Microcavity light-emitting diodes (MCLEDs) with top-emitting geometry have been grown by molecular beam epitaxy on GaAs and Si substrates. External quantum efficiencies of up to 10% were obtained for 420 x 420 mu m(2) homoepitaxial devices. The efficiency of 32 x 32 mu m(2) diodes was 4% for homoepitaxial devices and 1% for diodes realized on conformal GaAs-on-Si stripes. Much lower efficiencies were obtained for devices grown directly on GaAs-on-Si seed layers, demonstrating that the conformal epitaxy technique leads to an improvement of the GaAs-on-Si materials quality. These results underline the promise of the conformal epitaxy approach for the realization of monolithically integrated light emitters on silicon substrates. (C) 1999 Elsevier Science B.V. All rights reserved.