Selective plasma etching of Ge with a CF4/O2mixture is used to produce freestanding GaAs‐AlGaAs thin films. The etch rate of Ge substrates is as high as 150 &mgr;m/h at temperatures of 75 °C under optimized conditions and with a negligible etch rate for GaAs and AlGaAs materials. This plasma etching technique, combined with a lattice‐matched growth of GaAs‐AlGaAs structures on Ge substrates, has a variety of potential device applications. The characteristics of this etch process and the photoluminescence of freestanding AlGaAs‐GaAs structures are presented.