We use the 3ω method to measure the effective thermal conductivity of thin films of a-Ge with thicknesses of 20-150 nm in the temperature range of 30-300 K. By using a moving shadow mask, the films are grown on the same Si (001) substrate in a single deposition run to minimize changes in the microstructure. We observe a reduction in the effective conductivity of the films with the decreasing layer thickness. From the measured data we estimate values for both the film thermal conductivity and the thermal boundary resistance (TBR) between SiO2 /a-Ge/Si at the different temperatures. An experimental value of the interface resistance of 2× 10-8 m2 K/W is obtained at 300 K. The temperature dependence of the TBR differs appreciably from calculations based on the diffusive mismatch model. The values derived for the intrinsic thermal conductivity of the films, kfilm (300 K) =0.64 W/mK, agree with predictions from the minimum thermal conductivity model and with values measured by Cahill and Pohl [Phys. Rev. B 37, 8773 (1988)] for thicker films. © 2008 American Institute of Physics.