  We discuss the efficiency of single photon sources based on a single
quasi-monochromatic emitter (such as a semiconductor quantum dot) inserted in a
pillar microcavity. We show that their efficiency, which is in principle
excellent thanks to the Purcell effect, can be drastically limited by extrinsic
cavity losses, such as those related to the scattering by the sidewalls
roughness. We present novel design rules for micropillars in view of this
application and show that for the well-mastered GaAs/AlAs system more than 70%
of the emission can be concentrated into the collimated emission beam
associated with the fundamental cavity mode.
