Niobium and niobium nitride films have been deposited on n‐GaAs substrates using dc magnetron sputtering to form Nb/GaAs and NbN/GaAs Schottky diodes. Current‐voltage(I‐V) measurements were made on the diodes and the metal/GaAs interfaces were studied by Rutherford backscattering spectrometry. For the Nb/GaAs Schottky diodes, without rapid thermal annealing and after annealing for 10 s at temperatures up to 600 °C, the ideality factor and the Schottky barrier height as measured by the current‐voltage technique vary slightly from 1.01 to 1.03 and from 0.75 to 0.78 V, respectively. For annealing above 750 °C, evidence of interdiffusion between Nb and GaAs is observed. In contrast to the Nb/GaAs diodes, the NbN/GaAs Schottky diodes have poor characteristics without high‐temperature annealing; however, after annealing at 800 °C, the diodes show excellent forward I‐V characteristics with an ideality factor of 1.06 and a barrier height of 0.73 V. No trace of interdiffusion between NbN and GaAs is found after annealing up to 800 °C.