RAMAN INVESTIGATION OF SUBMICRO-GRAINED SI FILMS OBTAINED BY INCOHERENT UV PHOTO-CVD OF SILICON HYDRIDES

Silicon-based thin films have been obtained by the direct photodissoci ation of mono- or disilane gases in a windowless CVD reactor using the vacuum ultraviolet (VUV) argon emission generated by electrical disch arges. Atomic force microscopy, Raman scattering and elastic recoil de tection analysis techniques were applied to characterize the surface m orphology, the structural properties, and the hydrogen content of the deposits, respectively, as a function of the silicon hydride concentra tion in the feed gas and, to a lesser extent, of the substrate tempera ture. Independent of the hydride concentration, it is found that the l ayers have a low hydrogen content of the order of 5% to 10%. On the ot her hand, their surface roughness is limited to a few nanometers provi ding that the partial hydride gas pressure in the reactor does not exc eed 10 Pa. Raman investigations show that two different types of mater ial can be obtained: either a purely amorphous one for hydride concent rations not exceeding 10% or a mixture of Si submicrocrystals embedded in an amorphous medium for the higher concentrations. Given the condi tions of the growth technique used and the photodissociation mechanism s involved, it is concluded that a homogeneous nucleation process occu rs 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.