Electron energy loss spectroscopy and energy filtered transmission electron microscopy of hydrogenated nano-crystalline Si thin films deposited via hydrogen profiling during hot-wire CVD for application in photovoltaics

Abstract

Energy filtered transmission electron microscope imaging gives direct evidence of the nano-void network within hydrogenated nano-crystalline silicon thin films, deposited by reducing the hydrogen dilution as a function of deposition time (so called hydrogen profiling) during hot-wire chemical vapor deposition. Electron thickness maps show a t/λ value of approximately 0.38 in the void-rich areas compared to 0.47 in the continuous film, where t is the depth through which the electron beam is scattered and λ the electron mean free path in the Si thin film. Three-window elemental mapping shows no oxygen diffusion throughout the film architecture, whereas spectral imaging in combination with scanning transmission electron microscopy-coupled energy loss near edge fine-structure analyses along the growth direction of the film, across the tip and along the grain boundaries of the conically shaped crystals reveal changes in the Si-L3,2 and L1 energy-loss edges. In particular, the amorphous tissue between cone tips in the incubation layer typically shows a poorly defined L2 ionization edge, which progressively becomes more pronounced in the crystalline areas. Subtle variations in the L1 lineshape along the length of the film, characteristic of hydrogen passivation, confirm the profiling during synthesis.