Neodymium (Nd) doped amorphous silicon nitride films with various Si concentrations (Nd:SiNx) were fabricated by reactive magnetron co-sputtering followed by thermal annealing. The time dynamics of the energy transfer in Nd:SiNx was investigated, a systematic optimization of its 1.1 μm emission was performed, and the Nd excitation cross section in SiNx was measured. An active Nd:SiNx micro-disk resonator was fabricated and enhanced radiation rate at 1.1 μm was demonstrated due to stimulated emission at the whispering gallery resonant modes. These results provide an alternative approach for the engineering of Si-based optical amplifiers and lasers on a silicon nitride materials platform.