We have measured photon emission from individual luminescent states in GaInP/InP heterostructures, containing InP dots, using local injection from a scanning tunneling microscope tip. By changing the tip-sample bias we are able to see the Stark shift of the emission peaks, as well as the onset of impact ionization. We find that the exciton diffusion length is about 1 μm, while the minority carrier diffusion length is much less in our samples. Below the threshold for impact ionization the excitation is extremely local, limiting the excitation to one or a few quantum dots.