The spectral characteristics of spontaneous and stimulated luminescence of heavily silicon-doped Al (x) Ga1 - x N structures with the concentration n (Si) > 1020 cm(-3) are studied with pulsed optical pumping at lambda = 266 nm. The resulting dominant broadband radiation with a full width at half maximum of similar to 150 nm covers the whole visible spectral region. The radiation spectrum from the end of the structure is split into narrow components determined by the mode structure of the planar waveguide formed. The results indicate the stimulated character of the radiation. The optical gain for different structures fall in the range 20-70 cm(-1).