We present experimental measurements on silicon-on-insulator (SOI) photonic crystal slabs with an active layer containing Er 3+ ions-doped silicon nanoclusters (Si-nc), showing strong enhancement of 1.54 mum emission at room temperature. We provide a systematic theoretical analysis to interpret such results. In order to get further insight, we discuss experimental data on the guided luminescence of unpatterned SOI planar slot waveguides, which show enhanced light emission in transverse-magnetic (TM) modes over transverse-electric (TE) ones.