In this work we report the surface morphology of amorphous germanium (a-Ge) thin films (140 nm thickness) following thermal outgassing of SiO2/Si substrates. The thermal outgassing was performed by annealing the samples in air at different temperatures from 400 to 900 degrees C. Annealing at 400 degrees C in slow (2 degrees C/min) and fast (10 degrees C/min) modes promotes the formation of bubbles on the surface. A cross sectional view by transmission electron microscope taken of the sample slow annealed at 400 degrees C reveals traces of gas species embedded in the a-Ge film, allowing us to propose a possible mechanism for the formation of the bubbles. The calculated internal pressure and number of gas molecules for this sample are 30 MPa and 38 x 10(8), respectively. Over an area of 22 x 10(-3)cm(2) the density of bubbles obtained at slow annealing (9 x 10(3)cm(-2)) is smaller than that at rapid annealing (6.4 x 10(4)cm(-2)), indicating that the amount of liberated gas in both cases is only a fraction of the total gas contained in the substrate. After increasing the annealing temperature in the slow mode, bubbles of different diameters (from tens of nanometers up to tens of micrometers) randomly distribute over the Ge film and they grow with temperature. Vertical diffusion of the outgas species through the film dominates the annealing temperature interval 400-600 degrees C, whereas coalescence of bubbles caused by lateral diffusion is detected after annealing at 700 degrees C. The bubbles explode after annealing the samples at 800 degrees C. Annealing at higher temperatures, such as 900 degrees C, leads to surface migration of the film to the exploded bubbles and eventually forming islands of polycrystalline GeO2. (C) 2014 Elsevier B.V. All rights reserved.