Microstructural evolution in si1−x Gex:H thin films for photovoltaic applications

The growth of hydrogenated silicon germanium alloy (Si_1-xGe_x:H) films by plasma enhanced chemical vapor deposition (PECVD) has been investigated using real time spectroscopic ellipsometry (RTSE) to understand the effect of incorporated Ge on the relationship between the evolution of microcrystallinity and the optical properties, the latter in the form of the complex dielectric function spectra (ε = ε₁ + iε₂). The motivation is to explore the variations in the growth and properties of microcrystalline alloy films that arise due to Ge incorporation in materials suitable for integration into thin film Si:H based photovoltaic devices. Variations in the microcrystal evolution and the optical properties of microcrystalline silicon germanium (μc-Si_1-xGe_x:H) are extracted from films that initially nucleate microcrystallites from the amorphous phase at a thickness near 100-200 Å for alloy films spanning from Si:H to Ge:H. Although an increase in absorption is observed for the alloys, low Ge content films do not show the critical point features characteristic of crystalline Ge. Transmission electron micrographs (TEMs), the microstructural evolution obtained from RTSE, and a conical growth model for microcrystallites have been used to identify the average microcrystallite nucleation density and cone half angle. Monotonic decreases in the cone half angle with increasing Ge content are observed, indicating a reduced difference between the growth rates of the amorphous and microcrystalline phases with higher Ge incorporation. Also the nucleation density is lower in the alloy films with higher incorporation of Ge, which is consistent with a weaker tendency for microcrystallite formation expected on a more disordered alloy substrate.