Paper SE+TF+NC-ThM3
Randomness and Roughening in Glancing Angle Deposition

Thursday, October 23, 2008, 8:40 am, Room 204

The unique highly-porous nanostructures created with Glancing Angle Deposition are a direct result of the randomness inherent to the condensation of atomic or molecular vapors. This randomness arises through quantum indeterminism in the atom-by-atom evaporation of the source material. At one extreme of film growth conditions, when adatom diffusion is high, the influence of vapor-substrate geometry is minimized and the resulting films are typically dense with isotropic structure and properties - in essence this is the regime of molecular beam epitaxy. When adatom mobility is reduced (e.g. through reduced temperature or the introduction of a reactive gas) geometrical effects become increasingly important and a balance develops between roughening due to the random arrival of atoms and smoothing due to the reduced, but still finite, adatom diffusion. Films grown under these conditions with normal-incidence vapor can be quite dense, yet the film surface will always roughen due to the random arrival of the vapor atoms, eventually resulting in a cauliflower-like fractal morphology. The unique nanostructures of GLAD are created when geometrical shadowing is used to amplify randomness-induced roughening - requiring the vapor to arrive at an angle larger than approximately 70 degrees from the substrate normal. We present here the first experimental observation, through x-ray reflectivity (XRR) measurements of thin silicon films, of the transition to the glancing angle growth regime. We find that film porosity increases as a function of thickness in the GLAD regime, whereas it decreases with thickness under the same growth conditions yet with nearer-normal vapor incidence. Silicon films deposited at room temperature onto rapidly rotating substrates exhibit linearly increasing density as a function of thickness when deposited at vapor incidence angles of less than 70 degrees, and linearly decreasing density (increasing porosity) when deposited at incidence angles above 70 degrees. We also show that significant 'filling-in' can occur during glancing-angle growth, where vapor deposits some distance below the growing film surface. These XRR measurements provide valuable new insight into the glancing angle deposition growth process, and will help to refine film nanostructure simulation and design models.