Thin films of hydrogenated amorphous silicon (a-Si:H) have been deposited by plasma-enhanced chemical vapour deposition (PECVD), and the resulting topography measured in-situ on a nanometre scale using a scanning tunnelling microscope (STM). An island structure is observed on the surface of device quality a-Si:H, which can be quantitatively analysed using a one dimensional Fourier transform of the topography. Results suggest that deposition is limited by the creation of dangling bonds on the a-Si:H surface and not by the surface transport of SiH 3 radicals at the deposition temperature (598 K). Island nucleation takes place through the abstraction of hydrogen atoms from the a-Si:H surface by plasma etching and the subsequent attachment of an SiH 3 radicals to the available sites. A thermally activated hydrogen effusion process around the edge of each island, where the step edge causes a high local hydrogen concentration, then creates further dangling bonds which allow the islands to grow. A simulation has been constructed, which confirms this two stage mechanism.