A combined ion‐beam and molecular‐beam‐epitaxy (CIBMBE) system has been developed. This system consists of an ion implanter capable of producing ions in the energy range of 30 eV–30 keV and conventional solid‐source MBE. As a successful application of CIBMBE, low‐energy (100 eV) carbon ion (C+) irradiation during MBE growth of GaAs was carried out at substrate temperaturesTgbetween 500 and 590 °C. C+‐doped layers were characterized by low‐temperature (2 K) photoluminescence (PL), Raman scattering, and van der Pauw measurements. PL spectra of undoped GaAs grown by CIBMBE revealed that unintentional impurity incorporation into the epilayer is extremely small and precise doping effects are observable. CAsacceptor‐related emissions such as ‘‘g,’’ [g‐g], and [g‐g]&bgr;are observed and their spectra are significantly changed with increasing C+beam current densityIc. PL measurements showed that C atoms were efficiently incorporated during MBE growth by CIBMBE and were optically well activated as an acceptor in the as‐grown condition even forTgas low as 500 °C. Raman measurement showed negligible lattice damage of the epilayer bombarded with 100 eV C+with no subsequent heat treatment. These results indicate that contamination‐ and damage‐free impurity doping without postgrowth annealing can be achieved by the CIBMBE method. ©1995 American Institute of Physics.