X-ray reflectometry characterization of porous silicon films prepared by a glancing-angle deposition method

The competitive growth process of ballistic deposition is studied experimentally using x-ray reflectivity characterization of silicon thin films deposited with various vapor incidence angles. Linear profiles of film density with thickness are shown to reproduce the observed reflectivity spectra. A porosity of about 10% was observed in the first few atomic layers atop the substrate, with porosity decreasing with film thickness to near zero for vapor incidence between normal and 60 degrees, and porosity increasing to 50% and above for vapor incidence angles above 70 degrees-the regime of glancing-angle deposition. Our results support the model of glancing deposition as sequential atomic ballistic deposition, where the observed sign change in the slope of the density profile is understood to correspond to the geometric condition where the roughening caused by self-shadowing overtakes the smoothing effects of atomic surface diffusion. From the electron-density profiles derived from x-ray reflectivity measurements, we calculate the average porosity and using data on optical indices of refraction we estimate the amount of silicon oxide.