To achieve nanoscale infrared photodetector electrodes with low resistivity, ion-implantation is used to implant high dose of As ion into high-resistivity silicon, and followed by rapid thermal annealing (RTA). A 200 nm thick Si:As layer with resistivity of 10-4 Ω · cm is obtained. Characterization by atomic force microscopy shows that the surfaces of the ion-implanted samples are smooth with a root-mean-square (RMS) coarseness of 0.5 nm. Although introduction of As ions destroys the lattice structure of crystalline silicon and causes a plenty of defects, proper annealing can restore the crystal lattice, as evidenced by the HRTEM observation of the annealed sample prepared by using focused ion beam (FIB) technology. Besides, the measurements of hall effect and spreading resistance indicate that the Si:As layer has good electrical properties including high carrier concentrations 2.5 × 1020 cm-3, high electron mobilities 40 cm2/V · s, and high electrical conductivities. The low resistivity Si:As material obtained is suitable to be used as the back electrodes of silicon-based optoelectronic devices.