We have studied the thermal stability and spectroscopy of HN3 on Si(111)(7 × 7) in the temperature range from 120 to 1350 K. The results are similar to those observed on other two low-index Si surfaces. HN3 was found to molecularly adsorb on Si(111)(7 × 7) at 120 K, with the formation of dimers at higher dosages (≥ 2.0 L). At 270 K, HN3 began to decompose into HN and N2 species as indicated by the changes in the NH vibration mode in HREELS and the chemical shift of the N 1s XPS peak. Between 300 and 800 K, the NH species further decomposed into H and N on the surface, while N2 desorbed molecularly. In this temperature range, the steady increase of the SiNx and SiH peaks were noted and LEED exhibited a weak (1 × 1) pattern. When the surface was annealed at Ts > 800 K, H(a) recombined to desorb, with N remaining as the only species on the surface. Further annealing at higher temperatures caused the gradual transformation of the SiNx species into Si3N4, as confirmed by all the spectroscopic results, including LEED which showed an (8 × 8) pattern.