Abstract This work describes the temperature‐induced crystallization of amorphous Ge (a‐Ge) as a function of the thickness of the a‐Ge films (in the 12–2600 nm range), which were deposited both onto c‐Si substrates and c‐Si substrates covered with aluminium. After deposition, the samples were submitted to cumulative thermal annealing treatments. It is shown that the temperature of crystallization depends on the thickness of the a‐Ge films and to the presence (or not) of the Al layer. For an annealing temperature ( T a ) of ∼700 °C, for example, the Raman spectra of films thinner than ∼1000 nm and deposited onto c‐Si substrates are completely dominated by the sharp phonon mode of crystalline Si. Films with thicknesses equal to 300, 1000 and 2600 nm, deposited onto Al/c‐Si, and treated at T a = 600 °C, on the other hand, clearly display two additional peaks at 405 and 490 cm –1 . They correspond to the Raman modes of Si–Ge and Si–Si modes, suggesting the formation of a SiGe alloy during the thermal anneal of the films. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)