wafer-equivalent crystal quality, that is, poly-Si thin film crystals with large crystal grains, controlled crystal orientations and grain boundary characters. Such crystals will open a possibility for further heteroepitaxy of Si-based materials including BaSi 2 for solar cell applications [3]. Among various approaches, aluminum-induced layer exchange (ALILE) process has been regarded as one of the promising approaches since it proceeds with a relatively slow nucleation rate owing to the small amount of supersaturation, leading to high-quality large crystal grains in the order of 10–100 mm [4–6]. ALILE starts with stacked structures of amorphous Si(a-Si)/oxide/Al/substrate followed by annealing below the eutectic temperature (577 1C) in the Al–Si system. This leads to exchange of Al and a-Si layers and the initial a-Si is trans- formed into the continuous poly-Si films. Since ALILE process occurs by dissociation of Si atoms in Al through the a-Si/oxide/Al interface, the interfacial structure is considered to play an impor- tant role and many efforts have been done to play with the oxide at the interface. As another approach to modify the interface, we have recently reported that insertion of 10 nm-thick Ge layer at a-Si/Al interface affects the crystallization process by changing the driving force [7]. This confirms that the control of the interface can be a tool to control the growth of poly-Si by ALILE. Therefore, detailed study is required to clarify the impact of the initial structures of the interface on the growth mechanisms and microstructures of poly-Si thin films. In this paper, we report on the impact of the thickness of the Ge adlayer inserted at the a-Si/Al interface. An in-situ monitoring system disclosed that only a few nm-thick Ge adlayer remarkably reduces crystallization time and affects the grain size of poly-Si films. When the thickness of the Ge adlayer was 1 nm, the average grain size became almost twice compared with that of the control sample without Ge. However, further increase in Ge thickness lead to decrease of the grain size due to the increased nucleation rate. Avrami theory [8,9] was applied to interpret the growth mechanism depending on the thicknesses of the Ge adlayer. 2. Experiments Glass substrates were cleaned with acetone, methanol, and deionized water. The 100 nm-thick Al layers were deposited on the glass substrates using a thermal evaporator. Details of samples preparation are described elsewhere [10]. Then, samples were exposed to air for 48 h to form the native Al oxide layer. The Ge adlayer was deposited using a radio frequency (RF) sputter- ing system and the thickness was controlled as 1, 3, and 5 nm. Besides, a sample without Ge was prepared as the reference. Following to the deposition of the Ge layer, 100 nm-thick a-Si film was deposited in the same sputtering system. Finally, samples were annealed at 525 1C under argon gas ambient for 3 h. Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/jcrysgro Journal of Crystal Growth 0022-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2012.07.023 n Corresponding author at: Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, 980-8577, Japan. Tel.: þ81 22 215 2014. E-mail address: usa@imr.tohoku.ac.jp (N. Usami). Journal of Crystal Growth 362 (2013) 16–19