Maskless selective growth of Mg-doped GaAs films was performed by using a low-energy (30-200 eV) Mg-Ga focused ion beam (FIB) under the ambient of As-4 molecular beam. Under suitable As-4 pressure, all samples showed p-type and the carrier concentrations were around 10(17) cm(-3). We achieved very high sticking coefficient of Mg (k(Mg) of nearly 10(-1)) compared with the result obtained at Molecular beam epitaxy (MBE) growth (k(Mg) of nearly 10(-5)). A p-n junction was formed on n(+)-type GaAs substrate by maskless selective growth. The diode showed typical I V characteristics and the quality factor (n-value) was approximately 1.75 for the sample grown at 60 eV. These results indicate that this method would be suitable for making maskless selective micro-device fabrication. (c) 2006 Published by Elsevier B.V.