Superconducting thin films of Y‐Ba‐Cu‐O were fabricated on a CaF2/GaAs(100) substrate using the rf planar magnetron sputtering technique. Substrates were heated as low as 450 °C during deposition. The as‐deposited films without high‐temperature post‐annealing have a transition temperature of 75 K and zero resistance temperature of 45 K. The perovskite structure usually observed in this system was not observed. The critical current (Jc) value of 2×103A/cm2was measured at 4.2 K.