Phase evolution in nanocrystalline silicon films: Hydrogen dilution and the cone kinetics model

Abstract

Our cone kinetics model of heterogeneous thin film growth explains the evolution of the crystalline inclusions that form during chemical vapor deposition (CVD) growth of silicon films. Various morphologies, including isolated crystallites and conical nanocrystalline formations, form during plasma-enhanced (PE) and other CVD techniques when isotropic growth is coupled with the point nucleation of a second phase with a higher growth rate. Cone angles are determined by the relative growth rates alone. By generalizing the physics of cone formation, we create a qualitative phase diagram that predicts film morphology from two factors during deposition: the relative amorphous and crystalline growth rates and the rate of crystallite nucleation. It is well known that both of these factors are influenced by the H-dilution of the film precursor gases. Analysis of the statistics of cone geometries and densities in published experimental data on PECVD-grown nanocrystalline silicon shows that the cone angle increases monotonically with H-dilution. In the context of the cone kinetics model, this implies that increased H-dilution results in an increase of the ratio of the nanocrystalline silicon growth rate (νnc) to the amorphous silicon growth rate (νa). At a specific H-dilution, νnc/νa exceeds unity and nanocrystalline cone formation results.