Phase-separated Al-Si and Al-Ge films, composed of Al cylinders embedded in an amorphous Si and Ge matrix, respectively, were investigated by sputtering method with substrate bias voltage. In the AI-Si system, the period of Al cylinders was increased by applying a negative bias voltage to the substrate, which is consistent with the simulation results. On the other hand, the diameter of Al cylinders increased up to a bias voltage of -80 V, and then it began decreasing beyond -100 V accompanied by the appearance of a chain-like arrangement of Al cylinders. In the Al-Ge system, the period and diameter of Al cylinders increased by applying a negative bias voltage. However, in the Ge-rich film prepared at a high bias voltage, relatively large dots with low area density were observed. Transmission electron microscope observation revealed that the film was composed of AlGe gamma(2) phase as a matrix and Al-rich amorphous dots. (C) 2008 Elsevier B.V. All rights reserved.