In this work we investigate and compare the structural and optical properties of silicon-rich silicon oxide layers obtained by different deposition techniques, namely plasma enhanced chemical vapor deposition (PECVD) and RF magnetron sputtering. We demonstrate that, in contrast to what generally believed, properties of films grown by different methods are indeed very different as a result of the agglomeration properties. These data have also great implications on the performances of light emitting MOS devices whose active layer has been prepared by the two different techniques.