Electron-cyclotron-resonance plasma-excited molecular beam epitaxy (ECR-MBE) is a new technique for GaAs growth. This paper describes surface cleaning of GaAs and Si substrates at fairly low temperatures using hydrogen plasma, low temperature growth of GaAs on both substrates, and selective area growth of GaAs on both substrates partially covered with a silicon nitride (SiN) mask. The ability to clean and grow at low temperatures-assumed to be a benefit of using energetic particles—should permit us to grow layers on processed GaAs and/or Si substrates. The electrical properties of grown layers are also described. Selective area growth has been successfully carried out with no deposit on the mask or irregular growth at the mask edge. The desorption process introduced by impinging ions is found to play an important role in the selective area growth.