Experimental and theoretical study of amorphous silicon roughness evolution grown by low-temperature PECVD

Using real-time spectroscopic ellipsometry (RTSE) the evolution of the surface roughness in aSi:H thin-films grown by low-temperature plasma-enhanced chemical vapor deposition (PECVD) has been studied as a function of the hydrogen dilution ratio Rd =[H2]/[SiH4] with 15 ≤ Rd ≤ 60. To describe the roughness evolution, we have used a 3D linearized continuum equation which includes a negative surface tension term to take into account the destabilizing effects of short-range attraction and/or shadowing, as well as a smoothing term corresponding to surface diffusion. Using this model we have obtained very good agreement with experimental results for the evolution of the surface roughness in the case of large dilution ratio. However, our results indicate that for small dilution ratio the surface slopes are significantly larger, and as a result additional nonlinear terms need to be included at large thicknesses. Our results also indicate that surface diffusion plays an important role during PECVD film growth while the diffusion rate increases with increasing hydrogen dilution ratio. We also find that the early stages of island nucleation play an important role in determining the subsequent roughness evolution. In particular, the assumption of large wetting angles (θW ≃ 90) for the 3D islands formed in the initial stages leads to significantly better agreement with experiments than a smaller wetting angle (θW ≃ 45). This is consistent with recent experiments on liquid Si droplets on SiO2 [H. Kanai et al., J. Mater Sci. 42, 9529 (2007)] substrates in which a wetting angle of 90 was observed.