Raman investigation of submicro-grained Si films obtained by incoherent UV photo-CVD of silicon hydrides.

Abstract. Silicon-based thin films have been obtained by the direct photodissociation of mono- or disilane gases in a windowless CVD reactor using the vacuum ultraviolet (VUV) argon emission generated by electrical discharges. Atomic force microscopy, Raman scattering and elastic recoil detection analysis techniques were applied to characterize the surface morphology, the structural properties, and the hydrogen content of the deposits, respectively, as a function of the silicon hydride concentration in the feed gas and, to a lesser extent, of the substrate temperature. Independent of the hydride concentration, it is found that the layers have a low hydrogen content of the order of 5% to 10%. On the other hand, their surface roughness is limited to a few nanometers providing that the partial hydride gas pressure in the reactor does not exceed 10 Pa. Raman investigations show that two different types of material can be obtained: either a purely amorphous one for hydride concentrations not exceeding 10% or a mixture of Si submicrocrystals embedded in an amorphous medium for the higher concentrations. Given the conditions of the growth technique used and the photodissociation mechanisms involved, it is concluded that a homogeneous nucleation process occurs already within the gas phase, leading to the formation of nanosized Si grains, that may agglomerate before reaching the substrate where a further growing step eventually takes place. [ABSTRACT FROM AUTHOR]