Luminescence spectra of GaN epitaxial layers grown on sapphire display a strong intensity modulation of the below-band gap transitions and on the low-energy side of the near-band gap transition. The intensity modulation is attributed to a microcavity formed by the semiconductor-air and semiconductor-substrate interface. The microcavity effect is enhanced by using metallic reflectors which increase the cavity finesse. It is shown that microcavity effects can be used to determine the refractive index of the microcavity active material. Using this method, the GaN refractive index is determined and expressed analytically by a Sellmeir fit. © 1997 American Institute of Physics. ͓S0003-6951͑97͒00421-X͔ Microcavity effects in semiconductor optoelectronic devices have attracted much attention due to the potential of high-efficiency light-emitting diodes ͑LED͒, and low threshold lasers. 1 The enhancement of the spontaneous emission by microcavity effects has been demonstrated for resonantcavity LEDs in organic 2 as well as semiconducting 3 material systems. High-finesse GaN microcavities with distributed Bragg reflectors were recently realized by Redwing et al. 4 In the present study, the microcavity effects occurring in GaN epitaxial layers are analyzed and used for refractive index determination. Due to the refractive index step at the substrate-epilayer interface, the cavity effects are observed in GaN layers with a sufficiently small surface roughness. 5 By using metallic silver reflectors instead of the weakly reflecting semiconductor-air interface, the microcavity effects can be strongly enhanced. It is shown that the near-band gap transition of GaN is modulated on the low-energy shoulder only. In contrast, the entire band of below-band gap transitions are modulated. A new method is developed to determine the refractive index of the optically active material of microcavity structures. The usefulness of this method is demonstrated for GaN and the refractive index of GaN is expressed in analytic form by the Sellmeir equation.The GaN epitaxial layers were grown on ͑0001͒ oriented sapphire in an Emcore metal-organic vapor phase epitaxy ͑MOVPE͒ system. An initial 200-Å-thick GaN buffer layer was grown at 500°C after nitridation of the substrate. A homogenous 3-m-thick Si-doped GaN epitaxial layer (n ϭ2ϫ10 18 cm Ϫ3 ) was grown at 1050°C. After growth, the substrate was polished to allow for transmittance measurements. These measurements were performed using a broadband xenon light source. A polished sapphire substrate was used for reference measurements. The photoluminescence measurements were performed at room temperature with excitation by the 325 nm line of a HeCd laser. The very high luminescence intensity of the samples demonstrates the excellent quality and high radiative efficiency of the GaN epitaxial films. An excitation power density of 10 W/cm 2 on the sample surface was used. The luminescence was dispersed in a 0.75 nm monochromator and detected by a GaAs photo-multiplier connected to phase-sensitive amplifi...