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. (c) 2012 Elsevier B.V. All rights reserved.