Focused ion beam lithography combined with molecular-beam-epitaxial growth can be a useful tool for the formation of real-time patterned, embedded structures. For this purpose, sub-50 eV ion beams are essential to minimize ion induced damage and to ensure vertical localization of the deposited ions. The simultaneous patterning of a beam of dopant ions during wafer growth allows the realization of three-dimensional structures with doping profiles otherwise unattainable through conventional methods. This article reports on the successful fabrication of focused ion beam patterned Hall bars in epitaxially grown bulk GaAs and GaAs/AlGaAs heterostructures. The bulk Si2+ doped sample achieved a 77 K mobility of 4000 cm(2)V(-1) s(-1) for a carrier concentration of 3.4 X 10(17) cm(-3) while the heterostructure showed a 1.5 K mobility of 1. 8 X 10(5) cm(2) V-1 s(-1) at a carrier density of 5.5 X 10(11) cm(-2). It is also demonstrated that in situ device patterning reduces the number of required ex situ processing steps while maintaining the high quality of molecular-beam epitaxially grown material. Furthermore, the development of in-grown ohmic contact columns to buried structures is presented. A proposed three dimensionally integrated circuit and future applications of this new technology are also discussed.