A Fabry-Perot microcavity is used for the alteration of the spontaneous emission spectrum in hydrogenated amorphous silicon nitride. The modified photon density of states of the Fabry-Perot microcavity are responsible for the alteration of the spontaneous emission spectrum. The Fabry-Perot microcavity enhances the intensity of the spontaneous emission signal by a factor of 4 at the photon energies corresponding to the microcavity resonances. The 0.075 eV wide spontaneous emission linewidth of the Fabry-Perot microcavity resonances is 7 times smaller than the 0.5 eV wide spontaneous emission linewidth of the bulk hydrogenated amorphous silicon nitride. © 1998 Elsevier Science B.V. All rights reserved.