Monolithic integration of silicon micro-photonic devices with silicon microelectronics presents one potential solution to the interconnect bottleneck in deep submicron CMOS microprocessors. However, an inability to develop a robust silicon based electroluminescent (EL) technology continues to severely limit the feasibility of such a structure. Despite its promising photoluminescent (PL) properties, the erbium doped silicon rich silicon oxide material system has been only partly successful in EL devices owing to dielectric breakdown resulting from high field excitation requirements. However, PL data indicate that quantum confinement effects in this system enable it to overcome many of the luminescence quenching properties of bulk silicon based materials, while simultaneously offering the structural and chemical resilience which luminescent forms of silicon (eg. porous silicon) lack. However, it is now clear that the resolution of the current injection problem will require a detailed understanding of all known and especially potential luminescence excitation mechanisms. The present study compares PL measurements made for SiOx:Er (x less than or equal to 2) thin films grown by electron cyclotron resonance plasma enhanced chemical vapour deposition. Various compositions, containing erbium concentration below 1 atomic %, have been annealed over a range of temperatures between 600-1100 °C. The presence of excess silicon relative to SiO2 is found to result in a 100 fold increase of the PL emission near 1540 nm, resulting from the 4I13/2 to 4I15/2 transition of the trivalent erbium ion, relative to a sample containing no silicon excess. It has been found that luminescent silicon nano-clusters can be formed throughout the range of anneal temperatures studied. A dense array of extremely small amorphous silicon clusters is found to result in an optimized sensitization of the PL at 1540 nm for annealing at 800°C. It is proposed that in some cases, silicon nano-clusters coupled to erbium ions do not emit their intrinsic luminescence. Various oxygen-based luminescent defects are identified in this study, and they seem to exhibit a coupling to silicon nano-cluster and erbium-related centres. Possible application of these films to recently developed field-effect EL devices is discussed.